Here's an article about why we haven't seen evidence of aliens yet even though the universe is so big:
https://waitbutwhy.com/2014/05/fermi-paradox.html
The Fermi Paradox - Wait But Why
2014-05-21
Tim Urban
<div><div><p><em><strong>PDF: </strong>We made a fancy PDF of this post for printing and offline viewing. <a href="https://gum.co/wbw-fermi">Buy it here.</a> (Or see a <a href="https://waitbutwhy.com/wp-content/uploads/2016/04/fermi-paradox-preview.pdf">preview</a>.)</em></p>
<p>___________</p>
<p>Everyone feels something when they’re in a really good starry place on a really good starry night and they look up and see this:</p>
<p><a href="https://149909199.v2.pressablecdn.com/wp-content/uploads/2014/05/Stars-fixed.jpg"></a></p>
<p>Some people stick with the traditional, feeling struck by the epic beauty or blown away by the insane scale of the universe. Personally, I go for the old “existential meltdown followed by acting weird for the next half hour.” But everyone feels <em>something</em>.</p>
<p>Physicist Enrico Fermi felt something too—”Where is everybody?”</p>
<p>________________</p>
<p>A really starry sky seems <i>vast</i>—but all we’re looking at is our very local neighborhood. On the very best nights, we can see up to about 2,500 stars (roughly one hundred-millionth of the stars in our galaxy), and almost all of them are less than 1,000 light years away from us (or 1% of the diameter of the Milky Way). So what we’re really looking at is this:</p>
<p><a href="https://149909199.v2.pressablecdn.com/wp-content/uploads/2014/05/Milky-Way.jpg"></a></p>
<p>Galaxy image: <a href="http://skysurvey.org/">Nick Risinger</a></p>
<p>When confronted with the topic of stars and galaxies, a question that tantalizes most humans is, “Is there other intelligent life out there?” Let’s put some numbers to it—</p>
<p>As many stars as there are in our galaxy (100 – 400 billion), there are roughly an equal number of galaxies in the observable universe—so for every star in the colossal Milky Way, there’s a whole <em>galaxy</em> out there. All together, that comes out to the typically quoted range of <strong>between 10<sup>22</sup> and 10<sup>24</sup> total stars</strong>, which means that for every grain of sand on every beach on Earth, there are <em>10,000 stars</em> out there.</p>
<p>The science world isn’t in total agreement about what percentage of those stars are “sun-like” (similar in size, temperature, and luminosity)—opinions typically range from 5% to 20%. Going with the most conservative side of that (5%), and the lower end for the number of total stars (10<sup>22</sup>), gives us 500 quintillion, or <strong>500 billion billion sun-like stars. </strong></p>
<p>There’s also a debate over what percentage of those sun-like stars might be orbited by an Earth-like planet (one with similar temperature conditions that could have liquid water and potentially support life similar to that on Earth). Some say it’s as high as 50%, but let’s go with the more conservative 22% that came out of <a href="http://www.pnas.org/content/early/2013/10/31/1319909110.abstract">a recent PNAS study</a>. That suggests that there’s a potentially-habitable Earth-like planet orbiting at least 1% of the total stars in the universe—a total of <strong>100 billion billion Earth-like planets.</strong></p>
<p>So there are <em>100 Earth-like planets</em> for every grain of sand in the world. Think about that next time you’re on the beach.</p>
<p>Moving forward, we have no choice but to get completely speculative. Let’s imagine that after billions of years in existence, 1% of Earth-like planets develop life (if that’s true, every grain of sand would represent one planet with life on it). And imagine that on 1% of <em>those </em>planets, the life advances to an intelligent level like it did here on Earth. That would mean there were 10 quadrillion, or <strong>10 million billion intelligent civilizations in the observable universe.</strong></p>
<p>Moving back to just our galaxy, and doing the same math on the lowest estimate for stars in the Milky Way (100 billion), we’d estimate that there are <strong>1 billion Earth-like planets and 100,000 intelligent civilizations in our galaxy.</strong><a href="#footnote2-1-1567">1</a>
</p><p>SETI (Search for Extraterrestrial Intelligence) is an organization dedicated to listening for signals from other intelligent life. If we’re right that there are 100,000 or more intelligent civilizations in our galaxy, and even a fraction of them are sending out radio waves or laser beams or other modes of attempting to contact others, shouldn’t SETI’s <a href="https://upload.wikimedia.org/wikipedia/commons/6/63/USA.NM.VeryLargeArray.02.jpg">satellite dish array</a> pick up all kinds of signals?</p>
<p>But it hasn’t. Not one. Ever.</p>
<p><em>Where is everybody?</em></p>
<p>It gets stranger. Our sun is relatively young in the lifespan of the universe. There are far older stars with far older Earth-like planets, which should in theory mean civilizations far more advanced than our own. As an example, let’s compare our 4.54-billion-year-old Earth to a hypothetical 8-billion-year-old Planet X.</p>
<p><a href="https://149909199.v2.pressablecdn.com/wp-content/uploads/2014/05/timeline.png"></a></p>
<p>If Planet X has a similar story to Earth, let’s look at where their civilization would be today (using the orange timespan as a reference to show how huge the green timespan is):</p>
<p><a href="https://149909199.v2.pressablecdn.com/wp-content/uploads/2014/05/timeline-22.png"></a></p>
<p>The technology and knowledge of a civilization only 1,000 years ahead of us could be as shocking to us as our world would be to a medieval person. A civilization 1 million years ahead of us might be as incomprehensible to us as human culture is to chimpanzees. And Planet X is 3.4 <em>billion </em>years ahead of us…</p>
<p>There’s something called <a href="http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Kardashev_scale.html">The Kardashev Scale</a>, which helps us group intelligent civilizations into three broad categories by the amount of energy they use:</p>
<p>A <strong>Type I Civilization</strong> has the ability to use all of the energy on their <strong>planet</strong>. We’re not quite a Type I Civilization, but we’re close (Carl Sagan created a formula for this scale which puts us at a Type 0.7 Civilization).</p>
<p>A <strong>Type II Civilization </strong>can harness all of the energy of their <strong>host star</strong>. Our feeble Type I brains can hardly imagine how someone would do this, but we’ve tried our best, imagining things like a <a href="http://www.islandone.org/LEOBiblio/SETI1.HTM">Dyson Sphere</a>.</p>
<p><a href="https://149909199.v2.pressablecdn.com/wp-content/uploads/2014/05/Dyson-Sphere.png"></a></p>
<p>A <strong>Type III Civilization</strong> blows the other two away, accessing power comparable to that of <strong>the entire Milky Way galaxy</strong>.</p>
<p>If this level of advancement sounds hard to believe, remember Planet X above and their 3.4 <em>billion</em> years of further development. If a civilization on Planet X were similar to ours and were able to survive all the way to Type III level, the natural thought is that they’d probably have mastered inter-stellar travel by now, possibly even colonizing the entire galaxy.</p>
<p>One hypothesis as to how galactic colonization could happen is by creating <a href="http://www.rfreitas.com/Astro/ReproJBISJuly1980.htm">machinery</a> that can travel to other planets, spend 500 years or so self-replicating using the raw materials on their new planet, and then send two replicas off to do the same thing. Even without traveling anywhere near the speed of light, this process would colonize the whole galaxy in 3.75 million years, a relative blink of an eye when talking in the scale of billions of years:</p>
<p><a href="https://149909199.v2.pressablecdn.com/wp-content/uploads/2014/05/Colonize-Galaxy.png"></a></p>
<p><em>Source: <a href="http://www.nature.com/scientificamerican/journal/v283/n1/pdf/scientificamerican0700-38.pdf">Scientific American: “Where Are They”</a></em></p>
<p>Continuing to speculate, if 1% of intelligent life survives long enough to become a potentially galaxy-colonizing Type III Civilization, our calculations above suggest that there should be at least 1,000 Type III Civilizations in our galaxy alone—and given the power of such a civilization, their presence would likely be pretty noticeable. And yet, we see nothing, hear nothing, and we’re visited by no one.</p>
<p><strong>So where is everybody?</strong></p>
<p>_____________________</p>
<p>Welcome to the Fermi Paradox.</p>
<p>We have no answer to the Fermi Paradox—the best we can do is “possible explanations.” And if you ask ten different scientists what their hunch is about the correct one, you’ll get ten different answers. You know when you hear about humans of the past debating whether the Earth was round or if the sun revolved around the Earth or thinking that lightning happened because of Zeus, and they seem so primitive and in the dark? That’s about where we are with this topic.</p>
<p>In taking a look at some of the most-discussed possible explanations for the Fermi Paradox, let’s divide them into two broad categories—those explanations which assume that there’s no sign of Type II and Type III Civilizations because there <em>are </em>none of them out there, and those which assume they’re out there and we’re not seeing or hearing anything for other reasons.</p>
<p><strong>Explanation Group 1: There are no signs of higher (Type II and III) civilizations because there <em>are </em>no higher civilizations in existence.</strong></p>
<p>Those who subscribe to Group 1 explanations point to something called the non-exclusivity problem, which rebuffs any theory that says, “There are higher civilizations, but none of them have made any kind of contact with us because they all _____.” Group 1 people look at the math, which says there should be <em>so </em>many thousands (or millions) of higher civilizations, that at least <em>one </em>of them would be an exception to the rule. Even if a theory held for 99.99% of higher civilizations, the other .01% would behave differently and we’d become aware of their existence.</p>
<p>Therefore, say Group 1 explanations, it must be that there are no super-advanced civilizations. And since the math suggests that there are <em>thousands</em> of them just in our own galaxy, <em>something else must be going on</em>.</p>
<p>This something else is called <strong>The Great Filter.</strong></p>
<p>The Great Filter theory says that at some point from pre-life to Type III intelligence, there’s a wall that all or nearly all attempts at life hit. There’s some stage in that long evolutionary process that is extremely unlikely or impossible for life to get beyond. That stage is The Great Filter.</p>
<p><a href="https://149909199.v2.pressablecdn.com/wp-content/uploads/2014/05/Great-Filter1.png"></a></p>
<p>If this theory is true, the big question is, <em>Where in the timeline does the Great Filter occur? </em></p>
<p>It turns out that when it comes to the fate of humankind, this question is very important. Depending on where The Great Filter occurs, we’re left with three possible realities: <strong>We’re rare, we’re first, or we’re fucked.</strong></p>
<p><strong>1. We’re Rare (The Great Filter is <em>Behind </em>Us)</strong></p>
<p>One hope we have is that The Great Filter is behind us—we managed to <em>surpass</em> it, which would mean it’s <em>extremely rare </em>for life to make it to our level of intelligence. The diagram below shows only two species making it past, and we’re one of them.</p>
<p><a href="https://149909199.v2.pressablecdn.com/wp-content/uploads/2014/05/Great-Filter-RARE1.png"></a></p>
<p>This scenario would explain why there are no Type III Civilizations…but it would also mean that <em>we </em>could be one of the few exceptions now that we’ve made it this far. It would mean we have hope. On the surface, this sounds a bit like people 500 years ago suggesting that the Earth is the center of the universe—it implies that we’re <em>special</em>. However, something scientists call “observation selection effect” suggests that anyone who is pondering their own rarity is inherently part of an intelligent life “success story”—and whether they’re actually rare or quite common, the thoughts they ponder and conclusions they draw will be identical. This forces us to admit that being special is at least a possibility.</p>
<p>And if we are special, when exactly did we become special—i.e. <em>which step did we surpass that almost everyone else gets stuck on? </em></p>
<p><strong>One possibility: The Great Filter could be at the very beginning—it might be incredibly unusual for life to begin at all. </strong>This is a candidate because it took about a billion years of Earth’s existence to finally happen, and because we have tried extensively to replicate that event in labs and have never been able to do it. If this is indeed The Great Filter, it would mean that not only is there no intelligent life out there, there may be <em>no other life at all.</em></p>
<p><strong>Another possibility: The Great Filter could be the jump from the simple prokaryote cell to the complex eukaryote cell. </strong>After prokaryotes came into being, they remained that way for almost <em>two billion </em>years before making the evolutionary jump to being complex and having a nucleus. If this is The Great Filter, it would mean the universe is teeming with simple prokaryote cells and almost nothing beyond that.</p>
<p>There are a number of other possibilities—some even think the most recent leap we’ve made to our current intelligence is a Great Filter candidate. While the leap from semi-intelligent life (chimps) to intelligent life (humans) doesn’t at first seem like a miraculous step, Steven Pinker <a href="https://www.amazon.com/gp/product/B0042XA2XG/ref=as_li_tl?ie=UTF8&amp;camp=1789&amp;creative=390957&amp;creativeASIN=B0042XA2XG&amp;linkCode=as2&amp;tag=wabuwh00-20&amp;linkId=2GTTCWKITTD4G4MR">rejects the idea</a> of an inevitable “climb upward” of evolution: “Since evolution does not strive for a goal but just happens, it uses the adaptation most useful for a given ecological niche, and the fact that, on Earth, this led to technological intelligence only once so far may suggest that this outcome of natural selection is rare and hence by no means a certain development of the evolution of a tree of life.”</p>
<p>Most leaps do <em>not </em>qualify as Great Filter candidates. Any possible Great Filter must be one-in-a-billion type thing where one or more total freak occurrences need to happen to provide a crazy exception—for that reason, something like the jump from single-cell to multi-cellular life is ruled out, because it has occurred as many as <a href="http://www-eve.ucdavis.edu/grosberg/Grosberg%20pdf%20papers/2007%20Grosberg%20%26%20Strathmann.AREES.pdf">46 times</a>, in isolated incidents, just on this planet alone. For the same reason, if we were to find a fossilized eukaryote cell on Mars, it would rule the above “simple-to-complex cell” leap out as a possible Great Filter (as well as anything before that point on the evolutionary chain)—because if it happened on <em>both </em>Earth and Mars, it’s almost definitely not a one-in-a-billion freak occurrence.</p>
<p>If we are indeed rare, it could be because of a fluky biological event, but it also could be attributed to what is called the <strong>Rare Earth Hypothesis</strong>, which suggests that though there may be many Earth-<em>like </em>planets, the <em>particular</em> conditions on Earth—whether related to the specifics of this solar system, its relationship with the moon (a moon that large is unusual for such a small planet and contributes to our particular weather and ocean conditions), or something about the planet itself—are exceptionally friendly to life.</p>
<p><strong>2. We’re the First</strong></p>
<p><a href="https://149909199.v2.pressablecdn.com/wp-content/uploads/2014/05/Great-Filter-EARLY2.png"></a></p>
<p>For Group 1 Thinkers, if the Great Filter is <em>not </em>behind us, the <em>one </em>hope we have is that conditions in the universe are just recently, for the first time since the Big Bang, reaching a place that would allow intelligent life to develop. In that case, we and many other species may be <em>on our way </em>to super-intelligence, and it simply hasn’t happened yet. We happen to be here at the right time to become one of the first super-intelligent civilizations.</p>
<p>One example of a phenomenon that could make this realistic is the prevalence of gamma-ray bursts, insanely huge explosions that we’ve observed in distant galaxies. In the same way that it took the early Earth a few hundred million years before the asteroids and volcanoes died down and life became possible, it could be that the first chunk of the universe’s existence was full of cataclysmic events like gamma-ray bursts that would incinerate everything nearby from time to time and prevent any life from developing past a certain stage. Now, perhaps, we’re in the midst of an <a href="http://www.ncbi.nlm.nih.gov/pubmed/18855114">astrobiological phase transition</a> and this is the first time any life has been able to evolve for this long, uninterrupted.</p>
<p><strong>3. We’re Fucked (The Great Filter is <em>Ahead </em>of Us)</strong></p>
<p><a href="https://149909199.v2.pressablecdn.com/wp-content/uploads/2014/05/Great-Filter-FUCKED1.png"></a></p>
<p>If we’re neither rare nor early, Group 1 thinkers conclude that The Great Filter <em>must </em>be in our future. This would suggest that life regularly evolves to where we are, but that <em>something</em> prevents life from going much further and reaching high intelligence in almost all cases—and we’re unlikely to be an exception.</p>
<p>One possible future Great Filter is a regularly-occurring cataclysmic natural event, like the above-mentioned gamma-ray bursts, except they’re unfortunately not done yet and it’s just a matter of time before all life on Earth is suddenly wiped out by one. Another candidate is the possible inevitability that nearly all intelligent civilizations end up destroying themselves once a certain level of technology is reached.</p>
<p>This is why Oxford University philosopher Nick Bostrom <a href="http://www.nickbostrom.com/extraterrestrial.pdf">says that</a> “no news is good news.” The discovery of even simple life on Mars would be devastating, because it would cut out a number of potential Great Filters <em>behind </em>us. And if we were to find fossilized <em>complex</em> life on Mars, Bostrom says “it would be by far the worst news ever printed on a newspaper cover,” because it would mean The Great Filter is almost definitely <i>ahead </i>of us—ultimately dooming the species. Bostrom believes that when it comes to The Fermi Paradox, “the silence of the night sky is golden.”</p>
<p><strong>Explanation Group 2: Type II and III intelligent civilizations <em>are</em> out there—and there are logical reasons why we might not have heard from them.</strong></p>
<p>Group 2 explanations get rid of any notion that we’re rare or special or the first at anything—on the contrary, they believe in the <a href="https://books.google.com/books?id=ZkdqTFVNwmgC&amp;pg=PA20#v=onepage&amp;q&amp;f=false">Mediocrity Principle</a>, whose starting point is that there is nothing unusual or rare about our galaxy, solar system, planet, or level of intelligence, until evidence proves otherwise. They’re also much less quick to assume that the <em>lack</em> of evidence of higher intelligence beings is evidence of their nonexistence—emphasizing the fact that our search for signals stretches only about 100 light years away from us (0.1% across the galaxy) and suggesting a number of possible explanations. Here are 10:</p>
<p><strong>Possibility 1) Super-intelligent life could very well have already visited Earth, but before we were here. </strong>In the scheme of things, sentient humans have only been around for about 50,000 years, a little blip of time. If contact happened before then, it might have made some ducks flip out and run into the water and that’s it. Further, recorded history only goes back 5,500 years—a group of ancient hunter-gatherer tribes may have experienced some <em>crazy </em>alien shit, but they had no good way to tell anyone in the future about it.</p>
<p><strong>Possibility </strong><strong>2) The galaxy has been colonized, but we just live in some desolate rural area of the galaxy. </strong>The Americas may have been colonized by Europeans long before anyone in a small Inuit tribe in far northern Canada realized it had happened. There could be an urbanization component to the interstellar dwellings of higher species, in which all the neighboring solar systems in a certain area are colonized and in communication, and it would be impractical and purposeless for anyone to deal with coming all the way out to the random part of the spiral where we live.</p>
<p><strong>Possibility </strong><strong>3) The entire concept of physical colonization is a hilariously backward concept to a more advanced species. </strong>Remember the picture of the Type II Civilization above with the sphere around their star? With all that energy, they might have created a perfect environment for themselves that satisfies their every need. They might have crazy-advanced ways of reducing their need for resources and zero interest in leaving their happy utopia to explore the cold, empty, undeveloped universe.</p>
<p>An even more advanced civilization might view the <em>entire physical world</em> as a horribly primitive place, having long ago conquered their own biology and uploaded their brains to a virtual reality, eternal-life paradise. Living in the physical world of biology, mortality, wants, and needs might seem to them the way we view primitive ocean species living in the frigid, dark sea. FYI, thinking about another life form having bested mortality makes me incredibly jealous and upset.</p>
<p><strong>Possibility </strong><strong>4) There are scary predator civilizations out there, and most intelligent life knows better than to broadcast any outgoing signals and advertise their location. </strong>This is an unpleasant concept and would help explain the lack of any signals being received by the SETI satellites. It also means that we might be the super naive newbies who are being <em>unbelievably </em>stupid and risky by ever broadcasting outward signals. There’s a debate going on currently about whether we should engage in METI (Messaging to Extraterrestrial Intelligence—the reverse of SETI) or not, and most people say we should not. Stephen Hawking warns, “If aliens visit us, the outcome would be much as when Columbus landed in America, which didn’t turn out well for the Native Americans.” Even Carl Sagan (a general believer that any civilization advanced enough for interstellar travel would be altruistic, not hostile) <a href="http://www.kurzweilai.net/meti-should-we-be-shouting-at-the-cosmos">called</a> the practice of METI “deeply unwise and immature,” and recommended that “the newest children in a strange and uncertain cosmos should listen quietly for a long time, patiently learning about the universe and comparing notes, before shouting into an unknown jungle that we do not understand.” Scary.<a href="#footnote-1-1567">1</a>
</p><p><strong>Possibility </strong><strong>5) There’s only one instance of higher-intelligent life—a “superpredator” civilization (like humans are here on Earth)—that is <em>far</em> more advanced than everyone else and keeps it that way by exterminating any intelligent civilization once they get past a certain level. </strong>This would suck. The way it might work is that it’s an inefficient use of resources to exterminate all emerging intelligences, maybe because most die out on their own. But past a certain point, the super beings make their move—because to them, an emerging intelligent species becomes like a virus as it starts to grow and spread. This theory suggests that whoever was the <em>first</em> in the galaxy to reach intelligence won, and now no one else has a chance. This would explain the lack of activity out there because it would keep the number of super-intelligent civilizations to just one.</p>
<p><strong>Possibility </strong><strong>6) There’s plenty of activity and noise out there, but our technology is too primitive and we’re listening for the wrong things. </strong>Like walking into a modern-day office building, turning on a walkie-talkie, and when you hear no activity (which of course you wouldn’t hear because everyone’s texting, not using walkie-talkies), determining that the building must be empty. Or maybe, as Carl Sagan has pointed out, it could be that our minds work exponentially faster or slower than another form of intelligence out there—e.g. it takes them 12 years to say “Hello,” and when we hear that communication, it just sounds like white noise to us.</p>
<p><strong>Possibility </strong><b>7) We <em>are </em>receiving contact from other intelligent life, but the government is hiding it. </b>The more I learn about the topic, the more this seems like an idiotic theory, but I had to mention it because it’s talked about so much.</p>
<p><strong>Possibility </strong><b>8) Higher civilizations are aware of us and observing us (AKA the “Zoo Hypothesis”). </b>As far as we know, super-intelligent civilizations exist in a tightly-regulated galaxy, and our Earth is treated like part of a vast and protected national park, with a strict “Look but don’t touch” rule for planets like ours. We wouldn’t notice them, because if a far smarter species wanted to observe us, it would know how to easily do so without us realizing it. Maybe there’s a rule similar to the <i>Star Trek’s</i> “Prime Directive” which prohibits super-intelligent beings from making any open contact with lesser species like us or revealing themselves in any way, until the lesser species has reached a certain level of intelligence.</p>
<p><strong>Possibility </strong><strong>9) Higher civilizations are here, all around us. But we’re too primitive to perceive them. </strong>Michio Kaku <a href="http://dailygrail.com/features/michio-kaku-impossible-science">sums it up</a> like this: </p>
<p><em>Let’s say we have an anthill in the middle of the forest. And right next to the anthill, they’re building a ten-lane super-highway. And the question is “Would the ants be able to understand what a ten-lane super-highway is? Would the ants be able to understand the technology and the intentions of the beings building the highway next to them?”</em></p>
<p>So it’s not that we can’t pick up the signals from Planet X using our technology, it’s that we can’t even comprehend what the beings from Planet X <em>are</em> or what they’re trying to do. It’s <em>so </em>beyond us that even if they really wanted to enlighten us, it would be like trying to teach ants about the internet.</p>
<p>Along those lines, this may also be an answer to “Well if there are so many fancy Type III Civilizations, why haven’t they contacted us yet?” To answer that, let’s ask ourselves—when Pizarro made his way into Peru, did he stop for a while at an anthill to try to communicate? Was he magnanimous, trying to help the ants in the anthill? Did he become hostile and slow his original mission down in order to smash the anthill apart? Or was the anthill of complete and utter and eternal irrelevance to Pizarro? That might be our situation here.</p>
<p><strong>Possibility </strong><strong>10) We’re completely wrong about our reality. </strong>There are a lot of ways we could just be <em>totally </em>off with everything we think. The universe might appear one way and be something else entirely, like a <a href="http://www.nature.com/news/simulations-back-up-theory-that-universe-is-a-hologram-1.14328">hologram</a>. Or maybe <em>we’re</em> the aliens and we were planted here as an experiment or as a form of fertilizer. There’s even a chance that we’re all part of a computer simulation by some researcher from another world, and other forms of life simply weren’t programmed into the simulation.</p>
<p>________________</p>
<p>As we continue along with our possibly-futile search for extraterrestrial intelligence, I’m not really sure what I’m rooting for. Frankly, learning either that we’re officially alone in the universe or that we’re officially joined by others would be creepy, which is a theme with all of the surreal storylines listed above—<em>whatever</em> the truth actually is, it’s mindblowing.</p>
<p>Beyond its shocking science fiction component, The Fermi Paradox also leaves me with a deep humbling. Not just the normal “Oh yeah, I’m microscopic and my existence lasts for three seconds” humbling that the universe always triggers. The Fermi Paradox brings out a sharper, more personal humbling, one that can only happen after spending hours of research hearing your species’ most renowned scientists present <em>insane </em>theories, change their minds again and again, and wildly contradict each other—reminding us that future generations will look at us the same way we see the ancient people who were <em>sure </em>that the stars were the underside of the dome of heaven, and they’ll think “Wow they really had <em>no </em>idea what was going on.”</p>
<p>Compounding all of this is the blow to our species’ self-esteem that comes with all of this talk about Type II and III Civilizations. Here on Earth, we’re the king of our little castle, proud ruler of the huge group of imbeciles who share the planet with us. And in this bubble with no competition and no one to judge us, it’s rare that we’re ever confronted with the concept of being a dramatically inferior species to anyone. But after spending a lot of time with Type II and III Civilizations over the past week, our power and pride are seeming a bit David Brent-esque.</p>
<p>That said, given that my normal outlook is that humanity is a lonely orphan on a tiny rock in the middle of a desolate universe, the humbling fact that we’re probably not as smart as we think we are, and the possibility that a lot of what we’re sure about might be wrong, sounds wonderful. It opens the door just a crack that maybe, <em>just maybe</em>, there might be more to the story than we realize.</p>
<p><strong>Three more Wait But Why mind-benders:</strong></p>
<p><strong><a href="https://waitbutwhy.com/2015/08/how-and-why-spacex-will-colonize-mars.html">How (and Why) SpaceX Will Colonize Mars</a></strong> – A post I got to work on with Elon Musk and one that reframed my mental picture of the future.</p>
<p><strong><a href="https://waitbutwhy.com/2015/01/artificial-intelligence-revolution-1.html">The AI Revolution: Road to Superintelligence</a></strong> – Why any aliens that do visit us are likely to be artificial, not biological. We may not be the king of our castle for very much longer…</p>
<p><strong><a href="https://waitbutwhy.com/2014/12/what-makes-you-you.html">What Makes You You</a></strong> – It’s a harder question than you think.</p>
<p><strong>Some other popular Wait But Why posts:</strong></p>
<p><a href="https://waitbutwhy.com/2013/10/why-procrastinators-procrastinate.html">Why Procrastinators Procrastinate</a><a href="https://waitbutwhy.com/2014/02/pick-life-partner.html"><br />
How to Pick Your Life Partner<br />
</a><a href="https://waitbutwhy.com/2015/12/the-tail-end.html">The Tail End</a><br />
<a href="https://waitbutwhy.com/2014/05/life-weeks.html">Your Life in Weeks</a><br />
<a href="https://waitbutwhy.com/2014/10/religion-for-the-nonreligious.html">Religion for the Nonreligious</a></p>
<p>If you’re interested in supporting Wait But Why, <a href="https://www.patreon.com/waitbutwhy">here’s our Patreon</a>.<strong><br />
</strong></p>
<p><a href="https://gum.co/wbw-fermi"></a></p>
<p><strong>Sources:</strong><br />
PNAS: <a href="http://www.pnas.org/content/early/2013/10/31/1319909110.abstract">Prevalence of Earth-size planets orbiting Sun-like stars</a><br />
SETI: <a href="http://www.seti.org/drakeequation">The Drake Equation</a><br />
NASA: <a href="http://www.scribd.com/doc/58040076/Workshop-Report-on-the-Future-of-Intelligence-In-The-Cosmos-NASA-2007">Workshop Report on the Future of Intelligence In The Cosmos</a><br />
Keith Wiley: <a href="http://arxiv.org/abs/1111.6131">The Fermi Paradox, Self-Replicating Probes, and the Interstellar Transportation Bandwidth</a><br />
NCBI: <a href="http://www.ncbi.nlm.nih.gov/pubmed/18855114">Astrobiological phase transition: towards resolution of Fermi’s paradox</a><br />
André Kukla: <a href="https://books.google.com/books?id=ZkdqTFVNwmgC&amp;pg=PA20#v=onepage&amp;q&amp;f=false">Extraterrestrials: A Philosophical Perspective</a><br />
Nick Bostrom: <a href="http://www.nickbostrom.com/extraterrestrial.pdf">Where Are They?</a><br />
Science Direct: <a href="http://www.sciencedirect.com/science/article/pii/S1384107606000492">Galactic gradients, postbiological evolution and the apparent failure of SETI</a><br />
Nature: <a href="http://www.nature.com/news/simulations-back-up-theory-that-universe-is-a-hologram-1.14328">Simulations back up theory that Universe is a hologram</a><br />
Robin Hanson: <a href="http://hanson.gmu.edu/greatfilter.html">The Great Filter – Are We Almost Past It?</a><br />
John Dyson: <a href="http://www.islandone.org/LEOBiblio/SETI1.HTM">Search for Artificial Stellar Sources of Infrared Radiation</a></p>
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 The Fermi Paradox

 May 21, 2014 By Tim Urban


 PDF: We made a fancy PDF of this post for printing and offline viewing. Buy it here. (Or see a preview.)

 ___________

 Everyone feels something when they’re in a really good starry place on a really good starry night and they look up and see this:

 Some people stick with the traditional, feeling struck by the epic beauty or blown away by the insane scale of the universe. Personally, I go for the old “existential meltdown followed by acting weird for the next half hour.” But everyone feels something.

 Physicist Enrico Fermi felt something too—”Where is everybody?”

 ________________

 A really starry sky seems vast—but all we’re looking at is our very local neighborhood. On the very best nights, we can see up to about 2,500 stars (roughly one hundred-millionth of the stars in our galaxy), and almost all of them are less than 1,000 light years away from us (or 1% of the diameter of the Milky Way). So what we’re really looking at is this:

 Galaxy image: Nick Risinger

 When confronted with the topic of stars and galaxies, a question that tantalizes most humans is, “Is there other intelligent life out there?” Let’s put some numbers to it—

 As many stars as there are in our galaxy (100 – 400 billion), there are roughly an equal number of galaxies in the observable universe—so for every star in the colossal Milky Way, there’s a whole galaxy out there. All together, that comes out to the typically quoted range of between 1022 and 1024 total stars, which means that for every grain of sand on every beach on Earth, there are 10,000 stars out there.

 The science world isn’t in total agreement about what percentage of those stars are “sun-like” (similar in size, temperature, and luminosity)—opinions typically range from 5% to 20%. Going with the most conservative side of that (5%), and the lower end for the number of total stars (1022), gives us 500 quintillion, or 500 billion billion sun-like stars.

 There’s also a debate over what percentage of those sun-like stars might be orbited by an Earth-like planet (one with similar temperature conditions that could have liquid water and potentially support life similar to that on Earth). Some say it’s as high as 50%, but let’s go with the more conservative 22% that came out of a recent PNAS study. That suggests that there’s a potentially-habitable Earth-like planet orbiting at least 1% of the total stars in the universe—a total of 100 billion billion Earth-like planets.

 So there are 100 Earth-like planets for every grain of sand in the world. Think about that next time you’re on the beach.

 Moving forward, we have no choice but to get completely speculative. Let’s imagine that after billions of years in existence, 1% of Earth-like planets develop life (if that’s true, every grain of sand would represent one planet with life on it). And imagine that on 1% of those planets, the life advances to an intelligent level like it did here on Earth. That would mean there were 10 quadrillion, or 10 million billion intelligent civilizations in the observable universe.

 Moving back to just our galaxy, and doing the same math on the lowest estimate for stars in the Milky Way (100 billion), we’d estimate that there are 1 billion Earth-like planets and 100,000 intelligent civilizations in our galaxy.1

 SETI (Search for Extraterrestrial Intelligence) is an organization dedicated to listening for signals from other intelligent life. If we’re right that there are 100,000 or more intelligent civilizations in our galaxy, and even a fraction of them are sending out radio waves or laser beams or other modes of attempting to contact others, shouldn’t SETI’s satellite dish array pick up all kinds of signals?

 But it hasn’t. Not one. Ever.

 Where is everybody?

 It gets stranger. Our sun is relatively young in the lifespan of the universe. There are far older stars with far older Earth-like planets, which should in theory mean civilizations far more advanced than our own. As an example, let’s compare our 4.54-billion-year-old Earth to a hypothetical 8-billion-year-old Planet X.

 If Planet X has a similar story to Earth, let’s look at where their civilization would be today (using the orange timespan as a reference to show how huge the green timespan is):

 The technology and knowledge of a civilization only 1,000 years ahead of us could be as shocking to us as our world would be to a medieval person. A civilization 1 million years ahead of us might be as incomprehensible to us as human culture is to chimpanzees. And Planet X is 3.4 billion years ahead of us…

 There’s something called The Kardashev Scale, which helps us group intelligent civilizations into three broad categories by the amount of energy they use:

 A Type I Civilization has the ability to use all of the energy on their planet. We’re not quite a Type I Civilization, but we’re close (Carl Sagan created a formula for this scale which puts us at a Type 0.7 Civilization).

 A Type II Civilization can harness all of the energy of their host star. Our feeble Type I brains can hardly imagine how someone would do this, but we’ve tried our best, imagining things like a Dyson Sphere.

 A Type III Civilization blows the other two away, accessing power comparable to that of the entire Milky Way galaxy.

 If this level of advancement sounds hard to believe, remember Planet X above and their 3.4 billion years of further development. If a civilization on Planet X were similar to ours and were able to survive all the way to Type III level, the natural thought is that they’d probably have mastered inter-stellar travel by now, possibly even colonizing the entire galaxy.

 One hypothesis as to how galactic colonization could happen is by creating machinery that can travel to other planets, spend 500 years or so self-replicating using the raw materials on their new planet, and then send two replicas off to do the same thing. Even without traveling anywhere near the speed of light, this process would colonize the whole galaxy in 3.75 million years, a relative blink of an eye when talking in the scale of billions of years:

 Source: Scientific American: “Where Are They”

 Continuing to speculate, if 1% of intelligent life survives long enough to become a potentially galaxy-colonizing Type III Civilization, our calculations above suggest that there should be at least 1,000 Type III Civilizations in our galaxy alone—and given the power of such a civilization, their presence would likely be pretty noticeable. And yet, we see nothing, hear nothing, and we’re visited by no one.

 So where is everybody?

 _____________________

 Welcome to the Fermi Paradox.

 We have no answer to the Fermi Paradox—the best we can do is “possible explanations.” And if you ask ten different scientists what their hunch is about the correct one, you’ll get ten different answers. You know when you hear about humans of the past debating whether the Earth was round or if the sun revolved around the Earth or thinking that lightning happened because of Zeus, and they seem so primitive and in the dark? That’s about where we are with this topic.

 In taking a look at some of the most-discussed possible explanations for the Fermi Paradox, let’s divide them into two broad categories—those explanations which assume that there’s no sign of Type II and Type III Civilizations because there are none of them out there, and those which assume they’re out there and we’re not seeing or hearing anything for other reasons.

 Explanation Group 1: There are no signs of higher (Type II and III) civilizations because there are no higher civilizations in existence.

 Those who subscribe to Group 1 explanations point to something called the non-exclusivity problem, which rebuffs any theory that says, “There are higher civilizations, but none of them have made any kind of contact with us because they all _____.” Group 1 people look at the math, which says there should be so many thousands (or millions) of higher civilizations, that at least one of them would be an exception to the rule. Even if a theory held for 99.99% of higher civilizations, the other .01% would behave differently and we’d become aware of their existence.

 Therefore, say Group 1 explanations, it must be that there are no super-advanced civilizations. And since the math suggests that there are thousands of them just in our own galaxy, something else must be going on.

 This something else is called The Great Filter.

 The Great Filter theory says that at some point from pre-life to Type III intelligence, there’s a wall that all or nearly all attempts at life hit. There’s some stage in that long evolutionary process that is extremely unlikely or impossible for life to get beyond. That stage is The Great Filter.

 If this theory is true, the big question is, Where in the timeline does the Great Filter occur?

 It turns out that when it comes to the fate of humankind, this question is very important. Depending on where The Great Filter occurs, we’re left with three possible realities: We’re rare, we’re first, or we’re fucked.

 1. We’re Rare (The Great Filter is Behind Us)

 One hope we have is that The Great Filter is behind us—we managed to surpass it, which would mean it’s extremely rare for life to make it to our level of intelligence. The diagram below shows only two species making it past, and we’re one of them.

 This scenario would explain why there are no Type III Civilizations…but it would also mean that we could be one of the few exceptions now that we’ve made it this far. It would mean we have hope. On the surface, this sounds a bit like people 500 years ago suggesting that the Earth is the center of the universe—it implies that we’re special. However, something scientists call “observation selection effect” suggests that anyone who is pondering their own rarity is inherently part of an intelligent life “success story”—and whether they’re actually rare or quite common, the thoughts they ponder and conclusions they draw will be identical. This forces us to admit that being special is at least a possibility.

 And if we are special, when exactly did we become special—i.e. which step did we surpass that almost everyone else gets stuck on?

 One possibility: The Great Filter could be at the very beginning—it might be incredibly unusual for life to begin at all. This is a candidate because it took about a billion years of Earth’s existence to finally happen, and because we have tried extensively to replicate that event in labs and have never been able to do it. If this is indeed The Great Filter, it would mean that not only is there no intelligent life out there, there may be no other life at all.

 Another possibility: The Great Filter could be the jump from the simple prokaryote cell to the complex eukaryote cell. After prokaryotes came into being, they remained that way for almost two billion years before making the evolutionary jump to being complex and having a nucleus. If this is The Great Filter, it would mean the universe is teeming with simple prokaryote cells and almost nothing beyond that.

 There are a number of other possibilities—some even think the most recent leap we’ve made to our current intelligence is a Great Filter candidate. While the leap from semi-intelligent life (chimps) to intelligent life (humans) doesn’t at first seem like a miraculous step, Steven Pinker rejects the idea of an inevitable “climb upward” of evolution: “Since evolution does not strive for a goal but just happens, it uses the adaptation most useful for a given ecological niche, and the fact that, on Earth, this led to technological intelligence only once so far may suggest that this outcome of natural selection is rare and hence by no means a certain development of the evolution of a tree of life.”

 Most leaps do not qualify as Great Filter candidates. Any possible Great Filter must be one-in-a-billion type thing where one or more total freak occurrences need to happen to provide a crazy exception—for that reason, something like the jump from single-cell to multi-cellular life is ruled out, because it has occurred as many as 46 times , in isolated incidents, just on this planet alone. For the same reason, if we were to find a fossilized eukaryote cell on Mars, it would rule the above “simple-to-complex cell” leap out as a possible Great Filter (as well as anything before that point on the evolutionary chain)—because if it happened on both Earth and Mars, it’s almost definitely not a one-in-a-billion freak occurrence.

 If we are indeed rare, it could be because of a fluky biological event, but it also could be attributed to what is called the Rare Earth Hypothesis, which suggests that though there may be many Earth-like planets, the particular conditions on Earth—whether related to the specifics of this solar system, its relationship with the moon (a moon that large is unusual for such a small planet and contributes to our particular weather and ocean conditions), or something about the planet itself—are exceptionally friendly to life.

 2. We’re the First

 For Group 1 Thinkers, if the Great Filter is not behind us, the one hope we have is that conditions in the universe are just recently, for the first time since the Big Bang, reaching a place that would allow intelligent life to develop. In that case, we and many other species may be on our way to super-intelligence, and it simply hasn’t happened yet. We happen to be here at the right time to become one of the first super-intelligent civilizations.

 One example of a phenomenon that could make this realistic is the prevalence of gamma-ray bursts, insanely huge explosions that we’ve observed in distant galaxies. In the same way that it took the early Earth a few hundred million years before the asteroids and volcanoes died down and life became possible, it could be that the first chunk of the universe’s existence was full of cataclysmic events like gamma-ray bursts that would incinerate everything nearby from time to time and prevent any life from developing past a certain stage. Now, perhaps, we’re in the midst of an astrobiological phase transition and this is the first time any life has been able to evolve for this long, uninterrupted.

 3. We’re Fucked (The Great Filter is Ahead of Us)

 If we’re neither rare nor early, Group 1 thinkers conclude that The Great Filter must be in our future. This would suggest that life regularly evolves to where we are, but that something prevents life from going much further and reaching high intelligence in almost all cases—and we’re unlikely to be an exception.

 One possible future Great Filter is a regularly-occurring cataclysmic natural event, like the above-mentioned gamma-ray bursts, except they’re unfortunately not done yet and it’s just a matter of time before all life on Earth is suddenly wiped out by one. Another candidate is the possible inevitability that nearly all intelligent civilizations end up destroying themselves once a certain level of technology is reached.

 This is why Oxford University philosopher Nick Bostrom says that “no news is good news.” The discovery of even simple life on Mars would be devastating, because it would cut out a number of potential Great Filters behind us. And if we were to find fossilized complex life on Mars, Bostrom says “it would be by far the worst news ever printed on a newspaper cover,” because it would mean The Great Filter is almost definitely ahead of us—ultimately dooming the species. Bostrom believes that when it comes to The Fermi Paradox, “the silence of the night sky is golden.”

 Explanation Group 2: Type II and III intelligent civilizations are out there—and there are logical reasons why we might not have heard from them.

 Group 2 explanations get rid of any notion that we’re rare or special or the first at anything—on the contrary, they believe in the Mediocrity Principle, whose starting point is that there is nothing unusual or rare about our galaxy, solar system, planet, or level of intelligence, until evidence proves otherwise. They’re also much less quick to assume that the lack of evidence of higher intelligence beings is evidence of their nonexistence—emphasizing the fact that our search for signals stretches only about 100 light years away from us (0.1% across the galaxy) and suggesting a number of possible explanations. Here are 10:

 Possibility 1) Super-intelligent life could very well have already visited Earth, but before we were here. In the scheme of things, sentient humans have only been around for about 50,000 years, a little blip of time. If contact happened before then, it might have made some ducks flip out and run into the water and that’s it. Further, recorded history only goes back 5,500 years—a group of ancient hunter-gatherer tribes may have experienced some crazy alien shit, but they had no good way to tell anyone in the future about it.

 Possibility 2) The galaxy has been colonized, but we just live in some desolate rural area of the galaxy. The Americas may have been colonized by Europeans long before anyone in a small Inuit tribe in far northern Canada realized it had happened. There could be an urbanization component to the interstellar dwellings of higher species, in which all the neighboring solar systems in a certain area are colonized and in communication, and it would be impractical and purposeless for anyone to deal with coming all the way out to the random part of the spiral where we live.

 Possibility 3) The entire concept of physical colonization is a hilariously backward concept to a more advanced species. Remember the picture of the Type II Civilization above with the sphere around their star? With all that energy, they might have created a perfect environment for themselves that satisfies their every need. They might have crazy-advanced ways of reducing their need for resources and zero interest in leaving their happy utopia to explore the cold, empty, undeveloped universe.

 An even more advanced civilization might view the entire physical world as a horribly primitive place, having long ago conquered their own biology and uploaded their brains to a virtual reality, eternal-life paradise. Living in the physical world of biology, mortality, wants, and needs might seem to them the way we view primitive ocean species living in the frigid, dark sea. FYI, thinking about another life form having bested mortality makes me incredibly jealous and upset.

 Possibility 4) There are scary predator civilizations out there, and most intelligent life knows better than to broadcast any outgoing signals and advertise their location. This is an unpleasant concept and would help explain the lack of any signals being received by the SETI satellites. It also means that we might be the super naive newbies who are being unbelievably stupid and risky by ever broadcasting outward signals. There’s a debate going on currently about whether we should engage in METI (Messaging to Extraterrestrial Intelligence—the reverse of SETI) or not, and most people say we should not. Stephen Hawking warns, “If aliens visit us, the outcome would be much as when Columbus landed in America, which didn’t turn out well for the Native Americans.” Even Carl Sagan (a general believer that any civilization advanced enough for interstellar travel would be altruistic, not hostile) called the practice of METI “deeply unwise and immature,” and recommended that “the newest children in a strange and uncertain cosmos should listen quietly for a long time, patiently learning about the universe and comparing notes, before shouting into an unknown jungle that we do not understand.” Scary.1

 Possibility 5) There’s only one instance of higher-intelligent life—a “superpredator” civilization (like humans are here on Earth)—that is far more advanced than everyone else and keeps it that way by exterminating any intelligent civilization once they get past a certain level. This would suck. The way it might work is that it’s an inefficient use of resources to exterminate all emerging intelligences, maybe because most die out on their own. But past a certain point, the super beings make their move—because to them, an emerging intelligent species becomes like a virus as it starts to grow and spread. This theory suggests that whoever was the first in the galaxy to reach intelligence won, and now no one else has a chance. This would explain the lack of activity out there because it would keep the number of super-intelligent civilizations to just one.

 Possibility 6) There’s plenty of activity and noise out there, but our technology is too primitive and we’re listening for the wrong things. Like walking into a modern-day office building, turning on a walkie-talkie, and when you hear no activity (which of course you wouldn’t hear because everyone’s texting, not using walkie-talkies), determining that the building must be empty. Or maybe, as Carl Sagan has pointed out, it could be that our minds work exponentially faster or slower than another form of intelligence out there—e.g. it takes them 12 years to say “Hello,” and when we hear that communication, it just sounds like white noise to us.

 Possibility 7) We are receiving contact from other intelligent life, but the government is hiding it. The more I learn about the topic, the more t his seems like an idiotic theory, but I had to mention it because it’s talked about so much.

 Possibility 8) Higher civilizations are aware of us and observing us (AKA the “Zoo Hypothesis”). As far as we know, super-intelligent civilizations exist in a tightly-regulated galaxy, and our Earth is treated like part of a vast and protected national park, with a strict “Look but don’t touch” rule for planets like ours. We wouldn’t notice them, because i f a far smarter species wanted to observe us , it would know how to easi ly do so without us realizing it . Maybe there’s a rule similar to the Star Trek’s “Prime Directive” which prohibits super-intelligent beings from making any open contact with lesser species like us or revealing themselves in any way, until the lesser species has reached a certain level of intelligence.

 Possibility 9) Higher civilizations are here, all around us. But we’re too primitive to perceive them. Michio Kaku sums it up like this:

 Let’s say we have an anthill in the middle of the forest. And right next to the anthill, they’re building a ten-lane super-highway. And the question is “Would the ants be able to understand what a ten-lane super-highway is? Would the ants be able to understand the technology and the intentions of the beings building the highway next to them?”

 So it’s not that we can’t pick up the signals from Planet X using our technology, it’s that we can’t even comprehend what the beings from Planet X are or what they’re trying to do. It’s so beyond us that even if they really wanted to enlighten us, it would be like trying to teach ants about the internet.

 Along those lines, this may also be an answer to “Well if there are so many fancy Type III Civilizations, why haven’t they contacted us yet?” To answer that, let’s ask ourselves—when Pizarro made his way into Peru, did he stop for a while at an anthill to try to communicate? Was he magnanimous, trying to help the ants in the anthill? Did he become hostile and slow his original mission down in order to smash the anthill apart? Or was the anthill of complete and utter and eternal irrelevance to Pizarro? That might be our situation here.

 Possibility 10) We’re completely wrong about our reality . There are a lot of ways we could just be totally off with everything we think. The universe might appear one way and be something else entirely, like a hologram. Or maybe we’re the aliens and we were planted here as an experiment or as a form of fertilizer. There’s even a chance that we’re all part of a computer simulation by some researcher from another world, and other forms of life simply weren’t programmed into the simulation.

 ________________

 As we continue along with our possibly-futile search for extraterrestrial intelligence, I’m not really sure what I’m rooting for. Frankly, learning either that we’re officially alone in the universe or that we’re officially joined by others would be creepy, which is a theme with all of the surreal storylines listed above—whatever the truth actually is, it’s mindblowing.

 Beyond its shocking science fiction component, The Fermi Paradox also leaves me with a deep humbling. No t just the normal “Oh yeah, I’m microscopic and my existence lasts for three seconds” humbling that the universe always triggers. The Fermi Paradox brings out a sharper, more personal humbling, one that can only happen after spending hours of research hearing your species’ most renowned scientists present insane theories, change their minds again and again, and wildly contradict each other—reminding us that future generations will look at us the same way we see the ancient people who were sure that the stars were the underside of the dome of heaven, and they’ll think “Wow they really had no idea what was going on.”

 Compounding all of this is the blow to our species’ self-esteem that comes with all of this talk about Type II and III Civilizations. Here on Earth, we’re the king of our little castle, proud ruler of the huge group of imbeciles who share the planet with us. And in this bubble with no competition and no one to judge us, it’s rare that we’re ever confronted with the concept of being a dramatically inferior species to anyone. But after spending a lot of time with Type II and III Civilizations over the past week, our power and pride are seeming a bit David Brent-esque.

 That said, given that my normal outlook is that humanity is a lonely orphan on a tiny rock in the middle of a desolate universe, the humbling fact that we’re probably not as smart as we think we are, and the possibility that a lot of what we’re sure about might be wrong, sounds wonderful. It opens the door just a crack that maybe, just maybe, there might be more to the story than we realize.

 Three more Wait But Why mind-benders:

 How (and Why) SpaceX Will Colonize Mars – A post I got to work on with Elon Musk and one that reframed my mental picture of the future.

 The AI Revolution: Road to Superintelligence – Why any aliens that do visit us are likely to be artificial, not biological. We may not be the king of our castle for very much longer…

 What Makes You You – It’s a harder question than you think.

 Some other popular Wait But Why posts:

 Why Procrastinators Procrastinate
 How to Pick Your Life Partner
 The Tail End
 Your Life in Weeks
 Religion for the Nonreligious

 If you’re interested in supporting Wait But Why, here’s our Patreon.

 Sources:
 PNAS: Prevalence of Earth-size planets orbiting Sun-like stars
 SETI: The Drake Equation
 NASA: Workshop Report on the Future of Intelligence In The Cosmos
 Keith Wiley: The Fermi Paradox, Self-Replicating Probes, and the Interstellar Transportation Bandwidth
 NCBI: Astrobiological phase transition: towards resolution of Fermi’s paradox
 André Kukla: Extraterrestrials: A Philosophical Perspective
 Nick Bostrom: Where Are They?
 Science Direct: Galactic gradients, postbiological evolution and the apparent failure of SETI
 Nature: Simulations back up theory that Universe is a hologram
 Robin Hanson: The Great Filter – Are We Almost Past It?
 John Dyson: Search for Artificial Stellar Sources of Infrared Radiation

 1. Thinking about this logically, I think we should disregard all the warnings and get the outgoing signals rolling. If we catch the attention of super-advanced beings, yes, they might decide to wipe out our whole existence, but that’s not that different than our current fate (to each die within a century). And maybe, instead, they’d invite us to upload our brains into their eternal virtual utopia, which would solve the death problem and also probably allow me to achieve my childhood dream of bouncing around on the clouds. Sounds like a good gamble to me.↩

 1. The Drake Equation provides a formal method for this narrowing-down process we’re doing.↩

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Everyone feels something when they’re in a really good starry place on a really good starry night and they look up and see this:
   
Some people stick with the traditional, feeling struck by the epic beauty or blown away by the insane scale of the universe. Personally, I go for the old “existential meltdown followed by acting weird for the next half hour.” But everyone feels something.
Physicist Enrico Fermi felt something too—”Where is everybody?”
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A really starry sky seems vast—but all we’re looking at is our very local neighborhood. On the very best nights, we can see up to about 2,500 stars (roughly one hundred-millionth of the stars in our galaxy), and almost all of them are less than 1,000 light years away from us (or 1% of the diameter of the Milky Way). So what we’re really looking at is this:
   
Galaxy image: Nick Risinger 
When confronted with the topic of stars and galaxies, a question that tantalizes most humans is, “Is there other intelligent life out there?” Let’s put some numbers to it—
As many stars as there are in our galaxy (100 – 400 billion), there are roughly an equal number of galaxies in the observable universe—so for every star in the colossal Milky Way, there’s a whole galaxy out there. All together, that comes out to the typically quoted range of between 1022 and 1024 total stars, which means that for every grain of sand on every beach on Earth, there are 10,000 stars out there.
The science world isn’t in total agreement about what percentage of those stars are “sun-like” (similar in size, temperature, and luminosity)—opinions typically range from 5% to 20%. Going with the most conservative side of that (5%), and the lower end for the number of total stars (1022), gives us 500 quintillion, or 500 billion billion sun-like stars.  
There’s also a debate over what percentage of those sun-like stars might be orbited by an Earth-like planet (one with similar temperature conditions that could have liquid water and potentially support life similar to that on Earth). Some say it’s as high as 50%, but let’s go with the more conservative 22% that came out of a recent PNAS study. That suggests that there’s a potentially-habitable Earth-like planet orbiting at least 1% of the total stars in the universe—a total of 100 billion billion Earth-like planets. 
So there are 100 Earth-like planets for every grain of sand in the world. Think about that next time you’re on the beach.
Moving forward, we have no choice but to get completely speculative. Let’s imagine that after billions of years in existence, 1% of Earth-like planets develop life (if that’s true, every grain of sand would represent one planet with life on it). And imagine that on 1% of those planets, the life advances to an intelligent level like it did here on Earth. That would mean there were 10 quadrillion, or 10 million billion intelligent civilizations in the observable universe. 
Moving back to just our galaxy, and doing the same math on the lowest estimate for stars in the Milky Way (100 billion), we’d estimate that there are 1 billion Earth-like planets and 100,000 intelligent civilizations in our galaxy. 1
 SETI (Search for Extraterrestrial Intelligence) is an organization dedicated to listening for signals from other intelligent life. If we’re right that there are 100,000 or more intelligent civilizations in our galaxy, and even a fraction of them are sending out radio waves or laser beams or other modes of attempting to contact others, shouldn’t SETI’s satellite dish array pick up all kinds of signals?
But it hasn’t. Not one. Ever.
 Where is everybody? 
It gets stranger. Our sun is relatively young in the lifespan of the universe. There are far older stars with far older Earth-like planets, which should in theory mean civilizations far more advanced than our own. As an example, let’s compare our 4.54-billion-year-old Earth to a hypothetical 8-billion-year-old Planet X.
   
If Planet X has a similar story to Earth, let’s look at where their civilization would be today (using the orange timespan as a reference to show how huge the green timespan is):
   
The technology and knowledge of a civilization only 1,000 years ahead of us could be as shocking to us as our world would be to a medieval person. A civilization 1 million years ahead of us might be as incomprehensible to us as human culture is to chimpanzees. And Planet X is 3.4 billion years ahead of us…
There’s something called The Kardashev Scale, which helps us group intelligent civilizations into three broad categories by the amount of energy they use:
A Type I Civilization has the ability to use all of the energy on their planet. We’re not quite a Type I Civilization, but we’re close (Carl Sagan created a formula for this scale which puts us at a Type 0.7 Civilization).
A Type II Civilization can harness all of the energy of their host star. Our feeble Type I brains can hardly imagine how someone would do this, but we’ve tried our best, imagining things like a Dyson Sphere.
   
A Type III Civilization blows the other two away, accessing power comparable to that of the entire Milky Way galaxy.
If this level of advancement sounds hard to believe, remember Planet X above and their 3.4 billion years of further development. If a civilization on Planet X were similar to ours and were able to survive all the way to Type III level, the natural thought is that they’d probably have mastered inter-stellar travel by now, possibly even colonizing the entire galaxy.
One hypothesis as to how galactic colonization could happen is by creating machinery that can travel to other planets, spend 500 years or so self-replicating using the raw materials on their new planet, and then send two replicas off to do the same thing. Even without traveling anywhere near the speed of light, this process would colonize the whole galaxy in 3.75 million years, a relative blink of an eye when talking in the scale of billions of years:
   
 Source: Scientific American: “Where Are They”  
Continuing to speculate, if 1% of intelligent life survives long enough to become a potentially galaxy-colonizing Type III Civilization, our calculations above suggest that there should be at least 1,000 Type III Civilizations in our galaxy alone—and given the power of such a civilization, their presence would likely be pretty noticeable. And yet, we see nothing, hear nothing, and we’re visited by no one.
 So where is everybody? 
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Welcome to the Fermi Paradox.
We have no answer to the Fermi Paradox—the best we can do is “possible explanations.” And if you ask ten different scientists what their hunch is about the correct one, you’ll get ten different answers. You know when you hear about humans of the past debating whether the Earth was round or if the sun revolved around the Earth or thinking that lightning happened because of Zeus, and they seem so primitive and in the dark? That’s about where we are with this topic.
In taking a look at some of the most-discussed possible explanations for the Fermi Paradox, let’s divide them into two broad categories—those explanations which assume that there’s no sign of Type II and Type III Civilizations because there are none of them out there, and those which assume they’re out there and we’re not seeing or hearing anything for other reasons.
 Explanation Group 1: There are no signs of higher (Type II and III) civilizations because there are no higher civilizations in existence. 
Those who subscribe to Group 1 explanations point to something called the non-exclusivity problem, which rebuffs any theory that says, “There are higher civilizations, but none of them have made any kind of contact with us because they all _____.” Group 1 people look at the math, which says there should be so many thousands (or millions) of higher civilizations, that at least one of them would be an exception to the rule. Even if a theory held for 99.99% of higher civilizations, the other .01% would behave differently and we’d become aware of their existence.
Therefore, say Group 1 explanations, it must be that there are no super-advanced civilizations. And since the math suggests that there are thousands of them just in our own galaxy, something else must be going on.
This something else is called The Great Filter. 
The Great Filter theory says that at some point from pre-life to Type III intelligence, there’s a wall that all or nearly all attempts at life hit. There’s some stage in that long evolutionary process that is extremely unlikely or impossible for life to get beyond. That stage is The Great Filter.
   
If this theory is true, the big question is, Where in the timeline does the Great Filter occur?  
It turns out that when it comes to the fate of humankind, this question is very important. Depending on where The Great Filter occurs, we’re left with three possible realities: We’re rare, we’re first, or we’re fucked. 
 1. We’re Rare (The Great Filter is Behind Us) 
One hope we have is that The Great Filter is behind us—we managed to surpass it, which would mean it’s extremely rare for life to make it to our level of intelligence. The diagram below shows only two species making it past, and we’re one of them.
   
This scenario would explain why there are no Type III Civilizations…but it would also mean that we could be one of the few exceptions now that we’ve made it this far. It would mean we have hope. On the surface, this sounds a bit like people 500 years ago suggesting that the Earth is the center of the universe—it implies that we’re special. However, something scientists call “observation selection effect” suggests that anyone who is pondering their own rarity is inherently part of an intelligent life “success story”—and whether they’re actually rare or quite common, the thoughts they ponder and conclusions they draw will be identical. This forces us to admit that being special is at least a possibility.
And if we are special, when exactly did we become special—i.e. which step did we surpass that almost everyone else gets stuck on?  
 One possibility: The Great Filter could be at the very beginning—it might be incredibly unusual for life to begin at all. This is a candidate because it took about a billion years of Earth’s existence to finally happen, and because we have tried extensively to replicate that event in labs and have never been able to do it. If this is indeed The Great Filter, it would mean that not only is there no intelligent life out there, there may be no other life at all. 
 Another possibility: The Great Filter could be the jump from the simple prokaryote cell to the complex eukaryote cell. After prokaryotes came into being, they remained that way for almost two billion years before making the evolutionary jump to being complex and having a nucleus. If this is The Great Filter, it would mean the universe is teeming with simple prokaryote cells and almost nothing beyond that.
There are a number of other possibilities—some even think the most recent leap we’ve made to our current intelligence is a Great Filter candidate. While the leap from semi-intelligent life (chimps) to intelligent life (humans) doesn’t at first seem like a miraculous step, Steven Pinker rejects the idea of an inevitable “climb upward” of evolution: “Since evolution does not strive for a goal but just happens, it uses the adaptation most useful for a given ecological niche, and the fact that, on Earth, this led to technological intelligence only once so far may suggest that this outcome of natural selection is rare and hence by no means a certain development of the evolution of a tree of life.”
Most leaps do not qualify as Great Filter candidates. Any possible Great Filter must be one-in-a-billion type thing where one or more total freak occurrences need to happen to provide a crazy exception—for that reason, something like the jump from single-cell to multi-cellular life is ruled out, because it has occurred as many as 46 times, in isolated incidents, just on this planet alone. For the same reason, if we were to find a fossilized eukaryote cell on Mars, it would rule the above “simple-to-complex cell” leap out as a possible Great Filter (as well as anything before that point on the evolutionary chain)—because if it happened on both Earth and Mars, it’s almost definitely not a one-in-a-billion freak occurrence.
If we are indeed rare, it could be because of a fluky biological event, but it also could be attributed to what is called the Rare Earth Hypothesis, which suggests that though there may be many Earth-like planets, the particular conditions on Earth—whether related to the specifics of this solar system, its relationship with the moon (a moon that large is unusual for such a small planet and contributes to our particular weather and ocean conditions), or something about the planet itself—are exceptionally friendly to life.
 2. We’re the First 
   
For Group 1 Thinkers, if the Great Filter is not behind us, the one hope we have is that conditions in the universe are just recently, for the first time since the Big Bang, reaching a place that would allow intelligent life to develop. In that case, we and many other species may be on our way to super-intelligence, and it simply hasn’t happened yet. We happen to be here at the right time to become one of the first super-intelligent civilizations.
One example of a phenomenon that could make this realistic is the prevalence of gamma-ray bursts, insanely huge explosions that we’ve observed in distant galaxies. In the same way that it took the early Earth a few hundred million years before the asteroids and volcanoes died down and life became possible, it could be that the first chunk of the universe’s existence was full of cataclysmic events like gamma-ray bursts that would incinerate everything nearby from time to time and prevent any life from developing past a certain stage. Now, perhaps, we’re in the midst of an astrobiological phase transition and this is the first time any life has been able to evolve for this long, uninterrupted.
 3. We’re Fucked (The Great Filter is Ahead of Us) 
   
If we’re neither rare nor early, Group 1 thinkers conclude that The Great Filter must be in our future. This would suggest that life regularly evolves to where we are, but that something prevents life from going much further and reaching high intelligence in almost all cases—and we’re unlikely to be an exception.
One possible future Great Filter is a regularly-occurring cataclysmic natural event, like the above-mentioned gamma-ray bursts, except they’re unfortunately not done yet and it’s just a matter of time before all life on Earth is suddenly wiped out by one. Another candidate is the possible inevitability that nearly all intelligent civilizations end up destroying themselves once a certain level of technology is reached.
This is why Oxford University philosopher Nick Bostrom says that “no news is good news.” The discovery of even simple life on Mars would be devastating, because it would cut out a number of potential Great Filters behind us. And if we were to find fossilized complex life on Mars, Bostrom says “it would be by far the worst news ever printed on a newspaper cover,” because it would mean The Great Filter is almost definitely ahead of us—ultimately dooming the species. Bostrom believes that when it comes to The Fermi Paradox, “the silence of the night sky is golden.”
 Explanation Group 2: Type II and III intelligent civilizations are out there—and there are logical reasons why we might not have heard from them. 
Group 2 explanations get rid of any notion that we’re rare or special or the first at anything—on the contrary, they believe in the Mediocrity Principle, whose starting point is that there is nothing unusual or rare about our galaxy, solar system, planet, or level of intelligence, until evidence proves otherwise. They’re also much less quick to assume that the lack of evidence of higher intelligence beings is evidence of their nonexistence—emphasizing the fact that our search for signals stretches only about 100 light years away from us (0.1% across the galaxy) and suggesting a number of possible explanations. Here are 10:
 Possibility 1) Super-intelligent life could very well have already visited Earth, but before we were here. In the scheme of things, sentient humans have only been around for about 50,000 years, a little blip of time. If contact happened before then, it might have made some ducks flip out and run into the water and that’s it. Further, recorded history only goes back 5,500 years—a group of ancient hunter-gatherer tribes may have experienced some crazy alien shit, but they had no good way to tell anyone in the future about it.
 Possibility  2) The galaxy has been colonized, but we just live in some desolate rural area of the galaxy. The Americas may have been colonized by Europeans long before anyone in a small Inuit tribe in far northern Canada realized it had happened. There could be an urbanization component to the interstellar dwellings of higher species, in which all the neighboring solar systems in a certain area are colonized and in communication, and it would be impractical and purposeless for anyone to deal with coming all the way out to the random part of the spiral where we live.
 Possibility  3) The entire concept of physical colonization is a hilariously backward concept to a more advanced species. Remember the picture of the Type II Civilization above with the sphere around their star? With all that energy, they might have created a perfect environment for themselves that satisfies their every need. They might have crazy-advanced ways of reducing their need for resources and zero interest in leaving their happy utopia to explore the cold, empty, undeveloped universe.
An even more advanced civilization might view the entire physical world as a horribly primitive place, having long ago conquered their own biology and uploaded their brains to a virtual reality, eternal-life paradise. Living in the physical world of biology, mortality, wants, and needs might seem to them the way we view primitive ocean species living in the frigid, dark sea. FYI, thinking about another life form having bested mortality makes me incredibly jealous and upset.
 Possibility  4) There are scary predator civilizations out there, and most intelligent life knows better than to broadcast any outgoing signals and advertise their location. This is an unpleasant concept and would help explain the lack of any signals being received by the SETI satellites. It also means that we might be the super naive newbies who are being unbelievably stupid and risky by ever broadcasting outward signals. There’s a debate going on currently about whether we should engage in METI (Messaging to Extraterrestrial Intelligence—the reverse of SETI) or not, and most people say we should not. Stephen Hawking warns, “If aliens visit us, the outcome would be much as when Columbus landed in America, which didn’t turn out well for the Native Americans.” Even Carl Sagan (a general believer that any civilization advanced enough for interstellar travel would be altruistic, not hostile) called the practice of METI “deeply unwise and immature,” and recommended that “the newest children in a strange and uncertain cosmos should listen quietly for a long time, patiently learning about the universe and comparing notes, before shouting into an unknown jungle that we do not understand.” Scary.1
  Possibility  5) There’s only one instance of higher-intelligent life—a “superpredator” civilization (like humans are here on Earth)—that is far more advanced than everyone else and keeps it that way by exterminating any intelligent civilization once they get past a certain level. This would suck. The way it might work is that it’s an inefficient use of resources to exterminate all emerging intelligences, maybe because most die out on their own. But past a certain point, the super beings make their move—because to them, an emerging intelligent species becomes like a virus as it starts to grow and spread. This theory suggests that whoever was the first in the galaxy to reach intelligence won, and now no one else has a chance. This would explain the lack of activity out there because it would keep the number of super-intelligent civilizations to just one.
 Possibility  6) There’s plenty of activity and noise out there, but our technology is too primitive and we’re listening for the wrong things. Like walking into a modern-day office building, turning on a walkie-talkie, and when you hear no activity (which of course you wouldn’t hear because everyone’s texting, not using walkie-talkies), determining that the building must be empty. Or maybe, as Carl Sagan has pointed out, it could be that our minds work exponentially faster or slower than another form of intelligence out there—e.g. it takes them 12 years to say “Hello,” and when we hear that communication, it just sounds like white noise to us.
 Possibility  7) We are receiving contact from other intelligent life, but the government is hiding it. The more I learn about the topic, the more this seems like an idiotic theory, but I had to mention it because it’s talked about so much.
 Possibility  8) Higher civilizations are aware of us and observing us (AKA the “Zoo Hypothesis”). As far as we know, super-intelligent civilizations exist in a tightly-regulated galaxy, and our Earth is treated like part of a vast and protected national park, with a strict “Look but don’t touch” rule for planets like ours. We wouldn’t notice them, because if a far smarter species wanted to observe us, it would know how to easily do so without us realizing it. Maybe there’s a rule similar to the Star Trek’s “Prime Directive” which prohibits super-intelligent beings from making any open contact with lesser species like us or revealing themselves in any way, until the lesser species has reached a certain level of intelligence.
 Possibility  9) Higher civilizations are here, all around us. But we’re too primitive to perceive them. Michio Kaku sums it up like this: 
 Let’s say we have an anthill in the middle of the forest. And right next to the anthill, they’re building a ten-lane super-highway. And the question is “Would the ants be able to understand what a ten-lane super-highway is? Would the ants be able to understand the technology and the intentions of the beings building the highway next to them?” 
So it’s not that we can’t pick up the signals from Planet X using our technology, it’s that we can’t even comprehend what the beings from Planet X are or what they’re trying to do. It’s so beyond us that even if they really wanted to enlighten us, it would be like trying to teach ants about the internet.
Along those lines, this may also be an answer to “Well if there are so many fancy Type III Civilizations, why haven’t they contacted us yet?” To answer that, let’s ask ourselves—when Pizarro made his way into Peru, did he stop for a while at an anthill to try to communicate? Was he magnanimous, trying to help the ants in the anthill? Did he become hostile and slow his original mission down in order to smash the anthill apart? Or was the anthill of complete and utter and eternal irrelevance to Pizarro? That might be our situation here.
 Possibility  10) We’re completely wrong about our reality. There are a lot of ways we could just be totally off with everything we think. The universe might appear one way and be something else entirely, like a hologram. Or maybe we’re the aliens and we were planted here as an experiment or as a form of fertilizer. There’s even a chance that we’re all part of a computer simulation by some researcher from another world, and other forms of life simply weren’t programmed into the simulation.
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As we continue along with our possibly-futile search for extraterrestrial intelligence, I’m not really sure what I’m rooting for. Frankly, learning either that we’re officially alone in the universe or that we’re officially joined by others would be creepy, which is a theme with all of the surreal storylines listed above—whatever the truth actually is, it’s mindblowing.
Beyond its shocking science fiction component, The Fermi Paradox also leaves me with a deep humbling. Not just the normal “Oh yeah, I’m microscopic and my existence lasts for three seconds” humbling that the universe always triggers. The Fermi Paradox brings out a sharper, more personal humbling, one that can only happen after spending hours of research hearing your species’ most renowned scientists present insane theories, change their minds again and again, and wildly contradict each other—reminding us that future generations will look at us the same way we see the ancient people who were sure that the stars were the underside of the dome of heaven, and they’ll think “Wow they really had no idea what was going on.”
Compounding all of this is the blow to our species’ self-esteem that comes with all of this talk about Type II and III Civilizations. Here on Earth, we’re the king of our little castle, proud ruler of the huge group of imbeciles who share the planet with us. And in this bubble with no competition and no one to judge us, it’s rare that we’re ever confronted with the concept of being a dramatically inferior species to anyone. But after spending a lot of time with Type II and III Civilizations over the past week, our power and pride are seeming a bit David Brent-esque.
That said, given that my normal outlook is that humanity is a lonely orphan on a tiny rock in the middle of a desolate universe, the humbling fact that we’re probably not as smart as we think we are, and the possibility that a lot of what we’re sure about might be wrong, sounds wonderful. It opens the door just a crack that maybe, just maybe, there might be more to the story than we realize.
 Three more Wait But Why mind-benders: 
  How (and Why) SpaceX Will Colonize Mars  – A post I got to work on with Elon Musk and one that reframed my mental picture of the future.
  The AI Revolution: Road to Superintelligence  – Why any aliens that do visit us are likely to be artificial, not biological. We may not be the king of our castle for very much longer…
  What Makes You You  – It’s a harder question than you think.
 Some other popular Wait But Why posts: 
 Why Procrastinators Procrastinate  
How to Pick Your Life Partner
 The Tail End 
Your Life in Weeks 
Religion for the Nonreligious 
If you’re interested in supporting Wait But Why, here’s our Patreon. 
 
   
 Sources: 
PNAS: Prevalence of Earth-size planets orbiting Sun-like stars 
SETI: The Drake Equation 
NASA: Workshop Report on the Future of Intelligence In The Cosmos 
Keith Wiley: The Fermi Paradox, Self-Replicating Probes, and the Interstellar Transportation Bandwidth 
NCBI: Astrobiological phase transition: towards resolution of Fermi’s paradox 
André Kukla: Extraterrestrials: A Philosophical Perspective 
Nick Bostrom: Where Are They? 
Science Direct: Galactic gradients, postbiological evolution and the apparent failure of SETI 
Nature: Simulations back up theory that Universe is a hologram 
Robin Hanson: The Great Filter – Are We Almost Past It? 
John Dyson: Search for Artificial Stellar Sources of Infrared Radiation
The Fermi Paradox - Wait But Why
https://waitbutwhy.com/2014/05/fermi-paradox.html
https://www.space.com/25325-fermi-paradox.html
Fermi Paradox: Where Are the Aliens?
2021-12-17
Elizabeth Howell
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<p>The Fermi Paradox seeks to answer the question of where the aliens are. </p><p>Given that our <a href="https://www.space.com/16080-solar-system-planets.html">solar system</a> is quite young compared to the rest of the universe — roughly 4.5 billion years old, compared to 13.8 billion — and that interstellar travel might be fairly easy to achieve given enough time, Earth should have been visited by aliens already, the idea goes. </p><p>The paradox takes its name from Nobel Prize-winning physicist Enrico Fermi, who supposedly made the above points during a casual lunchtime conversation in 1950. And the implications have had astrobiologists and other scientists scratching their heads in the decades since.</p><p><strong>Related:</strong> <a href="https://www.space.com/alien-life-search.html">The search for alien life (reference)</a></p><p>"Fermi grasped that any civilization with a modest amount of rocket technology and an immodest amount of imperial incentive could rapidly colonize the entire galaxy," representatives of the Search For Extraterrestrial Intelligence (SETI) Institute in Mountain View, California, wrote in a <a href="https://www.seti.org/fermi-paradox-0">Fermi Paradox explainer</a> (opens in new tab). "Within a few tens of millions of years, every star system could be brought under the wing of empire. Tens of millions of years may sound like a long project, but in fact it’s quite short compared to the age of the galaxy, which is roughly a thousand times more."</p><p>Fermi died in 1954, so exploration and explication of the idea fell to other people, such as Michael Hart, who <a href="http://adsabs.harvard.edu/full/1975QJRAS..16..128H">wrote an article</a> (opens in new tab) titled "An explanation for the absence of extraterrestrials on Earth" in the Royal Astronomical Society (RAS) Quarterly Journal in 1975. (Some say this is the first such paper to explore the Fermi Paradox, although this claim is a bit hard to prove.)</p><p>"We observe that no intelligent beings from outer space are now present on <a href="https://www.space.com/54-earth-history-composition-and-atmosphere.html">Earth</a>," Hart wrote in the paper's abstract. "It is suggested that this fact can best be explained by the hypothesis that there are no other advanced civilizations in our galaxy." </p><p>He noted, however, that more research in biochemistry, planetary formation and atmospheres was needed to better narrow down the answer.</p><p>Hart argued that <a href="https://www.space.com/why-humans-search-intelligent-alien-life-SETI">intelligent aliens</a> could already have visited Earth at some point in our planet's history, unless they started their journey less than two million years ago. He thought the apparent lack of such visits is most likely due to the lack of intelligent aliens. But he outlined four other potential explanations as well: </p><ul><li>Aliens never came here because of a physical difficulty "that makes space travel infeasible," which could be related to astronomy, biology or engineering.</li><li>Aliens simply chose never to visit us.</li><li>Advanced civilizations beyond Earth arose too recently for aliens to reach us.</li><li>Aliens have visited Earth in the past, but we have not observed them.</li></ul><p>Frank Tipler, a professor of physics at Tulane University, followed up on Hart's argument in 1980 with a paper titled "<a href="http://adsbit.harvard.edu//full/1980QJRAS..21..267T/0000278.000.html">Extraterrestrial intelligent beings do not exist</a> (opens in new tab)," also published in the RAS Quarterly Journal. The bulk of his paper dealt with how to get resources for interstellar travel, which he suggested could be achieved by having some kind of self-replicating artificial intelligence move from star system to star system, creating copies of itself as it traveled. </p><p>Since evidence of such advanced machinery has never been found on Earth, Tipler argued that we are likely the only intelligence out there. He also wrote in the 1980 paper that those who believe in extraterrestrial intelligence are similar to <a href="https://www.space.com/ufos-uap-history-sightings-mysteries">UFO</a> (unidentified flying object) enthusiasts, because both camps believe "we are going to be saved from ourselves by some miraculous interstellar intervention."</p><p>Today, the topic of extraterrestrial intelligence is a popular one, with multiple papers appearing every year from different research groups. And the idea that advanced civilizations may exist beyond Earth has been buoyed by the ongoing exoplanet revolution.</p><h3>Plentiful planets</h3><p>The universe is incredibly vast and old. Data gathered by a variety of telescopes show that the observable universe is about <a href="https://www.space.com/24073-how-big-is-the-universe.html">92 billion light-years wide</a> (and growing faster and faster all the while). And separate measurements indicate it is <a href="https://www.space.com/24054-how-old-is-the-universe.html">about 13.82 billion years old</a>. </p><p>So alien civilizations have had plenty of time to arise and spread — but they also likely must cross a vast cosmic gulf to get to us.</p><p>When Fermi made his famous remark, the only planets scientists knew about were in our own solar system. But in 1992, astronomers spotted worlds circling a superdense stellar corpse known as a pulsar. And a few years later, the first <a href="https://www.space.com/17738-exoplanets.html">exoplanet</a> around a sunlike star was confirmed.</p><p>There are now <a href="https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html">more than 4,500 confirmed exoplanets</a> (opens in new tab), with more being found every year. The sheer number of alien worlds suggests that life may be plentiful throughout the cosmos.</p><p>Over time, with more advanced telescopes, scientists will be able to probe the chemical compositions of some nearby exoplanets' atmospheres. "Nearby" is a relative term, however; the closest known exoplanet, <a href="https://www.space.com/proxima-b-alien-planet-earth-sized-espresso.html">Proxima b</a>, lies about 4.2 light-years away, which is roughly 25 trillion miles (40 trillion kilometers). </p><p>The eventual goal is to understand how often rocky planets form in the "<a href="https://www.space.com/23910-habitable-zones-alien-planets-stars-infographic.html">habitable zone</a>" of their parent stars, which is traditionally defined as the range of orbital distances in which water can exist on a world's surface. Habitability isn't just about water, however. Other factors must be considered as well, such as the host star's activity and the planet's atmospheric composition. (And there are other reasons why the  habitable zone, as traditionally defined, is increasingly viewed as overly simplistic. For example, icy moons in our own solar system, such as Jupiter's <a href="https://www.space.com/15498-europa-sdcmp.html">Europa</a> and Saturn's Enceladus, lie far beyond the habitable zone and may still harbor life in their subsurface seas.) </p><p>Such caveats notwithstanding, there does seem to be plenty of habitable real estate out there. For example, a November 2013 study using data from NASA's <a href="https://www.space.com/24903-kepler-space-telescope.html">Kepler space telescope</a> suggested that <a href="https://www.space.com/23456-habitable-earthlike-alien-planets-common-kepler.html">one in five sunlike stars has a roughly Earth-size planet</a> orbiting in the habitable zone. A few months later, Kepler scientists released a "planet bonanza" of 715 newly discovered worlds. Many of these planets were confirmed using a new technique called "<a href="https://www.space.com/24824-alien-planets-population-doubles-nasa-kepler.html">verification by multiplicity</a>," which works partly on the logic of probability. (Objects seen to cross a star's face or tug at it gravitationally are more likely to be planets rather than companion stars, because a companion star at close proximity would likely destabilize the entire system over time.) </p><figure><div><p></p></div><figcaption>An artist's illustration of NASA's Kepler space telescope hunting for exoplanets. (Image credit: NASA)</figcaption></figure><p>Sunlike stars are the minority population in our galaxy, however; about three-quarters of Milky Way stars are small, dim burners known as <a href="https://www.space.com/23772-red-dwarf-stars.html">red dwarfs</a>. Astronomers have found multiple rocky worlds circling in the habitable zone of red dwarfs — Proxima b, for example, and three planets in the TRAPPIST-1 system, which lies about 39 light-years from Earth and harbors seven rocky worlds in total. But it's unclear how habitable such planets really are, because red dwarfs are incredibly volatile, especially in their youth. Stellar eruptions may therefore quickly blast away the nascent atmospheres of young "habitable zone" red dwarf planets, making it extremely <a href="https://www.space.com/superflares-thwart-alien-life-red-dwarf-stars">difficult for life to get a foothold there</a>. More study is required to better understand these stars and the ability of life to persist around them, scientists say.</p><p>Researchers are getting more tools with which to do such work. For example, NASA's <a href="https://www.space.com/39939-tess-satellite-exoplanet-hunter.html">Transiting Exoplanet Survey Satellite</a> (TESS) launched successfully in April 2018 and soon picked up the baton from Kepler, which was retired late that same year. The agency's highly-anticipated, $10 billion <a href="https://www.space.com/21925-james-webb-space-telescope-jwst.html">James Webb Space Telescope</a>, which is scheduled to launch in December 2021, will hunt for potential biosignature gases in the air of nearby exoplanets, among many other tasks. The European Space Agency's <a href="https://www.space.com/35741-esa-plato-facts.html">PLATO</a> (PLAnetary Transits and Oscillations of stars<a href="https://www.space.com/35741-esa-plato-facts.html">)</a> mission is expected to launch in 2026. Three huge ground-based observatories powerful enough to sniff exoplanet air — the Extremely Large Telescope, the <a href="https://www.space.com/giant-magellan-telescope-6th-mirror-casting">Giant Magellan Telescope</a> and the Thirty-Meter Telescope — are scheduled to come online later this decade as well. And one highly ambitious project, known as <a href="https://www.space.com/32554-breakthough-starshot-interstellar-alpha-centauri-nanocraft-pictures.html">Breakthrough Starshot</a>, aims to study Proxima b and other nearby worlds up close with swarms of tiny, laser-sailing nanoprobes. If technology development goes well, the first such interstellar robotic craft could launch around 2050.</p><p>Such missions and instruments will help scientists flesh out their understanding of astrobiology, which remains relatively primitive. For instance, we don't even know if there are life-hosting worlds in our own backyard. Studies here on Earth have shown that <a href="https://www.space.com/21295-coldest-temperature-bacteria-found-in-permafrost.html">microbes can survive in extreme environments</a>, suggesting that microbial life may well exist on Mars, Europa, Enceladus and/or the giant Saturn satellite <a href="https://www.space.com/15257-titan-saturn-largest-moon-facts-discovery-sdcmp.html">Titan</a>. But we haven't explored any of those worlds nearly well enough to know for sure.</p><p>The Fermi Paradox is thinking much bigger than microbes, however. To solve it, we need to know not only how commonly life evolves on alien planets, but also how often it gains the ability, and the desire, to communicate with other intelligent lifeforms or go sailing among the stars.</p><p><strong>Related:</strong> <a href="https://www.space.com/20155-hunting-intelligent-aliens-extreme-seti.html">13 ways to hunt intelligent aliens</a></p><p>The number of intelligent, detectable alien civilizations is estimated by the <a href="https://www.space.com/25219-drake-equation.html">Drake Equation</a>. In the words of the SETI Institute, the equation — written as "N = R* • fp • ne • fl • fi • fc • L" — has the following variables:</p><ul><li>"N: The number of civilizations in the Milky Way galaxy whose electromagnetic emissions are detectable.</li><li>R*: The rate of formation of stars suitable for the development of intelligent life (number per year).</li><li>fp: The fraction of those stars with planetary systems.</li><li>ne: The number of planets, per solar system, with an environment suitable for life.</li><li>fl: The fraction of suitable planets on which life actually appears.</li><li>fi: The fraction of life bearing planets on which intelligent life emerges.</li><li>fc: The fraction of civilizations that develop a technology that produces detectable signs of their existence.</li><li>L: The average length of time such civilizations produce such signs (years)."</li></ul><p>None of these values are known with any certainty right now, which makes predictions difficult. So the Fermi Paradox is fertile ground for speculation, and scientists and laypeople have advanced literally hundreds of possible explanations over the years.</p><h3>What could the answer be?</h3><p>These ideas run a very wide gamut. </p><p>For example, in 2015, scientists analyzing data from the <a href="https://www.space.com/15892-hubble-space-telescope.html">Hubble Space Telescope</a> and the Kepler space telescope concluded that Earth was likely an early bloomer, relatively speaking. Just 8% of all the potentially habitable worlds that will ever exist in the universe were around when Earth formed about 4.5 billion years ago, the researchers determined. So that's one possible explanation to the paradox: The aliens will come, <a href="https://www.space.com/30889-earth-bloomed-early-fermi-paradox-solution.html">but not for a while yet</a>.</p><p>Or perhaps <a href="https://www.space.com/31694-alien-life-extinct-fermi-paradox.html">life is too fragile to survive for long</a>. A 2016 study suggested that the early part of a rocky planet's history can be very conducive to life, which may commonly emerge starting just 500 million years or so after the planet cools down and liquid water becomes available. The history of our own Earth would seem to bolster that conclusion; there's (disputed) evidence that life had emerged here <a href="https://www.pnas.org/content/112/47/14518">by about 4.1 billion years ago</a> (opens in new tab) , and it was definitely established by 3.8 billion years ago. But those good times may not last long, thanks to a runaway greenhouse effect (as occurred on Venus long ago) or other climatic shifts.</p><p>"Between the early heat pulses, freezing, volatile content variation and runaway positive feedbacks, maintaining life on an initially wet rocky planet in the habitable zone may be like trying to ride a wild bull. Most life falls off," researchers Aditya Chopra and Charley Lineweaver wrote in <a href="https://www.liebertpub.com/doi/full/10.1089/ast.2015.1387">the study</a> (opens in new tab), which was published in the journal Astrobiology. "Life may be rare in the universe, not because it is difficult to get started, but because habitable environments are difficult to maintain during the first billion years."</p><p>Or maybe the bottleneck comes much later. A number of thinkers have suggested that civilizations may tend to extinguish themselves soon after becoming technologically competent. Again, Earth provides some support for this hypothesis: Humanity came alarmingly close to nuclear war during the <a href="https://www.atomicheritage.org/history/nuclear-close-calls-cuban-missile-crisis">Cuban Missile Crisis</a> (opens in new tab) in 1962, and we may be in the process of destroying ourselves, and much of the other life on the planet, right now via anthropogenic <a href="https://www.livescience.com/climate-change.html">climate change</a> (opens in new tab).</p><p>There are many other factors to consider as well. For example, planetary scientist Alan Stern, the leader of NASA's New Horizons mission to Pluto, recently posited that the most common life-hosting environments in the Milky Way galaxy <a href="https://www.space.com/38577-fermi-paradox-alien-life-buried-oceans.html">may be buried oceans</a>, such as the seas of Enceladus and Europa. Organisms that evolve in such locales seem unlikely to develop spacecraft; indeed, many of them may not even know there are other worlds out there to explore.</p><p>Alien psychology could be playing a role as well. Maybe there are lots of advanced alien civilizations out there, for example, but most of them have no desire to communicate with us or visit Earth. Perhaps Earth and its inhabitants just aren't interesting enough for them to trifle with — and won't be until humanity demonstrates enough intelligence and merit to be welcomed into the "<a href="https://www.scientificamerican.com/article/how-humanity-can-earn-the-respect-of-extraterrestrials/">galactic club</a> (opens in new tab)."</p><p>Or maybe most intelligent aliens tend to keep quiet as a general rule, concerned that making contact with their cosmic neighbors could bring on their own enslavement or annihilation. A number of researchers, including the late Stephen Hawking, have invoked such possibilities in arguing that humanity <a href="https://www.space.com/34184-stephen-hawking-afraid-alien-civilizations.html">shouldn't actively advertise its presence</a>.</p><p>Then there are the logistical difficulties of finding intelligent aliens. The universe is enormous and incredibly old. Humanity has been around for just 200,000 years, and we've been listening for possible radio signals from E.T. just since 1960. So the odds that we overlap in time and space with a detectable alien civilization don't seem great.</p><p>There probably is no single solution to the Fermi Paradox, most researchers say. A combination of factors — including, perhaps, some of the ones discussed above — are likely responsible for the "great silence" that currently confronts us. And the nature of those factors may start coming into clearer focus relatively soon.</p><p>For example, say scientists find evidence of past or present microbial life on <a href="https://www.space.com/47-mars-the-red-planet-fourth-planet-from-the-sun.html">Mars</a>, Europa or another body in our own solar system, and that those organisms represent a "second genesis" — something totally different than life on Earth. Such a discovery would strongly suggest that it's not hard for life to get going throughout the cosmos, allowing researchers to cross one possible Fermi Paradox explanation off the lengthy list.</p><h3>Additional resources</h3><ul><li>Read the SETI Institute's <a href="https://www.seti.org/fermi-paradox-0">discussion of the Fermi Paradox</a> (opens in new tab).</li><li>Learn more about <a href="https://www.nobelprize.org/prizes/physics/1938/fermi/biographical/">Enrico Fermi and his scientific achievements</a> (opens in new tab).</li><li>Read a study that investigates <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5767786/">how humanity might react</a> (opens in new tab) to the discovery of alien life.</li></ul><p><em>Follow us on Twitter </em><a href="https://twitter.com/SPACEdotcom"><em>@Spacedotcom</em></a> (opens in new tab)<em> or on </em><a href="https://www.facebook.com/spacecom/"><em>Facebook</em></a> (opens in new tab)<em>.</em></p>
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<div><p>Elizabeth Howell, Ph.D., is a staff writer in the spaceflight channel since 2022. She was contributing writer for <a href="http://space.com/">Space.com</a> (opens in new tab) for 10 years before that, since 2012. Elizabeth's reporting includes an exclusive with Office of the Vice-President of the United States, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and (soon) a Bachelor of History from Athabasca University. Elizabeth is also a post-secondary instructor in communications and science since 2015. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: <a href="https://qoto.org/@howellspace">https://qoto.org/@howellspace</a></p></div>
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 3. Search For Life

 Fermi Paradox: Where are the aliens?

 By Elizabeth Howell
 published 17 December 2021

 Is anybody out there?

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 Artist's illustration of the surface of a planet in the TRAPPIST-1 system, which hosts seven roughly Earth-size worlds. (Image credit: NASA/JPL-Caltech)
 Jump to:
 * There's a lot of potentially habitable real estate out there
 * And will we ever know?
 * Additional resources

 The Fermi Paradox seeks to answer the question of where the aliens are.

 Given that our solar system is quite young compared to the rest of the universe — roughly 4.5 billion years old, compared to 13.8 billion — and that interstellar travel might be fairly easy to achieve given enough time, Earth should have been visited by aliens already, the idea goes.

 The paradox takes its name from Nobel Prize-winning physicist Enrico Fermi, who supposedly made the above points during a casual lunchtime conversation in 1950. And the implications have had astrobiologists and other scientists scratching their heads in the decades since.

 Related: The search for alien life (reference)

 "Fermi grasped that any civilization with a modest amount of rocket technology and an immodest amount of imperial incentive could rapidly colonize the entire galaxy," representatives of the Search For Extraterrestrial Intelligence (SETI) Institute in Mountain View, California, wrote in a Fermi Paradox explainer (opens in new tab) . "Within a few tens of millions of years, every star system could be brought under the wing of empire. Tens of millions of years may sound like a long project, but in fact it’s quite short compared to the age of the galaxy, which is roughly a thousand times more."

 Fermi died in 1954, so exploration and explication of the idea fell to other people, such as Michael Hart, who wrote an article (opens in new tab) titled "An explanation for the absence of extraterrestrials on Earth" in the Royal Astronomical Society (RAS) Quarterly Journal in 1975. (Some say this is the first such paper to explore the Fermi Paradox, although this claim is a bit hard to prove.)

 "We observe that no intelligent beings from outer space are now present on Earth," Hart wrote in the paper's abstract. "It is suggested that this fact can best be explained by the hypothesis that there are no other advanced civilizations in our galaxy."

 He noted, however, that more research in biochemistry, planetary formation and atmospheres was needed to better narrow down the answer.

 Hart argued that intelligent aliens could already have visited Earth at some point in our planet's history, unless they started their journey less than two million years ago. He thought the apparent lack of such visits is most likely due to the lack of intelligent aliens. But he outlined four other potential explanations as well:

 * Aliens never came here because of a physical difficulty "that makes space travel infeasible," which could be related to astronomy, biology or engineering.
 * Aliens simply chose never to visit us.
 * Advanced civilizations beyond Earth arose too recently for aliens to reach us.
 * Aliens have visited Earth in the past, but we have not observed them.

 Frank Tipler, a professor of physics at Tulane University, followed up on Hart's argument in 1980 with a paper titled "Extraterrestrial intelligent beings do not exist (opens in new tab) ," also published in the RAS Quarterly Journal. The bulk of his paper dealt with how to get resources for interstellar travel, which he suggested could be achieved by having some kind of self-replicating artificial intelligence move from star system to star system, creating copies of itself as it traveled.

 Since evidence of such advanced machinery has never been found on Earth, Tipler argued that we are likely the only intelligence out there. He also wrote in the 1980 paper that those who believe in extraterrestrial intelligence are similar to UFO (unidentified flying object) enthusiasts, because both camps believe "we are going to be saved from ourselves by some miraculous interstellar intervention."

 Today, the topic of extraterrestrial intelligence is a popular one, with multiple papers appearing every year from different research groups. And the idea that advanced civilizations may exist beyond Earth has been buoyed by the ongoing exoplanet revolution.

 Plentiful planets

 The universe is incredibly vast and old. Data gathered by a variety of telescopes show that the observable universe is about 92 billion light-years wide (and growing faster and faster all the while). And separate measurements indicate it is about 13.82 billion years old.

 So alien civilizations have had plenty of time to arise and spread — but they also likely must cross a vast cosmic gulf to get to us.

 When Fermi made his famous remark, the only planets scientists knew about were in our own solar system. But in 1992, astronomers spotted worlds circling a superdense stellar corpse known as a pulsar. And a few years later, the first exoplanet around a sunlike star was confirmed.

 There are now more than 4,500 confirmed exoplanets (opens in new tab) , with more being found every year. The sheer number of alien worlds suggests that life may be plentiful throughout the cosmos.

 Over time, with more advanced telescopes, scientists will be able to probe the chemical compositions of some nearby exoplanets' atmospheres. "Nearby" is a relative term, however; the closest known exoplanet, Proxima b, lies about 4.2 light-years away, which is roughly 25 trillion miles (40 trillion kilometers).

 The eventual goal is to understand how often rocky planets form in the "habitable zone" of their parent stars, which is traditionally defined as the range of orbital distances in which water can exist on a world's surface. Habitability isn't just about water, however. Other factors must be considered as well, such as the host star's activity and the planet's atmospheric composition. (And there are other reasons why the habitable zone, as traditionally defined, is increasingly viewed as overly simplistic. For example, icy moons in our own solar system, such as Jupiter's Europa and Saturn's Enceladus, lie far beyond the habitable zone and may still harbor life in their subsurface seas.)

 Such caveats notwithstanding, there does seem to be plenty of habitable real estate out there. For example, a November 2013 study using data from NASA's Kepler space telescope suggested that one in five sunlike stars has a roughly Earth-size planet orbiting in the habitable zone. A few months later, Kepler scientists released a "planet bonanza" of 715 newly discovered worlds. Many of these planets were confirmed using a new technique called "verification by multiplicity," which works partly on the logic of probability. (Objects seen to cross a star's face or tug at it gravitationally are more likely to be planets rather than companion stars, because a companion star at close proximity would likely destabilize the entire system over time.)

 An artist's illustration of NASA's Kepler space telescope hunting for exoplanets. (Image credit: NASA)

 Sunlike stars are the minority population in our galaxy, however; about three-quarters of Milky Way stars are small, dim burners known as red dwarfs. Astronomers have found multiple rocky worlds circling in the habitable zone of red dwarfs — Proxima b, for example, and three planets in the TRAPPIST-1 system, which lies about 39 light-years from Earth and harbors seven rocky worlds in total. But it's unclear how habitable such planets really are, because red dwarfs are incredibly volatile, especially in their youth. Stellar eruptions may therefore quickly blast away the nascent atmospheres of young "habitable zone" red dwarf planets, making it extremely difficult for life to get a foothold there. More study is required to better understand these stars and the ability of life to persist around them, scientists say.

 Researchers are getting more tools with which to do such work. For example, NASA's Transiting Exoplanet Survey Satellite (TESS) launched successfully in April 2018 and soon picked up the baton from Kepler, which was retired late that same year. The agency's highly-anticipated, $10 billion James Webb Space Telescope, which is scheduled to launch in December 2021, will hunt for potential biosignature gases in the air of nearby exoplanets, among many other tasks. The European Space Agency's PLATO (PLAnetary Transits and Oscillations of stars) mission is expected to launch in 2026. Three huge ground-based observatories powerful enough to sniff exoplanet air — the Extremely Large Telescope, the Giant Magellan Telescope and the Thirty-Meter Telescope — are scheduled to come online later this decade as well. And one highly ambitious project, known as Breakthrough Starshot, aims to study Proxima b and other nearby worlds up close with swarms of tiny, laser-sailing nanoprobes. If technology development goes well, the first such interstellar robotic craft could launch around 2050.

 Such missions and instruments will help scientists flesh out their understanding of astrobiology, which remains relatively primitive. For instance, we don't even know if there are life-hosting worlds in our own backyard. Studies here on Earth have shown that microbes can survive in extreme environments, suggesting that microbial life may well exist on Mars, Europa, Enceladus and/or the giant Saturn satellite Titan. But we haven't explored any of those worlds nearly well enough to know for sure.

 The Fermi Paradox is thinking much bigger than microbes, however. To solve it, we need to know not only how commonly life evolves on alien planets, but also how often it gains the ability, and the desire, to communicate with other intelligent lifeforms or go sailing among the stars.

 Related: 13 ways to hunt intelligent aliens

 The number of intelligent, detectable alien civilizations is estimated by the Drake Equation. In the words of the SETI Institute, the equation — written as "N = R* • fp • ne • fl • fi • fc • L" — has the following variables:

 * "N: The number of civilizations in the Milky Way galaxy whose electromagnetic emissions are detectable.
 * R*: The rate of formation of stars suitable for the development of intelligent life (number per year).
 * fp: The fraction of those stars with planetary systems.
 * ne: The number of planets, per solar system, with an environment suitable for life.
 * fl: The fraction of suitable planets on which life actually appears.
 * fi: The fraction of life bearing planets on which intelligent life emerges.
 * fc: The fraction of civilizations that develop a technology that produces detectable signs of their existence.
 * L: The average length of time such civilizations produce such signs (years)."

 None of these values are known with any certainty right now, which makes predictions difficult. So the Fermi Paradox is fertile ground for speculation, and scientists and laypeople have advanced literally hundreds of possible explanations over the years.

 What could the answer be?

 These ideas run a very wide gamut.

 For example, in 2015, scientists analyzing data from the Hubble Space Telescope and the Kepler space telescope concluded that Earth was likely an early bloomer, relatively speaking. Just 8% of all the potentially habitable worlds that will ever exist in the universe were around when Earth formed about 4.5 billion years ago, the researchers determined. So that's one possible explanation to the paradox: The aliens will come, but not for a while yet.

 Or perhaps life is too fragile to survive for long. A 2016 study suggested that the early part of a rocky planet's history can be very conducive to life, which may commonly emerge starting just 500 million years or so after the planet cools down and liquid water becomes available. The history of our own Earth would seem to bolster that conclusion; there's (disputed) evidence that life had emerged here by about 4.1 billion years ago (opens in new tab) , and it was definitely established by 3.8 billion years ago. But those good times may not last long, thanks to a runaway greenhouse effect (as occurred on Venus long ago) or other climatic shifts.

 "Between the early heat pulses, freezing, volatile content variation and runaway positive feedbacks, maintaining life on an initially wet rocky planet in the habitable zone may be like trying to ride a wild bull. Most life falls off," researchers Aditya Chopra and Charley Lineweaver wrote in the study (opens in new tab) , which was published in the journal Astrobiology. "Life may be rare in the universe, not because it is difficult to get started, but because habitable environments are difficult to maintain during the first billion years."

 Or maybe the bottleneck comes much later. A number of thinkers have suggested that civilizations may tend to extinguish themselves soon after becoming technologically competent. Again, Earth provides some support for this hypothesis: Humanity came alarmingly close to nuclear war during the Cuban Missile Crisis (opens in new tab) in 1962, and we may be in the process of destroying ourselves, and much of the other life on the planet, right now via anthropogenic climate change (opens in new tab) .

 Related stories:

 — The search for alien life (reference)
 — The Kardashev Scale: Classifying alien civilizations
 — Does the Milky Way harbor dozens of alien civilizations?

 There are many other factors to consider as well. For example, planetary scientist Alan Stern, the leader of NASA's New Horizons mission to Pluto, recently posited that the most common life-hosting environments in the Milky Way galaxy may be buried oceans, such as the seas of Enceladus and Europa. Organisms that evolve in such locales seem unlikely to develop spacecraft; indeed, many of them may not even know there are other worlds out there to explore.

 Alien psychology could be playing a role as well. Maybe there are lots of advanced alien civilizations out there, for example, but most of them have no desire to communicate with us or visit Earth. Perhaps Earth and its inhabitants just aren't interesting enough for them to trifle with — and won't be until humanity demonstrates enough intelligence and merit to be welcomed into the "galactic club (opens in new tab) ."

 Or maybe most intelligent aliens tend to keep quiet as a general rule, concerned that making contact with their cosmic neighbors could bring on their own enslavement or annihilation. A number of researchers, including the late Stephen Hawking, have invoked such possibilities in arguing that humanity shouldn't actively advertise its presence.

 Then there are the logistical difficulties of finding intelligent aliens. The universe is enormous and incredibly old. Humanity has been around for just 200,000 years, and we've been listening for possible radio signals from E.T. just since 1960. So the odds that we overlap in time and space with a detectable alien civilization don't seem great.

 There probably is no single solution to the Fermi Paradox, most researchers say. A combination of factors — including, perhaps, some of the ones discussed above — are likely responsible for the "great silence" that currently confronts us. And the nature of those factors may start coming into clearer focus relatively soon.

 For example, say scientists find evidence of past or present microbial life on Mars, Europa or another body in our own solar system, and that those organisms represent a "second genesis" — something totally different than life on Earth. Such a discovery would strongly suggest that it's not hard for life to get going throughout the cosmos, allowing researchers to cross one possible Fermi Paradox explanation off the lengthy list.

 Additional resources

 * Read the SETI Institute's discussion of the Fermi Paradox (opens in new tab) .
 * Learn more about Enrico Fermi and his scientific achievements (opens in new tab) .
 * Read a study that investigates how humanity might react (opens in new tab) to the discovery of alien life.

 Follow us on Twitter @Spacedotcom (opens in new tab) or on Facebook (opens in new tab) .

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 Elizabeth Howell
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 Elizabeth Howell, Ph.D., is a staff writer in the spaceflight channel since 2022. She was contributing writer for Space.com (opens in new tab) for 10 years before that, since 2012. Elizabeth's reporting includes an exclusive with Office of the Vice-President of the United States, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and (soon) a Bachelor of History from Athabasca University. Elizabeth is also a post-secondary instructor in communications and science since 2015. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: https://qoto.org/@howellspace

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References
Search For Life
Artist's illustration of the surface of a planet in the TRAPPIST-1 system, which hosts seven roughly Earth-size worlds. 
(Image credit: NASA/JPL-Caltech)
The Fermi Paradox seeks to answer the question of where the aliens are.  Given that our solar system is quite young compared to the rest of the universe — roughly 4.5 billion years old, compared to 13.8 billion — and that interstellar travel might be fairly easy to achieve given enough time, Earth should have been visited by aliens already, the idea goes.  The paradox takes its name from Nobel Prize-winning physicist Enrico Fermi, who supposedly made the above points during a casual lunchtime conversation in 1950. And the implications have had astrobiologists and other scientists scratching their heads in the decades since.  Related: The search for alien life (reference)  "Fermi grasped that any civilization with a modest amount of rocket technology and an immodest amount of imperial incentive could rapidly colonize the entire galaxy," representatives of the Search For Extraterrestrial Intelligence (SETI) Institute in Mountain View, California, wrote in a Fermi Paradox explainer (opens in new tab). "Within a few tens of millions of years, every star system could be brought under the wing of empire. Tens of millions of years may sound like a long project, but in fact it’s quite short compared to the age of the galaxy, which is roughly a thousand times more." Fermi died in 1954, so exploration and explication of the idea fell to other people, such as Michael Hart, who wrote an article (opens in new tab) titled "An explanation for the absence of extraterrestrials on Earth" in the Royal Astronomical Society (RAS) Quarterly Journal in 1975. (Some say this is the first such paper to explore the Fermi Paradox, although this claim is a bit hard to prove.) "We observe that no intelligent beings from outer space are now present on Earth," Hart wrote in the paper's abstract. "It is suggested that this fact can best be explained by the hypothesis that there are no other advanced civilizations in our galaxy."  He noted, however, that more research in biochemistry, planetary formation and atmospheres was needed to better narrow down the answer. Hart argued that intelligent aliens could already have visited Earth at some point in our planet's history, unless they started their journey less than two million years ago. He thought the apparent lack of such visits is most likely due to the lack of intelligent aliens. But he outlined four other potential explanations as well:   Aliens never came here because of a physical difficulty "that makes space travel infeasible," which could be related to astronomy, biology or engineering. Aliens simply chose never to visit us. Advanced civilizations beyond Earth arose too recently for aliens to reach us. Aliens have visited Earth in the past, but we have not observed them.  Frank Tipler, a professor of physics at Tulane University, followed up on Hart's argument in 1980 with a paper titled "Extraterrestrial intelligent beings do not exist (opens in new tab)," also published in the RAS Quarterly Journal. The bulk of his paper dealt with how to get resources for interstellar travel, which he suggested could be achieved by having some kind of self-replicating artificial intelligence move from star system to star system, creating copies of itself as it traveled.  Since evidence of such advanced machinery has never been found on Earth, Tipler argued that we are likely the only intelligence out there. He also wrote in the 1980 paper that those who believe in extraterrestrial intelligence are similar to UFO (unidentified flying object) enthusiasts, because both camps believe "we are going to be saved from ourselves by some miraculous interstellar intervention." Today, the topic of extraterrestrial intelligence is a popular one, with multiple papers appearing every year from different research groups. And the idea that advanced civilizations may exist beyond Earth has been buoyed by the ongoing exoplanet revolution. Plentiful planets The universe is incredibly vast and old. Data gathered by a variety of telescopes show that the observable universe is about 92 billion light-years wide (and growing faster and faster all the while). And separate measurements indicate it is about 13.82 billion years old.  So alien civilizations have had plenty of time to arise and spread — but they also likely must cross a vast cosmic gulf to get to us. When Fermi made his famous remark, the only planets scientists knew about were in our own solar system. But in 1992, astronomers spotted worlds circling a superdense stellar corpse known as a pulsar. And a few years later, the first exoplanet around a sunlike star was confirmed. There are now more than 4,500 confirmed exoplanets (opens in new tab), with more being found every year. The sheer number of alien worlds suggests that life may be plentiful throughout the cosmos. Over time, with more advanced telescopes, scientists will be able to probe the chemical compositions of some nearby exoplanets' atmospheres. "Nearby" is a relative term, however; the closest known exoplanet, Proxima b, lies about 4.2 light-years away, which is roughly 25 trillion miles (40 trillion kilometers).  The eventual goal is to understand how often rocky planets form in the "habitable zone" of their parent stars, which is traditionally defined as the range of orbital distances in which water can exist on a world's surface. Habitability isn't just about water, however. Other factors must be considered as well, such as the host star's activity and the planet's atmospheric composition. (And there are other reasons why the  habitable zone, as traditionally defined, is increasingly viewed as overly simplistic. For example, icy moons in our own solar system, such as Jupiter's Europa and Saturn's Enceladus, lie far beyond the habitable zone and may still harbor life in their subsurface seas.)  Such caveats notwithstanding, there does seem to be plenty of habitable real estate out there. For example, a November 2013 study using data from NASA's Kepler space telescope suggested that one in five sunlike stars has a roughly Earth-size planet orbiting in the habitable zone. A few months later, Kepler scientists released a "planet bonanza" of 715 newly discovered worlds. Many of these planets were confirmed using a new technique called "verification by multiplicity," which works partly on the logic of probability. (Objects seen to cross a star's face or tug at it gravitationally are more likely to be planets rather than companion stars, because a companion star at close proximity would likely destabilize the entire system over time.)       An artist's illustration of NASA's Kepler space telescope hunting for exoplanets. (Image credit: NASA)  Sunlike stars are the minority population in our galaxy, however; about three-quarters of Milky Way stars are small, dim burners known as red dwarfs. Astronomers have found multiple rocky worlds circling in the habitable zone of red dwarfs — Proxima b, for example, and three planets in the TRAPPIST-1 system, which lies about 39 light-years from Earth and harbors seven rocky worlds in total. But it's unclear how habitable such planets really are, because red dwarfs are incredibly volatile, especially in their youth. Stellar eruptions may therefore quickly blast away the nascent atmospheres of young "habitable zone" red dwarf planets, making it extremely difficult for life to get a foothold there. More study is required to better understand these stars and the ability of life to persist around them, scientists say. Researchers are getting more tools with which to do such work. For example, NASA's Transiting Exoplanet Survey Satellite (TESS) launched successfully in April 2018 and soon picked up the baton from Kepler, which was retired late that same year. The agency's highly-anticipated, $10 billion James Webb Space Telescope, which is scheduled to launch in December 2021, will hunt for potential biosignature gases in the air of nearby exoplanets, among many other tasks. The European Space Agency's PLATO (PLAnetary Transits and Oscillations of stars) mission is expected to launch in 2026. Three huge ground-based observatories powerful enough to sniff exoplanet air — the Extremely Large Telescope, the Giant Magellan Telescope and the Thirty-Meter Telescope — are scheduled to come online later this decade as well. And one highly ambitious project, known as Breakthrough Starshot, aims to study Proxima b and other nearby worlds up close with swarms of tiny, laser-sailing nanoprobes. If technology development goes well, the first such interstellar robotic craft could launch around 2050. Such missions and instruments will help scientists flesh out their understanding of astrobiology, which remains relatively primitive. For instance, we don't even know if there are life-hosting worlds in our own backyard. Studies here on Earth have shown that microbes can survive in extreme environments, suggesting that microbial life may well exist on Mars, Europa, Enceladus and/or the giant Saturn satellite Titan. But we haven't explored any of those worlds nearly well enough to know for sure. The Fermi Paradox is thinking much bigger than microbes, however. To solve it, we need to know not only how commonly life evolves on alien planets, but also how often it gains the ability, and the desire, to communicate with other intelligent lifeforms or go sailing among the stars.  Related: 13 ways to hunt intelligent aliens  The number of intelligent, detectable alien civilizations is estimated by the Drake Equation. In the words of the SETI Institute, the equation — written as "N = R* • fp • ne • fl • fi • fc • L" — has the following variables:  "N: The number of civilizations in the Milky Way galaxy whose electromagnetic emissions are detectable. R*: The rate of formation of stars suitable for the development of intelligent life (number per year). fp: The fraction of those stars with planetary systems. ne: The number of planets, per solar system, with an environment suitable for life. fl: The fraction of suitable planets on which life actually appears. fi: The fraction of life bearing planets on which intelligent life emerges. fc: The fraction of civilizations that develop a technology that produces detectable signs of their existence. L: The average length of time such civilizations produce such signs (years)."  None of these values are known with any certainty right now, which makes predictions difficult. So the Fermi Paradox is fertile ground for speculation, and scientists and laypeople have advanced literally hundreds of possible explanations over the years. What could the answer be? These ideas run a very wide gamut.  For example, in 2015, scientists analyzing data from the Hubble Space Telescope and the Kepler space telescope concluded that Earth was likely an early bloomer, relatively speaking. Just 8% of all the potentially habitable worlds that will ever exist in the universe were around when Earth formed about 4.5 billion years ago, the researchers determined. So that's one possible explanation to the paradox: The aliens will come, but not for a while yet. Or perhaps life is too fragile to survive for long. A 2016 study suggested that the early part of a rocky planet's history can be very conducive to life, which may commonly emerge starting just 500 million years or so after the planet cools down and liquid water becomes available. The history of our own Earth would seem to bolster that conclusion; there's (disputed) evidence that life had emerged here by about 4.1 billion years ago (opens in new tab) , and it was definitely established by 3.8 billion years ago. But those good times may not last long, thanks to a runaway greenhouse effect (as occurred on Venus long ago) or other climatic shifts. "Between the early heat pulses, freezing, volatile content variation and runaway positive feedbacks, maintaining life on an initially wet rocky planet in the habitable zone may be like trying to ride a wild bull. Most life falls off," researchers Aditya Chopra and Charley Lineweaver wrote in the study (opens in new tab), which was published in the journal Astrobiology. "Life may be rare in the universe, not because it is difficult to get started, but because habitable environments are difficult to maintain during the first billion years." Or maybe the bottleneck comes much later. A number of thinkers have suggested that civilizations may tend to extinguish themselves soon after becoming technologically competent. Again, Earth provides some support for this hypothesis: Humanity came alarmingly close to nuclear war during the Cuban Missile Crisis (opens in new tab) in 1962, and we may be in the process of destroying ourselves, and much of the other life on the planet, right now via anthropogenic climate change (opens in new tab). There are many other factors to consider as well. For example, planetary scientist Alan Stern, the leader of NASA's New Horizons mission to Pluto, recently posited that the most common life-hosting environments in the Milky Way galaxy may be buried oceans, such as the seas of Enceladus and Europa. Organisms that evolve in such locales seem unlikely to develop spacecraft; indeed, many of them may not even know there are other worlds out there to explore. Alien psychology could be playing a role as well. Maybe there are lots of advanced alien civilizations out there, for example, but most of them have no desire to communicate with us or visit Earth. Perhaps Earth and its inhabitants just aren't interesting enough for them to trifle with — and won't be until humanity demonstrates enough intelligence and merit to be welcomed into the "galactic club (opens in new tab)." Or maybe most intelligent aliens tend to keep quiet as a general rule, concerned that making contact with their cosmic neighbors could bring on their own enslavement or annihilation. A number of researchers, including the late Stephen Hawking, have invoked such possibilities in arguing that humanity shouldn't actively advertise its presence. Then there are the logistical difficulties of finding intelligent aliens. The universe is enormous and incredibly old. Humanity has been around for just 200,000 years, and we've been listening for possible radio signals from E.T. just since 1960. So the odds that we overlap in time and space with a detectable alien civilization don't seem great. There probably is no single solution to the Fermi Paradox, most researchers say. A combination of factors — including, perhaps, some of the ones discussed above — are likely responsible for the "great silence" that currently confronts us. And the nature of those factors may start coming into clearer focus relatively soon. For example, say scientists find evidence of past or present microbial life on Mars, Europa or another body in our own solar system, and that those organisms represent a "second genesis" — something totally different than life on Earth. Such a discovery would strongly suggest that it's not hard for life to get going throughout the cosmos, allowing researchers to cross one possible Fermi Paradox explanation off the lengthy list. Additional resources  Read the SETI Institute's discussion of the Fermi Paradox (opens in new tab). Learn more about Enrico Fermi and his scientific achievements (opens in new tab). Read a study that investigates how humanity might react (opens in new tab) to the discovery of alien life.   Follow us on Twitter   @Spacedotcom  (opens in new tab) or on   Facebook  (opens in new tab). 
  Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: community@space.com.  
 Elizabeth Howell, Ph.D., is a staff writer in the spaceflight channel since 2022. She was contributing writer for Space.com (opens in new tab) for 10 years before that, since 2012. Elizabeth's reporting includes an exclusive with Office of the Vice-President of the United States, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and (soon) a Bachelor of History from Athabasca University. Elizabeth is also a post-secondary instructor in communications and science since 2015. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: https://qoto.org/@howellspace
Fermi Paradox: Where Are the Aliens?
https://www.space.com/25325-fermi-paradox.html
https://en.wikipedia.org/wiki/Fermi_paradox#Few.2C_if_any.2C_other_civilizations_currently_exist
Fermi paradox - Wikipedia
2001-12-03
None
<div><div>
<p>This article is about the absence of clear evidence for space aliens. For a type of estimation problem, see <a href="/wiki/Fermi_problem">Fermi problem</a>.</p>
<div><p><a href="/wiki/File:Arecibo_message.svg"></a></p><div><p>A graphical representation of the <a href="/wiki/Arecibo_message">Arecibo message</a>, humanity's first attempt to use radio waves to actively communicate its existence to alien civilizations</p></div></div>
<p>The <b>Fermi paradox</b> is the discrepancy between the lack of conclusive evidence of advanced <a href="/wiki/Extraterrestrial_life">extraterrestrial life</a> compared to the apparently high <a href="/wiki/A_priori_and_a_posteriori">a priori</a> likelihood of its existence.<sup><a href="#cite_note-INS-20190921-1">[1]</a></sup><sup><a href="#cite_note-Krauthammer2011-2">[2]</a></sup> As a 2015 article put it, "If life is so easy, someone from somewhere must have come calling by now."<sup><a href="#cite_note-NYT-20150803-3">[3]</a></sup>
</p><p><a href="/wiki/Italian-American">Italian-American</a> <a href="/wiki/Physicist">physicist</a> <a href="/wiki/Enrico_Fermi">Enrico Fermi</a>'s name is associated with the paradox because of a casual conversation in the summer of 1950 with fellow physicists <a href="/wiki/Edward_Teller">Edward Teller</a>, <a href="/wiki/Herbert_York">Herbert York</a>, and <a href="/wiki/Emil_Konopinski">Emil Konopinski</a>. While walking to lunch, the men discussed recent <a href="/wiki/UFO">UFO</a> reports and the possibility of <a href="/wiki/Faster-than-light_travel">faster-than-light travel</a>. The conversation moved on to other topics, until during lunch Fermi blurted out, "But where is everybody?" (although <a href="#Original_conversations">the exact quote is uncertain</a>).<sup><a href="#cite_note-NYT-20150803-3">[3]</a></sup><sup><a href="#cite_note-Eric-Jones-4">[4]</a></sup>
</p><p>There have been many attempts to resolve the Fermi paradox,<sup><a href="#cite_note-:0-5">[5]</a></sup><sup><a href="#cite_note-6">[6]</a></sup> such as suggesting that <a href="/wiki/Rare_Earth_hypothesis">intelligent extraterrestrial beings are extremely rare</a>, that the lifetime of such civilizations is short, or that they exist but (for various reasons) humans see no evidence.
</p>
<h2>Chain of reasoning[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=1">edit</a>]</h2>
<p>The following are some of the facts and hypotheses that together serve to highlight the apparent contradiction:
</p>
<ul><li>There are <a href="/wiki/Billion">billions</a> of stars in the <a href="/wiki/Milky_Way">Milky Way</a> similar to the <a href="/wiki/Sun">Sun</a>.<sup><a href="#cite_note-7">[7]</a></sup><sup><a href="#cite_note-8">[8]</a></sup></li>
<li>With high probability, some of these stars have Earth-like planets in a <a href="/wiki/Circumstellar_habitable_zone">circumstellar habitable zone</a>.<sup><a href="#cite_note-9">[9]</a></sup></li>
<li>Many of these stars, and hence their planets, are much older than the Sun.<sup><a href="#cite_note-10">[10]</a></sup><sup><a href="#cite_note-11">[11]</a></sup> If the Earth is typical, some may have developed <a href="/wiki/Human_intelligence">intelligent</a> life long ago.</li>
<li>Some of these <a href="/wiki/Civilization">civilizations</a> may have developed <a href="/wiki/Interstellar_travel">interstellar travel</a>, a step humans are investigating now.</li>
<li>Even at the slow pace of currently envisioned interstellar travel, the Milky Way galaxy could be completely traversed in a few million years.<sup><a href="#cite_note-Hart-12">[12]</a></sup></li>
<li>Since many of the Sun-like stars are billions of years older than the Sun, the Earth should have already been visited by extraterrestrial civilizations, or at least their probes.<sup><a href="#cite_note-13">[13]</a></sup></li>
<li>However, there is no convincing evidence that this has happened.<sup><a href="#cite_note-Hart-12">[12]</a></sup></li></ul>
<h2>History[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=2">edit</a>]</h2>
<p>Fermi was not the first to ask the question. An earlier implicit mention was by <a href="/wiki/Konstantin_Tsiolkovsky">Konstantin Tsiolkovsky</a> in an unpublished manuscript from 1933.<sup><a href="#cite_note-14">[14]</a></sup> He noted "people deny the presence of intelligent beings on the planets of the universe" because "(i) if such beings exist they would have visited Earth, and (ii) if such civilizations existed then they would have given us some sign of their existence." This was not a paradox for others, who took this to imply the absence of extraterrestrial life. But it was one for him, since he believed in extraterrestrial life and the possibility of space travel. Therefore, he proposed what is now known as the <a href="/wiki/Zoo_hypothesis">zoo hypothesis</a> and speculated that mankind is not yet ready for higher beings to contact us.<sup><a href="#cite_note-15">[15]</a></sup> That Tsiolkovsky himself may not have been the first to discover the paradox is suggested by his above-mentioned reference to other people's reasons for not accepting the premise that extraterrestrial civilizations exist.
</p><p>In 1975, <a href="/wiki/Michael_H._Hart">Michael H. Hart</a> published a detailed examination of the paradox, one of the first to do so.<sup><a href="#cite_note-Hart-12">[12]</a></sup><sup><a href="#cite_note-webb-16">[16]</a></sup><sup>: 27–28 </sup><sup><a href="#cite_note-forgan-17">[17]</a></sup><sup>: 6 </sup> He argued that if intelligent extraterrestrials exist, and are capable of space travel, then the galaxy could have been colonized in a time much less than that of the age of the Earth. However, there is no observable evidence they have been here, which Hart called "Fact A".<sup><a href="#cite_note-forgan-17">[17]</a></sup><sup>: 6 </sup>
</p><p>Other names closely related to Fermi's question ("Where are they?") include the Great Silence,<sup><a href="#cite_note-Brin-18">[18]</a></sup><sup><a href="#cite_note-Annis-19">[19]</a></sup><sup><a href="#cite_note-Hope-20">[20]</a></sup><sup><a href="#cite_note-Cirkovic2009-21">[21]</a></sup> and <i>silentium universi</i><sup><a href="#cite_note-Cirkovic2009-21">[21]</a></sup> (Latin for "silence of the universe"), though these only refer to one portion of the Fermi Paradox, that humans see no evidence of other civilizations.
</p>
<h3>Original conversations[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=3">edit</a>]</h3>
<div><p><a href="/wiki/File:Los_Alamos_aerial_view.jpeg"></a></p></div>
<p>In the summer of 1950 at <a href="/wiki/Los_Alamos_National_Laboratory">Los Alamos National Laboratory</a> in <a href="/wiki/New_Mexico">New Mexico</a>, Enrico Fermi and co-workers Emil Konopinski, Edward Teller, and Herbert York had one or several lunchtime conversations.<sup><a href="#cite_note-Eric-Jones-4">[4]</a></sup><sup><a href="#cite_note-22">[22]</a></sup> In one, Fermi suddenly blurted out, "Where is everybody?" (Teller's letter), or "Don't you ever wonder where everybody is?" (York's letter), or "But where is everybody?" (Konopinski's letter).<sup><a href="#cite_note-Eric-Jones-4">[4]</a></sup> Teller wrote, "The result of his question was general laughter because of the strange fact that in spite of Fermi's question coming from the clear blue, everybody around the table seemed to understand at once that he was talking about extraterrestrial life."<sup><a href="#cite_note-Eric-Jones-4">[4]</a></sup>
</p><p>In 1984 York wrote that Fermi "followed up with a series of calculations on the probability of earthlike planets, the probability of life given an earth, the probability of humans given life, the likely rise and duration of high technology, and so on. He concluded on the basis of such calculations that we ought to have been visited long ago and many times over."<sup><a href="#cite_note-Eric-Jones-4">[4]</a></sup> Teller remembers that not much came of this conversation "except perhaps a statement that the distances to the next location of living beings may be very great and that, indeed, as far as our galaxy is concerned, we are living somewhere in the sticks, far removed from the metropolitan area of the galactic center."<sup><a href="#cite_note-Eric-Jones-4">[4]</a></sup>
</p><p>Fermi died of cancer in 1954. However, in letters to the three surviving men decades later in 1984, Dr. Eric Jones of Los Alamos was able to partially put the original conversation back together. He informed each of the men that he wished to include a reasonably accurate version or composite in the written proceedings he was putting together for a previously held conference entitled "Interstellar Migration and the Human Experience".<sup><a href="#cite_note-Eric-Jones-4">[4]</a></sup><sup><a href="#cite_note-Interstellar_Migration_and_the_Human_Experience,_edited_by_Finney_and_Jones,_1985-23">[23]</a></sup> Jones first sent a letter to Edward Teller which included a secondhand account from <a href="/wiki/Hans_Mark">Hans Mark</a>. Teller responded, and then Jones sent Teller's letter to Herbert York. York responded, and finally, Jones sent both Teller's and York's letters to Emil Konopinski who also responded. Furthermore, Konopinski was able to later identify a cartoon which Jones found as the one involved in the conversation and thereby help to settle the time period as being the summer of 1950.<sup><a href="#cite_note-Eric-Jones-4">[4]</a></sup>
</p>
<h2>Basis[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=4">edit</a>]</h2>
<div><p><a href="/wiki/File:Enrico_Fermi_1943-49.jpg"></a></p></div>
<p>The Fermi <a href="/wiki/Paradox">paradox</a> is a conflict between the argument that <a href="/wiki/Scale_(spatial)">scale</a> and <a href="/wiki/Probability">probability</a> seem to favor intelligent life being common in the universe, and the total lack of <a href="/wiki/Evidence">evidence</a> of intelligent life having ever arisen anywhere other than on Earth.
</p><p>The first aspect of the Fermi paradox is a function of the scale or the large numbers involved: there are an estimated 200–400 billion stars in the Milky Way<sup><a href="#cite_note-24">[24]</a></sup> (2–4 × <a href="/wiki/Orders_of_magnitude">10<sup>11</sup></a>) and 70 sextillion (7×10<sup>22</sup>) in the <a href="/wiki/Observable_universe">observable universe</a>.<sup><a href="#cite_note-25">[25]</a></sup> Even if intelligent life occurs on only a minuscule percentage of planets around these stars, there might still be a great number of <a href="https://en.wiktionary.org/wiki/extant#English">extant</a> civilizations, and if the percentage were high enough it would produce a significant number of extant civilizations in the Milky Way. This assumes the <a href="/wiki/Mediocrity_principle">mediocrity principle</a>, by which Earth is a typical <a href="/wiki/Planet">planet</a>.
</p><p>The second aspect of the Fermi paradox is the argument of probability: given intelligent life's ability to overcome scarcity, and its tendency to colonize new <a href="/wiki/Habitat_(ecology)">habitats</a>, it seems possible that at least some civilizations would be technologically advanced, seek out new resources in space, and colonize their own <a href="/wiki/Star_system">star system</a> and, subsequently, surrounding star systems. Since there is no significant evidence on Earth, or elsewhere in the known universe, of other intelligent life after 13.8 billion years of the universe's history, there is a conflict requiring a resolution. Some examples of possible resolutions are that intelligent life is rarer than is thought, that assumptions about the general development or behavior of intelligent species are flawed, or, more radically, that current scientific understanding of the nature of the universe itself is quite incomplete.
</p><p>The Fermi paradox can be asked in two ways.<sup><a href="#cite_note-26">[note 1]</a></sup> The first is, "Why are no aliens or their artifacts found here on Earth, or in the <a href="/wiki/Solar_System">Solar System</a>?". If <a href="/wiki/Interstellar_travel">interstellar travel</a> is possible, even the "slow" kind nearly within the reach of Earth technology, then it would only take from 5 million to 50 million years to colonize the galaxy.<sup><a href="#cite_note-cr-27">[26]</a></sup> This is relatively brief on a <a href="/wiki/Geological_time">geological scale</a>, let alone a <a href="/wiki/Timeline_of_the_Big_Bang">cosmological one</a>. Since there are many stars older than the Sun, and since intelligent life might have evolved earlier elsewhere, the question then becomes why the galaxy has not been colonized already. Even if colonization is impractical or undesirable to all alien civilizations, large-scale exploration of the galaxy could be possible by <a href="#Conjectures_about_interstellar_probes">probes</a>. These might leave detectable artifacts in the Solar System, such as old probes or evidence of mining activity, but none of these have been observed.
</p><p>The second form of the question is "Why do we see no signs of intelligence elsewhere in the universe?". This version does not assume interstellar travel, but includes other galaxies as well. For distant galaxies, travel times may well explain the lack of alien visits to Earth, but a sufficiently advanced civilization could potentially be observable over a significant fraction of the <a href="/wiki/Observable_universe#Size">size of the observable universe</a>.<sup><a href="#cite_note-28">[27]</a></sup> Even if such civilizations are rare, the scale argument indicates they should exist somewhere at some point during the history of the universe, and since they could be detected from far away over a considerable period of time, many more potential sites for their origin are within range of human observation. It is unknown whether the paradox is stronger for the Milky Way galaxy or for the universe as a whole.<sup><a href="#cite_note-29">[28]</a></sup>
</p>
<h3>Drake equation[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=5">edit</a>]</h3>
<p>The theories and principles in the <a href="/wiki/Drake_equation">Drake equation</a> are closely related to the Fermi paradox.<sup><a href="#cite_note-30">[29]</a></sup> The equation was formulated by <a href="/wiki/Frank_Drake">Frank Drake</a> in 1961 in an attempt to find a systematic means to evaluate the numerous probabilities involved in the existence of alien life. The equation is presented as follows:
</p><p>
</p><p>Where is the number of technologically advanced civilizations in the Milky Way galaxy, and is asserted to be the product of 
</p>
<p>The fundamental problem is that the last four terms (, , , and ) are entirely unknown, rendering statistical estimates impossible.<sup><a href="#cite_note-31">[30]</a></sup>
</p><p>The Drake equation has been used by both optimists and pessimists, with wildly differing results. The first scientific meeting on the <a href="/wiki/Search_for_extraterrestrial_intelligence">search for extraterrestrial intelligence</a> (SETI), which had 10 attendees including Frank Drake and <a href="/wiki/Carl_Sagan">Carl Sagan</a>, speculated that the number of civilizations was roughly between 1,000 and 100,000,000 civilizations in the Milky Way galaxy.<sup><a href="#cite_note-32">[31]</a></sup> Conversely, <a href="/wiki/Frank_J._Tipler">Frank Tipler</a> and <a href="/wiki/John_D._Barrow">John D. Barrow</a> used pessimistic numbers and speculated that the average number of civilizations in a galaxy is much less than one.<sup><a href="#cite_note-33">[32]</a></sup> Almost all arguments involving the Drake equation suffer from the <a href="/wiki/Overconfidence_effect">overconfidence effect</a>, a common error of probabilistic reasoning about low-probability events, by guessing specific numbers for likelihoods of events whose mechanism is not yet understood, such as the likelihood of <a href="/wiki/Abiogenesis">abiogenesis</a> on an Earth-like planet, with current likelihood estimates varying over many hundreds of <a href="/wiki/Order_of_magnitude">orders of magnitude</a>. An analysis that takes into account some of the uncertainty associated with this lack of understanding has been carried out by <a href="/wiki/Anders_Sandberg">Anders Sandberg</a>, <a href="/wiki/Eric_Drexler">Eric Drexler</a> and <a href="/wiki/Toby_Ord">Toby Ord</a>,<sup><a href="#cite_note-34">[33]</a></sup> and suggests "a substantial <i><a href="/wiki/Ex_ante">ex ante</a></i> probability of there being no other intelligent life in our observable universe".
</p>
<h3>Great Filter[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=6">edit</a>]</h3>
<p>The Great Filter, a concept introduced by <a href="/wiki/Robin_Hanson">Robin Hanson</a> in 1996, represents whatever natural phenomena that would make it unlikely for life to evolve from inanimate matter to an <a href="/wiki/Kardashev_scale">advanced civilization</a>.<sup><a href="#cite_note-Hanson-35">[34]</a></sup><sup><a href="#cite_note-NYT-20150803-3">[3]</a></sup> The most commonly agreed-upon low probability event is <a href="/wiki/Abiogenesis">abiogenesis</a>: a gradual process of increasing complexity of the first self-replicating molecules by a randomly occurring chemical process. Other proposed great filters are the emergence of <a href="/wiki/Eukaryotes">eukaryotic cells</a><sup><a href="#cite_note-36">[note 2]</a></sup> or of <a href="/wiki/Meiosis">meiosis</a> or some of the steps involved in the evolution of a brain capable of complex logical deductions.<sup><a href="#cite_note-Lineweaver,_2009-37">[35]</a></sup>
</p><p>Astrobiologists <a href="/wiki/Dirk_Schulze-Makuch">Dirk Schulze-Makuch</a> and William Bains, reviewing the history of life on Earth, including <a href="/wiki/Convergent_evolution">convergent evolution</a>, concluded that transitions such as <a href="/wiki/Oxygenic_photosynthesis">oxygenic photosynthesis</a>, the <a href="/wiki/Eukaryote">eukaryotic cell</a>, <a href="/wiki/Multicellularity">multicellularity</a>, and <a href="/wiki/Tool">tool</a>-using <a href="/wiki/Intelligence">intelligence</a> are likely to occur on any Earth-like planet given enough time. They argue that the Great Filter may be abiogenesis, the rise of technological human-level intelligence, or an inability to settle other worlds because of self-destruction or a lack of resources.<sup><a href="#cite_note-38">[36]</a></sup>
</p>
<h2>Empirical evidence[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=7">edit</a>]</h2>
<p>There are two parts of the Fermi paradox that rely on empirical evidence—that there are many potential <a href="/wiki/Planetary_habitability">habitable planets</a>, and that humans see no evidence of life. The first point, that many suitable planets exist, was an assumption in Fermi's time but is now supported by the discovery that <a href="/wiki/Exoplanet">exoplanets</a> are common. Current models predict billions of habitable worlds in the Milky Way.<sup><a href="#cite_note-39">[37]</a></sup>
</p><p>The second part of the paradox, that humans see no evidence of extraterrestrial life, is also an active field of scientific research. This includes both efforts to find any indication of life,<sup><a href="#cite_note-40">[38]</a></sup> and efforts specifically directed to finding intelligent life. These searches have been made since 1960, and several are ongoing.<sup><a href="#cite_note-41">[note 3]</a></sup>
</p><p>Although astronomers do not usually search for extraterrestrials, they have observed phenomena that they could not immediately explain without positing an intelligent civilization as the source. For example, <a href="/wiki/Pulsar">pulsars</a>, when <a href="/wiki/PSR_B1919%2B21">first discovered</a> in 1967, were called <a href="/wiki/Little_green_men">little green men</a> (LGM) because of the precise repetition of their pulses.<sup><a href="#cite_note-42">[39]</a></sup> In all cases, explanations with no need for intelligent life have been found for such observations,<sup><a href="#cite_note-43">[note 4]</a></sup> but the possibility of discovery remains.<sup><a href="#cite_note-44">[40]</a></sup> Proposed examples include <a href="/wiki/Asteroid_mining">asteroid mining</a> that would change the appearance of debris disks around stars,<sup><a href="#cite_note-45">[41]</a></sup> or spectral lines from nuclear waste disposal in stars.<sup><a href="#cite_note-46">[42]</a></sup>
</p><p>Explanations based on <a href="/wiki/Technosignature">technosignatures</a>, such as <a href="/wiki/Radio_communication">radio communications</a>, have been presented.<sup><a href="#cite_note-AJ-20221222-47">[43]</a></sup>
</p>
<h3>Electromagnetic emissions[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=8">edit</a>]</h3>
<div><p><a href="/wiki/File:Parkes.arp.750pix.jpg"></a></p></div>
<p>Radio technology and the ability to construct a <a href="/wiki/Radio_telescope">radio telescope</a> are presumed to be a natural advance for technological species,<sup><a href="#cite_note-48">[44]</a></sup> theoretically creating effects that might be detected over interstellar distances. The careful searching for non-natural radio emissions from space may lead to the detection of alien civilizations. Sensitive alien observers of the Solar System, for example, would note unusually intense <a href="/wiki/Radio_wave">radio waves</a> for a <a href="/wiki/Star#Classification">G2 star</a> due to Earth's television and telecommunication broadcasts. In the absence of an apparent natural cause, alien observers might infer the existence of a terrestrial civilization. Such signals could be either "accidental" by-products of a civilization, or deliberate attempts to communicate, such as the <a href="/wiki/Arecibo_message">Arecibo message</a>. It is unclear whether "leakage", as opposed to a deliberate beacon, could be detected by an extraterrestrial civilization. The most sensitive radio telescopes on Earth, as of 2019, would not be able to detect non-directional radio signals even at a fraction of a <a href="/wiki/Light-year">light-year</a> away,<sup><a href="#cite_note-49">[45]</a></sup> but other civilizations could hypothetically have much better equipment.<sup><a href="#cite_note-50">[46]</a></sup>
</p><p>A number of astronomers and observatories have attempted and are attempting to detect such evidence, mostly through the SETI organization. Several decades of SETI analysis have not revealed any unusually bright or meaningfully repetitive radio emissions.<sup><a href="#cite_note-51">[47]</a></sup>
</p>
<h3>Direct planetary observation[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=9">edit</a>]</h3>
<div><p><a href="/wiki/File:Earth%27s_City_Lights_by_DMSP,_1994-1995_(large).jpg"></a></p><div><p>A composite picture of Earth at night, created using data from the <a href="/wiki/Defense_Meteorological_Satellite_Program">Defense Meteorological Satellite Program</a> (DMSP) Operational Linescan System (OLS). Large-scale artificial lighting produced by human civilization is detectable from space.</p></div></div>
<p>Exoplanet detection and classification is a very active sub-discipline in astronomy; the first possible <a href="/wiki/Terrestrial_planet">terrestrial planet</a> discovered within a star's <a href="/wiki/Habitable_zone">habitable zone</a> was found in 2007.<sup><a href="#cite_note-52">[48]</a></sup> New <a href="/wiki/Methods_of_detecting_extrasolar_planets#Other_possible_methods">refinements in exoplanet detection methods</a>, and use of existing methods from space (such as the <a href="/wiki/Kepler_space_telescope">Kepler</a> and <a href="/wiki/Transiting_Exoplanet_Survey_Satellite">TESS</a> missions) are starting to detect and characterize Earth-size planets, to determine whether they are within the habitable zones of their stars. Such observational refinements may allow for a better estimation of how common these potentially habitable worlds are.<sup><a href="#cite_note-53">[49]</a></sup>
</p>
<h3>Conjectures about interstellar probes[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=10">edit</a>]</h3>
<p>The <a href="/wiki/Hart-Tipler_conjecture">Hart-Tipler conjecture</a> is a form of <a href="/wiki/Contraposition">contraposition</a> which states that because no interstellar probes have been detected, there likely is no other intelligent life in the universe, as such life should be expected to eventually create and launch such probes.<sup><a href="#cite_note-Gray2015-54">[50]</a></sup><sup><a href="#cite_note-AliensSpaceTime2020-55">[51]</a></sup> Self-replicating probes could exhaustively explore a galaxy the size of the Milky Way in as little as a million years.<sup><a href="#cite_note-Hart-12">[12]</a></sup> If even a single civilization in the Milky Way attempted this, such probes could spread throughout the entire galaxy. Another speculation for contact with an alien probe—one that would be trying to find human beings—is an alien <a href="/wiki/Bracewell_probe">Bracewell probe</a>. Such a hypothetical device would be an autonomous space probe whose purpose is to seek out and communicate with alien civilizations (as opposed to von Neumann probes, which are usually described as purely exploratory). These were proposed as an alternative to carrying a slow <a href="/wiki/Speed-of-light">speed-of-light</a> dialogue between vastly distant neighbors. Rather than contending with the long delays a radio dialogue would suffer, a probe housing an <a href="/wiki/Artificial_intelligence">artificial intelligence</a> would seek out an alien civilization to carry on a close-range communication with the discovered civilization. The findings of such a probe would still have to be transmitted to the home civilization at light speed, but an information-gathering dialogue could be conducted in real time.<sup><a href="#cite_note-56">[52]</a></sup>
</p><p>Direct exploration of the Solar System has yielded no evidence indicating a visit by aliens or their probes. Detailed exploration of areas of the Solar System where resources would be plentiful may yet produce evidence of alien exploration,<sup><a href="#cite_note-AsteroidBelt-57">[53]</a></sup><sup><a href="#cite_note-58">[54]</a></sup> though the entirety of the Solar System is vast and difficult to investigate. Attempts to signal, attract, or activate hypothetical Bracewell probes in Earth's vicinity have not succeeded.<sup><a href="#cite_note-59">[55]</a></sup>
</p>
<h3>Searches for stellar-scale artifacts[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=11">edit</a>]</h3>
<div><p><a href="/wiki/File:Dyson_Sphere_Diagram-en.svg"></a></p><div><p>A variant of the speculative <a href="/wiki/Dyson_sphere">Dyson sphere</a>. Such large-scale artifacts would drastically alter the spectrum of a star.</p></div></div>
<p>In 1959, <a href="/wiki/Freeman_Dyson">Freeman Dyson</a> observed that every developing human civilization constantly increases its energy consumption, and he conjectured that a civilization might try to harness a large part of the energy produced by a star. He proposed a hypothetical "Dyson sphere" as a possible means: a shell or cloud of objects enclosing a star to absorb and utilize as much <a href="/wiki/Radiant_energy">radiant energy</a> as possible. Such a feat of <a href="/wiki/Astroengineering">astroengineering</a> would drastically alter the observed <a href="/wiki/Spectroscopy">spectrum</a> of the star involved, changing it at least partly from the normal <a href="/wiki/Emission_lines">emission lines</a> of a natural <a href="/wiki/Stellar_atmosphere">stellar atmosphere</a> to those of <a href="/wiki/Black-body_radiation">black-body radiation</a>, probably with a peak in the <a href="/wiki/Infrared">infrared</a>. Dyson speculated that advanced alien civilizations might be detected by examining the spectra of stars and searching for such an altered spectrum.<sup><a href="#cite_note-60">[56]</a></sup><sup><a href="#cite_note-G^I-61">[57]</a></sup><sup><a href="#cite_note-G^II-62">[58]</a></sup>
</p><p>There have been some attempts to find evidence of the existence of Dyson spheres that would alter the spectra of their core stars.<sup><a href="#cite_note-63">[59]</a></sup> Direct observation of thousands of galaxies has shown no explicit evidence of artificial construction or modifications.<sup><a href="#cite_note-G^I-61">[57]</a></sup><sup><a href="#cite_note-G^II-62">[58]</a></sup><sup><a href="#cite_note-G^III-64">[60]</a></sup><sup><a href="#cite_note-65">[61]</a></sup> In October 2015, there was some speculation that a dimming of light from star <a href="/wiki/KIC_8462852">KIC 8462852</a>, observed by the <a href="/wiki/Kepler_space_telescope">Kepler space telescope</a>, could have been a result of Dyson sphere construction.<sup><a href="#cite_note-66">[62]</a></sup><sup><a href="#cite_note-ATL-20151013-67">[63]</a></sup> However, in 2018, observations determined that the amount of dimming varied by the frequency of the light, pointing to dust, rather than an opaque object such as a Dyson sphere, as the culprit for causing the dimming.<sup><a href="#cite_note-68">[64]</a></sup><sup><a href="#cite_note-69">[65]</a></sup>
</p>
<h2>Hypothetical explanations for the paradox[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=12">edit</a>]</h2>
<h3>Rarity of intelligent life[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=13">edit</a>]</h3>
<h4>[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=14">edit</a>]</h4>
<p>Those who think that intelligent <a href="/wiki/Extraterrestrial_life">extraterrestrial life</a> is (nearly) impossible argue that the conditions needed for the evolution of life—or at least the <a href="/wiki/Evolution_of_biological_complexity">evolution of biological complexity</a>—are rare or even unique to Earth. Under this assumption, called the <a href="/wiki/Rare_Earth_hypothesis">rare Earth hypothesis</a>, a rejection of the <a href="/wiki/Mediocrity_principle">mediocrity principle</a>, complex multicellular life is regarded as exceedingly unusual.<sup><a href="#cite_note-rare-earth-70">[66]</a></sup>
</p><p>The rare Earth hypothesis argues that the evolution of biological complexity requires a host of fortuitous circumstances, such as a <a href="/wiki/Galactic_habitable_zone">galactic habitable zone</a>, a star and planet(s) having the requisite conditions, such as enough of a <a href="/wiki/Circumstellar_habitable_zone">continuous habitable zone</a>, the advantage of a giant guardian like Jupiter and a large <a href="/wiki/Natural_satellite">moon</a>, conditions needed to ensure the planet has a <a href="/wiki/Magnetosphere">magnetosphere</a> and <a href="/wiki/Plate_tectonics">plate tectonics</a>, the chemistry of the <a href="/wiki/Lithosphere">lithosphere</a>, <a href="/wiki/Atmosphere">atmosphere</a>, and oceans, the role of "evolutionary pumps" such as massive <a href="/wiki/Glaciation">glaciation</a> and rare <a href="/wiki/Meteoroid#Bolide">bolide</a> impacts. And perhaps most importantly, advanced life needs whatever it was that led to the transition of (some) <a href="/wiki/Prokaryote">prokaryotic cells</a> to <a href="/wiki/Eukaryote">eukaryotic cells</a>, <a href="/wiki/Sexual_reproduction">sexual reproduction</a> and the <a href="/wiki/Cambrian_explosion">Cambrian explosion</a>.
</p><p>In his book <i><a href="/wiki/Wonderful_Life_(book)">Wonderful Life</a></i> (1989), Stephen Jay Gould suggested that if the "tape of life" were rewound to the time of the Cambrian explosion, and one or two tweaks made, human beings most probably never would have evolved. Other thinkers such as Fontana, Buss, and Kauffman have written about the self-organizing properties of life.<sup><a href="#cite_note-71">[67]</a></sup>
</p>
<h4>[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=15">edit</a>]</h4>
<p>It is possible that even if complex life is common, intelligence (and consequently civilizations) is not.<sup><a href="#cite_note-Lineweaver,_2009-37">[35]</a></sup> While there are remote sensing techniques that could perhaps detect life-bearing planets without relying on the signs of technology,<sup><a href="#cite_note-72">[68]</a></sup><sup><a href="#cite_note-73">[69]</a></sup> none of them have any ability to tell if any detected life is intelligent. This is sometimes referred to as the "algae vs. alumnae" problem.<sup><a href="#cite_note-Tarter-NYAS-74">[70]</a></sup>
</p><p>Charles Lineweaver states that when considering any extreme trait in an animal, intermediate stages do not necessarily produce "inevitable" outcomes. For example, large brains are no more "inevitable", or convergent, than are the long noses of animals such as <a href="/wiki/Aardvark">aardvarks</a> and elephants. Humans, apes, whales, dolphins, octopuses, and squids are among the small group of definite or probable intelligence on Earth. And as he points out, "dolphins have had ~20 million years to build a radio telescope and have not done so".<sup><a href="#cite_note-Lineweaver,_2009-37">[35]</a></sup>
</p><p>In addition, Rebecca Boyle points out that of all the species who have ever evolved in the history of life on the planet Earth, only one—we human beings and only in the beginning stages—has ever become space-faring.<sup><a href="#cite_note-Rebecca_Boyle,_Quanta_Magazine-75">[71]</a></sup>
</p>
<h4>Periodic extinction by natural events[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=16">edit</a>]</h4>
<div><p><a href="/wiki/File:Coast_Impact.jpg"></a></p></div>
<p>New life might commonly die out due to runaway heating or cooling on their fledgling planets.<sup><a href="#cite_note-76">[72]</a></sup> On Earth, there have been numerous major <a href="/wiki/Extinction_event">extinction events</a> that destroyed the majority of complex species alive at the time; the <a href="/wiki/K-T_extinction">extinction of the non-avian dinosaurs</a> is the best known example. These are thought to have been caused by events such as impact from a large meteorite, massive volcanic eruptions, or astronomical events such as <a href="/wiki/Gamma-ray_burst">gamma-ray bursts</a>.<sup><a href="#cite_note-77">[73]</a></sup> It may be the case that such extinction events are common throughout the universe and periodically destroy intelligent life, or at least its civilizations, before the species is able to develop the technology to communicate with other intelligent species.<sup><a href="#cite_note-78">[74]</a></sup>
</p>
<h3>Evolutionary explanations[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=17">edit</a>]</h3>
<h4>Intelligent alien species have not developed advanced technologies[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=18">edit</a>]</h4>
<div><p><a href="/wiki/File:Le_Moustier.jpg"></a></p></div>
<p>It may be that while alien species with intelligence exist, they are primitive or have not reached the level of technological advancement necessary to communicate. Along with non-intelligent life, such civilizations would also be very difficult to detect.<sup><a href="#cite_note-Tarter-NYAS-74">[70]</a></sup> A trip using conventional rockets would take hundreds of thousands of years to reach the nearest stars.<sup><a href="#cite_note-79">[75]</a></sup>
</p><p>To skeptics, the fact that in the history of life on the Earth only one species has developed a civilization to the point of being capable of <a href="/wiki/Spaceflight">spaceflight</a> and radio technology lends more credence to the idea that technologically advanced civilizations are rare in the universe.<sup><a href="#cite_note-80">[76]</a></sup>
</p><p>Another hypothesis in this category is the "Water World hypothesis". According to author and scientist <a href="/wiki/David_Brin">David Brin</a>: "it turns out that our Earth skates the very inner edge of our sun’s continuously habitable—or 'Goldilocks'—zone. And Earth may be anomalous. It may be that because we are so close to our sun, we have an anomalously oxygen-rich atmosphere, and we have anomalously little ocean for a water world. In other words, 32 percent continental mass may be high among water worlds..."<sup><a href="#cite_note-DB_2012-81">[77]</a></sup> Brin continues, "In which case, the evolution of creatures like us, with hands and fire and all that sort of thing, may be rare in the galaxy. In which case, when we do build starships and head out there, perhaps we’ll find lots and lots of life worlds, but they’re all like Polynesia. We’ll find lots and lots of intelligent lifeforms out there, but they’re all dolphins, whales, squids, who could never build their own starships. What a perfect universe for us to be in, because nobody would be able to boss us around, and we’d get to be the voyagers, the <i>Star Trek</i> people, the starship builders, the policemen, and so on."<sup><a href="#cite_note-DB_2012-81">[77]</a></sup>
</p>
<h4>It is the nature of intelligent life to destroy itself[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=19">edit</a>]</h4>
<div><p><a href="/wiki/File:Operation_Upshot-Knothole_-_Badger_001.jpg"></a></p></div>
<p>This is the argument that technological civilizations may usually or invariably destroy themselves before or shortly after developing radio or spaceflight technology. The astrophysicist <a href="/wiki/Sebastian_von_Hoerner">Sebastian von Hoerner</a> stated that the progress of science and technology on <a href="/wiki/Earth">Earth</a> was driven by two factors—the struggle for domination and the desire for an easy life. The former potentially leads to complete destruction, while the latter may lead to biological or mental degeneration.<sup><a href="#cite_note-Hoerner-82">[78]</a></sup> Possible means of annihilation via major global issues, where global interconnectedness actually makes humanity more vulnerable than resilient,<sup><a href="#cite_note-83">[79]</a></sup> are many,<sup><a href="#cite_note-84">[80]</a></sup> including war, accidental environmental contamination or damage, the development of <a href="/wiki/Biotechnology_risk">biotechnology</a>,<sup><a href="#cite_note-85">[81]</a></sup> <a href="/wiki/Synthetic_life">synthetic life</a> like <a href="/wiki/Mirror_life">mirror life</a>,<sup><a href="#cite_note-86">[82]</a></sup> <a href="/wiki/Resource_depletion">resource depletion</a>, <a href="/wiki/Global_warming">climate change</a>,<sup><a href="#cite_note-87">[83]</a></sup> or <a href="/wiki/Existential_risk_from_artificial_general_intelligence">poorly-designed artificial intelligence</a>. This general theme is explored both in fiction and in scientific hypothesizing.<sup><a href="#cite_note-88">[84]</a></sup>
</p><p>In 1966, Sagan and <a href="/wiki/Iosif_Shklovsky">Shklovskii</a> speculated that technological civilizations will either tend to destroy themselves within a century of developing interstellar communicative capability or master their self-destructive tendencies and survive for billion-year timescales.<sup><a href="#cite_note-89">[85]</a></sup> Self-annihilation may also be viewed in terms of <a href="/wiki/Thermodynamics">thermodynamics</a>: insofar as life is an ordered <a href="/wiki/System">system</a> that can sustain itself against the <a href="/wiki/Entropy">tendency to disorder</a>, Stephen Hawking's "external transmission" or interstellar communicative phase, where <a href="/wiki/Knowledge_economy">knowledge production</a> and <a href="/wiki/Knowledge_management">knowledge management</a> is more important than transmission of information via <a href="/wiki/Evolution">evolution</a>, may be the point at which the system becomes unstable and self-destructs.<sup><a href="#cite_note-90">[86]</a></sup><sup><a href="#cite_note-91">[87]</a></sup> Here, Hawking emphasizes self-design of the <a href="/wiki/Human_genome">human genome</a> (<a href="/wiki/Transhumanism">transhumanism</a>) or enhancement via machines (e.g., <a href="/wiki/Brain%E2%80%93computer_interface">brain–computer interface</a>) to enhance <a href="/wiki/Human_intelligence">human intelligence</a> and reduce <a href="/wiki/Aggression">aggression</a>, without which he implies human civilization may be too stupid collectively to survive an increasingly unstable system. For instance, the development of technologies during the "external transmission" phase, such as <a href="/wiki/Weaponization">weaponization</a> of <a href="/wiki/Artificial_general_intelligence">artificial general intelligence</a> or <a href="/wiki/Antimatter">antimatter</a>, may not be met by concomitant increases in human ability to manage its own inventions. Consequently, disorder increases in the system: <a href="/wiki/Global_governance">global governance</a> may become increasingly destabilized, worsening humanity's ability to manage the possible means of annihilation listed above, resulting in global <a href="/wiki/Societal_collapse">societal collapse</a>.
</p>
<div><p><a href="/wiki/File:Frank_climate_model,_2018.svg"></a></p><div><p>Possible trajectories of anthropogenic climate change in a model by Frank <i>et al</i>., 2018.</p></div></div>
<p>Using extinct civilizations such as <a href="/wiki/Easter_Island">Easter Island (Rapa Nui)</a> as models, a study conducted in 2018 by <a href="/wiki/Adam_Frank">Adam Frank</a> <i>et al.</i> posited that <a href="/wiki/Climate_change">climate change</a> induced by "energy intensive" civilizations may prevent sustainability within such civilizations, thus explaining the paradoxical lack of evidence for intelligent extraterrestrial life. According to his model, possible outcomes of climate change include gradual <a href="/wiki/Population_decline">population decline</a> until an equilibrium is reached; a scenario where <a href="/wiki/Sustainability">sustainability</a> is attained and both population and surface temperature level off; and societal collapse, including scenarios where a <a href="/wiki/Tipping_points_in_the_climate_system">tipping point</a> is crossed.<sup><a href="#cite_note-billings-92">[88]</a></sup>
</p><p>A less theoretical example might be the resource-depletion issue on Polynesian islands, of which Easter Island is only the best known. David Brin points out that during the expansion phase from 1500 BC to 800 AD there were cycles of overpopulation followed by what might be called periodic cullings of adult males through war or ritual. He writes, "There are many stories of islands whose men were almost wiped out—sometimes by internal strife, and sometimes by invading males from other islands."<sup><a href="#cite_note-93">[89]</a></sup>
</p>
<h4>It is the nature of intelligent life to destroy others[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=20">edit</a>]</h4>
<p>Another hypothesis is that an intelligent species beyond a certain point of technological capability will destroy other intelligent species as they appear, perhaps by using <a href="/wiki/Self-replicating_spacecraft">self-replicating probes</a>. Science fiction writer <a href="/wiki/Fred_Saberhagen">Fred Saberhagen</a> has explored this idea in his <i><a href="/wiki/Berserker_(novel_series)">Berserker</a></i> series, as has physicist <a href="/wiki/Gregory_Benford">Gregory Benford</a><sup><a href="#cite_note-94">[90]</a></sup> and, as well, science fiction writer <a href="/wiki/Greg_Bear">Greg Bear</a> in his <i><a href="/wiki/The_Forge_of_God">The Forge of God</a></i> novel,<sup><a href="#cite_note-95">[91]</a></sup> and later <a href="/wiki/Liu_Cixin">Liu Cixin</a> in his <i><a href="/wiki/The_Three-Body_Problem_(novel)">The Three-Body Problem</a></i> series.
</p><p>A species might undertake such extermination out of expansionist motives, greed, paranoia, or aggression. In 1981, cosmologist <a href="/wiki/Edward_Robert_Harrison">Edward Harrison</a> argued that such behavior would be an act of prudence: an intelligent species that has overcome its own self-destructive tendencies might view any other species bent on galactic expansion as a threat.<sup><a href="#cite_note-96">[92]</a></sup> It has also been suggested that a successful alien species would be a <a href="/wiki/Apex_predator">superpredator</a>, as are humans.<sup><a href="#cite_note-97">[93]</a></sup><sup><a href="#cite_note-webb02-98">[94]</a></sup><sup>: 112 </sup> Another possibility invokes the "<a href="/wiki/Tragedy_of_the_commons">tragedy of the commons</a>" and the <a href="/wiki/Anthropic_principle">anthropic principle</a>: the first lifeform to achieve interstellar travel will necessarily (even if unintentionally) prevent competitors from arising, and humans simply happen to be first.<sup><a href="#cite_note-ARX-20180327-99">[95]</a></sup><sup><a href="#cite_note-100">[96]</a></sup>
</p>
<h4>Civilizations only broadcast detectable signals for a brief period of time[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=21">edit</a>]</h4>
<p>It may be that alien civilizations are detectable through their radio emissions for only a short time, reducing the likelihood of spotting them. The usual assumption is that civilizations outgrow radio through technological advancement.<sup><a href="#cite_note-101">[97]</a></sup> However, there could be other leakage such as that from microwaves used to transmit power from solar satellites to ground receivers.<sup><a href="#cite_note-Stephenson1984-102">[98]</a></sup>
</p><p>Regarding the first point, in a 2006 <i><a href="/wiki/Sky_%26_Telescope">Sky &amp; Telescope</a></i> article, <a href="/wiki/Seth_Shostak">Seth Shostak</a> wrote, "Moreover, radio leakage from a planet is only likely to get weaker as a civilization advances and its communications technology gets better. Earth itself is increasingly switching from broadcasts to leakage-free cables and fiber optics, and from primitive but obvious carrier-wave broadcasts to subtler, hard-to-recognize spread-spectrum transmissions."<sup><a href="#cite_note-Seth_Shostak,_Sky_&amp;_Telescope,_2006-103">[99]</a></sup>
</p><p>More hypothetically, advanced alien civilizations may evolve beyond broadcasting at all in the electromagnetic spectrum and communicate by technologies not developed or used by mankind.<sup><a href="#cite_note-104">[100]</a></sup> Some scientists have hypothesized that advanced civilizations may send <a href="/wiki/Neutrino">neutrino</a> signals.<sup><a href="#cite_note-105">[101]</a></sup> If such signals exist, they could be detectable by <a href="/wiki/Neutrino_detector">neutrino detectors</a> that are now under construction for other goals.<sup><a href="#cite_note-106">[102]</a></sup>
</p>
<h4>Alien life may be too alien[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=22">edit</a>]</h4>
<div><p><a href="/wiki/File:TerrestrialMicrowaveWindow.jpg"></a></p><div><p>Microwave window as seen by a ground-based system. From NASA report SP-419: SETI – the Search for Extraterrestrial Intelligence</p></div></div>
<p>Another possibility is that human theoreticians have underestimated how much alien life might differ from that on Earth. Aliens may be psychologically unwilling to attempt to communicate with human beings. Perhaps human mathematics is <a href="/wiki/Parochialism">parochial</a> to Earth and not shared by other life,<sup><a href="#cite_note-107">[103]</a></sup> though others argue this can only apply to abstract math since the math associated with physics must be similar (in results, if not in methods).<sup><a href="#cite_note-108">[104]</a></sup>
</p><p>Physiology might also cause a communication barrier. Carl Sagan speculated that an alien species might have a thought process orders of magnitude slower (or faster) than that of humans.<sup><a href="#cite_note-109">[105]</a></sup> A message broadcast by that species might well seem like random background noise to humans, and therefore go undetected.
</p><p>Another thought is that technological civilizations invariably experience a <a href="/wiki/Technological_singularity">technological singularity</a> and attain a post-biological character.<sup><a href="#cite_note-110">[106]</a></sup> Hypothetical civilizations of this sort may have advanced drastically enough to render communication impossible.<sup><a href="#cite_note-111">[107]</a></sup><sup><a href="#cite_note-112">[108]</a></sup><sup><a href="#cite_note-113">[109]</a></sup>
</p><p>In his 2009 book, SETI scientist <a href="/wiki/Seth_Shostak">Seth Shostak</a> wrote, "Our experiments [such as plans to use drilling rigs on Mars] are still looking for the type of extraterrestrial that would have appealed to <a href="/wiki/Percival_Lowell#Canals_of_Mars">Percival Lowell</a> [astronomer who believed he had observed canals on Mars]."<sup><a href="#cite_note-114">[110]</a></sup>
</p><p><a href="/wiki/Paul_Davies">Paul Davies</a> states that 500 years ago the very idea of a computer doing work merely by manipulating internal data may not have been viewed as a technology at all. He writes, "Might there be a still <i>higher</i> level ... If so, this 'third level' would never be manifest through observations made at the informational level, still less the matter level. There is no vocabulary to describe the third level, but that doesn't mean it is non-existent, and we need to be open to the possibility that alien technology may operate at the third level, or maybe the fourth, fifth ... levels."<sup><a href="#cite_note-115">[111]</a></sup>
</p>
<h3>Sociological explanations[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=23">edit</a>]</h3>
<h4>Colonization is not the cosmic norm[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=24">edit</a>]</h4>
<p>In response to Tipler's idea of self-replicating probes, Stephen Jay Gould wrote, "I must confess that I simply don’t know how to react to such arguments. I have enough trouble predicting the plans and reactions of the people closest to me. I am usually baffled by the thoughts and accomplishments of humans in different cultures. I’ll be damned if I can state with certainty what some extraterrestrial source of intelligence might do."<sup><a href="#cite_note-Beyond_Fermi's_Paradox_II,_Stephen_Jay_Gould_quote,_April_2015-116">[112]</a></sup><sup><a href="#cite_note-If_the_Universe_Is_Teeming_._._._75_Solutions,_Stephen_Webb,_2015-117">[113]</a></sup>
</p>
<h4>Alien species may have only settled part of the galaxy[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=25">edit</a>]</h4>
<p>A February 2019 article in <i>Popular Science</i> states, "Sweeping across the Milky Way and establishing a unified galactic empire might be inevitable for a monolithic super-civilization, but most cultures are neither monolithic nor super—at least if our experience is any guide."<sup><a href="#cite_note-Struggling_and_hustling,_just_like_us,_Popular_Science,_Feb._2019-118">[114]</a></sup> Astrophysicist Adam Frank, along with co-authors such as astronomer Jason Wright, ran a variety of simulations in which they varied such factors as settlement lifespans, fractions of suitable planets, and recharge times between launches. They found many of their simulations seemingly resulted in a "third category" in which the Milky Way remains partially settled indefinitely.<sup><a href="#cite_note-Struggling_and_hustling,_just_like_us,_Popular_Science,_Feb._2019-118">[114]</a></sup> The abstract to their 2019 paper states, "These results break the link between Hart's famous 'Fact A' (no interstellar visitors on Earth now) and the conclusion that humans must, therefore, be the only technological civilization in the galaxy. Explicitly, our solutions admit situations where our current circumstances are consistent with an otherwise settled, steady-state galaxy."<sup><a href="#cite_note-119">[115]</a></sup>
</p>
<h4>Alien species may not live on planets[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=26">edit</a>]</h4>
<p>Some colonization scenarios predict spherical expansion across star systems, with continued expansion coming from the systems just previously settled. It has been suggested that this would cause a strong <a href="/wiki/Natural_selection">selection</a> process among the colonization front favoring cultural or biological <a href="/wiki/Adaptation">adaptations</a> to living in starships or space habitats. As a result, they may forgo living on planets.<sup><a href="#cite_note-David_Brin,_1983,_abandonment_of_planet-dwelling-120">[116]</a></sup>
</p><p>This may result in the destruction of terrestrial planets in these systems for use as building materials, thus preventing the development of life on those worlds. Or, they may have an ethic of protection for "nursery worlds", and protect them in a similar fashion to the <a href="/wiki/Zoo_hypothesis">zoo hypothesis</a>.<sup><a href="#cite_note-David_Brin,_1983,_abandonment_of_planet-dwelling-120">[116]</a></sup>
</p>
<h4>Alien species may isolate themselves from the outside world[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=27">edit</a>]</h4>
<p>It has been suggested that some advanced beings may divest themselves of physical form, create massive artificial virtual environments, transfer themselves into these environments through <a href="/wiki/Mind_uploading">mind uploading</a>, and exist totally within virtual worlds, ignoring the external physical universe.<sup><a href="#cite_note-121">[117]</a></sup>
</p><p>It may also be that intelligent alien life develops an "increasing disinterest" in their outside world.<sup><a href="#cite_note-webb02-98">[94]</a></sup><sup>: 86 </sup> Possibly any sufficiently advanced society will develop highly engaging media and entertainment well before the capacity for advanced space travel, with the rate of appeal of these social contrivances being destined, because of their inherent reduced complexity, to overtake any desire for complex, expensive endeavors such as space exploration and communication. Once any sufficiently advanced civilization becomes able to master its environment, and most of its physical needs are met through technology, various "social and entertainment technologies", including virtual reality, are postulated to become the primary drivers and motivations of that civilization.<sup><a href="#cite_note-122">[118]</a></sup>
</p>
<h3>Economic explanations[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=28">edit</a>]</h3>
<h4>Lack of resources needed to physically spread throughout the galaxy[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=29">edit</a>]</h4>
<p>The ability of an alien culture to colonize other star systems is based on the idea that interstellar travel is technologically feasible. While the current understanding of physics rules out the possibility of <a href="/wiki/Faster-than-light">faster-than-light</a> travel, it appears that there are no major theoretical barriers to the construction of "slow" interstellar ships, even though the engineering required is considerably beyond present capabilities. This idea underlies the concept of the Von Neumann probe and the Bracewell probe as a potential evidence of extraterrestrial intelligence.
</p><p>It is possible, however, that present scientific knowledge cannot properly gauge the feasibility and costs of such interstellar colonization. Theoretical barriers may not yet be understood, and the resources needed may be so great as to make it unlikely that any civilization could afford to attempt it. Even if interstellar travel and colonization are possible, they may be difficult, leading to a colonization model based on <a href="/wiki/Percolation_theory">percolation theory</a>.<sup><a href="#cite_note-landis-123">[119]</a></sup><sup><a href="#cite_note-kinouchi-124">[120]</a></sup>
</p><p>Colonization efforts may not occur as an unstoppable rush, but rather as an uneven tendency to "percolate" outwards, within an eventual slowing and termination of the effort given the enormous costs involved and the expectation that colonies will inevitably develop a culture and civilization of their own. Colonization may thus occur in "clusters", with large areas remaining uncolonized at any one time.<sup><a href="#cite_note-landis-123">[119]</a></sup><sup><a href="#cite_note-kinouchi-124">[120]</a></sup>
</p>
<h4>Information is cheaper to transmit than matter is to transfer[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=30">edit</a>]</h4>
<p>If a human-capability machine construct, such as via <a href="/wiki/Mind_uploading">mind uploading</a>, is possible, and if it is possible to transfer such constructs over vast distances and rebuild them on a remote machine, then it might not make strong economic sense to travel the galaxy by spaceflight. After the first civilization has physically explored or colonized the galaxy, as well as sent such machines for easy exploration, then any subsequent civilizations, after having contacted the first, may find it cheaper, faster, and easier to explore the galaxy through intelligent mind transfers to the machines built by the first civilization, which is cheaper than spaceflight by a factor of 10<sup>8</sup>–10<sup>17</sup>. However, since a star system needs only one such remote machine, and the communication is most likely highly directed, transmitted at high-frequencies, and at a minimal power to be economical, such signals would be hard to detect from Earth.<sup><a href="#cite_note-125">[121]</a></sup>
</p>
<h3>[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=31">edit</a>]</h3>
<h4>Humans have not listened properly[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=32">edit</a>]</h4>
<p>There are some assumptions that underlie the <a href="/wiki/SETI">SETI</a> programs that may cause searchers to miss signals that are present. Extraterrestrials might, for example, transmit signals that have a very high or low data rate, or employ unconventional (in human terms) <a href="/wiki/Frequency">frequencies</a>, which would make them hard to distinguish from background noise. Signals might be sent from non-<a href="/wiki/Main_sequence">main sequence</a> star systems that humans search with lower priority; current programs assume that most alien life will be orbiting <a href="/wiki/Solar_twins">Sun-like stars</a>.<sup><a href="#cite_note-126">[122]</a></sup>
</p><p>The greatest challenge is the sheer size of the radio search needed to look for signals (effectively spanning the entire observable universe), the limited amount of resources committed to SETI, and the sensitivity of modern instruments. SETI estimates, for instance, that with a radio telescope as sensitive as the <a href="/wiki/Arecibo_Observatory">Arecibo Observatory</a>, Earth's television and radio broadcasts would only be detectable at distances up to 0.3 light-years, less than 1/10 the distance to the nearest star. A signal is much easier to detect if it consists of a deliberate, powerful transmission directed at Earth. Such signals could be detected at ranges of hundreds to tens of thousands of light-years distance.<sup><a href="#cite_note-127">[123]</a></sup> However, this means that detectors must be listening to an appropriate range of frequencies, and be in that region of space to which the beam is being sent. Many SETI searches assume that extraterrestrial civilizations will be broadcasting a deliberate signal, like the Arecibo message, in order to be found.
</p><p>Thus, to detect alien civilizations through their radio emissions, Earth observers either need more sensitive instruments or must hope for fortunate circumstances: that the broadband radio emissions of alien radio technology are much stronger than humanity's own; that one of SETI's programs is listening to the correct frequencies from the right regions of space; or that aliens are deliberately sending focused transmissions in Earth's general direction.
</p>
<h4>Humans have not listened for long enough[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=33">edit</a>]</h4>
<p>Humanity's ability to detect intelligent extraterrestrial life has existed for only a very brief period—from 1937 onwards, if the invention of the <a href="/wiki/Radio_telescope">radio telescope</a> is taken as the dividing line—and <i><a href="/wiki/Homo_sapiens">Homo sapiens</a></i> is a geologically recent species. The whole period of modern human existence to date is a very brief period on a cosmological scale, and radio transmissions have only been propagated since 1895. Thus, it remains possible that human beings have neither existed long enough nor made themselves sufficiently detectable to be found by extraterrestrial intelligence.<sup><a href="#cite_note-128">[124]</a></sup>
</p>
<h4>Intelligent life may be too far away[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=34">edit</a>]</h4>
<div><p><a href="/wiki/File:Terrestrial_Planet_Finder_PIA04499.jpg"></a></p></div>
<p>It may be that non-colonizing technologically capable alien civilizations exist, but that they are simply too far apart for meaningful two-way communication.<sup><a href="#cite_note-webb02-98">[94]</a></sup><sup>: 62–71 </sup> Sebastian von Hoerner estimated the average duration of civilization at 6,500 years and the average distance between civilizations in the Milky Way at 1,000 light years.<sup><a href="#cite_note-Hoerner-82">[78]</a></sup> If two civilizations are separated by several thousand light-years, it is possible that one or both cultures may become extinct before meaningful dialogue can be established. Human searches may be able to detect their existence, but communication will remain impossible because of distance. It has been suggested that this problem might be ameliorated somewhat if contact and communication is made through a <a href="/wiki/Bracewell_probe">Bracewell probe</a>. In this case at least one partner in the exchange may obtain meaningful information. Alternatively, a civilization may simply broadcast its knowledge, and leave it to the receiver to make what they may of it. This is similar to the transmission of information from ancient civilizations to the present,<sup><a href="#cite_note-129">[125]</a></sup> and humanity has undertaken similar activities like the <a href="/wiki/Arecibo_message">Arecibo message</a>, which could transfer information about Earth's intelligent species, even if it never yields a response or does not yield a response in time for humanity to receive it. It is possible that observational signatures of self-destroyed civilizations could be detected, depending on the destruction scenario and the timing of human observation relative to it.<sup><a href="#cite_note-130">[126]</a></sup>
</p><p>A related speculation by Sagan and Newman suggests that if other civilizations exist, and are transmitting and exploring, their signals and probes simply have not arrived yet.<sup><a href="#cite_note-131">[127]</a></sup> However, critics have noted that this is unlikely, since it requires that humanity's advancement has occurred at a very special point in time, while the Milky Way is in transition from empty to full. This is a tiny fraction of the lifespan of a galaxy under ordinary assumptions, so the likelihood that humanity is in the midst of this transition is considered low in the paradox.<sup><a href="#cite_note-David_Brin,_1983,_pages_287_and_298-132">[128]</a></sup>
</p><p>Some SETI skeptics may also believe that humanity is at a very special point of time. Specifically, a transitional period from no space-faring societies to one space-faring society, namely that of human beings.<sup><a href="#cite_note-David_Brin,_1983,_pages_287_and_298-132">[128]</a></sup>
</p>
<h4>Intelligent life may exist hidden from view[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=35">edit</a>]</h4>
<p>Planetary scientist Alan Stern put forward the idea that there could be a number of worlds with subsurface oceans (such as Jupiter's <a href="/wiki/Europa_(moon)">Europa</a> or Saturn's <a href="/wiki/Enceladus">Enceladus</a>). The surface would provide a large degree of protection from such things as cometary impacts and nearby supernovae, as well as creating a situation in which a much broader range of orbits are acceptable. Life, and potentially intelligence and civilization, could evolve. Stern states, "If they have technology, and let's say they're broadcasting, or they have city lights or whatever—we can't see it in any part of the spectrum, except maybe very-low-frequency [radio]."<sup><a href="#cite_note-133">[129]</a></sup><sup><a href="#cite_note-134">[130]</a></sup>
</p>
<h4>Advanced civilizations may limit their search for life to technological signatures[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=36">edit</a>]</h4>
<p>If life is abundant in the universe but the cost of space travel is high, an advanced civilization may choose focus its search not on signs of life in general, but on those of other advanced civilizations, and specifically on <a href="/wiki/Radio">radio</a> signals. Since humanity has <a href="/wiki/History_of_radio">only recently</a> began to use radio communication, its signals may have yet to arrive to other inhabited planets, and if they have, <a href="/wiki/Space_probe">probes</a> from those planets may have yet to arrive on Earth.<sup><a href="#cite_note-135">[131]</a></sup>
</p>
<h3>Willingness to communicate[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=37">edit</a>]</h3>
<h4>Everyone is listening but no one is transmitting[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=38">edit</a>]</h4>
<p>Alien civilizations might be technically capable of contacting Earth, but could be only listening instead of transmitting.<sup><a href="#cite_note-webbwhere-136">[132]</a></sup> If all or most civilizations act in the same way, the galaxy could be full of civilizations eager for contact, but everyone is listening and no one is transmitting. This is the so-called <i><a href="/wiki/SETI">SETI</a> Paradox</i>.<sup><a href="#cite_note-137">[133]</a></sup>
</p><p>The only civilization known, humanity, does not <a href="/wiki/Active_SETI">explicitly transmit</a>, except for a few small efforts.<sup><a href="#cite_note-webbwhere-136">[132]</a></sup> Even these efforts, and certainly any attempt to expand them, are controversial.<sup><a href="#cite_note-138">[134]</a></sup> It is not even clear humanity would respond to a detected signal—the official policy within the SETI community<sup><a href="#cite_note-139">[135]</a></sup> is that "[no] response to a signal or other evidence of extraterrestrial intelligence should be sent until appropriate international consultations have taken place". However, given the possible impact of any reply,<sup><a href="#cite_note-140">[136]</a></sup> it may be very difficult to obtain any consensus on who would speak and what they would say.
</p>
<h4>Communication is dangerous[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=39">edit</a>]</h4>
<p>An alien civilization might feel it is too dangerous to communicate, either for humanity or for them. It is argued that when very different civilizations have met on Earth, the results have often been disastrous for one side or the other, and the same may well apply to interstellar contact.<sup><a href="#cite_note-141">[137]</a></sup> Even contact at a safe distance could lead to infection by computer code<sup><a href="#cite_note-142">[138]</a></sup> or even ideas themselves.<sup><a href="#cite_note-143">[139]</a></sup> Perhaps prudent civilizations actively hide not only from Earth but from everyone, out of <a href="#It_is_the_nature_of_intelligent_life_to_destroy_others">fear of other civilizations</a>.<sup><a href="#cite_note-HawkingFearTheologyAndScience-144">[140]</a></sup>
</p><p>Perhaps the Fermi paradox itself—or the alien equivalent of it—is the reason for any civilization to avoid contact with other civilizations, even if no other obstacles existed. From any one civilization's point of view, it would be unlikely for them to be the first ones to make first contact. Therefore, according to this reasoning, it is likely that previous civilizations faced fatal problems with first contact and doing so should be avoided. So perhaps every civilization keeps quiet because of the possibility that there is a real reason for others to do so.<sup><a href="#cite_note-Brin-18">[18]</a></sup>
</p><p>In <a href="/wiki/Liu_Cixin">Liu Cixin</a>'s 2008 novel <i><a href="/wiki/The_Dark_Forest">The Dark Forest</a></i>, the author proposes a literary explanation for the Fermi paradox in which many multiple alien civilizations exist, but are both silent and paranoid, destroying any nascent lifeforms loud enough to make themselves known.<sup><a href="#cite_note-ChinaFileCixinLiu-145">[141]</a></sup> This is because any other intelligent life may represent a future threat. As a result, Liu's universe contains a plethora of quiet civilizations which do not reveal themselves, as in a "dark forest"...filled with "armed hunter(s) stalking through the trees like a ghost".<sup><a href="#cite_note-146">[142]</a></sup><sup><a href="#cite_note-Dark_Forest_Answer_to_Star_Wars_Optimism-147">[143]</a></sup><sup><a href="#cite_note-Technology's_dark_forest-148">[144]</a></sup> This idea has come to be known as the <i>dark forest hypothesis</i>.<sup><a href="#cite_note-UnivTodayDarkForest-149">[145]</a></sup><sup><a href="#cite_note-NHPR-150">[146]</a></sup><sup><a href="#cite_note-151">[147]</a></sup>
</p>
<h4>Earth is deliberately being avoided[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=40">edit</a>]</h4>
<p>The <a href="/wiki/Zoo_hypothesis">zoo hypothesis</a> states that intelligent extraterrestrial life exists and does not contact life on Earth to allow for its natural evolution and development.<sup><a href="#cite_note-:1-152">[148]</a></sup> A variation on the zoo hypothesis is the laboratory hypothesis, where humanity has been or is being subject to experiments,<sup><a href="#cite_note-:1-152">[148]</a></sup><sup><a href="#cite_note-:0-5">[5]</a></sup> with Earth or the Solar System effectively serving as a laboratory. The zoo hypothesis may break down under the <a href="/wiki/Uniformity_of_motive">uniformity of motive</a> flaw: all it takes is a single culture or civilization to decide to act contrary to the imperative within humanity's range of detection for it to be abrogated, and the probability of such a violation of hegemony increases with the number of civilizations,<sup><a href="#cite_note-cr-27">[26]</a></sup><sup><a href="#cite_note-153">[149]</a></sup> tending not towards a 'Galactic Club' with a unified foreign policy with regard to life on Earth but multiple 'Galactic Cliques'.<sup><a href="#cite_note-154">[150]</a></sup> However, if <a href="/wiki/Artificial_superintelligence">artificial superintelligences</a> dominate galactic life, and if it is true that such intelligences tend towards merged hegemonic behavior, then this would address the uniformity of motive flaw by dissuading rogue behavior.<sup><a href="#cite_note-155">[151]</a></sup>
</p><p>Analysis of the inter-arrival times between civilizations in the galaxy based on common astrobiological assumptions suggests that the initial civilization would have a commanding lead over the later arrivals. As such, it may have established what has been termed the <i>zoo hypothesis</i> through force or as a galactic or universal norm and the resultant "paradox" by a cultural <a href="/wiki/Founder_effect">founder effect</a> with or without the continued activity of the founder.<sup><a href="#cite_note-156">[152]</a></sup>
</p><p>It is possible that a civilization advanced enough to travel between solar systems could be actively visiting or observing Earth while remaining undetected or unrecognized.<sup><a href="#cite_note-157">[153]</a></sup>
</p>
<h4>Earth is deliberately being isolated[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=41">edit</a>]</h4>
<p>A related idea to the zoo hypothesis is that, beyond a certain distance, the perceived universe is a <a href="/wiki/Simulated_reality">simulated reality</a>. The planetarium hypothesis<sup><a href="#cite_note-158">[154]</a></sup> speculates that beings may have created this simulation so that the universe appears to be empty of other life.
</p>
<h3>Alien life is already here, unacknowledged[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=42">edit</a>]</h3>
<p>A significant fraction of the population believes that at least some UFOs (Unidentified Flying Objects) are spacecraft piloted by aliens.<sup><a href="#cite_note-159">[155]</a></sup><sup><a href="#cite_note-160">[156]</a></sup> While most of these are unrecognized or mistaken interpretations of mundane phenomena, there are those that remain puzzling even after investigation. The consensus scientific view is that although they may be unexplained, they do not rise to the level of convincing evidence.<sup><a href="#cite_note-161">[157]</a></sup>
</p><p>Similarly, it is theoretically possible that SETI groups are not reporting positive detections, or governments have been blocking signals or suppressing publication. This response might be attributed to security or economic interests from the potential use of advanced extraterrestrial technology. It has been suggested that the detection of an extraterrestrial radio signal or technology could well be the most highly secret information that exists.<sup><a href="#cite_note-162">[158]</a></sup> Claims that this has already happened are common in the popular press,<sup><a href="#cite_note-163">[159]</a></sup><sup><a href="#cite_note-164">[160]</a></sup> but the scientists involved report the opposite experience—the press becomes informed and interested in a potential detection even before a signal can be confirmed.<sup><a href="#cite_note-165">[161]</a></sup>
</p><p>Regarding the idea that aliens are in secret contact with governments, David Brin writes, "Aversion to an idea, simply because of its long association with crackpots, gives crackpots altogether too much influence."<sup><a href="#cite_note-166">[162]</a></sup>
</p>
<h2>See also[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=43">edit</a>]</h2>
<div>
<ul><li><a href="/wiki/Aestivation_hypothesis">Aestivation hypothesis</a> – Hypothesized solution to the Fermi paradox</li>
<li><a href="/wiki/Anthropic_principle">Anthropic principle</a> – Philosophical principle about the occurrence of sapient life in the Universe</li>
<li><a href="/wiki/Astrobiology">Astrobiology</a> – Science concerned with life in the universe</li>
<li><a href="/wiki/Calculating_God">Calculating God</a> – 2000 novel by Robert J. Sawyer</li>
<li><a href="/wiki/Fermi_problem">Fermi problem</a> – Estimation problem in physics or engineering education</li>
<li><a href="/wiki/Interstellar_travel">Interstellar travel</a> – Hypothetical travel between stars or planetary systems</li>
<li><a href="/wiki/Panspermia">Panspermia</a> – Hypothesis on the interstellar spreading of primordial life</li>
<li><a href="/wiki/Rare_Earth_hypothesis">Rare Earth hypothesis</a> – Hypothesis that complex extraterrestrial life is improbable and extremely rare</li>
<li><a href="/wiki/The_Martians_(scientists)">The Martians (scientists)</a> – Group of prominent Hungarian scientists</li>
<li><a href="/wiki/Wow!_signal">Wow! signal</a> – 1977 narrowband radio signal from SETI</li></ul>
</div>
<h2>Notes[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=44">edit</a>]</h2>
<div><ol>
<li><b><a href="#cite_ref-26">^</a></b> See Hart for an example of "no aliens are here", and Webb for an example of the more general "We see no signs of intelligence anywhere".
</li>
<li><b><a href="#cite_ref-36">^</a></b> Eukaryotes also include plants, animals, fungi, and algae.
</li>
<li><b><a href="#cite_ref-41">^</a></b> See, for example, the <a href="/wiki/SETI_Institute">SETI Institute</a>, <a href="http://seti.harvard.edu/seti/">The Harvard SETI Home Page</a> <a href="https://web.archive.org/web/20100816170426/http://seti.harvard.edu/seti/">Archived</a> August 16, 2010, at the <a href="/wiki/Wayback_Machine">Wayback Machine</a>, or <a href="http://seti.berkeley.edu/">The Search for Extra Terrestrial Intelligence at Berkeley</a> <a href="https://web.archive.org/web/20190406232538/http://seti.berkeley.edu/">Archived</a> April 6, 2019, at the <a href="/wiki/Wayback_Machine">Wayback Machine</a>
</li>
<li><b><a href="#cite_ref-43">^</a></b> Pulsars are now attributed to neutron stars, and Seyfert galaxies to an end-on view of the accretion onto the black holes.
</li>
</ol></div>
<h2>References[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=45">edit</a>]</h2>
<div><ol>
<li><b><a href="#cite_ref-INS-20190921_1-0">^</a></b> <cite>Woodward, Avlin (September 21, 2019). <a href="https://www.insider.com/why-no-contact-with-aliens-2019-9">"A winner of this year's Nobel prize in physics is convinced we'll detect alien life in 100 years. Here are 13 reasons why we haven't made contact yet"</a>. <i><a href="/wiki/Insider_Inc">Insider Inc</a></i>. Retrieved September 21, 2019.</cite>
</li>
<li><b><a href="#cite_ref-Krauthammer2011_2-0">^</a></b> <cite>Krauthammer, Charles (December 29, 2011). <a href="https://www.washingtonpost.com/opinions/are-we-alone-in-the-universe/2011/12/29/gIQA2wSOPP_story.html">"Are We Alone in the Universe?"</a>. <i><a href="/wiki/The_Washington_Post">The Washington Post</a></i>. <a href="https://web.archive.org/web/20141210160035/http://www.washingtonpost.com/opinions/are-we-alone-in-the-universe/2011/12/29/gIQA2wSOPP_story.html">Archived</a> from the original on December 10, 2014. Retrieved January 6, 2015.</cite>
</li>
<li>^ <a href="#cite_ref-NYT-20150803_3-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-NYT-20150803_3-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-NYT-20150803_3-2"><sup><i><b>c</b></i></sup></a> <cite><a href="/wiki/Dennis_Overbye">Overbye, Dennis</a> (August 3, 2015). <a href="https://www.nytimes.com/2015/08/04/science/space/the-flip-side-of-optimism-about-life-on-other-planets.html">"The Flip Side of Optimism About Life on Other Planets"</a>. <i><a href="/wiki/The_New_York_Times">The New York Times</a></i>. <a href="https://web.archive.org/web/20190919003259/https://www.nytimes.com/2015/08/04/science/space/the-flip-side-of-optimism-about-life-on-other-planets.html">Archived</a> from the original on September 19, 2019. Retrieved October 29, 2015.</cite>
</li>
<li>^ <a href="#cite_ref-Eric-Jones_4-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Eric-Jones_4-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-Eric-Jones_4-2"><sup><i><b>c</b></i></sup></a> <a href="#cite_ref-Eric-Jones_4-3"><sup><i><b>d</b></i></sup></a> <a href="#cite_ref-Eric-Jones_4-4"><sup><i><b>e</b></i></sup></a> <a href="#cite_ref-Eric-Jones_4-5"><sup><i><b>f</b></i></sup></a> <a href="#cite_ref-Eric-Jones_4-6"><sup><i><b>g</b></i></sup></a> <a href="#cite_ref-Eric-Jones_4-7"><sup><i><b>h</b></i></sup></a> <a href="http://www.fas.org/sgp/othergov/doe/lanl/la-10311-ms.pdf">"Where is everybody?": An account of Fermi's question"</a> <a href="https://web.archive.org/web/20070629174738/http://www.fas.org/sgp/othergov/doe/lanl/la-10311-ms.pdf">Archived</a> June 29, 2007, at the <a href="/wiki/Wayback_Machine">Wayback Machine</a>, Dr. Eric M. Jones, Los Alamos technical report, March 1985. Jones wrote to Edward Teller on July 13, 1984, Herbert York on Sept. 4, and Emil Konopinski on Sept. 24, 1984.
</li>
<li>^ <a href="#cite_ref-:0_5-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-:0_5-1"><sup><i><b>b</b></i></sup></a> <a href="https://books.google.com/books?id=Y111CQAAQBAJ&amp;pg=PP11"><i>If the Universe Is Teeming with Aliens ... WHERE IS EVERYBODY?: Seventy-Five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life, Second Edition</i></a>, Stephen Webb, foreword by Martin Rees, Heidelberg, New York, Dordrecht, London: Springer International Publishing, 2002, 2015.
</li>
<li><b><a href="#cite_ref-6">^</a></b> <cite>Urban, Tim (June 17, 2014). <a href="http://www.huffingtonpost.com/wait-but-why/the-fermi-paradox_b_5489415.html">"The Fermi Paradox"</a>. <i>Huffington Post</i>. <a href="https://web.archive.org/web/20170402042005/http://www.huffingtonpost.com/wait-but-why/the-fermi-paradox_b_5489415.html">Archived</a> from the original on April 2, 2017. Retrieved January 6, 2015.</cite>
</li>
<li><b><a href="#cite_ref-7">^</a></b> <cite><a href="https://www.britannica.com/topic/star-astronomy">"Star (astronomy)"</a>. <i>Encyclopædia Britannica</i>. <a href="https://web.archive.org/web/20160301055912/https://www.britannica.com/topic/star-astronomy">Archived</a> from the original on March 1, 2016. Retrieved February 4, 2016.</cite> "With regard to mass, size, and intrinsic brightness, the Sun is a typical star." Technically, the sun is near the middle of the main sequence of the <a href="/wiki/Hertzsprung%E2%80%93Russell_diagram">Hertzsprung–Russell diagram</a>. This sequence contains 80–90% of the stars of the galaxy. <a href="http://astro.unl.edu/naap/hr/hr_background3.html">[1]</a> <a href="https://web.archive.org/web/20110716170751/http://astro.unl.edu/naap/hr/hr_background3.html">Archived</a> July 16, 2011, at the <a href="/wiki/Wayback_Machine">Wayback Machine</a>
</li>
<li><b><a href="#cite_ref-8">^</a></b> <cite>Grevesse, N.; Noels, A.; Sauval, A. J. (1996). "Standard abundances". <i>ASP Conference Series</i>. Vol. 99. p. 117. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1996ASPC...99..117G">1996ASPC...99..117G</a>. <q>The Sun is a normal star, though dispersion exists.</q></cite>
</li>
<li><b><a href="#cite_ref-9">^</a></b> <cite>Buchhave, Lars A.; Latham, David W.; Johansen, Anders; et al. (2012). "An abundance of small exoplanets around stars with a wide range of metallicities". <i><a href="/wiki/Nature_(journal)">Nature</a></i>. <b>486</b> (7403): 375–377. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2012Natur.486..375B">2012Natur.486..375B</a>. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1038%2Fnature11121">10.1038/nature11121</a>. <a href="/wiki/ISSN_(identifier)">ISSN</a> <a href="//www.worldcat.org/issn/0028-0836">0028-0836</a>. <a href="/wiki/PMID_(identifier)">PMID</a> <a href="//pubmed.ncbi.nlm.nih.gov/22722196">22722196</a>. <a href="/wiki/S2CID_(identifier)">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:4427321">4427321</a>.</cite>
</li>
<li><b><a href="#cite_ref-10">^</a></b> <cite>Schilling, G. (June 13, 2012). <a href="https://www.science.org/content/article/scienceshot-alien-earths-have-been-around-while">"ScienceShot: Alien Earths Have Been Around for a While"</a>. <i><a href="/wiki/Science_(journal)">Science</a></i>. <a href="https://web.archive.org/web/20150809002119/http://news.sciencemag.org/2012/06/scienceshot-alien-earths-have-been-around-while">Archived</a> from the original on August 9, 2015. Retrieved January 6, 2015.</cite>
</li>
<li><b><a href="#cite_ref-11">^</a></b> <cite>Aguirre, V. Silva; G. R. Davies; S. Basu; J. Christensen-Dalsgaard; O. Creevey; T. S. Metcalfe; T. R. Bedding; et al. (2015). "Ages and fundamental properties of Kepler exoplanet host stars from asteroseismology". <i>Monthly Notices of the Royal Astronomical Society</i>. <b>452</b> (2): 2127–2148. <a href="/wiki/ArXiv_(identifier)">arXiv</a>:<a href="//arxiv.org/abs/1504.07992">1504.07992</a>. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2015MNRAS.452.2127S">2015MNRAS.452.2127S</a>. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1093%2Fmnras%2Fstv1388">10.1093/mnras/stv1388</a>. <a href="/wiki/S2CID_(identifier)">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:85440256">85440256</a>.</cite> Accepted for publication in MNRAS. See Figure 15 in particular.
</li>
<li>^ <a href="#cite_ref-Hart_12-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Hart_12-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-Hart_12-2"><sup><i><b>c</b></i></sup></a> <a href="#cite_ref-Hart_12-3"><sup><i><b>d</b></i></sup></a> <cite>Hart, Michael H. (1975). "Explanation for the Absence of Extraterrestrials on Earth". <i><a href="/wiki/Quarterly_Journal_of_the_Royal_Astronomical_Society">Quarterly Journal of the Royal Astronomical Society</a></i>. <b>16</b>: 128–135. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1975QJRAS..16..128H">1975QJRAS..16..128H</a>.</cite>
</li>
<li><b><a href="#cite_ref-13">^</a></b> <cite>Chris Impe (2011). <i>The Living Cosmos: Our Search for Life in the Universe</i>. Cambridge University Press. p. 282. <a href="/wiki/ISBN_(identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-521-84780-3"><bdi>978-0-521-84780-3</bdi></a>.</cite>
</li>
<li><b><a href="#cite_ref-14">^</a></b> Tsiolkovsky, K. (1933). <i>The Planets are Occupied by Living Beings</i>, Archives of the Tsiolkovsky State Museum of the History of Cosmonautics, Kaluga, Russia. See <a href="https://en.wikisource.org/wiki/ru:%D0%9F%D0%BB%D0%B0%D0%BD%D0%B5%D1%82%D1%8B_%D0%B7%D0%B0%D1%81%D0%B5%D0%BB%D0%B5%D0%BD%D1%8B_%D0%B6%D0%B8%D0%B2%D1%8B%D0%BC%D0%B8_%D1%81%D1%83%D1%89%D0%B5%D1%81%D1%82%D0%B2%D0%B0%D0%BC%D0%B8_(%D0%A6%D0%B8%D0%BE%D0%BB%D0%BA%D0%BE%D0%B2%D1%81%D0%BA%D0%B8%D0%B9)">original text</a> in Russian <a href="/wiki/Wikisource">Wikisource</a>.
</li>
<li><b><a href="#cite_ref-15">^</a></b> <cite>Lytkin, V.; Finney, B.; Alepko, L. (December 1995). "Tsiolkovsky – Russian Cosmism and Extraterrestrial Intelligence". <i>Quarterly Journal of the Royal Astronomical Society</i>. <b>36</b> (4): 369. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1995QJRAS..36..369L">1995QJRAS..36..369L</a>.</cite>
</li>
<li><b><a href="#cite_ref-webb_16-0">^</a></b> <cite>Webb, Stephen (2015). <a href="https://www.springer.com/gp/book/9783319132358"><i>If the Universe Is Teeming with Aliens ... WHERE IS EVERYBODY?: Seventy-Five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life</i></a> (2 ed.). Springer International Publishing. <a href="/wiki/ISBN_(identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-3-319-13235-8"><bdi>978-3-319-13235-8</bdi></a>. <a href="https://web.archive.org/web/20200612035828/https://www.springer.com/gp/book/9783319132358">Archived</a> from the original on June 12, 2020. Retrieved June 12, 2020.</cite>
</li>
<li>^ <a href="#cite_ref-forgan_17-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-forgan_17-1"><sup><i><b>b</b></i></sup></a> <cite>Forgan, Duncan H. (2019). <a href="https://books.google.com/books?id=5U-PDwAAQBAJ"><i>Solving Fermi's Paradox</i></a>. Cambridge University Press. <a href="/wiki/ISBN_(identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-1-316-73231-1"><bdi>978-1-316-73231-1</bdi></a>. <a href="https://web.archive.org/web/20200625083852/https://books.google.com/books?id=5U-PDwAAQBAJ">Archived</a> from the original on June 25, 2020. Retrieved June 12, 2020.</cite>
</li>
<li>^ <a href="#cite_ref-Brin_18-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Brin_18-1"><sup><i><b>b</b></i></sup></a> <cite><a href="/wiki/David_Brin">Brin, Glen David</a> (August 1983). <a href="https://ui.adsabs.harvard.edu/abs/1983QJRAS..24..283B/abstract">"The Great Silence - The Controversy Concerning Extraterrestrial Intelligent Life"</a>. <i>Quarterly Journal of the Royal Astronomical Society</i>. <b>24</b> (3): 283–309. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1983QJRAS..24..283B">1983QJRAS..24..283B</a>.</cite>
</li>
<li><b><a href="#cite_ref-Annis_19-0">^</a></b> <cite>Annis, James (1999). "An Astrophysical Explanation for the Great Silence". <i>Journal of the British Interplanetary Society</i>. <b>52</b> (1): 19. <a href="/wiki/ArXiv_(identifier)">arXiv</a>:<a href="//arxiv.org/abs/astro-ph/9901322">astro-ph/9901322</a>. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1999JBIS...52...19A">1999JBIS...52...19A</a>.</cite>
</li>
<li><b><a href="#cite_ref-Hope_20-0">^</a></b> <cite><a href="/wiki/Nick_Bostrom">Bostrom, Nick</a> (May 2008). <a href="http://www.nickbostrom.com/papers/fermi.pdf">"In Great Silence there is Great Hope"</a> (PDF). <i>MIT Technology Review</i>. pp. 72–77. <a href="https://web.archive.org/web/20110228111236/http://www.nickbostrom.com/papers/fermi.pdf">Archived</a> (PDF) from the original on February 28, 2011. Retrieved September 6, 2010.</cite>
</li>
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<li><b><a href="#cite_ref-96">^</a></b> <cite>Soter, Steven (2005). <a href="https://web.archive.org/web/20070929092545/http://www.astrobio.net/news/modules.php?op=modload&amp;name=News&amp;file=article&amp;sid=1745">"SETI and the Cosmic Quarantine Hypothesis"</a>. <i>Astrobiology Magazine</i>. Space.com. Archived from the original on September 29, 2007. Retrieved May 3, 2006.</cite><code>{{<a href="/wiki/Template:Cite_web">cite web</a>}}</code>: CS1 maint: unfit URL (<a href="/wiki/Category:CS1_maint:_unfit_URL">link</a>)
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<li><b><a href="#cite_ref-97">^</a></b> <cite>Archer, Michael (1989). "Slime Monsters Will Be Human Too". <i>Aust. Nat. Hist</i>. <b>22</b>: 546–547.</cite>
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<li>^ <a href="#cite_ref-webb02_98-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-webb02_98-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-webb02_98-2"><sup><i><b>c</b></i></sup></a> <cite>Webb, Stephen (2002). <a href="https://books.google.com/books?id=UhzjBwAAQBAJ"><i>If the Universe Is Teeming with Aliens... Where Is Everybody? Fifty solutions to the Fermi Paradox and the Problem of Extraterrestrial Life</i></a>. Copernicus Books. <a href="/wiki/ISBN_(identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-387-95501-8"><bdi>978-0-387-95501-8</bdi></a>. <a href="https://web.archive.org/web/20150903224843/https://books.google.com/books?id=UhzjBwAAQBAJ">Archived</a> from the original on September 3, 2015. Retrieved July 21, 2015.</cite>
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<li><b><a href="#cite_ref-ARX-20180327_99-0">^</a></b> <cite>Berezin, Alexander (March 27, 2018). "'First in, last out' solution to the Fermi Paradox". <a href="/wiki/ArXiv_(identifier)">arXiv</a>:<a href="//arxiv.org/abs/1803.08425v2">1803.08425v2</a> [<a href="//arxiv.org/archive/physics.pop-ph">physics.pop-ph</a>].</cite>
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<li><b><a href="#cite_ref-100">^</a></b> <cite>Dockrill, Peter (June 2, 2019). <a href="https://www.sciencealert.com/a-physicist-has-proposed-a-pretty-depressing-explanation-for-why-we-never-see-aliens">"A Physicist Has Proposed a Pretty Depressing Explanation For Why We Never See Aliens"</a>. <i>ScienceAlert</i>. <a href="https://web.archive.org/web/20190602213542/https://www.sciencealert.com/a-physicist-has-proposed-a-pretty-depressing-explanation-for-why-we-never-see-aliens">Archived</a> from the original on June 2, 2019. Retrieved June 2, 2019.</cite>
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<li><b><a href="#cite_ref-101">^</a></b> <cite>Marko Horvat (2007). "Calculating the probability of detecting radio signals from alien civilizations". <i>International Journal of Astrobiology</i>. <b>5</b> (2): 143–149. <a href="/wiki/ArXiv_(identifier)">arXiv</a>:<a href="//arxiv.org/abs/0707.0011">0707.0011</a>. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2006IJAsB...5..143H">2006IJAsB...5..143H</a>. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1017%2FS1473550406003004">10.1017/S1473550406003004</a>. <a href="/wiki/S2CID_(identifier)">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:54608993">54608993</a>.</cite> "There is a specific time interval during which an alien civilization uses radio communications. Before this interval, radio is beyond the civilization's technical reach, and after this interval radio will be considered obsolete."
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<li><b><a href="#cite_ref-Stephenson1984_102-0">^</a></b> <cite>Stephenson, D. G. (1984). "Solar Power Satellites as Interstellar Beacons". <i><a href="/wiki/Quarterly_Journal_of_the_Royal_Astronomical_Society">Quarterly Journal of the Royal Astronomical Society</a></i>. <b>25</b> (1): 80. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1984QJRAS..25...80S">1984QJRAS..25...80S</a>.</cite>
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<li><b><a href="#cite_ref-Seth_Shostak,_Sky_&amp;_Telescope,_2006_103-0">^</a></b> <a href="https://www.skyandtelescope.com/astronomy-news/the-future-of-seti/">The Future of SETI</a> <a href="https://web.archive.org/web/20190524193142/https://www.skyandtelescope.com/astronomy-news/the-future-of-seti/">Archived</a> May 24, 2019, at the <a href="/wiki/Wayback_Machine">Wayback Machine</a>, <i>Sky &amp; Telescope</i>, Seth Shostak, July 19, 2006. This article also discusses strategy for optical SETI.
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<li><b><a href="#cite_ref-104">^</a></b> <cite><a href="/wiki/Caleb_Scharf">Scharf, Caleb</a> (August 10, 2022). <a href="https://nautil.us/we-might-already-speak-the-same-language-as-et-22841/">"We Might Already Speak the Same Language As ET"</a>. <i><a href="/wiki/Nautilus_Quarterly">Nautilus Quarterly</a></i>. Retrieved August 11, 2022.</cite>
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<li><b><a href="#cite_ref-105">^</a></b> <cite><a href="http://www.bigear.org/vol1no3/neutrino.htm">"Cosmic Search Vol. 1 No. 3"</a>. Bigear.org. September 21, 2004. <a href="https://web.archive.org/web/20101027210854/http://bigear.org/vol1no3/neutrino.htm">Archived</a> from the original on October 27, 2010. Retrieved July 3, 2010.</cite>
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<li><b><a href="#cite_ref-106">^</a></b> <cite>Learned, J; Pakvasa, S; Zee, A (2009). "Galactic neutrino communication". <i>Physics Letters B</i>. <b>671</b> (1): 15–19. <a href="/wiki/ArXiv_(identifier)">arXiv</a>:<a href="//arxiv.org/abs/0805.2429">0805.2429</a>. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2009PhLB..671...15L">2009PhLB..671...15L</a>. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1016%2Fj.physletb.2008.11.057">10.1016/j.physletb.2008.11.057</a>. <a href="/wiki/S2CID_(identifier)">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:118453255">118453255</a>.</cite>
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<li><b><a href="#cite_ref-107">^</a></b> Schombert, James. <a href="http://abyss.uoregon.edu/~js/cosmo/lectures/lec28.html">"Fermi's paradox (i.e. Where are they?)"</a> <a href="https://web.archive.org/web/20111107220554/http://abyss.uoregon.edu/~js/cosmo/lectures/lec28.html">Archived</a> November 7, 2011, at the <a href="/wiki/Wayback_Machine">Wayback Machine</a> <i>Cosmology Lectures</i>, University of Oregon.
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<li><b><a href="#cite_ref-108">^</a></b> <cite>Hamming, RW (1998). "Mathematics on a distant planet". <i>The American Mathematical Monthly</i>. <b>105</b> (7): 640–650. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.2307%2F2589247">10.2307/2589247</a>. <a href="/wiki/JSTOR_(identifier)">JSTOR</a> <a href="//www.jstor.org/stable/2589247">2589247</a>.</cite>
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<li><b><a href="#cite_ref-109">^</a></b> <cite>Carl Sagan. <i>Contact</i>.</cite> Chapter 3, p. 49.
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<li><b><a href="#cite_ref-110">^</a></b> <cite><a href="/wiki/Zoltan_Istvan">Istvan, Zoltan</a> (March 16, 2016). <a href="https://motherboard.vice.com/en_us/article/vv7bkb/why-havent-we-met-aliens-yet-because-theyve-evolved-into-ai">"Why Haven't We Met Aliens Yet? Because They've Evolved into AI"</a>. <i>Motherboard</i>. <a href="/wiki/Vice_Media">Vice Media</a>. <a href="https://archive.today/20171230231127/https://motherboard.vice.com/en_us/article/vv7bkb/why-havent-we-met-aliens-yet-because-theyve-evolved-into-ai">Archived</a> from the original on December 30, 2017. Retrieved December 30, 2017.</cite>
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<li><b><a href="#cite_ref-111">^</a></b> <cite>Long, K. F. (2011). <a href="https://books.google.com/books?id=EAD2-GrarkEC&amp;q=fermi+paradox+technological+singularity&amp;pg=PA114"><i>Deep Space Propulsion: A Roadmap to Interstellar Flight</i></a>. p. 114. <a href="/wiki/ISBN_(identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-1-4614-0607-5"><bdi>978-1-4614-0607-5</bdi></a>. Retrieved June 23, 2015.</cite>
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<li><b><a href="#cite_ref-112">^</a></b> <cite>Cook, Stephen P. (2012). "SETI: Assessing Imaginative Proposals". <i>Life on Earth and other Planetary Bodies</i>. p. 54. <a href="/wiki/ISBN_(identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-94-007-4966-5"><bdi>978-94-007-4966-5</bdi></a>.</cite>
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<li><b><a href="#cite_ref-113">^</a></b> <cite>Istvan, Zoltan (August 26, 2016). <a href="https://web.archive.org/web/20190717040808/https://www.vice.com/en_us/article/jpgvxx/the-language-of-aliens-will-always-be-indecipherable">"The Language of Aliens Will Always Be Indecipherable"</a>. <i><a href="/wiki/Vice_Media">Vice Media</a></i>. Archived from <a href="https://www.vice.com/en_us/article/jpgvxx/the-language-of-aliens-will-always-be-indecipherable">the original</a> on July 17, 2019. Retrieved July 17, 2019.</cite>
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<li><b><a href="#cite_ref-114">^</a></b> <a href="https://books.google.com/books?id=jFcJHatXiWIC&amp;pg=PT212&amp;lpg=PT212&amp;dq=%22Our+experiments+are+still+looking+for+the+type+of+extraterrestrial+that+would+have+appealed+to+Percival+Lowell%22&amp;source=bl&amp;ots=nDU7_FnIwS&amp;sig=ACfU3U01lTCJadQuBtBO7JtZ7g2veCMtSA&amp;hl=en&amp;sa=X&amp;ved=2ahUKEwit94S0gafnAhXUQs0KHZKEC-oQ6AEwAHoECAEQAQ#v=onepage&amp;q=%22Our%20experiments%20are%20still%20looking%20for%20the%20type%20of%20extraterrestrial%20that%20would%20have%20appealed%20to%20Percival%20Lowell%22&amp;f=false">Confessions of an Alien Hunter: A Scientist's Search for Extraterrestrial Intelligence</a>, Seth Shostak (Senior Astronomer, SETI Institute), Ch. 7 "Beyond Gray and Hairless," p. 264, published by National Geographic, 2009.
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<li><b><a href="#cite_ref-115">^</a></b> <a href="https://archive.org/details/eeriesilencerene00davi/page/145"><i>The Eerie Silence: Renewing Our Search for Alien Intelligenc</i>e</a>, Paul Davies (Beyond Center for Fundamental Concepts in Science, Arizona State University), Boston, New York: Houghton Mifflin Harcourt, 2010, pp. 144–145.
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<li><b><a href="#cite_ref-Beyond_Fermi's_Paradox_II,_Stephen_Jay_Gould_quote,_April_2015_116-0">^</a></b> <a href="https://www.universetoday.com/tag/carl-sagan/">Beyond “Fermi’s Paradox” II: Questioning the Hart-Tipler Conjecture</a> <a href="https://web.archive.org/web/20190322205106/https://www.universetoday.com/tag/carl-sagan/">Archived</a> March 22, 2019, at the <a href="/wiki/Wayback_Machine">Wayback Machine</a> (middle of page), <i>Universe Today</i>, April 8, 2015.
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<li><b><a href="#cite_ref-If_the_Universe_Is_Teeming_._._._75_Solutions,_Stephen_Webb,_2015_117-0">^</a></b> <i>If the Universe Is Teeming...</i>, Stephen Webb, <a href="https://books.google.com/books?id=Y111CQAAQBAJ&amp;pg=PA28">p. 28</a>.
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<li>^ <a href="#cite_ref-Struggling_and_hustling,_just_like_us,_Popular_Science,_Feb._2019_118-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Struggling_and_hustling,_just_like_us,_Popular_Science,_Feb._2019_118-1"><sup><i><b>b</b></i></sup></a> <cite>Wood, Charlie (February 22, 2019). <a href="https://www.popsci.com/where-are-aliens-new-model">"Where are all the aliens? Struggling and hustling, just like us"</a>. <i>Popular Science</i>. <a href="https://web.archive.org/web/20190222214111/https://www.popsci.com/where-are-aliens-new-model">Archived</a> from the original on February 22, 2019.</cite>
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<li><b><a href="#cite_ref-119">^</a></b> <cite>Carrol-Nellenback, Jonathan; Frank, Adam; Wright, Jason; Scharf, Caleb (2019). <a href="https://iopscience.iop.org/article/10.3847/1538-3881/ab31a3">"The Fermi Paradox and the Aurora Effect: Exo-civilization Settlement, Expansion, and Steady States"</a>. <i>The Astronomical Journal</i> (published August 20, 2019). <b>158</b> (3): 117. <a href="/wiki/ArXiv_(identifier)">arXiv</a>:<a href="//arxiv.org/abs/1902.04450">1902.04450</a>. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2019AJ....158..117C">2019AJ....158..117C</a>. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.3847%2F1538-3881%2Fab31a3">10.3847/1538-3881/ab31a3</a>. <a href="/wiki/S2CID_(identifier)">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:119185080">119185080</a>. Retrieved December 31, 2021.</cite>
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<li>^ <a href="#cite_ref-David_Brin,_1983,_abandonment_of_planet-dwelling_120-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-David_Brin,_1983,_abandonment_of_planet-dwelling_120-1"><sup><i><b>b</b></i></sup></a> "The Great Silence: the Controversy Concerning Extraterrestrial Intelligent Life" (15-page paper), <i>Quarterly J. Royal Astron. Soc.,</i> David Brin, 1983, <a href="http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000300.000.html">p. 300 " …. abandonment of planet-dwelling …. "</a> <a href="https://web.archive.org/web/20190406165950/http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000300.000.html">Archived</a> April 6, 2019, at the <a href="/wiki/Wayback_Machine">Wayback Machine</a>.
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<li><b><a href="#cite_ref-121">^</a></b> <cite>Bostrom, Nick (April 22, 2008). <a href="https://www.technologyreview.com/2008/04/22/220999/where-are-they/">"Where Are They?"</a>. <i>MIT Technology Review</i>. Retrieved October 5, 2020.</cite>
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<li><b><a href="#cite_ref-122">^</a></b> <cite>Webb, Stephen (2015). <a href="https://books.google.com/books?id=QWKyrQEACAAJ"><i>If the Universe Is Teeming with Aliens... Where Is Everybody? Seventy five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life</i></a> (2nd ed.). Copernicus Books. <a href="/wiki/ISBN_(identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-3-319-13235-8"><bdi>978-3-319-13235-8</bdi></a>. <a href="https://web.archive.org/web/20150903224843/https://books.google.com/books?id=QWKyrQEACAAJ">Archived</a> from the original on September 3, 2015. Retrieved July 21, 2015.</cite> Chapter 15: "They Stay at Home and Surf the Web"
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<li>^ <a href="#cite_ref-landis_123-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-landis_123-1"><sup><i><b>b</b></i></sup></a> <cite><a href="/wiki/Geoffrey_A._Landis">Landis, Geoffrey</a> (1998). <a href="http://www.sff.net/people/Geoffrey.Landis/percolation.htp">"The Fermi Paradox: An Approach Based on Percolation Theory"</a>. <i>Journal of the British Interplanetary Society</i>. <b>51</b> (5): 163–166. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1998JBIS...51..163L">1998JBIS...51..163L</a>. <a href="https://web.archive.org/web/20060927154115/http://www.sff.net/people/Geoffrey.Landis/percolation.htp">Archived</a> from the original on September 27, 2006. Retrieved June 6, 2004.</cite>
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<li>^ <a href="#cite_ref-kinouchi_124-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-kinouchi_124-1"><sup><i><b>b</b></i></sup></a> <cite>Galera, E.; Galanti, G. R.; Kinouchi, O. (2018). "Invasion Percolation Solves Fermi Paradox but Challenges SETI Projects". <i>International Journal of Astrobiology</i>. <b>*</b> (4): 316–322. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1017%2FS1473550418000101">10.1017/S1473550418000101</a>. <a href="/wiki/S2CID_(identifier)">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:126238563">126238563</a>.</cite>
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<li><b><a href="#cite_ref-125">^</a></b> <cite>Scheffer, L.K. (1994). "Machine Intelligence, the Cost of Interstellar Travel and Fermi's Paradox". <i><a href="/wiki/Quarterly_Journal_of_the_Royal_Astronomical_Society">Quarterly Journal of the Royal Astronomical Society</a></i>. <b>35</b>: 157. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1994QJRAS..35..157S">1994QJRAS..35..157S</a>.</cite>
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<li><b><a href="#cite_ref-126">^</a></b> <cite>Turnbull, Margaret C.; Tarter, Jill C. (2003). <a href="http://www.projectrho.com/HabCat.pdf">"Target Selection for SETI. I. A Catalog of Nearby Habitable Stellar Systems"</a> (PDF). <i>The Astrophysical Journal Supplement Series</i>. <b>145</b> (1): 181–198. <a href="/wiki/ArXiv_(identifier)">arXiv</a>:<a href="//arxiv.org/abs/astro-ph/0210675">astro-ph/0210675</a>. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2003ApJS..145..181T">2003ApJS..145..181T</a>. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1086%2F345779">10.1086/345779</a>. <a href="/wiki/S2CID_(identifier)">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:14734094">14734094</a>. <a href="https://web.archive.org/web/20100614030445/http://www.projectrho.com/HabCat.pdf">Archived</a> (PDF) from the original on June 14, 2010. Retrieved August 19, 2010.</cite>
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<li><b><a href="#cite_ref-127">^</a></b> <cite>The Staff at the National Astronomy and Ionosphere Center (December 1975). "The Arecibo message of November, 1974". <i>Icarus</i>. <b>26</b> (4): 462–466. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1975Icar...26..462.">1975Icar...26..462.</a>. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1016%2F0019-1035%2875%2990116-5">10.1016/0019-1035(75)90116-5</a>.</cite> "A radio telescope in M13 operating at the transmission frequency, and pointed toward the Sun at the time the message arrives at the receiving site will observe a flux density from the message which will exceed the flux density of the Sun itself by a factor of roughly 10<sup>7</sup>. Indeed, at that unique time, the Sun will appear to the receptors to be by far the brightest star of the Milky Way."
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<li><b><a href="#cite_ref-128">^</a></b> <cite>Seth D. Baum; Jacob D. Haqq-Misra; Shawn D. Domagal-Goldman (2011). <a href="http://sethbaum.com/ac/2011_ET-Scenarios.pdf">"Would contact with extraterrestrials benefit or harm humanity? A scenario analysis"</a> (PDF). <i>Acta Astronautica</i>. <b>68</b> (11): 2114–2129. <a href="/wiki/ArXiv_(identifier)">arXiv</a>:<a href="//arxiv.org/abs/1104.4462">1104.4462</a>. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2011AcAau..68.2114B">2011AcAau..68.2114B</a>. <a href="/wiki/CiteSeerX_(identifier)">CiteSeerX</a> <a href="//citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.592.1341">10.1.1.592.1341</a>. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1016%2Fj.actaastro.2010.10.012">10.1016/j.actaastro.2010.10.012</a>. <a href="/wiki/S2CID_(identifier)">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:16889489">16889489</a>. <a href="https://web.archive.org/web/20180721225331/http://sethbaum.com/ac/2011_ET-Scenarios.pdf">Archived</a> (PDF) from the original on July 21, 2018. Retrieved August 1, 2018.</cite> "If ETI search for us just as we search for them, i.e. by scanning the sky at radio and optical wavelengths [...] the radiation that has been unintentionally leaking and intentionally transmitted from Earth may have already alerted any nearby ETI to our presence and may eventually alert more distant ETI. Once ETI become alerted to our presence, it will take at least as many years for us to realize that they know."
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<li><b><a href="#cite_ref-129">^</a></b> <cite>Vakoch, Douglas (November 15, 2001). <a href="https://web.archive.org/web/20090523030456/https://www.space.com/searchforlife/seti_decode_011115.html">"Decoding E.T.: Ancient Tongues Point Way To Learning Alien Languages"</a>. SETI Institute. Archived from <a href="http://www.space.com/searchforlife/seti_decode_011115.html">the original</a> on May 23, 2009. Retrieved August 19, 2010.</cite>
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<li><b><a href="#cite_ref-130">^</a></b> <cite>Adam Stevens; Duncan Forgan; Jack O'Malley James (2015). "Observational Signatures of Self–Destructive Civilisations". <i>International Journal of Astrobiology</i>. <b>15</b> (4): 333–344. <a href="/wiki/ArXiv_(identifier)">arXiv</a>:<a href="//arxiv.org/abs/1507.08530">1507.08530</a>. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1017%2FS1473550415000397">10.1017/S1473550415000397</a>. <a href="/wiki/S2CID_(identifier)">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:118428874">118428874</a>.</cite>
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<li><b><a href="#cite_ref-131">^</a></b> <cite>Newman, W.T.; Sagan, C. (1981). "Galactic civilizations: Population. dynamics and interstellar diffusion". <i>Icarus</i>. <b>46</b> (3): 293–327. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1981Icar...46..293N">1981Icar...46..293N</a>. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1016%2F0019-1035%2881%2990135-4">10.1016/0019-1035(81)90135-4</a>. <a href="/wiki/Hdl_(identifier)">hdl</a>:<a href="//hdl.handle.net/2060%2F19790011801">2060/19790011801</a>.</cite>
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<li>^ <a href="#cite_ref-David_Brin,_1983,_pages_287_and_298_132-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-David_Brin,_1983,_pages_287_and_298_132-1"><sup><i><b>b</b></i></sup></a> "The Great Silence: the Controversy . . " (15-page paper), <i>Quart. Journ. Royal Astronomical Soc.,</i> David Brin, 1983, <a href="http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000287.000.html">page 287, sixth paragraph, "Equilibrium is another concept which weaves through the new SETI debate … . "</a> <a href="https://web.archive.org/web/20190411223152/http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000287.000.html">Archived</a> April 11, 2019, at the <a href="/wiki/Wayback_Machine">Wayback Machine</a>, as well as <a href="http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000298.000.html">page 298, third paragraph, "Newman &amp; Sagan (<b>4</b>) have suggested that population pressure is not . . . "</a> <a href="https://web.archive.org/web/20190411223717/http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000298.000.html">Archived</a> April 11, 2019, at the <a href="/wiki/Wayback_Machine">Wayback Machine</a>.
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<li><b><a href="#cite_ref-133">^</a></b> <cite>Wall, Mike (October 26, 2017). <a href="https://www.space.com/38577-fermi-paradox-alien-life-buried-oceans.html">"Where Are All the Intelligent Aliens? Maybe They're Trapped in Buried Oceans"</a>. <i>Space.com</i>.</cite>
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<li><b><a href="#cite_ref-134">^</a></b> <a href="https://ui.adsabs.harvard.edu/abs/2017DPS....4920203S/abstract">An Answer to Fermi’s Paradox In the Prevalence of Ocean Worlds</a> <a href="https://web.archive.org/web/20191221201926/https://ui.adsabs.harvard.edu/abs/2017DPS....4920203S/abstract">Archived</a> December 21, 2019, at the <a href="/wiki/Wayback_Machine">Wayback Machine</a>, S. Alan Stern, American Astronomical Society, Division for Planetary Sciences Meeting Abstracts #49, October 2017. " . . We suggest another—namely that the great majority of worlds with biology and civilizations are interior water ocean worlds (WOWs). . "
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<li><b><a href="#cite_ref-135">^</a></b> <cite>Wandel, Amri (December 1, 2022). <a href="https://iopscience.iop.org/article/10.3847/1538-4357/ac9e00">"The Fermi Paradox Revisited: Technosignatures and the Contact Era"</a>. <i>The Astrophysical Journal</i>. <b>941</b> (2): 184. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.3847%2F1538-4357%2Fac9e00">10.3847/1538-4357/ac9e00</a>. <a href="/wiki/ISSN_(identifier)">ISSN</a> <a href="//www.worldcat.org/issn/0004-637X">0004-637X</a>. <a href="/wiki/S2CID_(identifier)">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:254096277">254096277</a>.</cite>
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<li>^ <a href="#cite_ref-webbwhere_136-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-webbwhere_136-1"><sup><i><b>b</b></i></sup></a> <cite>Webb, Stephen (2015). <a href="https://books.google.com/books?id=Y111CQAAQBAJ&amp;q=fermi+paradox+everybody+listening+nobody+sending&amp;pg=PA148"><i>If the Universe Is Teeming with Aliens … WHERE IS EVERYBODY?: Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life</i></a>. <a href="/wiki/ISBN_(identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-387-95501-8"><bdi>978-0-387-95501-8</bdi></a>. Retrieved June 21, 2015.</cite>
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<li><b><a href="#cite_ref-137">^</a></b> <cite>Alexander Zaitsev (2006). "The SETI paradox". <a href="/wiki/ArXiv_(identifier)">arXiv</a>:<a href="//arxiv.org/abs/physics/0611283">physics/0611283</a>.</cite>
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<li><b><a href="#cite_ref-138">^</a></b> <cite>The Associated Press (February 13, 2015). <a href="https://web.archive.org/web/20150906144552/http://www.nytimes.com/aponline/2015/02/13/science/ap-us-sci-calling-the-cosmos.html">"Should We Call the Cosmos Seeking ET? Or Is That Risky?"</a>. <i>The New York Times</i>. Archived from <a href="https://www.nytimes.com/aponline/2015/02/13/science/ap-us-sci-calling-the-cosmos.html">the original</a> on September 6, 2015. Retrieved March 1, 2017.</cite>
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<li><b><a href="#cite_ref-139">^</a></b> <cite><a href="https://web.archive.org/web/20150718122415/http://www.seti.org/post-detection.html">"Protocols for an ETI Signal Detection: Concerning Activities Following the Detection of Extraterrestrial Intelligence"</a>. SETI Institute. Archived from <a href="http://www.seti.org/post-detection.html">the original</a> on July 18, 2015. Retrieved July 12, 2015.</cite>
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<li><b><a href="#cite_ref-140">^</a></b> <cite>Michaud, M. (2003). "Ten decisions that could shake the world". <i>Space Policy</i>. <b>19</b> (2): 131–950. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2003SpPol..19..131M">2003SpPol..19..131M</a>. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1016%2FS0265-9646%2803%2900019-5">10.1016/S0265-9646(03)00019-5</a>.</cite>
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<li><b><a href="#cite_ref-142">^</a></b> <cite>Carrigan, Richard A. (2006). "Do potential SETI signals need to be decontaminated?". <i>Acta Astronautica</i>. <b>58</b> (2): 112–117. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2006AcAau..58..112C">2006AcAau..58..112C</a>. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1016%2Fj.actaastro.2005.05.004">10.1016/j.actaastro.2005.05.004</a>.</cite>
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<li><b><a href="#cite_ref-146">^</a></b> <cite>Liu, Cixin (2015). <i>The dark forest</i> (First ed.). New York. p. 484. <a href="/wiki/ISBN_(identifier)">ISBN</a> <a href="/wiki/Special:BookSources/9780765377081"><bdi>9780765377081</bdi></a>.</cite>
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<li><b><a href="#cite_ref-Dark_Forest_Answer_to_Star_Wars_Optimism_147-0">^</a></b> <cite>Hsu, Jeremy (October 31, 2015). <a href="https://www.discovermagazine.com/the-sciences/chinas-dark-forest-answer-to-star-wars-optimism">"China's 'Dark Forest' Answer to 'Star Wars' Optimism"</a>. <i>Discover Magazine</i>.</cite>
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<li><b><a href="#cite_ref-NHPR_150-0">^</a></b> <cite>Paradis, Justine (February 18, 2022). <a href="https://www.nhpr.org/environment/2022-02-18/what-is-the-dark-forest-theory-outside-in">"Outside/In[box]: What is the Dark Forest Theory?"</a>. <i>New Hampshire Public Radio</i>. Retrieved October 18, 2022.</cite>
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<li>^ <a href="#cite_ref-:1_152-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-:1_152-1"><sup><i><b>b</b></i></sup></a> <cite>Ball, J (1973). "The zoo hypothesis". <i>Icarus</i>. <b>19</b> (3): 347–349. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1973Icar...19..347B">1973Icar...19..347B</a>. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1016%2F0019-1035%2873%2990111-5">10.1016/0019-1035(73)90111-5</a>.</cite>
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<li><b><a href="#cite_ref-153">^</a></b> <cite>Forgan, Duncan H. (June 8, 2011). "Spatio-temporal constraints on the zoo hypothesis, and the breakdown of total hegemony". <i>International Journal of Astrobiology</i>. <b>10</b> (4): 341–347. <a href="/wiki/ArXiv_(identifier)">arXiv</a>:<a href="//arxiv.org/abs/1105.2497">1105.2497</a>. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2011IJAsB..10..341F">2011IJAsB..10..341F</a>. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1017%2Fs147355041100019x">10.1017/s147355041100019x</a>. <a href="/wiki/ISSN_(identifier)">ISSN</a> <a href="//www.worldcat.org/issn/1473-5504">1473-5504</a>. <a href="/wiki/S2CID_(identifier)">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:118431252">118431252</a>.</cite>
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<li><b><a href="#cite_ref-154">^</a></b> <cite>Forgan, Duncan H. (November 28, 2016). "The Galactic Club or Galactic Cliques? Exploring the limits of interstellar hegemony and the Zoo Hypothesis". <i>International Journal of Astrobiology</i>. <b>16</b> (4): 349–354. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1017%2Fs1473550416000392">10.1017/s1473550416000392</a>. <a href="/wiki/Hdl_(identifier)">hdl</a>:<a href="//hdl.handle.net/10023%2F10869">10023/10869</a>. <a href="/wiki/ISSN_(identifier)">ISSN</a> <a href="//www.worldcat.org/issn/1473-5504">1473-5504</a>. <a href="/wiki/S2CID_(identifier)">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:59041278">59041278</a>.</cite>
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<li><b><a href="#cite_ref-155">^</a></b> <cite>Visscher, Alex De (2020). <a href="https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/abs/artificial-versus-biological-intelligence-in-the-cosmos-clues-from-a-stochastic-analysis-of-the-drake-equation/7BC4932CD94887E338B2DA55488DE8C6">"Artificial versus biological intelligence in the Cosmos: clues from a stochastic analysis of the Drake equation"</a>. <i>International Journal of Astrobiology</i>. <b>19</b> (5): 353–359. <a href="/wiki/ArXiv_(identifier)">arXiv</a>:<a href="//arxiv.org/abs/2001.11644">2001.11644</a>. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2020IJAsB..19..353D">2020IJAsB..19..353D</a>. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1017%2FS1473550420000129">10.1017/S1473550420000129</a>. <a href="/wiki/ISSN_(identifier)">ISSN</a> <a href="//www.worldcat.org/issn/1473-5504">1473-5504</a>. <a href="/wiki/S2CID_(identifier)">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:211003646">211003646</a>.</cite>
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<li><b><a href="#cite_ref-156">^</a></b> <cite>Hair, Thomas W. (February 25, 2011). "Temporal dispersion of the emergence of intelligence: an inter-arrival time analysis". <i>International Journal of Astrobiology</i>. <b>10</b> (2): 131–135. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2011IJAsB..10..131H">2011IJAsB..10..131H</a>. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1017%2FS1473550411000024">10.1017/S1473550411000024</a>. <a href="/wiki/S2CID_(identifier)">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:53681377">53681377</a>.</cite>
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<li><b><a href="#cite_ref-158">^</a></b> <cite>Baxter, Stephen (2001). "The Planetarium Hypothesis: A Resolution of the Fermi Paradox". <i>Journal of the British Interplanetary Society</i>. <b>54</b> (5/6): 210–216. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2001JBIS...54..210B">2001JBIS...54..210B</a>.</cite>
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<li><b><a href="#cite_ref-159">^</a></b> <cite>Ray Villard (August 10, 2012). <a href="http://news.discovery.com/space/why-do-people-believe-in-ufos-120810.htm">"Why Do People Believe in UFOs?"</a>. Discovery News. <a href="https://web.archive.org/web/20160328235420/http://news.discovery.com/space/why-do-people-believe-in-ufos-120810.htm">Archived</a> from the original on March 28, 2016. Retrieved March 18, 2016.</cite>
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<li><b><a href="#cite_ref-160">^</a></b> <cite>Paul Speigel (October 18, 2012). <a href="http://www.huffingtonpost.com/2012/10/15/alien-believers-outnumber-god_n_1968259.html">"More Believe in Space Aliens Than in God According To U.K. Survey"</a>. <i>Huffington Post</i>. <a href="https://web.archive.org/web/20170409112145/http://www.huffingtonpost.com/2012/10/15/alien-believers-outnumber-god_n_1968259.html">Archived</a> from the original on April 9, 2017. Retrieved April 8, 2017.</cite>
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<li><b><a href="#cite_ref-161">^</a></b> <cite>Shermer, Michael (2011). "UFOs, UAPs and CRAPs". <i>Scientific American</i>. <b>304</b> (4): 90. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2011SciAm.304d..90S">2011SciAm.304d..90S</a>. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1038%2Fscientificamerican0411-90">10.1038/scientificamerican0411-90</a>. <a href="/wiki/PMID_(identifier)">PMID</a> <a href="//pubmed.ncbi.nlm.nih.gov/21495489">21495489</a>.</cite>
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<li><b><a href="#cite_ref-162">^</a></b> <cite>A. Tough (1990). "A critical examination of factors that might encourage secrecy". <i>Acta Astronautica</i>. <b>21</b> (2): 97–102. <a href="/wiki/Bibcode_(identifier)">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1990AcAau..21...97T">1990AcAau..21...97T</a>. <a href="/wiki/Doi_(identifier)">doi</a>:<a href="https://doi.org/10.1016%2F0094-5765%2890%2990134-7">10.1016/0094-5765(90)90134-7</a>.</cite>
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<li><b><a href="#cite_ref-163">^</a></b> <cite>Ashlee Vance (July 31, 2006). <a href="https://www.theregister.co.uk/2006/07/31/signals_seti/">"SETI urged to fess up over alien signals"</a>. <i>The Register</i>. <a href="https://web.archive.org/web/20070402080142/https://www.theregister.co.uk/2006/07/31/signals_seti/">Archived</a> from the original on April 2, 2007. Retrieved August 10, 2017.</cite>
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<li><b><a href="#cite_ref-165">^</a></b> <cite>G. Seth Shostak (2009). <a href="https://archive.org/details/confessionsofali00shos/page/17"><i>Confessions of an Alien Hunter: A Scientist's Search for Extraterrestrial Intelligence</i></a>. National Geographic. p. <a href="https://archive.org/details/confessionsofali00shos/page/17">17</a>. <a href="/wiki/ISBN_(identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-1-4262-0392-3"><bdi>978-1-4262-0392-3</bdi></a>.</cite>
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<li><b><a href="#cite_ref-166">^</a></b> "The Great Silence: the Controversy . . " (15-page paper), <i>Quarterly J. Royal Astron. Soc.,</i> David Brin, 1983, <a href="http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000299.000.html">p. 299 bottom</a> <a href="https://web.archive.org/web/20190411223152/http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000299.000.html">Archived</a> April 11, 2019, at the <a href="/wiki/Wayback_Machine">Wayback Machine</a>.
</li>
</ol></div>
<h2>Further reading[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=46">edit</a>]</h2>
<ul><li><cite>Rebecca Boyle and Quanta Magazine (March 10, 2019). <a href="https://www.theatlantic.com/science/archive/2019/03/star-movement-fermi-paradox-alien-intelligence/584518/">"Moving Stars Might Speed the Spread of Alien Life"</a>. <i><a href="/wiki/The_Atlantic">The Atlantic</a></i>.</cite></li>
<li>Ćirković, Milan <a href="http://nautil.us/issue/63/horizons/our-attitude-toward-aliens-proves-we-still-think-were-special">[2]</a> <a href="https://web.archive.org/web/20210505080417/http://nautil.us/issue/63/horizons/our-attitude-toward-aliens-proves-we-still-think-were-special">Archived</a> May 5, 2021, at the <a href="/wiki/Wayback_Machine">Wayback Machine</a> Why we downplay Fermi's paradox. <a href="/wiki/Nautilus_Quarterly">Nautilus</a></li>
<li>Ćirković, Milan <a href="https://global.oup.com/academic/product/the-great-silence-9780199646302?cc=gb&amp;lang=en&amp;">The Great Silence: Science and Philosophy of Fermi's Paradox</a> The Great Silence <a href="/wiki/Oxford_University_Press">Oxford University Press</a></li>
<li><cite>Crowe, Michael J. (2008). <i>The Extraterrestrial Life Debate, Antiquity to 1915</i>. <a href="/wiki/University_of_Notre_Dame_Press">University of Notre Dame Press</a>. <a href="/wiki/ISBN_(identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-268-02368-3"><bdi>978-0-268-02368-3</bdi></a>.</cite></li>
<li><cite>Forgan, Duncan H. (2019). <i>Solving Fermi's Paradox</i>. Cambridge University Press. <a href="/wiki/ISBN_(identifier)">ISBN</a> <a href="/wiki/Special:BookSources/9781107163652"><bdi>9781107163652</bdi></a>.</cite></li>
<li><cite>Michaud, Michael (2006). <a href="https://archive.org/details/contactwithalien0000mich"><i>Contact with Alien Civilizations: Our Hopes and Fears about Encountering Extraterrestrials</i></a>. Copernicus Books. <a href="/wiki/ISBN_(identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-387-28598-6"><bdi>978-0-387-28598-6</bdi></a>.</cite></li>
<li><cite><a href="/wiki/Benjamin_Zuckerman">Zuckerman, Ben</a>; Hart, Michael H. (1995). <i>Extraterrestrials: Where Are They?</i>. Cambridge University Press. <a href="/wiki/ISBN_(identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-521-44803-1"><bdi>978-0-521-44803-1</bdi></a>.</cite></li></ul>
<h2>External links[<a href="/w/index.php?title=Fermi_paradox&amp;action=edit&amp;section=47">edit</a>]</h2>
<div><p></p><p>These audio files were created from a revision of this article dated 29 May 2008, and do not reflect subsequent edits.</p></div>
<ul><li>Kestenbaum, David. <a href="https://www.thisamericanlife.org/617/fermis-paradox">"Three people grapple with the question, 'Are we alone?'"</a>, <i>This American Life</i> radio show, hosted by Ira Glass. This episode's first 22 minutes discusses the Fermi Paradox. See also the show's <a href="https://www.thisamericanlife.org/617/transcript">May 19, 2017 transcript</a>.</li>
<li><cite><a href="http://www.setileague.org/press/silence.htm"><i>Overcome the Great Silence</i></a>. Translated by <a href="/wiki/Aleksandr_Leonidovich_Zaitsev">Leonidovich Zaitsev, Aleksandr</a> (Translation of the documentary ed.).</cite></li>
<li><a href="https://www.youtube.com/watch?v=sNhhvQGsMEc">The Fermi Paradox — Where Are All The Aliens? (2015), Kurzgesagt – In a Nutshell</a></li></ul>
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 Contents

 move to sidebar hide
 * (Top)
 * 1 Chain of reasoning
 * 2 History
 + 2.1 Original conversations
 * 3 Basis
 + 3.1 Drake equation
 + 3.2 Great Filter
 * 4 Empirical evidence
 + 4.1 Electromagnetic emissions
 + 4.2 Direct planetary observation
 + 4.3 Conjectures about interstellar probes
 + 4.4 Searches for stellar-scale artifacts
 * 5 Hypothetical explanations for the paradox
 + 5.1 Rarity of intelligent life
 o 5.1.1 Extraterrestrial life is rare or non-existent
 o 5.1.2 Extraterrestrial intelligence is rare or non-existent
 o 5.1.3 Periodic extinction by natural events
 + 5.2 Evolutionary explanations
 o 5.2.1 Intelligent alien species have not developed advanced technologies
 o 5.2.2 It is the nature of intelligent life to destroy itself
 o 5.2.3 It is the nature of intelligent life to destroy others
 o 5.2.4 Civilizations only broadcast detectable signals for a brief period of time
 o 5.2.5 Alien life may be too alien
 + 5.3 Sociological explanations
 o 5.3.1 Colonization is not the cosmic norm
 o 5.3.2 Alien species may have only settled part of the galaxy
 o 5.3.3 Alien species may not live on planets
 o 5.3.4 Alien species may isolate themselves from the outside world
 + 5.4 Economic explanations
 o 5.4.1 Lack of resources needed to physically spread throughout the galaxy
 o 5.4.2 Information is cheaper to transmit than matter is to transfer
 + 5.5 Discovery of extraterrestrial life is too difficult
 o 5.5.1 Humans have not listened properly
 o 5.5.2 Humans have not listened for long enough
 o 5.5.3 Intelligent life may be too far away
 o 5.5.4 Intelligent life may exist hidden from view
 o 5.5.5 Advanced civilizations may limit their search for life to technological signatures
 + 5.6 Willingness to communicate
 o 5.6.1 Everyone is listening but no one is transmitting
 o 5.6.2 Communication is dangerous
 o 5.6.3 Earth is deliberately being avoided
 o 5.6.4 Earth is deliberately being isolated
 + 5.7 Alien life is already here, unacknowledged
 * 6 See also
 * 7 Notes
 * 8 References
 * 9 Further reading
 * 10 External links
 Toggle the table of contents

 Fermi paradox

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 From Wikipedia, the free encyclopedia
 This article is about the absence of clear evidence for space aliens. For a type of estimation problem, see Fermi problem.

 A graphical representation of the Arecibo message, humanity's first attempt to use radio waves to actively communicate its existence to alien civilizations

 The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life compared to the apparently high a priori likelihood of its existence.[1][2] As a 2015 article put it, "If life is so easy, someone from somewhere must have come calling by now."[3]

 Italian-American physicist Enrico Fermi's name is associated with the paradox because of a casual conversation in the summer of 1950 with fellow physicists Edward Teller, Herbert York, and Emil Konopinski. While walking to lunch, the men discussed recent UFO reports and the possibility of faster-than-light travel. The conversation moved on to other topics, until during lunch Fermi blurted out, "But where is everybody?" (although the exact quote is uncertain).[3][4]

 There have been many attempts to resolve the Fermi paradox,[5][6] such as suggesting that intelligent extraterrestrial beings are extremely rare, that the lifetime of such civilizations is short, or that they exist but (for various reasons) humans see no evidence.

 Chain of reasoning [ edit ]

 The following are some of the facts and hypotheses that together serve to highlight the apparent contradiction:

 * There are billions of stars in the Milky Way similar to the Sun.[7][8]
 * With high probability, some of these stars have Earth-like planets in a circumstellar habitable zone.[9]
 * Many of these stars, and hence their planets, are much older than the Sun.[10][11] If the Earth is typical, some may have developed intelligent life long ago.
 * Some of these civilizations may have developed interstellar travel, a step humans are investigating now.
 * Even at the slow pace of currently envisioned interstellar travel, the Milky Way galaxy could be completely traversed in a few million years.[12]
 * Since many of the Sun-like stars are billions of years older than the Sun, the Earth should have already been visited by extraterrestrial civilizations, or at least their probes.[13]
 * However, there is no convincing evidence that this has happened.[12]

 History [ edit ]

 Fermi was not the first to ask the question. An earlier implicit mention was by Konstantin Tsiolkovsky in an unpublished manuscript from 1933.[14] He noted "people deny the presence of intelligent beings on the planets of the universe" because "(i) if such beings exist they would have visited Earth, and (ii) if such civilizations existed then they would have given us some sign of their existence." This was not a paradox for others, who took this to imply the absence of extraterrestrial life. But it was one for him, since he believed in extraterrestrial life and the possibility of space travel. Therefore, he proposed what is now known as the zoo hypothesis and speculated that mankind is not yet ready for higher beings to contact us.[15] That Tsiolkovsky himself may not have been the first to discover the paradox is suggested by his above-mentioned reference to other people's reasons for not accepting the premise that extraterrestrial civilizations exist.

 In 1975, Michael H. Hart published a detailed examination of the paradox, one of the first to do so.[12][16] : 27–28 [17] : 6 He argued that if intelligent extraterrestrials exist, and are capable of space travel, then the galaxy could have been colonized in a time much less than that of the age of the Earth. However, there is no observable evidence they have been here, which Hart called "Fact A".[17] : 6

 Other names closely related to Fermi's question ("Where are they?") include the Great Silence,[18][19][20][21] and silentium universi[21] (Latin for "silence of the universe"), though these only refer to one portion of the Fermi Paradox, that humans see no evidence of other civilizations.

 Original conversations [ edit ]

 Los Alamos National Laboratory
 Los Alamos, New Mexico, United States

 In the summer of 1950 at Los Alamos National Laboratory in New Mexico, Enrico Fermi and co-workers Emil Konopinski, Edward Teller, and Herbert York had one or several lunchtime conversations.[4][22] In one, Fermi suddenly blurted out, "Where is everybody?" (Teller's letter), or "Don't you ever wonder where everybody is?" (York's letter), or "But where is everybody?" (Konopinski's letter).[4] Teller wrote, "The result of his question was general laughter because of the strange fact that in spite of Fermi's question coming from the clear blue, everybody around the table seemed to understand at once that he was talking about extraterrestrial life."[4]

 In 1984 York wrote that Fermi "followed up with a series of calculations on the probability of earthlike planets, the probability of life given an earth, the probability of humans given life, the likely rise and duration of high technology, and so on. He concluded on the basis of such calculations that we ought to have been visited long ago and many times over."[4] Teller remembers that not much came of this conversation "except perhaps a statement that the distances to the next location of living beings may be very great and that, indeed, as far as our galaxy is concerned, we are living somewhere in the sticks, far removed from the metropolitan area of the galactic center."[4]

 Fermi died of cancer in 1954. However, in letters to the three surviving men decades later in 1984, Dr. Eric Jones of Los Alamos was able to partially put the original conversation back together. He informed each of the men that he wished to include a reasonably accurate version or composite in the written proceedings he was putting together for a previously held conference entitled "Interstellar Migration and the Human Experience".[4][23] Jones first sent a letter to Edward Teller which included a secondhand account from Hans Mark. Teller responded, and then Jones sent Teller's letter to Herbert York. York responded, and finally, Jones sent both Teller's and York's letters to Emil Konopinski who also responded. Furthermore, Konopinski was able to later identify a cartoon which Jones found as the one involved in the conversation and thereby help to settle the time period as being the summer of 1950.[4]

 Basis [ edit ]

 Enrico Fermi (1901–1954)

 The Fermi paradox is a conflict between the argument that scale and probability seem to favor intelligent life being common in the universe, and the total lack of evidence of intelligent life having ever arisen anywhere other than on Earth.

 The first aspect of the Fermi paradox is a function of the scale or the large numbers involved: there are an estimated 200–400 billion stars in the Milky Way[24] (2–4 × 1011) and 70 sextillion (7×1022) in the observable universe.[25] Even if intelligent life occurs on only a minuscule percentage of planets around these stars, there might still be a great number of extant civilizations, and if the percentage were high enough it would produce a significant number of extant civilizations in the Milky Way. This assumes the mediocrity principle, by which Earth is a typical planet.

 The second aspect of the Fermi paradox is the argument of probability: given intelligent life's ability to overcome scarcity, and its tendency to colonize new habitats, it seems possible that at least some civilizations would be technologically advanced, seek out new resources in space, and colonize their own star system and, subsequently, surrounding star systems. Since there is no significant evidence on Earth, or elsewhere in the known universe, of other intelligent life after 13.8 billion years of the universe's history, there is a conflict requiring a resolution. Some examples of possible resolutions are that intelligent life is rarer than is thought, that assumptions about the general development or behavior of intelligent species are flawed, or, more radically, that current scientific understanding of the nature of the universe itself is quite incomplete.

 The Fermi paradox can be asked in two ways.[note 1] The first is, "Why are no aliens or their artifacts found here on Earth, or in the Solar System?". If interstellar travel is possible, even the "slow" kind nearly within the reach of Earth technology, then it would only take from 5 million to 50 million years to colonize the galaxy.[26] This is relatively brief on a geological scale, let alone a cosmological one. Since there are many stars older than the Sun, and since intelligent life might have evolved earlier elsewhere, the question then becomes why the galaxy has not been colonized already. Even if colonization is impractical or undesirable to all alien civilizations, large-scale exploration of the galaxy could be possible by probes. These might leave detectable artifacts in the Solar System, such as old probes or evidence of mining activity, but none of these have been observed.

 The second form of the question is "Why do we see no signs of intelligence elsewhere in the universe?". This version does not assume interstellar travel, but includes other galaxies as well. For distant galaxies, travel times may well explain the lack of alien visits to Earth, but a sufficiently advanced civilization could potentially be observable over a significant fraction of the size of the observable universe.[27] Even if such civilizations are rare, the scale argument indicates they should exist somewhere at some point during the history of the universe, and since they could be detected from far away over a considerable period of time, many more potential sites for their origin are within range of human observation. It is unknown whether the paradox is stronger for the Milky Way galaxy or for the universe as a whole.[28]

 Drake equation [ edit ]

 Main article: Drake equation

 The theories and principles in the Drake equation are closely related to the Fermi paradox.[29] The equation was formulated by Frank Drake in 1961 in an attempt to find a systematic means to evaluate the numerous probabilities involved in the existence of alien life. The equation is presented as follows:

 Where is the number of technologically advanced civilizations in the Milky Way galaxy, and is asserted to be the product of

 * , the rate of formation of stars in the galaxy;
 * , the fraction of those stars with planetary systems;
 * , the number of planets, per solar system, with an environment suitable for organic life;
 * , the fraction of those suitable planets whereon organic life actually appears;
 * , the fraction of habitable planets whereon intelligent life actually appears;
 * , the fraction of civilizations that reach the technological level whereby detectable signals may be dispatched; and
 * , the length of time that those civilizations dispatch their signals.

 The fundamental problem is that the last four terms (, , , and ) are entirely unknown, rendering statistical estimates impossible.[30]

 The Drake equation has been used by both optimists and pessimists, with wildly differing results. The first scientific meeting on the search for extraterrestrial intelligence (SETI), which had 10 attendees including Frank Drake and Carl Sagan, speculated that the number of civilizations was roughly between 1,000 and 100,000,000 civilizations in the Milky Way galaxy.[31] Conversely, Frank Tipler and John D. Barrow used pessimistic numbers and speculated that the average number of civilizations in a galaxy is much less than one.[32] Almost all arguments involving the Drake equation suffer from the overconfidence effect, a common error of probabilistic reasoning about low-probability events, by guessing specific numbers for likelihoods of events whose mechanism is not yet understood, such as the likelihood of abiogenesis on an Earth-like planet, with current likelihood estimates varying over many hundreds of orders of magnitude. An analysis that takes into account some of the uncertainty associated with this lack of understanding has been carried out by Anders Sandberg, Eric Drexler and Toby Ord,[33] and suggests "a substantial ex ante probability of there being no other intelligent life in our observable universe".

 Great Filter [ edit ]

 Main article: Great Filter

 The Great Filter, a concept introduced by Robin Hanson in 1996, represents whatever natural phenomena that would make it unlikely for life to evolve from inanimate matter to an advanced civilization.[34][3] The most commonly agreed-upon low probability event is abiogenesis: a gradual process of increasing complexity of the first self-replicating molecules by a randomly occurring chemical process. Other proposed great filters are the emergence of eukaryotic cells[note 2] or of meiosis or some of the steps involved in the evolution of a brain capable of complex logical deductions.[35]

 Astrobiologists Dirk Schulze-Makuch and William Bains, reviewing the history of life on Earth, including convergent evolution, concluded that transitions such as oxygenic photosynthesis, the eukaryotic cell, multicellularity, and tool-using intelligence are likely to occur on any Earth-like planet given enough time. They argue that the Great Filter may be abiogenesis, the rise of technological human-level intelligence, or an inability to settle other worlds because of self-destruction or a lack of resources.[36]

 Empirical evidence [ edit ]

 Main articles: Search for extraterrestrial intelligence and Technosignature

 There are two parts of the Fermi paradox that rely on empirical evidence—that there are many potential habitable planets, and that humans see no evidence of life. The first point, that many suitable planets exist, was an assumption in Fermi's time but is now supported by the discovery that exoplanets are common. Current models predict billions of habitable worlds in the Milky Way.[37]

 The second part of the paradox, that humans see no evidence of extraterrestrial life, is also an active field of scientific research. This includes both efforts to find any indication of life,[38] and efforts specifically directed to finding intelligent life. These searches have been made since 1960, and several are ongoing.[note 3]

 Although astronomers do not usually search for extraterrestrials, they have observed phenomena that they could not immediately explain without positing an intelligent civilization as the source. For example, pulsars, when first discovered in 1967, were called little green men (LGM) because of the precise repetition of their pulses.[39] In all cases, explanations with no need for intelligent life have been found for such observations,[note 4] but the possibility of discovery remains.[40] Proposed examples include asteroid mining that would change the appearance of debris disks around stars,[41] or spectral lines from nuclear waste disposal in stars.[42]

 Explanations based on technosignatures, such as radio communications, have been presented.[43]

 Electromagnetic emissions [ edit ]

 Further information: Project Phoenix (SETI), SERENDIP, and Allen Telescope Array
 Radio telescopes are often used by SETI projects.

 Radio technology and the ability to construct a radio telescope are presumed to be a natural advance for technological species,[44] theoretically creating effects that might be detected over interstellar distances. The careful searching for non-natural radio emissions from space may lead to the detection of alien civilizations. Sensitive alien observers of the Solar System, for example, would note unusually intense radio waves for a G2 star due to Earth's television and telecommunication broadcasts. In the absence of an apparent natural cause, alien observers might infer the existence of a terrestrial civilization. Such signals could be either "accidental" by-products of a civilization, or deliberate attempts to communicate, such as the Arecibo message. It is unclear whether "leakage", as opposed to a deliberate beacon, could be detected by an extraterrestrial civilization. The most sensitive radio telescopes on Earth, as of 2019, would not be able to detect non-directional radio signals even at a fraction of a light-year away,[45] but other civilizations could hypothetically have much better equipment.[46]

 A number of astronomers and observatories have attempted and are attempting to detect such evidence, mostly through the SETI organization. Several decades of SETI analysis have not revealed any unusually bright or meaningfully repetitive radio emissions.[47]

 Direct planetary observation [ edit ]

 A composite picture of Earth at night, created using data from the Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS). Large-scale artificial lighting produced by human civilization is detectable from space.

 Exoplanet detection and classification is a very active sub-discipline in astronomy; the first possible terrestrial planet discovered within a star's habitable zone was found in 2007.[48] New refinements in exoplanet detection methods, and use of existing methods from space (such as the Kepler and TESS missions) are starting to detect and characterize Earth-size planets, to determine whether they are within the habitable zones of their stars. Such observational refinements may allow for a better estimation of how common these potentially habitable worlds are.[49]

 Conjectures about interstellar probes [ edit ]

 Further information: Hart–Tipler conjecture, Von Neumann probe, and Bracewell probe

 The Hart-Tipler conjecture is a form of contraposition which states that because no interstellar probes have been detected, there likely is no other intelligent life in the universe, as such life should be expected to eventually create and launch such probes.[50][51] Self-replicating probes could exhaustively explore a galaxy the size of the Milky Way in as little as a million years.[12] If even a single civilization in the Milky Way attempted this, such probes could spread throughout the entire galaxy. Another speculation for contact with an alien probe—one that would be trying to find human beings—is an alien Bracewell probe. Such a hypothetical device would be an autonomous space probe whose purpose is to seek out and communicate with alien civilizations (as opposed to von Neumann probes, which are usually described as purely exploratory). These were proposed as an alternative to carrying a slow speed-of-light dialogue between vastly distant neighbors. Rather than contending with the long delays a radio dialogue would suffer, a probe housing an artificial intelligence would seek out an alien civilization to carry on a close-range communication with the discovered civilization. The findings of such a probe would still have to be transmitted to the home civilization at light speed, but an information-gathering dialogue could be conducted in real time.[52]

 Direct exploration of the Solar System has yielded no evidence indicating a visit by aliens or their probes. Detailed exploration of areas of the Solar System where resources would be plentiful may yet produce evidence of alien exploration,[53][54] though the entirety of the Solar System is vast and difficult to investigate. Attempts to signal, attract, or activate hypothetical Bracewell probes in Earth's vicinity have not succeeded.[55]

 Searches for stellar-scale artifacts [ edit ]

 Further information: Dyson sphere, Stellar engine, and Kardashev scale
 A variant of the speculative Dyson sphere. Such large-scale artifacts would drastically alter the spectrum of a star.

 In 1959, Freeman Dyson observed that every developing human civilization constantly increases its energy consumption, and he conjectured that a civilization might try to harness a large part of the energy produced by a star. He proposed a hypothetical "Dyson sphere" as a possible means: a shell or cloud of objects enclosing a star to absorb and utilize as much radiant energy as possible. Such a feat of astroengineering would drastically alter the observed spectrum of the star involved, changing it at least partly from the normal emission lines of a natural stellar atmosphere to those of black-body radiation, probably with a peak in the infrared. Dyson speculated that advanced alien civilizations might be detected by examining the spectra of stars and searching for such an altered spectrum.[56][57][58]

 There have been some attempts to find evidence of the existence of Dyson spheres that would alter the spectra of their core stars.[59] Direct observation of thousands of galaxies has shown no explicit evidence of artificial construction or modifications.[57][58][60][61] In October 2015, there was some speculation that a dimming of light from star KIC 8462852, observed by the Kepler space telescope, could have been a result of Dyson sphere construction.[62][63] However, in 2018, observations determined that the amount of dimming varied by the frequency of the light, pointing to dust, rather than an opaque object such as a Dyson sphere, as the culprit for causing the dimming.[64][65]

 Hypothetical explanations for the paradox [ edit ]

 Rarity of intelligent life [ edit ]

 Extraterrestrial life is rare or non-existent [ edit ]

 Main articles: Rare Earth hypothesis and Firstborn hypothesis

 Those who think that intelligent extraterrestrial life is (nearly) impossible argue that the conditions needed for the evolution of life—or at least the evolution of biological complexity—are rare or even unique to Earth. Under this assumption, called the rare Earth hypothesis, a rejection of the mediocrity principle, complex multicellular life is regarded as exceedingly unusual.[66]

 The rare Earth hypothesis argues that the evolution of biological complexity requires a host of fortuitous circumstances, such as a galactic habitable zone, a star and planet(s) having the requisite conditions, such as enough of a continuous habitable zone, the advantage of a giant guardian like Jupiter and a large moon, conditions needed to ensure the planet has a magnetosphere and plate tectonics, the chemistry of the lithosphere, atmosphere, and oceans, the role of "evolutionary pumps" such as massive glaciation and rare bolide impacts. And perhaps most importantly, advanced life needs whatever it was that led to the transition of (some) prokaryotic cells to eukaryotic cells, sexual reproduction and the Cambrian explosion.

 In his book Wonderful Life (1989), Stephen Jay Gould suggested that if the "tape of life" were rewound to the time of the Cambrian explosion, and one or two tweaks made, human beings most probably never would have evolved. Other thinkers such as Fontana, Buss, and Kauffman have written about the self-organizing properties of life.[67]

 Extraterrestrial intelligence is rare or non-existent [ edit ]

 It is possible that even if complex life is common, intelligence (and consequently civilizations) is not.[35] While there are remote sensing techniques that could perhaps detect life-bearing planets without relying on the signs of technology,[68][69] none of them have any ability to tell if any detected life is intelligent. This is sometimes referred to as the "algae vs. alumnae" problem.[70]

 Charles Lineweaver states that when considering any extreme trait in an animal, intermediate stages do not necessarily produce "inevitable" outcomes. For example, large brains are no more "inevitable", or convergent, than are the long noses of animals such as aardvarks and elephants. Humans, apes, whales, dolphins, octopuses, and squids are among the small group of definite or probable intelligence on Earth. And as he points out, "dolphins have had ~20 million years to build a radio telescope and have not done so".[35]

 In addition, Rebecca Boyle points out that of all the species who have ever evolved in the history of life on the planet Earth, only one—we human beings and only in the beginning stages—has ever become space-faring.[71]

 Periodic extinction by natural events [ edit ]

 An asteroid impact may trigger an extinction event.
 See also: Global catastrophic risk

 New life might commonly die out due to runaway heating or cooling on their fledgling planets.[72] On Earth, there have been numerous major extinction events that destroyed the majority of complex species alive at the time; the extinction of the non-avian dinosaurs is the best known example. These are thought to have been caused by events such as impact from a large meteorite, massive volcanic eruptions, or astronomical events such as gamma-ray bursts.[73] It may be the case that such extinction events are common throughout the universe and periodically destroy intelligent life, or at least its civilizations, before the species is able to develop the technology to communicate with other intelligent species.[74]

 Evolutionary explanations [ edit ]

 Intelligent alien species have not developed advanced technologies [ edit ]

 Le Moustier Neanderthals (Charles R. Knight, 1920)

 It may be that while alien species with intelligence exist, they are primitive or have not reached the level of technological advancement necessary to communicate. Along with non-intelligent life, such civilizations would also be very difficult to detect.[70] A trip using conventional rockets would take hundreds of thousands of years to reach the nearest stars.[75]

 To skeptics, the fact that in the history of life on the Earth only one species has developed a civilization to the point of being capable of spaceflight and radio technology lends more credence to the idea that technologically advanced civilizations are rare in the universe.[76]

 Another hypothesis in this category is the "Water World hypothesis". According to author and scientist David Brin: "it turns out that our Earth skates the very inner edge of our sun’s continuously habitable—or 'Goldilocks'—zone. And Earth may be anomalous. It may be that because we are so close to our sun, we have an anomalously oxygen-rich atmosphere, and we have anomalously little ocean for a water world. In other words, 32 percent continental mass may be high among water worlds..."[77] Brin continues, "In which case, the evolution of creatures like us, with hands and fire and all that sort of thing, may be rare in the galaxy. In which case, when we do build starships and head out there, perhaps we’ll find lots and lots of life worlds, but they’re all like Polynesia. We’ll find lots and lots of intelligent lifeforms out there, but they’re all dolphins, whales, squids, who could never build their own starships. What a perfect universe for us to be in, because nobody would be able to boss us around, and we’d get to be the voyagers, the Star Trek people, the starship builders, the policemen, and so on."[77]

 It is the nature of intelligent life to destroy itself [ edit ]

 See also: Technological utopianism § Criticisms
 A 23-kiloton tower shot called BADGER, fired as part of the Operation Upshot–Knothole nuclear test series

 This is the argument that technological civilizations may usually or invariably destroy themselves before or shortly after developing radio or spaceflight technology. The astrophysicist Sebastian von Hoerner stated that the progress of science and technology on Earth was driven by two factors—the struggle for domination and the desire for an easy life. The former potentially leads to complete destruction, while the latter may lead to biological or mental degeneration.[78] Possible means of annihilation via major global issues, where global interconnectedness actually makes humanity more vulnerable than resilient,[79] are many,[80] including war, accidental environmental contamination or damage, the development of biotechnology,[81] synthetic life like mirror life,[82] resource depletion, climate change,[83] or poorly-designed artificial intelligence. This general theme is explored both in fiction and in scientific hypothesizing.[84]

 In 1966, Sagan and Shklovskii speculated that technological civilizations will either tend to destroy themselves within a century of developing interstellar communicative capability or master their self-destructive tendencies and survive for billion-year timescales.[85] Self-annihilation may also be viewed in terms of thermodynamics: insofar as life is an ordered system that can sustain itself against the tendency to disorder, Stephen Hawking's "external transmission" or interstellar communicative phase, where knowledge production and knowledge management is more important than transmission of information via evolution, may be the point at which the system becomes unstable and self-destructs.[86][87] Here, Hawking emphasizes self-design of the human genome (transhumanism) or enhancement via machines (e.g., brain–computer interface) to enhance human intelligence and reduce aggression, without which he implies human civilization may be too stupid collectively to survive an increasingly unstable system. For instance, the development of technologies during the "external transmission" phase, such as weaponization of artificial general intelligence or antimatter, may not be met by concomitant increases in human ability to manage its own inventions. Consequently, disorder increases in the system: global governance may become increasingly destabilized, worsening humanity's ability to manage the possible means of annihilation listed above, resulting in global societal collapse.

 Possible trajectories of anthropogenic climate change in a model by Frank et al., 2018.

 Using extinct civilizations such as Easter Island (Rapa Nui) as models, a study conducted in 2018 by Adam Frank et al. posited that climate change induced by "energy intensive" civilizations may prevent sustainability within such civilizations, thus explaining the paradoxical lack of evidence for intelligent extraterrestrial life. According to his model, possible outcomes of climate change include gradual population decline until an equilibrium is reached; a scenario where sustainability is attained and both population and surface temperature level off; and societal collapse, including scenarios where a tipping point is crossed.[88]

 A less theoretical example might be the resource-depletion issue on Polynesian islands, of which Easter Island is only the best known. David Brin points out that during the expansion phase from 1500 BC to 800 AD there were cycles of overpopulation followed by what might be called periodic cullings of adult males through war or ritual. He writes, "There are many stories of islands whose men were almost wiped out—sometimes by internal strife, and sometimes by invading males from other islands."[89]

 It is the nature of intelligent life to destroy others [ edit ]

 See also: Berserker hypothesis, Technological singularity, and Von Neumann probe

 Another hypothesis is that an intelligent species beyond a certain point of technological capability will destroy other intelligent species as they appear, perhaps by using self-replicating probes. Science fiction writer Fred Saberhagen has explored this idea in his Berserker series, as has physicist Gregory Benford[90] and, as well, science fiction writer Greg Bear in his The Forge of God novel,[91] and later Liu Cixin in his The Three-Body Problem series.

 A species might undertake such extermination out of expansionist motives, greed, paranoia, or aggression. In 1981, cosmologist Edward Harrison argued that such behavior would be an act of prudence: an intelligent species that has overcome its own self-destructive tendencies might view any other species bent on galactic expansion as a threat.[92] It has also been suggested that a successful alien species would be a superpredator, as are humans.[93][94] : 112 Another possibility invokes the "tragedy of the commons" and the anthropic principle: the first lifeform to achieve interstellar travel will necessarily (even if unintentionally) prevent competitors from arising, and humans simply happen to be first.[95][96]

 Civilizations only broadcast detectable signals for a brief period of time [ edit ]

 It may be that alien civilizations are detectable through their radio emissions for only a short time, reducing the likelihood of spotting them. The usual assumption is that civilizations outgrow radio through technological advancement.[97] However, there could be other leakage such as that from microwaves used to transmit power from solar satellites to ground receivers.[98]

 Regarding the first point, in a 2006 Sky & Telescope article, Seth Shostak wrote, "Moreover, radio leakage from a planet is only likely to get weaker as a civilization advances and its communications technology gets better. Earth itself is increasingly switching from broadcasts to leakage-free cables and fiber optics, and from primitive but obvious carrier-wave broadcasts to subtler, hard-to-recognize spread-spectrum transmissions."[99]

 More hypothetically, advanced alien civilizations may evolve beyond broadcasting at all in the electromagnetic spectrum and communicate by technologies not developed or used by mankind.[100] Some scientists have hypothesized that advanced civilizations may send neutrino signals.[101] If such signals exist, they could be detectable by neutrino detectors that are now under construction for other goals.[102]

 Alien life may be too alien [ edit ]

 Microwave window as seen by a ground-based system. From NASA report SP-419: SETI – the Search for Extraterrestrial Intelligence

 Another possibility is that human theoreticians have underestimated how much alien life might differ from that on Earth. Aliens may be psychologically unwilling to attempt to communicate with human beings. Perhaps human mathematics is parochial to Earth and not shared by other life,[103] though others argue this can only apply to abstract math since the math associated with physics must be similar (in results, if not in methods).[104]

 Physiology might also cause a communication barrier. Carl Sagan speculated that an alien species might have a thought process orders of magnitude slower (or faster) than that of humans.[105] A message broadcast by that species might well seem like random background noise to humans, and therefore go undetected.

 Another thought is that technological civilizations invariably experience a technological singularity and attain a post-biological character.[106] Hypothetical civilizations of this sort may have advanced drastically enough to render communication impossible.[107][108][109]

 In his 2009 book, SETI scientist Seth Shostak wrote, "Our experiments [such as plans to use drilling rigs on Mars] are still looking for the type of extraterrestrial that would have appealed to Percival Lowell [astronomer who believed he had observed canals on Mars]."[110]

 Paul Davies states that 500 years ago the very idea of a computer doing work merely by manipulating internal data may not have been viewed as a technology at all. He writes, "Might there be a still higher level ... If so, this 'third level' would never be manifest through observations made at the informational level, still less the matter level. There is no vocabulary to describe the third level, but that doesn't mean it is non-existent, and we need to be open to the possibility that alien technology may operate at the third level, or maybe the fourth, fifth ... levels."[111]

 Sociological explanations [ edit ]

 Colonization is not the cosmic norm [ edit ]

 In response to Tipler's idea of self-replicating probes, Stephen Jay Gould wrote, "I must confess that I simply don’t know how to react to such arguments. I have enough trouble predicting the plans and reactions of the people closest to me. I am usually baffled by the thoughts and accomplishments of humans in different cultures. I’ll be damned if I can state with certainty what some extraterrestrial source of intelligence might do."[112][113]

 Alien species may have only settled part of the galaxy [ edit ]

 A February 2019 article in Popular Science states, "Sweeping across the Milky Way and establishing a unified galactic empire might be inevitable for a monolithic super-civilization, but most cultures are neither monolithic nor super—at least if our experience is any guide."[114] Astrophysicist Adam Frank, along with co-authors such as astronomer Jason Wright, ran a variety of simulations in which they varied such factors as settlement lifespans, fractions of suitable planets, and recharge times between launches. They found many of their simulations seemingly resulted in a "third category" in which the Milky Way remains partially settled indefinitely.[114] The abstract to their 2019 paper states, "These results break the link between Hart's famous 'Fact A' (no interstellar visitors on Earth now) and the conclusion that humans must, therefore, be the only technological civilization in the galaxy. Explicitly, our solutions admit situations where our current circumstances are consistent with an otherwise settled, steady-state galaxy."[115]

 Alien species may not live on planets [ edit ]

 Some colonization scenarios predict spherical expansion across star systems, with continued expansion coming from the systems just previously settled. It has been suggested that this would cause a strong selection process among the colonization front favoring cultural or biological adaptations to living in starships or space habitats. As a result, they may forgo living on planets.[116]

 This may result in the destruction of terrestrial planets in these systems for use as building materials, thus preventing the development of life on those worlds. Or, they may have an ethic of protection for "nursery worlds", and protect them in a similar fashion to the zoo hypothesis.[116]

 Alien species may isolate themselves from the outside world [ edit ]

 It has been suggested that some advanced beings may divest themselves of physical form, create massive artificial virtual environments, transfer themselves into these environments through mind uploading, and exist totally within virtual worlds, ignoring the external physical universe.[117]

 It may also be that intelligent alien life develops an "increasing disinterest" in their outside world.[94] : 86 Possibly any sufficiently advanced society will develop highly engaging media and entertainment well before the capacity for advanced space travel, with the rate of appeal of these social contrivances being destined, because of their inherent reduced complexity, to overtake any desire for complex, expensive endeavors such as space exploration and communication. Once any sufficiently advanced civilization becomes able to master its environment, and most of its physical needs are met through technology, various "social and entertainment technologies", including virtual reality, are postulated to become the primary drivers and motivations of that civilization.[118]

 Economic explanations [ edit ]

 Lack of resources needed to physically spread throughout the galaxy [ edit ]

 See also: Project Daedalus, Project Orion (nuclear propulsion), and Project Longshot

 The ability of an alien culture to colonize other star systems is based on the idea that interstellar travel is technologically feasible. While the current understanding of physics rules out the possibility of faster-than-light travel, it appears that there are no major theoretical barriers to the construction of "slow" interstellar ships, even though the engineering required is considerably beyond present capabilities. This idea underlies the concept of the Von Neumann probe and the Bracewell probe as a potential evidence of extraterrestrial intelligence.

 It is possible, however, that present scientific knowledge cannot properly gauge the feasibility and costs of such interstellar colonization. Theoretical barriers may not yet be understood, and the resources needed may be so great as to make it unlikely that any civilization could afford to attempt it. Even if interstellar travel and colonization are possible, they may be difficult, leading to a colonization model based on percolation theory.[119][120]

 Colonization efforts may not occur as an unstoppable rush, but rather as an uneven tendency to "percolate" outwards, within an eventual slowing and termination of the effort given the enormous costs involved and the expectation that colonies will inevitably develop a culture and civilization of their own. Colonization may thus occur in "clusters", with large areas remaining uncolonized at any one time.[119][120]

 Information is cheaper to transmit than matter is to transfer [ edit ]

 If a human-capability machine construct, such as via mind uploading, is possible, and if it is possible to transfer such constructs over vast distances and rebuild them on a remote machine, then it might not make strong economic sense to travel the galaxy by spaceflight. After the first civilization has physically explored or colonized the galaxy, as well as sent such machines for easy exploration, then any subsequent civilizations, after having contacted the first, may find it cheaper, faster, and easier to explore the galaxy through intelligent mind transfers to the machines built by the first civilization, which is cheaper than spaceflight by a factor of 108–1017. However, since a star system needs only one such remote machine, and the communication is most likely highly directed, transmitted at high-frequencies, and at a minimal power to be economical, such signals would be hard to detect from Earth.[121]

 Discovery of extraterrestrial life is too difficult [ edit ]

 Humans have not listened properly [ edit ]

 There are some assumptions that underlie the SETI programs that may cause searchers to miss signals that are present. Extraterrestrials might, for example, transmit signals that have a very high or low data rate, or employ unconventional (in human terms) frequencies, which would make them hard to distinguish from background noise. Signals might be sent from non-main sequence star systems that humans search with lower priority; current programs assume that most alien life will be orbiting Sun-like stars.[122]

 The greatest challenge is the sheer size of the radio search needed to look for signals (effectively spanning the entire observable universe), the limited amount of resources committed to SETI, and the sensitivity of modern instruments. SETI estimates, for instance, that with a radio telescope as sensitive as the Arecibo Observatory, Earth's television and radio broadcasts would only be detectable at distances up to 0.3 light-years, less than 1/10 the distance to the nearest star. A signal is much easier to detect if it consists of a deliberate, powerful transmission directed at Earth. Such signals could be detected at ranges of hundreds to tens of thousands of light-years distance.[123] However, this means that detectors must be listening to an appropriate range of frequencies, and be in that region of space to which the beam is being sent. Many SETI searches assume that extraterrestrial civilizations will be broadcasting a deliberate signal, like the Arecibo message, in order to be found.

 Thus, to detect alien civilizations through their radio emissions, Earth observers either need more sensitive instruments or must hope for fortunate circumstances: that the broadband radio emissions of alien radio technology are much stronger than humanity's own; that one of SETI's programs is listening to the correct frequencies from the right regions of space; or that aliens are deliberately sending focused transmissions in Earth's general direction.

 Humans have not listened for long enough [ edit ]

 Humanity's ability to detect intelligent extraterrestrial life has existed for only a very brief period—from 1937 onwards, if the invention of the radio telescope is taken as the dividing line—and Homo sapiens is a geologically recent species. The whole period of modern human existence to date is a very brief period on a cosmological scale, and radio transmissions have only been propagated since 1895. Thus, it remains possible that human beings have neither existed long enough nor made themselves sufficiently detectable to be found by extraterrestrial intelligence.[124]

 Intelligent life may be too far away [ edit ]

 NASA's conception of the Terrestrial Planet Finder

 It may be that non-colonizing technologically capable alien civilizations exist, but that they are simply too far apart for meaningful two-way communication.[94] : 62–71 Sebastian von Hoerner estimated the average duration of civilization at 6,500 years and the average distance between civilizations in the Milky Way at 1,000 light years.[78] If two civilizations are separated by several thousand light-years, it is possible that one or both cultures may become extinct before meaningful dialogue can be established. Human searches may be able to detect their existence, but communication will remain impossible because of distance. It has been suggested that this problem might be ameliorated somewhat if contact and communication is made through a Bracewell probe. In this case at least one partner in the exchange may obtain meaningful information. Alternatively, a civilization may simply broadcast its knowledge, and leave it to the receiver to make what they may of it. This is similar to the transmission of information from ancient civilizations to the present,[125] and humanity has undertaken similar activities like the Arecibo message, which could transfer information about Earth's intelligent species, even if it never yields a response or does not yield a response in time for humanity to receive it. It is possible that observational signatures of self-destroyed civilizations could be detected, depending on the destruction scenario and the timing of human observation relative to it.[126]

 A related speculation by Sagan and Newman suggests that if other civilizations exist, and are transmitting and exploring, their signals and probes simply have not arrived yet.[127] However, critics have noted that this is unlikely, since it requires that humanity's advancement has occurred at a very special point in time, while the Milky Way is in transition from empty to full. This is a tiny fraction of the lifespan of a galaxy under ordinary assumptions, so the likelihood that humanity is in the midst of this transition is considered low in the paradox.[128]

 Some SETI skeptics may also believe that humanity is at a very special point of time. Specifically, a transitional period from no space-faring societies to one space-faring society, namely that of human beings.[128]

 Intelligent life may exist hidden from view [ edit ]

 Planetary scientist Alan Stern put forward the idea that there could be a number of worlds with subsurface oceans (such as Jupiter's Europa or Saturn's Enceladus). The surface would provide a large degree of protection from such things as cometary impacts and nearby supernovae, as well as creating a situation in which a much broader range of orbits are acceptable. Life, and potentially intelligence and civilization, could evolve. Stern states, "If they have technology, and let's say they're broadcasting, or they have city lights or whatever—we can't see it in any part of the spectrum, except maybe very-low-frequency [radio]."[129][130]

 Advanced civilizations may limit their search for life to technological signatures [ edit ]

 If life is abundant in the universe but the cost of space travel is high, an advanced civilization may choose focus its search not on signs of life in general, but on those of other advanced civilizations, and specifically on radio signals. Since humanity has only recently began to use radio communication, its signals may have yet to arrive to other inhabited planets, and if they have, probes from those planets may have yet to arrive on Earth.[131]

 Willingness to communicate [ edit ]

 Everyone is listening but no one is transmitting [ edit ]

 Alien civilizations might be technically capable of contacting Earth, but could be only listening instead of transmitting.[132] If all or most civilizations act in the same way, the galaxy could be full of civilizations eager for contact, but everyone is listening and no one is transmitting. This is the so-called SETI Paradox.[133]

 The only civilization known, humanity, does not explicitly transmit, except for a few small efforts.[132] Even these efforts, and certainly any attempt to expand them, are controversial.[134] It is not even clear humanity would respond to a detected signal—the official policy within the SETI community[135] is that "[no] response to a signal or other evidence of extraterrestrial intelligence should be sent until appropriate international consultations have taken place". However, given the possible impact of any reply,[136] it may be very difficult to obtain any consensus on who would speak and what they would say.

 Communication is dangerous [ edit ]

 See also: Dark forest hypothesis

 An alien civilization might feel it is too dangerous to communicate, either for humanity or for them. It is argued that when very different civilizations have met on Earth, the results have often been disastrous for one side or the other, and the same may well apply to interstellar contact.[137] Even contact at a safe distance could lead to infection by computer code[138] or even ideas themselves.[139] Perhaps prudent civilizations actively hide not only from Earth but from everyone, out of fear of other civilizations.[140]

 Perhaps the Fermi paradox itself—or the alien equivalent of it—is the reason for any civilization to avoid contact with other civilizations, even if no other obstacles existed. From any one civilization's point of view, it would be unlikely for them to be the first ones to make first contact. Therefore, according to this reasoning, it is likely that previous civilizations faced fatal problems with first contact and doing so should be avoided. So perhaps every civilization keeps quiet because of the possibility that there is a real reason for others to do so.[18]

 In Liu Cixin's 2008 novel The Dark Forest, the author proposes a literary explanation for the Fermi paradox in which many multiple alien civilizations exist, but are both silent and paranoid, destroying any nascent lifeforms loud enough to make themselves known.[141] This is because any other intelligent life may represent a future threat. As a result, Liu's universe contains a plethora of quiet civilizations which do not reveal themselves, as in a "dark forest"...filled with "armed hunter(s) stalking through the trees like a ghost".[142][143][144] This idea has come to be known as the dark forest hypothesis.[145][146][147]

 Earth is deliberately being avoided [ edit ]

 Main article: Zoo hypothesis

 The zoo hypothesis states that intelligent extraterrestrial life exists and does not contact life on Earth to allow for its natural evolution and development.[148] A variation on the zoo hypothesis is the laboratory hypothesis, where humanity has been or is being subject to experiments,[148][5] with Earth or the Solar System effectively serving as a laboratory. The zoo hypothesis may break down under the uniformity of motive flaw: all it takes is a single culture or civilization to decide to act contrary to the imperative within humanity's range of detection for it to be abrogated, and the probability of such a violation of hegemony increases with the number of civilizations,[26][149] tending not towards a 'Galactic Club' with a unified foreign policy with regard to life on Earth but multiple 'Galactic Cliques'.[150] However, if artificial superintelligences dominate galactic life, and if it is true that such intelligences tend towards merged hegemonic behavior, then this would address the uniformity of motive flaw by dissuading rogue behavior.[151]

 Analysis of the inter-arrival times between civilizations in the galaxy based on common astrobiological assumptions suggests that the initial civilization would have a commanding lead over the later arrivals. As such, it may have established what has been termed the zoo hypothesis through force or as a galactic or universal norm and the resultant "paradox" by a cultural founder effect with or without the continued activity of the founder.[152]

 It is possible that a civilization advanced enough to travel between solar systems could be actively visiting or observing Earth while remaining undetected or unrecognized.[153]

 Earth is deliberately being isolated [ edit ]

 Main article: Planetarium hypothesis

 A related idea to the zoo hypothesis is that, beyond a certain distance, the perceived universe is a simulated reality. The planetarium hypothesis[154] speculates that beings may have created this simulation so that the universe appears to be empty of other life.

 Alien life is already here, unacknowledged [ edit ]

 Main article: UFO conspiracy theories

 A significant fraction of the population believes that at least some UFOs (Unidentified Flying Objects) are spacecraft piloted by aliens.[155][156] While most of these are unrecognized or mistaken interpretations of mundane phenomena, there are those that remain puzzling even after investigation. The consensus scientific view is that although they may be unexplained, they do not rise to the level of convincing evidence.[157]

 Similarly, it is theoretically possible that SETI groups are not reporting positive detections, or governments have been blocking signals or suppressing publication. This response might be attributed to security or economic interests from the potential use of advanced extraterrestrial technology. It has been suggested that the detection of an extraterrestrial radio signal or technology could well be the most highly secret information that exists.[158] Claims that this has already happened are common in the popular press,[159][160] but the scientists involved report the opposite experience—the press becomes informed and interested in a potential detection even before a signal can be confirmed.[161]

 Regarding the idea that aliens are in secret contact with governments, David Brin writes, "Aversion to an idea, simply because of its long association with crackpots, gives crackpots altogether too much influence."[162]

 See also [ edit ]

 * Aestivation hypothesis – Hypothesized solution to the Fermi paradox
 * Anthropic principle – Philosophical principle about the occurrence of sapient life in the Universe
 * Astrobiology – Science concerned with life in the universe
 * Calculating God – 2000 novel by Robert J. Sawyer
 * Fermi problem – Estimation problem in physics or engineering education
 * Interstellar travel – Hypothetical travel between stars or planetary systems
 * Panspermia – Hypothesis on the interstellar spreading of primordial life
 * Rare Earth hypothesis – Hypothesis that complex extraterrestrial life is improbable and extremely rare
 * The Martians (scientists) – Group of prominent Hungarian scientists
 * Wow! signal – 1977 narrowband radio signal from SETI

 Notes [ edit ]

 1. ^ See Hart for an example of "no aliens are here", and Webb for an example of the more general "We see no signs of intelligence anywhere".
 2. ^ Eukaryotes also include plants, animals, fungi, and algae.
 3. ^ See, for example, the SETI Institute , The Harvard SETI Home Page Archived August 16, 2010, at the Wayback Machine , or The Search for Extra Terrestrial Intelligence at Berkeley Archived April 6, 2019, at the Wayback Machine
 4. ^ Pulsars are now attributed to neutron stars, and Seyfert galaxies to an end-on view of the accretion onto the black holes.

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 159. ^ Ashlee Vance (July 31, 2006). "SETI urged to fess up over alien signals". The Register. Archived from the original on April 2, 2007 . Retrieved August 10, 2017 .
 160. ^ Speigel, Lee (December 6, 2011). "UFO Hunters Keep Pressing White House For Answers Through 'We The People' Petitions". The Huffington Post. Archived from the original on April 15, 2013 . Retrieved April 16, 2013 .
 161. ^ G. Seth Shostak (2009). Confessions of an Alien Hunter: A Scientist's Search for Extraterrestrial Intelligence. National Geographic. p. 17. ISBN 978-1-4262-0392-3.
 162. ^ "The Great Silence: the Controversy . . " (15-page paper), Quarterly J. Royal Astron. Soc., David Brin, 1983, p. 299 bottom Archived April 11, 2019, at the Wayback Machine .

 Further reading [ edit ]

 Library resources about 
 Fermi paradox 
 * Online books 
 * Resources in your library 
 * Resources in other libraries
 
 * Rebecca Boyle and Quanta Magazine (March 10, 2019). "Moving Stars Might Speed the Spread of Alien Life". The Atlantic.
 * Ćirković, Milan [2] Archived May 5, 2021, at the Wayback Machine Why we downplay Fermi's paradox. Nautilus
 * Ćirković, Milan The Great Silence: Science and Philosophy of Fermi's Paradox The Great Silence Oxford University Press
 * Crowe, Michael J. (2008). The Extraterrestrial Life Debate, Antiquity to 1915. University of Notre Dame Press. ISBN 978-0-268-02368-3.
 * Forgan, Duncan H. (2019). Solving Fermi's Paradox. Cambridge University Press. ISBN 9781107163652.
 * Michaud, Michael (2006). Contact with Alien Civilizations: Our Hopes and Fears about Encountering Extraterrestrials. Copernicus Books. ISBN 978-0-387-28598-6.
 * Zuckerman, Ben; Hart, Michael H. (1995). Extraterrestrials: Where Are They?. Cambridge University Press. ISBN 978-0-521-44803-1.

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 * Kestenbaum, David. "Three people grapple with the question, 'Are we alone?'", This American Life radio show, hosted by Ira Glass. This episode's first 22 minutes discusses the Fermi Paradox. See also the show's May 19, 2017 transcript.
 * Overcome the Great Silence. Translated by Leonidovich Zaitsev, Aleksandr (Translation of the documentary ed.).
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This article is about the absence of clear evidence for space aliens. For a type of estimation problem, see Fermi problem.
      A graphical representation of the Arecibo message, humanity's first attempt to use radio waves to actively communicate its existence to alien civilizations  
The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life compared to the apparently high a priori likelihood of its existence. [1]   [2]  As a 2015 article put it, "If life is so easy, someone from somewhere must have come calling by now." [3] 
  Italian-American physicist Enrico Fermi's name is associated with the paradox because of a casual conversation in the summer of 1950 with fellow physicists Edward Teller, Herbert York, and Emil Konopinski. While walking to lunch, the men discussed recent UFO reports and the possibility of faster-than-light travel. The conversation moved on to other topics, until during lunch Fermi blurted out, "But where is everybody?" (although the exact quote is uncertain). [3]   [4] 
 There have been many attempts to resolve the Fermi paradox, [5]   [6]  such as suggesting that intelligent extraterrestrial beings are extremely rare, that the lifetime of such civilizations is short, or that they exist but (for various reasons) humans see no evidence.
Chain of reasoning[edit]
The following are some of the facts and hypotheses that together serve to highlight the apparent contradiction:
 There are billions of stars in the Milky Way similar to the Sun. [7]   [8]  
With high probability, some of these stars have Earth-like planets in a circumstellar habitable zone. [9]  
Many of these stars, and hence their planets, are much older than the Sun. [10]   [11]  If the Earth is typical, some may have developed intelligent life long ago.
Some of these civilizations may have developed interstellar travel, a step humans are investigating now.
Even at the slow pace of currently envisioned interstellar travel, the Milky Way galaxy could be completely traversed in a few million years. [12]  
Since many of the Sun-like stars are billions of years older than the Sun, the Earth should have already been visited by extraterrestrial civilizations, or at least their probes. [13]  
However, there is no convincing evidence that this has happened. [12]   
History[edit]
Fermi was not the first to ask the question. An earlier implicit mention was by Konstantin Tsiolkovsky in an unpublished manuscript from 1933. [14]  He noted "people deny the presence of intelligent beings on the planets of the universe" because "(i) if such beings exist they would have visited Earth, and (ii) if such civilizations existed then they would have given us some sign of their existence." This was not a paradox for others, who took this to imply the absence of extraterrestrial life. But it was one for him, since he believed in extraterrestrial life and the possibility of space travel. Therefore, he proposed what is now known as the zoo hypothesis and speculated that mankind is not yet ready for higher beings to contact us. [15]  That Tsiolkovsky himself may not have been the first to discover the paradox is suggested by his above-mentioned reference to other people's reasons for not accepting the premise that extraterrestrial civilizations exist.
 In 1975, Michael H. Hart published a detailed examination of the paradox, one of the first to do so. [12]   [16]  : 27–28   [17]  : 6  He argued that if intelligent extraterrestrials exist, and are capable of space travel, then the galaxy could have been colonized in a time much less than that of the age of the Earth. However, there is no observable evidence they have been here, which Hart called "Fact A". [17]  : 6 
 Other names closely related to Fermi's question ("Where are they?") include the Great Silence, [18]   [19]   [20]   [21]  and silentium universi  [21]  (Latin for "silence of the universe"), though these only refer to one portion of the Fermi Paradox, that humans see no evidence of other civilizations.
Original conversations[edit]
     
In the summer of 1950 at Los Alamos National Laboratory in New Mexico, Enrico Fermi and co-workers Emil Konopinski, Edward Teller, and Herbert York had one or several lunchtime conversations. [4]   [22]  In one, Fermi suddenly blurted out, "Where is everybody?" (Teller's letter), or "Don't you ever wonder where everybody is?" (York's letter), or "But where is everybody?" (Konopinski's letter). [4]  Teller wrote, "The result of his question was general laughter because of the strange fact that in spite of Fermi's question coming from the clear blue, everybody around the table seemed to understand at once that he was talking about extraterrestrial life." [4] 
 In 1984 York wrote that Fermi "followed up with a series of calculations on the probability of earthlike planets, the probability of life given an earth, the probability of humans given life, the likely rise and duration of high technology, and so on. He concluded on the basis of such calculations that we ought to have been visited long ago and many times over." [4]  Teller remembers that not much came of this conversation "except perhaps a statement that the distances to the next location of living beings may be very great and that, indeed, as far as our galaxy is concerned, we are living somewhere in the sticks, far removed from the metropolitan area of the galactic center." [4] 
 Fermi died of cancer in 1954. However, in letters to the three surviving men decades later in 1984, Dr. Eric Jones of Los Alamos was able to partially put the original conversation back together. He informed each of the men that he wished to include a reasonably accurate version or composite in the written proceedings he was putting together for a previously held conference entitled "Interstellar Migration and the Human Experience". [4]   [23]  Jones first sent a letter to Edward Teller which included a secondhand account from Hans Mark. Teller responded, and then Jones sent Teller's letter to Herbert York. York responded, and finally, Jones sent both Teller's and York's letters to Emil Konopinski who also responded. Furthermore, Konopinski was able to later identify a cartoon which Jones found as the one involved in the conversation and thereby help to settle the time period as being the summer of 1950. [4] 
Basis[edit]
     
The Fermi paradox is a conflict between the argument that scale and probability seem to favor intelligent life being common in the universe, and the total lack of evidence of intelligent life having ever arisen anywhere other than on Earth.
 The first aspect of the Fermi paradox is a function of the scale or the large numbers involved: there are an estimated 200–400 billion stars in the Milky Way [24]  (2–4 × 1011 ) and 70 sextillion (7×1022) in the observable universe. [25]  Even if intelligent life occurs on only a minuscule percentage of planets around these stars, there might still be a great number of extant civilizations, and if the percentage were high enough it would produce a significant number of extant civilizations in the Milky Way. This assumes the mediocrity principle, by which Earth is a typical planet.
 The second aspect of the Fermi paradox is the argument of probability: given intelligent life's ability to overcome scarcity, and its tendency to colonize new habitats, it seems possible that at least some civilizations would be technologically advanced, seek out new resources in space, and colonize their own star system and, subsequently, surrounding star systems. Since there is no significant evidence on Earth, or elsewhere in the known universe, of other intelligent life after 13.8 billion years of the universe's history, there is a conflict requiring a resolution. Some examples of possible resolutions are that intelligent life is rarer than is thought, that assumptions about the general development or behavior of intelligent species are flawed, or, more radically, that current scientific understanding of the nature of the universe itself is quite incomplete.
 The Fermi paradox can be asked in two ways. [note 1]  The first is, "Why are no aliens or their artifacts found here on Earth, or in the Solar System?". If interstellar travel is possible, even the "slow" kind nearly within the reach of Earth technology, then it would only take from 5 million to 50 million years to colonize the galaxy. [26]  This is relatively brief on a geological scale, let alone a cosmological one. Since there are many stars older than the Sun, and since intelligent life might have evolved earlier elsewhere, the question then becomes why the galaxy has not been colonized already. Even if colonization is impractical or undesirable to all alien civilizations, large-scale exploration of the galaxy could be possible by probes. These might leave detectable artifacts in the Solar System, such as old probes or evidence of mining activity, but none of these have been observed.
 The second form of the question is "Why do we see no signs of intelligence elsewhere in the universe?". This version does not assume interstellar travel, but includes other galaxies as well. For distant galaxies, travel times may well explain the lack of alien visits to Earth, but a sufficiently advanced civilization could potentially be observable over a significant fraction of the size of the observable universe. [27]  Even if such civilizations are rare, the scale argument indicates they should exist somewhere at some point during the history of the universe, and since they could be detected from far away over a considerable period of time, many more potential sites for their origin are within range of human observation. It is unknown whether the paradox is stronger for the Milky Way galaxy or for the universe as a whole. [28] 
Drake equation[edit]
The theories and principles in the Drake equation are closely related to the Fermi paradox. [29]  The equation was formulated by Frank Drake in 1961 in an attempt to find a systematic means to evaluate the numerous probabilities involved in the existence of alien life. The equation is presented as follows:
 
 Where is the number of technologically advanced civilizations in the Milky Way galaxy, and is asserted to be the product of 
The fundamental problem is that the last four terms (, , , and ) are entirely unknown, rendering statistical estimates impossible. [30] 
 The Drake equation has been used by both optimists and pessimists, with wildly differing results. The first scientific meeting on the search for extraterrestrial intelligence (SETI), which had 10 attendees including Frank Drake and Carl Sagan, speculated that the number of civilizations was roughly between 1,000 and 100,000,000 civilizations in the Milky Way galaxy. [31]  Conversely, Frank Tipler and John D. Barrow used pessimistic numbers and speculated that the average number of civilizations in a galaxy is much less than one. [32]  Almost all arguments involving the Drake equation suffer from the overconfidence effect, a common error of probabilistic reasoning about low-probability events, by guessing specific numbers for likelihoods of events whose mechanism is not yet understood, such as the likelihood of abiogenesis on an Earth-like planet, with current likelihood estimates varying over many hundreds of orders of magnitude. An analysis that takes into account some of the uncertainty associated with this lack of understanding has been carried out by Anders Sandberg, Eric Drexler and Toby Ord, [33]  and suggests "a substantial  ex ante  probability of there being no other intelligent life in our observable universe".
Great Filter[edit]
The Great Filter, a concept introduced by Robin Hanson in 1996, represents whatever natural phenomena that would make it unlikely for life to evolve from inanimate matter to an advanced civilization. [34]   [3]  The most commonly agreed-upon low probability event is abiogenesis: a gradual process of increasing complexity of the first self-replicating molecules by a randomly occurring chemical process. Other proposed great filters are the emergence of eukaryotic cells  [note 2]  or of meiosis or some of the steps involved in the evolution of a brain capable of complex logical deductions. [35] 
 Astrobiologists Dirk Schulze-Makuch and William Bains, reviewing the history of life on Earth, including convergent evolution, concluded that transitions such as oxygenic photosynthesis, the eukaryotic cell, multicellularity, and tool-using intelligence are likely to occur on any Earth-like planet given enough time. They argue that the Great Filter may be abiogenesis, the rise of technological human-level intelligence, or an inability to settle other worlds because of self-destruction or a lack of resources. [36] 
Empirical evidence[edit]
There are two parts of the Fermi paradox that rely on empirical evidence—that there are many potential habitable planets, and that humans see no evidence of life. The first point, that many suitable planets exist, was an assumption in Fermi's time but is now supported by the discovery that exoplanets are common. Current models predict billions of habitable worlds in the Milky Way. [37] 
 The second part of the paradox, that humans see no evidence of extraterrestrial life, is also an active field of scientific research. This includes both efforts to find any indication of life, [38]  and efforts specifically directed to finding intelligent life. These searches have been made since 1960, and several are ongoing. [note 3] 
 Although astronomers do not usually search for extraterrestrials, they have observed phenomena that they could not immediately explain without positing an intelligent civilization as the source. For example, pulsars, when first discovered in 1967, were called little green men (LGM) because of the precise repetition of their pulses. [39]  In all cases, explanations with no need for intelligent life have been found for such observations, [note 4]  but the possibility of discovery remains. [40]  Proposed examples include asteroid mining that would change the appearance of debris disks around stars, [41]  or spectral lines from nuclear waste disposal in stars. [42] 
 Explanations based on technosignatures, such as radio communications, have been presented. [43] 
Electromagnetic emissions[edit]
     
Radio technology and the ability to construct a radio telescope are presumed to be a natural advance for technological species, [44]  theoretically creating effects that might be detected over interstellar distances. The careful searching for non-natural radio emissions from space may lead to the detection of alien civilizations. Sensitive alien observers of the Solar System, for example, would note unusually intense radio waves for a G2 star due to Earth's television and telecommunication broadcasts. In the absence of an apparent natural cause, alien observers might infer the existence of a terrestrial civilization. Such signals could be either "accidental" by-products of a civilization, or deliberate attempts to communicate, such as the Arecibo message. It is unclear whether "leakage", as opposed to a deliberate beacon, could be detected by an extraterrestrial civilization. The most sensitive radio telescopes on Earth, as of 2019, would not be able to detect non-directional radio signals even at a fraction of a light-year away, [45]  but other civilizations could hypothetically have much better equipment. [46] 
 A number of astronomers and observatories have attempted and are attempting to detect such evidence, mostly through the SETI organization. Several decades of SETI analysis have not revealed any unusually bright or meaningfully repetitive radio emissions. [47] 
Direct planetary observation[edit]
      A composite picture of Earth at night, created using data from the Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS). Large-scale artificial lighting produced by human civilization is detectable from space.  
Exoplanet detection and classification is a very active sub-discipline in astronomy; the first possible terrestrial planet discovered within a star's habitable zone was found in 2007. [48]  New refinements in exoplanet detection methods, and use of existing methods from space (such as the Kepler and TESS missions) are starting to detect and characterize Earth-size planets, to determine whether they are within the habitable zones of their stars. Such observational refinements may allow for a better estimation of how common these potentially habitable worlds are. [49] 
Conjectures about interstellar probes[edit]
The Hart-Tipler conjecture is a form of contraposition which states that because no interstellar probes have been detected, there likely is no other intelligent life in the universe, as such life should be expected to eventually create and launch such probes. [50]   [51]  Self-replicating probes could exhaustively explore a galaxy the size of the Milky Way in as little as a million years. [12]  If even a single civilization in the Milky Way attempted this, such probes could spread throughout the entire galaxy. Another speculation for contact with an alien probe—one that would be trying to find human beings—is an alien Bracewell probe. Such a hypothetical device would be an autonomous space probe whose purpose is to seek out and communicate with alien civilizations (as opposed to von Neumann probes, which are usually described as purely exploratory). These were proposed as an alternative to carrying a slow speed-of-light dialogue between vastly distant neighbors. Rather than contending with the long delays a radio dialogue would suffer, a probe housing an artificial intelligence would seek out an alien civilization to carry on a close-range communication with the discovered civilization. The findings of such a probe would still have to be transmitted to the home civilization at light speed, but an information-gathering dialogue could be conducted in real time. [52] 
 Direct exploration of the Solar System has yielded no evidence indicating a visit by aliens or their probes. Detailed exploration of areas of the Solar System where resources would be plentiful may yet produce evidence of alien exploration, [53]   [54]  though the entirety of the Solar System is vast and difficult to investigate. Attempts to signal, attract, or activate hypothetical Bracewell probes in Earth's vicinity have not succeeded. [55] 
Searches for stellar-scale artifacts[edit]
      A variant of the speculative Dyson sphere. Such large-scale artifacts would drastically alter the spectrum of a star.  
In 1959, Freeman Dyson observed that every developing human civilization constantly increases its energy consumption, and he conjectured that a civilization might try to harness a large part of the energy produced by a star. He proposed a hypothetical "Dyson sphere" as a possible means: a shell or cloud of objects enclosing a star to absorb and utilize as much radiant energy as possible. Such a feat of astroengineering would drastically alter the observed spectrum of the star involved, changing it at least partly from the normal emission lines of a natural stellar atmosphere to those of black-body radiation, probably with a peak in the infrared. Dyson speculated that advanced alien civilizations might be detected by examining the spectra of stars and searching for such an altered spectrum. [56]   [57]   [58] 
 There have been some attempts to find evidence of the existence of Dyson spheres that would alter the spectra of their core stars. [59]  Direct observation of thousands of galaxies has shown no explicit evidence of artificial construction or modifications. [57]   [58]   [60]   [61]  In October 2015, there was some speculation that a dimming of light from star KIC 8462852, observed by the Kepler space telescope, could have been a result of Dyson sphere construction. [62]   [63]  However, in 2018, observations determined that the amount of dimming varied by the frequency of the light, pointing to dust, rather than an opaque object such as a Dyson sphere, as the culprit for causing the dimming. [64]   [65] 
Hypothetical explanations for the paradox[edit]
Rarity of intelligent life[edit]
[edit]
Those who think that intelligent extraterrestrial life is (nearly) impossible argue that the conditions needed for the evolution of life—or at least the evolution of biological complexity—are rare or even unique to Earth. Under this assumption, called the rare Earth hypothesis, a rejection of the mediocrity principle, complex multicellular life is regarded as exceedingly unusual. [66] 
 The rare Earth hypothesis argues that the evolution of biological complexity requires a host of fortuitous circumstances, such as a galactic habitable zone, a star and planet(s) having the requisite conditions, such as enough of a continuous habitable zone, the advantage of a giant guardian like Jupiter and a large moon, conditions needed to ensure the planet has a magnetosphere and plate tectonics, the chemistry of the lithosphere, atmosphere, and oceans, the role of "evolutionary pumps" such as massive glaciation and rare bolide impacts. And perhaps most importantly, advanced life needs whatever it was that led to the transition of (some) prokaryotic cells to eukaryotic cells, sexual reproduction and the Cambrian explosion.
 In his book  Wonderful Life  (1989), Stephen Jay Gould suggested that if the "tape of life" were rewound to the time of the Cambrian explosion, and one or two tweaks made, human beings most probably never would have evolved. Other thinkers such as Fontana, Buss, and Kauffman have written about the self-organizing properties of life. [67] 
[edit]
It is possible that even if complex life is common, intelligence (and consequently civilizations) is not. [35]  While there are remote sensing techniques that could perhaps detect life-bearing planets without relying on the signs of technology, [68]   [69]  none of them have any ability to tell if any detected life is intelligent. This is sometimes referred to as the "algae vs. alumnae" problem. [70] 
 Charles Lineweaver states that when considering any extreme trait in an animal, intermediate stages do not necessarily produce "inevitable" outcomes. For example, large brains are no more "inevitable", or convergent, than are the long noses of animals such as aardvarks and elephants. Humans, apes, whales, dolphins, octopuses, and squids are among the small group of definite or probable intelligence on Earth. And as he points out, "dolphins have had ~20 million years to build a radio telescope and have not done so". [35] 
 In addition, Rebecca Boyle points out that of all the species who have ever evolved in the history of life on the planet Earth, only one—we human beings and only in the beginning stages—has ever become space-faring. [71] 
Periodic extinction by natural events[edit]
     
New life might commonly die out due to runaway heating or cooling on their fledgling planets. [72]  On Earth, there have been numerous major extinction events that destroyed the majority of complex species alive at the time; the extinction of the non-avian dinosaurs is the best known example. These are thought to have been caused by events such as impact from a large meteorite, massive volcanic eruptions, or astronomical events such as gamma-ray bursts. [73]  It may be the case that such extinction events are common throughout the universe and periodically destroy intelligent life, or at least its civilizations, before the species is able to develop the technology to communicate with other intelligent species. [74] 
Evolutionary explanations[edit]
Intelligent alien species have not developed advanced technologies[edit]
     
It may be that while alien species with intelligence exist, they are primitive or have not reached the level of technological advancement necessary to communicate. Along with non-intelligent life, such civilizations would also be very difficult to detect. [70]  A trip using conventional rockets would take hundreds of thousands of years to reach the nearest stars. [75] 
 To skeptics, the fact that in the history of life on the Earth only one species has developed a civilization to the point of being capable of spaceflight and radio technology lends more credence to the idea that technologically advanced civilizations are rare in the universe. [76] 
 Another hypothesis in this category is the "Water World hypothesis". According to author and scientist David Brin: "it turns out that our Earth skates the very inner edge of our sun’s continuously habitable—or 'Goldilocks'—zone. And Earth may be anomalous. It may be that because we are so close to our sun, we have an anomalously oxygen-rich atmosphere, and we have anomalously little ocean for a water world. In other words, 32 percent continental mass may be high among water worlds..." [77]  Brin continues, "In which case, the evolution of creatures like us, with hands and fire and all that sort of thing, may be rare in the galaxy. In which case, when we do build starships and head out there, perhaps we’ll find lots and lots of life worlds, but they’re all like Polynesia. We’ll find lots and lots of intelligent lifeforms out there, but they’re all dolphins, whales, squids, who could never build their own starships. What a perfect universe for us to be in, because nobody would be able to boss us around, and we’d get to be the voyagers, the Star Trek people, the starship builders, the policemen, and so on." [77] 
It is the nature of intelligent life to destroy itself[edit]
     
This is the argument that technological civilizations may usually or invariably destroy themselves before or shortly after developing radio or spaceflight technology. The astrophysicist Sebastian von Hoerner stated that the progress of science and technology on Earth was driven by two factors—the struggle for domination and the desire for an easy life. The former potentially leads to complete destruction, while the latter may lead to biological or mental degeneration. [78]  Possible means of annihilation via major global issues, where global interconnectedness actually makes humanity more vulnerable than resilient, [79]  are many, [80]  including war, accidental environmental contamination or damage, the development of biotechnology, [81]  synthetic life like mirror life, [82]  resource depletion, climate change, [83]  or poorly-designed artificial intelligence. This general theme is explored both in fiction and in scientific hypothesizing. [84] 
 In 1966, Sagan and Shklovskii speculated that technological civilizations will either tend to destroy themselves within a century of developing interstellar communicative capability or master their self-destructive tendencies and survive for billion-year timescales. [85]  Self-annihilation may also be viewed in terms of thermodynamics: insofar as life is an ordered system that can sustain itself against the tendency to disorder, Stephen Hawking's "external transmission" or interstellar communicative phase, where knowledge production and knowledge management is more important than transmission of information via evolution, may be the point at which the system becomes unstable and self-destructs. [86]   [87]  Here, Hawking emphasizes self-design of the human genome (transhumanism) or enhancement via machines (e.g., brain–computer interface) to enhance human intelligence and reduce aggression, without which he implies human civilization may be too stupid collectively to survive an increasingly unstable system. For instance, the development of technologies during the "external transmission" phase, such as weaponization of artificial general intelligence or antimatter, may not be met by concomitant increases in human ability to manage its own inventions. Consequently, disorder increases in the system: global governance may become increasingly destabilized, worsening humanity's ability to manage the possible means of annihilation listed above, resulting in global societal collapse.
      Possible trajectories of anthropogenic climate change in a model by Frank et al., 2018.  
Using extinct civilizations such as Easter Island (Rapa Nui) as models, a study conducted in 2018 by Adam Frank et al. posited that climate change induced by "energy intensive" civilizations may prevent sustainability within such civilizations, thus explaining the paradoxical lack of evidence for intelligent extraterrestrial life. According to his model, possible outcomes of climate change include gradual population decline until an equilibrium is reached; a scenario where sustainability is attained and both population and surface temperature level off; and societal collapse, including scenarios where a tipping point is crossed. [88] 
 A less theoretical example might be the resource-depletion issue on Polynesian islands, of which Easter Island is only the best known. David Brin points out that during the expansion phase from 1500 BC to 800 AD there were cycles of overpopulation followed by what might be called periodic cullings of adult males through war or ritual. He writes, "There are many stories of islands whose men were almost wiped out—sometimes by internal strife, and sometimes by invading males from other islands." [89] 
It is the nature of intelligent life to destroy others[edit]
Another hypothesis is that an intelligent species beyond a certain point of technological capability will destroy other intelligent species as they appear, perhaps by using self-replicating probes. Science fiction writer Fred Saberhagen has explored this idea in his  Berserker  series, as has physicist Gregory Benford  [90]  and, as well, science fiction writer Greg Bear in his  The Forge of God  novel, [91]  and later Liu Cixin in his  The Three-Body Problem  series.
 A species might undertake such extermination out of expansionist motives, greed, paranoia, or aggression. In 1981, cosmologist Edward Harrison argued that such behavior would be an act of prudence: an intelligent species that has overcome its own self-destructive tendencies might view any other species bent on galactic expansion as a threat. [92]  It has also been suggested that a successful alien species would be a superpredator, as are humans. [93]   [94]  : 112  Another possibility invokes the "tragedy of the commons" and the anthropic principle: the first lifeform to achieve interstellar travel will necessarily (even if unintentionally) prevent competitors from arising, and humans simply happen to be first. [95]   [96] 
Civilizations only broadcast detectable signals for a brief period of time[edit]
It may be that alien civilizations are detectable through their radio emissions for only a short time, reducing the likelihood of spotting them. The usual assumption is that civilizations outgrow radio through technological advancement. [97]  However, there could be other leakage such as that from microwaves used to transmit power from solar satellites to ground receivers. [98] 
 Regarding the first point, in a 2006  Sky &amp; Telescope  article, Seth Shostak wrote, "Moreover, radio leakage from a planet is only likely to get weaker as a civilization advances and its communications technology gets better. Earth itself is increasingly switching from broadcasts to leakage-free cables and fiber optics, and from primitive but obvious carrier-wave broadcasts to subtler, hard-to-recognize spread-spectrum transmissions." [99] 
 More hypothetically, advanced alien civilizations may evolve beyond broadcasting at all in the electromagnetic spectrum and communicate by technologies not developed or used by mankind. [100]  Some scientists have hypothesized that advanced civilizations may send neutrino signals. [101]  If such signals exist, they could be detectable by neutrino detectors that are now under construction for other goals. [102] 
Alien life may be too alien[edit]
      Microwave window as seen by a ground-based system. From NASA report SP-419: SETI – the Search for Extraterrestrial Intelligence  
Another possibility is that human theoreticians have underestimated how much alien life might differ from that on Earth. Aliens may be psychologically unwilling to attempt to communicate with human beings. Perhaps human mathematics is parochial to Earth and not shared by other life, [103]  though others argue this can only apply to abstract math since the math associated with physics must be similar (in results, if not in methods). [104] 
 Physiology might also cause a communication barrier. Carl Sagan speculated that an alien species might have a thought process orders of magnitude slower (or faster) than that of humans. [105]  A message broadcast by that species might well seem like random background noise to humans, and therefore go undetected.
 Another thought is that technological civilizations invariably experience a technological singularity and attain a post-biological character. [106]  Hypothetical civilizations of this sort may have advanced drastically enough to render communication impossible. [107]   [108]   [109] 
 In his 2009 book, SETI scientist Seth Shostak wrote, "Our experiments [such as plans to use drilling rigs on Mars] are still looking for the type of extraterrestrial that would have appealed to Percival Lowell [astronomer who believed he had observed canals on Mars]." [110] 
  Paul Davies states that 500 years ago the very idea of a computer doing work merely by manipulating internal data may not have been viewed as a technology at all. He writes, "Might there be a still higher level ... If so, this 'third level' would never be manifest through observations made at the informational level, still less the matter level. There is no vocabulary to describe the third level, but that doesn't mean it is non-existent, and we need to be open to the possibility that alien technology may operate at the third level, or maybe the fourth, fifth ... levels." [111] 
Sociological explanations[edit]
Colonization is not the cosmic norm[edit]
In response to Tipler's idea of self-replicating probes, Stephen Jay Gould wrote, "I must confess that I simply don’t know how to react to such arguments. I have enough trouble predicting the plans and reactions of the people closest to me. I am usually baffled by the thoughts and accomplishments of humans in different cultures. I’ll be damned if I can state with certainty what some extraterrestrial source of intelligence might do." [112]   [113] 
Alien species may have only settled part of the galaxy[edit]
A February 2019 article in Popular Science states, "Sweeping across the Milky Way and establishing a unified galactic empire might be inevitable for a monolithic super-civilization, but most cultures are neither monolithic nor super—at least if our experience is any guide." [114]  Astrophysicist Adam Frank, along with co-authors such as astronomer Jason Wright, ran a variety of simulations in which they varied such factors as settlement lifespans, fractions of suitable planets, and recharge times between launches. They found many of their simulations seemingly resulted in a "third category" in which the Milky Way remains partially settled indefinitely. [114]  The abstract to their 2019 paper states, "These results break the link between Hart's famous 'Fact A' (no interstellar visitors on Earth now) and the conclusion that humans must, therefore, be the only technological civilization in the galaxy. Explicitly, our solutions admit situations where our current circumstances are consistent with an otherwise settled, steady-state galaxy." [115] 
Alien species may not live on planets[edit]
Some colonization scenarios predict spherical expansion across star systems, with continued expansion coming from the systems just previously settled. It has been suggested that this would cause a strong selection process among the colonization front favoring cultural or biological adaptations to living in starships or space habitats. As a result, they may forgo living on planets. [116] 
 This may result in the destruction of terrestrial planets in these systems for use as building materials, thus preventing the development of life on those worlds. Or, they may have an ethic of protection for "nursery worlds", and protect them in a similar fashion to the zoo hypothesis. [116] 
Alien species may isolate themselves from the outside world[edit]
It has been suggested that some advanced beings may divest themselves of physical form, create massive artificial virtual environments, transfer themselves into these environments through mind uploading, and exist totally within virtual worlds, ignoring the external physical universe. [117] 
 It may also be that intelligent alien life develops an "increasing disinterest" in their outside world. [94]  : 86  Possibly any sufficiently advanced society will develop highly engaging media and entertainment well before the capacity for advanced space travel, with the rate of appeal of these social contrivances being destined, because of their inherent reduced complexity, to overtake any desire for complex, expensive endeavors such as space exploration and communication. Once any sufficiently advanced civilization becomes able to master its environment, and most of its physical needs are met through technology, various "social and entertainment technologies", including virtual reality, are postulated to become the primary drivers and motivations of that civilization. [118] 
Economic explanations[edit]
Lack of resources needed to physically spread throughout the galaxy[edit]
The ability of an alien culture to colonize other star systems is based on the idea that interstellar travel is technologically feasible. While the current understanding of physics rules out the possibility of faster-than-light travel, it appears that there are no major theoretical barriers to the construction of "slow" interstellar ships, even though the engineering required is considerably beyond present capabilities. This idea underlies the concept of the Von Neumann probe and the Bracewell probe as a potential evidence of extraterrestrial intelligence.
 It is possible, however, that present scientific knowledge cannot properly gauge the feasibility and costs of such interstellar colonization. Theoretical barriers may not yet be understood, and the resources needed may be so great as to make it unlikely that any civilization could afford to attempt it. Even if interstellar travel and colonization are possible, they may be difficult, leading to a colonization model based on percolation theory. [119]   [120] 
 Colonization efforts may not occur as an unstoppable rush, but rather as an uneven tendency to "percolate" outwards, within an eventual slowing and termination of the effort given the enormous costs involved and the expectation that colonies will inevitably develop a culture and civilization of their own. Colonization may thus occur in "clusters", with large areas remaining uncolonized at any one time. [119]   [120] 
Information is cheaper to transmit than matter is to transfer[edit]
If a human-capability machine construct, such as via mind uploading, is possible, and if it is possible to transfer such constructs over vast distances and rebuild them on a remote machine, then it might not make strong economic sense to travel the galaxy by spaceflight. After the first civilization has physically explored or colonized the galaxy, as well as sent such machines for easy exploration, then any subsequent civilizations, after having contacted the first, may find it cheaper, faster, and easier to explore the galaxy through intelligent mind transfers to the machines built by the first civilization, which is cheaper than spaceflight by a factor of 108–1017. However, since a star system needs only one such remote machine, and the communication is most likely highly directed, transmitted at high-frequencies, and at a minimal power to be economical, such signals would be hard to detect from Earth. [121] 
[edit]
Humans have not listened properly[edit]
There are some assumptions that underlie the SETI programs that may cause searchers to miss signals that are present. Extraterrestrials might, for example, transmit signals that have a very high or low data rate, or employ unconventional (in human terms) frequencies, which would make them hard to distinguish from background noise. Signals might be sent from non-main sequence star systems that humans search with lower priority; current programs assume that most alien life will be orbiting Sun-like stars. [122] 
 The greatest challenge is the sheer size of the radio search needed to look for signals (effectively spanning the entire observable universe), the limited amount of resources committed to SETI, and the sensitivity of modern instruments. SETI estimates, for instance, that with a radio telescope as sensitive as the Arecibo Observatory, Earth's television and radio broadcasts would only be detectable at distances up to 0.3 light-years, less than 1/10 the distance to the nearest star. A signal is much easier to detect if it consists of a deliberate, powerful transmission directed at Earth. Such signals could be detected at ranges of hundreds to tens of thousands of light-years distance. [123]  However, this means that detectors must be listening to an appropriate range of frequencies, and be in that region of space to which the beam is being sent. Many SETI searches assume that extraterrestrial civilizations will be broadcasting a deliberate signal, like the Arecibo message, in order to be found.
 Thus, to detect alien civilizations through their radio emissions, Earth observers either need more sensitive instruments or must hope for fortunate circumstances: that the broadband radio emissions of alien radio technology are much stronger than humanity's own; that one of SETI's programs is listening to the correct frequencies from the right regions of space; or that aliens are deliberately sending focused transmissions in Earth's general direction.
Humans have not listened for long enough[edit]
Humanity's ability to detect intelligent extraterrestrial life has existed for only a very brief period—from 1937 onwards, if the invention of the radio telescope is taken as the dividing line—and  Homo sapiens  is a geologically recent species. The whole period of modern human existence to date is a very brief period on a cosmological scale, and radio transmissions have only been propagated since 1895. Thus, it remains possible that human beings have neither existed long enough nor made themselves sufficiently detectable to be found by extraterrestrial intelligence. [124] 
Intelligent life may be too far away[edit]
     
It may be that non-colonizing technologically capable alien civilizations exist, but that they are simply too far apart for meaningful two-way communication. [94]  : 62–71  Sebastian von Hoerner estimated the average duration of civilization at 6,500 years and the average distance between civilizations in the Milky Way at 1,000 light years. [78]  If two civilizations are separated by several thousand light-years, it is possible that one or both cultures may become extinct before meaningful dialogue can be established. Human searches may be able to detect their existence, but communication will remain impossible because of distance. It has been suggested that this problem might be ameliorated somewhat if contact and communication is made through a Bracewell probe. In this case at least one partner in the exchange may obtain meaningful information. Alternatively, a civilization may simply broadcast its knowledge, and leave it to the receiver to make what they may of it. This is similar to the transmission of information from ancient civilizations to the present, [125]  and humanity has undertaken similar activities like the Arecibo message, which could transfer information about Earth's intelligent species, even if it never yields a response or does not yield a response in time for humanity to receive it. It is possible that observational signatures of self-destroyed civilizations could be detected, depending on the destruction scenario and the timing of human observation relative to it. [126] 
 A related speculation by Sagan and Newman suggests that if other civilizations exist, and are transmitting and exploring, their signals and probes simply have not arrived yet. [127]  However, critics have noted that this is unlikely, since it requires that humanity's advancement has occurred at a very special point in time, while the Milky Way is in transition from empty to full. This is a tiny fraction of the lifespan of a galaxy under ordinary assumptions, so the likelihood that humanity is in the midst of this transition is considered low in the paradox. [128] 
 Some SETI skeptics may also believe that humanity is at a very special point of time. Specifically, a transitional period from no space-faring societies to one space-faring society, namely that of human beings. [128] 
Intelligent life may exist hidden from view[edit]
Planetary scientist Alan Stern put forward the idea that there could be a number of worlds with subsurface oceans (such as Jupiter's Europa or Saturn's Enceladus). The surface would provide a large degree of protection from such things as cometary impacts and nearby supernovae, as well as creating a situation in which a much broader range of orbits are acceptable. Life, and potentially intelligence and civilization, could evolve. Stern states, "If they have technology, and let's say they're broadcasting, or they have city lights or whatever—we can't see it in any part of the spectrum, except maybe very-low-frequency [radio]." [129]   [130] 
Advanced civilizations may limit their search for life to technological signatures[edit]
If life is abundant in the universe but the cost of space travel is high, an advanced civilization may choose focus its search not on signs of life in general, but on those of other advanced civilizations, and specifically on radio signals. Since humanity has only recently began to use radio communication, its signals may have yet to arrive to other inhabited planets, and if they have, probes from those planets may have yet to arrive on Earth. [131] 
Willingness to communicate[edit]
Everyone is listening but no one is transmitting[edit]
Alien civilizations might be technically capable of contacting Earth, but could be only listening instead of transmitting. [132]  If all or most civilizations act in the same way, the galaxy could be full of civilizations eager for contact, but everyone is listening and no one is transmitting. This is the so-called  SETI Paradox. [133] 
 The only civilization known, humanity, does not explicitly transmit, except for a few small efforts. [132]  Even these efforts, and certainly any attempt to expand them, are controversial. [134]  It is not even clear humanity would respond to a detected signal—the official policy within the SETI community [135]  is that "[no] response to a signal or other evidence of extraterrestrial intelligence should be sent until appropriate international consultations have taken place". However, given the possible impact of any reply, [136]  it may be very difficult to obtain any consensus on who would speak and what they would say.
Communication is dangerous[edit]
An alien civilization might feel it is too dangerous to communicate, either for humanity or for them. It is argued that when very different civilizations have met on Earth, the results have often been disastrous for one side or the other, and the same may well apply to interstellar contact. [137]  Even contact at a safe distance could lead to infection by computer code [138]  or even ideas themselves. [139]  Perhaps prudent civilizations actively hide not only from Earth but from everyone, out of fear of other civilizations. [140] 
 Perhaps the Fermi paradox itself—or the alien equivalent of it—is the reason for any civilization to avoid contact with other civilizations, even if no other obstacles existed. From any one civilization's point of view, it would be unlikely for them to be the first ones to make first contact. Therefore, according to this reasoning, it is likely that previous civilizations faced fatal problems with first contact and doing so should be avoided. So perhaps every civilization keeps quiet because of the possibility that there is a real reason for others to do so. [18] 
 In Liu Cixin's 2008 novel  The Dark Forest , the author proposes a literary explanation for the Fermi paradox in which many multiple alien civilizations exist, but are both silent and paranoid, destroying any nascent lifeforms loud enough to make themselves known. [141]  This is because any other intelligent life may represent a future threat. As a result, Liu's universe contains a plethora of quiet civilizations which do not reveal themselves, as in a "dark forest"...filled with "armed hunter(s) stalking through the trees like a ghost". [142]   [143]   [144]  This idea has come to be known as the dark forest hypothesis. [145]   [146]   [147] 
Earth is deliberately being avoided[edit]
The zoo hypothesis states that intelligent extraterrestrial life exists and does not contact life on Earth to allow for its natural evolution and development. [148]  A variation on the zoo hypothesis is the laboratory hypothesis, where humanity has been or is being subject to experiments, [148]   [5]  with Earth or the Solar System effectively serving as a laboratory. The zoo hypothesis may break down under the uniformity of motive flaw: all it takes is a single culture or civilization to decide to act contrary to the imperative within humanity's range of detection for it to be abrogated, and the probability of such a violation of hegemony increases with the number of civilizations, [26]   [149]  tending not towards a 'Galactic Club' with a unified foreign policy with regard to life on Earth but multiple 'Galactic Cliques'. [150]  However, if artificial superintelligences dominate galactic life, and if it is true that such intelligences tend towards merged hegemonic behavior, then this would address the uniformity of motive flaw by dissuading rogue behavior. [151] 
 Analysis of the inter-arrival times between civilizations in the galaxy based on common astrobiological assumptions suggests that the initial civilization would have a commanding lead over the later arrivals. As such, it may have established what has been termed the zoo hypothesis through force or as a galactic or universal norm and the resultant "paradox" by a cultural founder effect with or without the continued activity of the founder. [152] 
 It is possible that a civilization advanced enough to travel between solar systems could be actively visiting or observing Earth while remaining undetected or unrecognized. [153] 
Earth is deliberately being isolated[edit]
A related idea to the zoo hypothesis is that, beyond a certain distance, the perceived universe is a simulated reality. The planetarium hypothesis [154]  speculates that beings may have created this simulation so that the universe appears to be empty of other life.
Alien life is already here, unacknowledged[edit]
A significant fraction of the population believes that at least some UFOs (Unidentified Flying Objects) are spacecraft piloted by aliens. [155]   [156]  While most of these are unrecognized or mistaken interpretations of mundane phenomena, there are those that remain puzzling even after investigation. The consensus scientific view is that although they may be unexplained, they do not rise to the level of convincing evidence. [157] 
 Similarly, it is theoretically possible that SETI groups are not reporting positive detections, or governments have been blocking signals or suppressing publication. This response might be attributed to security or economic interests from the potential use of advanced extraterrestrial technology. It has been suggested that the detection of an extraterrestrial radio signal or technology could well be the most highly secret information that exists. [158]  Claims that this has already happened are common in the popular press, [159]   [160]  but the scientists involved report the opposite experience—the press becomes informed and interested in a potential detection even before a signal can be confirmed. [161] 
 Regarding the idea that aliens are in secret contact with governments, David Brin writes, "Aversion to an idea, simply because of its long association with crackpots, gives crackpots altogether too much influence." [162] 
See also[edit]
  Aestivation hypothesis – Hypothesized solution to the Fermi paradox
 Anthropic principle – Philosophical principle about the occurrence of sapient life in the Universe
 Astrobiology – Science concerned with life in the universe
 Calculating God – 2000 novel by Robert J. Sawyer
 Fermi problem – Estimation problem in physics or engineering education
 Interstellar travel – Hypothetical travel between stars or planetary systems
 Panspermia – Hypothesis on the interstellar spreading of primordial life
 Rare Earth hypothesis – Hypothesis that complex extraterrestrial life is improbable and extremely rare
 The Martians (scientists) – Group of prominent Hungarian scientists
 Wow! signal – 1977 narrowband radio signal from SETI 
Notes[edit]
 
  ^  See Hart for an example of "no aliens are here", and Webb for an example of the more general "We see no signs of intelligence anywhere".
  ^  Eukaryotes also include plants, animals, fungi, and algae.
  ^  See, for example, the SETI Institute, The Harvard SETI Home Page Archived August 16, 2010, at the Wayback Machine, or The Search for Extra Terrestrial Intelligence at Berkeley Archived April 6, 2019, at the Wayback Machine
  ^  Pulsars are now attributed to neutron stars, and Seyfert galaxies to an end-on view of the accretion onto the black holes.
 
References[edit]
 
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^    a       b    Webb, Stephen (2015).  If the Universe Is Teeming with Aliens … WHERE IS EVERYBODY?: Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life . ISBN  978-0-387-95501-8 . Retrieved June 21, 2015.
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^    a       b    Ball, J (1973). "The zoo hypothesis". Icarus. 19 (3): 347–349. Bibcode:1973Icar...19..347B. doi:10.1016/0019-1035(73)90111-5.
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  ^  Baxter, Stephen (2001). "The Planetarium Hypothesis: A Resolution of the Fermi Paradox". Journal of the British Interplanetary Society. 54 (5/6): 210–216. Bibcode:2001JBIS...54..210B.
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  ^  A. Tough (1990). "A critical examination of factors that might encourage secrecy". Acta Astronautica. 21 (2): 97–102. Bibcode:1990AcAau..21...97T. doi:10.1016/0094-5765(90)90134-7.
  ^  Ashlee Vance (July 31, 2006). "SETI urged to fess up over alien signals". The Register. Archived from the original on April 2, 2007. Retrieved August 10, 2017.
  ^  Speigel, Lee (December 6, 2011). "UFO Hunters Keep Pressing White House For Answers Through 'We The People' Petitions". The Huffington Post. Archived from the original on April 15, 2013. Retrieved April 16, 2013.
  ^  G. Seth Shostak (2009).  Confessions of an Alien Hunter: A Scientist's Search for Extraterrestrial Intelligence . National Geographic. p. 17. ISBN  978-1-4262-0392-3 .
  ^  "The Great Silence: the Controversy . . " (15-page paper), Quarterly J. Royal Astron. Soc., David Brin, 1983, p. 299 bottom Archived April 11, 2019, at the Wayback Machine.
 
Further reading[edit]
  Rebecca Boyle and Quanta Magazine (March 10, 2019). "Moving Stars Might Speed the Spread of Alien Life".  The Atlantic . 
Ćirković, Milan [2] Archived May 5, 2021, at the Wayback Machine Why we downplay Fermi's paradox. Nautilus 
Ćirković, Milan The Great Silence: Science and Philosophy of Fermi's Paradox The Great Silence Oxford University Press 
 Crowe, Michael J. (2008). The Extraterrestrial Life Debate, Antiquity to 1915. University of Notre Dame Press. ISBN  978-0-268-02368-3 . 
 Forgan, Duncan H. (2019). Solving Fermi's Paradox. Cambridge University Press. ISBN  9781107163652 . 
 Michaud, Michael (2006).  Contact with Alien Civilizations: Our Hopes and Fears about Encountering Extraterrestrials . Copernicus Books. ISBN  978-0-387-28598-6 . 
  Zuckerman, Ben; Hart, Michael H. (1995). Extraterrestrials: Where Are They?. Cambridge University Press. ISBN  978-0-521-44803-1 .  
External links[edit]
   These audio files were created from a revision of this article dated 29 May 2008, and do not reflect subsequent edits. 
 Kestenbaum, David. "Three people grapple with the question, 'Are we alone?'", This American Life radio show, hosted by Ira Glass. This episode's first 22 minutes discusses the Fermi Paradox. See also the show's May 19, 2017 transcript.
   Overcome the Great Silence . Translated by Leonidovich Zaitsev, Aleksandr (Translation of the documentary ed.). 
 The Fermi Paradox — Where Are All The Aliens? (2015), Kurzgesagt – In a Nutshell
Fermi paradox - Wikipedia
https://en.wikipedia.org/wiki/Fermi_paradox
http://blog.practicalethics.ox.ac.uk/2022/03/the-aliens-are-coming/
Practical Ethics
2022-03-03
Paul D VanPelt says
<div><div>
	 
	 <div><p><a href="http://blog.practicalethics.ox.ac.uk/wp-content/uploads/2022/03/alienspaceship.jpg"></a></p><p>UFO against the sky. Free public domain CC0 photo</p></div>
<p><strong>By Charles Foster</strong></p>
<p><a href="https://www.theguardian.com/world/2022/feb/05/ufos-america-aliens-government-report">It’s said that 2022 is going to be a bumper year for UFO revelations</a>. Secret archives are going to be opened and the skies are going to be probed as never before for signs of extraterrestrial life.</p>
<p>This afternoon we might be presented with irrefutable evidence not just of life beyond the Earth, but of intelligences comparable in power and subtlety to our own. What then? Would it change our view of ourselves and the universe we inhabit? If so, how? Would it change our behaviour? If so how?</p>
<p>Much would depend, no doubt, on what we knew or supposed about the nature and intentions of the alien intelligences. If they seemed hostile, intent on colonising Planet Earth and enslaving us, our reactions would be fairly predictable. But what if the reports simply disclosed the existence of other intelligences, together with the fact that those intelligences knew about and were interested in us?</p>
<p>Many would feel relief that we were not alone. The night sky would seem a less scarily empty place, and our position, balanced on this ball of spinning rock, less precarious. Many would be glad, too, that the responsibilities associated with being the sole possessors of intelligence in the universe could now be shared with others. Some would be worried that, true to our recent form, we would seek to kill and exploit the aliens: to farm them, eat them, steal and patent their secrets, destroy their innocence, teach them the morality of the free market, and set them to work in sweat shops.</p>
<p>Others would see the aliens, however benign, as a threat to human supremacy. Many humans like to think of themselves as the best that the universe has birthed. They are the ones who, in their own ontological insecurity, tend to deny the consciousness and the moral significance of non-human animals, because (they think) to credit any non-human with any human attribute is to diminish humankind (by which they really mean themselves). These, I imagine, would address their insecurities by trying to become alien-farmers.</p>
<p>In the sphere of religion the revelation of alien intelligence <em>shouldn’t</em> have much of an effect. It should merely complete the Copernican Revolution. C. S. Lewis, in his Space Triology, dealt squarely and convincingly with the suggestion that Christianity would be challenged by extraterrestrial life.<sup>1</sup> Other religions, tending to be less anthropocentric than Christianity, have always been less troubled by the thought that there might be theologically significant organisms with bodies that do not look like the body of Jesus.</p>
<p>And yet, of course, the Copernican Revolution, though it might have triumphed in cosmology, has not triumphed in (particularly fundamentalist Christian) psychology. There are some would-be alien-ranchers who would try to deal with their theological queasiness at sharing the world with intelligent non-humans by declaring that so long as they were able to control the aliens, outer space was simply a suburb of the Earth, and the Earth continued to be, really, the centre of the universe. This would create a theological and psychological mandate for subduing the aliens.</p>
<p>Some, of course, would simply be fascinated. What sort of anatomy and physiology do the aliens have? And, perhaps most intriguing of all, what sort of <em>being </em>do they have? We tend to assume that our kind of <em>being </em>(embodied, sensual, machines plus souls or, in our more desolate moments, just advanced computers) is the only imaginable type of being. It is not necessarily so. The religious literature is full of accounts of other kinds. And wouldn’t we expect that radically different ontologies might mean radically different epistemologies too? The universe might turn out to be a much more interesting place than we suspect.</p>
<p>Most of us, though, will read with interest the article announcing the proof of alien life, and will then get on with making the kids’ tea, writing the long-overdue paper, ringing up a friend, or worrying about the plight of earthly refugees. I’m not at all sure that those aren’t the best responses.</p>
<p><strong><u>References</u></strong></p>
<p><strong><u> </u></strong><em>Out of the Silent Planet, Perelandra, </em>and <em>That Hideous Strength.</em></p>
	 
	 </div>||||I|||| Practical Ethics


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 « Three Observations about Justifying AI
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 The Aliens Are Coming

 Published March 3, 2022 | By Charles Foster

 UFO against the sky. Free public domain CC0 photo

 By Charles Foster

 It’s said that 2022 is going to be a bumper year for UFO revelations. Secret archives are going to be opened and the skies are going to be probed as never before for signs of extraterrestrial life.

 This afternoon we might be presented with irrefutable evidence not just of life beyond the Earth, but of intelligences comparable in power and subtlety to our own. What then? Would it change our view of ourselves and the universe we inhabit? If so, how? Would it change our behaviour? If so how?

 Much would depend, no doubt, on what we knew or supposed about the nature and intentions of the alien intelligences. If they seemed hostile, intent on colonising Planet Earth and enslaving us, our reactions would be fairly predictable. But what if the reports simply disclosed the existence of other intelligences, together with the fact that those intelligences knew about and were interested in us?

 Many would feel relief that we were not alone. The night sky would seem a less scarily empty place, and our position, balanced on this ball of spinning rock, less precarious. Many would be glad, too, that the responsibilities associated with being the sole possessors of intelligence in the universe could now be shared with others. Some would be worried that, true to our recent form, we would seek to kill and exploit the aliens: to farm them, eat them, steal and patent their secrets, destroy their innocence, teach them the morality of the free market, and set them to work in sweat shops.

 Others would see the aliens, however benign, as a threat to human supremacy. Many humans like to think of themselves as the best that the universe has birthed. They are the ones who, in their own ontological insecurity, tend to deny the consciousness and the moral significance of non-human animals, because (they think) to credit any non-human with any human attribute is to diminish humankind (by which they really mean themselves). These, I imagine, would address their insecurities by trying to become alien-farmers.

 In the sphere of religion the revelation of alien intelligence shouldn’t have much of an effect. It should merely complete the Copernican Revolution. C. S. Lewis, in his Space Triology, dealt squarely and convincingly with the suggestion that Christianity would be challenged by extraterrestrial life.1 Other religions, tending to be less anthropocentric than Christianity, have always been less troubled by the thought that there might be theologically significant organisms with bodies that do not look like the body of Jesus.

 And yet, of course, the Copernican Revolution, though it might have triumphed in cosmology, has not triumphed in (particularly fundamentalist Christian) psychology. There are some would-be alien-ranchers who would try to deal with their theological queasiness at sharing the world with intelligent non-humans by declaring that so long as they were able to control the aliens, outer space was simply a suburb of the Earth, and the Earth continued to be, really, the centre of the universe. This would create a theological and psychological mandate for subduing the aliens.

 Some, of course, would simply be fascinated. What sort of anatomy and physiology do the aliens have? And, perhaps most intriguing of all, what sort of being do they have? We tend to assume that our kind of being (embodied, sensual, machines plus souls or, in our more desolate moments, just advanced computers) is the only imaginable type of being. It is not necessarily so. The religious literature is full of accounts of other kinds. And wouldn’t we expect that radically different ontologies might mean radically different epistemologies too? The universe might turn out to be a much more interesting place than we suspect.

 Most of us, though, will read with interest the article announcing the proof of alien life, and will then get on with making the kids’ tea, writing the long-overdue paper, ringing up a friend, or worrying about the plight of earthly refugees. I’m not at all sure that those aren’t the best responses.

 References

 Out of the Silent Planet, Perelandra, and That Hideous Strength.

 * 
 * 
 * 
 Posted in Bioethics, Charles Foster's Posts, Current Affairs, Decision Making, Epistemic Ethics, Ethics, Reflections, Religion, Science

 23 Responses to The Aliens Are Coming

 * Paul D. VanPelt says:
 March 3, 2022 at 11:59 am

 There is an awful lot of hoopla about an awful lot of things now. Aliens had to re-emerge, mas o menos, But, to mention them in the same breath with artificial intelligence seems presumptive, if not outright counterintuitive. I mean, if we suppose they (aliens) are life forms, it feels insulting to bring them into an AI discussion. The notion of something we have called human consciousness is also taking a beating…never mind that we cannot even agree on what it is or whether it exists.: some are now characterizing it as hallucination. Presumptions upon presumptions. I have been considering and evaluating a concept I have called contextual reality—sorta like an idea John Perry floated a number of years ago (levels-of-reality). The premise is relatively simple. Reality changes, over time, with what we know, or, in a more modern context, newer facts and information override older notions of what is real and what is not. This is tricky, however. Because interest groups, believing their ideas sound, are fooled by faulty facts and/or erroneous information. This manifests in our social institutions, mass and popular culture for example. I contend it has also wrought havoc in our political system.
 More later, I hope…

 * Charles Foster says:
 March 3, 2022 at 12:11 pm

 Many thanks, Paul. But I said nothing at all about artificial intelligence in this post.

 * Paul D. VanPelt says:
 March 3, 2022 at 2:20 pm

 Dear Mr. Foster:
 Point taken. You did not write about consciousness or reality either. My remarks were not a critique of your article; were not intended as such anyway. I was drawing comparisons among certain concepts and ideas, either currently or perennially popular in philosophy and neuroscience, good, bad or indifferent. And introducing a notion of my own regarding our collective views on reality. Hope this clarifies my intention.
 Warmly,
 PDV.

 * Keith Tayler says:
 March 3, 2022 at 7:18 pm

 This may be of some interest to you as it shows that speculation about alien life has a long and what we might call an almost ‘matter of fact’ history.

 I have a copy of the 11th ed. (1799) of Richard Brookes’, ‘The General Gazetteer or, Compendious Geographical Dictionary’ (First published 1750). In the Introduction Brookes gives a brilliant little description of 18th century cosmology which he concludes with a speculation about the possibility of intelligent life on other planets.

 ‘It cannot be imagined, therefore, that the omnipotent Creator, who acts with infinite wisdom, and never acts in vain, shoud have created so many glorious suns, fitted for so many important purposes, and placed at such distances from each other, without suitable objects sufficiently near them to be benefited by their influence. On the contrary, it is reasonable to conclude, that they were created for the same purposes with our Sun; to bestow light, heat, and vegetation, on a certain number of planets revolving round them. And, from analogy we may infer, that all these innumerable systems are with equal wisdom contrived for the accommodation of rational inhabitants; perhaps of still higher orders of intelligent beings, all capable, in the different scales of existence, of a perpetual progression in knowledge and virtue, in perfection and felicity.’

 The only mention of ‘Omnipotence’ (not Creator or God) before this passage is the usual deist move of how the immensity and grandeur of universe must ‘exalt our ideas of the wonders of Omnipotence, and the inconceivable extent of the creation’. We should not forget that Brookes’ remarkable argument by analogy was in a best selling *factual* gazetteer that was in continuous print for nearly seventy years.

 + Andy says:
 May 17, 2022 at 1:33 am

 And it should be noted that Richard Brookes’ gazetteer is far from unique among works in the mid-18th century discussing intelligent life on other planets; there is at least one historical monograph on this subject, Michael J. Crowe’s The Extraterrestrial Life Debate 1750–1900: The Idea of a Plurality of Worlds from Kant to Lowell (Cambridge University Press, 1986). Among many works from the same time period, Crowe quotes Benjamin Franklin, author of the “Poor Richard” almanacs, who in 1728 expressed ideas very similar to those in the passage above from Brookes’ gazetteer. Crowe notes that Franklin was “far from being the only almanac author advocating pluralism” of inhabited planets; others in the 18th century include Nathaniel Ames, Benjamin West (pseudonym Isaac Bickerstaff), Samuel Ellsworth, Benjamin Banneker, and Philip Freneau.

 As the title of Crowe’s book suggests, Kant was famously confident about extraterrestrials. Kant made regular references to aliens throughout his works, and Peter Szendy’s book Kant in the Land of Extraterrestrials: Cosmopolitical Philosofictions (Fordham University Press, 2013) goes so far as to give aliens a “central role” in Kant’s thought, connecting alien personhood with Kant’s interest in the possibility of perpetual peace on Earth.

 Kant’s enthusiasm for aliens was even mentioned in Bertrand Russell’s A History of Western Philosophy, and its Connection with Political and Social Circumstances from the Earliest Times to the Present Day (1945), where Russell threw some cold water on Kant’s enthusiasm:

 “The most important of [Kant’s] scientific writings is his General Natural History and Theory of the Heavens (1755), which anticipates Laplace’s nebular hypothesis, and sets forth a possible origin of the solar system. Parts of this work have a remarkable Miltonic sublimity. It has the merit of inventing what proved a fruitful hypothesis, but it does not, as Laplace did, advance serious arguments in its favour. In parts it is purely fanciful, for instance in the doctrine that all planets are inhabited, and that the most distant planets have the best inhabitants—a view to be praised for its terrestrial modesty, but not supported by any scientific grounds.”

 o Keith Tayler says:
 May 17, 2022 at 8:54 am

 Andy. Thanks for this. I vaguely remember Kant’s interest in extraterrestrials as it was I recall a topic Gerd Buchdahl covered in his Kant lectures. These were well before the Michael Crowe and Peter Szendy books you mention which I shall track down. Russell threw a lot of cold water over Kant but I forget this drenching. Thanks again.

 - Paul D. Van Pelt says:
 May 17, 2022 at 11:32 am

 Good job, Mr. Tayler. Others who have commented here were not as thoughtful on this topic I try to keep an open mind.

 * Paul D. VanPelt says:
 March 3, 2022 at 9:05 pm

 Thanks, but no. Someone else may benefit from your comments. I am not interested.

 + Keith Tayler says:
 March 3, 2022 at 9:58 pm

 I did not hit ‘reply’, so I was addressing Charles not you. I did not post *my* comments, I posted an 18th century comment by Brookes on alien life which I thought might interest Charles or anyone else with the slightest interest in intellectual history and/or the possibility of alien life.

 I must say your “comments” looks like something an AI system might generate.

 * j harrison says:
 March 3, 2022 at 11:18 pm

 Prescient – “…rational inhabitants; perhaps of still higher orders of intelligent beings, all capable, in the different scales of existence, of a perpetual progression in knowledge and virtue, in perfection and felicity.”

 * Charles Foster says:
 March 4, 2022 at 7:48 am

 Keith: very many thanks. Fascinating.

 * Mathew says:
 March 4, 2022 at 1:16 pm

 Aliens certainly exist, proving to be appearing in public more and more often. The question is, will it come with good intentions or bad?

 * MC says:
 March 31, 2022 at 9:28 am

 Interesting.

 + MC says:
 May 19, 2022 at 12:42 pm

 I am still following this blog, mainly because Mr. Foster had me at “It’s said that 2022 is going to be a bumper year for UFO revelations.” As a school teacher and follower of Christ, I am always seeking new information and enlightenment to broaden my horizons on education and spirituality. God is so mysterious and powerful that there has to be more to everything beyond the human experiences.
 Thank you,
 Mary

 * Ria says:
 April 1, 2022 at 11:10 am

 Humans are so fed up with their lives

 * Marte says:
 April 6, 2022 at 9:15 pm

 @Ria It’s the other way around; THEY are fed up with humans, and the way “we” (sorry, but I’m not a sheep..) trash this “world”, both mentally and in any other way. They will get rid of bad energy, and darkness, and I thank them, from the bottom of my heart.. Finally <3

 * jaydo says:
 April 28, 2022 at 5:26 pm

 are they real

 * jewel says:
 April 28, 2022 at 5:28 pm

 yes uwu

 * Onwodi nonso says:
 May 11, 2022 at 7:18 pm

 Wow so I am actually going to see an alien won’t that be cool

 * Itskicken says:
 May 16, 2022 at 2:41 pm

 Mental insidiblety is a problem slap yourself wake from your dream alliens are gov.deceptions

 + pat laurie says:
 May 19, 2022 at 7:50 pm

 MANMADEDEMONS.. EVEN YOURE LOVED ONES WILL GO COLD.. how do you not not ur talking to another bean from outter earth?? RISKY..

 * iyaana says:
 May 17, 2022 at 11:04 pm

 i don’t think this is true it’s fake fr bruh theyy cap

 + pat laurie says:
 May 19, 2022 at 8:00 pm

 EVEN FISH IN THE SEA ALMOST IS HUMAN >>SOME COULD PASS AS ALIENS.. its funny but true..

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 Copyright © the University of Oxford 2023. All Rights Reserved.

UFO against the sky. Free public domain CC0 photo 
 By Charles Foster 
 It’s said that 2022 is going to be a bumper year for UFO revelations. Secret archives are going to be opened and the skies are going to be probed as never before for signs of extraterrestrial life.
This afternoon we might be presented with irrefutable evidence not just of life beyond the Earth, but of intelligences comparable in power and subtlety to our own. What then? Would it change our view of ourselves and the universe we inhabit? If so, how? Would it change our behaviour? If so how?
Much would depend, no doubt, on what we knew or supposed about the nature and intentions of the alien intelligences. If they seemed hostile, intent on colonising Planet Earth and enslaving us, our reactions would be fairly predictable. But what if the reports simply disclosed the existence of other intelligences, together with the fact that those intelligences knew about and were interested in us?
Many would feel relief that we were not alone. The night sky would seem a less scarily empty place, and our position, balanced on this ball of spinning rock, less precarious. Many would be glad, too, that the responsibilities associated with being the sole possessors of intelligence in the universe could now be shared with others. Some would be worried that, true to our recent form, we would seek to kill and exploit the aliens: to farm them, eat them, steal and patent their secrets, destroy their innocence, teach them the morality of the free market, and set them to work in sweat shops.
Others would see the aliens, however benign, as a threat to human supremacy. Many humans like to think of themselves as the best that the universe has birthed. They are the ones who, in their own ontological insecurity, tend to deny the consciousness and the moral significance of non-human animals, because (they think) to credit any non-human with any human attribute is to diminish humankind (by which they really mean themselves). These, I imagine, would address their insecurities by trying to become alien-farmers.
In the sphere of religion the revelation of alien intelligence shouldn’t have much of an effect. It should merely complete the Copernican Revolution. C. S. Lewis, in his Space Triology, dealt squarely and convincingly with the suggestion that Christianity would be challenged by extraterrestrial life.1 Other religions, tending to be less anthropocentric than Christianity, have always been less troubled by the thought that there might be theologically significant organisms with bodies that do not look like the body of Jesus.
And yet, of course, the Copernican Revolution, though it might have triumphed in cosmology, has not triumphed in (particularly fundamentalist Christian) psychology. There are some would-be alien-ranchers who would try to deal with their theological queasiness at sharing the world with intelligent non-humans by declaring that so long as they were able to control the aliens, outer space was simply a suburb of the Earth, and the Earth continued to be, really, the centre of the universe. This would create a theological and psychological mandate for subduing the aliens.
Some, of course, would simply be fascinated. What sort of anatomy and physiology do the aliens have? And, perhaps most intriguing of all, what sort of being do they have? We tend to assume that our kind of being (embodied, sensual, machines plus souls or, in our more desolate moments, just advanced computers) is the only imaginable type of being. It is not necessarily so. The religious literature is full of accounts of other kinds. And wouldn’t we expect that radically different ontologies might mean radically different epistemologies too? The universe might turn out to be a much more interesting place than we suspect.
Most of us, though, will read with interest the article announcing the proof of alien life, and will then get on with making the kids’ tea, writing the long-overdue paper, ringing up a friend, or worrying about the plight of earthly refugees. I’m not at all sure that those aren’t the best responses.
  References  
     Out of the Silent Planet, Perelandra, and That Hideous Strength.
Practical Ethics
https://blog.practicalethics.ox.ac.uk/2022/03/the-aliens-are-coming/
https://www.loc.gov/collections/finding-our-place-in-the-cosmos-with-carl-sagan/articles-and-essays/life-on-other-worlds/ufos-and-aliens-among-us
UFOs and Aliens Among Us | Life on Other Worlds | Articles and Essays | Finding Our Place in the Cosmos: From Galileo to Sagan and Beyond | Digital Collections | Library of Congress
2015-01-01
None
<div><div>
	<p>In the 1940s and 50s reports of "flying saucers" became an American cultural phenomena. Sightings of strange objects in the sky became the raw materials for Hollywood to present visions of potential threats. <a href="/item/2002720224/">Posters for films, like <em>Earth vs. the Flying Saucers</em></a> from 1956 illustrate these fears.  Connected to ongoing ideas about life <a href="/collections/finding-our-place-in-the-cosmos-with-carl-sagan/articles-and-essays/life-on-other-worlds/peoples-and-creatures-of-the-moon">on</a> the Moon, the <a href="/collections/finding-our-place-in-the-cosmos-with-carl-sagan/articles-and-essays/life-on-other-worlds/seeing-and-interpreting-martian-oceans-and-canals/">canals</a> on Mars, and ideas about Martian Civilizations, flying saucers have come to represent the hopes and fears of the modern world. </p>
	<p>Are these alleged visitors from other worlds peaceful and benevolent or would they attack and destroy humanity? The destructive power of the Atomic bomb called into question the <a href="/collections/finding-our-place-in-the-cosmos-with-carl-sagan/articles-and-essays/carl-sagan-and-the-tradition-of-science/sagans-youth-and-the-progressive-promise-of-space">progressive potential of technology</a>. Fear of the possibilities for destruction in the Cold War-era proved fertile ground for terrestrial anxieties to manifest visions of flying saucers and visitors from other worlds who might be hidden among us in plain sight.</p>
	<h2>Aliens Among us and Fears of the Other</h2>
	<p>If UFOs were visiting our world, where were these extraterrestrials? Could they be hidden among us? Comic books and television illustrates how the possibility of extraterrestrial visitors reflected anxieties of that era. </p>
	<p>The 1962 comic <a href="/item/acd2008000008/PP/"><em>There are Martians Among Us</em></a>, from Amazing Fantasy #15, illustrates the way fear of extraterrestrials could reflect Cold War anxieties. In the comic, a search party gathers around a landed alien craft, but it can find no sign of alien beings. Radio announcers warn those nearby to stay indoors. The action shifts to a husband and wife as he prepares to leave their home despite a television announcer's warning to remain indoors. As he waves goodbye he reminds his wife to stay inside. The wife however decides to slip out to the store and is attacked and dragged off. The husband returns home and finding it empty runs towards the telephone in a panic. In a twist, the anxious husband reveals that he and his wife are the Martians. </p>
	<p>The fear that there might be alien enemies in our midst resonates with fears of Soviets and communists from the McCarthy era. Ultimately, in this story, the humans are the ones who accost and capture the alien woman. The shift in perspective puts the humans in the position of the monsters.</p>
	<h2>UFOs as Contemporary Folklore</h2>
	<p>Aside from depictions of UFOs in media, UFOs are also part of American folk culture. Ideas of aliens and flying saucers are a part of the mythology of America. You can find documentation of these kinds of experiences in folk life collections. An interview with Howard Miller about hunting and hound dogs, collected as part of <a href="/collections/folklife-and-landscape-in-southern-west-virginia/about-this-collection/overview"><em>Tending the Commons: Folklife and Landscape in Southern West Virginia</em></a> collection, documents an individual's experience with a potential UFO sighting.</p>
	<p>In <a href="/item/cmns001679">A mysterious light</a>, a segment of an ethnographic interview, Miller describes a strange light he saw once while hunting with his dogs in 1966 "All at once it was daylight, and I looked up to see what happened. There was a light about that big, going up, drifting up the hill. When I looked and seen it just faded out. I've been in the Marines, and know what airplane lights look like, and it was too big for that." When asked if he knew what it was he offered, "I don't know what it was" but went on to explain, "If there is any such thing as a UFO that's what that was." This unexplained light on a walk in the woods is typical of many stories of these kinds of encounters. It's not only the media that tells stories and represents these kinds of ideas, documentation of the experiences and stories Americans tell each other is similarly important for understanding and interpreting what UFOs meant to 20th century America.</p>
	<h2>Skepticism of UFOs and Alien Encounters</h2>
	<p>Scientists and astronomers express varying degrees of enthusiasm for the possibility of intelligent life in the universe. However, scientists generally dismiss the idea that there are aliens visiting Earth. In <a href="/item/cosmos000044"><em>Pale Blue Dot: A Vision of the Human Future in Space</em></a>, Carl Sagan reviews the possibilities of alien visitors to Earth, and suggests that there is good reason to be skeptical of them. Much of Sagan's work focuses on debunking folk stories and beliefs and tries to encourage more rigorous and skeptical thought. He similarly discussed criticism of beliefs in alien visitors in his earlier book, <a href="/item/cosmos000005"><em>Demon Haunted World: Science as a Candle in the Dark</em></a>.</p>
	<p>This zealous criticism of belief in UFOs from Sagan, who was well known for his speculative ideas about the likelihood of alien civilizations, might seem to be a contradiction. Sagan himself had even speculated on the possibilities of visits by ancient aliens in his essay from the early 60s <a href="/item/cosmos000012"><em>Direct Contact among Galactic Civilizations by Relativistic Interstellar Spaceflight</em></a>.</p>
	<p>How do we reconcile Sagan the skeptic with the imaginative Sagan? Far from a contradiction, these two parts of Sagan's perspective offer a framework for understanding him and the interchange between science and myth about life on other worlds. Skepticism and speculative imagination come together as two halves of the whole. It's essential to entertain and explore new ideas, however strange, while at the same time testing and evaluating the validity of those ideas. </p>
	</div>||||I||||Top of page

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 UFOs and Aliens Among Us

 In the 1940s and 50s reports of "flying saucers" became an American cultural phenomena. Sightings of strange objects in the sky became the raw materials for Hollywood to present visions of potential threats. Posters for films, like Earth vs. the Flying Saucers from 1956 illustrate these fears. Connected to ongoing ideas about life on the Moon, the canals on Mars, and ideas about Martian Civilizations, flying saucers have come to represent the hopes and fears of the modern world.

 Are these alleged visitors from other worlds peaceful and benevolent or would they attack and destroy humanity? The destructive power of the Atomic bomb called into question the progressive potential of technology. Fear of the possibilities for destruction in the Cold War-era proved fertile ground for terrestrial anxieties to manifest visions of flying saucers and visitors from other worlds who might be hidden among us in plain sight.

 Aliens Among us and Fears of the Other

 If UFOs were visiting our world, where were these extraterrestrials? Could they be hidden among us? Comic books and television illustrates how the possibility of extraterrestrial visitors reflected anxieties of that era.

 The 1962 comic There are Martians Among Us, from Amazing Fantasy #15, illustrates the way fear of extraterrestrials could reflect Cold War anxieties. In the comic, a search party gathers around a landed alien craft, but it can find no sign of alien beings. Radio announcers warn those nearby to stay indoors. The action shifts to a husband and wife as he prepares to leave their home despite a television announcer's warning to remain indoors. As he waves goodbye he reminds his wife to stay inside. The wife however decides to slip out to the store and is attacked and dragged off. The husband returns home and finding it empty runs towards the telephone in a panic. In a twist, the anxious husband reveals that he and his wife are the Martians.

 The fear that there might be alien enemies in our midst resonates with fears of Soviets and communists from the McCarthy era. Ultimately, in this story, the humans are the ones who accost and capture the alien woman. The shift in perspective puts the humans in the position of the monsters.

 UFOs as Contemporary Folklore

 Aside from depictions of UFOs in media, UFOs are also part of American folk culture. Ideas of aliens and flying saucers are a part of the mythology of America. You can find documentation of these kinds of experiences in folk life collections. An interview with Howard Miller about hunting and hound dogs, collected as part of Tending the Commons: Folklife and Landscape in Southern West Virginia collection, documents an individual's experience with a potential UFO sighting.

 In A mysterious light, a segment of an ethnographic interview, Miller describes a strange light he saw once while hunting with his dogs in 1966 "All at once it was daylight, and I looked up to see what happened. There was a light about that big, going up, drifting up the hill. When I looked and seen it just faded out. I've been in the Marines, and know what airplane lights look like, and it was too big for that." When asked if he knew what it was he offered, "I don't know what it was" but went on to explain, "If there is any such thing as a UFO that's what that was." This unexplained light on a walk in the woods is typical of many stories of these kinds of encounters. It's not only the media that tells stories and represents these kinds of ideas, documentation of the experiences and stories Americans tell each other is similarly important for understanding and interpreting what UFOs meant to 20th century America.

 Skepticism of UFOs and Alien Encounters

 Scientists and astronomers express varying degrees of enthusiasm for the possibility of intelligent life in the universe. However, scientists generally dismiss the idea that there are aliens visiting Earth. In Pale Blue Dot: A Vision of the Human Future in Space, Carl Sagan reviews the possibilities of alien visitors to Earth, and suggests that there is good reason to be skeptical of them. Much of Sagan's work focuses on debunking folk stories and beliefs and tries to encourage more rigorous and skeptical thought. He similarly discussed criticism of beliefs in alien visitors in his earlier book, Demon Haunted World: Science as a Candle in the Dark.

 This zealous criticism of belief in UFOs from Sagan, who was well known for his speculative ideas about the likelihood of alien civilizations, might seem to be a contradiction. Sagan himself had even speculated on the possibilities of visits by ancient aliens in his essay from the early 60s Direct Contact among Galactic Civilizations by Relativistic Interstellar Spaceflight.

 How do we reconcile Sagan the skeptic with the imaginative Sagan? Far from a contradiction, these two parts of Sagan's perspective offer a framework for understanding him and the interchange between science and myth about life on other worlds. Skepticism and speculative imagination come together as two halves of the whole. It's essential to entertain and explore new ideas, however strange, while at the same time testing and evaluating the validity of those ideas.

 Movies like Earth vs. The Flying Saucers capitalized on and further spread ideas about the possibilities of flying saucers visiting and potentially attacking the Earth. Flying Saucers Attack! 1956. Prints and Photographs Division

 Alien visitors to earth provide a powerful means to express fear of others. There are Martians among us! 1962. Prints and Photographs Division

 Howard Miller, describing his experiences hunting with hound dogs in West Virginia. In one section of the ethnographic interview he recounts observations of a mysterious light. Howard Miller telling a story about his 'coon hounds. 1995. Archive of Folk Culture, American Folklife Center

 In this hand written edit to a draft of his book Pale Blue Dot, Carl Sagan suggests that stories of alien abduction are primarily part of American culture. Image 84, Pale Blue Dot: A Vision of the Human Future in Space: Draft 1997. Manuscript Division

 Here Sagan offers a series of examples of how to explain away UFO sightings, noting some are psychological aberrations while others are hoaxes. Image 60, Pale Blue Dot: A Vision of the Human Future in Space: Draft Manuscript Division

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In the 1940s and 50s reports of "flying saucers" became an American cultural phenomena. Sightings of strange objects in the sky became the raw materials for Hollywood to present visions of potential threats. Posters for films, like Earth vs. the Flying Saucers  from 1956 illustrate these fears.  Connected to ongoing ideas about life on the Moon, the canals on Mars, and ideas about Martian Civilizations, flying saucers have come to represent the hopes and fears of the modern world. 
	Are these alleged visitors from other worlds peaceful and benevolent or would they attack and destroy humanity? The destructive power of the Atomic bomb called into question the progressive potential of technology. Fear of the possibilities for destruction in the Cold War-era proved fertile ground for terrestrial anxieties to manifest visions of flying saucers and visitors from other worlds who might be hidden among us in plain sight.
	Aliens Among us and Fears of the Other
	If UFOs were visiting our world, where were these extraterrestrials? Could they be hidden among us? Comic books and television illustrates how the possibility of extraterrestrial visitors reflected anxieties of that era. 
	The 1962 comic  There are Martians Among Us , from Amazing Fantasy #15, illustrates the way fear of extraterrestrials could reflect Cold War anxieties. In the comic, a search party gathers around a landed alien craft, but it can find no sign of alien beings. Radio announcers warn those nearby to stay indoors. The action shifts to a husband and wife as he prepares to leave their home despite a television announcer's warning to remain indoors. As he waves goodbye he reminds his wife to stay inside. The wife however decides to slip out to the store and is attacked and dragged off. The husband returns home and finding it empty runs towards the telephone in a panic. In a twist, the anxious husband reveals that he and his wife are the Martians. 
	The fear that there might be alien enemies in our midst resonates with fears of Soviets and communists from the McCarthy era. Ultimately, in this story, the humans are the ones who accost and capture the alien woman. The shift in perspective puts the humans in the position of the monsters.
	UFOs as Contemporary Folklore
	Aside from depictions of UFOs in media, UFOs are also part of American folk culture. Ideas of aliens and flying saucers are a part of the mythology of America. You can find documentation of these kinds of experiences in folk life collections. An interview with Howard Miller about hunting and hound dogs, collected as part of  Tending the Commons: Folklife and Landscape in Southern West Virginia  collection, documents an individual's experience with a potential UFO sighting.
	In A mysterious light, a segment of an ethnographic interview, Miller describes a strange light he saw once while hunting with his dogs in 1966 "All at once it was daylight, and I looked up to see what happened. There was a light about that big, going up, drifting up the hill. When I looked and seen it just faded out. I've been in the Marines, and know what airplane lights look like, and it was too big for that." When asked if he knew what it was he offered, "I don't know what it was" but went on to explain, "If there is any such thing as a UFO that's what that was." This unexplained light on a walk in the woods is typical of many stories of these kinds of encounters. It's not only the media that tells stories and represents these kinds of ideas, documentation of the experiences and stories Americans tell each other is similarly important for understanding and interpreting what UFOs meant to 20th century America.
	Skepticism of UFOs and Alien Encounters
	Scientists and astronomers express varying degrees of enthusiasm for the possibility of intelligent life in the universe. However, scientists generally dismiss the idea that there are aliens visiting Earth. In  Pale Blue Dot: A Vision of the Human Future in Space , Carl Sagan reviews the possibilities of alien visitors to Earth, and suggests that there is good reason to be skeptical of them. Much of Sagan's work focuses on debunking folk stories and beliefs and tries to encourage more rigorous and skeptical thought. He similarly discussed criticism of beliefs in alien visitors in his earlier book,  Demon Haunted World: Science as a Candle in the Dark .
	This zealous criticism of belief in UFOs from Sagan, who was well known for his speculative ideas about the likelihood of alien civilizations, might seem to be a contradiction. Sagan himself had even speculated on the possibilities of visits by ancient aliens in his essay from the early 60s  Direct Contact among Galactic Civilizations by Relativistic Interstellar Spaceflight .
	How do we reconcile Sagan the skeptic with the imaginative Sagan? Far from a contradiction, these two parts of Sagan's perspective offer a framework for understanding him and the interchange between science and myth about life on other worlds. Skepticism and speculative imagination come together as two halves of the whole. It's essential to entertain and explore new ideas, however strange, while at the same time testing and evaluating the validity of those ideas.
UFOs and Aliens Among Us | Life on Other Worlds | Articles and Essays | Finding Our Place in the Cosmos: From Galileo to Sagan and Beyond | Digital Collections | Library of Congress
https://www.loc.gov/collections/finding-our-place-in-the-cosmos-with-carl-sagan/articles-and-essays/life-on-other-worlds/ufos-and-aliens-among-us
https://www.washingtonpost.com/outlook/ufo-report-aliens-seti/2021/06/09/1402f6a8-c899-11eb-81b1-34796c7393af_story.html
Perspective | Contacting aliens could end all life on earth. Let’s stop trying.
2021-06-09
Mark Buchanan
<div><div><div><p>In April 2020, the Defense Department <a href="https://www.defense.gov/Newsroom/Releases/Release/Article/2165713/statement-by-the-department-of-defense-on-the-release-of-historical-navy-videos/">released videos</a> recorded by infrared cameras on U.S. Navy aircraft that documented the planes’ encounters with a variety of  “unidentified aerial phenomena.” Pilots reported seeing objects flying across the sky at hypersonic speeds and changing direction almost instantaneously, capabilities far beyond that of any known aircraft. </p></div><p>What were the pilots seeing? Bizarre atmospheric phenomena? Alien spacecraft? Something else? Several branches of the government<a href="https://www.nbcnews.com/science/science-news/ufos-are-make-way-us-senate-know-rcna973"> have been investigating</a> the events, motivated in part by concern that adversaries such as Russia or China might have made some spectacular technological advance, and later this month, the government plans to publish a report revealing what they know. Reportedly, the government will say there’s <a href="https://www.washingtonpost.com/national-security/report-does-not-confirm-or-rule-out-ufos-in-unexplained-aerial-events/2021/06/03/87654f70-c4d3-11eb-8c34-f8095f2dc445_story.html?itid=lk_inline_manual_3">no proof of extraterrestrial activity</a>, but that the incidents remain unexplained.</p><p>Chances are, though, that we should all be grateful that we don’t yet have any evidence of contact with alien civilizations. Attempting to communicate with extraterrestrials, if they do exist, could be extremely dangerous for us. We need to figure out whether it’s wise — or safe — and how to handle such attempts in an organized manner.</p><div><figure><figcaption><p>The USS Russell and the USS Omaha captured video appearing to show UFOs flying, hovering and splashing into the ocean. (Video: Jeremy Corbell)</p></figcaption></figure></div><p>Some scientific circles have already been debating questions around whether to try to contact other civilizations. It’s a topic of profound importance for the entire planet. For 60 years, scientists have been searching with radio telescopes, listening in for possible signals coming from other civilizations on planets orbiting distant stars. These efforts have largely been organized by the <a href="https://www.seti.org/">SETI institute</a> in California — the acronym stands for Search for ExtraTerrestrial Intelligence — and so far, they’ve had no success. Getting impatient, <a href="http://meti.org/">some other scientists</a> are now pushing for a more active program — METI, for Messaging ExtraTerrestrial Intelligence — that wouldn’t just listen, but actually send out powerful messages toward other stars, seeking to make contact.</p><p>The search for aliens has reached a stage of technological sophistication and associated risk that it needs strict regulation at national and international levels. Without oversight, even one person — with access to powerful transmitting technology — could take actions affecting the future of the entire planet.</p><p>That’s because any aliens we ultimately encounter will likely be far more technologically advanced than we are, for a simple reason: Most stars in our galaxy are much older than the sun. If civilizations arise fairly frequently on some planets, then there ought to be many civilizations in our galaxy millions of years more advanced than our own. Many of these would likely have taken significant steps to begin exploring and possibly colonizing the galaxy.</p><p>Hence, it’s a profound mystery — known as <a href="https://en.wikipedia.org/wiki/Fermi_paradox">the Fermi Paradox</a>, after the Italian physicist Enrico Fermi — why we haven’t yet seen any such aliens. Many resolutions of the paradox have been proposed, among them the suggestion that all civilizations, once reaching sufficient technological capacity, eventually destroy themselves. Or perhaps aliens are so alien and unlike humans that we simply cannot interact with them.</p><p>More alarming is the possibility that alien civilizations are remaining out of contact because they know something: that sending out signals is catastrophically risky. Our history on Earth has given us many examples of what can happen when civilizations with unequal technology meet — generally, the technologically more advanced has destroyed or enslaved the other. A cosmic version of this reality might have convinced many alien civilizations to remain silent. Exposing yourself is an invitation to be preyed upon and devoured.</p><p><a href="https://www.nature.com/articles/nphys3852">I’ve written</a> about METI in the past, suggesting such activity takes a huge risk for very little gain. But these concerns don’t convince supporters of trying it, who have <a href="https://www.nature.com/articles/nphys3897.epdf?author_access_token=xVbYXTjQLe1keDJgyRgAtdRgN0jAjWel9jnR3ZoTv0MUBFM4JewRDNYiUp1JrAsJMNJt2SUlTaREDPqYjiPlx0ILnhiGZwA4l_ie5tUD1GaZds2R-WkpsTNfv6oOVAXF">some counterarguments</a>. Douglas Vakoch of METI International argues that it’s unrealistic to worry about the danger of an alien invasion. We have, after all, been sending radio and television emissions into space for a century, and a civilization far more advanced than our own will probably have already detected these. If they wanted to invade, they already would have.</p><p>He also argues that, in assessing risks, it’s important not only to consider the risk coming from taking an action, but also from not taking that action. Our world faces a number of potentially existential threats, including global warming and destabilization of the environment, and it’s possible that far more advanced civilizations may have already faced these issues and found solutions. If we don’t send out signals, Vakosh writes, we risk “missing guidance that could enhance our own civilization’s sustainability.” It’s also conceivable, he suggests, that we’re making a spectacular misjudgment — and some super-advanced alien civilization may attack us precisely because we haven’t reached out.</p><p>For obvious reasons, much of the thinking about these issues has to be rather speculative. The best way forward, perhaps, is to broaden the discussion. If all of humanity is exposed to the possible consequences trying to contact alien civilizations, then more people should be involved in making decisions about what is wise and what isn’t. It shouldn’t be left to a handful of radio astronomers.</p><p>One vocal critic of the idea of reaching out to aliens proactively — astronomer John Gertz of SETI — has developed <a href="https://arxiv.org/abs/1701.08422">proposals</a> to move toward more inclusive public consideration of these activities. What we need, he suggests, are laws and international treaties to govern more explicit contact attempts. Without prior broad agreement from some globally representative body, Gertz says, contacting extraterrestrials should be considered “as the reckless endangerment of all mankind, and be absolutely proscribed with criminal consequences, presumably as exercised at the national level, or administered through the International Court of Justice in The Hague.”</p><p>Currently, no such prohibitions exist. Some <a href="https://www.seti.org/protocols-eti-signal-detection">informal protocols</a> for interacting with alien civilizations have been adopted by researchers involved in SETI, but these are far from legally binding governmental regulations. That’s mostly because, up to now, talking about meeting or contacting aliens has seemed widely speculative — if not a little deranged — despite the apparent scientific plausibility of such an event.</p><p>It’s not easy to weigh the pros and cons of activities around which so much remains unknown. We don’t know if there are any aliens. They might be friendly. They might not be. Given the potential risks involved with trying to make contact, perhaps it would be safer and wiser to just wait — we can always reach out later, and meanwhile, our abilities to do passive listening are rapidly growing more powerful.</p><p>In 2015, SETI launched a new 10-year program called <a href="https://spectrum.ieee.org/aerospace/astrophysics/breakthrough-listen-seti">Breakthrough Listen</a>, funded by a $100 million donation from Israeli-Russian billionaire Yuri Milner. As a result, SETI is now recording more signals than ever before, over a frequency range some tenfold larger, and bringing more computational power to bear on analyzing the recorded signals. It’s impossible to know how close or far from making a discovery we may be, but Gertz estimates that our chances are at least 100 times greater than they used to be.</p><p>The search is also benefiting from astronomers’ knowledge of exoplanets — planets in orbit around stars other than the sun. Since the first exoplanet was found in 1992, we’ve identified nearly 5,000 more, and the rate of discovery is accelerating. Each one give SETI researchers new promising targets to scrutinize.</p><p>Personally, all of this makes me dead-set against any experimentation with attempting to contact other civilizations. Why take cosmic risks when we may have a far safer pathway to discovering them, if they’re out there? Of course, even listening comes with some potentially fraught governance issues also: If and when someone really identifies an alien signal, we’ll need to decide if we should reply — and if so, how. Surely such an act — putting all of humanity at risk — ought to be the result of some collective decision. But there’s no mechanism to encourage that now. Any individual or nation could take the human response into their own hands.</p><p>Both paths — listening for aliens or trying to call them — have reached the stage where they require broader public discussion, with an eye to developing sensible regulation. That’s going to take the efforts of leaders from many nations, presumably coordinated through the United Nations or some similar international body. It should happen now. Or soon. Before it’s too late.</p><p>Twitter: @Mark_Buchanan</p></div>||||I|||| Accessibility statementSkip to main content
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 Outlook

 Contacting aliens could end all life on earth. Let’s stop trying.

 Whatever the UFO report says, it’s time to set some rules for talking to extraterrestrials

 Perspective by Mark Buchanan
 Mark Buchanan is a physicist and science writer based in Europe.
 June 10, 2021 at 6:00 a.m. EDT

 The Milky Way stretches in the sky above the Blue Ridge Mountains in Shenandoah National Park. If there are aliens out there, they’ll probably be far more technologically advanced than we are — and therefore dangerous. (Jonathan Newton/The Washington Post)

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 In April 2020, the Defense Department released videos recorded by infrared cameras on U.S. Navy aircraft that documented the planes’ encounters with a variety of “unidentified aerial phenomena.” Pilots reported seeing objects flying across the sky at hypersonic speeds and changing direction almost instantaneously, capabilities far beyond that of any known aircraft.

 Sign up for a weekly roundup of thought-provoking ideas and debates

 What were the pilots seeing? Bizarre atmospheric phenomena? Alien spacecraft? Something else? Several branches of the government have been investigating the events, motivated in part by concern that adversaries such as Russia or China might have made some spectacular technological advance, and later this month, the government plans to publish a report revealing what they know. Reportedly, the government will say there’s no proof of extraterrestrial activity, but that the incidents remain unexplained.

 Chances are, though, that we should all be grateful that we don’t yet have any evidence of contact with alien civilizations. Attempting to communicate with extraterrestrials, if they do exist, could be extremely dangerous for us. We need to figure out whether it’s wise — or safe — and how to handle such attempts in an organized manner.

 Advertisement

 The USS Russell and the USS Omaha captured video appearing to show UFOs flying, hovering and splashing into the ocean. (Video: Jeremy Corbell)

 Some scientific circles have already been debating questions around whether to try to contact other civilizations. It’s a topic of profound importance for the entire planet. For 60 years, scientists have been searching with radio telescopes, listening in for possible signals coming from other civilizations on planets orbiting distant stars. These efforts have largely been organized by the SETI institute in California — the acronym stands for Search for ExtraTerrestrial Intelligence — and so far, they’ve had no success. Getting impatient, some other scientists are now pushing for a more active program — METI, for Messaging ExtraTerrestrial Intelligence — that wouldn’t just listen, but actually send out powerful messages toward other stars, seeking to make contact.

 The search for aliens has reached a stage of technological sophistication and associated risk that it needs strict regulation at national and international levels. Without oversight, even one person — with access to powerful transmitting technology — could take actions affecting the future of the entire planet.

 The military keeps encountering UFOs. Why doesn’t the Pentagon care?

 That’s because any aliens we ultimately encounter will likely be far more technologically advanced than we are, for a simple reason: Most stars in our galaxy are much older than the sun. If civilizations arise fairly frequently on some planets, then there ought to be many civilizations in our galaxy millions of years more advanced than our own. Many of these would likely have taken significant steps to begin exploring and possibly colonizing the galaxy.

 Advertisement

 Hence, it’s a profound mystery — known as the Fermi Paradox, after the Italian physicist Enrico Fermi — why we haven’t yet seen any such aliens. Many resolutions of the paradox have been proposed, among them the suggestion that all civilizations, once reaching sufficient technological capacity, eventually destroy themselves. Or perhaps aliens are so alien and unlike humans that we simply cannot interact with them.

 More alarming is the possibility that alien civilizations are remaining out of contact because they know something: that sending out signals is catastrophically risky. Our history on Earth has given us many examples of what can happen when civilizations with unequal technology meet — generally, the technologically more advanced has destroyed or enslaved the other. A cosmic version of this reality might have convinced many alien civilizations to remain silent. Exposing yourself is an invitation to be preyed upon and devoured.

 I’ve written about METI in the past, suggesting such activity takes a huge risk for very little gain. But these concerns don’t convince supporters of trying it, who have some counterarguments. Douglas Vakoch of METI International argues that it’s unrealistic to worry about the danger of an alien invasion. We have, after all, been sending radio and television emissions into space for a century, and a civilization far more advanced than our own will probably have already detected these. If they wanted to invade, they already would have.

 Advertisement

 He also argues that, in assessing risks, it’s important not only to consider the risk coming from taking an action, but also from not taking that action. Our world faces a number of potentially existential threats, including global warming and destabilization of the environment, and it’s possible that far more advanced civilizations may have already faced these issues and found solutions. If we don’t send out signals, Vakosh writes, we risk “missing guidance that could enhance our own civilization’s sustainability.” It’s also conceivable, he suggests, that we’re making a spectacular misjudgment — and some super-advanced alien civilization may attack us precisely because we haven’t reached out.

 For obvious reasons, much of the thinking about these issues has to be rather speculative. The best way forward, perhaps, is to broaden the discussion. If all of humanity is exposed to the possible consequences trying to contact alien civilizations, then more people should be involved in making decisions about what is wise and what isn’t. It shouldn’t be left to a handful of radio astronomers.

 A new frontier is opening in the search for extraterrestrial life

 One vocal critic of the idea of reaching out to aliens proactively — astronomer John Gertz of SETI — has developed proposals to move toward more inclusive public consideration of these activities. What we need, he suggests, are laws and international treaties to govern more explicit contact attempts. Without prior broad agreement from some globally representative body, Gertz says, contacting extraterrestrials should be considered “as the reckless endangerment of all mankind, and be absolutely proscribed with criminal consequences, presumably as exercised at the national level, or administered through the International Court of Justice in The Hague.”

 Advertisement

 Currently, no such prohibitions exist. Some informal protocols for interacting with alien civilizations have been adopted by researchers involved in SETI, but these are far from legally binding governmental regulations. That’s mostly because, up to now, talking about meeting or contacting aliens has seemed widely speculative — if not a little deranged — despite the apparent scientific plausibility of such an event.

 It’s not easy to weigh the pros and cons of activities around which so much remains unknown. We don’t know if there are any aliens. They might be friendly. They might not be. Given the potential risks involved with trying to make contact, perhaps it would be safer and wiser to just wait — we can always reach out later, and meanwhile, our abilities to do passive listening are rapidly growing more powerful.

 In 2015, SETI launched a new 10-year program called Breakthrough Listen, funded by a $100 million donation from Israeli-Russian billionaire Yuri Milner. As a result, SETI is now recording more signals than ever before, over a frequency range some tenfold larger, and bringing more computational power to bear on analyzing the recorded signals. It’s impossible to know how close or far from making a discovery we may be, but Gertz estimates that our chances are at least 100 times greater than they used to be.

 Advertisement

 The search is also benefiting from astronomers’ knowledge of exoplanets — planets in orbit around stars other than the sun. Since the first exoplanet was found in 1992, we’ve identified nearly 5,000 more, and the rate of discovery is accelerating. Each one give SETI researchers new promising targets to scrutinize.

 Personally, all of this makes me dead-set against any experimentation with attempting to contact other civilizations. Why take cosmic risks when we may have a far safer pathway to discovering them, if they’re out there? Of course, even listening comes with some potentially fraught governance issues also: If and when someone really identifies an alien signal, we’ll need to decide if we should reply — and if so, how. Surely such an act — putting all of humanity at risk — ought to be the result of some collective decision. But there’s no mechanism to encourage that now. Any individual or nation could take the human response into their own hands.

 Both paths — listening for aliens or trying to call them — have reached the stage where they require broader public discussion, with an eye to developing sensible regulation. That’s going to take the efforts of leaders from many nations, presumably coordinated through the United Nations or some similar international body. It should happen now. Or soon. Before it’s too late.

 Twitter: @Mark_Buchanan

 Read more from Outlook:

 When disgraced theories are respectable again

 Five myths about space

 What it means for us to actually ‘see’ a black hole

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In April 2020, the Defense Department released videos recorded by infrared cameras on U.S. Navy aircraft that documented the planes’ encounters with a variety of  “unidentified aerial phenomena.” Pilots reported seeing objects flying across the sky at hypersonic speeds and changing direction almost instantaneously, capabilities far beyond that of any known aircraft.   What were the pilots seeing? Bizarre atmospheric phenomena? Alien spacecraft? Something else? Several branches of the government have been investigating the events, motivated in part by concern that adversaries such as Russia or China might have made some spectacular technological advance, and later this month, the government plans to publish a report revealing what they know. Reportedly, the government will say there’s no proof of extraterrestrial activity, but that the incidents remain unexplained. Chances are, though, that we should all be grateful that we don’t yet have any evidence of contact with alien civilizations. Attempting to communicate with extraterrestrials, if they do exist, could be extremely dangerous for us. We need to figure out whether it’s wise — or safe — and how to handle such attempts in an organized manner.    The USS Russell and the USS Omaha captured video appearing to show UFOs flying, hovering and splashing into the ocean. (Video: Jeremy Corbell)    Some scientific circles have already been debating questions around whether to try to contact other civilizations. It’s a topic of profound importance for the entire planet. For 60 years, scientists have been searching with radio telescopes, listening in for possible signals coming from other civilizations on planets orbiting distant stars. These efforts have largely been organized by the SETI institute in California — the acronym stands for Search for ExtraTerrestrial Intelligence — and so far, they’ve had no success. Getting impatient, some other scientists are now pushing for a more active program — METI, for Messaging ExtraTerrestrial Intelligence — that wouldn’t just listen, but actually send out powerful messages toward other stars, seeking to make contact. The search for aliens has reached a stage of technological sophistication and associated risk that it needs strict regulation at national and international levels. Without oversight, even one person — with access to powerful transmitting technology — could take actions affecting the future of the entire planet. That’s because any aliens we ultimately encounter will likely be far more technologically advanced than we are, for a simple reason: Most stars in our galaxy are much older than the sun. If civilizations arise fairly frequently on some planets, then there ought to be many civilizations in our galaxy millions of years more advanced than our own. Many of these would likely have taken significant steps to begin exploring and possibly colonizing the galaxy. Hence, it’s a profound mystery — known as the Fermi Paradox, after the Italian physicist Enrico Fermi — why we haven’t yet seen any such aliens. Many resolutions of the paradox have been proposed, among them the suggestion that all civilizations, once reaching sufficient technological capacity, eventually destroy themselves. Or perhaps aliens are so alien and unlike humans that we simply cannot interact with them. More alarming is the possibility that alien civilizations are remaining out of contact because they know something: that sending out signals is catastrophically risky. Our history on Earth has given us many examples of what can happen when civilizations with unequal technology meet — generally, the technologically more advanced has destroyed or enslaved the other. A cosmic version of this reality might have convinced many alien civilizations to remain silent. Exposing yourself is an invitation to be preyed upon and devoured.  I’ve written about METI in the past, suggesting such activity takes a huge risk for very little gain. But these concerns don’t convince supporters of trying it, who have some counterarguments. Douglas Vakoch of METI International argues that it’s unrealistic to worry about the danger of an alien invasion. We have, after all, been sending radio and television emissions into space for a century, and a civilization far more advanced than our own will probably have already detected these. If they wanted to invade, they already would have. He also argues that, in assessing risks, it’s important not only to consider the risk coming from taking an action, but also from not taking that action. Our world faces a number of potentially existential threats, including global warming and destabilization of the environment, and it’s possible that far more advanced civilizations may have already faced these issues and found solutions. If we don’t send out signals, Vakosh writes, we risk “missing guidance that could enhance our own civilization’s sustainability.” It’s also conceivable, he suggests, that we’re making a spectacular misjudgment — and some super-advanced alien civilization may attack us precisely because we haven’t reached out. For obvious reasons, much of the thinking about these issues has to be rather speculative. The best way forward, perhaps, is to broaden the discussion. If all of humanity is exposed to the possible consequences trying to contact alien civilizations, then more people should be involved in making decisions about what is wise and what isn’t. It shouldn’t be left to a handful of radio astronomers. One vocal critic of the idea of reaching out to aliens proactively — astronomer John Gertz of SETI — has developed proposals to move toward more inclusive public consideration of these activities. What we need, he suggests, are laws and international treaties to govern more explicit contact attempts. Without prior broad agreement from some globally representative body, Gertz says, contacting extraterrestrials should be considered “as the reckless endangerment of all mankind, and be absolutely proscribed with criminal consequences, presumably as exercised at the national level, or administered through the International Court of Justice in The Hague.” Currently, no such prohibitions exist. Some informal protocols for interacting with alien civilizations have been adopted by researchers involved in SETI, but these are far from legally binding governmental regulations. That’s mostly because, up to now, talking about meeting or contacting aliens has seemed widely speculative — if not a little deranged — despite the apparent scientific plausibility of such an event. It’s not easy to weigh the pros and cons of activities around which so much remains unknown. We don’t know if there are any aliens. They might be friendly. They might not be. Given the potential risks involved with trying to make contact, perhaps it would be safer and wiser to just wait — we can always reach out later, and meanwhile, our abilities to do passive listening are rapidly growing more powerful. In 2015, SETI launched a new 10-year program called Breakthrough Listen, funded by a $100 million donation from Israeli-Russian billionaire Yuri Milner. As a result, SETI is now recording more signals than ever before, over a frequency range some tenfold larger, and bringing more computational power to bear on analyzing the recorded signals. It’s impossible to know how close or far from making a discovery we may be, but Gertz estimates that our chances are at least 100 times greater than they used to be. The search is also benefiting from astronomers’ knowledge of exoplanets — planets in orbit around stars other than the sun. Since the first exoplanet was found in 1992, we’ve identified nearly 5,000 more, and the rate of discovery is accelerating. Each one give SETI researchers new promising targets to scrutinize. Personally, all of this makes me dead-set against any experimentation with attempting to contact other civilizations. Why take cosmic risks when we may have a far safer pathway to discovering them, if they’re out there? Of course, even listening comes with some potentially fraught governance issues also: If and when someone really identifies an alien signal, we’ll need to decide if we should reply — and if so, how. Surely such an act — putting all of humanity at risk — ought to be the result of some collective decision. But there’s no mechanism to encourage that now. Any individual or nation could take the human response into their own hands. Both paths — listening for aliens or trying to call them — have reached the stage where they require broader public discussion, with an eye to developing sensible regulation. That’s going to take the efforts of leaders from many nations, presumably coordinated through the United Nations or some similar international body. It should happen now. Or soon. Before it’s too late. Twitter: @Mark_Buchanan
Perspective | Contacting aliens could end all life on earth. Let’s stop trying.
https://www.washingtonpost.com/outlook/ufo-report-aliens-seti/2021/06/09/1402f6a8-c899-11eb-81b1-34796c7393af_story.html
https://www.scientificamerican.com/article/astronomer-avi-loeb-says-aliens-have-visited-and-hes-not-kidding1/
Astronomer Avi Loeb Says Aliens Have Visited, and He’s Not Kidding
2021-02-01
Lee Billings
<div><div><p>Avi Loeb is no stranger to controversy. The prolific Harvard University astrophysicist has produced pioneering and provocative research on black holes, gamma-ray bursts, the early universe and other standard topics of his field. But for more than a decade he has also courted a more contentious subject—namely, space aliens, including how to find them. Until relatively recently, Loeb’s most high-profile work in that regard was his involvement with <a href="https://www.scientificamerican.com/article/inside-the-breakthrough-starshot-mission-to-alpha-centauri/">Breakthrough Starshot</a>, a project funded by Silicon Valley billionaire Yuri Milner to send laser-boosted, gossamer-thin mirrorlike spacecraft called “light sails” on high-speed voyages to nearby stars. All that began to change in late 2017, however, when astronomers around the world <a href="https://www.scientificamerican.com/article/astronomers-spot-first-ever-space-rock-from-another-star/">scrambled to study an enigmatic interstellar visitor</a>—the first ever seen—that briefly came within range of their telescopes.</p>
<p>The object’s discoverers dubbed it <a href="https://www.scientificamerican.com/article/everything-scientists-know-so-far-about-the-first-interstellar-objects-ever-detected/">‘Oumuamua</a>—a Hawaiian term that roughly translates to “scout.” The unavoidably cursory examinations of this celestial passerby showed it had <a href="https://blogs.scientificamerican.com/observations/6-strange-facts-about-the-interstellar-visitor-oumuamua/">several properties that defied easy natural explanation</a>. ‘Oumuamua’s apparent shape—which was like a 100-meter-long cigar or pancake—did not closely resemble any known asteroid or comet. Neither did its brightness, which revealed ‘Oumuamua was at least 10 times more reflective than one of our solar system’s typical space rocks—shiny enough to suggest the gleam of burnished metal. Most strangely, as it zoomed off after swooping by the sun, the object <a href="https://www.nature.com/articles/s41586-018-0254-4">sped up faster than could be explained by our star’s waning gravitational grip alone</a>. Run-of-the-mill comets can exhibit similar accelerations because of the rocketlike effect of evaporating gases jetting from their sunlight-warmed icy surfaces. But no signs of such jets were seen around ‘Oumuamua.</p>
<p>To Loeb, the most plausible explanation was as obvious as it was sensational: taken together with its possibly pancakelike shape and high reflectivity, ‘Oumuamua’s anomalous acceleration made perfect sense if the object was in fact a light sail—perhaps a derelict from some long-expired galactic culture. Primed by years spent pondering how we might someday find evidence of cosmic civilizations in the sky’s depths, he became increasingly convinced that, with ‘Oumuamua, the evidence had instead found us. In late 2018 Loeb and his co-author Shmuel Bialy, a Harvard postdoctoral fellow, published <a href="https://iopscience.iop.org/article/10.3847/2041-8213/aaeda8">a paper</a> in the <em>Astrophysical Journal Letters</em> arguing that ‘Oumuamua had been nothing less than humanity’s first contact with an artifact of extraterrestrial intelligence.</p>
<p>The paper has been a smash hit with journalists but has <a href="https://planetplanet.net/2019/07/01/oumuamua-was-it-aliens-spoiler-no/">fallen flat</a> with most of Loeb’s astrobiology-focused peers, who insist that, while strange, ‘Oumuamua’s properties still place it <a href="https://www.nature.com/articles/s41550-019-0816-x">well within the realm of natural phenomena</a>. To claim otherwise, Loeb’s critics say, is cavalier at best and destructive at worst for the long struggle to remove the stigma of credulous UFO and alien-abduction reports from what should unquestionably be a legitimate field of scientific inquiry.</p>
<p>Loeb has now taken his case to the public with the book <em><a href="https://www.amazon.com/Extraterrestrial-First-Intelligent-Beyond-Earth/dp/0358278147">Extraterrestrial: The First Sign of Intelligent Life beyond Earth</a></em><em>,</em> which is just as much about the author’s life story as it is about ‘Oumuamua’s fundamental mysteries. <em>Scientific American</em> spoke with Loeb about the book, his controversial hypothesis and why he believes science is in crisis.</p>
<p>[<em>An edited transcript of the interview follows</em>.]</p>
<p><strong>Hi Avi. How are you?</strong></p>
<p>I’m good, but I have been losing sleep, because in order to cope with all the media requests, I’ve been doing interviews with, for example, <em>Good Morning Britain</em> at 1:50 A.M. and <em>Coast to Coast AM</em> at 3 A.M.—plus appearances on U.S. network and cable television. I’ve got about 100 podcast interviews to do in the next few weeks. And I already recorded long conversations with [podcasters] Lex Fridman and Joe Rogan for their shows. I’ve never seen anything like this; there has been so much interest in the book. I mean, there were 10 filmmakers and producers from Hollywood who contacted me over the past few weeks! I joked with my literary agent that if a film comes out of this, I want to be played by Brad Pitt.</p>
<p><strong>Ha, indeed, the resemblance is uncanny. Based on your productivity, I’ve never gotten the sense that you get a lot of sleep anyway.</strong></p>
<p>My routine is to wake up each morning at 5 A.M. and go jogging. It’s really beautiful when nobody's outside—just me and the birds, ducks and rabbits. And, yes, because of the pandemic, the past 10 months have been the most productive in my career. I don’t need to commute to work. I don’t need to meet so many people. And most importantly, I don’t need to think about what’s wrong with all the things that other people say!</p><figure>
<p>
</p>
<p>Sign up for <em>Scientific American</em>’s free newsletters.</p>
</figure>
<p><strong>Speaking of important things, here is one I think we both agree on: in science, we must keep each other honest. </strong><strong>I mention it only because there’s a point in <em>Extraterrestrial</em> where you claim you don’t want the limelight and that you’re not interested in self-promotion. How can that be true?</strong></p>
<p>Let me explain. I think talking to the media is an important opportunity because it allows me to share my message with a broader audience that otherwise would not have exposure to it.</p>
<p><strong>What is your message, exactly? I take it you’re talking about more than ‘Oumuamua.</strong></p>
<p>Yes. My message is that something is wrong with the scientific community today in terms of its health.</p>
<p>Too many scientists are now mostly motivated by ego, by getting honors and awards, by showing their colleagues how smart they are. They treat science as a monologue about themselves rather than a dialogue with nature. They build echo chambers using students and postdocs who repeat their mantras so that their voice will be louder and their image will be promoted. But that’s not the purpose of science. Science is not about us; it’s not about empowering ourselves or making our image great. It’s about trying to understand the world, and it’s meant to be a learning experience in which we take risks and make mistakes along the way. You can never tell in advance, when you work on the frontier, what is the right path forward. You only learn that by getting feedback from experiments.</p>
<p>Which is the other problem with science today: people are not only motivated by the wrong reasons; they are also no longer guided by evidence. Evidence keeps you modest because you predict something, you test it, and the evidence sometimes shows you’re wrong. Right now you have many celebrated scientists doing mathematical gymnastics about lots of untestable things: string theory, the multiverse, even <a href="https://www.scientificamerican.com/article/cosmic-inflation-theory-faces-challenges/">the theory of cosmic inflation</a>. Once, in a public forum, <a href="https://youtu.be/V6rUi65qLqc?t=7193">I asked [physicist] Alan Guth, who originated the theory, “Is inflation falsifiable?” And he said it’s a silly question</a>, because for whatever cosmological data an experiment gives us, a model of inflation can be found that accommodates it. And therefore, <a href="https://www.scientificamerican.com/article/readers-respond-to-the-february-and-march-2017-issues/">inflation is in a very strong position</a> because it can explain anything! But I see this as a very weak position because a theory of everything is sometimes a theory of nothing. There may be no difference between the two.</p>
<p>To me, this bubble of imaginary stuff is like being high on drugs: You can get high and imagine that you’re wealthier than Elon Musk, who is now the richest person in the world. That’s a very fun thought. You can feel really good about it and talk about it with your friends. And if you’re part of a big like-minded community, everyone can support and respect one another, and you give one another awards, and that’s great, right? But then if you go to withdraw funds, if you want to really spend that money you think you have, you realize that you don’t actually have anything. Just like going to an ATM, doing experiments can serve as a reality check. And in science, it’s essential that we have that check—that we make testable predictions and put some skin in the game—because otherwise we won’t learn anything new. I don’t think that’s properly recognized anymore.</p>
<figure>
<figcaption>Marked with a blue circle, ‘Oumuamua appears as a faint dot in the center of this image, which is one of the best available and combines observations from multiple different telescopes. Credit: <a href="https://www.eso.org/public/images/eso1737b/">ESO/K. Meech et al.</a></figcaption>
</figure>
<p><strong>So speculating about string theory and multiverses is bad, but speculating about alien civilizations and their artifacts passing through the solar system is okay? You could say appealing to “aliens” can explain anything, too.</strong></p>
<p>The difference is: you can make predictions and test for the latter, and the speculations come from a conservative position.</p>
<p>If ‘Oumuamua is a member of a population of objects moving on random trajectories, then based on its discovery with the <a href="https://en.wikipedia.org/wiki/Pan-STARRS">Pan-STARRS</a> telescope, you can estimate that we should very soon begin finding, on average, one of these objects per month after the <a href="https://en.wikipedia.org/wiki/Vera_C._Rubin_Observatory">Vera C. Rubin Observatory</a> comes online. We can also establish a system of instruments—satellites, maybe—that would not only monitor the sky but also be able to <a href="https://www.scientificamerican.com/article/european-comet-interceptor-could-visit-an-interstellar-object/">react to the approach of such objects</a> so we can get photographs of them as they come in rather than chasing them as they go out, because they move very fast. Not all this work needs to be in space, either: You can imagine <a href="https://www.scientificamerican.com/article/did-a-meteor-from-another-star-strike-earth-in-2014/">meteors of interstellar origin</a> as well, and we can search for those. And if you find any that ended up on Earth’s surface, you might even be able to examine them with your own hands.</p>
<p>People ask why I get this media attention. The only reason is because my colleagues are not using common sense. Contrast string theory and multiverses with what I and many others say, which is that based on the data from NASA’s <a href="https://en.wikipedia.org/wiki/Kepler_space_telescope">Kepler mission</a>, roughly <a href="https://arxiv.org/abs/2010.14812">half of the galaxy’s sunlike stars</a> have a planet about the size of the Earth, at about the same distance of the Earth from the sun, so that you can have liquid water on the surface and the chemistry of life as we know it. So if you roll the dice on life billions of times in the Milky Way, what is the chance that we are alone? Minuscule, most likely! To say that if you arrange for similar circumstances, you get similar outcomes is, to me, the most conservative statement imaginable. So I would expect most people to endorse that, to hug me and say, “Great, Avi, you’re correct. We should look for these things because they must be very likely.” Instead what I see is a backlash that shows a loss of an intellectual compass—because how else can you explain working on string theory’s extra dimensions or the multiverse when we have no clue for their existence? But <em>that </em>is considered mainstream? That’s <em>crazy</em>.</p>
<p>Allow me to put this in a very specific context. I’m obviously not a rebel outsider; I’m in leadership positions. I chair the <a href="https://www.nationalacademies.org/bpa/about">Board on Physics and Astronomy</a> of the National Academies [of Sciences, Engineering, and Medicine], okay? That board is overseeing the <a href="https://www.nationalacademies.org/our-work/decadal-survey-on-astronomy-and-astrophysics-2020-astro2020">Astronomy and Astrophysics Decadal Survey</a>, which will set major science priorities for NASA and the [National Science Foundation] when it is released later this year. Now, I see astronomers talking about <a href="https://www.scientificamerican.com/article/nasa-considers-its-next-flagship-space-telescope/">future telescopes costing billions of dollars</a>, with the main motivation being to find life by looking for oxygen in the atmospheres of exoplanets. That is a noble wish. But if you look at <a href="https://www.scientificamerican.com/article/a-new-recipe-for-hunting-alien-life/">the Earth for its first two billion years or so</a>, the planet did not have much oxygen in its atmosphere even though it had a lot of microbial life. That’s point number one. Point number two is that even if you have oxygen, you can get it from <a href="https://iopscience.iop.org/article/10.1088/0004-637X/792/2/90/meta">natural processes such as breaking apart water molecules</a>. So even if you if you spend these billions and <a href="https://en.wikipedia.org/wiki/Biosignature#Atmospheric_disequilibrium">find oxygen and maybe even find methane along with it</a>, people will still argue about it forever. Look at how much discussion there has been about the potential <a href="https://www.scientificamerican.com/article/venus-might-host-life-new-discovery-suggests/">detection of phosphine on Venus</a>, which is a very unusual molecule, compared with oxygen. Anyway, my point is that with these same instruments—you don’t need any extra investment of funds—you can actually get conclusive evidence for life, intelligence and technology. What would that be? <a href="https://iopscience.iop.org/article/10.1088/2041-8205/792/1/L7/meta">Industrial pollution</a> in the same atmosphere. You could, for instance, look for chlorofluorocarbons, these complex molecules only produced on Earth for refrigeration systems. If you found that on another planet, there is just no way nature would produce these molecules naturally. You would have conclusive evidence that life—and more—existed there.</p>
<p>So what is the problem with saying that looking for industrial pollution is a worthwhile thing to do? What other than some sort of psychological barrier that prevents some scientists from admitting they want the search for technological signatures of alien civilizations to be at the periphery, with very little funding? What I’m saying is that these sorts of things should be prioritized and that they are conservative things to do because they will bring us the most information about the existence of alien life. And yet the opposite is being done right now.</p>
<p><strong>You write about a concept you call “‘Oumuamua’s wager,” after <a href="https://en.wikipedia.org/wiki/Pascal%27s_wager">Pascal’s wager</a>, 17th-century mathematician Blaise Pascal’s argument that the benefits of assuming God exists outweigh the drawbacks. Similarly, you say believing ‘Oumuamua is an alien artifact would be a net good because it could catalyze a revolution in space science and technology centered around a more vigorous search for life and intelligence beyond Earth. Even if that hunt finds no aliens, your reasoning goes, we’d still gain a much deeper understanding of our cosmic context. And the investments behind it would enhance our ability to answer other questions about the universe and perhaps even help stave off our own extinction.</strong></p>
<p><strong>But if the stakes are so high, what about the counterargument that going “all in” on promoting ‘Oumuamua’s putative artificial nature is reckless and dangerous? Your critics say you are doing more harm than good. For instance, you mentioned you appeared on Joe Rogan’s podcast, one of the most popular in the world. That’s great for selling books. But given Rogan’s reputation for <a href="https://www.insider.com/joe-rogan-alex-jones-spread-conspiracy-theories-challenge-podcast-2020-10">spreading</a> <a href="https://www.newsweek.com/bill-burr-joe-rogan-podcast-covid-19-masks-coronavirus-1511488">dangerous</a> <a href="https://www.forbes.com/sites/danidiplacido/2020/09/18/joe-rogan-uncharacteristically-apologies-for-spreading-misinformation/?sh=2eeb47001619">misinformation</a> on his podcast, is that sort of thing a wise move? Would you also agree to be a speaker at a gathering of UFO “true believers” outside Area 51? Where do you draw the line for public outreach that risks enhancing the so-called <a href="https://arxiv.org/abs/1708.05318">giggle factor</a> that has stymied progress in the search for extraterrestrial intelligence (SETI) for decades?</strong></p>
<p>Okay, here is my point of view. By and large, the public funds science. And the public is extremely interested in the search for alien life. So I must ask: If scientists are supported by the public, how dare they shy away from this question that can be addressed with the technologies they are developing?</p>
<p>There are, of course, science-fiction stories about aliens, and there are many unsubstantiated UFO reports. Now, suppose there was some literature about the magical properties of COVID-19 that had no bearing in reality. Would that mean scientists should never work on finding a vaccine to this pandemic? No! I don’t see the search for technological signatures any differently from the search for the nature of dark matter. We have invested hundreds of millions of dollars in searching for weakly interacting massive particles, a leading dark matter candidate. And so far <a href="https://www.scientificamerican.com/article/physics-confronts-its-heart-of-darkness/">those searches have failed</a>. That doesn’t mean they were a waste: going down dark alleys is part of the scientific process.</p>
<p>And in terms of risk, in science, we are supposed to put everything on the table. We cannot just avoid certain ideas because we worry about the consequences of discussing them, because there is great risk in that, too. That would be similar to telling Galileo not to speak about Earth moving around the sun and to avoid looking in his telescope because it was <a href="https://en.wikipedia.org/wiki/Galileo_affair">dangerous to the philosophy of the day</a>. We should not want to repeat that experience. We need an open dialogue among scientists where people present different ideas and then allow evidence to dictate which one is right. In the context of ‘Oumuamua, I say the available evidence suggests this particular object is artificial, and the way to test this is to find more [examples] of the same and examine them. It’s as simple as that.</p>
<p>So how do you change this situation? I think the answer is to bring it to the public as much as I can.</p>
<figure>
<figcaption>Credit: <a href="https://www.hmhbooks.com/shop/books/Extraterrestrial/9780358274551">Houghton Mifflin Harcourt</a></figcaption>
</figure>
<p><strong>In your book, you link your outspokenness about ‘Oumuamua with a phrase, an ethos, you learned when you were a conscript in the Israel Defense Forces: “To lay your body on the barbed wire.” That is, to make personal sacrifices for the greater good. Are you to be a martyr for this cause, then? Have you lost friends or stature over it?</strong></p>
<p>No one has violently assaulted me or anything like that. Maybe people talk behind my back, which would make more sense, given my leadership positions. But I don’t really know. I have zero footprint on social media. Although I should say that I think my critics who are most vocal with nasty remarks on Twitter and elsewhere are relatively mediocre scientists. Most really good scientists would not behave that way—they would make arguments for or against my claims, and that would be enough. Nasty remarks don’t make sense—except, well, deep inside, I would not be surprised if many of these critics are actually quite intrigued by this possibility that ‘Oumuamua is artificial. But they don’t want to admit it. So they loudly say the opposite.</p>
<p>Unfortunately, my situation is different from that of the young postdocs who I’ve worked with because they need to apply for jobs. I’m sure that people have approached them and said, “Look, this is dangerous for you.” And so they froze and basically stopped working on anything related. This isn’t surprising. If you create a hostile intellectual culture where something like SETI is not being honored, then young, bright people will not go there. But don’t step on the grass and then complain it doesn’t grow as you stand on it. Don’t block brilliant researchers from working on SETI and then say, “Look, nothing is being found. SETI is a failure!”</p>
<p>None of this means all of space science should be about SETI. If you look at the commercial world, companies such as Bell Labs in the past or Google now, they incentivize and allow for their personnel to pursue innovative “blue-sky” research that is not immediately applicable for profit. But if you look at academia, it’s much more conservative than the commercial sector. That doesn’t make sense.</p>
<p><strong>How do you respond to the idea that for a person with a hammer, everything looks like a nail? Someone could uncharitably say what you are really doing here is attempting to curry further favor with wealthy benefactors, such as Yuri Milner, because you are an adviser for his Breakthrough Initiatives programs, which fund research related to SETI and light sails.</strong></p>
<p>It’s true for me—and everyone else, I think—that my imagination is limited by what I know. I can’t deny the fact that my involvement in Breakthrough was influential here. I was the one who suggested the light sail [<a href="https://arxiv.org/abs/1604.01356">proposed by physicist Philip Lubin</a>] to Yuri Milner as a promising concept for interstellar spacecraft in the first place. So I had it in my vocabulary, and as a result of that, I imagined it as applied to ‘Oumuamua. Now, you might ask, “Okay, well, isn’t that a biased view?” I would say this occurs again and again in physics and in SETI. In the context of SETI, you know, once we developed radio technology, we started searching the sky looking for radio signals. It was the same for lasers. It’s just natural that once you work on some technology that you imagine maybe it exists out there and search for it. So I would not deny that the reason the light sail idea was in my brain is because I had previously worked on it, yeah. But in terms of trying to motivate Yuri, that has nothing to do with it. Why would I do it this way when I can just approach him directly whenever I want to advocate my views? And it is not as if my work on ‘Oumuamua was coordinated with or supported by Breakthrough Initiatives. They have issued no press releases about my ideas. If anything, they might be worried—they have their own reputation to preserve and so forth. On this issue, I’ve had zero support from or communication with them. This was me being curious, not using ‘Oumuamua as some sort of a political vehicle in the context of Breakthrough. That has nothing to do with my motivation.</p>
<p><strong>After this, what comes next for you? Do you have plans?</strong></p>
<p>I just stepped down from being chair of Harvard’s astronomy department, so I really do have the ability now to move to the next phase. And the question is: What would it be? Life, of course, is not always what you’ve planned, but another leadership opportunity would be so tempting because I could try to shape reality in a way others would not. I couldn’t pass that up. But maybe we should exclude leadership possibilities from this. Maybe I won’t be offered anything again because of my ideas about ‘Oumuamua! That’s a possibility. Then I’d write more books, do more research and continue to jog every morning.</p></div>||||I||||Skip to main content
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 Space & Physics

 Astronomer Avi Loeb Says Aliens Have Visited, and He’s Not Kidding

 In conversation, the Harvard University professor explains his shocking hypothesis—and calls out what he sees as a crisis in science

 * By Lee Billings on February 1, 2021
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 Astrophysicist Avi Loeb at the unveiling of the Breakthrough Starshot initiative in New York City on April 12, 2016. Credit: Jemal Countess Getty Images

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 Avi Loeb is no stranger to controversy. The prolific Harvard University astrophysicist has produced pioneering and provocative research on black holes, gamma-ray bursts, the early universe and other standard topics of his field. But for more than a decade he has also courted a more contentious subject—namely, space aliens, including how to find them. Until relatively recently, Loeb’s most high-profile work in that regard was his involvement with Breakthrough Starshot, a project funded by Silicon Valley billionaire Yuri Milner to send laser-boosted, gossamer-thin mirrorlike spacecraft called “light sails” on high-speed voyages to nearby stars. All that began to change in late 2017, however, when astronomers around the world scrambled to study an enigmatic interstellar visitor—the first ever seen—that briefly came within range of their telescopes.

 The object’s discoverers dubbed it ‘Oumuamua—a Hawaiian term that roughly translates to “scout.” The unavoidably cursory examinations of this celestial passerby showed it had several properties that defied easy natural explanation. ‘Oumuamua’s apparent shape—which was like a 100-meter-long cigar or pancake—did not closely resemble any known asteroid or comet. Neither did its brightness, which revealed ‘Oumuamua was at least 10 times more reflective than one of our solar system’s typical space rocks—shiny enough to suggest the gleam of burnished metal. Most strangely, as it zoomed off after swooping by the sun, the object sped up faster than could be explained by our star’s waning gravitational grip alone. Run-of-the-mill comets can exhibit similar accelerations because of the rocketlike effect of evaporating gases jetting from their sunlight-warmed icy surfaces. But no signs of such jets were seen around ‘Oumuamua.

 To Loeb, the most plausible explanation was as obvious as it was sensational: taken together with its possibly pancakelike shape and high reflectivity, ‘Oumuamua’s anomalous acceleration made perfect sense if the object was in fact a light sail—perhaps a derelict from some long-expired galactic culture. Primed by years spent pondering how we might someday find evidence of cosmic civilizations in the sky’s depths, he became increasingly convinced that, with ‘Oumuamua, the evidence had instead found us. In late 2018 Loeb and his co-author Shmuel Bialy, a Harvard postdoctoral fellow, published a paper in the Astrophysical Journal Letters arguing that ‘Oumuamua had been nothing less than humanity’s first contact with an artifact of extraterrestrial intelligence.

 The paper has been a smash hit with journalists but has fallen flat with most of Loeb’s astrobiology-focused peers, who insist that, while strange, ‘Oumuamua’s properties still place it well within the realm of natural phenomena. To claim otherwise, Loeb’s critics say, is cavalier at best and destructive at worst for the long struggle to remove the stigma of credulous UFO and alien-abduction reports from what should unquestionably be a legitimate field of scientific inquiry.

 Loeb has now taken his case to the public with the book Extraterrestrial: The First Sign of Intelligent Life beyond Earth, which is just as much about the author’s life story as it is about ‘Oumuamua’s fundamental mysteries. Scientific American spoke with Loeb about the book, his controversial hypothesis and why he believes science is in crisis.

 [An edited transcript of the interview follows.]

 Hi Avi. How are you?

 I’m good, but I have been losing sleep, because in order to cope with all the media requests, I’ve been doing interviews with, for example, Good Morning Britain at 1:50 A.M. and Coast to Coast AM at 3 A.M.—plus appearances on U.S. network and cable television. I’ve got about 100 podcast interviews to do in the next few weeks. And I already recorded long conversations with [podcasters] Lex Fridman and Joe Rogan for their shows. I’ve never seen anything like this; there has been so much interest in the book. I mean, there were 10 filmmakers and producers from Hollywood who contacted me over the past few weeks! I joked with my literary agent that if a film comes out of this, I want to be played by Brad Pitt.

 Ha, indeed, the resemblance is uncanny. Based on your productivity, I’ve never gotten the sense that you get a lot of sleep anyway.

 My routine is to wake up each morning at 5 A.M. and go jogging. It’s really beautiful when nobody's outside—just me and the birds, ducks and rabbits. And, yes, because of the pandemic, the past 10 months have been the most productive in my career. I don’t need to commute to work. I don’t need to meet so many people. And most importantly, I don’t need to think about what’s wrong with all the things that other people say!

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 Speaking of important things, here is one I think we both agree on: in science, we must keep each other honest. I mention it only because there’s a point in Extraterrestrial where you claim you don’t want the limelight and that you’re not interested in self-promotion. How can that be true?

 Let me explain. I think talking to the media is an important opportunity because it allows me to share my message with a broader audience that otherwise would not have exposure to it.

 What is your message, exactly? I take it you’re talking about more than ‘Oumuamua.

 Yes. My message is that something is wrong with the scientific community today in terms of its health.

 Too many scientists are now mostly motivated by ego, by getting honors and awards, by showing their colleagues how smart they are. They treat science as a monologue about themselves rather than a dialogue with nature. They build echo chambers using students and postdocs who repeat their mantras so that their voice will be louder and their image will be promoted. But that’s not the purpose of science. Science is not about us; it’s not about empowering ourselves or making our image great. It’s about trying to understand the world, and it’s meant to be a learning experience in which we take risks and make mistakes along the way. You can never tell in advance, when you work on the frontier, what is the right path forward. You only learn that by getting feedback from experiments.

 Which is the other problem with science today: people are not only motivated by the wrong reasons; they are also no longer guided by evidence. Evidence keeps you modest because you predict something, you test it, and the evidence sometimes shows you’re wrong. Right now you have many celebrated scientists doing mathematical gymnastics about lots of untestable things: string theory, the multiverse, even the theory of cosmic inflation. Once, in a public forum, I asked [physicist] Alan Guth, who originated the theory, “Is inflation falsifiable?” And he said it’s a silly question, because for whatever cosmological data an experiment gives us, a model of inflation can be found that accommodates it. And therefore, inflation is in a very strong position because it can explain anything! But I see this as a very weak position because a theory of everything is sometimes a theory of nothing. There may be no difference between the two.

 To me, this bubble of imaginary stuff is like being high on drugs: You can get high and imagine that you’re wealthier than Elon Musk, who is now the richest person in the world. That’s a very fun thought. You can feel really good about it and talk about it with your friends. And if you’re part of a big like-minded community, everyone can support and respect one another, and you give one another awards, and that’s great, right? But then if you go to withdraw funds, if you want to really spend that money you think you have, you realize that you don’t actually have anything. Just like going to an ATM, doing experiments can serve as a reality check. And in science, it’s essential that we have that check—that we make testable predictions and put some skin in the game—because otherwise we won’t learn anything new. I don’t think that’s properly recognized anymore.

 Marked with a blue circle, ‘Oumuamua appears as a faint dot in the center of this image, which is one of the best available and combines observations from multiple different telescopes. Credit: ESO/K. Meech et al.

 So speculating about string theory and multiverses is bad, but speculating about alien civilizations and their artifacts passing through the solar system is okay? You could say appealing to “aliens” can explain anything, too.

 The difference is: you can make predictions and test for the latter, and the speculations come from a conservative position.

 If ‘Oumuamua is a member of a population of objects moving on random trajectories, then based on its discovery with the Pan-STARRS telescope, you can estimate that we should very soon begin finding, on average, one of these objects per month after the Vera C. Rubin Observatory comes online. We can also establish a system of instruments—satellites, maybe—that would not only monitor the sky but also be able to react to the approach of such objects so we can get photographs of them as they come in rather than chasing them as they go out, because they move very fast. Not all this work needs to be in space, either: You can imagine meteors of interstellar origin as well, and we can search for those. And if you find any that ended up on Earth’s surface, you might even be able to examine them with your own hands.

 People ask why I get this media attention. The only reason is because my colleagues are not using common sense. Contrast string theory and multiverses with what I and many others say, which is that based on the data from NASA’s Kepler mission, roughly half of the galaxy’s sunlike stars have a planet about the size of the Earth, at about the same distance of the Earth from the sun, so that you can have liquid water on the surface and the chemistry of life as we know it. So if you roll the dice on life billions of times in the Milky Way, what is the chance that we are alone? Minuscule, most likely! To say that if you arrange for similar circumstances, you get similar outcomes is, to me, the most conservative statement imaginable. So I would expect most people to endorse that, to hug me and say, “Great, Avi, you’re correct. We should look for these things because they must be very likely.” Instead what I see is a backlash that shows a loss of an intellectual compass—because how else can you explain working on string theory’s extra dimensions or the multiverse when we have no clue for their existence? But that is considered mainstream? That’s crazy.

 Allow me to put this in a very specific context. I’m obviously not a rebel outsider; I’m in leadership positions. I chair the Board on Physics and Astronomy of the National Academies [of Sciences, Engineering, and Medicine], okay? That board is overseeing the Astronomy and Astrophysics Decadal Survey, which will set major science priorities for NASA and the [National Science Foundation] when it is released later this year. Now, I see astronomers talking about future telescopes costing billions of dollars, with the main motivation being to find life by looking for oxygen in the atmospheres of exoplanets. That is a noble wish. But if you look at the Earth for its first two billion years or so, the planet did not have much oxygen in its atmosphere even though it had a lot of microbial life. That’s point number one. Point number two is that even if you have oxygen, you can get it from natural processes such as breaking apart water molecules. So even if you if you spend these billions and find oxygen and maybe even find methane along with it, people will still argue about it forever. Look at how much discussion there has been about the potential detection of phosphine on Venus, which is a very unusual molecule, compared with oxygen. Anyway, my point is that with these same instruments—you don’t need any extra investment of funds—you can actually get conclusive evidence for life, intelligence and technology. What would that be? Industrial pollution in the same atmosphere. You could, for instance, look for chlorofluorocarbons, these complex molecules only produced on Earth for refrigeration systems. If you found that on another planet, there is just no way nature would produce these molecules naturally. You would have conclusive evidence that life—and more—existed there.

 So what is the problem with saying that looking for industrial pollution is a worthwhile thing to do? What other than some sort of psychological barrier that prevents some scientists from admitting they want the search for technological signatures of alien civilizations to be at the periphery, with very little funding? What I’m saying is that these sorts of things should be prioritized and that they are conservative things to do because they will bring us the most information about the existence of alien life. And yet the opposite is being done right now.

 You write about a concept you call “‘Oumuamua’s wager,” after Pascal’s wager, 17th-century mathematician Blaise Pascal’s argument that the benefits of assuming God exists outweigh the drawbacks. Similarly, you say believing ‘Oumuamua is an alien artifact would be a net good because it could catalyze a revolution in space science and technology centered around a more vigorous search for life and intelligence beyond Earth. Even if that hunt finds no aliens, your reasoning goes, we’d still gain a much deeper understanding of our cosmic context. And the investments behind it would enhance our ability to answer other questions about the universe and perhaps even help stave off our own extinction.

 But if the stakes are so high, what about the counterargument that going “all in” on promoting ‘Oumuamua’s putative artificial nature is reckless and dangerous? Your critics say you are doing more harm than good. For instance, you mentioned you appeared on Joe Rogan’s podcast, one of the most popular in the world. That’s great for selling books. But given Rogan’s reputation for spreading dangerous misinformation on his podcast, is that sort of thing a wise move? Would you also agree to be a speaker at a gathering of UFO “true believers” outside Area 51? Where do you draw the line for public outreach that risks enhancing the so-called giggle factor that has stymied progress in the search for extraterrestrial intelligence (SETI) for decades?

 Okay, here is my point of view. By and large, the public funds science. And the public is extremely interested in the search for alien life. So I must ask: If scientists are supported by the public, how dare they shy away from this question that can be addressed with the technologies they are developing?

 There are, of course, science-fiction stories about aliens, and there are many unsubstantiated UFO reports. Now, suppose there was some literature about the magical properties of COVID-19 that had no bearing in reality. Would that mean scientists should never work on finding a vaccine to this pandemic? No! I don’t see the search for technological signatures any differently from the search for the nature of dark matter. We have invested hundreds of millions of dollars in searching for weakly interacting massive particles, a leading dark matter candidate. And so far those searches have failed. That doesn’t mean they were a waste: going down dark alleys is part of the scientific process.

 And in terms of risk, in science, we are supposed to put everything on the table. We cannot just avoid certain ideas because we worry about the consequences of discussing them, because there is great risk in that, too. That would be similar to telling Galileo not to speak about Earth moving around the sun and to avoid looking in his telescope because it was dangerous to the philosophy of the day. We should not want to repeat that experience. We need an open dialogue among scientists where people present different ideas and then allow evidence to dictate which one is right. In the context of ‘Oumuamua, I say the available evidence suggests this particular object is artificial, and the way to test this is to find more [examples] of the same and examine them. It’s as simple as that.

 So how do you change this situation? I think the answer is to bring it to the public as much as I can.

 Credit: Houghton Mifflin Harcourt

 In your book, you link your outspokenness about ‘Oumuamua with a phrase, an ethos, you learned when you were a conscript in the Israel Defense Forces: “To lay your body on the barbed wire.” That is, to make personal sacrifices for the greater good. Are you to be a martyr for this cause, then? Have you lost friends or stature over it?

 No one has violently assaulted me or anything like that. Maybe people talk behind my back, which would make more sense, given my leadership positions. But I don’t really know. I have zero footprint on social media. Although I should say that I think my critics who are most vocal with nasty remarks on Twitter and elsewhere are relatively mediocre scientists. Most really good scientists would not behave that way—they would make arguments for or against my claims, and that would be enough. Nasty remarks don’t make sense—except, well, deep inside, I would not be surprised if many of these critics are actually quite intrigued by this possibility that ‘Oumuamua is artificial. But they don’t want to admit it. So they loudly say the opposite.

 Unfortunately, my situation is different from that of the young postdocs who I’ve worked with because they need to apply for jobs. I’m sure that people have approached them and said, “Look, this is dangerous for you.” And so they froze and basically stopped working on anything related. This isn’t surprising. If you create a hostile intellectual culture where something like SETI is not being honored, then young, bright people will not go there. But don’t step on the grass and then complain it doesn’t grow as you stand on it. Don’t block brilliant researchers from working on SETI and then say, “Look, nothing is being found. SETI is a failure!”

 None of this means all of space science should be about SETI. If you look at the commercial world, companies such as Bell Labs in the past or Google now, they incentivize and allow for their personnel to pursue innovative “blue-sky” research that is not immediately applicable for profit. But if you look at academia, it’s much more conservative than the commercial sector. That doesn’t make sense.

 How do you respond to the idea that for a person with a hammer, everything looks like a nail? Someone could uncharitably say what you are really doing here is attempting to curry further favor with wealthy benefactors, such as Yuri Milner, because you are an adviser for his Breakthrough Initiatives programs, which fund research related to SETI and light sails.

 It’s true for me—and everyone else, I think—that my imagination is limited by what I know. I can’t deny the fact that my involvement in Breakthrough was influential here. I was the one who suggested the light sail [proposed by physicist Philip Lubin] to Yuri Milner as a promising concept for interstellar spacecraft in the first place. So I had it in my vocabulary, and as a result of that, I imagined it as applied to ‘Oumuamua. Now, you might ask, “Okay, well, isn’t that a biased view?” I would say this occurs again and again in physics and in SETI. In the context of SETI, you know, once we developed radio technology, we started searching the sky looking for radio signals. It was the same for lasers. It’s just natural that once you work on some technology that you imagine maybe it exists out there and search for it. So I would not deny that the reason the light sail idea was in my brain is because I had previously worked on it, yeah. But in terms of trying to motivate Yuri, that has nothing to do with it. Why would I do it this way when I can just approach him directly whenever I want to advocate my views? And it is not as if my work on ‘Oumuamua was coordinated with or supported by Breakthrough Initiatives. They have issued no press releases about my ideas. If anything, they might be worried—they have their own reputation to preserve and so forth. On this issue, I’ve had zero support from or communication with them. This was me being curious, not using ‘Oumuamua as some sort of a political vehicle in the context of Breakthrough. That has nothing to do with my motivation.

 After this, what comes next for you? Do you have plans?

 I just stepped down from being chair of Harvard’s astronomy department, so I really do have the ability now to move to the next phase. And the question is: What would it be? Life, of course, is not always what you’ve planned, but another leadership opportunity would be so tempting because I could try to shape reality in a way others would not. I couldn’t pass that up. But maybe we should exclude leadership possibilities from this. Maybe I won’t be offered anything again because of my ideas about ‘Oumuamua! That’s a possibility. Then I’d write more books, do more research and continue to jog every morning.

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 Lee Billings is a senior editor for space and physics at Scientific American. Credit: Nick Higgins

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Avi Loeb is no stranger to controversy. The prolific Harvard University astrophysicist has produced pioneering and provocative research on black holes, gamma-ray bursts, the early universe and other standard topics of his field. But for more than a decade he has also courted a more contentious subject—namely, space aliens, including how to find them. Until relatively recently, Loeb’s most high-profile work in that regard was his involvement with Breakthrough Starshot, a project funded by Silicon Valley billionaire Yuri Milner to send laser-boosted, gossamer-thin mirrorlike spacecraft called “light sails” on high-speed voyages to nearby stars. All that began to change in late 2017, however, when astronomers around the world scrambled to study an enigmatic interstellar visitor—the first ever seen—that briefly came within range of their telescopes.
The object’s discoverers dubbed it ‘Oumuamua—a Hawaiian term that roughly translates to “scout.” The unavoidably cursory examinations of this celestial passerby showed it had several properties that defied easy natural explanation. ‘Oumuamua’s apparent shape—which was like a 100-meter-long cigar or pancake—did not closely resemble any known asteroid or comet. Neither did its brightness, which revealed ‘Oumuamua was at least 10 times more reflective than one of our solar system’s typical space rocks—shiny enough to suggest the gleam of burnished metal. Most strangely, as it zoomed off after swooping by the sun, the object sped up faster than could be explained by our star’s waning gravitational grip alone. Run-of-the-mill comets can exhibit similar accelerations because of the rocketlike effect of evaporating gases jetting from their sunlight-warmed icy surfaces. But no signs of such jets were seen around ‘Oumuamua.
To Loeb, the most plausible explanation was as obvious as it was sensational: taken together with its possibly pancakelike shape and high reflectivity, ‘Oumuamua’s anomalous acceleration made perfect sense if the object was in fact a light sail—perhaps a derelict from some long-expired galactic culture. Primed by years spent pondering how we might someday find evidence of cosmic civilizations in the sky’s depths, he became increasingly convinced that, with ‘Oumuamua, the evidence had instead found us. In late 2018 Loeb and his co-author Shmuel Bialy, a Harvard postdoctoral fellow, published a paper in the Astrophysical Journal Letters arguing that ‘Oumuamua had been nothing less than humanity’s first contact with an artifact of extraterrestrial intelligence.
The paper has been a smash hit with journalists but has fallen flat with most of Loeb’s astrobiology-focused peers, who insist that, while strange, ‘Oumuamua’s properties still place it well within the realm of natural phenomena. To claim otherwise, Loeb’s critics say, is cavalier at best and destructive at worst for the long struggle to remove the stigma of credulous UFO and alien-abduction reports from what should unquestionably be a legitimate field of scientific inquiry.
Loeb has now taken his case to the public with the book  Extraterrestrial: The First Sign of Intelligent Life beyond Earth  , which is just as much about the author’s life story as it is about ‘Oumuamua’s fundamental mysteries. Scientific American spoke with Loeb about the book, his controversial hypothesis and why he believes science is in crisis.
[An edited transcript of the interview follows.]
 Hi Avi. How are you? 
I’m good, but I have been losing sleep, because in order to cope with all the media requests, I’ve been doing interviews with, for example, Good Morning Britain at 1:50 A.M. and Coast to Coast AM at 3 A.M.—plus appearances on U.S. network and cable television. I’ve got about 100 podcast interviews to do in the next few weeks. And I already recorded long conversations with [podcasters] Lex Fridman and Joe Rogan for their shows. I’ve never seen anything like this; there has been so much interest in the book. I mean, there were 10 filmmakers and producers from Hollywood who contacted me over the past few weeks! I joked with my literary agent that if a film comes out of this, I want to be played by Brad Pitt.
 Ha, indeed, the resemblance is uncanny. Based on your productivity, I’ve never gotten the sense that you get a lot of sleep anyway. 
My routine is to wake up each morning at 5 A.M. and go jogging. It’s really beautiful when nobody's outside—just me and the birds, ducks and rabbits. And, yes, because of the pandemic, the past 10 months have been the most productive in my career. I don’t need to commute to work. I don’t need to meet so many people. And most importantly, I don’t need to think about what’s wrong with all the things that other people say! 
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 Speaking of important things, here is one I think we both agree on: in science, we must keep each other honest.  I mention it only because there’s a point in Extraterrestrial where you claim you don’t want the limelight and that you’re not interested in self-promotion. How can that be true? 
Let me explain. I think talking to the media is an important opportunity because it allows me to share my message with a broader audience that otherwise would not have exposure to it.
 What is your message, exactly? I take it you’re talking about more than ‘Oumuamua. 
Yes. My message is that something is wrong with the scientific community today in terms of its health.
Too many scientists are now mostly motivated by ego, by getting honors and awards, by showing their colleagues how smart they are. They treat science as a monologue about themselves rather than a dialogue with nature. They build echo chambers using students and postdocs who repeat their mantras so that their voice will be louder and their image will be promoted. But that’s not the purpose of science. Science is not about us; it’s not about empowering ourselves or making our image great. It’s about trying to understand the world, and it’s meant to be a learning experience in which we take risks and make mistakes along the way. You can never tell in advance, when you work on the frontier, what is the right path forward. You only learn that by getting feedback from experiments.
Which is the other problem with science today: people are not only motivated by the wrong reasons; they are also no longer guided by evidence. Evidence keeps you modest because you predict something, you test it, and the evidence sometimes shows you’re wrong. Right now you have many celebrated scientists doing mathematical gymnastics about lots of untestable things: string theory, the multiverse, even the theory of cosmic inflation. Once, in a public forum, I asked [physicist] Alan Guth, who originated the theory, “Is inflation falsifiable?” And he said it’s a silly question, because for whatever cosmological data an experiment gives us, a model of inflation can be found that accommodates it. And therefore, inflation is in a very strong position because it can explain anything! But I see this as a very weak position because a theory of everything is sometimes a theory of nothing. There may be no difference between the two.
To me, this bubble of imaginary stuff is like being high on drugs: You can get high and imagine that you’re wealthier than Elon Musk, who is now the richest person in the world. That’s a very fun thought. You can feel really good about it and talk about it with your friends. And if you’re part of a big like-minded community, everyone can support and respect one another, and you give one another awards, and that’s great, right? But then if you go to withdraw funds, if you want to really spend that money you think you have, you realize that you don’t actually have anything. Just like going to an ATM, doing experiments can serve as a reality check. And in science, it’s essential that we have that check—that we make testable predictions and put some skin in the game—because otherwise we won’t learn anything new. I don’t think that’s properly recognized anymore.
Marked with a blue circle, ‘Oumuamua appears as a faint dot in the center of this image, which is one of the best available and combines observations from multiple different telescopes. Credit: ESO/K. Meech et al. 
 So speculating about string theory and multiverses is bad, but speculating about alien civilizations and their artifacts passing through the solar system is okay? You could say appealing to “aliens” can explain anything, too. 
The difference is: you can make predictions and test for the latter, and the speculations come from a conservative position.
If ‘Oumuamua is a member of a population of objects moving on random trajectories, then based on its discovery with the Pan-STARRS telescope, you can estimate that we should very soon begin finding, on average, one of these objects per month after the Vera C. Rubin Observatory comes online. We can also establish a system of instruments—satellites, maybe—that would not only monitor the sky but also be able to react to the approach of such objects so we can get photographs of them as they come in rather than chasing them as they go out, because they move very fast. Not all this work needs to be in space, either: You can imagine meteors of interstellar origin as well, and we can search for those. And if you find any that ended up on Earth’s surface, you might even be able to examine them with your own hands.
People ask why I get this media attention. The only reason is because my colleagues are not using common sense. Contrast string theory and multiverses with what I and many others say, which is that based on the data from NASA’s Kepler mission, roughly half of the galaxy’s sunlike stars have a planet about the size of the Earth, at about the same distance of the Earth from the sun, so that you can have liquid water on the surface and the chemistry of life as we know it. So if you roll the dice on life billions of times in the Milky Way, what is the chance that we are alone? Minuscule, most likely! To say that if you arrange for similar circumstances, you get similar outcomes is, to me, the most conservative statement imaginable. So I would expect most people to endorse that, to hug me and say, “Great, Avi, you’re correct. We should look for these things because they must be very likely.” Instead what I see is a backlash that shows a loss of an intellectual compass—because how else can you explain working on string theory’s extra dimensions or the multiverse when we have no clue for their existence? But that is considered mainstream? That’s crazy.
Allow me to put this in a very specific context. I’m obviously not a rebel outsider; I’m in leadership positions. I chair the Board on Physics and Astronomy of the National Academies [of Sciences, Engineering, and Medicine], okay? That board is overseeing the Astronomy and Astrophysics Decadal Survey, which will set major science priorities for NASA and the [National Science Foundation] when it is released later this year. Now, I see astronomers talking about future telescopes costing billions of dollars, with the main motivation being to find life by looking for oxygen in the atmospheres of exoplanets. That is a noble wish. But if you look at the Earth for its first two billion years or so, the planet did not have much oxygen in its atmosphere even though it had a lot of microbial life. That’s point number one. Point number two is that even if you have oxygen, you can get it from natural processes such as breaking apart water molecules. So even if you if you spend these billions and find oxygen and maybe even find methane along with it, people will still argue about it forever. Look at how much discussion there has been about the potential detection of phosphine on Venus, which is a very unusual molecule, compared with oxygen. Anyway, my point is that with these same instruments—you don’t need any extra investment of funds—you can actually get conclusive evidence for life, intelligence and technology. What would that be? Industrial pollution in the same atmosphere. You could, for instance, look for chlorofluorocarbons, these complex molecules only produced on Earth for refrigeration systems. If you found that on another planet, there is just no way nature would produce these molecules naturally. You would have conclusive evidence that life—and more—existed there.
So what is the problem with saying that looking for industrial pollution is a worthwhile thing to do? What other than some sort of psychological barrier that prevents some scientists from admitting they want the search for technological signatures of alien civilizations to be at the periphery, with very little funding? What I’m saying is that these sorts of things should be prioritized and that they are conservative things to do because they will bring us the most information about the existence of alien life. And yet the opposite is being done right now.
 You write about a concept you call “‘Oumuamua’s wager,” after Pascal’s wager, 17th-century mathematician Blaise Pascal’s argument that the benefits of assuming God exists outweigh the drawbacks. Similarly, you say believing ‘Oumuamua is an alien artifact would be a net good because it could catalyze a revolution in space science and technology centered around a more vigorous search for life and intelligence beyond Earth. Even if that hunt finds no aliens, your reasoning goes, we’d still gain a much deeper understanding of our cosmic context. And the investments behind it would enhance our ability to answer other questions about the universe and perhaps even help stave off our own extinction. 
 But if the stakes are so high, what about the counterargument that going “all in” on promoting ‘Oumuamua’s putative artificial nature is reckless and dangerous? Your critics say you are doing more harm than good. For instance, you mentioned you appeared on Joe Rogan’s podcast, one of the most popular in the world. That’s great for selling books. But given Rogan’s reputation for spreading dangerous misinformation on his podcast, is that sort of thing a wise move? Would you also agree to be a speaker at a gathering of UFO “true believers” outside Area 51? Where do you draw the line for public outreach that risks enhancing the so-called giggle factor that has stymied progress in the search for extraterrestrial intelligence (SETI) for decades? 
Okay, here is my point of view. By and large, the public funds science. And the public is extremely interested in the search for alien life. So I must ask: If scientists are supported by the public, how dare they shy away from this question that can be addressed with the technologies they are developing?
There are, of course, science-fiction stories about aliens, and there are many unsubstantiated UFO reports. Now, suppose there was some literature about the magical properties of COVID-19 that had no bearing in reality. Would that mean scientists should never work on finding a vaccine to this pandemic? No! I don’t see the search for technological signatures any differently from the search for the nature of dark matter. We have invested hundreds of millions of dollars in searching for weakly interacting massive particles, a leading dark matter candidate. And so far those searches have failed. That doesn’t mean they were a waste: going down dark alleys is part of the scientific process.
And in terms of risk, in science, we are supposed to put everything on the table. We cannot just avoid certain ideas because we worry about the consequences of discussing them, because there is great risk in that, too. That would be similar to telling Galileo not to speak about Earth moving around the sun and to avoid looking in his telescope because it was dangerous to the philosophy of the day. We should not want to repeat that experience. We need an open dialogue among scientists where people present different ideas and then allow evidence to dictate which one is right. In the context of ‘Oumuamua, I say the available evidence suggests this particular object is artificial, and the way to test this is to find more [examples] of the same and examine them. It’s as simple as that.
So how do you change this situation? I think the answer is to bring it to the public as much as I can.
Credit: Houghton Mifflin Harcourt 
 In your book, you link your outspokenness about ‘Oumuamua with a phrase, an ethos, you learned when you were a conscript in the Israel Defense Forces: “To lay your body on the barbed wire.” That is, to make personal sacrifices for the greater good. Are you to be a martyr for this cause, then? Have you lost friends or stature over it? 
No one has violently assaulted me or anything like that. Maybe people talk behind my back, which would make more sense, given my leadership positions. But I don’t really know. I have zero footprint on social media. Although I should say that I think my critics who are most vocal with nasty remarks on Twitter and elsewhere are relatively mediocre scientists. Most really good scientists would not behave that way—they would make arguments for or against my claims, and that would be enough. Nasty remarks don’t make sense—except, well, deep inside, I would not be surprised if many of these critics are actually quite intrigued by this possibility that ‘Oumuamua is artificial. But they don’t want to admit it. So they loudly say the opposite.
Unfortunately, my situation is different from that of the young postdocs who I’ve worked with because they need to apply for jobs. I’m sure that people have approached them and said, “Look, this is dangerous for you.” And so they froze and basically stopped working on anything related. This isn’t surprising. If you create a hostile intellectual culture where something like SETI is not being honored, then young, bright people will not go there. But don’t step on the grass and then complain it doesn’t grow as you stand on it. Don’t block brilliant researchers from working on SETI and then say, “Look, nothing is being found. SETI is a failure!”
None of this means all of space science should be about SETI. If you look at the commercial world, companies such as Bell Labs in the past or Google now, they incentivize and allow for their personnel to pursue innovative “blue-sky” research that is not immediately applicable for profit. But if you look at academia, it’s much more conservative than the commercial sector. That doesn’t make sense.
 How do you respond to the idea that for a person with a hammer, everything looks like a nail? Someone could uncharitably say what you are really doing here is attempting to curry further favor with wealthy benefactors, such as Yuri Milner, because you are an adviser for his Breakthrough Initiatives programs, which fund research related to SETI and light sails. 
It’s true for me—and everyone else, I think—that my imagination is limited by what I know. I can’t deny the fact that my involvement in Breakthrough was influential here. I was the one who suggested the light sail [proposed by physicist Philip Lubin] to Yuri Milner as a promising concept for interstellar spacecraft in the first place. So I had it in my vocabulary, and as a result of that, I imagined it as applied to ‘Oumuamua. Now, you might ask, “Okay, well, isn’t that a biased view?” I would say this occurs again and again in physics and in SETI. In the context of SETI, you know, once we developed radio technology, we started searching the sky looking for radio signals. It was the same for lasers. It’s just natural that once you work on some technology that you imagine maybe it exists out there and search for it. So I would not deny that the reason the light sail idea was in my brain is because I had previously worked on it, yeah. But in terms of trying to motivate Yuri, that has nothing to do with it. Why would I do it this way when I can just approach him directly whenever I want to advocate my views? And it is not as if my work on ‘Oumuamua was coordinated with or supported by Breakthrough Initiatives. They have issued no press releases about my ideas. If anything, they might be worried—they have their own reputation to preserve and so forth. On this issue, I’ve had zero support from or communication with them. This was me being curious, not using ‘Oumuamua as some sort of a political vehicle in the context of Breakthrough. That has nothing to do with my motivation.
 After this, what comes next for you? Do you have plans? 
I just stepped down from being chair of Harvard’s astronomy department, so I really do have the ability now to move to the next phase. And the question is: What would it be? Life, of course, is not always what you’ve planned, but another leadership opportunity would be so tempting because I could try to shape reality in a way others would not. I couldn’t pass that up. But maybe we should exclude leadership possibilities from this. Maybe I won’t be offered anything again because of my ideas about ‘Oumuamua! That’s a possibility. Then I’d write more books, do more research and continue to jog every morning.
Astronomer Avi Loeb Says Aliens Have Visited, and He’s Not Kidding
https://www.scientificamerican.com/article/astronomer-avi-loeb-says-aliens-have-visited-and-hes-not-kidding1/
https://nineplanets.org/questions/how-big-is-the-universe/
How Big is the Universe? | Size, Why So Big, Infinite & Facts
2020-09-29
None
<div><div>
	<main>
	
<article>
	<div>
	<p>Think about this for a second; it takes us around three days to reach the <a href="https://nineplanets.org/the-moon/">Moon</a>, approximately seven months to get the closest <a href="https://nineplanets.org/planets/">planet</a> to us, namely <a href="https://nineplanets.org/mars/">Mars</a>, 15 months to reach <a href="https://nineplanets.org/venus/">Venus</a>, six years to reach <a href="https://nineplanets.org/jupiter/">Jupiter</a>, seven to reach <a href="https://nineplanets.org/saturn/">Saturn</a>, 8.5 years to reach <a href="https://nineplanets.org/uranus/">Uranus</a>, 9.5 years to reach <a href="https://nineplanets.org/pluto/">Pluto</a> – the closest <a href="https://nineplanets.org/planets/dwarf-planets/">dwarf planet</a>, and twelve years to get to <a href="https://nineplanets.org/neptune/">Neptune</a>, the farthest planet.</p>
<p>The <a href="https://nineplanets.org/the-sun/">Sun</a> is 0.00001581 light-years away, and in the best-case scenario, we could reach it in 25 days. So how big is the <a href="https://nineplanets.org/the-universe/">Universe</a>? Its around 93 billion light-years. How much is that? Well, let’s think about the Sun again.</p>
<p></p>
<p>The Sun is one astronomical unit (AU) away from us. One astronomical unit is 149,598,000 km / 92,955,887 miles, and in our top shape, we could reach it in 25 days. Now, the Universe is 93 billion light-years across, and one, just one light-year, is equivalent to 63,000 astronomical units. </p>
<p>As such, one light-year is the equivalent to 9 trillion kilometers / 6 trillion miles, and our Universe is 93 billion light-years in diameter. That’s how big our Universe is, and that’s not even the end of it. The 93 billion years is just the observable Universe, the Universe, which we can currently see. The whole Universe might very well be 250 times larger than the observable Universe, or at least 7 trillion light-years in diameter.</p>
<h2><b>Why is the Universe so Big?</b></h2>
<p>The Universe is so big because it is constantly expanding, and it does so at a speed that even exceeds the speed of light. Space itself is actually growing, and this is going on for around 14 billion years or so.</p>
<p>In this amount of time, with speed greater than the speed of light, the Universe gradually grew, and it still expands even to this day. There isn’t actually an answer to why the Universe is this big.</p>
<p>Think about how big you are compared to an ant or an atom. There is a huge difference even there. It all has to do with our perception, and even in our current modern age, our perception of the Universe is vastly limited.</p>
<p></p>
<p>Think of it this way, we might see the Universe as being big now, but in the far future, who knows how future generations would view it. Our ancestors didn’t have cars or planes, and they would traverse the world in many months, even years. For them, traveling from point a to b seemed incredibly difficult, and they themselves might have thought, why is the <a href="https://nineplanets.org/earth/">Earth</a> so big?</p>
<p>Nowadays, the world doesn’t seem so big anymore. You can reach point b from point a in a couple of minutes, hours, or in the worst case, days. Our perception is the only thing at play here. </p>
<p>In the future, who knows, maybe we could actually invent something that could travel at the speed of light. Perhaps we might even invent teleportation, or we could use wormholes.</p>
<p>If such technologies are there for us to grab, then will the Universe truly seem so big then? Perhaps it will, but if the vast distances which we should traverse are more easily done, and if the man of the future will find a way to prolong his life further, then the Universe will certainly begin to feel much smaller. Again, perception is everything at play here. We are asking the wrong questions; nature is what it is. </p>
<h2><b>Is the Universe Really Infinite?</b></h2>
<p>Many believe that our Universe is just 13.8 billion years old. However, this is uncertain until proven with extreme accuracy. Sometimes, we can’t even pinpoint the certain age of an object here on Earth, let alone our Universe.</p>
<p>The Universe may be infinite, or it might not be, but again, our perception is at play here. If we will analyze how many <a href="https://nineplanets.org/stars/">stars</a>, planets, and the distances involved for reaching them, and the fact that our Universe is expanding, then it certainly seems infinite.</p>
<p></p>
<p>Even if our Universe weren’t infinite, we would view it as such due to its vastness, and the time it would take to explore everything in it. Our perception creates endless problems in the end.</p>
<p>Now, as you observe, our world is created from dualistic elements—day and night, hot or cold, love or hate, etc. Everything in our little world seems finite, so why shouldn’t there be an infinite element, such as our Universe? Many are afraid to admit that something is infinite, but no matter how you look at it, even if you would reach the end of the Universe, it would still seem infinite regardless.</p>
<h2><b>How Big is the Universe Compared to the Observable Universe?</b></h2>
<p>The observable Universe is 93 billion light-years in diameter. Some scientists believe its true size is even scarier than that. By using the Bayesian model averaging, scientists estimated that the Universe is at least 250 times larger than the observable Universe, or at least 7 trillion light-years in diameter.</p>
<p></p>
<p>The Bayesian model focuses on how likely a model is to be correct, given the data, rather than asking how well the model itself fits the data. Now, this might not be the best method of estimating the true size of our Universe. Still, chances are very high that our Universe is nonetheless, bigger than the observable Universe.</p>
<h2><b>Does the Universe End?</b></h2>
<p>Many cosmologists agree that the Universe is flat and might expand forever. Others theorize that the Universe will end one day. Perhaps the most logical answer is that when all the stars reach the end of their life spans, the Universe will end as the light will end, everything will be covered in darkness.</p>
<p>Maybe the Universe won’t end there if other means of survival are available. Some believe that the Universe will begin to cool as it expands, and life will cease to exist because of this. This theory is named the Big Freeze.</p>
<p></p>
<p>Another popular scenario suggests that our Universe will stop expanding, and will actually reverse this process. When this happens, the Universe will re-collapse, and it will possibly lead to a reformation that will start with another Big Bang. This scenario is called the Big Crunch.</p>
<p>A third popular theory is named the Big Rip. This theory states that everything will be torn to shreds, including atoms. It will happen when the theoretical energy known as dark energy, becomes stronger than gravity. No matter the case, what is certain is that these apocalyptic scenarios won’t happen for billions of years to come.</p>
<h2><b>Where Does Space End?</b></h2>
<p>Space doesn’t end because it is constantly expanding, faster than the speed of light itself. From our point of view, space end at approximately 93 billion light-years away from us, but that isn’t the end of the Universe. It is only how far we can see.</p>
<h2><b>Will the Universe Last Forever?</b></h2>
<p>It is unknown if the Universe will last forever, but most likely, we won’t even be there to see it. We currently do not know if the Univers will stop expanding, and if it does, what would this imply.</p>
<p>Many have proposed several apocalyptic scenarios, like the ones aforementioned above, but isn’t that just typical of us? The Universe might just well last forever, but one thing is true. We are very far from answering such questions.</p>
<p></p>
<p>Everything in the Universe is in motion, and it appears that many celestial objects, such as <a href="https://nineplanets.org/galaxies/">galaxies</a>, are moving away from us. Perhaps, this is the true end of the Universe, when things will be so far apart that nothing could be reached anymore, and nothing could be concluded as being the Universe anymore since everything will be so far apart, we wouldn’t even know its there.</p>
<h2><b>Did you Know?</b></h2>
<ul>
<li>Around 550 people have been into Space, and only three of them have died in accidents.</li>
<li>The smallest thing in the Universe that we currently know of is the atom.</li>
<li>The biggest thing we have discovered so far in our Universe is the Hercules-Corona Borealis Great Wall. It is a supercluster which has a diameter of around 10 billion light-years.</li>
<li>Many believe that our Universe is only one of a set of separate universes, collectively known as the multiverse.</li>
<li>The word cosmos, rather than Universe, implies viewing the Universe as a complex and orderly system or deity – the opposite of chaos.</li>
<li>The observable Universe is 93 billion light-years, yet, our galaxy, the <a href="https://nineplanets.org/milky-way/">Milky Way</a>, is just 100,000 light-years in diameter. It would take us endless generations just to explore our galaxy, let alone the Universe.</li>
<li>Another ancient structure is the galaxy supercluster known as the Hyperion Supercluster. This celestial object has more than four quadrillion solar masses, and many estimate that it formed just 2 billion years after the Big Bang.</li>
<li>Universe means “whole,” and it comes from the Latin word “universus.”</li>
</ul>
<h4><b>Sources:</b></h4>
<ol>
<li><a href="https://en.wikipedia.org/wiki/Universe">Wikipedia</a></li>
<li><a href="https://www.nasa.gov/audience/foreducators/5-8/features/F_How_Big_is_Our_Universe.html">NASA</a></li>
<li><a href="https://www.space.com/24073-how-big-is-the-universe.html">Space</a></li>
<li><a href="https://www.livescience.com/how-big-universe.html">Livescience</a></li>
<li><a href="https://scaleofuniverse.com/">Scaleofuniverse</a></li>
<li><a href="https://phys.org/news/2015-10-big-universe.html">Phys</a></li>
<li><a href="https://coolcosmos.ipac.caltech.edu/ask/237-How-big-is-the-Universe-#:~:text=We%20can%20only%20see%20out,years%20in%20diameter%20(across).">Coolcosmos</a></li>
</ol>
<h4><b>Image Sources:</b></h4>
<ul>
<li><a href="https://miro.medium.com/max/670/1*73_LKv_ioTH3TF5Hc0LFYg.jpeg">https://miro.medium.com/max/670/1*73_LKv_ioTH3TF5Hc0LFYg.jpeg</a></li>
<li><a href="https://thumbor.forbes.com/thumbor/960x0/https%3A%2F%2Fblogs-images.forbes.com%2Fstartswithabang%2Ffiles%2F2016%2F07%2F060915_CMB_Timeline150-1200x819.jpg">https://thumbor.forbes.com/thumbor/960×0/https%3A%2F%2Fblogs-images.forbes.com%2Fstartswithabang%2Ffiles%2F2016%2F07%2F060915_CMB_Timeline150-1200×819.jpg</a></li>
<li><a href="https://cdn.images.express.co.uk/img/dynamic/151/590x/1-695868.jpg">https://cdn.images.express.co.uk/img/dynamic/151/590x/1-695868.jpg</a></li>
<li><a href="https://thumbor.forbes.com/thumbor/960x0/https%3A%2F%2Fblogs-images.forbes.com%2Fstartswithabang%2Ffiles%2F2017%2F05%2FSDSS_Deep.jpg">https://thumbor.forbes.com/thumbor/960×0/https%3A%2F%2Fblogs-images.forbes.com%2Fstartswithabang%2Ffiles%2F2017%2F05%2FSDSS_Deep.jpg</a></li>
<li><a href="https://cdn.iflscience.com/images/4a22cda7-0ef5-5226-9ed3-e80c4cf053c7/default-1510334289-cover-image.jpg">https://cdn.iflscience.com/images/4a22cda7-0ef5-5226-9ed3-e80c4cf053c7/default-1510334289-cover-image.jpg</a></li>
<li><a href="https://images.theconversation.com/files/93501/original/image-20150901-25717-1y3igv.jpg?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=496&amp;fit=clip">https://images.theconversation.com/files/93501/original/image-20150901-25717-1y3igv.jpg?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=496&amp;fit=clip</a></li>
</ul>
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</article>
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 Home » Space Questions » How Big is the Universe?

 Think about this for a second; it takes us around three days to reach the Moon , approximately seven months to get the closest planet to us, namely Mars , 15 months to reach Venus , six years to reach Jupiter , seven to reach Saturn , 8.5 years to reach Uranus , 9.5 years to reach Pluto – the closest dwarf planet , and twelve years to get to Neptune , the farthest planet.

 The Sun is 0.00001581 light-years away, and in the best-case scenario, we could reach it in 25 days. So how big is the Universe ? Its around 93 billion light-years. How much is that? Well, let’s think about the Sun again.

 The Sun is one astronomical unit (AU) away from us. One astronomical unit is 149,598,000 km / 92,955,887 miles, and in our top shape, we could reach it in 25 days. Now, the Universe is 93 billion light-years across, and one, just one light-year, is equivalent to 63,000 astronomical units.

 As such, one light-year is the equivalent to 9 trillion kilometers / 6 trillion miles, and our Universe is 93 billion light-years in diameter. That’s how big our Universe is, and that’s not even the end of it. The 93 billion years is just the observable Universe, the Universe, which we can currently see. The whole Universe might very well be 250 times larger than the observable Universe, or at least 7 trillion light-years in diameter.

 Why is the Universe so Big?

 The Universe is so big because it is constantly expanding, and it does so at a speed that even exceeds the speed of light. Space itself is actually growing, and this is going on for around 14 billion years or so.

 In this amount of time, with speed greater than the speed of light, the Universe gradually grew, and it still expands even to this day. There isn’t actually an answer to why the Universe is this big.

 Think about how big you are compared to an ant or an atom. There is a huge difference even there. It all has to do with our perception, and even in our current modern age, our perception of the Universe is vastly limited.

 Think of it this way, we might see the Universe as being big now, but in the far future, who knows how future generations would view it. Our ancestors didn’t have cars or planes, and they would traverse the world in many months, even years. For them, traveling from point a to b seemed incredibly difficult, and they themselves might have thought, why is the Earth so big?

 Nowadays, the world doesn’t seem so big anymore. You can reach point b from point a in a couple of minutes, hours, or in the worst case, days. Our perception is the only thing at play here.

 In the future, who knows, maybe we could actually invent something that could travel at the speed of light. Perhaps we might even invent teleportation, or we could use wormholes.

 If such technologies are there for us to grab, then will the Universe truly seem so big then? Perhaps it will, but if the vast distances which we should traverse are more easily done, and if the man of the future will find a way to prolong his life further, then the Universe will certainly begin to feel much smaller. Again, perception is everything at play here. We are asking the wrong questions; nature is what it is.

 Is the Universe Really Infinite?

 Many believe that our Universe is just 13.8 billion years old. However, this is uncertain until proven with extreme accuracy. Sometimes, we can’t even pinpoint the certain age of an object here on Earth, let alone our Universe.

 The Universe may be infinite, or it might not be, but again, our perception is at play here. If we will analyze how many stars , planets, and the distances involved for reaching them, and the fact that our Universe is expanding, then it certainly seems infinite.

 Even if our Universe weren’t infinite, we would view it as such due to its vastness, and the time it would take to explore everything in it. Our perception creates endless problems in the end.

 Now, as you observe, our world is created from dualistic elements—day and night, hot or cold, love or hate, etc. Everything in our little world seems finite, so why shouldn’t there be an infinite element, such as our Universe? Many are afraid to admit that something is infinite, but no matter how you look at it, even if you would reach the end of the Universe, it would still seem infinite regardless.

 How Big is the Universe Compared to the Observable Universe?

 The observable Universe is 93 billion light-years in diameter. Some scientists believe its true size is even scarier than that. By using the Bayesian model averaging, scientists estimated that the Universe is at least 250 times larger than the observable Universe, or at least 7 trillion light-years in diameter.

 The Bayesian model focuses on how likely a model is to be correct, given the data, rather than asking how well the model itself fits the data. Now, this might not be the best method of estimating the true size of our Universe. Still, chances are very high that our Universe is nonetheless, bigger than the observable Universe.

 Does the Universe End?

 Many cosmologists agree that the Universe is flat and might expand forever. Others theorize that the Universe will end one day. Perhaps the most logical answer is that when all the stars reach the end of their life spans, the Universe will end as the light will end, everything will be covered in darkness.

 Maybe the Universe won’t end there if other means of survival are available. Some believe that the Universe will begin to cool as it expands, and life will cease to exist because of this. This theory is named the Big Freeze.

 Another popular scenario suggests that our Universe will stop expanding, and will actually reverse this process. When this happens, the Universe will re-collapse, and it will possibly lead to a reformation that will start with another Big Bang. This scenario is called the Big Crunch.

 A third popular theory is named the Big Rip. This theory states that everything will be torn to shreds, including atoms. It will happen when the theoretical energy known as dark energy, becomes stronger than gravity. No matter the case, what is certain is that these apocalyptic scenarios won’t happen for billions of years to come.

 Where Does Space End?

 Space doesn’t end because it is constantly expanding, faster than the speed of light itself. From our point of view, space end at approximately 93 billion light-years away from us, but that isn’t the end of the Universe. It is only how far we can see.

 Will the Universe Last Forever?

 It is unknown if the Universe will last forever, but most likely, we won’t even be there to see it. We currently do not know if the Univers will stop expanding, and if it does, what would this imply.

 Many have proposed several apocalyptic scenarios, like the ones aforementioned above, but isn’t that just typical of us? The Universe might just well last forever, but one thing is true. We are very far from answering such questions.

 Everything in the Universe is in motion, and it appears that many celestial objects, such as galaxies , are moving away from us. Perhaps, this is the true end of the Universe, when things will be so far apart that nothing could be reached anymore, and nothing could be concluded as being the Universe anymore since everything will be so far apart, we wouldn’t even know its there.

 Did you Know?

 * Around 550 people have been into Space, and only three of them have died in accidents.
 * The smallest thing in the Universe that we currently know of is the atom.
 * The biggest thing we have discovered so far in our Universe is the Hercules-Corona Borealis Great Wall. It is a supercluster which has a diameter of around 10 billion light-years.
 * Many believe that our Universe is only one of a set of separate universes, collectively known as the multiverse.
 * The word cosmos, rather than Universe, implies viewing the Universe as a complex and orderly system or deity – the opposite of chaos.
 * The observable Universe is 93 billion light-years, yet, our galaxy, the Milky Way , is just 100,000 light-years in diameter. It would take us endless generations just to explore our galaxy, let alone the Universe.
 * Another ancient structure is the galaxy supercluster known as the Hyperion Supercluster. This celestial object has more than four quadrillion solar masses, and many estimate that it formed just 2 billion years after the Big Bang.
 * Universe means “whole,” and it comes from the Latin word “universus.”

 Sources:

 1. Wikipedia
 2. NASA
 3. Space
 4. Livescience
 5. Scaleofuniverse
 6. Phys
 7. Coolcosmos

 Image Sources:

 * https://miro.medium.com/max/670/1*73_LKv_ioTH3TF5Hc0LFYg.jpeg
 * https://thumbor.forbes.com/thumbor/960×0/https%3A%2F%2Fblogs-images.forbes.com%2Fstartswithabang%2Ffiles%2F2016%2F07%2F060915_CMB_Timeline150-1200×819.jpg
 * https://cdn.images.express.co.uk/img/dynamic/151/590x/1-695868.jpg
 * https://thumbor.forbes.com/thumbor/960×0/https%3A%2F%2Fblogs-images.forbes.com%2Fstartswithabang%2Ffiles%2F2017%2F05%2FSDSS_Deep.jpg
 * https://cdn.iflscience.com/images/4a22cda7-0ef5-5226-9ed3-e80c4cf053c7/default-1510334289-cover-image.jpg
 * https://images.theconversation.com/files/93501/original/image-20150901-25717-1y3igv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip

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Think about this for a second; it takes us around three days to reach the Moon, approximately seven months to get the closest planet to us, namely Mars, 15 months to reach Venus, six years to reach Jupiter, seven to reach Saturn, 8.5 years to reach Uranus, 9.5 years to reach Pluto – the closest dwarf planet, and twelve years to get to Neptune, the farthest planet.
The Sun is 0.00001581 light-years away, and in the best-case scenario, we could reach it in 25 days. So how big is the Universe? Its around 93 billion light-years. How much is that? Well, let’s think about the Sun again.
 
The Sun is one astronomical unit (AU) away from us. One astronomical unit is 149,598,000 km / 92,955,887 miles, and in our top shape, we could reach it in 25 days. Now, the Universe is 93 billion light-years across, and one, just one light-year, is equivalent to 63,000 astronomical units. 
As such, one light-year is the equivalent to 9 trillion kilometers / 6 trillion miles, and our Universe is 93 billion light-years in diameter. That’s how big our Universe is, and that’s not even the end of it. The 93 billion years is just the observable Universe, the Universe, which we can currently see. The whole Universe might very well be 250 times larger than the observable Universe, or at least 7 trillion light-years in diameter.
 Why is the Universe so Big? 
The Universe is so big because it is constantly expanding, and it does so at a speed that even exceeds the speed of light. Space itself is actually growing, and this is going on for around 14 billion years or so.
In this amount of time, with speed greater than the speed of light, the Universe gradually grew, and it still expands even to this day. There isn’t actually an answer to why the Universe is this big.
Think about how big you are compared to an ant or an atom. There is a huge difference even there. It all has to do with our perception, and even in our current modern age, our perception of the Universe is vastly limited.
 
Think of it this way, we might see the Universe as being big now, but in the far future, who knows how future generations would view it. Our ancestors didn’t have cars or planes, and they would traverse the world in many months, even years. For them, traveling from point a to b seemed incredibly difficult, and they themselves might have thought, why is the Earth so big?
Nowadays, the world doesn’t seem so big anymore. You can reach point b from point a in a couple of minutes, hours, or in the worst case, days. Our perception is the only thing at play here. 
In the future, who knows, maybe we could actually invent something that could travel at the speed of light. Perhaps we might even invent teleportation, or we could use wormholes.
If such technologies are there for us to grab, then will the Universe truly seem so big then? Perhaps it will, but if the vast distances which we should traverse are more easily done, and if the man of the future will find a way to prolong his life further, then the Universe will certainly begin to feel much smaller. Again, perception is everything at play here. We are asking the wrong questions; nature is what it is. 
 Is the Universe Really Infinite? 
Many believe that our Universe is just 13.8 billion years old. However, this is uncertain until proven with extreme accuracy. Sometimes, we can’t even pinpoint the certain age of an object here on Earth, let alone our Universe.
The Universe may be infinite, or it might not be, but again, our perception is at play here. If we will analyze how many stars, planets, and the distances involved for reaching them, and the fact that our Universe is expanding, then it certainly seems infinite.
 
Even if our Universe weren’t infinite, we would view it as such due to its vastness, and the time it would take to explore everything in it. Our perception creates endless problems in the end.
Now, as you observe, our world is created from dualistic elements—day and night, hot or cold, love or hate, etc. Everything in our little world seems finite, so why shouldn’t there be an infinite element, such as our Universe? Many are afraid to admit that something is infinite, but no matter how you look at it, even if you would reach the end of the Universe, it would still seem infinite regardless.
 How Big is the Universe Compared to the Observable Universe? 
The observable Universe is 93 billion light-years in diameter. Some scientists believe its true size is even scarier than that. By using the Bayesian model averaging, scientists estimated that the Universe is at least 250 times larger than the observable Universe, or at least 7 trillion light-years in diameter.
 
The Bayesian model focuses on how likely a model is to be correct, given the data, rather than asking how well the model itself fits the data. Now, this might not be the best method of estimating the true size of our Universe. Still, chances are very high that our Universe is nonetheless, bigger than the observable Universe.
 Does the Universe End? 
Many cosmologists agree that the Universe is flat and might expand forever. Others theorize that the Universe will end one day. Perhaps the most logical answer is that when all the stars reach the end of their life spans, the Universe will end as the light will end, everything will be covered in darkness.
Maybe the Universe won’t end there if other means of survival are available. Some believe that the Universe will begin to cool as it expands, and life will cease to exist because of this. This theory is named the Big Freeze.
 
Another popular scenario suggests that our Universe will stop expanding, and will actually reverse this process. When this happens, the Universe will re-collapse, and it will possibly lead to a reformation that will start with another Big Bang. This scenario is called the Big Crunch.
A third popular theory is named the Big Rip. This theory states that everything will be torn to shreds, including atoms. It will happen when the theoretical energy known as dark energy, becomes stronger than gravity. No matter the case, what is certain is that these apocalyptic scenarios won’t happen for billions of years to come.
 Where Does Space End? 
Space doesn’t end because it is constantly expanding, faster than the speed of light itself. From our point of view, space end at approximately 93 billion light-years away from us, but that isn’t the end of the Universe. It is only how far we can see.
 Will the Universe Last Forever? 
It is unknown if the Universe will last forever, but most likely, we won’t even be there to see it. We currently do not know if the Univers will stop expanding, and if it does, what would this imply.
Many have proposed several apocalyptic scenarios, like the ones aforementioned above, but isn’t that just typical of us? The Universe might just well last forever, but one thing is true. We are very far from answering such questions.
 
Everything in the Universe is in motion, and it appears that many celestial objects, such as galaxies, are moving away from us. Perhaps, this is the true end of the Universe, when things will be so far apart that nothing could be reached anymore, and nothing could be concluded as being the Universe anymore since everything will be so far apart, we wouldn’t even know its there.
 Did you Know? 
Around 550 people have been into Space, and only three of them have died in accidents.
The smallest thing in the Universe that we currently know of is the atom.
The biggest thing we have discovered so far in our Universe is the Hercules-Corona Borealis Great Wall. It is a supercluster which has a diameter of around 10 billion light-years.
Many believe that our Universe is only one of a set of separate universes, collectively known as the multiverse.
The word cosmos, rather than Universe, implies viewing the Universe as a complex and orderly system or deity – the opposite of chaos.
The observable Universe is 93 billion light-years, yet, our galaxy, the Milky Way, is just 100,000 light-years in diameter. It would take us endless generations just to explore our galaxy, let alone the Universe.
Another ancient structure is the galaxy supercluster known as the Hyperion Supercluster. This celestial object has more than four quadrillion solar masses, and many estimate that it formed just 2 billion years after the Big Bang.
Universe means “whole,” and it comes from the Latin word “universus.”
 Sources: 
 Wikipedia 
 NASA 
 Space 
 Livescience 
 Scaleofuniverse 
 Phys 
 Coolcosmos 
 Image Sources: 
 https://miro.medium.com/max/670/1*73_LKv_ioTH3TF5Hc0LFYg.jpeg 
 https://thumbor.forbes.com/thumbor/960×0/https%3A%2F%2Fblogs-images.forbes.com%2Fstartswithabang%2Ffiles%2F2016%2F07%2F060915_CMB_Timeline150-1200×819.jpg 
 https://cdn.images.express.co.uk/img/dynamic/151/590x/1-695868.jpg 
 https://thumbor.forbes.com/thumbor/960×0/https%3A%2F%2Fblogs-images.forbes.com%2Fstartswithabang%2Ffiles%2F2017%2F05%2FSDSS_Deep.jpg 
 https://cdn.iflscience.com/images/4a22cda7-0ef5-5226-9ed3-e80c4cf053c7/default-1510334289-cover-image.jpg 
 https://images.theconversation.com/files/93501/original/image-20150901-25717-1y3igv.jpg?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=496&amp;fit=clip
How Big is the Universe? | Size, Why So Big, Infinite & Facts
https://nineplanets.org/questions/how-big-is-the-universe/#:~:text=The%20Universe%20is%20so%20big,14%20billion%20years%20or%20so.
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