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The Multiverse postulates the existence of multiple universes parallel to our own. This pluraility of universes has been proposed by physicists to explain why the universe appears to be finely tuned for our existence.


  “ Call it a fluke, a mystery, a miracle. Or call it the biggest problem in physics. Short of invoking a benevolent creator, many physicists see only one possible explanation: Our universe may be but one of perhaps infinitely many universes in an inconceivably vast multi­verse. Most of those universes are barren, but some, like ours, have conditions suitable for life.

The idea is controversial. Critics say it doesn’t even qualify as a scientific theory because the existence of other universes cannot be proved or disproved. ”
                  —Tim Folger, November 10, 2008, Discover Magazine Online (Archive)

  “ But the main reason for believing in an ensemble of universes is that it could explain why the laws governing our Universe appear to be so finely tuned for our existence....This fine-tuning has two possible explanations. Either the Universe was designed specifically for us by a creator or there is a multitude of universes--a "multiverse". ”
                  —Marcus Chown, New Scientist, 06 June 1998.

  “ Should the fine-tuning turn out to be real, what are we to make of it? There are two widely-discussed possibilities: either God fine-tuned the universe for us to be here, or there are (as string theory implies) a large number of universes, each with different laws of physics, and we happen to find ourselves in a universe where the laws happen to be just right for us to live. After all, how could we not? ”
                  —Marcus Chown, New Scientist, 19 May 2010 (Archive)

  “ WHAT would you rather believe in, God or the multiverse? It sounds like an instance of cosmic apples and oranges, but increasingly we are being told it’s a choice we must make. Take the dialogue earlier this year between Richard Dawkins and physicist Steven Weinberg in Austin, Texas. Discussing the fact that the universe appears fine-tuned for our existence, Weinberg told Dawkins: “If you discovered a really impressive fine-tuning… I think you’d really be left with only two explanations: a benevolent designer or a multiverse.” ”
                  —Amanda Gefter, New Scientist, 3 December 2008

  “ However the very nature of the scientific enterprise is at stake in the multiverse debate: the multiverse proponents are proposing weakening the nature of scientific proof in order to claim that multiverses provide a scientific explanation. This is a dangerous tactic (note that we are concerned with really existing multiverses, not potential or hypothetical).

Two central scientific virtues are testability and explanatory power. In the cosmological context, these are often in conflict with each other (Ellis [12]). The extreme case is multiverse proposals, where no direct observational tests of the hypothesis are possible, as the supposed other universes cannot be seen by any observations whatever, and the assumed underlying physics is also untested and indeed probably untestable.

In this context one must re-evaluate what the core of science is: can one maintain one has a genuine scientific theory when direct and indeed indirect tests of the theory are impossible? If one claims this, one is altering what one means by science. One should be very careful before so doing. ”
                  —George F R Ellis, "Dark matter and dark energy proposals:maintaining cosmology as a true science?" November 21, 2008. (Archive)

  “ The multiverse idea is not provable either by observation, or as an implication of well established physics (cf. Gardner [13]). It may be true, but cannot be shown to be true by observation or experiment. However it does have great explanatory power: it does provide an empirically based rationalization for fine tuning, developing from known physical principles.

Here one must distinguish between explanation and prediction. Successful scientific theories make predictions, which can then be tested. The multiverse theory can’t make any predictions because it can explain anything at all. Any theory that is so flexible is not testable because almost any observation can be accommodated. I conclude that multiverse proposals are good empirically-based philosophical proposals for the nature of what exists, but are not strictly within the domain of science because they are not testable. I emphasize that there is nothing wrong with empirically-based philosophical explanation, indeed it is of great value, provided it is labeled for what it is.

I suggest that cosmologists should be very careful not to make methodological proposals that erode the essential nature of science in their enthusiasm to support such theories as being scientific (cf. Tegmark [27, 28]), for if they do so, there will very likely be unintended consequences in other areas where the boundaries of science are in dispute. It is dangerous to weaken the grounds of scientific proof in order to include multiverses under the mantle of ‘tested science’ for there are many other theories standing in the wings that would also like to claim that mantle. ”
                  —George F R Ellis, "Dark matter and dark energy proposals:maintaining cosmology as a true science?", November 21, 2008.(Archive)

  “ We have shown that most of the global time cutoff measures of the multiverse suffer from severe inconsistencies and developed a new framework which allows us to study the measure problem from a completely different perspective. In the emerging picture an infinite multiverse is replaced with a finite geocentric region... ”
                  —'Geocentric cosmology: a new look at the measure problem', Mahdiyar Noorbala and Vitaly Vanchurin, Department of Physics, Stanford University, Stanford, CA 94305, 2011. (Archive)

  “ Here's the dilemma: if the universe began with a quantum particle blipping into existence, inflating godlessly into space-time and a whole zoo of materials, then why is it so well suited for life?

For medieval philosophers, the purported perfection of the universe was the key to proving the existence of God. The universe is so fit for intelligent life that it must be the product of a powerful, benevolent external deity. Or, as popular theology might put it today: all this can’t be an accident.

Modern physics has also wrestled with this “fine-tuning problem”, and supplies its own answer. If only one universe exists, then it is strange to find it so hospitable to life, when nearly any other value for the gravitational or cosmological constants would have produced nothing at all. But if there is a “multiverse” of many universes, all with different constants, the problem vanishes: we’re here because we happen to be in one of the universes that works.

No miracles, no plan, no creator. As the cosmologist Bernard Carr puts it: “If you don’t want God, you’d better have a multiverse." ”
                  —New Scientist, God vs the multiverse: The 2500-year war

  “ The theory of the multiverse has seductively great explanatory power (while it has almost no predictive power), which is a major reason why many physicists and cosmologists find it attractive. On the other hand, other physicists dismiss it as pseudoscience because it is practically untestable.

It is common among supporters of the multiverse to conceive it as an alternative to a divinely created world and ideas of natural theology. Because it represents our universe as a chance universe, special only by the fact that we live in it, the multiverse has been likened to another and more famous anti-design theory, neo-Darwinianism. Weinberg puts it as follows: “Just as Darwin and Wallace explained how the wonderful adaption of living forms could arise without supernatural intervention, so the string landscape may explain how the constants of nature that we observe can take values suitable for life without being fine-tuned by a benevolent creator” (Weinberg 2007, 39). ”
                  —Stanford Encyclopedia of Philosophy (Archive)

Scientific American

Multiverse Theories Are Bad for Science - Scientific American (Archive)
Nov 2019
John Horgan, Stevens Institute of Technology

New books by a physicist and science journalist mount aggressive but ultimately unpersuasive defenses of multiverses

  “ In 1990 I wrote a bit of fluff for Scientific American about whether our cosmos might be just one in an “infinitude,” as several theories of physics implied. I titled my piece “Here a Universe, There a Universe . . .” and kept the tone light, because I didn’t want readers to take these cosmic conjectures too seriously. After all, there was no way of proving, or disproving, the existence of other universes.

Today, physicists still lack evidence of other universes, or even good ideas for obtaining evidence. Many nonetheless insist our cosmos really is just a mote of dust in a vast “multiverse.” One especially eloquent and passionate multiverse theorist is Sean Carroll. His faith in the multiverse stems from his faith in quantum mechanics, which he sees as our best account of reality.

In his book Something Deeply Hidden, Carroll asserts that quantum mechanics describes not just very small things but everything, including us. “As far as we currently know,” he writes, “quantum mechanics isn’t just an approximation to the truth; it is the truth.” And however preposterous it might seem, a multiverse, Carroll argues, is an inescapable consequence of quantum mechanics.

To make his case, he takes us deep into the surreal quantum world. Our world! The basic quantum equation, called a wave function, shows a particle—an electron, say—inhabiting many possible positions, with different probabilities assigned to each one. Aim an instrument at the electron to determine where it is, and you’ll find it in just one place. You might reasonably assume that the wave function is just a statistical approximation of the electron’s behavior, which can’t be more precise because electrons are tiny and our instruments crude. But you would be wrong, according to Carroll. The electron exists as a kind of probabilistic blur until you observe it, when it “collapses,” in physics lingo, into a single position.

Physicists and philosophers have been arguing about this “measurement problem” for almost a century now. Various other explanations have been proposed, but most are either implausible, making human consciousness a necessary component of reality, or kludgy, requiring ad hoc tweaks of the wave function. The only solution that makes sense to Carroll—because it preserves quantum mechanics in its purest form—was proposed in 1957 by a Princeton graduate student, Hugh Everett III. He conjectured that the electron actually inhabits all the positions allowed by the wave function, but in different universes.

This hypothesis, which came to be called the many-worlds theory, has been refined over the decades. It no longer entails acts of measurement, or consciousness (sorry New Agers). The universe supposedly splits, or branches, whenever one quantum particle jostles against another, making their wave functions collapse. This process, called “decoherence,” happens all the time, everywhere. It is happening to you right now. And now. And now. Yes, zillions of your doppelgangers are out there at this very moment, probably having more fun than you. Asked why we don’t feel ourselves splitting, Everett replied, “Do you feel the motion of the earth?”

Carroll addresses the problem of evidence, sort of. He says philosopher Karl Popper, who popularized the notion that scientific theories should be precise enough to be testable, or falsifiable, “had good things to say about” Everett’s hypothesis, calling it “a completely objective discussion of quantum mechanics.” (Popper, I must add, had doubts about natural selection, so his taste wasn’t irreproachable.)

Carroll proposes furthermore that because quantum mechanics is falsifiable, the many-worlds hypothesis “is the most falsifiable theory ever invented”—even if we can never directly observe any of those many worlds. The term “many,” by the way, is a gross understatement. The number of universes created since the big bang, Carroll estimates, is 2 to the power of 10 to the power of 112. Like I said, an infinitude.

And that’s just the many-worlds multiverse. Physicists have proposed even stranger multiverses, which science writer Tom Siegfried describes in his book The Number of the Heavens. String theory, which posits that all the forces of nature stem from stringy thingies wriggling in nine or more dimensions, implies that our cosmos is just a hillock in a sprawling “landscape” of universes, some with radically different laws and dimensions than ours. Chaotic inflation, a supercharged version of the big bang theory, suggests that our universe is a minuscule bubble in a boundless, frothy sea.

In addition to describing these and other multiverses, Siegfried provides a history of the idea of other worlds, which goes back to the ancient Greeks. (Is there anything they didn’t think of first?) Acknowledging that “nobody can say for sure” whether other universes exist, Siegfried professes neutrality on their existence. But he goes on to construct an almost comically partisan defense of the multiverse, declaring that “it makes much more sense for a multiverse to exist than not."

Siegfried blames historical resistance to the concept of other worlds on Aristotle, who “argued with Vulcan-like assuredness” that earth is the only world. Because Aristotle was wrong about that, Siegfried seems to suggest, maybe modern multiverse skeptics are wrong too. After all, the known universe has expanded enormously since Aristotle’s era. We learned only a century ago that the Milky Way is just one of many galaxies.

The logical next step, Siegfried contends, would be for us to discover that our entire cosmos is one of many. Rebutting skeptics who call multiverse theories “unscientific” because they are untestable, Siegfried retorts that the skeptics are unscientific, because they are “pre-supposing a definition of science that rules out multiverses to begin with.” He calls skeptics “deniers”—a term usually linked to doubts about real things, like vaccines, climate change and the Holocaust.

I am not a multiverse denier, any more than I am a God denier. Science cannot resolve the existence of either God or the multiverse, making agnosticism the only sensible position. I see some value in multiverse theories. Particularly when presented by a writer as gifted as Sean Carroll, they goad our imaginations and give us intimations of infinity. They make us feel really, really small—in a good way.

But I’m less entertained by multiverse theories than I once was, for a couple of reasons. First, science is in a slump, for reasons both internal and external. Science is ill-served when prominent thinkers tout ideas that can never be tested and hence are, sorry, unscientific. Moreover, at a time when our world, the real world, faces serious problems, dwelling on multiverses strikes me as escapism—akin to billionaires fantasizing about colonizing Mars. Shouldn’t scientists do something more productive with their time?

Maybe in another universe Carroll and Siegfried have convinced me to take multiverses seriously, but I doubt it. ”