The Significance of Yesterday’s Gravitational Wave Announcement: an FAQ










Why are their mergers the best sources for gravitational waves?





What happens when these objects merge?


How do we know this isn’t science fiction?






























72 responses to “The Significance of Yesterday’s Gravitational Wave Announcement: an FAQ

  1. Thank you. I shared this widely and even people who are not usually interested in this subject are engaged and amazed.
    Also, the links you provided, “in computer simulations such as this one and this one.” did not work. Thanks again.

  2. Colin N Keenan

    How likely would it be to see something similar to a kilonova somewhere in the regions they were looking if it was just any random night and not immediately following the gravitational wave detection. I’m trying to understand how strong the evidence is that the electromagnetic detection is the same event as the gravitational wave detection.

    • Hi Colin, very unlikely. But the key is the combination of several things. First, a gamma ray burst signal at almost the same instant as the peak of the gravitational wave signal. Second, a bright transient in the rough region of the sky that behaves somewhat like other post-gamma-ray-burst afterglows. Third, the moderately good agreement of the afterglow (especially in visible and infrared light) with previous theory on how neutron star mergers ought to behave. Fourth, no other reasonable candidate gamma ray burst at around that time form that region of the sky.

      • Thank you for the quick response. I was hoping for a quantitative answer though. Do they do statistics on this kind of experiment?

        • Michael Kelsey

          Short GRBs (as GRB170817A was) have a rate of roughly one per galaxy per million years (LIGO quotes the high end of their visible NS-NS merger rate at about 10k Gpc^-1 yr^-1). The combined LIGO/VIRGO/Fermi/et al. paper ( estimated the probability of a chance coincidence at 5 x 10^-8.

      • Perhaps the likeliness combination to look at is first the odds of an approximate coincidence between a GW signal and a GRB signal, this is fairly likely and has happened for instance to the first GW signal detected. Then the odds that the triangulation of the GW and the GRB overlap. And then the odds of observing an optical transient roughly in that area. The latter might be depending on how strong the previous evidence on short GRBs are associated with kilonovas. That would leave the overall likeliness basically to the chance of GRB and GW signal coinciding temporally within seconds and with overlapping rough source direction after enough interferometer observación time.

  3. Thank you Matt. ………does this proves that space is a physical real medium that can be vibrated

    • I am cautious about answering this. If you are asking: is space a medium for waves in just the same way that water is a medium for waves, the answer is: no. If you are asking: is space a medium for waves, though in a way that does not, in key features, resemble water, then the answer is yes. Space is, in a sense, a medium, but it’s not like any one you’ve ever seen or felt. In particular, the speed of the waves as they pass you is the same — equal to the cosmic speed limit — no matter how you choose to move. That is not true of water waves.

  4. brendan wallace

    The neutron capture process for heavy nucleii does seem a bit monkeys and shotguns compared to the moderation by the weak force for a few hundred million years. We can look forward to new exotic processes for their formation in kilonovas.

  5. Colin: Kilonovas aren’t that common, and most observed are very distant.
    The distance of the gravity wave source can be read directly and had a pattern indicating neutron star masses; the optical component had a similar direction and the same distance. I THINK the directional sensitivity is about 5 degrees (but I don’t know how axis affects that). The GRB and LIGO triangulations overlapped; the optical component was found within that. The GRB and LIGO observations were essentially simultaneous and the following observations all fit the same model for intensity and luminosity curve.

    It’s a good question, but It seems unlikely. Of course LIGO is building up frequency data, which will help to refine those odds. It might be a while, though, before we see another within the 300m light year region for such observations. Roll on more sensitive equipment.

  6. It is not clear to me why the gamma rays, in the bursts, arrive at the same time as the gravitations waves. In Supernova SN1987, neutrinos arrived earlier than light waves, which interacted with charged particles, on their way to earth. I should have expected the same thing to happen to the gamma ray bursts, versus gravitational waves. Apparently this is wrong. Are the gamma rays so energetic that charged particles aren’t “seen” by them, accounting for their simultaneous arrival?

    • Yes, the gamma rays are energetic enough that they aren’t affected. But the reason that the light from Supernova 1987a was seen later than the neutrinos is not because the light was affected by charged particles. It’s that nobody was looking for it. It was seen when sharp-eyed astronomers, outdoors, noticed something was funny-looking in the Magellenic Cloud. With modern techniques the glow of the supernova would have been picked up earlier.

  7. Peter Stiphout

    Would the energy of a black hole be sufficient to change all that enters into energy of one dimension?

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  9. Abraham Sternlieb

    At the beginning of your talk you state that a black hole is matter that collapses “endlessly”.Well,nothing in physics is endless(infinite),so please kindly be carefull because such statement is simply false.
    Yours sincerely

    • You are absolutely correct. The problem that I faced when writing that statement is that making a statement which is both clear and correct is very difficult indeed. What happens to the matter as it collapses depends on the perspective of the observer, and also on whether the matter has angular momentum, charge, etc. Let me think about whether I can word this in a way that isn’t false. It’s very difficult to do it in a way that is strictly true, and I didn’t want to get involved in a complicated discussion of the subtle details of how black holes form.

      • Hello Matt. There is another significant problem with supposing that black holes can crush down to an infinitesimal speck of zero volume. We know from considerations of the geometry of forces of massive bodies that the center gravitational forces are zero; the gravitational pull from the mass from one direction is countered by the pull from the opposite direction. The net result is zero. This is true for every object in inertial motion, a moon, planet, star, neutron star, and a black hole. When we realize the truth of this we face the problem of explaining how something could have the most massive gravity (spacetime curvature) at the same center point where gravity (spacetime curvature) should be, in fact, zero. This is a significant contradiction for those who think that a singularity means zero or near zero volume. In my understanding, a singularity is a mathematical issue only for those who do not understand how to think through the physics over a spacetime horizon. To wit, black holes must have a real physical size just like neutron stars.

        When we look for where felt gravity is strongest we find this to be surfaces that are sufficiently strong to stop another object that has reached the surface from penetrating through to the center. Objects with distant surfaces are essential in my view for understanding how any object can cause gravity (spacetime curvature). Even when looking at this from the perspective of the equivalence principle we are always dealing with extended objects.

    • Can we say this collapse as, “until the known physics works” (equivalence principle) .. or
      … collapse of, “numerical equality between the inertial and Gravitational mass” – by the disturbance in the so called spacetime ?

  10. The LIGO/VIRGO results paper had over 3,600 collaborative authors, think it’s safe to say it has been proofread. Given that we7ll soon be able to observe in greater detail hundreds of thousands to millions of galaxies, observations of these rare neutron star collisions should become more frequent. If we see the lightning and .0000000000000000002 seconds later we hear the thunder, it should tell us we’re close.

  11. Claude Deschenes

    Thank you Matt. Very clear and comprehensive.

  12. The idea that GW could in the future give an early warning of an impending NS merger is fascinating. Optical and radio instruments could then be oriented to pick up signals of the entire merger process. It’s not clear though what extra info could be gleaned from that. any ideas?

  13. federicoalbertoceccopieri

    Dear Matt,
    I have a rather fast question. Why a binary system spiralyzes and eventually collapses? Does che system loose energy and decreases its angular momentum because it is emitting gravitational waves ( in analogy with a accelerated charged particle emitting EM radiation? ) . Or the reason of the collapse is induced by other mechanisms?

    • Several things cause the system to collapse. When the stars are distant gravitational wave emission is very weak. In that case friction with interstellar gas and nudges by nearby stars will slowly push the two together. As they get closer gravitational radiation grows stronger and eventually takes over, being responsible for the bulk of their orbital decay once their speed hits a few percent that of light’s.

      For supermassive black holes this ‘swapping’ actually causes an issue, the “final parsec problem”.

  14. Petri Hirvonen

    Would the current detectors be able to capture a merger between two white dwarfs under any circumstances? If so, from how far could such collisions be detected?
    Thanks a lot!

    • @Petri, Based on this paper,, It seems that white-dwarf mergers could only be detected in our galaxy and then only with a space-based interferometer like LISA. The GW frequencies from merging white dwarfs is too low for ground based observatories to detect.

  15. Petri Hirvonen

    Oh, and another question: What sized black holes would make the biggest splash (highest frequency, largest energy output)? The current data is bit too slim to try extrapolate for tiny hypothetical primordial black holes with masses less than the Sun’s mass, and for supermassive black holes. Tiny black holes can orbit very close to one another, but have relatively little mass so maybe their orbital velocity and gravitational waves aren’t that great after all? Supermassive black holes on the other hand have huuuuuuge masses, but are also very large and can’t get too close before merging. So how would the maximum frequency and energy output scale as a function of the binary’s mass (for simplicity, let’s assume the two components have identical masses).
    A big thanks for your time!

  16. Hi Matt,
    Thank you very much for your articles! It is always a pleasure to read them!

    I have a question about the delay of x-ray and radio waves. The explanation given at the press conference was that they were late because we are not viewing the jets head on but rather at an angle. Initially I thought what they were hinting at was that it took a while for the jets to disperse enough to widen the angle and include earth in its extended path. But then I am still not clear why would that cause a delay for the x-ray and radio only? Why it did not affect other frequencies? Or did I completely misunderstand what they were saying?

    • The idea is that the main jets of the merging pair have missed Earth entirely; we see the gamma rays because they were emitted powerfully in most directions. But we’ll never see ‘direct’ X-ray and radio photons from the jets.

      Instead the light ‘sideways’ from the jets heats material nearby which then emits photons in all directions, some of which we see. It takes time for the jets to encounter enough material for the ‘secondary glow’ to be bright enough to see. We’d expect z-rays to have a small delay (Being the most energetic secondary emission.) with radio waves having a longer delay. (Being the result of more dispersed,lower-energy processes.)

  17. “gravitational waves _directly_ provide the distance to the galaxy”

    Pls explain — how & why is that possible? Even with the 1st LIGO obseevation couple years ago, distance to the event was cited, but AFAIK was never explained in the reports..

    Also, do gravitational waves experience either doppler or cosmological “red shift” equivalent?

    • Michael Kelsey

      The absolute amplitude of a GW signal (at the source) can be calculated directly from the masses, their separation, and their orbital period (the latter two are redundant, of course). That absolute amplitude can be directly compared to the amplitude measured by the detector. The amplitude falls off like 1/R for gravitational waves so you can extract R from the ratio.

      Yes, gravitational waves do experience both Doppler and cosmological redshifts, since they’re waves like any other wave. However, all of the detections so far have been close enough that the difference is small (for example, the GW170817 signal was at 40 Mpc, corresponding to z ~< 0.1).

  18. Mat,
    could you explain please ” there are two different methods for making the measurement, and they disagree” – what are they, and by how much they disagree? Also, how do “the gravitational waves directly provide the distance to the galaxy”?

    • The various measured values of Hubble’s constant are listed here:

      Of this the topmost three are the ones of note, 71.9, 67.6 and LIGO\VIRGO’s 70.0.

      The highest vaules come from the Hubble Space Telescope and relies on using gravitationally lensed objects; these can produce multiple images that have different path lengths so that light takes longer to reach us from one image than another. This time delay gives us the path difference and geometry gives us the linear distance away from us.

      The lower value comes from BOSS, a survey to map the distribution of galaxies and from this get an idea of the strength f primordial sound waves in the early universe. Both methods have considerable uncertainty, but do not really overlap, suggesting an error, bias or unknown factor in one of the measurements.

  19. A day late with my question. How does neutron-star matter behave when not confined at degenerate pressures? Would the nuclear strong force would keep large clumps of neutronium (my word) stable? I visualize chunks of neutron-star crust being flung off during merger rather than fragmenting into individual neutrons. Is that accurate? Followed by furious beta-decay to synthesize the elements from heavest to lighter elements? Are we able to detect spectroscopically if any “stable” elements with A>260 (aka theoretical island of stability for very heavy elements)? Or are emission lines impossible to know in advance for such super-heavy elements?

  20. Matt,
    Is there any indication as to the size of the black holes. Some believe they are infinitesimally small point objects or do they have a finite size.

    • The big problem is that if black holes have a ‘size’ it is likely smaller than their event horizon. Due to the nature of the horizon we can’t say much about what happens below it; if black hole ‘cores’ really are smaller than their horizons these measurements won’t tell us that. Only if they were larger (Not impossible but not favored) would we see something amiss.

      As it is you can mostly treat the size of a black hole as the size of its horizon, the ‘no return’ point. It’s not a size in the way we usually think of it, but shares some properties.


  22. Have we made enough spectroscopic observations of regular supernovas to compare how heavy element formation happens in them vs. in this kilonova? How are they different?

    • There are a host of problems with spectroscopic observations. Chief among these is the swamping of rare elements by those like iron and carbon that can be millions of times more abundant.

      Models predict that the abundance of an element should fall off exponentially with an increase in atomic weight, drastically enough to cause the aforementioned abundance problems. What evidence we have shows a lot of elements too faint to get a real idea of their abundance; this matches our models for the most part but there’s enough wriggle room in the orders of magnitude for vastly more elements to be produced by some poorly understood mechanism while providing similar spectroscopic results.

      As such our standing (Until now) has been ‘theory predicts and abundance problem and observations are consistent with that.’ Now we have confirmation (And testing and refinement) of an additional angle that relaxes the demands put on supernovae.

      The main differences between the two novae are the neutron flux and the starting material. Supernovae start with iron and related elements, mostly lighter, and a neutron flux produced by breakdown of nuclei in situ (as it were.) A kilonova starts with iron (from the neutron star’s ‘crust’) that has already endured a supernova, enriching it in heavy elements and a neutron flux generated by tearing apart neutron-rich material. This should (in theory now looking increasingly proven) a much higher flux for a longer time allowing heavier nuclei to build up.

  23. One other reason this is important, at least in my mind, is that it eliminates once and for all any tiny lingering doubt about whether LIGO/VIRGO are detecting real events, rather than instrumental artefacts.

  24. Could it be that the recent finding of millions of BHs in our MW halo are produced by such mergers? for millions of BHs see:
    If so, could we suggest a relation between black holes and Dark Matter in our galaxy halo?

  25. according to Sabine Hossenfelder , space only expands between galaxies and clusters . then we face a question ; there must be something that happens at the boundary of non expanding and expanding space …..what could that be since as you said that space IS an unknown physical medium with unknown properties , does GW change velocity ? …….thanks .

    • Petri Hirvonen

      Correct me if I’m wrong: Space expands uniformly, but it’s significant only on length scales of millions of light years and more?

    • There are two ways of looking at this, both negate the need for a boundary of some kind.

      The first is to view all space as expanding, but with bound systems resisting this expansion. In this view the space you occupy right now is expanding, albeit very slightly, but the far more powerful forces binding your atoms together resist this, keeping their bound size. Because of this objects don’t move apart and so the expansion of space cannot be detected.

      Conversely space in gravity dominated systems are best described using he Schwarzschild metric, which doesn’t expand. In this case we can consider the effect of gravity itself as swamping the effect. As you leave such a system the geometry of space changes as gravity weakens, slowly giving way to the expanding space of low density voids.

      • Vincent Sauve

        That is very fair description of the standard view of expanding space. I just don’t believe that all the evidence favors that interpretation for the cosmological redshift. But my position is in the minority.

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  27. A pity to mention firewalls. The consensus nowadays is that they don’t exist. The problem is just to explain why. Not in any way relevant to this topic.

  28. Vincent Suave, you gave a good answer.
    If you dig into the archives of Scientific American, you will find an October 2008 article by Martin Bojowald “Follow the Bouncing Universe” He (and others) postulates that our universe bounced into being from the collapse of a preexisting universe. When the universe collapsed, there was a limit on how small it got (not a point), the physics of the matter was “scrambled”, and it bounced to form our universe (the Big Bounce). This negates the Big Bang theory in which our universe sprang into being from an infinitely small point.
    The point of this is that it means a black Hole does have size and I contend that the only singularities in our entire universe are the origins of coordinate systems.
    The only problem I have with the Big Bounce theory is why an entire universe had to collapse. Why could it not have been a partial collapse and our universe be inside the remains of the preexisting universe.

  29. I also need to add that a partial collapse of a preexisting universe gives a very simple answer to what is dark matter and dark energy. Dark matter is preexisting universe matter and dark energy is the “gravitational” force of that matter.

    • How does that work? Does it involve additional dimensions, or is the dark matter ‘scrambled’ relative to our own and so unable to interact with light? Can it be disconnected from gravity also?

  30. It is just the opposite, our matter was scrambled at the Big Bounce. There was an accretion limit, which I coined the Bojowald Limit, at the time of the Big Bounce. However, additional matter was still on its way in and I am theorizing that it’s momentum caused our matter’s Planck Length to shorten slightly, causing the interaction problem. If this is true, there are still multiples of wavelengths that are the same, perhaps 1 out of 100,000, that can be detected because they will interact. To prove this would require a detector with an extremely narrow bandwidth.

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  32. Matt,

    Thanks a lot for your brief on the subject, as well as the FAQ that follows!

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  34. Can a black hole “choke” on a star of much larger mass?
    By the way, my understanding is that gravitational waves do not bend space – they bend space-time.

    • Black holes are very compact. This means that there is only so much matter that can fall into them at once. When this is exceeded (And it needn’t be by a much larger mass.) the matter starts to ‘pile up’ at the center. For supermassive black holes in the centers of galaxies this matter can be ejected at the poles as two highly energetic ‘jets’ that are a feature of quasars.

      Stellar mass black holes merely get an odd accretion disk, the matter lacks the energy to escape but is too energetic to fall in; it has to emit energy as EM radiation. In the extreme a black hole could collide with a star and devour its core with the energy released supporting its outer layers, much like how a traditional star’s core works. Such stars would be stable for hundreds of thousands of years at most and it is possible we may have detected one. (PSR J1740—3052 looks like it may BECOME one in future.)

  35. minor typo
    look for “even those these”

  36. Almost all the planets, including our Earth are having curvature of space time. This curvature gives balance to the planet. Whether a planet is perpendicular or tilt is decided by this curvature only.

    Does the GW tilt this balance (disturb equivalence principle) ?

  37. “Best test so far……”

    Why not identical under the same circumstances?

    • /The physical reality of Time dilation and Rest mass cannot coexist/.
      Extreme values ( like near infinity, near c) at one side of Numerical equation (equivalence principle) – will unbalance position or momentum.
      So the physical reality is only a balance.

  38. So many of the observations stemming from this event seem to match predicted behavior very well. Since that seems to be the case, can this event be viewed as further affirmation of the Standard Model of particle physics?

  39. The design of these very large instruments (LIGO/VIRGO) is based on General Relativity (GR). GR is a classical theory, in the sense that there is only one single story, one single outcome.

    On the Contrary, the Standard Model is a mish-mash of Quantum Field Theories (QFTs) like Quantum Electro Dynamics and Quantum Chromo Dynamics, and it (SM) explicitly excludes an explanation of gravitation, even though SM in a way postulates that gravitation is a fundamental force.

    SM, being a collection of QFTs is clearly NOT a classical theory: there are multiple stories, multiple outcomes, each possible outcome has its own probability of ocurrence (this is exactly what we see and measure in particle accelerators like the LHC).

    For SM there are four fundamental forces: Electromagnetism, the Weak Nuclear Force, the Strong Nuclear Force and Gravitation.

    Kind regards, GEN

  40. I would not argue that these types of experiments are a confirmation of SM.

  41. /The physical reality of Time dilation and Rest mass cannot coexist/.
    Big crunch and big rip in Paradox.
    If you go to past tense, there might be more mass and gravity near big bang. But the time slowed – means, *zero rest mass and only Energy.

    .) The term classical theory means, a theory where reduced Planck’s constant h-bar = 0 is zero.
    Angular frequency (gravity?) prevents space expansion ?

    If there is *zero rest mass, then, only – mass from motion (change in form of energy) – the “Kugelblitz” ?

  42. Doug McDonald

    How much closer to us would this event have to have been for the two Ligo detectors to have seen it all the way to merger and ringdown? (as a ratio of distances).