Recent reports claim that the gravitational waves detected by LIGO match up with the signal expected from two black holes merging as predicted by general relativity. Additionally, the masses of both black holes were estimated.


  1. How certain are we that the detected of gravitational waves are necessarily from two black holes merging?

  2. Since supermassive black holes are typically at the center of a host galaxy, what happened to the galaxies that contained the two merging black holes? For example, did their host galaxies merge?


After reading the answers, I understand that the signals detected by LIGO are proof of the validity of gravitational wave generational artifacts as predicted by general relativity. When additionally gravitational waves, caused by the merger of the supermassive black holes (those which ARE indeed located in the centers of the merging galaxies) will be detected, since mergers of galaxies could be observed by the methods independent of gravitational waves such as registering the fact of the quasar appearance (source).

Additional questions:

  1. Could someone offer me the historical ASTRONOMY precedent when the second degree of inference (from the mathematical model, being associated with the theory of Physics) was accepted as the discovery of the astronomical object?

  2. Could someone give a reference to trustable scientific publication, with the subject of study being discussion of requirements satisfying claim of astronomical discovery of an astronomical object?

  3. Have independent scientific sources (outside of members of the LIGO team) analyzed methodology and results of the LIGO signal detection, and published their conclusions with regards of what was actually discovered?

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    $\begingroup$ What is the source of the quote? Is it from arxiv.org/abs/astro-ph/9803211? $\endgroup$ – HDE 226868 Feb 14 '16 at 17:15
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    $\begingroup$ Alex, you may find it helpful to read about the philosophy of science. It will help explain why those who have answered sidestep your requests for wording involving "proof" or "truth." Science in its purist form is not a religion, It dabbles not in truth, but in observations and evidence. It avoids the word true for the same reason the legal system talks of facts that are "beyond reasonable doubt," rather than "true." In addition, the burden of proof can be even higher in science. The legal system has to collect evidence about a singular event while science is interested in repeatable... $\endgroup$ – Cort Ammon Feb 14 '16 at 20:04
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    $\begingroup$ ... events, making the gathering of evidence easier. This is why the answers you have received instead talk of theories an evidence. I would dare say that a scientist who replies to a direct question as to whether a theory is true with either "yes" or "no" is either blowing you off, or has a vested interest in people believing in said theory, such as a lot of grant money at stake. The real answer will always be one of evidecne and uncertainty. $\endgroup$ – Cort Ammon Feb 14 '16 at 20:07
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    $\begingroup$ My answer to this other question shows the similarity of the theorized signal and the observed one. There is little room for this sideshow you're presenting Alex. $\endgroup$ – Kyle Kanos Mar 3 '16 at 14:14
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    $\begingroup$ @KyleKanos - I do not have proper qualifications to call my inquiry the "scrutiny" - but I am asking whether independent scientific sources (outside of members of the LIGO team) conducted such scrutiny and published their conclusions with regards of what was actually discovered. $\endgroup$ – Alex Mar 3 '16 at 15:52

The two black holes observed by LIGO were around 30 solar masses each - they were formed from stellar sources - that is, a supernova or similar event. They are not the same "kind" of black holes which are found in the middle of galaxies.

(sidenote: The fact that they are 30 solar masses is actually interesting. In this paper they discuss how the environment had to be a little bit special for these black holes to form).

In regards to the condition of "truth", it conforms to established scientific norms. For instance, the detector has been very well-modeled and every reasonable error has been accounted for, so we have very good reason to believe that the signal is real (to say nothing about the fact that it was observed in TWO detectors, one in Louisiana and one in Washington, and the signals are nearly identical). To determine the details of the merger, people have been working very hard over the past decade to develop a library of signals, for a variety of objects (neutron stars and black holes) and a wide variety of parameters (masses and orbital parameters). So they determined the characteristics of the merger by comparison with those models.

Of course, we aren't in a spaceship floating over this merger viewing it with our own eyes. But on the basis of the scientific method (hypothesis testing and independent verification), this establishes the existence of gravitational waves.

(for the full paper talking about the observation)

EDIT: I'm going to try to tackle your clarifying questions.

  1. This one is slightly tricky, since all (extra-solar) astronomy is indirect in this way - we only observe the cosmos via the light we receive from it. For example, the existence of the star Polaris is indirect, and depends on the assumption that stars produce light (which is on very solid footing, obviously). Some examples that might be closer to what you're thinking of - Dark matter is only detected via it's gravitational influence (never directly), but most people consider it to be a real phenomena. Pulsars being associated to neutron stars is mostly theoretical - although we can associate them to SNR sometimes. And actually, the vast majority of extrasolar planets are detected indirectly, via the Doppler shift or transit methods.

  2. I think the answer is "no". You would have to explore each one individually, since the argument in each case is rather unique. I once listened to an interesting podcast about how astronomy is observational, not experimental. I think it's here. I think the best you can do is list evidence for discovery and let the community decide. This is not a unique problem, BTW - no one has ever seen a Higgs particle, in the traditional sense - we inferred it's existence at a level sufficient for the scientific community.

  3. LIGO releases it's data to the public at proscribed times. Here's a list of projects using LIGO data. I don't think I see specifically what you are interested in ("We checked LIGO, it's right!"), but this list is only the past few months.

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    $\begingroup$ Well that's exactly right - there are no known mechanisms to create a signal like this except for the gravitational waves produced by two ~30 solar mass black holes. $\endgroup$ – levitopher Feb 14 '16 at 16:29
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    $\begingroup$ Alex, all physics models are just validated by being fitted to the data. It is as much of a fact as the 5.1 sigma probability makes it a fact, so not as much a fact as a predicted solar eclipse, but good enough , in analogy, for establishing a Higgs and validating the standard model. $\endgroup$ – anna v Feb 14 '16 at 17:20
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    $\begingroup$ @annav It is not the randomness of the signal I am discussing - it is definitely not the noise, it is indeed caused by some physical phenomena. The question is - is it result of the two black holes merge or something else ... $\endgroup$ – Alex Feb 15 '16 at 16:19
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    $\begingroup$ @Alex "it is definitely not the noise". There is about a 1 in 3.5 million chance that it could have been caused by the anticipated patterns of noise in both detectors. There is nothing definite in physical experiments. $\endgroup$ – ProfRob Feb 22 '16 at 14:25
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    $\begingroup$ @RobJeffries - you just have said: "There is nothing definite in physical experiments" ... I would add - there is nothing absolutely definite in conclusions, derived from such experiments, especially in such conclusion, which represents second degree inference from GR's mathematical model. $\endgroup$ – Alex Feb 22 '16 at 16:16

levitopher's answer clears up a bit of your confusion regarding the type of black hole involved. The two black holes thought to have produced the detected gravitational waves were ~36 and ~29 solar masses in mass, nowhere near the mass of typical supermassive black holes. They are instead relatively massive stellar-mass black holes.

Regarding your other question - it is indeed quite likely that the source is in fact a pair of merging black holes. Regrettably, no other gravitational wave detectors were online at the time of detection by LIGO. Additionally, the Swift gamma-ray detector reported that it found no counterpart to the merger in the small part of the electromagnetic spectrum it surveyed (Evans et al. (2016)) two days after the gravitational wave detection, and the ANTARES and Ice Cube neutrino detectors detected very few neutrinos at the time of gravitational wave detection (here). However, these results are not disappointing, as Swift's results came later, and the neutrino detections helped put a "concrete limit on neutrino emission from this GW source type", according to the paper.

However, as Kyle Kanos pointed out, the Fermi Gamma-ray Bust Monitor detected a short source of photons which may be connected to GW150914.

The LIGO results present very strong evidence that the event is what it is predicted to be - the merger of two stellar-mass black holes. It passes the 5-sigma threshold confidence level. See here for a short explanation of the team's conclusions as to the nature of the event.

It is also worth mentioning that analysis shows that the observations match all predictions from general relativity.

  • $\begingroup$ Thanks - taking in account the choice of words used in your answer, such as, "quite likely" and "very strong evidence" - is it an established scientific astronomical (cosmological) fact or just a best possible implied assumption? $\endgroup$ – Alex Feb 14 '16 at 17:20
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    $\begingroup$ @Alex I wouldn't describe it as either a "fact" or an "assumption". It's closer to the former, because quite a lot of evidence points to it. It's not an assumption, though, because people did not widely assume it to be true before the detection event was analyzed. $\endgroup$ – HDE 226868 Feb 14 '16 at 17:30
  • $\begingroup$ Well, after I was educated by the answers (which cleared my black holes classification confusion :-) - my apologies for lack of prior knowledge on that), wouldn't it be more look like as definite proof when additionally gravitational waves, caused by merge of the supermassive black holes (those which ARE indeed located in the centers of the merging galactics) will be detected, since merges of galactics could be observed by the methods independent of gravitational waves? scienceworldreport.com/articles/10307/20131018/… $\endgroup$ – Alex Feb 14 '16 at 18:04
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    $\begingroup$ @levitopher True, although this is the first instance of direct detection. $\endgroup$ – HDE 226868 Feb 14 '16 at 20:20
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    $\begingroup$ Fermi GBM observed a few $\gamma$-ray photons: arxiv.org/abs/1602.03920 $\endgroup$ – Kyle Kanos Mar 3 '16 at 14:12

I think it is highly relevant that in 2005 the numerical simulation of more than one orbit of a binary black hole merger was successfully achieved by several groups (examples: 1; 2). These papers solved a problem after 30 years of concerted effort and moreover they presented the expected expected signal that a gravitational wave interferometer should see given such an event. That is the basis of the interpretation of the chirp signal that was seen.

So, this was very much an "if Einstein's theory is correct and if you detect a black hole merger, this is what it will look like" moment: in other words a prediction of a possible outcome of the LIGO experiment. If you like the Popperian approach to science (of which I am not a strong supporter) you could say that Einstein's theory made a precise prediction that was later verified.

My conclusion is that, taken together, the numerical experiments show that Einstein's theory looks better all the time, numerical simulation has advanced in leaps and bounds, and the LIGO experiment is a triumph of human engineering and scientific foresight. I personally don't think "truth" has anything to do with it.

  • $\begingroup$ I disagree that the conclusion of the PRL follows a Popperian approach. As I would frame it, a Popperian approach would say "if GR is correct and we know that 2 black holes have merged, then we will also observe a specific signal of the event on an interferometer". Since they received only the signal, they "concluded" that both premises are true. But not receiving any signal would not have refuted in any way GR...so it cannot be a Popperian argument I think. $\endgroup$ – gatsu Mar 3 '16 at 18:54
  • $\begingroup$ @gatsu There are also tests of GR by LIGO, where they modify the PN terms to account for changes in GR. $\endgroup$ – Otto Nov 25 '17 at 1:43

I'd like to point out that the recent discovery by LIGO was specifically one of gravitational waves. The claim that the cause of the waves was a merger of two stellar kerr black holes is more inferential than by virtue of direct detection. The precise form of the chirp and the ringdown definitively show that the involved objects are compact masses (instead of say, neutron stars, which would have different frequency modulation even prior to the collision). As the signal also includes parts of the orbit where the gravity is relatively weak (where post-newtonian approximations still suffice), one can still distinguish compact masses from fluid spheres even without invoking all of GR in complete detail (the fact that any alternate theory of gravity must still reduce to relativistically corrected newtonian gravity, implies this is very strong evidence of the objects involved in the merger being compact).

As for whether the two colliding objects are black holes, that in my opinion is still an open question. Conclusively detecting a black hole would involve pinning down the event horizon, which at the level of the present detection sensitivity is not possible. The quasi-normal modes post the merger do hold crucial information on the existence of an event horizon. The fact that we see exponentially decaying oscillations at the end is by far the greatest evidence for the final object being a black hole. The issue here is that to conclusively verify this, one needs to be able to resolve a bit more of the frequency spectrum of the quasi-normal modes. Extracting this information by fitting the signal to numerical relativity simulations pre-assumes the validity of GR in the strong-field while perturbative analytical calculations become valid in a regime where the signal dies off into the noise. Hopefully with further upgrades, when LIGO begins to function at design sensitivity, we should be able to resolve this better. The current argument put forth is one of ignorance, that we do not know of any astrophysical mechanisms and/or exotic matter that do not lead to a collapse to a black hole (at ~30 solar masses).

As for the validity of the detection itself, I think quite a few people have answered before me about this and hence I shall not say any more on it.

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    $\begingroup$ This is an objective answer, which pretty much goes alone with my understanding of the situation, therefore I am awarding my bounty to this answer. $\endgroup$ – Alex Mar 4 '16 at 18:15
  • $\begingroup$ Here is the snippet from HDE 226868 answer , made at astronomy.stackexchange.com/questions/6066 "Only by using a multitude of different observations, and careful analysis, can an object finally be designated a black hole" $\endgroup$ – Alex Mar 6 '16 at 19:14
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    $\begingroup$ Or you could put it this way. The GW signature is perfectly well explained using GR and the merger of two black holes. Alternative interpretations would mean there is something wrong with our current understanding of gravity and/or something much more exotic than a black hole. But there is no evidence at all for this, so the simple interpretation is preferred. Proof is for Maths. Theory can only be falsified in physics and GR plus black holes passes this potential banana skin with flying colours. $\endgroup$ – ProfRob Mar 14 '16 at 1:03
  • $\begingroup$ FYI - Per hypothesis described in sciencedaily.com/releases/2016/06/160615134951.htm in both cases of GW LIGO detection (09/15/2016 and 12/25/2016) colliding black holes are not formed by the collapse of neutron stars but are embodiments of the "dark matter". $\endgroup$ – Alex Jun 21 '16 at 20:39
  • $\begingroup$ To what extent is the ring-down signal "seeing the event horizon"--sure, it's just the motion of the event horizon and not "the event horizon"--but it's kinda seeing the event horizon. I'd award a Nobel prize just for that part of signal in addition to one for the in-spiral. $\endgroup$ – JEB Jun 1 '18 at 1:52

Just as an addendum to everything else, LIGO is tuned to observe mergers of stellar-mass black holes. The frequency of the gravitational waves is determined by the mass of the system, and since SMBH will have much larger masses than stellar mass black holes (the LIGO holes were 30 solar masses, the SMBH in the Milky way is something like ${10}^6$ stellar masses), they will have frequencies much lower than what LIGO is sensitive for.

This, in fact, is the motivation for LISA, which WILL be sensitive to events related to SMBH and to low-frequency cosmological background gravitational waves.


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