Gravitational Wave Problem

I just watched this video - I’m curious if this gravitational wave problem has been resolved. Have we really measured gravitational waves? - YouTube

From what I understand, she’s saying that scientists believe they are measuring the gravitational wave from the wake of another boat. But we can’t yet be sure that the gravitational wave isn’t just our own terrestrial boat bouncing around: Scientists see a gravitational wave and go looking for a source. But she is saying that to be certain, they need to see a source, predict what the gravitational wave should be, and then find it in the data.

Am I understanding that right?

I watched a few other videos with her and like her personality. :grinning:

This video is by Sabine Hossenfelder. I’m curious what the @physicists think.

That’s been done:


I’m confused why she neglects this…

There was GW170817 that already corroborated that the gravitational wave is astrophysical by nature.

Sabine did not neglect GW170817; she was pointing out that because the detection of the EM counterpart was circulated 40 minutes before the gravitational wave counterpart, this leaves the option that either:

  1. The LIGO team saw the EM alert and then lied; they made up a claim that LIGO detected a gravitational wave
  2. The LIGO team saw the EM alert and then changed their assumptions in the data search such that they accidentally biased their analysis, and thus also claimed that they detected a gravitational wave

Charge 1) is more severe than charge 2),though both requires the LIGO team to be either dishonest about their science, or in the case of 2), they might also be incompetent. I don’t think these charges are believable because:

  1. LIGO is a large collaboration with >1000 members representing >100 institutions and many countries, I find it unbelieveable that all these people conspired together to lie, or that none of them are competent enough to know that they are cooking the data analysis accidentally
  2. The signals that LIGO detect are excellent matches to the theoretical predictions. Despite what some LIGO critics are saying, it is actually not easy to generate these signals through noise (systematic or otherwise)

Note that Sabine kept reiterating in the video that she herself also does not believe that the LIGO detections are fake.


Virgo is yet another gravity wave detector being run by the Europeans, and it is even more sensitive than LIGO. It is also worth mentioning that LIGO is actually two detectors, so if there is local noise causing the observations then the waves should not be observed at both locations with equal intensity. The lag between observations at the separate LIGO sites (Washington, Lousiana) is also consistent with the speed of light which is the speed at which gravitational waves should propagate. If these were Earth tremors then they would be vastly delayed. Add the more sensitive Virgo in Italy to the mix and you have even more correction for local noise.


More discussion on this on Sabine’s blog here.

@PdotdQ is someone who actually works on gravity, so he probably has a more informed opinion. That being said, I’m skeptical of two things:

First, Sabine doesn’t neglect the neutron star-black hole merger event. She just argues that because the LIGO detection alert was released after the gamma ray burst alert, that doesn’t count as a true verified prediction but only a post-diction. I don’t think her argument is strong. First, the difference postdiction vs. prediction is overrated (see Laura Snyder’s Is Evidence Historical? for a good historical argument that it has not been as important as some people think). Instead, what’s important is a (relatively) simple, coherent theory to explain as much of the evidence as possible. At the end of the day, one needs to explain why something detected by multiple GW detectors and a bunch of EM components seem to match a coherent set of theoretical predictions, namely that the event was an NS-BH merger. This is the case regardless of which detector first released their alert. (See @PdotdQ’s second option.) I don’t think Sabine has offered a compelling alternate hypothesis, other than simply saying that this event doesn’t count and raising general skepticism of LIGO. Thus, the theory that the GW detectors did detect something of extraterrestrial origin is much more compelling.

Second, I disagree with Sabine’s later claim that because LIGO can’t understand all of the sources of noise/glitches in their detector, that they are not practicing good science and cherry-picking their data. Experimentalists working in precision measurement know that it’s usually not possible to understand all of the sources of noise in your detector, no matter how good it is. In fact, the better your detectors get, the more weird and varied your noise sources will be, unsurprisingly. To counteract this, people design detectors which can give robust data even if there are unknown sources of noise or glitches. In this case, one huge check to LIGO GW detections is that there are multiple GW detectors, and any GW detection has to be correlated with all of them. Thus, it doesn’t matter if say, LIGO Hanford has a few unknown, peculiar glitches. As long as those glitches are relatively infrequent and not correlated with the other GW detectors, they do not pose a problem and can be safely ignored. (It is a different story if the glitches or unknown noise is so prevalent and large that it compromises a large part of your main signal.)

Now, perhaps Sabine wants to argue that there are some sources of correlated noise between detectors which have not been properly accounted for by the LIGO collaboration. Or perhaps the LIGO team is not properly taking into account the look-elsewhere effect in their statistical analysis, such that they are picking up random noise which happens to be correlated among the 2 or 3 detectors by chance and mistakenly labeling this as a GW detection. But she hasn’t made any technical argument of that sort on her video nor her blog. So I’m not very convinced with her general argument. I don’t know the ins-and-outs of LIGO but at the moment I don’t see any reason to doubt their general integrity and competence.


Let me expand further on the postdiction vs. prediction issue. This is actually something that may be relevant for an experiment like mine, when we’re trying to measure a quantity at a precision that no one has done before.

Suppose a detector with method A finds a statistically significant signal of new physics (say a phenomena called X). The scientists make a convincing case that they understand the physics of their detector well (including accounting for sources of systematic errors), even if their detector has never been operated at this sensitivity before. However, because this is a very radical result if true, not everyone is fully convinced. Perhaps there are some unknown systematic errors that detector A overlooked.

A few months later, another detector with a different method B also finds a signal of X that matches A’s findings. Similar with A, detector B uses well-understood physics, even if the detector has never operated at this sensitivity before.

A year later, the same phenomenon happens with detector C (with yet another method), which also detects signal X.

What is the “proper” epistemic attitude towards X, A, B, and C? First, I think most would agree that it is rational to believe that X is real, as it has been detected by three independent methods.

Now, if we only accept prediction and not postdiction (as Sabine argues), then it seems that when detector B detected X, that only increased our confidence in the correctness of detector A’s result. Since detector B only “postdicted” X, we have no idea if B is seeing anything real. It is only when detector C detects X that we become confident of detector B. And even then, we would not be sure in the confidence of detector C, only A and B.

Furthermore, after seeing the successful prediction of A when we detected the result of B, since we don’t know for sure if X is real, one might even argue that even this successful prediction doesn’t have much epistemic worth either. The same argument would apply to C, D, E, and any further detectors that see X - their epistemic worth would all collapse like a pile of dominos. This would seem to prohibit the possibility of science and inductive reasoning altogether.

I hope that the above example suffices to demonstrate the untenability of this position. The proper epistemic attitude is that after seeing both A and B detect X and agreeing with each other, that increases our confidence in both method A and B of detecting X. When C also detects X, that increases our confidence in C as well. Whether A, B, and C’s agreement has anything to do with say, fudging their data has to be judged on their own merits regardless of the temporal order of the detections.


From what I understand, she’s not exactly arguing whether X is real, she’s saying we don’t actually know the source of X.

In my earlier 5D universe thought experiment, I was suggesting that we live in a bubble that is not stable. I think scientists are assuming we don’t move, and the astronomical event washes over us. I was suggesting that we do move and the astronomical event nudges us.

After I decided we and black holes just live in a bunch of bubbles, :rofl: I was reminded of a ball pit that my kids have jumped into. Because the balls fill the space, when something moves them, they make a little wave and then come to a dead stop. It’s very different than how it would work in water. That’s what the wave reminds me of, even though I don’t understand the science behind it.

The difference is whether earth is fixed within spacetime or if earth and spacetime bend together I suppose. And I thought her unwillingness to concede that it could not be a terrestrial effect was interesting.

When I wrote “X is real” I meant it as a shorthand for “GWs were really detected”, meaning extraterrestrial origin.


Wiki has a nice list of detections…
List of Gravitational Wave Observations


To expand further. The principle that agreement between multiple detectors can increase our epistemic confidence in what each of them are claiming is not just a theoretical proposal. I argue that we’ve seen a nice demonstration of this principle in action in the case of the proton radius puzzle, which started around 2010 when the value of the the proton radius measured by a new method (muonic hydrogen) disagreed starkly with previous values.

A flurry of further precise measurements on the proton radius using different methods were then pursued over the next decade, with a greater level of precision and care than attempted before. The majority of them agreed with the new value. Finally, in 2019 the results of a very different method (electron-scattering) also agreed with the new value. At this point people started saying that the puzzle had been solved.

Now, if we only valued predictions and not postdictions, then all of the results after 2010 would be epistemically meaningless, even if pursued with very different methods. After all, they already knew the new value that they “wanted”, and was biased towards it.

But that’s a simplistic take. In reality, many careful practices are put into place to reduce the effect of personal bias. For example, many precision measurement groups blind the absolute value of the quantity they are measuring and only reveal it at the very end, after all of the analysis is finished. This prevents tweaking with the parameters of the data analysis until they agree with what one “thinks” is the right answer. This practice is also carried out in LIGO in the form of “false injections”, where a separate team has the job of secretly sending in fake signals into the apparatus and testing to see if those pass the analysis routine.

These careful, bias-reducing practices further justify why we don’t seem to care so much about the temporal order of results. What matters is whether a measurement is precise, rigorous, carefully studied, and well-understood, regardless of the time when it was performed. A more precise measurement that comes more recently is always going to be valued over an older measurement that is cruder.


Congratulations, you are a Bayesian! :slight_smile:


The real pickle of a question, however, is which group, A, B, or C, has best standing to getting a Nobel Prize.


Which group was the first to rush to the patent claims office?

There are no patents for science this basic. Now what?

Which group is based in closest physical proximity to Cambridge, MA?

This is indeed the correct view on prediction vs postdiction, assuming that B is not lying or accidentally biasing their data analysis after seeing A, which is only possible in the case of a postdiction. I am pointing this out because in the field, a well known “criticism” against the LIGO detection of GW170817 (birthday today, btw) by a small sect of anti-LIGO folks is that LIGO is guilty of the second charge (some even claim the first). They seem to think that you can give the LIGO pipeline any values of the parameters (e.g., chirp mass, reduced spin, etc), and the data will produce a detection out of the noise. So, when GW170817 was seen in the EM, all the LIGO team needs to do is to plug in the parameters inferred from the EM counterpart and get a GW detection. This is of course not the case.


Another great example is the measurement of the Higgs boson at the LHC. Two independent experiments both arrived at the same measurements.


This one is interesting: Gravitational wave scientists grapple with the cosmic mystery of GW190814 | EurekAlert! Science News

Maybe part of what she was challenging?

In a new paper, published in the Astrophysical Journal Letters , the team has announced that the signal was generated by a compact object (a neutron star or a black hole) 2.6 times the mass of our sun (2.6 solar masses), merging with a black hole of 23 solar masses.

The new observation is important because it challenges astrophysicists’ understanding both of how stars die and how they pair up into binary systems. Although the precise nature of the lighter member of the binary that generated GW190814 is unknown, scientists have confirmed that it is a record breaker: it is more massive than any neutron star and lighter than any black hole yet observed…

From the very outset it was clear that this was a special event," says Dr Geraint Pratten, a researcher at the Institute for Gravitational Wave Astronomy, who was involved in producing the initial sky-maps for optical telescopes’ follow-ups. "It highlights the need for ever better theoretical models of the emitted gravitational-wave signal, such as those produced here in Birmingham, to mine as much information as possible from the data and understand how such high mass-ratio binaries are formed."

This may be incredibly unscientific and a stupid question because I don’t really understand any of this :sweat_smile:…but I was wondering if the gravitational wave glitches were fast radio bursts? The shape looks similar to what Sabine had on her blog. A mystery solved? Fast Radio Burst detected within Milky Way | Space | EarthSky

This is from Sabine’s blog that someone linked… I just noticed it related to the GW190814 I posted about.

Since April, the collaboration has issued 33 alerts for new events, but so-far no electromagnetic counterparts have been seen. You can check the complete list for yourself here. 9 of the 33 events have meanwhile been downgraded because they were identified as likely of terrestrial origin, and been retracted…

The number of retractions is fairly high partly because the collaboration is still coming to grips with the upgraded detector. This is new scientific territory and the researchers themselves are still learning how to best analyze and interpret the data. A further difficulty is that the alerts must go out quickly in order for telescopes to be swung around and point at the right location in the sky. This does not leave much time for careful analysis.

In the third observation run, the collaboration has so-far seen one high-significance binary neutron star candidate (S190425z). But the associated electromagnetic signal for this event has not been found. This may be for various reasons. For example, the analysis of the signal revealed that the event must have been far away, about 4 times farther than the 2017 neutron-star event. This means that any electromagnetic signal would have been fainter by a factor of about 16. In addition, the location in the sky was rather uncertain. So, the electromagnetic signal was plausibly hard to detect.

More recently, on August 14th, the collaboration reported a neutron-star black hole merger. Again the electromagnetic counterpart is missing. In this case they were able to locate the origin to better precision. But they still estimate the source is about 7 times farther away than the 2017 neutron-star event, meaning it would have been fainter by a factor of about 50."

So she is also saying something unusual is going on. Perhaps all the signals they’re throwing out because they can’t see the EM signal actually ARE something, and we just don’t know what it is. In that case, they’re throwing away data just because they don’t understand it?

Anyway, scientists are driving me nuts :rofl: :upside_down_face: it seems to me that Gunnar Nordstrom is so obviously right here and it does describe our world: “The above point of view provides, as we have seen, some formal advantages as it permits the eletromagnetic and the gravitational fields to be expressed as a single field. Of course the equations have not thereby gained any new physical meaning. Yet I do not think it can be excluded that this symmetry may have a deeper foundation. However, I will not here explore the possibilities that one might conjecture regarding this. Summary: it is shown that a unifying treatment of the electromagnetic and gravitational fields is possible if one considers the four dimensional spacetime-world to be a surface in a five dimensional world.”

Here Matt O’Down is talking about the extra dimension being compactified or looped in another. So what if the gravitational waves are often so compacted within the electromagnetic field we can’t see it? But when they’re not we can? I thought c^4 should show up somewhere if they’re layered up and then I was watching a video from Sabine and one of Einstein’s Field Equations flashed on the screen that had c^4. Someone please figure it out for me and put me out of my misery. :joy: because I don’t understand any of it really, but it seems there’s something so obviously there…