Uncovering the Truth Behind LIGO's Gravity Wave Detection: A Critical Analysis

[auou]

What's needed is a response from members of the LIGO team to that claim.

There was already a response on Sunday and a link to it was already posted here above: http://www.preposterousuniverse.com...the-time-lags-of-the-ligo-signals-guest-post/

It doesn't hurt to read what other people write.

 

[PeterDonis]

Since the black hole merger was a billion light years away, isn't it probable the event actually happened a billion or millions of light years ago?

In an expanding universe, you can't just "read off" time from distance this way. The billion light-years is a "distance now", i.e., how far away the merger site is from Earth at this particular instant of time in standard cosmological coordinates. But when the gravitational waves from the merger were emitted, the merger site was closer to where Earth was then; and while the waves were traveling, the universe was expanding, so the actual travel time of the waves is something in between the light travel time equivalents of the distance "then" and the distance "now". The exact relationship depends on the details of the universe's expansion history.

Travelling at the speed of light, it takes 3 billion years (of time) to travel 3 billion light years (of distance).

Careful. In an expanding universe, it isn't this simple, because "distance" isn't this simple. See above.

 

[RockyMarciano]

Here's a link to a discussion of the paper in reddit: https://www.reddit.com/r/Physics/comments/6hs618/casting_doubt_on_all_three_ligo_detections/?
A mentor from this site(mfb) who hasn't commented anything in this thread participates there.

 

[Vanadium 50]

Like Vanadium50 said above. It's not reasonable to expect an immediate reply.

To put things in perspective:

• Time between observation and publication: 172 days
• Time between LIGO and NBI publications: 363 days
• Time between NBI publication and LIGO response: even a single day is too long! We want answers now!

Hmmm...

In any event, we have an unofficial answer through LIGO's outreach group. NBI assumed that the template subtracted data is free of correlations. Because the templates are not perfect, this is not true. LIGO was aware of this and considered this in their significance calculation. To me, this sounds like a plausible response, even though I have no details on exactly how this was taken into account.

If this is unsatisfactory, the way this is addressed is that the NBI group (or someone else, citing the NBI group) publishes a comment in PRL, and the authors reply to it. The procedure is here: https://journals.aps.org/prl/authors/comments-physical-review-letters

 

[RockyMarciano]

In any event, we have an unofficial answer through LIGO's outreach group. NBI assumed that the template subtracted data is free of correlations. Because the templates are not perfect, this is not true. LIGO was aware of this and considered this in their significance calculation. To me, this sounds like a plausible response, even though I have no details on exactly how this was taken into account.If this is unsatisfactory,

NBI didn't assume that and their paper is not about that, so maybe a second reading would help there. LIGO unofficial response is useless for the NBI until officially released by the collaboration. As for people expecting a fast official response by LIGO, who was that addressed to? I'm certainly not one of them. I think Ligo is going to take its time or even avoid a published answer that might compromise them as much as they can.

the way this is addressed is that the NBI group (or someone else, citing the NBI group) publishes a comment in PRL, and the authors reply to it. The procedure is here: https://journals.aps.org/prl/authors/comments-physical-review-letters

That is the procedure for PRL, not a general procedure.

 

[Vanadium 50]

NBI didn't assume that

As a consequence of this result, it is assumed that, in the absence of a gravitational wave event, the records of the Livingston and Hanford detectors will be uncorrelated

so maybe a second reading would help there.

Quite.

 

[RockyMarciano]

Quite.

That quote is in the context of what LIGO is assuming according to Creswell et al. as a consequence of LIGO analysis.
Try reading in context instead of quoting out of it.

 

[auou]

There's a new article on this topic from the forbes blog that brought some attention on the paper of the Danish group:
https://www.forbes.com/sites/startswithabang/2017/06/28/how-uncertain-are-ligos-first-gravitational-wave-detections

And perhaps even more interesting it has a link to a response of the Danish group on the comment by Ian Harry of the LIGO team:
http://www.nbi.ku.dk/gravitational-waves/

 

[PeterDonis]

If the same external hardware 'process of elimination" methodology which was applied to all environmental factors is also applied to celestial claims of cause, then the lack of external visual or neutrino confirmation can be used to eliminate celestial sources from further consideration, and these three signals then end up in the "unknown origin" category, not the discovery category.

You are missing a huge additional factor here: the signals which LIGO identifies as gravitational wave signals appear in both detectors with a light travel time delay consistent with the distance between the detectors. Signals which look like GW signals but only appear in one detector, which is how an "unknown origin" signal would be expected to behave, don't pass this filter. The paper you link to makes this clear in the introduction:

"For LIGO, the fundamental signature of a transient gravitational wave signal is a near-simultaneous signal with consistent waveforms in the two detectors."

This is a positive claim; it is not a claim of the form "anything that we can't attribute to an environmental factor is assumed to be from a celestial cause", and it does not give celestial causes a "free pass"--the criteria for something being due to a celestial (GW) cause are clear and specific. All of the "process of elimination" methodology you refer to is to make sure that the correct waveforms, if any, in the two detectors are being compared to see if they are consistent and near-simultaneous.

 

[MichaelMo]

You are missing a huge additional factor here: the signals which LIGO identifies as gravitational wave signals appear in both detectors with a light travel time delay consistent with the distance between the detectors. Signals which look like GW signals but only appear in one detector, which is how an "unknown origin" signal would be expected to behave, don't pass this filter. The paper you link to makes this clear in the introduction:

"For LIGO, the fundamental signature of a transient gravitational wave signal is a near-simultaneous signal with consistent waveforms in the two detectors."

I'm not "missing" the point that the signal travels at high speed or that it's observed by both detectors. I'm simply doubting that 'blip transients' cannot have that same effect on both *upgraded* detectors, particularly during the 'engineering run" that is supposed to be finding that out. Even if we assume that prior to the upgrades LIGO had *never* (as opposed to seldom) observed a blip transient in both detectors within a 10 ms window, how can we know that is still the case once the detectors have been upgraded by a factor of 10 in terms of distance, and 1000 in terms of volume space, during the engineering run no less?

This is a positive claim;

It sounds like a positive claim about the nature of blip transients which seems hard to logically justify since they don't know the actual cause of blip transients, they have no veto to remove them, and the equipment has just been significantly upgraded.

it is not a claim of the form "anything that we can't attribute to an environmental factor is assumed to be from a celestial cause",and it does not give celestial causes a "free pass"--the criteria for something being due to a celestial (GW) cause are clear and specific.

For the record, I never accused them of the strawman quote that you came up with. :) Their methodology does give celestial claims as to cause a "free pass" compared to electrical discharge related activity for instance. They "eliminated" lightning strikes as a potential cause based on a *lack* of external support in auxiliary hardware. GW claims as to cause lack that same external validation. That is an ihherent bias in favor of celestial origin claims in correlated noise, and a bias against electrical discharge activity (or anything else).

All of the "process of elimination" methodology you refer to is to make sure that the correct waveforms, if any, in the two detectors are being compared to see if they are consistent and near-simultaneous.

IMO all that demonstrates is that it's a real noise pattern that appears in both detectors which probably travels at C. I still can't eliminate anything which might travel at C as the potential cause of the signal.

Given a visual confirmation however, it would be easy to show that one option passes an external test, whereas the other one doesn't. As it stands, nothing seems to pass any external test, so if we apply the same methodology consistently, these signals should go into the "unknown origin" category, not the "GW wave discovery" category. Anything else is a bias.

If the Danish group is correct that there is a 6.9 MS noise correlation, that only makes the discovery claim that much more suspicious. The US power grid comes to mind as a potential source of correlated noise sources.

 

[MichaelMo]

If you require that, you would defeat much of the potential value of LIGO.

I'm not sure I'd make it a requirement forever, but I'd at least like to know for a fact that at least one such "chirp signal" can be correlated to an actual celestial event, otherwise I have no idea where the source really came from. I could 'speculate", but nothing seems to enjoy any clear external support. Keep in mind that even Einstein himself had doubts as to the existence of singularities and gravitational waves:

http://www.astronomy.com/news/2016/02/even-einstein-had-his-doubts-about-gravitational-waves

For many of the sought events, including the ones so far detected, there is no expected visual signal.

Wouldn't we have to know something about the *type* of black holes that merged in order to make that assessment?

https://journals.aps.org/prd/abstract/10.1103/PhysRevD.89.044008

According to that published paper, charged black hole mergers might be expected to emit more energy in the EM spectrum than in gravitational waves.

This would be like saying radio astronomy should discount all observations not accompanied by visible light; or that x-rays are useless when there is no visual verification (cut open your chest to verify a chest x-ray, anyone?).

I'm pretty sure that the reason that we had confidence that x-rays were useful to see inside of human tissue is because someone did in fact cut open a few humans to see what was inside to verify that it "matched' what we observed in x-ray images. :)

 

[Nugatory]

A number of posts that could be charitably be described as not fully consistent with the Physics Forums rule about acceptable sources have been removed from this thread.

 

[PeterDonis]

Moderator's note: an off topic subthread has been deleted.

 

[auou]

Signals which look like GW signals but only appear in one detector, which is how an "unknown origin" signal would be expected to behave, don't pass this filter.

So it could have mathematically been only possible for a lightning storm "exactly" in between the detectors, all other storms don't pass the filter because they don't make the 1ms time limit and/or they have a different wave pattern. In any case there was such a storm/candidate for the first detection.

Would there be a list of 'single' strong detections that correspond to lightning storms, which could define a certain signal pattern correlation and exclude the possibility of it being a storm, or is the amount of single noise data to start doing this too immense?

 

[mfb]

all other storms don't pass the filter because they don't make the 1ms time limit and/or they have a different wave pattern.

They would still appear in the background estimate if you shift the data by more than 1 ms. But there is nothing even remotely similar there.

Would there be a list of 'single' strong detections that correspond to lightning storms, which could define a certain signal pattern correlation and exclude the possibility of it being a storm, or is the amount of single noise data to start doing this too immense?

Based on the likelihood profile they published, they don't have anything else as strong as the signal in the individual detectors (apart from the other GW events) - so even without the time correlation the event stands out in both detectors.

 

[auou]

they don't have anything else as strong as the signal in the individual detectors

That's not clear to me, they have published the results of events they have filtered out as 'PeterDonis' said:

Signals which look like GW signals but only appear in one detector, which is how an "unknown origin" signal would be expected to behave, don't pass this filter.

There is no talk of possible other strong signals, that doesn't mean that they have none, and that was exactly my question.

Yes, we know for those events that the signal was more than clear. But what outside of these filtered time periods?

 

[mfb]

That filter is used to search for signal events, not for the background estimate.

There is no talk of possible other strong signals, that doesn't mean that they have none, and that was exactly my question.

They have a histogram of the significance distribution.

 

[auou]

They have a histogram of the significance distribution.

What is this, do you have a link?

 

[mfb]

In the original publication, figure 4.

Note how the background estimate at intermediate detection statistics in 4b is completely dominated by combining one of the signal spectra with random noise from the other detector.

 

[auou]

The graph gives the filtered results that 'PeterDonis' mentioned and not the individual signals that I asked about:

The search reconstructs signal waveforms consistent with a common gravitational-wave signal in both detectors using a multidetector maximum likelihood method.

Do you know of a reference of other non-filtered events?

 

[mfb]

There is nothing filtered - apart from data not used in either the search or the background estimate. The whole dataset used to search for events is used for the background estimate as well.

 

[auou]

There is nothing filtered - apart from data not used in either the search or the background estimate. The whole dataset used to search for events is used for the background estimate as well.

Sorry, but I am not seeing it in Fig. 4 of the paper. The first graph has value ηc and the second one ρˆc and both formulas are based on combined data of the two detectors:

• "The statistic ηc thus quantifies the SNR of the event and the consistency of the data between the two detectors."

• "The final step enforces coincidence between detectors by selecting event pairs that occur within a 15-ms window and come from the same template. The 15-ms window is determined by the 10-ms intersite propagation time plus 5 ms for uncertainty in arrival time of weak signals. We rank coincident events based on the quadrature sum ρˆc of the ρˆ from both detectors."

 

[mfb]

If you combine a strong signal-like pattern in one detector with random noise in the other, you get intermediate $\hat \rho_c$ values sometimes. That's what you see in 4b between 13 and 21 for this value: Combinations of the GW signal from one detector with random data from the other (from a different point in time). After they remove the two signal-like patterns from GW150914 from the analysis, nothing of this remains. If there would be another similar signal-like pattern (from a single detector) somewhere in the dataset you would still get some entries in this intermediate range. But there is absolutely nothing.

 

[auou]

Yes, but this doesn't answer my original question:

Would there be a list of 'single' strong detections …

Fig. 4 doesn't tell us anything about any other signals that appeared in only one of the detectors. The graphs are both about combinations as you point out.

I am not sure if you understood my question.

 

[mfb]

Fig. 4 doesn't tell us anything about any other signals that appeared in only one of the detectors.

It does. The graph would look completely different if there would be other signal-like features (of comparable strength) in only one of the detectors.

 

[auou]

It does. The graph would look completely different if there would be other signal-like features (of comparable strength) in only one of the detectors.

Let's say there was such a signal in only one of the detectors where would it be positioned, because as mentioned in my previous post the formulae used are based on data of the two detectors. So if it doesn't show up in the other the 'statistic' goes all the way down.

This intrigues me as to were an exactly similar signal detected at only one detector would show up in the graph?

 

[mfb]

If there would have been such a signal, figure 4b "without GW150914" would have entries in the intermediate test statistics range. It does not have them.

So if it doesn't show up in the other the 'statistic' goes all the way down.

It is lower than 32, sure. But still higher than all the background they actually see.

This intrigues me as to were an exactly similar signal detected at only one detector would show up in the graph?

In the intermediate test statistics range. The entries "with GW150914" are exactly the result of such a signal from one detector correlated with unrelated data from the other one.

 

[auou]

If there would have been such a signal, figure 4b "without GW150914" would have entries in the intermediate test statistics range. It does not have them.

Mh, this seems to conflicts with what the Danish group wrote in their paper:

"There are, however, a few events that are morphologically similar to the GW150914 event for the 0.1 s case with cross correlators at the level of 0.3–0.6. In order to illustrate these features of the H4096 and L4096 records, we have considered the following four events from Fig. 7 with the approximate coordinates A = (0.6,0.4), B = (0.5,−0.5), C = (0, −0.7) and D = (−0.65, −0.3). All of these events are characterized by a relatively large high level of positive or negative cross-correlations with GW150914. Event A is pointed in the direction of GW150914 and seems to be morphologically close to it." - https://arxiv.org/abs/1609.08346

 

[mfb]

What the Danish group did is not what LIGO did. The Danish group didn't even consider how LIGO estimated the background.
You can get large correlation coefficients with noise alone. That is not the right way to search for signals. You have to consider the amplitude. LIGO did that, the Danish group did not.

Where exactly is the text you quoted?

 

[auou]

Where exactly is the text you quoted?

Sorry, I had pasted the link of their other paper it should have been this one: https://arxiv.org/pdf/1604.06211.pdf?

at topic 5 'The search for similar morphology in the 4096 second records' on page 9

 

[mfb]

Thanks. See above: That is not the right way to estimate a background.

 

[auou]

Thanks. See above: That is not the right way to estimate a background.

Mh, have you read the response from the Danish group to Harry that I posted earlier:
http://www.nbi.ku.dk/gravitational-waves/gravitational-waves.html

In short they say that there's an issue with the cleaning of the noise. The correlation of the noise is in the same time window as the signal, while:

The purpose in having two independent detectors is precisely to ensure that, after sufficient cleaning, the only genuine correlations between them will be due to gravitational wave effects.

Anyway to come back to my question, there is also a new article by 'Quantamagazine' where they write:

Lots of little bumps and vibrations can mimic a gravitational-wave signal … https://www.quantamagazine.org/strange-noise-in-gravitational-wave-data-sparks-debate-20170630/

 

[mfb]

They make a lot of claims.
Their claim that you should see exactly (?) zero correlation in the residuals is one of the worst ones. You only expect this if the template matches exactly. No one expects that, and LIGO explored possible deviations. Again something the Danish authors seem to ignore.

the only genuine correlations between them will be due to gravitational wave effects.

And random fluctuations. Not every non-zero correlation is directly something interesting. You expect a lot of non-zero correlation events in noise just by random chance. They will all have small amplitudes. Did I mention that you have to look at the amplitude?

Lots of little bumps and vibrations can mimic a gravitational-wave signal

But not with the signal strength LIGO saw.

 

[auou]

They make a lot of claims.
Their claim that you should see exactly (?) zero correlation in the residuals is one of the worst ones.

…

But not with the signal strength LIGO saw.

The article claimed the same thing:

The noise at each detector should be completely uncorrelated — a jackhammer going off in the town near one detector won’t show up as noise in the other.

BTW the signal of 'a jackhammer going off' should at least have the same 'signal strength' if not stronger. I speak of personal experience they have been doing construction works in my street the past few weeks and at times I could feel my apartment shake, in contrary I have never felt a GW.

 

[mfb]

The article claimed the same thing:

It is not the same thing, and the difference is what the Danish authors are missing.

"has no expected correlation" and "will be measured with a correlation coefficient of exactly zero" are different things, and "noise" and "residuals after template subtraction" are different things as well.

BTW the signal of 'a jackhammer going off' should at least have the same 'signal strength' if not stronger.

And the seismic sensors detect it and it doesn't enter data analysis.

they have been doing construction works in my street the past few weeks and at times I could feel my apartment shake, in contrary I have never felt a GW.

Can we keep the discussion scientific please?