Strange Noise in Gravitational-Wave Data Sparks Debate
quantamagazine.org
quantamagazine.org
Even more I'm-not-a-physicist-y speculation; could the additional correlation be some secondary gravitational wave effect hidden under the main signal, pointing to another aspect of the phenomenon?
But, LIGO ran their detectors for 8 years at a lower sensitivity with no detections. Almost as soon as they turned on the upgraded detectors in September 2015, they detected the first event (GW150914) which appeared at a level that would have been undetectable previously. Also, they have made two other detections in the 1.5 years since. My guess is that if these events were due to flawed analysis, then we should have seen something during those first 8 years.
Also, it is not just a correlation that has been observed - it is a correlation that has a specific form, a rising fundamental frequency that happens to in line with what we would expect from merging black holes. I know there's an element of circularity in the argument, but what are the chances of that particular sort of signal arising from a spurious correlation?
What? That's an incredibly dangerous position for a scientist of any stripe to take.
The claim is that only the people working in the project are qualified to criticize; that's almost the opposite. People who are working on the project are more often than not the LEAST qualified to criticize, because they're heads down in the weeds and have too much invested in the particular details and experimental choices that were made.
Moreover, for LIGO specifically, not every scientist is motivated to work for a high-risk, long-time-till-result endeavor. In fact, I'd suspect the people most likely to approach things with the right critical mindset are antiselected from working on the project.
page 3:
"The only persons who have the right critical mindset to assess this paper are in the LIGO Scientific Collaboration"
I believe instead it is simply saying:
"The paper has no obvious flaws that anyone unfamiliar with LIGO could point out. Therefore the best people who could assess it are the LIGO team"
What's the point of peer review, then? Why bother publishing detailed experimental reports and providing access to data if the team gives it the two thumbs up and no one on the team sees any flaws. Their word should be good enough.
The fact that one team is more qualified to analyse a paper that another team has produced has nothing to do with whether or not that paper should be independently reviewed.
The quote has nothing to do with who is trained in signal processing in the abstract, and everything to do with who has a detailed understanding of this particular machine.
That kind of gives the lie to the conspiracy that many would like to believe... And before you go demanding that they release the raw data (in addition to the processed data), I hope you are aware of how many disk drives you're going to need to fit into that station wagon.
the 'truckloads' of data are basically all the uninteresting parts of the timeseries. Of course there is statistically going to be random events where the noises correlate at the level seen by the critique, and maybe the truckloads of data show that those are frequent enough that say 1/3 of the bloops should have that correlated noise superimposed on the 'actual signal'; and maybe that bloops 2 and 3 do not have the correlated noise if you apply the same analysis to it.
How much more 'raw data' do you need to reach a conclusions that refutes the criticism?
https://notebooks.azure.com/losc/libraries/tutorials
1. click on a notebook to see the html rendering 2. or clone+run to execute/edit the code (need to create login)
original criticism: http://www.nbi.ku.dk/gravitational-waves/correlations.html
response: https://github.com/spxiwh/response_to_1706_04191/blob/master...
definitely click through the quanta article's links.. fascinating reads all the way down
"Finally, LIGO runs 'unmodelled' searches, which do not search for specific signals, but instead look for any coherent non-Gaussian behaviour in the observatories. These searches actually were the first to find GW150914, and did so with remarkably consistent parameters to the modelled searches..."
http://www.preposterousuniverse.com/blog/2017/06/18/a-respon...
The 2nd team is arguing the presence of residual correlations after the signal is removed means the total correlation isn't well described by a gravity wave signature.
This is experimental data, so there's clearly going to be noise and there's clearly going to be complexities in the detector response to the event. The question boils down to whether the residuals are significant enough and distinct enough from expected detector response behaviors to call the detection into question or suggest something unexpected is contained inside the detected signal.
There are specific signal shapes/characteristics that would be producable by a gravitational wave. Anything that doesn't match that shape/doesn't have those characteristics by definition isn't from a gravity wave. Ignoring those characteristics and just saying "if it's correlated it's from a gravity wave" would be a highly incorrect way to analyze the data, which is why none of the scientists in the debate would advocate doing that.
This discussion would then simply be about how well the combined signal matches a gravitational wave, which seems a lot more sensible.
This sort of thing is much more about statistics and error propagation than one encounters in more traditional signal processing.
Cosmic background radiation is noise but it can be mapped out into a pattern. Could the correlation be a view of a type of cosmic background gravitational waves?
> The main claim of Jackson’s team is that there appears to be correlated noise in the detectors at the time of the gravitational-wave signal. This might mean that, at worst, the gravitational-wave signal might not have been a true signal at all, but just louder noise.
If that's an accurate characterization, "louder noise" does seem a lot like signal.
Couldn't the "correlated noise" just be a superposition of numerous small gravitational-wave signals?
Scientists keep saying and repeating how correlation doesn't mean anything to the point it's become a joke
Now they are dabbling about a spurious correlation in a small section of the data? For real?
No, really, if anyone would be trying to prove anything based on that correlation alone they would have been laughed and dismissed by the majority of the scientific community
But suddenly when it's to reject a result then it's important?
> points out that Jackson’s team could have misused a common data-processing technique called the Fourier transform... The error, Harry writes, has to do with the technical assumption that the input data signal be “cyclical,”
Correct. Not to mention the sample is too short (and the even is short as well and there's noise and the sampling rate is not too big).
If you play with FFT on these conditions you start seeing things that "aren't there"
If the basic model underlying the original LIGO finding would predict no correlation between the recordings, then a significant amount of correlation should lead scientists to revise the model, and reexamine the findings. Very much standard scientific practice.
But the signal is still stronger than the noise
Besides the "poking the data in weird ways until it stops meaning anything" issue
Correlation as used here and put simply means that when the signal coming out of one the detectors increases, the signal coming out of the other detector (which is physically separated from the first detector by a very large distance) also increases, and when the signal coming out of the first detector decreases the signal coming out of the second detector also decreases.
The existence of that mathematical correlation between those two signals coming out of those two remote detectors absolutely implies there is a causation to the correlation (yes, mathematical correlation can imply a causation, even while it alone is not able to indicate what that causation is).
The question at hand is not whether there is a correlation or whether there is a causation. The question is what is the causation for the correlation and whether that causation is a gravity wave or something else.
The odds are extremely good that that causation is a gravity wave, but it's a big enough experiment and a big enough result that it's worth looking very closely at the analyses that argue in favor of it being a gravity wave (and those that argue against it).
The more important point, however, is that a correlation between A and B does not imply that A causes B or B causes A. It could also be that unknown factor C causes A and B.
Your point is a very good one, though. If we view causation through a causal graph, such that vertices are variables and direct causal links are arrows (directed edges), a statement "correlation doesn't imply causation" can be taken to mean several things:
* From correlation, we cannot determine the direction of the arrow.
* From correlation, we cannot claim there *is* a (direct) arrow between two vertices.
* From correlation, we cannot claim there is even a path from one vertex to the other.