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Strange Noise in Gravitational-Wave Data Sparks Debate
- raverbashing 9y agoWait 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"
- deleted 9y ago[deleted]
- phreeza 9y agoScientists don't say correlation doesn't mean anything. What they do say is that correlation doesn't imply causation. 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.
- raverbashing 9y agoThere are several possible sources of correlation on the "impulse response" of the LIGO detectors (the whole detector, from building construction to the electronics and detectors in itself) so it is not surprising that might happen. Or maybe it's something in the source of the signal But the signal is still stronger than the noise Besides the "poking the data in weird ways until it stops meaning anything" issue
- yodon 9y agoNo. The word "Correlation" as used in the article to discuss correlations in the data generated by two remote detectors has a very specific technical meaning. It's not simply one of the words in the trope "correlation does not imply causation." 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).
- catamorphismic 9y ago... what? This is precisely the correlation meant in the aphorism you mentioned and is also the only sense of the word correlation of which I am aware of. Signals can be similar (correlated) purely by chance and this cannot be taken to imply a causative link between them.
- phreeza 9y agoIf a correlation is consistent, some sort of causation is quite likely. There are of cause spurious correlations but in physics experiments we can mostly get rid of them by repeating the experiment several times, unlike in economics for example. 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.
- justifier 9y agocode links: original criticism: http://www.nbi.ku.dk/gravitational-waves/correlations.html http://www.nbi.ku.dk/gravitational-waves/correlations.html response: https://github.com/spxiwh/response_to_1706_04191/blob/master/On_the_time_lags.ipynb https://github.com/spxiwh/response_to_1706_04191/blob/master... definitely click through the quanta article's links.. fascinating reads all the way down
- hcrisp 9y agoIt is fascinating, and I learned that one of the events was first detected by an unmodeled search for strange events in the signals: "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-response-to-on-the-time-lags-of-the-ligo-signals-guest-post/ http://www.preposterousuniverse.com/blog/2017/06/18/a-respon...
- unlikelymordant 9y agoI have done lots of signal processing. You cant just subtract out a noisy signal and complain the residual is correlated, there will be bits remaining. If they want to show correlated noise, why cant they use any other part of the signal?
- yodon 9y agoThe correlated remainder is only in the vicinity of the signal event. 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.
- chmike 9y agoIt all boils down to what they subtract. It is said that they subtract the wave signal. Is this signal known for sure or very predictable ? It also depends on how they subtract it. Is it in the fequency domain or the space doman ? I didn't read the arXiv article and don't plan too. Just commenting and asking.
- yodon 9y agoThink of it as a matched filter tuned to the impulse response of a gravity wave. If that filter matches the observed signal then it's a detection, if it doesn't then it's not. The debate is over the significance of the inevitable remainder after subtracting the model of the signal. Is it an expected amount of remainder or an unexpectedly large amount of remainder.
- contravariant 9y agoIf any correlation in the residual signals would be enough to doubt the discovery then I don't understand why they didn't simply combined the signals into one by minimizing the residual correlation.
- dnautics 9y ago“The only persons qualified to analyze this paper are in the LIGO Scientific Collaboration” What? That's an incredibly dangerous position for a scientist of any stripe to take.
- arca_vorago 9y agoI've yet to find a good solution to this increasingly more prevelant sentiment in the current highly-specialized niche type of scientific advancement. I always looked up to the autodidacts of history and wanted to emulate them, but all I hear is "that's impossible these days".
- jessriedel 9y agoIt's an empirical claim, not a normative one.
- dnautics 9y agoit's a terrible claim. You can't claim that for all of the subdisciplines in the massive LIGO project, every single field is covered by the 'topmost expert'. Especially for a criticism about signal processing - there are so many adjacent or even distant scientific fields that are likely to have individuals with superior 'expertise', in the same class of algorithms used for the signal processing. 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.
- kurthr 9y agoPlease reread. Those other experts are apparently Fourier analyzing processed data to "remove the signal". Whether they are doing it correctly or not, any real check will require looking at the underlying data, which is what the non-LIGO associated scientist said. Misquoting someone is not a good way to start an argument.
- maxander 9y agoDo they propose a mechanism for how there could be simultaneous correlations between the two detectors that aren't gravitational waves? There's few enough forces that can act through kilometers of solid earth that I'd be willing to take it as a given that they're uncorrelated, absent a signal... but then, I'm not a physicist. 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?
- fleitz 9y agoIf gravity is waves, it stands to reason that there would be lots of noise that correlated as various events in the universe reach earth.
- spuz 9y agoI think the argument is that any correlation in the noise must be simply coincidental. In other words, the detection could be explained by chance rather than some true physical phenomenon measured by both detectors. 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.
- smortaz 9y agoIf you want to look at the data and run their code w/o installing any software, check out their ready to view/run Jupyter notebooks: https://notebooks.azure.com/losc/libraries/tutorials 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)
- sleavey 9y agoLet's wait and see if their article gets through peer review in a similarly respected journal to the three published detection papers'.
- snarfy 9y agoIs it really noise or just more signal? 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?
- dboreham 9y agoThe pattern is in a different dimension than the noise (time vs spatial). A pattern by definition isn't noise.
- kobeya 9y agoNot by definition. There is such a thing as structured noise.
- mirimir 9y agoWell, except: > 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?
- shamaku 9y agowhere there's smoke...
- PhantomGremlin 9y agoObviously the simplest solution is to have a third detector, located far away from the other two. I'm surprised the article didn't even mention that.