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A LIGO Was It All Just Noise?

  1. Jun 17, 2017 #1
    Hi,

    I was looking for a second opinion on this article regarding LIGO's gravity waves and if it made sense or not.

    https://www.forbes.com/sites/starts...dent-analysis-casts-doubt-on-ligos-detections

    "Analysis of the data attributed the signal to a black hole merger that happened several billion light years away. But what if there wasn't a signal at all, but rather patterns and correlations in the noise that fooled us into believing we were seeing something that wasn't real? A group of Danish researchers just submitted a paper arguing that the celebration might have been premature."
     
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  3. Jun 17, 2017 #2

    Vanadium 50

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    Forbes is going to be useless as a source. You should find the primary source and post it here. (And yes, I could probably figure out where it is - but it's your question)
     
  4. Jun 17, 2017 #3
    Here is the primary source:
    arXiv.org > astro-ph > arXiv:1706.04191
     
    Last edited by a moderator: Jun 17, 2017
  5. Jun 17, 2017 #4

    pervect

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    A direct link to the article is https://arxiv.org/abs/1706.04191. I don't have an opinion on the paper at this point, but I thought I'd post a better link to it.
     
  6. Jun 17, 2017 #5
    As a source yes, what was 'interesting' about the article was that the researchers at LIGO didn't want to respond, why?
     
  7. Jun 17, 2017 #6

    PeterDonis

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    Forbes in general is not a great source for scientific news, but the particular article in question was written by Sabine Hossenfelder, who is knowledgeable in the field. From skimming the primary source paper, her presentation of the issue appears to be reasonable.

    According to the article, the LIGO collaboration scientists "do not agree on the methods being used and thus with the conclusions". The article also says: "A major shortcoming of the Danish group’s analysis that they [the LIGO scientists] pointed out to me is that the Danes use methods based on tutorials from the LIGO Website, but these methods do not reach the quality standard of the – more intricate – data analysis that was used to obtain the published results." So the LIGO group appears to believe that the Danish group's findings are an artifact of a flawed method of analysis, not an actual problem with the LIGO findings.
     
  8. Jun 17, 2017 #7
    Alright, so that was easily settled with a bit of hand waving.

    I guess the Danish group might have been annoyingly more critical because LIGO resembled the pride of Denmark, LEGO (The name 'LEGO' is an abbreviation of the two Danish words "leg godt", meaning "play well") :biggrin:
     
  9. Jun 17, 2017 #8

    etudiant

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  10. Jun 17, 2017 #9
    As far as I know the forward plan is to have multiple detectors similar to LIGO.
    In that case local background noise should be easy to eliminate
     
  11. Jun 17, 2017 #10

    atyy

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    Motl's main point seems reasonable to me:

    "Andrew Jackson et al. is saying that "something is wrong" because "the residual noise from LIGO-LA and LIGO-WA detectors are correlated and have the same delay" but this correlation between these two "noises" shouldn't exist.

    Except that this statement is wrong. The residuals aren't just noise. They're the difference between the best fit and the actual observation. But the best fit isn't the same thing as the actual gravitational wave."

    One way to check Motl's argument is to see whether the blind injections also have the same feature that Jackson et al think are problematic. Alternatively, Jackson et al could check their paper by analyzing the blind injections and showing that they lack the supposedly problematic features.

    http://www.ligo.org/news/blind-injection.php
    http://www.ligo.org/scientists/GW100916/index.php (publicly available blind injection data)
     
  12. Jun 17, 2017 #11

    Vanadium 50

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    A. It's not published.
    B. It was released Wednesday, and it's only Saturday.
    C. Normally the way things happen if a formal response from the collaboration is required is that the NBI group would submit this as a comment to the journal that published the original result. This isn't, so far, what's happened.
     
  13. Jun 18, 2017 #12
    Not sure what to think of his article, he only briefly mentions what's wrong with the paper of the Danish group, and than goes on over his on own theory of 'different polarizations'.
     
  14. Jun 18, 2017 #13
    Alright, this makes sense.
     
  15. Jun 18, 2017 #14

    vanhees71

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    Well, I'm not an expert in data analysis, and I must say, I can't decide, what Jackson et al really did. There are many words, a nice plot and not much clear mathematical analysis there. Also, one should be as careful to believe analyses of outsiders (i.e., people who are not in the collaboration of the experiment) as one has to take criticism seriously. I hope, the LIGO group will carefully analyse how Jackson et al came to their conclusion.

    Without really understanding Jackson's paper, I only can guess, what's really behind it, assuming that there's really a signal for gravitational waves there (which I still believe since I think the LIGO collaboration was very careful before publishing), what Jackson et al take as noise is the difference between the full signal (applying band filters to it) and a fit with a gravitational-wave signal for both detectors. The signal of one detector was shifted (and inverted) to match it with the signal of the other, and LIGO says that with high significance they detected gravitational waves, particularly the time shift necessary is compatible with the propagation of the grav. wave with the speed of light. Now it's clear that in such a case the difference taken as "noise" of both detectors are correlated due to this fitting procedure. It doesn't mean that the noise of the detectors is really correlated, because you don't know the true noise. That's what I guess is behind these findings by Jackson et al. Nevertheless, this must be carefully checked by experts, and I'm pretty sure, if they send their paper to a good journal, they'll send it to several referees to carefully check it (hopefully to both members of the LIGO collab. and independent experts).
     
  16. Jun 18, 2017 #15
    Actually no, this is not solely what they take as noise, they look at the raw band filtered full signal at the relevant time window, and also to the full signal with the GW attributed signal fit substracted and get similar results(the GW fit signal is very weak compared to the full signal, there is correlation between Hanford and Louisville, with a lag of 7ms. Now this is quite shocking as only the GW signal should have this correlation. The supposed noise associated to this signal should be stochastic. If the full signal is correlated then it is no longer just the noise of a true signal buried within it, it is all signal to the extent it is correlated. And the correlated signal certainly doesn't have a shape that one can attribute to a GW.

    The key point here is that the analysis by Jackson et al is done without the templates, wich are a biasing element.
     
  17. Jun 18, 2017 #16
    The analysis by Motl is quite bizarre, first he is taking for granted that the residual noise is affected by the GW signal for reasons that (he says it himself nd asks for recognition of this basic detail that according to him has gone unnoticed for every other expert) basically contradict LIGO's own basis for their analysis leading to conclude the signal detected is a GW. And second he is working under the assumption that no other source could conceivably produce a signal like it's been detected.
     
  18. Jun 18, 2017 #17
    Curiously enough that's the information they have put out for people to look up and find objections to(and after thousands of citations this seems like the first group that actually has taken the effort to do a critical analysis and publish it. That's weird, as if all scientists are willing to accept such outstanding discovery just on faith), and when someone does find an objection their answer is: "Well that is not really the best place to look at. The good stuff with the right methods is only in our posession".

    Even if this was the case the correct thing to do would be to release that "more intricate method", and in any case explain exactly why what Jackson et al are saying is flawed.
    So far I would say the only difference in methodology is that Jackson is not using the templates, basically because one can see the signal correlation between the detectors perfectly without the templates. And the templates are an obvious biasing element when one is not convinced beforehand that the signal corresponds to a GW, which should be the starting point in any experiment that claims to be a seminal discovery.
     
  19. Jun 18, 2017 #18
    Would you care to elaborate?
     
  20. Jun 18, 2017 #19

    PeterDonis

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    I think they already have, in their own data analysis (published at the time of the announcements of GW detection). But I agree it would be nice to have a direct statement from the LIGO group in response to the Danish group's criticisms, instead of just an indirect quote in an article written by a physicist who is not part of that group.
     
  21. Jun 18, 2017 #20
    Doesn't help that there's only two LIGO stations.
    Issue may soon resolve itself given that several countries are now building 'LIGO-alike' GW observatories. Looks like the Chinese pair will be on-line first. This should provide a tetrahedron of detectors, giving both a longer base-line and better 3D direction finding. Okay, given the configuration, some angles will have much better error-bars than others but, as other stations come on-line, coverage will improve...
     
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