2017-09-30

LIGO noise correlations

I spent some weekend science time reading this paper on LIGO noise that claims that the time delays in the LIGO detections (between the Louisiana and Washington sites) are seen in the noise too—that is, that the time delays or coincidence aspects of LIGO detections are suspect. I don't understand the paper completely, but they show plots (Figure 3) that show very strong phase–frequency relationships in data that are supposed to be noise-dominated. That's strange; if there are strong phase–frequency relationships, then there are almost always visible structures in real space. (To see this, imagine what happens as you modify the zero of time: The phases wind up!) Indeed, it is the phases that encode real-space structure. I don't have an opinion on the bigger question yet, but I would like to have seen the real-space structures creating the phase–frequency correlations they show.

3 comments:

  1. About the phases encoding real-space structure: isn't that just because we are comparing two structures that should have a very similar power spectrum? Every face has a nose, mouth, eyes etc. with approximately the same length scales. So yes, telling one face from another seems to be a matter mostly of phases; that doesn't tell us anything about the differences between more diverse structures being encoded predominantly in the phase information.

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  2. (I assume you are referring to Figure 3 in the Cresswell paper about the phases?) Note that the paper did not do any special windowing to produce these phase plots; so, by default, they are using a square window in the time domain on data that are definitely not periodic. Hence the strong correlations between phases. The actual LIGO analysis uses appropriate windowing. (Also note that Creswell make a number of claims about the filtering used in the LIGO analysis that are not correct---LIGO uses a number of different filters in its various analyses, and none of them correspond to the filter used by Creswell or shown at the LIGO open data tutorial on which Creswell's is based.)

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  3. See https://telescoper.wordpress.com/2017/08/10/on-the-time-lags-of-the-ligo-signals/.

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