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In late 2015, the LIGO project team announced the detection of gravitational waves. The detector is (very basically) a laser measuring the distance between two mirrors in vacuum, over a large-enough distance to detect very small changes.

The two mirrors are installed on Earth, and a couple kilometers apart.

I believe, perhaps mistakenly, that tectonic activities (if only vibrations) may change the mirror distance over time, so the LIGO project has probably used some model and technology to nullify their effects.

How can that be done? If there is any mistake there, I wonder about how to validate the experiment results.

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There are four aspects to this:

  • You suspend the mirrors and other components using the best suspension mechanism that has ever been built. This is very effective at isolating the experiment.
  • There are two detectors thousands of km apart. To claim a genuine signal it should be seen identically in both in less than the light travel time between them.
  • Many of the astrophysical GW sources (e.g. merging black holes) produce characteristic signals that look nothing like random seismic activity - for instance a correlated increase in frequency and amplitude terminating in a "chirp".
  • Lastly, LIGO is not immune to seismic noise - it is this that partly limits its sensitivity below frequencies of around 30 cycles per second. The seismic noise component at higher frequencies is much smaller.

The way that LIGO isolates the astrophysical signal from the various noise components that are present is described in this paper by the LIGO and VIRGO collaborations. A crude characterisation of the process is that you cross-correlate your signal against a library of expected signatures from astrophysical GW sources and look for a high degree of correlation (which you wouldn't expect in random noise).

The characterisation of the noise (including seismic noise), a description of how it is assessed and monitored (including how data is rejected when deemed too noisy) is found in a further LIGO publication.

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  • $\begingroup$ Thank you for your answer. Links would be very interesting to explore. Do you have information on (in the frame of the question) how noise is filtered out, then? That would address the "How can that be done?" part of the question. $\endgroup$ – Eric Platon Feb 22 '16 at 6:03
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From what I remember from a visit/tour I took of the LIGO facility in WA back in 2000, they said something to the effect that non-GW events (earthquakes, car driving down the road, etc) have a completely different wave signatures or whatever than what was theorized with GW.

Also, detections in WA are compared against what the LIGO in LA may have detected at the same time (or within ~10 milliseconds–time for light/GW to travel between the two observatories?). Any tectonic activity would take much longer to propagate between the two sites, thus eliminating it as a GW.

I'm sure there's more to it than that, but that's the basic gist as my non-scientist mind recalls from 15 years ago.

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  • $\begingroup$ Fantastic, thank you for your answer. Any memory about the how? $\endgroup$ – Eric Platon Feb 22 '16 at 6:05

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