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I was very interested in gravitational wave detectors and how they work exactly. From what I know they set up two lasers with mirrors set up to cancel each other out. Then if movement is detected from gravitational wave changes then light from the laser can pass through.

So can't vibrations of the earth or some other medium of energy be affecting this like changes in air pressure, earth plate movement, etc.?

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    $\begingroup$ Yes $\endgroup$ – rob Feb 4 at 5:33
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    $\begingroup$ There are thousands of fracking wells in Nebraska exactly in the middle between Livingston and Hanford. $\endgroup$ – safesphere Feb 4 at 6:13
  • $\begingroup$ I think I saw in a documentary once that they pick up the ocean waves rolling in on the coast. $\endgroup$ – Arthur Feb 4 at 12:41
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    $\begingroup$ Certainly, the whole point of the amazing gravitational wave detectors, is that, they are devices which can deal with such false signals. You've described the very nature of gravitational wave detectors, OP ! $\endgroup$ – Fattie Feb 4 at 12:48
  • $\begingroup$ @safesphere Are you sure about that? Environment America says there were only five in 2015 (appendix, p32), in a document that's pretty strongly anti-fracking. $\endgroup$ – David Richerby Feb 5 at 1:20
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Summary Yes they can. False positives arising from the acoustic sources you name are ruled out by seismological analysis and the examination of correlation between the separate gravitational wave detection stations.


Pretty obviously, the LIGO detectors are probably the most sensitive microphones ever built. So how do we know they are detecting gravitational waves?

Part of the answer is exquisitely sophisticated vibration isolation engineering. The kind of issues that are addressed by this vibration are explained in the Einstein Telescope design document:

M. Abernathy (& about 30 others), "Einstein gravitational wave Telescope conceptual design study", Published by the Einstein Telescope collaboration comprising the institutions listed in Table 1 of this document, 2011

in chapter 4 "Suspension Systems" and Appendix C.

However, it is impossible to get rid of all the effects of vibration.

So a major part of the experimental design is the fact that there are two LIGO detector stations almost as far apart as one can make them in the United States: one in Louisiana in the South East and one in Washington State in the North West.

Therefore, we look for signals that are present in both interferometers at once. If we accept the relativistic conclusion that no signal can travel faster than $c$, then no the effect on or near to the ground (mining, traffic on roads, and all the human made clatter that we make as a species) that influences one interferometer can possibly influence the other in the time it takes to glean a reading, because those influences travel much, much slower than lightspeed.

This "correlation test" rules most false positives arising from effects other than gravitational waves out. There are then only two possible sources for correlated signals arising at both detector stations with a delay less than 100 milliseconds: (1) acoustic signals arising from a common source within the Earth on the bisecting plane midway between the two stations or (2) from outer space.

Careful and thorough seismometry continuously monitors all seismic waves that arrive at the detector stations. This is enough to rule false positives from source (1) out.

The only other possible source of a signal common to both stations is therefore a disturbance from outer space. When we see such a signal that is absent from the seismometers, we know it is something that modulates the interferometer path lengths simultaneously (or near enough thereto that all other Earthly effects are ruled out) that is coming from outer space.

There are no other known natural effects that can easily explain such correlated detections.

Furthermore, the spectacular gravitational wave event GW170817 was the simultaneous observation of a gravitational wave event by both the LIGO detectors in the United States and the Virgo detector in Italy as well as a gamma ray burst observation within 1.7 seconds by the Fermi telescope. Given that gamma ray bursts are detected by Fermi about once every few days and are observed to arrive as a Poisson process (i.e. they are equally likely to arrive at all times and the statistics conditioned on any event are exactly the same as the unconditioned statistics), the probability of a gamma ray burst within 1.7 seconds of the gravitational wave detection by pure co-incidence is of the order of $10^{-5}$. So GW170817 was a spectacular corroboration of the hypothesis that our gravitational wave detectors are indeed detecting gravitational waves. It is almost certain that the gamma rays and whatever it was that "shook" the LIGO and Virgo detectors in GW170817 was the same source.

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    $\begingroup$ The spirit of this is right, but there is still a lot of work to rule out coincidental sources of noise and (1) is not correct. Anything that happens near the line equi-distant to the 2 detectors is hard to remove. $\endgroup$ – drjpizzle Feb 4 at 13:44
  • $\begingroup$ If one would build and use 4 detectors instead of 2 (3 including Virgo) that should suffice to rule out events that originated on earth with sufficient probability. Does humanity have 4+ such detectors? $\endgroup$ – WorldSEnder Feb 4 at 14:00
  • $\begingroup$ @WorldSEnder you could still not rule out seismic events originating in Earth's core. And as far as I'm aware, we do only have three. $\endgroup$ – John Dvorak Feb 4 at 14:31
  • $\begingroup$ @JohnDvorak The timing is probably tight enough that you could check if the delay between detectors thousands of km apart matches a wavefront coming from outer space and sweeping through the planet at light speed. I suspect that the propagation speed of seismic waves varies enough that they couldn't give a false positive. $\endgroup$ – JollyJoker Feb 4 at 15:08
  • $\begingroup$ I like that idea. You still do need four detectors, though. $\endgroup$ – John Dvorak Feb 4 at 15:09
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Yes they can, and the designers of the LIGO system took extraordinary pains to get the very best isolation numbers they could so as to minimize the influence of external noise on their data. It's worth reading about.

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    $\begingroup$ Or going on a LIGO tour! If you're able to. I did: It was great. To be on topic with this discussion: It was pointed out at the tour that they originally had a noise problem due to light planes using one of the LIGO arms to line up with a nearby airport ... $\endgroup$ – davidbak Feb 4 at 18:57

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