# Is there absolute proof for gravitational waves?

As you probably know gravitational waves seem to have been proven unless you've been living under a rock for the past day. The experiment involves the reflecting off mirrors and waves of light and it is best you look because it is a little bit complicated but not too difficult. The effects are very subtle, and I am wondering if they could be a result of any Earth movements or activity above the Earth's surface. However, it seems most likely that if it is not gravitational waves, it is Earth movements because the experiment took place deep underground. Is there any proof that this is not a result of Earth movements and the only possible explanation is gravitational waves?

I don't want to be the stick in the mud here and spoil the party. However, I do like to know that what we are seeing is the truth as the effects require highly advanced equipment to be observed.

After reading all the great responses, the proof that gravitational waves do exist appears rock solid, and it seems Einstein was right again.

• There is never "absolute proof" for any interpretation of any experimental result. Empirical science doesn't deal in proofs. Feb 11, 2016 at 17:55
• One of the things they stated in their release was that the event was measured separately by the two detectors, separated in time by the amount of time you would expect gravitational waves to traverse that distance from the target. So that is at least consistent with it being gravitational waves (among other evidence). You wouldn't expect this type of correlation if it were related to local geologic activity. Feb 11, 2016 at 17:56
• Great effort was made to attenuate earth vibrations (they suspended the mirrors like pendulums). Moreover, the agreement between the measured and predicted signals is extremely compelling. Feb 11, 2016 at 18:00
• Looks like StackExchange is itself a gravitational wave detector. If a gravitational wave of sufficient strength hits earth, the number of question asked about the topic will rise :) Feb 11, 2016 at 18:03
• "deep underground" -- I think you're confusing LIGO with neutrino detectors. Look up pictures -- you can see the thing from the air :)
– user10851
Feb 11, 2016 at 18:54

Yes, LIGO's detectors include a slew of detectors attempting to detect just about every possible form of noise, including noises due to rumblings of the Earth. They also include elaborate suspension systems which attempt to damp out these effects. There is an easy way to explain how this works if you have a "slinky" toy: hold it from one edge and let it hang down, then vibrate its edge very rapidly: you will notice that the lowest end shakes but does not move up-down very dramatically. Now move its edge up and down very slowly: you will notice that the lowest end follows your hand's motion. Only when you shake it up and down near a very precise frequency, the resonance frequency of the system, do you get a dramatic result where the bottom goes up and down more independently of the top. This is called a "band pass filter", we can ensure that any waveforms outside of this particular band of frequencies are very strongly damped-out, and then measure the frequencies that are allowed to pass through the suspension very precisely. They do not use these "slinkies" precisely that way, but the cable systems which suspend the mirrors involved have essentially the same role in removing geological vibrations.

The detectors:

1. Measure all sorts of other phenomena, and this effect was not attributable to any of those other phenomena;
2. Measured the exact same effect in tandem with a small time delay on two different places on Earth separated by 3,000km: so this effect was not local to any single part of the Earth but seemed to "wash over it";
3. Measured a signal which was exactly in line with what the radiation from a certain known astrophysics event predicts: slow oscillations ramping up in a certain precise way in frequency, leading to a sudden crescendo of vibration followed by an immediate cut-off -- so this is not a typical signal for any other typical terrestrial events;
4. Measured a signal which visibly appears above the normal "noise floor" that you can observe in the detectors normally -- so this is not part of their typical noise that we can observe;
5. Measured a signal very soon after they were turned on, so this is not a likely statistical anomaly visible in any large swath of data.

For these reasons, it seems to be a signal from space in the exact regime that the detectors were built for, not a terrestrial signal that somehow messed with everything in the right way.

• Thanks for the great answer - in point (2) I've been wondering, how the hell do they sync the two distant stations so precisely? Are they connected by fiber or copper of known length, or, do they use dead reckoning with some sort of clocks?! I mean I find it hard enough to sync a video game to a ms :O Feb 12, 2016 at 13:47
• @JoeBlow I do not know precisely how they have done it. Many scientific setups use the fact that there exists a broad network of atomic-clock-satellites orbiting around the Earth which is constantly broadcasting the current time (with general-relativistic corrections) to every place on its surface. This network is called "GPS" because by measuring how out-of-sync a constellation of 3+ clocks is, you can triangulate your position to high certainty -- using position you can then also determine the true time delay and the local time. But maybe they just use NTP over their high-bandwidth networks. Feb 12, 2016 at 15:25
• @JoeBlow Actually, I can confirm that: this paper discusses how they get 1-microsecond accuracy even though there's 13 light-microseconds distance between the two detectors in Hanford, WA and Livingston, LA. Each facility has their own atomic clocks and each second they adjust it slightly to make sure that it matches the GPS time. Feb 12, 2016 at 15:46
• Totally fascinating, and the paper is the exact answer to my question! Thanks! To be sure to be sure, as I understood it the two different sites (Louisiana; Washington state - 3002 (? see below ?) km apart) need to be in sync - that's correct? Feb 12, 2016 at 16:52
• (I've just realised this wiki article en.wikipedia.org/wiki/LIGO#Observatories states the observatories are 3002 km apart. But I wonder if whoever calculated that did so, unthinkingly, as a great circle route? Surely the relevant figure is the literally straight line distance (ie, through the earth) between them? Feb 12, 2016 at 16:54

There is no absolute proof. I think we can be confident that a speed-of-light pulse from the direction of southern hemisphere hit the Earth. However, as I noted on another post (and got a lot of heat and dinged), there is only one laboratory doing the measurements, and "science" requires more than one laboratory to provide proof, even that a pulse hit. The Livingston-Hanford detectors together make up one laboratory. One without the other is useless. That's why two were built simultaneously.

What may be true is that other pulses have been detected that have not been announced. Let's wait to be true believers. Maybe they were gamma ray bursts. It may only take weeks. No hurry, no worry.

• you're right, they are completely wrong : the resulting BH weighs 0.25 solar mass less than announced ! hence relativity is wrong, general relativity is wrong and quantum teleportation is wrong ( benefits for all ) .
– user46925
Feb 14, 2016 at 5:29
• And now there is Virgo, which has also made a detection. Apr 8, 2018 at 8:18