# What did LIGO Actually See? (Gravitational waves discovery)

I am trying to find an original video/image of what LIGO actually saw, but all I can find is artist renditions of gravitational waves.

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As discussed in my answer below, LIGO is more like a microphone than a camera; so it makes more sense to talk about what we heard rather than what we saw. You can listen to the signal here: youtube.com/watch?v=TWqhUANNFXw – Chris Mueller Feb 17 at 14:29
Wouldn't a better metaphor be a seismometer? – user151841 Feb 17 at 22:42
@user151841 Not really. Seismometers have three output data streams: acceleration in x, y , and z. Also, I think microphones are more intuitively familiar to the non-science public than seismometers. The LIGO detectors are also especially well suited to being compared to a microphone because the sensitive band of the detectors is completely within the range of human hearing. – Chris Mueller Feb 17 at 23:43

The actual image isn't much. I was able to find it from Science, and this is all it is:

It's just a ripple, seen at slightly different times from two different observatories. The shift fits perfectly by shifting it by the speed of light difference in their locations. Thus is the proof of gravity waves.

It should be noted that the reason there are two instruments is to provide a cross check against other vibration sources. Each observatory works by detecting vibrations on a 4 km scale, down to a very small order of magnitude (1/10,000 the width of a proton). When the two are compared, then one can assume the signal must have come from a non-local source, which only Gravity Waves fit that definition.

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The original source is journals.aps.org/prl/pdf/10.1103/… – OrangeDog Feb 17 at 12:03
"The actual image isn't much", "this is all it is." Your tone understates how awesome it actually is IMO ;). Of course, I'm a little bit biased. – Chris Mueller Feb 17 at 19:31
How do the two observation locations coordinate their times relative to a shared or common clock? Are they referring to the same atomic clock and making adjustments for "latency", the time it takes to get the time? – TRomano Feb 18 at 21:28
@TRomano We use GPS which is accurate to 10s of nanoseconds. You can read more about the aLIGO timing system here: authors.library.caltech.edu/20471/1/… – Chris Mueller Feb 19 at 14:03
@ChrisMueller: I suspected it was GPS, but didn't have time to look it up at that moment. Thanks! – PearsonArtPhoto Feb 19 at 14:07

First of all, I think your question belies a misunderstanding of the nature of the LIGO observatories. The nature of the detectors is that they act like a microphone, as opposed to a camera. What that means is that they are sensitive to gravitational waves coming in from most directions, but don't have an ability to distinguish where the waves came from. By using multiple detectors (which is also necessary for a confident detection) the difference in time between the detectors can be used to give some idea of the location of the source. It also means that the output of the detectors is a single stream of data.

This image from the paper in Physical Review Letters (not behind a paywall) is a better summary of what LIGO heard than the current accepted answer. I'll explain the panes from top to bottom.

1. The top panes show the 'raw' signals measured in the two detectors with the H1 data overlayed on the L1 data on the right.
2. The second row of panes shows a number of different simulations of what general relativity (Einstein's theory) predicts for the gravitational waves. These simulations is how LIGO is able to claim that they know that the wave was caused by two merging black holes.
3. The third row of panes is the 'raw' data minus the simulations.
4. The bottom panes are simply another way of plotting the 'raw' data called a time-frequency plot. Time is on the x axis and frequency is on the y axis. To a person from the field this signal is the most recognizable characteristic of a merger, known as a chirp. As time moves forward, the frequency shifts higher. You can actually listen to the 'raw' chirp here.

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It's not behind a paywall because the paper is open content—it is licensed under CC BY 3.0. – bwDraco Feb 17 at 19:25
@bwDraco Good point. – Chris Mueller Feb 17 at 19:28

LIGO didn't "see" anything. It monitors the relative lengths of the paths taken by two laser beams in vacuum pipes about 4km long (although the laser path consists of about 75 trips up and down the arms) and at right angles to each other.

A gravitational wave, travelling at the speed of light, changes the ratio of these lengths (one gets shorter, one gets larger, then they swap) by about plus or minus 1 part in $10^{21}$ (a billion trillion) about 30-200 times per second as it passes through the instrument.

The whole event lasted about 0.3 seconds and the trace (which has been all over the news) simply records the fraction by which the length of the arms changes as a function of time.

The event was (nearly) simultaneously recorded by two almost identical setups in different parts of the USA. The detection of the same signal in both detectors rules out a local cause of the disturbance, and the small time delay between the detections allows a rough location of the gravitational wave source on the sky.

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According to the GW150914 tutorial, this is what Advanced LIGO L1 and H1 detectors originally saw:

The other answers show already processed (whitened, filtered, shifted by 7 ms, inverted) waveforms.

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You are correct that this is what the raw data streams out of the detectors look like (note that I was careful in my answer to keep 'raw' in quotes). The sensitive band of the detectors ranges from 10 Hz to 100 kHz, but the raw data stream is dominated by the incredibly large (for LIGO) noise below 10 Hz. You can see this by comparing the units on your plot to those in the plots I posted. Part of the technologies LIGO employs to reach its unprecedented goal involve advanced signal processing. – Chris Mueller Feb 18 at 12:42
You can see the actual noise curves of the detectors around the time of the detection here: dcc.ligo.org/public/0119/G1500623/001/… – Chris Mueller Feb 18 at 12:55

The actual mechanism of measurement that LIGO used is laser interferometry, so one reasonable interpretation of what LIGO "saw" would be the interference pattern caused by the gravity waves, which would "look" something like this:

Unfortunately I could not find an image of the actual laser interference that LIGO mentioned; it's probably too small for photography anyway.

All the other graphs people are linking do are just graphs of the interference pattern data. Showing a graph of the LIGO data as an answer to this question is like showing an image histogram as an answer to the question, "What does the Hubble space telescope see?"

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This is actually the interference pattern of two overlapping laser beams with different curvatures, and is what one might expect to see in a cheap interferometer (see e.g. Newton's rings). However, LIGO has incredibly well made mirrors so the interference at the output of the detector does not have any rings, and, in fact, is completely black on the scale of this image. – Chris Mueller Feb 18 at 12:46

I don't know whether it's interesting for you, but here is the link of the paper that was published about those observations:

http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.116.061102

Once the answer above is quite straightforward! What the paper says (in short) is that LIGO has observed a transient gravitational-wave signal, and these observations match with the predictions of waveform derived by General Relativity for the system involving two black holes.

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Welcome to Astronomy! However, link only answers are usually dis encouraged. If you have something new to add, please sum it up in a few paragraphs. – Hohmannfan Feb 16 at 20:39