GW150914: this may not mean anything to you but I can assure you that any gravitational wave physicist is just melting when seeing this codename.


GW150914 is the name of the source observed by the LIGO experiment on 2015, September (09) 14. And, of course GW stands for Gravitational Wave. This is the first source ever discovered by gravitational waves, the first of hopefully a very long series. And their name will follow the same pattern, GW followed by the date of detection.


And what an amazing source: two black holes of respective masses 29 and 36 solar masses which merge into a single one, of 62 solar masses. Add things up: you are missing three solar masses. This corresponds to a release in a few hundredths of second of the corresponding mass-energy in the form of gravitational waves, close to 1050 Watts! This distorts space and time around, and this distortion propagates in all directions…


…1.3 billion years later, exactly on September 14, 2015, at 09:50:45 UTC, this distortion reached Earth and was detected in sequence by the two LIGO detectors, first in Livingston (Louisiana) then in Hanford (Washington state), separated by 7 milliseconds.


Here is the beautiful signal, as shown in the press conference of February 11, and in the article published in Physical Review Letters, and its comparison with the theoretical predictions:

fig1In the top panels, you can see the signals arrived first in Livingston (right), then in Hanford (left): see on the right how well they match! In the medium panel, you find the prediction from relativity for the mass parameters given above. Even with the eye, you see by comparing with the plots above how well observation and prediction match. In the bottom panels, you have the difference between signal and prediction i.e. what we physicists call the noise. Does it seem “noisy” to you? Well, look at the numbers on the vertical axis on the left: the signal is clearly dominating. This is in part because those were massive black holes: the event was a very powerful one.


Note also the horizontal time axis: everything happened in less than half a second. Isn’t that an awfully short time for a cosmic event? Well, you have to remember that the LIGO detector is sensitive to waves in a certain frequency range, basically 10 Hz to a few thousand Hz. The frequency of gravitational waves are directly related to the frequency of rotation of the two black holes. As they get closer over centuries and years, their frequency increases until it is picked up by the detector (when the frequency reaches 10 Hz) but that is only a fraction of a second before the final plunge.


You can see this on the next plot, again provided by the authors of the discovery paper:


You see here the three phases (see post A black hole pas de deux) and to which oscillations in the detector they correspond to. On the bottom panel, you see the evolution with time of the distance between the black hole, in units of the so-called Schwarzschild radius. The Schwarzschild radius is basically the radius of the horizon of the final black hole, approximately 200 km. You also see their relative velocity in units of the velocity of light: it evolves from 1/3 the velocity of light to 2/3 the velocity of light at touchdown! Really an amazing event!


The LIGO collaboration has shown at the press conference the following video showing the collision of these two black holes. Enjoy!

Now, you are ready to get a closer look at the discovery paper. It is written for the scientific community, but the first part is rather accessible. If you have to read only one scientific paper in your lifetime, this may be the one.

And if you are a physics student, with at least some background in classical mechanics, here are a few notes for you to understand better this remarkable event. If you feel dizzy when you see a maths equation, stay away!

Understanding GW150914 notes


One comment

  • Peter Gehrke

    Dear Gravity! team

    Isn’t it a funny coincidence that at the same moment that gained an overwhelming interest in your MOOC on gravity, the over 100-year search for gravitational waves has come to a successful end. Or maybe it is not a coincidence at all, but you felt “that something is in the air” and it was time to raise the attention of a broader public to this fundamental force of nature. Thank you for making us aware and capable to understand right on cue.

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