Skip to 0 minutes and 12 secondsWe have detected gravitational waves. We did it!

Skip to 0 minutes and 20 secondsSo these gravitational waves were produced by two colliding black holes that came together, merged to form a single black hole about 1.3 billion years ago. They were detected by LIGO, the Laser Interferometer Gravitational-Wave Observatory. As the black holes spin around each other, all right, the stars behind them are warped and that's because the strong gravitational fields bend the light that comes around. But what I want you to pay attention to in this video is the fact that as they orbit, the black holes are getting closer and closer to one another. The orbit is speeding up and eventually they're going to merge. The event horizons are going to join. Boom.

Skip to 1 minute and 5 secondsSo here are the predictions, the prediction for the event. So remember, you have the two black holes going round another. They get closer and closer. They go faster and faster until the two horizons touch one another. And this is a merger phase or the ring-down phase. And here is a prediction of the theorists.

Skip to 1 minute and 30 secondsNow you can read many things. For example, you can see that the frequency of oscillation is increasing. You see the frequency of a gravitational wave, suddenly, it gets smaller and smaller. That's directly related to the fact that the black holes are, as I said, when they get closer and closer they get faster and faster so then you see it on the signal. The signal of a merger is fairly simple. For many years people didn't know how to compute this. One had to compute this on computers, because it was extremely difficult. You can imagine two black holes touching, that's very difficult. This is also a place where gravity is very strong.

Skip to 2 minutes and 11 secondsAnd so clearly for a while people didn't know how to compute this. It was even called the grand challenge in the 1990s. And lots of money were put into this problem and nobody could solve it. And suddenly in 2005, an American physicist of the name Frans Pretorius could compute the first oscillations. And then everybody could do that probably because also the computers were much more faster and more powerful. So now one can predict all this signal. OK, you see down there you have two interesting things. First, the separation of the black holes. You know those black holes have been going around forever, for many years, 100,000 years, million years, hundred million years and so on.

Skip to 3 minutes and 11 secondsAnd they got closer and closer. And now it's only when they got very close to one another that the signal detection was very strong. And so the signal was powerful enough to be detected. And so it's actually the last 200 milliseconds before the final plunge that when they identified the signal in the detector. At that time the separation was a few hundred kilometres. So you remember those are objects of 30 solar masses. You will see in a second which are very compact and which are close by, close to a few hundred kilometres. And the velocity is impressive. When the signal was picked up the velocity of rotation was 0.3, one third of the velocity of light.

Skip to 4 minutes and 8 secondsAnd it went all the way to half the velocity of light. So these are gigantic-- well they are not gigantic because are very compact. But these very massive objects where really you're going around one another like crazy.

Skip to 4 minutes and 26 secondsSo what about the signal? So it happened in Paris on September 14

Skip to 4 minutes and 33 secondsat 9:50, 9 hours 50 minutes and 45 seconds

Skip to 4 minutes and 38 secondsuniversal time which was two hours, 11:50 Paris time. And so they first detected one signal in the Livingston detector, and then another one in the Hanford detector. There are 3,000 kilometres difference between the two. So basically light travels around 10 milliseconds-- it takes light 10 milliseconds to go from one site to the other. OK, they saw the signal in Livingston and then seven milliseconds later they saw the signal in Hanford in Washington state. Here's the signal. So first look, the first signal is the blue one in Livingston. Then seven milliseconds later you have the red one in Hanford. OK, they provided you with the comparison.

Skip to 5 minutes and 31 secondsSo the orange it shows the same one as the red, but put on the same plot. So you see that the signals look very much the same. And so that's very striking. You can also see that you have little wiggles, for example, at the end. OK, this correspond to the sort of standard noise of the detector. And so you see this is much bigger than the standard noise of the detector, a kind of effect that the detector is we have little motions and all that. So they studied that for years. But then suddenly there's a big signal as you can see.

Skip to 6 minutes and 8 secondsAnd what is most impressive it is exactly the same signal-- except for the delay-- exactly the same signal in the Livingston detector and the Hanford detector and they are 3,000 kilometres difference. After 10 minutes-- of course, there is automatic detection, so that is automatic data treatment, data processing-- and so after 10 minutes the automatic data processing identified a signal which is the greyish one. Now in blue this is the theoretical prediction that I was telling you. And so you see that clearly there is an amazing-- of course with some parameters-- but there is an amazing proximity between the first analysis after 10 minutes and the prediction corresponding to two black holes of something like 30 solar masses.

Skip to 7 minutes and 13 secondsAnd even more amazing is the fact that you get exactly the same signal in two detectors which are 3,000 kilometres away from one another. So that was in September. Of course it took them time-- and you'll get more about that this afternoon-- it took them time to analyse all this, to see what were the parameters and so on. So they could identify the mass of the two black holes, 36 and 29 for the initial black holes, 62 solar masses for the final black hole. One can identify this was the final black hole is a rotating black hole. And one can identify the distance which is 1.3 billion light years. And if you add up 36 plus 29 that makes 65.

Skip to 8 minutes and 4 secondsYou get 62 in the end, so you're missing three solar masses. And this three solar masses was an energy which was released in the form of gravitational waves. OK, so that's a huge energy. I was just saying it was announced in the press conference that if you talk in terms of power this is three solar masses released in 0.2 seconds. So that's as much power as the total power which we receive from the stars-- from the light of stars of the whole universe. So that's a huge energy which is releasing these gravitational waves. And basically you see that almost does not interact with us. It's maybe better that it doesn't because otherwise we might not be around.

Skip to 8 minutes and 53 secondsNow we have been searching for these gravitational waves for a full 100 years and suddenly we have discovered them. So that sort of closes the search for gravitational waves and that was extremely important. We didn't really doubt that there were gravitational waves, but anyway it was important to discover them. So it's a very important date, if you want. For the first time it test general relativity in the strong regime. You have seen that two gravitational waves-- I'm sorry, that two black holes are so close that gravity is very strong. And in all the test of general relativity, so far we were in regimes where gravity was weak.

Skip to 9 minutes and 48 secondsSo we didn't know if Einstein's theory was applying to this very strong regime of the gravitational force. The fact that you get exact agreement between the prediction and the observation shows that general relativity is for the first time tested in a strong field regime. And finally, the discovery of this binary source of black holes-- and you have seen the details-- and there are many things one can read of course as you did with the fact that, indeed these gravitational waves are going through the universe for 1.3 billion years without being touched, if you want.

Skip to 10 minutes and 38 secondsSo encoded in these gravitational waves in a very clean manner you have all the information of the distortion that the event created in space time 1.3 billion light years ago. So that's certainly opening a full new field anyway that's called gravitational wave astronomy where you can directly study the gravitational universe. And remember that gravitation is really the engine of the universe. For first time we'll see directly what is behind the dynamics of the universe.

Skip to 11 minutes and 26 secondsOur best chance of seeing a black hole is through the detection of gravitational waves. And probably the most fascinating event will be associated with the collision of two galaxies and the fusion of very central black holes. With the eLISA satellite, we should be able to follow this process from the collision up to the end where the two black holes are merging into a single one, what we call the coalescence of black holes. In the following simulation we'll see precisely that event. On both sides, you have two galaxies and each luminous dot is a star of one of these two galaxies. Now you see that they are colliding and so that is a big mess of stars during the collision.

Skip to 12 minutes and 24 secondsAnd now we are zooming on the central part. And you see the two black holes with their accretion disc that are caught gravitationally with one another and are rotating. Now that system is emitting gravitational waves, so it's losing energy. They get closer and closer. They turn faster and faster until the moment we have the two horizons touch one another. They turn into a single black hole, that's a coalescence. And that black hole is giving away some of its decrease of freedom-- some of its hair-- in the form, again, of gravitational waves.

Skip to 13 minutes and 7 secondsIt is this event that should be seen by the eLISA satellite and that one can bet that day most of the telescopes in the world will be turned in the direction of this event.

Skip to 13 minutes and 25 secondsWe might have to wait a few centuries before we sent out astronauts to the horizon of a black hole and we let one astronaut cross the horizon of a black hole. But we can do almost as well using gravitational waves and that should be done in the next years or in the next decade. The idea is the following, just consider a very massive black hole at the centre of a galaxy. There are stars, stellar object that keep falling into the horizon. Now before falling into the horizon these objects are orbiting around the horizon. So, for example, you could have stellar mass black hole, a small black hole that would be orbiting around the horizon of a black hole.

Skip to 14 minutes and 9 secondsIt loses energy by emitting gravitational waves. We can, of course, detect these gravitational waves. And because it loses energy it gets closer and closer. But at the same time it is mapping the structure of space-time around the horizon. And then at some point it gets into the horizon, the signal in gravitational waves disappears and that's a proof of the existence of an horizon around the massive black holes. In the simulation that follows you're going to see precisely this. Of course, it's accelerated. But you'll see stellar mass black hole orbiting something like 100,000 times around the horizon of a very massive black hole. And you have to wait till the last second because at some point the signal disappears.

Skip to 15 minutes and 5 secondsThe stellar mass black hole has disappeared through the horizon of a massive black hole.

Skip to 16 minutes and 12 secondsAnd you have noticed that suddenly the signal has disappeared. This is a sign that just like the astronaut would cross the horizon of the black hole, that the small black hole has disappeared through the horizon which is a proof that the massive black hole is surrounded by an horizon.

# The discovery of gravitational waves

Here comes the climax of this course: the discovery of gravitational waves on September 2015 thanks to an event that took place 1.3 billion years ago: the fusion of two massive stellar black holes. Share the excitement by watching excerpts of the press conference announcing the discovery (February 11, 2016) and of a special event organized two weeks later especially for the learners of Gravity!

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