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6.9

Skip to 0 minutes and 14 secondsWe have seen that black holes, at least the black holes of general relativity are very simple objects, like an egg.

Skip to 0 minutes and 25 secondsThey are defined with just three numbers: the charge, the mass, and the angular momentum. Now this is quite different in the real world, if you want, in astrophysics. The astrophysical black holes are much more complicated objects, not so much the central black hole itself but the environment of a black hole. And so this is what we are going to discuss now, because you'll see that this is what allows us to see a black hole.

Skip to 0 minutes and 57 secondsI told you earlier that one thinks now that black holes are everywhere in the Universe. Well, it was not always this way. The first black hole identified is Cygnus X-1, which actually is a black hole in a binary system with a companion star. It was discovered in 1965, and it took 10 years to identify it as a candidate black hole. But for a long time, one was talking about black holes in quotation marks. And it is really as we have said only in the early 2000 that one identified a black hole at the centre of our own galaxy. And from then on, all of the candidate black holes turned into sort of real black holes.

Skip to 1 minute and 57 secondsNow these astrophysical black holes have a rich matter environment that allows us, as a matter of a fact, to identify them. One can see around the horizon of a black hole, of course, outside the horizon of a black hole. One could see dust in the form of a torus. A torus is a geometrical figure which is just the shape of a doughnut. We have also accretion disc. It is just matter falling into the black hole and which forms a disc. And even more surprisingly, we have jets of particles, energetic particles-- so jets which find their origin very close to the horizon of the black hole and which are sending very energetic particles on both sides.

Skip to 2 minutes and 52 secondsNow let us illustrate this with a few pictures. This first picture shows you an accretion disc around a black hole. In this second picture, you see the two jets of particles, which are originating from the close vicinity of the black hole horizon.

Skip to 3 minutes and 19 secondsAnd in the third picture, you have the whole system. So on the left-hand side, you see the black hole, its environment-- so the galaxy at the centre of which is the black hole-- and the jets of particles. On the right-hand side we are zooming toward the centre, and so you see the torus of dust. Inside the torus, you have the accretion disc. And right at the centre of the accretion disc, you have this zone which seems to have more light. Well, this is due to the fact that this is matter falling into the black hole, into the horizon of the black hole. So it's being accelerated. It emits light.

Skip to 4 minutes and 7 secondsAnd so if you could see inside this sort of light, you'd have a completely dark object, which would be the horizon of the black hole.

Skip to 4 minutes and 21 secondsSo let us illustrate this with an animation where you will see all these different structures around a black hole and at this time it's a black hole, a massive black hole, at the centre of a galaxy.

Skip to 4 minutes and 37 secondsSo we are in a spiral galaxy, like our own Milky Way, and you see that we are plunging toward the centre of a galaxy where there is much light. At the centre, you see the torus of dust and at the centre of this torus an accretion disc, jets of particles, and right at the centre you have this black sphere, which is just the horizon of the black hole.

Skip to 5 minutes and 5 secondsIt is striking that one finds a similar environment with all black holes in the Universe. You have here a picture of three different examples. At the centre, you recognise what we have just seen. So that's a black hole at the centre of a galaxy with a torus of dust, an accretion disc. The size of the accretion disc is about one billion kilometre. And you have these, you know, energetic jets of particles. And this is why one calls this type of black holes active galactic nuclei. And it's precisely the jets that allow us to see these black holes. Now on the left-hand side, you have a much smaller black hole, a stellar mass black hole.

Skip to 5 minutes and 46 secondsSo that means the massive black hole is of the order of the mass of the Sun, for example. And you see this is similar to Cygnus X-1. This black hole is in a binary system, and it keeps sucking matter out of the companion star. But again, you find a torus of dust, an accretion disc-- this time much smaller, something like 1,000 kilometres-- and again jets of particles. On the right-hand side, you have a more massive star that collapsed into a black hole. This is what is known as a gamma ray burst. And in this case, it's really the core of a star that collapsed into the black hole.

Skip to 6 minutes and 30 secondsAgain, a stellar mass black hole with accretion disc and jets of particles. And these jets of particles are hitting the outer layers of the star, which make them explode. And this is precisely the explosion that we call a gamma ray burst.

Skip to 6 minutes and 54 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 a collision of two galaxies and the fusion of their 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.

Skip to 7 minutes and 32 secondsIn the following simulation, we'll see precisely that event. On both sides, you have the two galaxies and each luminous dot is a star of one of these two galaxies. Now you see that they are colliding and so it is a big mess of stars during the collision. And 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 where the two horizons touch one another. They turn into a single black hole. That's the coalescence.

Skip to 8 minutes and 24 secondsAnd that black hole is giving away some of its degrees of freedom, some of its hair, in the form, again, of gravitational waves. It is this event that should be seen by the eLISA satellite. And one can bet that that day most of the telescopes in the world will be turned in the direction of this event.

Skip to 8 minutes and 52 secondsWe might have to wait a few centuries before we send 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 objects, 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 a stellar mass black hole, a small black hole that would be orbiting around the horizon of the black hole.

Skip to 9 minutes and 37 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 spacetime around the horizon. And then at some point, it gets into the horizon, the signal in gravitational waves disappears, and that's the proof of the existence of an horizon around the massive black holes. In the simulation that follows, you are going to see precisely this. Of course, it's accelerated, but you'll see a stellar mass black hole orbiting something like 100,000 times around the horizon of a very massive black hole. And you have to wait untill the last second, because at some point the signal disappears.

Skip to 10 minutes and 32 secondsThe stellar mass black hole has disappeared through the horizon of a massive black hole.

Skip to 11 minutes and 40 secondsAnd you have noticed that suddenly the signal has disappeared. This is a sign that, just like the astronaut would cross the horizon of a 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.

Skip to 12 minutes and 5 secondsTo summarise, we have seen that black holes are the sort of quintessential gravitational objects. The black holes of general activity are very simple objects. They have even been compared with bald heads. Their vicinity is the location of very complex phenomena which on the other hand, allow to see these black holes in an indirect way. Of course, the direct way of detecting black holes will be through the observation of the gravitational waves which are produced, for example, when two black holes are merging in order to form a single one or when an astrophysical object falls into the horizon of a black hole. And in this way, we realise, in a certain way, the trip to the horizon of a black hole.

How to see black holes

Astrophysical black holes have a rich matter environment: they organise the matter in their close vicinity. It is this very specific environment that allows us to identify black holes. Ultimately, gravitational wave detection will allow us to characterise black holes directly, to see the fusion of two black holes, and to make the trip through the horizon a reality! (13:16)

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This video is from the free online course:

Gravity! From the Big Bang to Black Holes

Paris Diderot

Course highlights Get a taste of this course before you join:

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