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How to see black holes

Pierre Binétruy explains how the effect on their immediate environment allows to "see" black holes, before direct studies with gravitational waves.
We have seen that black holes, at least the black holes of general relativity are very simple objects, like an egg.
They 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.
I 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.
Now 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.
Now 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.
And 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.
And 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.
So 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.
So 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.
It 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.
So 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.
Again, 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.
To summarise, we have seen that black holes are the sort of quintessential gravitational objects. The black holes of general relativity 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.
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! (7:43)
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Gravity! The Big Bang, Black Holes and Gravitational Waves

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