﻿ The black hole, a quintessential gravitational object

# The black hole, a quintessential gravitational object

Pierre Binétruy tells the story of black holes from the 18th century to the discovery that our own Galaxy hosts a gigantic black hole in its centre
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One has been talking about black holes since the end of the 18th century. In 1783, John Mitchell in England and in 1796, Pierre-Simon de Laplace in France identified the concept of a black star. So let me give you the reasoning that they followed. Let’s come back to the Earth.
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We have seen how to put an object into orbit. Now let us consider a rocket. And now we would like to send the rocket out of the gravitational attraction of the Earth. In this case, we have to give the rocket a velocity which is larger than 11,200 metres per second. This is called the escape velocity.
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Now what Mitchell and Laplace imagined is a star Not the Earth, but a star which is so massive and so compact that the escape velocity is not this one, but it’s much larger. It’s even larger than velocity of light. So that means that the light, which in those days according to Newton, consisted of little, tiny, very tiny microscopic objects. Well the objects that compose the light could not escape the gravitational velocity, the gravitational attraction of the star. And so would fall back into the star. So even the light could not escape the attraction of the star. The star would be black. It would not emit any light.
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We meet again black holes 130 years later. We are in 1915. This is the year that Einstein is publishing his work on general relativity. On the German front– remember that this is World War I– a German physicist by the name of Karl Schwarzschild probably gets bored because he tries to solve Einstein’s equations. In a very specific context, he tries to solve Einstein’s equations just outside a spherical star. This is in order, for example, to look at the geometry of space-time outside the spherical star and the way the light rays are curved. Now he identifies a solution, which is a solution which actually will be presented by Einstein himself because Schwarzschild dies in 1916.
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But that solution has a very interesting property. In the case of a very massive and very compact star, the geometry is so curved that light rays that are emitted by the star return to the star, are extremely curved and return to the star. So it means that the light cannot escape the star. So those are the modern equivalent of the black star of Mitchell and Laplace and they are the modern black holes.
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For a long time black holes were just a curiosity of Einstein’s theory. And people were using quotation marks to write the term “black hole.” Until the beginning of the years 2000, one identified a black hole, a gigantic black hole at the very centre of our own galaxy. Now you have here a picture of again the Milky Way, our own galaxy. And we believe now that at the exact centre of the Milky Way, there is a very large black hole. And on this other picture, you have– this is a picture of the electromagnetic radiation in the radio frequency wave. And you see that the very centre of the galaxy is the location of very intense activity.
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Now that might be surprising that there is a large electromagnetic emission very close to a black hole, and we’ll come back to that later, but you see that there are very interesting phenomena at the very centre of our own galaxy.
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We are here at the centre of our own galaxy close to Sagittarius A star. And you see the position of stars very close to that centre, which is represented by a red cross. Now you’re going to see this position evolve from 1992 until 2006. So you see that the star that is coming from the top left behind me suddenly changes direction, as if it was attracted by a very massive object. And the star at the centre, one could determine its positions very accurately. And so reconstruct the gravitational trajectory.
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And by doing that, one was able to identify the existence of a very compact and very massive object of mass of the order of 3 million solar masses right at the centre of our galaxy. This is the black hole at the centre of our galaxy.
Listen to the story of black holes: from the concept of a black star in the 18th century, to its resurrection in the context of general relativity and the recent discovery that the centre of our galaxy hosts a gigantic black hole. (6.36)