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  • Definition | Black hole – Supermassive black holes – Stellar black holes

Definition | Black hole – Supermassive black holes – Stellar black holes

The term " black hole Was invented by the physicist American John Wheeler, in 1967, to describe a concentration of mass-energy which collapsed gravitatively under its own force of attraction and which became so compact that even photons cannot evade this gravitational force.

Jean-Pierre Luminet, Research Director at CNRS, and Françoise Combes, Professor at the Collège com France, tell us about black holes. © Hugot Foundation of the Collège com France

Black hole, horizon of events and Einstein's general relativity

The idea had already been conceived in the XVIIIe century by John Michell and Pierre-Simon com Laplace. But, for a physicist and a astrophysicist modern a black hole is, first and foremost, characterized by the existence of an event horizon, which, in this specific case, is a spherical surface delimiting a region of thespace-time including even the light can't go out.

As part of equations of the general relativity, such an object is described by a unique family of solutions of the equations ofEinstein say of Kerr-Newman and which correspond to a rotating black hole having a cinematic moment, a mass and an electrical charge.

Black holes from Kerr, Schwarzschild and Reissner-Nordström

  • when a black hole turns but is unloaded, we speak of a Kerr black hole;

  • when a black hole does not rotate and is unloaded, we speak of a Schwarzschild black hole;

  • when a black hole does not rotate but has a charge, it is described by the Reissner-Nordström solution.

Although a singularity of space-time is present in these solutions, it does not characterize a black hole. Astrophysicists have reason to believe that a quantum treatment of space-time and material inside a black hole removes this singularity which can be roughly described as a point of infinite density where the curvature of space-time is also infinite. A quantum black hole model leads to the concept ofplanck star.

Stellar black holes, supermassive black holes and black mini holes

  • The stellar black holes are formed on the occasion ofcollapse gravitational of certain massive stars which explode in supernovae.

  • We know that there are, at the heart of some galaxies, so-called supermassive black holes. They contain a few million to a few billion solar masses, but it is not clear how they are formed.

  • There could also be black mini holes from the very primitive phases ofuniverse. Man could create them thanks to collisions of particles in accelerators.

Astrophysicists have reason to believe that charged black holes discharge spontaneously and very quickly. So those who exist in the universe should only be black holes from Kerr or Schwarzschild.

A presentation of the success ofEvent Horizon Telescope which has just delivered the first image of a black hole, that of the supermassive one which is at the heart of the elliptical galaxy M87 at about 53 million light years from the Milky Way. To obtain a fairly faithful translation into French, click on the white rectangle at the bottom right. English subtitles should then appear. Then click on the nut to the right of the rectangle, then on "Subtitles" and finally on "Translate automatically". Choose "French". © Perimeter Institute for Theoretical Physics

Black holes: the Nobel Prize in Physics for Chandrasekhar

Subrahmanyan Chandrasekhar had planned the black hole formation before everyone at the beginning of the 1930s when discovering that a star having exhausted its nuclear fuel and whose mass exceeded 1.44 solar mass had to collapse on itself. Although such a gravitational collapse can sometimes simply lead to the formation of a neutron star, it can also lead to that of a black hole, as Robert Oppenheimer and George Volkoff showed with Hartland Snyder.

The black hole theory was the subject of impressive work by Chandrasekhar during the 1970s. With his discovery of what is now called "the chandrasekhar mass ", They were partly behind his physic's Nobel price, obtained in 1983. As usual for the presentation of this prize, the winner gave a conference. At the end of it, the great Indian astrophysicist made fascinating remarks concerning the mathematical theory of black holes:

“I don't know if the whole scope of what I said is clear. Let me explain. The black holes are macroscopic objects with masses varying from a few solar masses to billions of solar masses. When they can be considered stationary and isolated, they are all, each of them, described exactly by the Kerr's solution. This is the only known case where we have an exact description of a macroscopic object.

The macroscopic objects all around us are governed by a variety of forces, described by various approximations of several physical theories. (…) On the other hand, the only elements of construction black holes are our basic concepts of space and time. They are thus, almost by definition, the most perfect macroscopic objects in the universe. And, since the theory of general relativity provides us with a family of solutions depending only on two parameters for their description, they are also the simplest objects in the universe ".

Black holes: Hawking's radiance

These considerations are problematic because they lead us to believe that all the information contained in objects falling into a black hole (starting with that contained in a star turning into a black hole) is definitively destroyed, or at the very least inaccessible. A black hole should therefore have a entropy and, like any object with entropy, it should have a temperature and radiate. This is the conclusion that has come to Stephen Hawking by applying the Quantum mechanics with black holes, which enabled him to discover that these had to emit radiation like a black body heated.

Black holes should therefore evaporate by Hawking radiation. However, no observation supports this theory to date, although it seems very solid on the basics of the physical current theoretical. However, it leads to paradoxes, such as the information paradox and the information paradox. firewall, whose solutions would revolutionize physics.

Astrophysicists seek to demonstrate that objects that behave like black holes really are, that is, they have horizons of events. The study of gravitational waves issued by black hole mergers, with Ligo but especially eLisa, and maybe also the Event Horizon Telescope (EHT), could give us answers on this subject. Ligo detected directly on Earth for the first time the waves emitted by such a fusion of black holes in 2015 and the members of the EHT collaboration revealed a first image of a supermassive black hole with its horizon of events April 10, 2019. The image is that of the black hole in the center of the galaxy M87 and it conforms to that predicted on computer for the first time by Jean-Pierre Luminet in 1979.

This will also interest you

Stephen Hawking, the astrophysicist who made science love The great physicist Stephen Hawking died on March 14, 2018. A true physics legend, he was also a very talented popularizer. A look back at the extraordinary life of this scientist who has made himself loved by the public and made his scientific research accessible: black holes, superstring theory, Hawking radiation, theorems on singularities … Science tells him a huge thank you.

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