Black hole
In astrophysics, a black hole is a massive object whose gravitational field is so intense that it prevents any form of matter or radiation to escape. Such objects do not emit light and are then black. Black holes are described by the theory of general relativity.
They are not directly observable, but several techniques of indirect observation in different wavelengths have been developed and used to study phenomena that they induce on their environment. In particular, the material that is caught by a black hole is heated to significant temperatures before being swallowed and sends thus a large amount of X-rays Thus, even if a black hole does not emit itself radiation, it may still be detectable by its action on the environment. The existence of black holes is a certainty for almost all of the relevant scientific community (astrophysics and theoretical physicists).
A black hole has a given mass concentrated at a point called gravitational singularity. This mass to define a sphere called horizon of the black hole, centered on singularity and whose radius is a limit below which the black hole prevents radiation from escaping. This sphere is something of the spatial extension of the black hole. For a black hole of mass equal to the mass of the Sun, its radius is about 3 kilometers away. In an interstellar distance (million kilometers), a black hole does not exert more attraction than any other body of the same mass; so it is not a "vacuum cleaner" irresistible. For example, if the Sun was replaced by a black hole of the same mass, the orbits of its planets would remain unchanged.
It is impossible to directly observe a black hole. However, it is possible to infer its presence by its gravitational action on the environment, especially within microquasars and active galactic nuclei, where the nearby material falling into the black hole will be greatly heated and emit strong radiation X . observations and can detect the presence of massive objects and very small. The only objects that these observations imply and are consistent within the framework of general relativity are black holes.
properties
A black hole is an astrophysical object as another. It is characterized by the fact that it is very difficult to observe directly, and the central region can not be described satisfactorily by physical theories in their state of the early twenty-first century as it boasts a gravitational singularity. The latter can only be described in the context of a theory of quantum gravity, missing today. By cons, we know perfectly describe the physical conditions in the immediate vicinity, as well as its influence on the environment, which can detect through various indirect methods.
Moreover, black holes are amazing in that they are described by a very small number of parameters. Indeed, their description in the universe we live in, is up to three parameters: mass, electric charge and angular momentum. All other parameters of the black hole (eg size or shape) are fixed by those. By comparison, the description of a planet involves hundreds of parameters (chemical composition, differentiation of its elements, convection, atmosphere, etc.). The reason that a black hole is described by these three parameters has been known since 1967: it is the no-hair theorem demonstrated by Werner Israel. He explains that the only long-range interactions are fundamental gravitation and electromagnetism, the only measurable properties of black holes are given by the parameters describing these interactions, namely mass, angular momentum and electric charge.
For a black hole mass and electric charge are the usual properties that describes classical physics (i.e. non-relativistic) the black hole has a gravitational field proportional to its mass and a proportional electric field dependent. The influence of angular momentum is against specific to general relativity. This one stipulates that a rotating body will tend to "train" space-time in its vicinity. This phenomenon not yet observed at present in the solar system because of its extreme weakness for non compact stars, is known as the Lense-Thirring effect (also called frame dragging, in English). It takes a considerable amplitude near a rotating black hole, to the point that an observer located in the immediate vicinity would inevitably be driven in the direction of rotation of the black hole. The area where this occurs is called ergorégion.
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