Even Isaac Newton asked himself whether his gravitaional law could be applied to light, i.e. if the light rays could be bent in the gravitational field of a massive body. In 1783 J.Michell suggested to Cavendish that it might be possible to obtain the mass of a star by measuring the change of the speed of light leaving the star. Cavendish got the solution and was thus the first one to calculate the deflection of light. This encouraged Laplace to imagine such a massive body which even light could not escape. It was a pure coincidence that he got the correct result for the size of such body of mass M: RSch=2GM/c2, which is now known as the Schwarzschild radius. Some typical values for RSch are: for Sun 2.96 km, for Earth 8 mm, and for a man 10-25 m. The surface 4 pi R2Sch is known as the event horizon and encloses the area from which no information can be gained by an outside observer. All the mass of the black hole is concentrated at the center at r=0.
The theoretical grounds for black holes were set by Einstein at the beginning of the 20th century with his general theory of relativity. Soon after the theory was published, Schwarzschild found the solutions of Einstein equations, which turned out to describe the nonrotating black holes. Later, in 1963, Kerr found solutions describing the rotating black holes.
An important step towards understanding the formation of black holes was made by Oppenhimer. He discovered that stars, which are more massive than 2-3 MSun at the final stages of their evolution, collapse into black holes. One of the most important discoveries from 60's is that "black holes have no hair". This simply means that it is impossible for an outside observer to determine any details of the black hole except its mass, angular momentum and charge.
Phenomena in strong gravitational field
Accretion disk at the center of the galaxy
Since black holes do not emmit any light, they can be detected only by measuring the effects they have on their surrounding due to the strong gravitational fioeld. The first experimental evidence for existence of black holes was obtained in 70's. The best way to detect black holes turned out to be the detection of x-rays, which are emitted when material accretes onto a black hole. The first image of an accretion disk has been made by Hubble space telescope.
Gravitational lensing: Due to strong lensing
effect 5 images of a lensed object are seen
instead of only one.
A typical phenomenon in gravitational field is the gravitational lensing. This effect is noticable already in weak fields. As a matter of fact, the first time it was measured, it was masured in the gravitational field of the Sun (during a solar eclipse). This was also one of the first evidence for the general theory of relativity. Beside the gravitational lensing there is also the gravitational redshift - due to strong gravitational field the wavelength of the light, emitted near a black hole, is changed. As a consequence, all objects in the vicinity of black holes appear redder.
What happens to a body, which approaches a black hole? The result depends on the size of both of them and on the minimum distance between the body and the black hole. If the body approaches to a distaace of tidal radius, the tidal interaction completely disupts the body. Of course, this happens only if the tidal radius lies outside Schwarzschild radius. From expressions for tidal radius rR=(M/m)1/2r* and Schwarzschild radius it follows, that the Sun can be tidally disrupted only by a black hole which is less massive than 108MSun. If the black hole is more massive than that, the tidal radius lies beyond the event horizon. As a consequence, the star is not deformed and falls into the black hole intact.
The final proof for theory of general relativity would be the detection of gravitational waves, which are emitted by any moving body. Currently there are many experiments going on, trying to detect gravitaional waves:
So far, no gravitational waves have been detected by any detector.