We are studying an object, that people may think from recent conception, but that it was conceived more than two centuries ago.
In their initial conception, a black hole was an object with a gravity force in their surface so big that nothing could escape from him; not even light if it were affected by gravity (this was not known 200 years ago). Before measuring the speed of light and before the theory of relativity, by means of which it was demonstrated that nothing could surpass the speed of light, people thought that a body could reach an infinite speed and therefore the black hole was a body in which the scape speed was also infinite. This could only happen in the case of a infinite mass star or infinite density star. This cases were illogical cases and the scientists did not give any importance to the matter, falling it in the oblivion.
But with the theory of the special relativity the maximum speed that a body could reach is the speed of light, and then one could think that a black hole could already have a finite volume and mass, since the speed of escape would be finite.
As we will see, in further posts, the special relativity take us again to a punctual black hole, because the scape speed from a relativistic point of view could never overcome the speed of light.
Anyway it was discovered that the light is not simply a particle, and thus we can’t apply the idea of escape speed. But it’s from the point of view of the Einstein’s general relativity theory when the most interesting consequences for the bodies with extreme mass are deduced, being again feasible the idea of a non-punctual black hole. The so called event horizon appears, region of the space around the hole whose space time bending impedes that nothing escapes; not even the light.
Furthermore it’s no longer thought to be absurd the fact that a body collapses until it only takes only the volume of a point. In order to clarify ideas we’ll begin seeing how can be formed the black holes, continuing after with a relativistic analysis of black holes.