Black Hole

Black hole is a .hypothetical astronomical system. When a star near the end of its history contracts under gravitation and becomes smaller than a certain critical radius(proportional to the star’s mass),then , according to Einstein’s theory of relativity,no radiation can escape from it ;as far as the rest of the universe is concerned it becomes unobservable- -its escape velocity exceeds the speed of light. Such a black hole will still exert gravitational force, so that its existence could be inferred from the motion of neighboring bodies; several black holes have been tentatively identified in this way . What happens to the matter falling into a black hole? Does it collapse to a point of infinite density, or is this prevented by quantum mechanics, or does matter re-explode， appearing as a "white hole" in another universe? These are unsolved problems.