Black holes exist only because of Einstein's General Theory of Relativity. According to this theory, time slows down due to gravity. A clock on a satellite at an altitude of 10,000 km should run 4.5 parts in 1010 faster than one on the Earth. This is important for the GPS, Global Positioning System. Without taking into account this small slowdown due to relativity, GPS accuracy would be miles instead of feet.

 

A black hole is such a huge mass that time slows down so much that it actually stops at the surface. This means that it takes forever for an object to reach the surface of the black hole.

 

Some textbooks say that a black hole is an object from which light cannot escape. This is not correct. Since it takes forever to reach the surface, we cannot talk about the "inside" of the black hole. This confusing concept is because General Relativity tells us that gravitation changes geometry. Space and time are different near a black hole.