"After the nuclear fuel is used up, the star goes into a state of gravitational collapse. All parts of the star fall more or less freely inward... [Y]ou would imagine that the freefall could not continue... because the falling material would... arrive at the center... But Einstein's equations have the peculiar consequence... permanent freefall without ever reaching the bottom... what we call a black hole. ...[T]he space ...is so strongly curved that space and time become interchanged... time becomes space and... space becomes time. More precisely, if you observe... from the outside, you see... motion slow down and stop because the direction of time inside... is perpendicular to the direction of time as seen from the outside. The collapsing star can continue to fall freely forever..."