Re: How do we know what's in a black hole?

Excellent question - and you are exactly correct, we don't actually know what the space "inside" the black hole is like. Here, "inside" the black hole refers to a location within the event horizon of the hole. This is the radius at which the escape speed becomes equal to the speed of light. Because gravity gets stronger the closer you get to the object, inside the event horizon, you would need to move faster than the speed of light to escape the pull of the black hole. Because nothing can move faster than the speed of light, including any kind of electromagnetic radiation, no object or information can leave the event horizon. Thus, we can never observe what happens at the center of the black hole.

The reason that one would expect the center of the black hole to have a singularity (meaning a point of infinite density, sometimes interpreted as a "tear" in spacetime) is that the equations of general relativity predict there to be one. Essentially, gravity is so strong that all of the mass in the hole gets pulled toward the exact center. The singularity is, at least in some ways, a mathematical artifact of the equations of general relativity. The equations predict it to be there, but there is no way to observe it, so who really knows?

To many physicists, the existence of a singularity is distasteful. They have been working on theories that unify gravity and quantum mechanics, and it is hoped that the final, successful theory will remove singularities inside black holes by offering a "smoother" geometry or distribution of matter. Recently, significant progress in this direction has been made by string theorists. They have described (mathematically) how black holes can so severely warp space without creating any singularities. The initial tear is "sewn up" because the strings of matter inside the hole shield the central point from the pile-up to infinite density that we might expect. In fact, understanding the intricacies of black holes has been one of the main thrusts in recent work in string theory - and it has not only helped us to understand black holes, but also to understand string theory itself!

For more about the classical view of black holes (with some mathematics), see d'Inverno's Introducing Einstein's Relativity. For an excellent nontechnical discussion of the recent work in string theory trying to show that there isn't a true singularity at all, see Brian Greene's The Elegant Universe.

[Moderator's Note: there is also an introduction to string theory on the web: http://www.superstringtheory.com - Jim O'Donnell]