Re: What happens to string theory when you enter a black hole?

Well, to be precise about it, we don't know whether or not general
relativity and/or quantum mechanics break down in black holes, as there is
no way of observing what goes on within a black hole. It is true that we
have no successful theory of quantum gravity - a theory which would unite
general relativity and quantum mechanics - indeed, it is possible that no
such theory is possible, and that both quantum mechanics and general
relativity need modification when dealing with very high energies, small
distances, short times, and high gravitational field gradients.

If we extrapolate our current theories, they do indeed break down as we get
closer and closer to the central singularity in a black hole (the place
where the gravitational force becomes infinite). But we don't know if there
really is a singularity in there; some recent work suggests that, due to
the relativistic time dilation, black holes never really quite form - it
takes an infinite amount of time for anything to fall in.

In order to be able to make scientifically sound statements about what
might happen in certain circumstances (such as inside the event horizon of
a black hole), it is necessary to be able to make testable predictions.
However, since no signal can escape a black hole, we can never observe what
happens in there, and can never test our theories.

String theory is doubly problematic in this regard - it doesn't make any
testable predictions at all, whether or not you are inside a black hole.
This is why some physicists are becoming reluctant to view string theory as
a true theory - so far, at least, it is not testable, putting it more in
the realm of philosophy than of science. So, if actual predictions ever
come out of string theory, will they still be sensible inside a black hole?
It's impossible to say right now.

If there were a "universal equation of everything", it would have to cover
what happens inside a black hole by definition, now wouldn't it? Physicists
have dreamed of a "theory of everything" for many years (although it would
presumably involve many equations), but it remains an elusive goal.

Recent thinking on black holes:
 http://www.scienceagogo.com/news/black_hole_redux.shtml

Two on string theory:
 http://cosmicvariance.com/2005/07/21/two-cheers-for-string-theory/
 http://www.math.columbia.edu/~woit/wordpress/