From: "Jennifer C"

Q Ever since I can remember I have been absolutely fascinated with space and, for some strange reason, especially the black hole. Don't get me wrong, everything about space fascinates me, but for some reason, the information in encyclopedias and on the internet just does not satisfy my mind. One guestion I have always wanted to ask and hopefully get an answer to is What is on the other side of a black hole? I read that basically a black hole sucks things into it once they pass its horizon and within minutes they are destroyed, but don't you think that mabey on the other side of a black could there possible be a "white hole"? My mind goes crazy thinking about things sometimes, but like I said, my mind just is not satisfied with information give by books and internet. I know that no one really knows what is on the other side of a black hole because no one could ever survive the trip through, but I still needed to ask and hopefully get someones feedback on my thoughts. Please e-mail me with your thoughts.

A Good question, Jennifer - many of us in physics and astronomy got interested
in our subjects by thinking about the same kinds of questions!

The short answer is that white holes and wormholes (tunnels in spacetime
that connect black holes to white holes) are possible in theory, but most
physicists who work on this subject think that they don't really exist in
nature. Let me explain what I mean.

Almost a century ago, Einstein wrote down a set of equations that (as far as
we know) describe how gravity works. When you play around with them, you
discover that the equations allow you to construct objects which are so
dense that light cannot escape from them - black holes. Black holes are
thus theoretically possible - they work in the equations. We also think
black holes exist in the real world - they can form when huge stars die and
collapse under their own gravity, and astronomers have found a lot of funny
things in the sky that are almost certain to be black holes.

A white hole is a little different. Imagine you watch some stuff fall into
a black hole and disappear. Now suppose you play the movie backwards -
you'll see a bunch of stuff come flying out of the "black hole". Since it
turns out that Einstein's equations don't care about which direction you
play the movie (whether time goes forward or backward), this must work in
the equations too. This is called a "white hole", the time-reversed version
of a black hole. In fact, the equations indicate that under some
circumstances (spinning black holes, or black holes with electric charge,
for example) these two objects can be linked by a "wormhole".

The big difference is that there seems to be no way to for white holes and
wormholes to be formed in the real world. They're very delicate solutions
to the equations - even a tiny disturbance (such as you sitting at your
computer on the other side of the universe) is enough to prevent a wormhole
from forming. No wormhole or white hole is made when a real star collapses
to form a black hole, since the infalling gas from the star itself is too
big a disturbance. Realistic black holes have no "other side" - the stuff
that falls in never makes it out and is eventually destroyed when it reaches
the singularity at the hole's center. A black hole isn't a hole that leads
anywhere, it's more like a room with a turnstile at the entrance - once you
go in, you can never leave.

So we're not likely to have Star Trek-style wormhole rapid transit anytime
soon, but then again we still have a lot to learn about gravity.

Hope that helps. Keep thinking about the universe, Jennifer - it's an
interesting place!

Jeff Filippini
Berkeley Cosmology Group