## Question

“Are white holes real, and what would happen if one collided with a black hole?”

## Answer

Chris asked Carolin Crawford to separate fact from fiction with this question from Stella.

Chris - So firstly, what is a white hole and would there be such a thing as collision between a white hole and a black hole?

Carolin - Let’s start off with a black hole. Lets remind everyone that’s a region of extreme gravity that comes out of Einstein’s equations of general relativity and one of the really nice mathematical things about these equations is that they’re perfectly symmetric. The same equations that predict a black hole also predict, it’s like an opposite to black hole, which is the white hole so instead of attracting things in it spits things out.

You’ve got the same mathematical equation so you end up with the same geometry of a singularity. You have what’s called the event horizon and a black hole at the point where once you cross over that you’re sucked into the black hole. With a white hole you can’t return, you get spat out once you cross that event horizon. So, so far so good.

However the catch is that symmetric in time. So when time runs forward you get a black hole, you need to run time backwards to get the white holes. So even though the mathematical equations predict it, we can’t figure out any way to get a white hole. We see astrophysical black holes on all scales all through the universe. We see nowhere anything that resembles a white hole. So it is just something that is predicted from the equations - there’s no evidence, as yet, for one existing.

Chris - Michio Kaku did a talk here in Cambridge a little while ago to coincide with a book launch he was doing and his talk was all about parallel universes. He mentioned something about the idea that you might have a black hole in one universe and the end of that black hole is a white hole, which is a big bang in the new universe. Could that be possible?

Carolin - I think once you get to that kind of speculation, anything’s possible. There are ideas that you could have an extreme rotating black hole that smears out singularity. You could perhaps fall into the black hole, miss the singularity, go through a wormhole, be spat out. Ideally a white hole in our universe, the possibility also, could be it creates a new universe, as you were saying. These are ideas and we still don't quite know how you’d be able to form one, and there’s certainly no evidence. To be honest, if you get spat out in another universe, I'm not sure we’re going to know about it in this one.

Chris - Andrew Ponson, I think it was on this show said “that with maths you can prove anything you like with theoretical physics.” So everything's up for grabs.

Carolin - It doesn’t mean something necessarily exists.

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