What are black holes made of?

Thanks to Cambridge University's Ben Allanach for the answer!

David - What are black holes made of? Would it be based on subatomic particles?

James - To help us this week, I'm joined by Ben Allanach, professor of Theoretical Physics at the University of Cambridge. How are you, Ben?

Ben - I'm very good. Thank you. James. I'm on holiday in Cornwall.

James - Well thank you for taking a little bit of time out to speak with us. We better start, Ben, with what a black hole actually is.

Ben - What happens is that things collapse in the universe through gravity, interstellar gas, that's just hanging around in the universe and stars. They collapse and gravity pulls it all together. And so it forms a very dense bit of matter. And sometimes that matter is so dense that it will suck light back into itself. And so that's what happens basically. The black is the absence of light because it's just been sucked back in by the gravity. So no light hits your eyeball and so the absence of light looks black. So that's what a black hole is. It's a surface around a very dense object, which is in the middle of it, probably, where the light gets sucked back into the middle.

James - So the short answer is it's made up of the things, the gas, the objects that it sucks towards the middle of it.

Ben - Exactly. But the problem is they all end up being very densely compressed in the middle, and we don't fully know what goes on there. What's likely to happen is instead of atoms and things, you've got subatomic particles. But they're at such high pressures and temperatures. We don't really know how they behave right in the middle. You can describe it with equations and the simple equations have infinities in it, and it's probably a lot more complicated than we know. So we are reaching the edge of scientific knowledge about what's happening right in the centre. But the bit that we see as a black hole that we understand, it's this sphere around it where the light gets sucked back in.

James - And you touched on the second part of David's question, which is the particles that are getting sucked into the singularity. The, for all intents and purposes, dot in the middle are getting changed into subatomic particles when they're in there. And that's up for debate.

Ben - They certainly will when they get close to the centre, because they'll get heated up. The temperature will break the atoms apart. So there will be a stage at least where they're subatomic particles. And actually we know, in the sense that we know how they behave. But when they get heated up more beyond any experiments we've been able to do and things we've been able to test, then we're not really sure how they're going to behave after that. Something quite unusual might happen. We don't know.