Black holes and Hawking Radiation

Despite his diagnosis of motor neurone disease (MND) and the grim prognosis it carried, which might have seen him survive for only a few more years, Stephen Hawking continued his research with fervour. A lot of this was exploring the intricacies of black holes and how they worked. Cosmologist Andrew Pontzen, from University College London, explains to Chris Smith what Hawking was working on...

Andrew -  As we heard from Martin Rees earlier on, he was particularly interested in black holes for a variety of reasons. But I think, once you start getting into them, they remain tantalising because they are the most extreme objects that we can really think of in physics. You know, they are objects where gravity has gone slightly crazy. To actually understand what makes them tick is, I think, something that many cosmologists would like to do. And gravity is a very mysterious force actually, it really doesn’t behave in the way that the other forces that we know about behave, so understanding a black hole is about drilling down, understanding gravity. It sort of becomes a fantastic playground almost for theoretical ideas.

Chris - I like that analogy. One of the things that Stephen Hawking did do was to highlight some of the potential energy balance issues concerned with black holes. Tell us a bit about that.

Andrew - A black hole has something like a surface area - you can think of it as a sphere sitting in space. In principle, you could go and measure the surface area of that sphere and he showed that, if you take two black holes and throw them together, then in the end, you’ll end up with a black hole whose total area that you end up with has got to be greater than the area of the two black holes that you started off with summed. It’s a sort of thing where you add two things together and you know that that total area can never go downwards.

Chris - It sounds like the national debt!

Andrew - It is a bit like the national debt but, in fact, it’s also a bit like something else we know about in physics. Another physicists, Jacob Bekenstein, pointed out that this is very much like what we call “entropy.” Entropy is a sort of measure of disorder in the universe and there’s a fundamental law in what we call “thermodynamics” which states that entropy also must always increase. That is if you’re summing all the entropy in the universe, and the overall entropy of the universe increases. In other words, the universe is getting more, and more, and more messy as time goes on. Jacob Bekenstein pointed out actually that this is a very close connection and went as far as suggesting that perhaps that means that black holes themselves, what we’re seeing in terms of this ever increasing area is, in fact, another manifestation of that idea that entropy in the universe also has to increase.

Chris - But equally, the other, I think, striking thing that now bears Stephen Hawking’s name is the whole idea that black holes don’t just draw stuff in, they do give things off. There’s this Hawking Radiation, isn’t there?

Andrew - Yes. This was Stephen Hawking’s next big contribution really. He took this idea of Jacob Beckenstein’s black holes have entropy, and he says well, if that’s true then they must be taking part in other processes in physics. Entropy in physics, we normally associate with disorder; we normally associate it with having lots of little particles of matter or chunks of energy that are getting themselves into a complete mess and getting into disorder. So, he suggested that if black holes possess entropy, they must, they must also be able to generate disordered particles of radiation. It was a very strange conclusion to come to.

Chris - In essence then, we’ve got this material which is leaking out of a black hole, for want of a better phrase. This is slightly paradoxical because everyone thought black holes don’t leak anything. They’re black for a reason, they’re soaking up everything including light. So what is the stuff that effectively is coming out and at what sort of rate, what sort of volume?

Andrew - You’re absolutely right that, of course, the famous thing about a black hole is supposed to be that it’s black. That nothing can ever come out of a black hole including light. This is an example of what we call a quantum correction that although in the sort of classical picture of physics a black hole would be completely black. When you put in quantum mechanics you get a very, very small correction to that picture. The kind of rate we’re talking about stuff leaking out of the black hole is incredibly slow.

The way I like to think about this is imagine the Sun, and I want you to imagine that the Sun gets dimmer, and dimmer, and dimmer, and dimmer until it’s about as bright as a torch - that’s an enormous factor. If you had a black hole that was as massive as a sun, it wouldn’t be as dim as the torch, you’d have to keep going dimmer, and dimmer, and dimmer from there by the same factor again, and that is the kind of level of stuff that’s leaking back out of the black hole, so it’s incredibly slow. It will take many many times the age of the universe before a black hole actually appreciably shrinks due to leaking this material back out.

Chris - Can we use that material though, and information intrinsic to it to infer things about the black hole that released it so it give us an insight into what that black hole is doing and what it’s eating?

Andrew - The honest answer to that question is we don’t know at the moment. It’s something called the “information paradox” and it’s a subject of active research.

Chris - And maybe we’ll find out what that is later on...