Will black holes ever become stars again?

We put Alan's question to cosmologist Andrew Pontzen... Andrew - Hawking radiation is really one of these breakthroughs. It happened in the mid 1970s named after Stephen Hawking, of course, and it was a breakthrough in our understanding of the Universe. Because according to Einstein's famous theory of general relativity, which is our best understanding of the way that gravity works, it's possible to cram so much stuff into such a tiny space that nothing can get out at all from that region of space. It's just trapped in there by the sheer strength of gravity and that's what we call a black hole.

What Hawking radiation is all about is showing that actually, when you combine that theory of general relativity with the other major theory of the twentieth century which is quantum mechanics, which is all about the way that very tiny bits of matter behave in the universe, you get a really remarkable result. Which is that, actually, very, very slowly things can trickle back out of a black hole, and we call that trickle of stuff coming back out of a black hole Hawking radiation.

So with this question saying, can a black hole start shining? In a sense, what Hawking radiation is saying is it already is shining, it's just shining so very, very faintly that we can't really see it. But one of the interesting predictions of Hawking radiation is the rate at which stuff drifts back out of black hole. In other words the rate at which it's shining, increases as a black hole gets less massive. So you can imagine what happens is, if you leave black hole for long enough, very slowly there's this trickle of stuff coming out so slowly it's losing it's mass is just seeping away into the Universe. So it get's smaller and smaller and, as the black hole gets smaller and smaller of course, the brightness, the rate at which all of this stuff is coming out is going up. And so, actually, it gets brighter, and brighter as time goes on.

Now for this to really happen, you need to leave the black hole well alone. If you just chuck a bit of extra stuff in the black hole, of course you'll easily cancel this out. So you need to take a black hole, isolate it for many, many times the age of the Universe, in fact, something like ten to the fifty-three times the age of the Universe.

Chris - It's not going to happen any time soon then?

Andrew - It's not happening any time soon. Then eventually, you will see it. You will see it literally light up and at the last 0.1 seconds of it's life it will actually pretty much explode. Because this process of stuff leaking out and generating energy becomes so efficient it explodes with something like a million H bombs. We're measuring everything in H bombs today. It will be something like a million H bombs..

Chris - Only a million. So the Neutron star thimble can knock this into next week then?

Andrew - That's absolutely right. In astronomical terms this is a tiny, tiny amount of energy but still, it's a million H bombs in the last tenth of a second in the life of a black hole.

Chris - Thank you Andrew. Ed Wilson on twitter @nakedscientists has tweeted in. He's actually thinking about the practicalities of, going back to the Neutron star, of getting the thimble full of said Neutron star material to Earth. And says, well that's going to take a huge amount of energy in and of itself, so that's another tricky aspect of that.

Andrew - Yeah. And you would also need to make a very, very, well reinforced thimble. You go and scoop this thing up and then somehow you actually have to keep it together until you get it to Earth in order to destroy the Earth. It would be the worst Bond movie ever.

Chris - Kerstin.

Kerstin - I was wondering, can you actually measure this? Do we have any chance of observing the black holes radiating?

Andrew - It would be extremely hard to observe an actual black hole radiating for the reason that real black hole in the Universe, and they do exist, but the real ones tend to be surrounded by gas, and dust, and stars, and things that will be putting out radiation much, much, much faster than this tiny trickle. But, people are trying to create in a laboratory sort of analogues to black holes where they make very, very small systems which behave in a very similar way to black holes and, in that context, people have actually seen this behaviour.