What does a black hole look like?

07 March 2017

Question

What does a black hole look like?

Answer

We put Tim's question to Michael...

Michael - In a way, the clue is in the name. The actual black hole itself will look black - there won’t be any light coming to us from that. But that’s just part of what’s going to be happening around the black hole because, obviously, the gravity around the black hole is quite strong. You’ll have all this other material which is getting sucked up and pulled towards the black hole and, in some cases, orbiting it.

That’s how we recognise that there are black holes already. We haven’t seen one directly so much as seeing the effects of one on other planets of other solar systems, so all of this material swirling around it. It crashes into each other, it gets hot. Hot things will start to give out light and so you’ll see this kind of swirl of light around the black hole.

Of course, this is even more complicated than that because black holes warp space and time around themselves and so around, near the black hole, some of the things that you’ll see when you're looking nearly at it will be stuff that’s behind the black hole. The light that has come from all of the material crashing into each other behind it will bend around the black hole. To the light it seems like it’s going in a straight line - we know light goes in straight line but the actual space is warped around the black hole and so we’ll see some of the light from behind it, and this is called gravitational lensing. In some cases we’ve imaged other galaxies, I believe, from behind black holes or just large concentrations of mass.

Chris - Why is it helpful to be able to see an image and shape of these black holes in this way?

Michael - We’ve got lots of predictions about what is going on in the vicinity of a black hole. Einstein’s general theory of relativity is what we need to use in order to describe a black hole. Basically it's a case to say “have we got this right, can we actually check this against the universe”? Because if we see something that we don’t expect, then our theories are wrong.

Chris - Blimey! That would be a worry, wouldn’t it? Mind you, some physicists say it’s kind of good to be wrong because it shows that you don’t understand everything and it gives you something to aim at. You then know know what the next question is you have to ask.

Michael - There’s a brilliant line I saw Dara O’Brien the comedian give in one of his shows which was “science knows that it’s wrong sometimes because, if it wasn’t, it would stop and that would be it.” It’s always exciting even if you think that the results that you didn’t expect might just be experimental error like the one about neutrinos possibly traveling faster than the speed of light that we had a few years ago. Even when you think that it is a mistake, there’s still a chance that it might be some new physics that we haven’t worked out yet, and that’s really exciting.

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