How to find a black hole

If black holes cannot be seen, how do we know that they are there?
18 April 2023

Interview with 

James Nightingale, University of Durham


Artists impression of a black hole in space


Now that we know what I’m dealing with, we need to know where to find a black hole. The first instance of a recorded black hole was in 1971, and we’ve discovered dozens since then. But how do you spot a black hole in a mostly black universe. Well, perhaps by asking someone who has done exactly that. Dr James Nightingale is a physicist at the University of Durham and recently found a supermassive black hole 33 billion times the mass of our sun, and spoke about the different ways you can go black hole spotting.

James - There's many different ways that we discover black holes. I'll specifically focus on the discovery of some of the biggest black holes, which are called these supermassive black holes. And these were first discovered from sort of the 1970s onwards when we started looking at the stars in the centres of galaxies nearby the Milky Way. So if a galaxy's very close to the Milky Way, we can actually resolve all of its individual stars as little point sources, and we can actually measure the velocities of those stars so we can determine how fast those stars are either moving away from us on earth or coming towards us, and we can monitor those velocities over the course of years or even decades. And what we basically found in the 1970s was that towards the centres of these nearby galaxies, the velocities of the stars were showing sudden increases. As a star moved through or towards the centre of a galaxy, it began to really, really speed up. And the only real explanation one could have for this is that something must be causing the stars to speed up some sort of intense gravitational field, exerting Newton's second law, bringing the stars further towards them, which was ultimately decided that therefore it must be giant black holes, some millions of times the mass of the sun that was causing the stars to speed up in this way.

Will - So it's never really about seeing them directly, it's about being able to infer them from movements of things we can see.

James - Yeah, exactly. Obviously you can never see a black hole directly, and I don't think all of the ways that we discover black holes, all of the ways that astronomers assert their existence and measure their masses are reliant on typically, typically reliant on their intense gravitational fields, having knock-on effects or consequences, which are what we observe. So in this case, it's the gravitational field of the black hole changing the orbits of the stars around it. Basically, all of the ways that we infer the presence of black holes are somehow related to the knock-on effect of the black hole's intense gravitational field.

Will - As you say, there's a bit of a resolution problem if we're trying to find something perhaps more distant than inside our Milky Way. What are we doing now to look for more distant black holes?

James - In order to infer the presence of these black holes, though, we have to look for a different phenomena. And so in the centres of galaxies, as we've said, there are these supermassive black holes and they sometimes are feeding on the material in their galaxy. They're in what's called an active state. So these supermassive black holes at the centre of the galaxies, they might be eating the stars in the galaxy surrounding them. They might be eating its gas, they might be eating its dust, they might even be eating other black holes within that galaxy. And the key thing is because they're bringing all of the galaxy's material to their centre because their gravitational field is bringing in all of this stuff, basically this material at the centre of the galaxy begins to undergo intense levels of friction, intense levels of heating, and therefore begins to emit extremely bright amounts of energy across the entire electromagnetic spectrum at the centre of the galaxy. So in a very small subset of galaxies, we observe this extremely bright emission at their centre, which is of course indicative of the black hole being there.

Will - So it's almost paradoxical then that in looking for black holes, we look for the lightest parts that we can see.

James - I think you always have to be careful here because although black holes are obviously famously invisible, famously don't emit light, they are also the brightest things in the universe, but it's specifically the stuff that's outside the event horizon, but still very, very close to the black hole that does that. So it's not actually a paradox, it's simply that the stuff we're seeing is slightly beyond the event horizon of the actual supermassive black hole at the centre.

Will - And you pretty recently spotted your own supermassive black hole. How did you go about spotting yours?

James - The paper myself and my team put out used a technique called gravitational lensing and was actually the first time that the mass of a black hole or a black hole in general has been discovered using this technique. And so gravitational lensing is a phenomenon where, basically, as the light of a distant galaxy travels through the universe, imagine the light ray of a photon just traveling through the universe. The path of that light ray will be slightly distorted and deflected by any of the mass it encounters along the way, the gravitational fields, or other galaxies. That will cause that light ray to come slightly towards those galaxies. Using the Hubble space telescope, we found this wonderful gravitational lens where we observed it had four distinct light rays that had all been distorted and deflected and warped by another galaxy that was perfectly in front of it. But what was particularly special about this gravitational lens is that one of these light rays that has traveled through the universe, one of these gravitational lens collections of photons traveled right next to the centre of another galaxy that was responsible for the gravitational lensing. And it traveled so close to the centre of that galaxy that not only was it deflected by the stars within that galaxy, but the black hole at the centre of that galaxy also caused an additional bending of light, an additional contribution to this gravitational lensing effect. And so we basically observed with Hubble this image of a galaxy that was so distorted, so deflected, so bent by the material in the universe that the only way that we could explain it was the presence of an extremely large 33 billion solar mass. That's 33 billion times the mass of the sun black hole at the centre of this galaxy that the lightweight just happened to pass by.

Will - So purely hypothetically, of course, if there was someone out there hoping to go to a black hole for research purposes, would you recommend them heading towards the centre of a galaxy for the brightest bit? Would that be your best bet?

James - I mean, yeah, I think so. I think that the smaller a black hole, the more rapidly changing its gravitational field. So if you head into a black hole that's only a couple of times the mass of our sun, you will be ripped apart or spaghettification, the common term used, almost instantly. As black holes grow larger, the gravitational field actually changes at a much slower rate or a much more gradual rate, which means you could feasibly fall into a black hole as large as the one I've discovered and not be ripped apart by the intense gravitational forces. Now don't get me wrong, you'll probably have a lot of other very nasty consequences to deal with, but at least the gravity itself would probably just about be bearable for a human. So I would definitely recommend if, if you are set on traveling to the centre of a black hole, definitely pick one of the largest possible.


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