What are black holes?

How do they form, what kinds of black holes are there, and how do they mess with time?
18 April 2023

Interview with 

Chris Reynolds, University of Cambridge


An artist's impression of a black hole accretion disk.


Black holes are one of the most extreme things in the universe. Their gravitational pull is so strong, they can bend light and even time. So were someone to find one and fall into it…what would happen to them? To help find out more about what to should expect,  Chris Reynolds, a professor of astronomy at the University of Cambridge gave the rundown on how these mysterious regions of spacetime work.

Chris - The best way to think about what a black hole is to think about how it might form. Gravity is an incredibly powerful force and the key thing in nature is that gravity can actually, in some circumstances, overwhelm all other forces. So if you have an object that has a lot of mass and the mass is collected into one place, then you can have so much mass collected into one place. The gravitational pull of that mass, the gravitational force that's sort of crushing that mass down, can actually overwhelm all the other physical forces. And the object just starts falling in on itself and it falls in on itself and keeps on falling in on itself. Nothing can resist that, and theoretically it falls all the way into a point. Now in doing that, something really interesting happens in that it actually seals itself off from the rest of the universe. The speed of light is a fundamental speed limit on, on anything. So speed limit of course, how fast light can go, but it's also a speed limit on how fast information can be communicated from one place to another. And as the object starts to fold in on itself. The escape velocity of that object, in other words the velocity that you have to leave with in order to actually escape the object, goes up and up and up. And at some point the object is now so dense that you would need to be thrown off it at the speed of light or faster to actually leave. And there's a point of no return that surrounds it, called the event horizon. And when you think of a black hole and you think of that black sphere, that's what you're looking at. You're looking at that, that event horizon, the point at which nothing can actually get out.

Will - So are all black holes the same size?

Chris - That's a good question. We know of at least two classes of black holes. There's what I would call small ones, which are from the collapse of stars, the core of the star undergoes a collapse and boom, you have a black hole. In doing that, the energy released in forming that black hole can blow the rest of the star up in a supernova. What you left with is a black hole that is maybe about 10 times the mass of the sun. Now the other class of black holes we know about are the supermassive black holes. These are black holes that are anywhere between a million to sometimes up to 10 billion times the mass of the sun. And they sit in the centre of galaxies.

Will - Are all black holes active. Is there such a thing as a dormant black hole?

Chris - It's probably just a matter of degree. Any black hole, there'll be some trickle of gas or dust or whatever surrounding it into the black hole. As that gas is falling in, it will be releasing some of its energy, it'll be emitting light. And so there'll be a little bit of what we would call activity from that black hole as that gas is falling into the black hole. However, there are certainly a subset of black holes. There's a small fraction, you know, maybe one to 10% of super massive black holes for which there's a lot of gas falling into them. There's a lot of matter falling into them, and they really light up. You can see them across, you know, vast distances. So those are what we normally think of as the active black holes, the terminology is active galactic nuclei.

Will - Perhaps the most mysterious aspect of the black hole is the middle. Do we know what's going on in the middle of a black hole? Can it even be called the middle?

Chris - Well, yes, that is the profound mystery of what's happening in the very centre of the black hole. This is known as the spacetime singularity. It's the technical term for it. It's a very hard region to understand firstly because theoretically there are such great uncertainties about it but also we can't see it, so we can't get data on it, which makes it of course, very hard to understand. In fact, some people would even say it's not science because we can't get data on it. The nature of the spacetime singularity is actually very closely wrapped up with some of the profound questions about black holes that Stephen Hawking asked. Stephen Hawking was asking questions about how black holes and information sort of played with each other. There was this profound mystery that according to our standard theory of gravity, if you were to take a black hole and then throw, you know, a book hard disk into the black hole, the information on that book with the hard disk should just be completely lost. You just destroy that information. However, quantum mechanics, the other theory that underlies modern physics tells us that's impossible. You can't destroy information. So how do those two pillars of physics play with each other? That was a big concern of Hawking and, and many other theoretical physicists in the past few decades. And it gets very closely wrapped up with issues of that singularity.

Will - Many of us have seen films like Interstellar, which make great plays on how black holes affect time. Presumably it's all to do with that immense gravitational pull. But in your own words, how does a black hole mess with the concepts that we have of time?

Chris - Yes. So black holes do mess with time in a very interesting way. Fundamentally, as you get closer to a black hole, time starts to slow down. What does that mean? What that means is if you are a long way from a black hole and you are watching a friend going into a black hole or getting close to a black hole, and you can sort of see the clock that they carry with them, you start to see their clock running more slowly and you start to see them aging more slowly. They of course don't think anything's going wrong. They see time passing perfectly normally for them. If they're looking out at you though, they would see your clock running fast and they'll see you aging fast. So that's what the theory tells us. And indeed, one way to think about the event horizon, this is that point of no return, is that all these effects get stronger and stronger and stronger until you get to the event horizon. And then if you're looking from the outside, that's the point where time basically seems to stop. If your friend is right at the event horizon, you see them basically frozen in time.


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