Taking a trip into a black hole

What are black holes, how do you find one, and what would happen if you fell in?
18 April 2023
Presented by Will Tingle
Production by Will Tingle.


An artist's impression of a black hole


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 you to find one and fall into it…what would happen?

In this episode

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

00:50 - What are black holes?

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

What are black holes?
Chris Reynolds, University of Cambridge

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.

Artists impression of a black hole in space

07:22 - How to find a black hole

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

How to find a black hole
James Nightingale, University of Durham

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.

An artist's impression of a black hole

15:27 - What happens if you fall into a black hole?

Would it be as simple as being squashed, or is there more to it?

What happens if you fall into a black hole?
Ed Bloomer, Royal Observatory Greenwich

It's time to jump into a black hole. I’ve now been sufficiently prepared for my voyage into the unknown and I am now coming to you from just outside of a supermassive black hole. I went with 'Sagittarius A', the supermassive black hole at the centre of the Milky Way. It has a mass of 4 million suns, and I’ll explain why I picked this one later, but for now let’s do this.

Will - The effects start slow. A black hole this big has probably been gravitationally influencing me even slightly for millions of miles at this point. Although up until now, I will have been able to escape it. The chances of escape, however diminishing the closer I get to the orbit of the black hole. How close before it's a noticeable pull, of course, depends on the size of the black hole. And I recall the words of wisdom from the royal observatory at Greenwich's Ed Bloomer...

Ed - I tried to do some back of the envelope calculation and I was working with a black hole that had twice the mass of the sun because that's about the lower limit of black holes you can traditionally make with supernovae. Once you're about five and a half million kilometers away, then the pull on your body just from that black hole would be about the same that you'd experience right now here on earth. So you feel like you weighed one standard earth gravity just because of that black hole.

Will - This black hole, however, is many, many times the size of that one though. So the feeling of earth-like gravity started a lot further away. The pool is increasing and it feels like I'm beginning to accelerate.

Ed - It's the nature of the gravity itself. The closer you are to the gravitating body, the stronger the pool and that accelerates somebody.

Will - More time passes, although not nearly as much time as we'll be passing on earth with such a massive time dilation. One minute here is about 700 years on our planet. So sorry if this goes out a bit late. Physics is starting to get really funky now because light is orbiting the black hole in a circle. If I look around, I might be able to see the back of me. The brightest object in the galaxy could now be my bald spot, but I can take solace in the fact that anyone observing my fault is seeing some pretty weird stuff too.

Ed - I don't want to go near this black hole. So let's say I watch you fall into it. From my point of view, I think you are slowing down and the time itself is passing at a different rate. As, as in fact it is. But also gravitational redshift is happening. The immense gravity of the black hole means that these electromagnetic radiation waves, whether that's radio or whether you're shining a torch at me or whatever, they're being stretched to larger and larger wave strengths. So actually, in fact you're not only just changing colour, eventually your radio signals are gonna be stretched out to the point where I can't actually receive, uh, anything from you or indeed see you anymore. Eventually you would lose vision as well.

Will - Good job. I'm recording this on a handheld device. Then eventually I hit the point of no return. If I were made of light, this would be the event horizon or the Schwarzschild radius. But given that I'm not, and my bathroom scales assure me that, it's a bit further out,

Ed - There's a point where you cannot maintain a stable orbit. And it depends on the situation. But let's say in the best case scenario about four and a half times what we call the Schwarzschild radius, which people sort of tend to think as the really the last stopping point, but actually about four and a half times further out than that. That's the point at which you are not going to be able to prevent yourself from falling into that black hole.

Will - Passing over this point. And then the event horizon, the forces of gravity coming from the singularity at the centre are conspiring to pull me apart. The difference in gravitational pull and the distance between my head and feet is so large that I would be pulled apart. Think of the centre of a black hole as a tube and the smaller the black hole, the smaller the tube I would be squeezed through. It's called spaghettification. It sounds like a grim way to go. And in smaller black holes that would be the case. But this is why I picked a supermassive black hole.

Ed - When you have much more mass in the black hole, the size of the black hole scale up with that mass. And what that means is when you have truly, truly massive black holes, the gradient is a little bit more gentle for you. And in theory, depending on the mass of the black hole, if you've got truly massive black holes, you could have a gradient that's gentle enough that spaghettification won't be the end of you, which means that you could actually get pretty close to the event horizon. But it would only be, I think about 25 times Earth's gravity.

Will - 25 times the gravity of Earth is barely anything. So assuming I've survived being squeezed, I am curious as to how I might meet my end.

Ed - It depends what else the black hole is doing because in fact as other objects fall into it, they accelerate and they produce x-rays and even gamma rays. So in fact, if other dust clouds and things like that, little particles are falling into the black hole, possibly long before you have a problem with, you know, the actual mechanics on your body, you're actually being blasted by gamma rays from the black hole. So incoming radiation might actually be the thing that kills you, I'm afraid.

Will - So I may well go out as a well done steak rather than a piece of spaghetti. Assuming neither of those happen, I enter the circle of the black hole as it consumes me. The rest of the universe now only looks like a circle of distant light that is rapidly fading. I mentioned physics was funky earlier, but now it kicks into overdrive. Due to the nature of the singularity's gravity, every direction is now inward. Any attempt to move away from the black hole would actually bring me closer towards the centre as we enter the singularity. What happens next is contentious. And depends if you think a black hole completely destroys the matter that it takes in or just compresses it down to near infinity. But either way, I'm stripped of my being and the atoms that made up my existence are crushed and joined the infinitely dense point. I become part of the black hole.

An artist's impression of a supermassive black hole

21:15 - Black holes and white holes

If black holes exist, could they have white counterparts?

Black holes and white holes
Carlo Rovelli, Centre de Physique Theorique de Luminy

White holes are, for now, the theory that if a body can exist from which nothing can escape - that’s black holes - then there could be something into which nothing can enter. And if you subscribe to the theory that black holes do not destroy information, there might be a way of it coming back out. Perhaps our best chance of understanding this might lie with theoretical physicist and author of the upcoming book White Holes, Carlo Rovelli.

Carlo - What is a white hole? It's a black hole as if it would look like if you could film it and show the film backwards. So it's sort of a black hole coming back. So the idea here is that a white hole could be like a ball falling to the ground and bouncing up. So everything that falls in bounces and comes out through the white hole.

Will - How would a white hole form then? Is it just a black hole that's almost collapsed in on itself and then for lack of a better word, exploded back outwards again?

Carlo - Yes, exactly. So one way of visualizing a black hole, what's going on inside is that there is a horizon, a surface. The thing of the black hole we see on the outside, but inside there's a long tube that becomes longer and longer and longer and narrower until the point in which it's so narrow that quantum effects make it bounce back. And so this long tube starts becoming thicker again and shorter comes back. So the formation of this tube bounces into a coming back of the tube itself, and that's a white hole from the outside. First we see the horizon where everything comes falling in, and then we see the white hole, which is the same horizon, the same position in the sky, the same star if you want. But now instead of an horizon where everything can only go in, it's an horizon for which everything can only go out. So whatever is inside now can bounce out, and that's a white hole.

Will - And what would cause the black hole to sort of flip in a quantum sense to start throwing stuff back out?

Carlo - One of the things that quantum mechanics tell us about the world is that many things that we think are continuous are actually granular. So light is made by photons. It's not a continuous wave, it's just a few grains, which are the photons. And if applied quantum mechanics to space itself, it tells us that space is granular. So a black hole cannot squeeze things down to infinity. There's no infinity small. At some point you get to the minimum size, and once you get to the minimum size, the fall is blocked, so to say. And you cannot fall it more and it has to stop. And when things stop falling, typically they bounce. If you want black holes are a solution of Einstein's equations, which we understand very well except how they end. And white holes are a solution of Einstein equations that we understand very well except how it starts. So it's extremely plausible to imagine that when a black hole ends, that's exactly the quantum position to a white hole being born.

Will - What kind of matter would it spit out? Would it be recognizable? Would it just be pure energy? What kind of things would we be looking at?

Carlo - We would expect that what spits out is just ordinary matter radiation, more likely like electromagnetic radiation. Very much unrecognizable with respect to what fell in because things are gonna be squashed horrendously by falling into the black hole. Calculations suggest that what could come out is very low energy radiation. So like light of very, very low energy, very low frequency, which is one of the signals. There are many of these things. It could be another of the signals that one might search to identify the existence of this object.

Will - We have to stress at this point that we haven't actually observed a white hole yet. This is all theoretical for now. But is that simply because we don't really know what to look for yet?

Carlo - No, it's more than that. We don't know if this scenario is correct or not. We don't know if white holes exist in the universe or not. We have to remember that for almost a century, black holes were predicted by general relativity and many people didn't believe they existed. It was an open question. When I was a student, I studied black holes and my teacher in my textbook said, well, it's implausible that these things exist in the universe. So we are in the same situation now with white holes. They're predicted by generativity, but we don't know whether they exist in the universe or not. To me, it seems very plausible that they do, and it seems very plausible that the end of a black hole is the birth of the white hole. But until we are actually seeing them or seeing an effect of them or a consequence of them, we have to consider white hole hypothetical as an idea.

Will - There's lots of theories going on about white holes and black holes and their relationship. And one of them is that white holes don't exist in this universe. They exist in another universe, and there's some form of door that exists between black holes and white holes. Do you subscribe to that theory or do you think there's something else going on?

Carlo - I think there's something else going on. I think that when the black holes die, three things can happen. One is that just everything disappears magically, but that seems implausible to me. The second possibility is that it ends up in another universe. Somehow. It creates another universe. This is a beautiful, enchanting idea, but it doesn't seem plausible to me. I mean, creating another universe might not be so simple as collapsing a star. And the third possibility we just talked about namely, that whatever falls in can come out because a black hole becomes a white hole. So the entire process of formation of a black hole evaporation and then formation of white hole and coming out is just like a ball that falls to the ground and then bounces up. One of the fascinating aspects of this story for me is that in general activity, time goes at a very different speed. As we know, time goes faster in the mountains than near the sea on earth, but that's a tiny difference. Now, if you work out the speed of time inside the black hole and the white hole is enormously lower than outside. So the process seen from the outside of a star collapsing and then becoming white hole and coming out may last billions of years. But if you fall inside, it takes just a few seconds or minutes for you to get to the center and come out as a white hole. So black hole to white hole might be just like a shortcut to the very distant future. If you survived, you wouldn't be washed by the enormous forces inside the black hole. You could just jump to the black hole and come out from a white hole. But very, very far in the future, millions or even billions of years in the future.


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