Brian Cox - The Final Mystery of Black Holes

Added: 2026-05-13

[00:00:00]The idea of black holes goes back a long way actually back into the 1780s and 1790s. There were there were two um physicists, mathematicians, natural philosophers, whatever you want to call them working at the time that had the same idea apparently independently of each other to understand why black holes cause so many conceptual problems. It it might be worth just describing very very briefly what a black hole looks like.

[00:00:27]Black holes are interesting uh because going back to the work of Steven Hawking in the 1970s, it turns out that they demand that we think about both quantum theory and general relativity together. And the quest to unify those two great pillars of 20th and 21st century physics into what's often referred to as a quantum theory of gravity is in some sense a holy grail for theoretical physicists.

[00:00:59]But the problem has always been well is there anywhere in nature that we can look to to observe something that requires us to merge those two theories together? And black holes really are the unique place as far as we can tell in nature where we can see a thing just sitting there in the sky that demands that we consider those two theories uh working together to hopefully reveal a deeper theory.

[00:01:30]The idea of black holes goes back a long way actually back into the 1780s and 1790s. There were there were two physicists, mathematicians, natural philosophers, whatever you want to call them working at the time that had the same idea apparently independently of each other. One was a clergyman, an English clergyman called Mitchell and the other was the great French mathematician Llass and they were both thinking in terms of an idea called escape velocity. So the escape velocity is the speed you have to travel to completely escape the gravitational pull of something a planet or a star. For the earth for example the escape velocity from the surface of the earth is around 8 m a second 11 km a second. If you go bigger, you make a bigger more massive thing, let's go up to a star for example, like the sun, then the escape velocity increases because the gravitational pull at the surface increases. And actually for the sun, it's somewhere in the region of 400 m a second. It's really fast. What Mitchell and the Plast thought and I think it's a very beautiful idea is they imagined in their mind's eye, well, can you go bigger? Can you imagine more and more massive stars, giant stars such that the gravitational pull is so large at the surface that the escape velocity exceeds the speed of light and then you wouldn't be able to see them. There's a wonderful quote actually in Llass's paper where he says that the largest objects in the universe may go unseen by reason of their magnitude and this is back in the 1780s or 1790s. So he's imagining stars where the gravitational pull is so vast that even light can't escape and you couldn't see it. dark stars. I think he referred to them as. Now we know that such objects do not exist in the universe in that sense in the sense that Mitchell and Plant. But actually uh they miss something which is not surprising because it sounds almost paradoxical but you can also increase the escape velocity the surface of an object by squashing it. And it turns out that if you take the Earth and you squash it down and squash it down and squash it down until it's about that big, the radius just less than a centimeter, then the gravitational pull at the surface will be so great that light couldn't escape. And that is essentially the modern concept of a black hole. Now Mitchell and the Pl's um calculations or imaginings were based on Newtonian physics. So pre Einstein we come forward to 1915 Einstein published his general theory of relativity which is a different model a better theory of gravity and it turns out that black holes also exist in general relativity.

[00:04:27]Now if we come forward to 1915 Newton's theory of gravity is replaced by a better model a better theory which is Einstein's general theory of relativity. But the idea that there can be objects that can be compressed such that they trap light is also present in Einstein's theory. The first physicist to predict such a thing or at least derive the mathematics that describes such a thing as a black hole although he didn't know that they existed was a man called KL Schwarz. What did was provide a an exact solution to Einstein's equations that describes space and time, the distortion of space and time in the region of a star or or at least an idealized star which is a perfectly spherical non-spinning ball of matter.

[00:05:24]It's a model a simple model of a star.

[00:05:26]Now, Schwarzel described or discovered the solutions to Einstein's equations that describes what happens to spacetime outside such a thing way back actually 1916 just after the theory was published. What Schwarzill's solution also describes though, although he didn't think in these terms at the time, was what that space looks like if you completely remove the star but leave its imprint in the in the fabric of the universe behind. And that is essentially the the theoretical description or the model of a modern relativistic black hole.

[00:06:06]Now while Swatchel's solution and we're by solution I mean we're to picture a distortion in space and time a distortion in the fabric of the universe. While Schwarz solution does indeed describe the simplest possible black hole that we can model in the universe people didn't really think in those terms at all until later. You see in the 1930s Einstein and a colleague of his Rosen, for example, exploring that spacetime and building models of what that spacetime might look like. But I think it's true to say that really certainly until the late 1930s and actually arguably post war until the 1960s most physicists thought that such things would not exist in nature. So they were theoretically interesting, perhaps not practically interesting. And the reason is that you have to create such a thing. So it's one thing to have a model of space and time that describes this object called a black hole from which not even light can escape. But it's another thing for nature to actually make it. So if you go through the 1930s there's a lot of papers actually Robert Oppenheimer and a student of this Snyder wrote a very famous paper just before the war which explored whether a real star in the universe at the end of its life could collapse and collapse without limit to form this geometry this thing that we call a black hole. Just before the war, Oppenheimer and Schneider showed that under certain assumptions, a star could behave in such a way. Um, but it wasn't really until the work of people like Roger Penrose and Steven Hawking and several others in the 1960s that it really began to look as if nature would build these things. There's this great quote I remember with some fondness actually Arthur Edington a colleague of Einstein's who was very English very proper physicist and he said nature will prevent such absurdities from existing just that's it nature will prevent it well it turns out nature doesn't prevent it we now know we've observed them that stars do collapse to form black holes and then theoretical physics moves on so people accept that these things should exist.

[00:08:30]Although it's true to say that we haven't actually imaged one until the 21st century. But in any case, people accepted these things should exist. More and more evidence mounted that they do exist at the centers of galaxies and at the sightes of collapsed stars. But then we have to face the consequences. What does it mean for our understanding of the universe if there are these objects where space and time behave in a very strange way where where light is trapped and where it would seem that anything that falls in is at the very least locked away from the universe forever.

[00:09:07]So to understand the conceptual problems that are faced if these things exist then it might be worth just describing very briefly the Einsteinian description the pure description in general relativity a black hole what do you see from the outside well there's an event horizon surrounding the black hole in some sense it defines the boundary between the external universe and the interior of the black hole. The event horizon is um very simply and a bit handwavingly but it's a reasonable description. It's just if you can imagine a sphere in space and if you go across the boundary into the interior of this sphere then even if you can travel as fast as the speed of light you can't escape. So the event horizon separates the interior of the black hole from the external universe. But we'll see a bit later why that's a bit of a handwavy description.

[00:10:05]But another description of the event horizon which confused people all the way through the history of black hole research actually certainly to the the early papers in the 1930s and perhaps even postwar was the idea that the event horizon when viewed from the outside is a place in space where time stops. Now, that's a direct prediction of Einstein's theory of relativity from the external perspective. If you watched, for example, an astronaut falling in towards a black hole, then from your external perspective, you'd see their time pass more slowly, slower and slower and slower as the astronaut approached the black hole until on the horizon you would see their time stop. That suggested to many people in the early days that a star couldn't collapse to form a black hole. It confused them.

[00:11:00]They thought well if a star is collapsing then does it not freeze forever in some sense on the horizon. So all sorts of initial early conceptual problems which ultimately was solved.

[00:11:11]The thing about relativity just the one sentence thing to understand is that time can stop from one perspective but time can pass at the usual rate from another perspective. And indeed uh from the perspective of an astronaut falling into a black hole then for a sufficiently large black hole like the ones that we find at the centers of galaxies the astronaut would notice nothing at all as they fell across the horizon into the interior of the black hole. So time passes at 1 second per second on the watch of an astronaut falling in. But from the external perspective time freezes on the horizon.

[00:11:50]So black holes are full of these um apparent conceptual challenges which are actually not conceptual challenges at all. They're just a central part of Einstein's general theory of relativity.

[00:12:01]So that confusion was eventually dealt with and solved and people understood certainly by the 1960s what these things are and how general relativity models them. There is a central problem though which is still not solved which is you put it this way. What lies at the center? And I'll be careful with my language. What lies at the center of a black hole. Now in pure just in Einstein's general theory of relativity actually it's not right to talk about the center of a black hole really. So what are we picturing? It's this thing called the singularity. You might think of it an infinitely dense point to which this massive star collapses. It's kind of the natural way to think of it. But actually, just even in pure general relativity, when you look at a nice map of a black hole, the so-called Penrose diagram named after Roger Penrose, what you see is that the singularity is not really a place in space at all. It's a moment in time and actually it's the end of time. So one way of picturing what's happened when a star collapses to form a black hole is that space and time are so distorted that in a sense their roles swap. And so what we thought of as an infinitely dense point, a place in space at the center at the center of the collapse of the star if you like actually becomes a moment in time and the end of time the singularity. But the nature of that thing uh was not and is still not understood. So that's a great mystery and it's been long accepted that we will need a so-called quantum theory of gravity, a deeper theory of gravity in order to explain the singularity. And for many many years actually until quite recently then people thought well there we are. We have a problem with the singularity. We don't really have any access to it. We don't have the conceptual tools to explore it. So it may remain a mystery for a century to come. is it's not clear what to do for a perfectly spherical non- spinning ball of matter. It's the simplest thing you could do which tells you how space and time are distorted by it. And that's a model for a star. It's the simplest thing you could do. So he he solved the equations. It's a remarkable thing. In those equations there is a description of a black hole although it wasn't realized at the time. It's a remarkable piece simple piece of mathematics actually. Um so essentially what's the idea behind a black hole? One way to think about it is that you could remove could you remove the star from from this fabric but leave the distortion behind.

[00:14:47]So if you if you do that you get the description of a black hole. But you might say what what do you mean that how can that be formed in nature? So you think about what a star is then a star is a balancing act. So it's a it's a mainly hydrogen and helium collapsing under its own gravity. That's how our sun formed 4 and a half billion years ago. So it's collapsing. So what stops it collapsing? Well, as it collapses, the core heats up and that initiates nuclear fusion reactions in the core. In in the case of our sun, it's hydrogen being fused into helium that releases energy which creates a pressure which holds the thing up. So it's balancing.

[00:15:26]So, but it needs the fuel and and it's not infinitely big of course. So, at some point it runs out of nuclear fuel and ultimately no more fusion reactions can occur in any star and so the star will resume its collapse. So, the question is well is there something that stops it? Because if there isn't something that stops the collapse then it will collapse without limit. And and so actually if you look at the history of physics in the 20s and 30s people were saying well we'd like to avoid this idea that the thing will collapse without limit because if it does then Schwarzel's equation predicts some very strange things indeed and so people kind of are trying to avoid it. It was really actually Oppenheimer and his student Schneider in the late 30s just before the Second World War that showed that really with some assumptions it looks like a massive enough star could actually collapse without limit. So what does that mean? Collapses without limit.

[00:16:26]It means that essentially it does what I said. You essentially remove the star from the fabric of the universe leaving the distortion behind. And the black hole, the idea behind the black hole is, let's say you take, let's say you take the sun, a star, the mass of the sun, and you just collapse it and you keep on collapsing it. You get to a point when the the radius of the sun is not 700,000 kilometers, which is what is in miles, half a million miles, and you squash it down, and you squash it and squash it until its radius becomes 3 km. three.

[00:17:04]Then there are several ways to look at this. One is that on the surface, the speed you'd have to travel to escape its gravitational pull, it's called the escape velocity, would exceed the speed of light. That's one way to think about it. So even light rays emitted from the surface, if you could squash it down that far, would just stay there. They would not escape because they'd be trying to go at the speed of light and and the escape velocity is the speed of light and they just stop.

[00:17:33]So what what happens then if a star collapses inside that number which for the mass of the sun 3 km is called the swatchual radius then it will collapse without limit nothing will stop it and so all you will get is essentially the geometry of space and time the curvature and that's a black hole. So it's a thing that traps light in that sense. So you think about that if you have this surface uh and space and time are so distorted there that if you go in across that surface it's called the event horizon of the black hole then you can't get out you you one way to think about it is you'd have to travel faster than light to get out. Another way to think about it is that um there's a beautiful model which is my favorite model. It's called the river model of a black hole.

[00:18:24]You can write the equations as as a space being like a river that flows into this thing almost like a sinkhole or something in space and the river of space flows at the speed of light inwards on the horizon and then faster than light inside. So if you imagine that you're a a photon a particle of light you're like a little fish swimming against the tide. But if the tide's going at the speed of as fast as you can swim the speed of light you can't get out. Not only can you not get out, but you're going inwards towards something.

[00:18:58]And this thing, the the something is called the singularity. You say, "What is this thing the singularity?" And I think it's really tempting to picture it as some infinitely dense point to which this star collapsed. When you draw a map of space and time, what you see really clearly is that this singularity thing is not a place in space. It's a moment in time. And it's in fact the end of time in Einstein's theory. So, so a way that I often kind of picture it or explain it to myself is that space and time have become so distorted that when you look at it from the outside, they flip roles. So space has be space has become time and time has become space in the mathematics. If you put a little graphic up, you'll see that the plus and minus signs in the sports equip and so they they change. So what you thought of as as an infinitely end place in space to which the star collapsed has become a moment in time. Uh even black holes the am the sun the the sun will not form a black hole. When it runs out of nuclear fuel it will collapse and there's something that can stop it collapsing. So when we're falling in we might as well be this is the Einstein's equivalence principle in action. we might as well be in flat space because the distortion it's like saying uh on the surface of the earth um if you look at a mile a square mile of the surface of the earth you don't feel the you don't see the curvature right you have to go it's a bigger distances to see that you're on a curved surface it's kind of like that so so the the distortion the difference in gravitational pull as you said or the distortion you don't feel it until it becomes very distorted or there's a big gravitational pull when you get very close to this And then you start to see that and actually it works. It's not only stretching, it's also a squashing. So the way the tidal gravity works is to squash in one direction and pull in the other direction.

[00:20:52]>> So we are getting >> so you feel it. So you start to feel this strange sort of sensation of being stretched and squashed and and and as you go closer and closer to the singularity those effects become much more extreme until they're so extreme and then the atoms get get separated and then the protons the quarks inside the protons will get separated and ultimately according to Einstein's theory the tidal forces become infinite so that in formally infinite and so everything is gone everything's been ripped apart and that this is what we call the singularity.

[00:21:28]So, so the whole thing kind of gets very extreme and breaks down ultimately in Einstein's picture. So, our window onto this deeper theory, call it quantum gravity if you like it has been very simple questions actually about black holes which are real things. That very simple question from Stephen Hawking.

[00:21:50]That's why Hawing's calculation is so important because it's the first glimpse of a problem with our picture of the world and it's a very very precise glimpse.

[00:22:05]It's a it's a precise question. Does this thing destroy information or not?

[00:22:10]If it doesn't, how does the information get out? That's a simple question. But it's lead and is still leading which is why I'm waving my hands around a lot.

[00:22:20]You you would get a different different pictures from any any you know expert who does the calculations. I'm not one of those right but if you talk to someone who does the calculations they would not be certain about the physical picture. I think it's fair to say is it that but could it be that you can describe the whole universe in terms of a quantum theory living on a boundary of some description and I think the guess is yes the the guess but we don't even know what we mean by the boundary >> right um it's one of the problems here so the the reason that Maldina was able to show this is this works for this very specific thing called ads space is because there's a boundary you can identify in that particular geometry whereas our universe is dissitter right there isn't it's not obvious what you mean by a boundary it's not really obvious what question you know as you said it's hard to find the words it is we don't really know what question we're asking but that rough picture that There could be a a a theory of a quantum theory somehow. This network of cubits that gives rise to geometry, space, time is accepted broadly. Right? So we but when you try to get into the detail of it, it's only been fully realized for a very specific model which is kind of a toy model. It's not our universe. So it could be that our universe does not admit that description. Could be. could be the >> but it's certainly beyond us >> at the moment technically but it's a wonderful thought and actually there's a paper >> uh recently that so people are beginning to use the the Google chip the willow chip >> um which is a very powerful it's not a quantum computer it's a proto quantum computer kind of thing so and people have started to use it because what it is is a load of cubits that you can entangle and you can set it up and it's very well controlled. So whilst we don't know how to do quantum calculations on the thing really what you can do is try to say well could I set them up so that it's like space emerges >> from them and there was a paper recently where something that was described in the paper as a one-dimensional wormhole was made it wouldn't be in our universe this thing it would be but it kind of emerged from this structure >> and that's the kind of picture we're trying to get to it's a it's a good paper. It's been peer- reviewviewed.

[00:25:06]It's a controversial paper. You'll see loads of stuff online, but it's worth looking at those papers and by the time this is uh sent out there might be some other paper. A lot of people are working on it. So, and also actually building little clocks, little quantum clocks or tiny clocks because we don't even know what a clock is right at the most fundamental level because we don't know what time is. So, so about very fundamental questions about reality in this research. Um, so it's very counterintuitive, but it's about polarity that there's a property of particles called spin where this is realized. So they're like quantum coins really and and we use it now in quantum cryptography for example. So this is becoming technology now. this this idea of using entanglement and also in quantum computing extremely exciting. So so we're now this is not just wishy-washy wild stuff. It's it's fundamental. We use it we use it in laboratories as it's people think of entanglement as an information resource now a resource that we can use to build computers. Um, but it's profound because it it really does seem to be the case that entanglement builds up spaceime. So that it's almost like it's the underpinning structure. I mean, imagine a world where at the fundamental level space, which is this thing we take for granted. It's the room that we're in. We talk about things being one mile apart or something. We think we know what we mean by distance and separation.

[00:26:40]Actually no. Underpinning that it does seem is a is a world of quantum entanglement in which there is no concept of distance or separation or space. It emerges. It's like it's like saying um you know you I talk about us right we you right what are you you're a human being and you're thinking and feeling and we're having this conversation but actually there's another level which you could call a deeper level where you're just atoms.

[00:27:10]So, you're just protons and neutrons and electrons and the protons and neutrons are made of quarks and they're all stuck together and that's a description of you. But it isn't you. This isn't it feels like it isn't all there is to you, right? There's also you a human being with thoughts and feelings and you know the most wonderful structure in the universe which is what human beings are.

[00:27:32]In the same way we're saying that um that space and time are like that and and it's it's as incomprehensible but then again no less incomprehensible is that the fact that a human being can be made of atoms what we're saying is space and time can be made of something else which seems to be quantum entanglement.

[00:27:50]It's a theory that isn't it? I mean so there's a theory there and so you can get the little things and we will analyze them and have a look. I mean I wouldn't I it's funny because on you mentioned social media I think you occasionally on social media I I'll quite often I'll tweet something and and there'll be quite a few people who disagree reasonably strongly with what I said and one of them is is the the UFO thing you know that I mean there are people who really believe that there are UFOs visiting the earth and I always say you know I haven't seen any evidence of that that I think is strong evidence it's a huge claim that there are other civilizations out there that are visiting But I wouldn't be surprised in a in a sense in a strict sense that if I said to someone the other day, you know, if a big UFO came now, we walk outside and over Westminster there's a spaceship hovering, I wouldn't been the least bit surprised because I know that there are trillions of planets in the Milky Way galaxy alone and hundreds billions of stars and there's been a lot of time.

[00:28:53]And one of the great mysteries actually in physics is why we don't seem to see much out there. Anything we haven't, you know, there's strong there's strong evidence of nothing out there at all at the moment. We we have no strong evidence of any life beyond Earth. And that's a puzzle and a paradox. So, it's a it's about with with those claims, you don't rule them out. If someone says, "Well, I've got this I found this thing at the bottom of the sea and I think it's really weird." Um then the correct thing to do is go okay we'll put it in a lab and get an electron microscope and pro it around and find out just how weird it is. And nature fineman again said the thing to remember is nature does not care at all what you think.

[00:29:35]Nature just doesn't it doesn't matter who you are or how famous you are. Any letters you've got before or after your name, whatever. It doesn't matter.

[00:29:43]Nature just is. So, if indeed an alien spacecraft crashed into wherever it was that they found these things a billion years ago and left all the fragments there and we've dug them up, then that's interesting. The way I've described it, the way Einstein's theory describes it is somehow that stuff goes to the singularity, whatever that thing is, the end of time, a region of spaceime that's so convoluted and distorted that we don't understand how to describe it at all. But then one day, the whole thing is gone. All that's left in the far far future is Hawking radiation. Those particles that were produced in the vicinity of the event horizon. The question is, is it possible if you could collect all that radiation, all the Hawin radiation through the whole life of the black hole, is it somehow possible in principle that the information about everything that fell into the black hole throughout its history is imprinted in that radiation in the far future. Is that true or is it not true? You might say, why did I ask that question? Seems like a bit of a random question. It's a very important question. Let's say that I take anything in in here in this room, a book, a table, the camera, right? Anything at all. And I set fire to it. I incinerate it. I destroy it any way that I can. Could throw it into a furnace. I could I could put it in the heart of a nuclear bomb and explode it, whatever. Just completely incinerate it. In physics, in basic fundamental physics, then it turns out that if you could collect every piece of that thing that I detonated or incinerated, every quantum of radiation, every photon, every particle, everything in principle, if I could just collect it all and I was clever enough, then I could reconstruct the thing that I had destroyed.

[00:31:48]information is conserved in the universe as far as we know. So every law of nature that we have says that information is conserved. The problem was that Steven Hawings initial calculation of the way those black holes evaporate away said that information is not conserved. Said that black holes are erasers of information. To put it very bluntly, the the calculation said that once that black hole had gone, then even in principle, there is absolutely no way you could learn anything, reconstruct anything about the things that fell in, including the star that collapsed to to to form it. So information erasers, the only information erasers that we know of in nature. That was the initial picture of black holes as Steven Hawking understood them in back in the 1970s and 1980s. This became known as the black hole information paradox.

[00:32:50]We have a situation the 1980s, 1990s, 2000s, where it appears that there's a fundamental problem with our understanding of something. Black holes, quantum mechanics, general relativity, when you put them together, you have this apparent prediction that these things erase information from the universe. Some physicists, Steven Hawing, initially actually felt that that was the case. that maybe these things do erase information. Maybe we don't care about that. Maybe that's the way the quantum theory of gravity is.

[00:33:27]And other physicists, people like Leonard Suskin initially, for example, Jared Tahuft, the the great Nobel Prize winning particle physicist and others felt that no, there's something wrong with our understanding that there's no way in which we can allow these these things to erase information from the universe, thus challenging our understanding of the basic laws of nature. So that debate was was vigorous and went on for decade after decade after decade. The reason it's interesting is because there's a very precise problem posed. So it's not some kind of thing like trying to understand a singularity or understand the big bang or where you can just say well we're miles away from understanding. the challenges were posed in a region of space, the vicinity of the event horizon where we thought we had control of the physics. And so that's really useful because it means that it should be the case that calculations can be performed to resolve. The reason this challenge, th this apparent contradiction, this fundamental problem so garnered so much attention is because it it occurred that the problem came from physics that everybody thought they understood. So calculating in a region of space under under conditions in the universe where we thought and assumed that we had full control of the mathematics of the theories. That's really interesting because it some means you have a chance catching a glimpse of some kind of deeper theory by resolving this contradiction. So that's why more and more people got interested in the black hole information paradox. It turns out if we fast forward to the present day and a series of papers still being written, so this is still research that's happening as we speak. But it turns out now that the general view, I would say the generally accepted view is that black holes do not erase information from the universe. So in principle if you could collect all that Hawking radiation emitted over eons right the lifetimes of some of these black holes by the way 10 to the^ 120 years plus for some of the big ones it's one with 120 knots after it the universe is only one with 10 knots after it years old at the moment 10 billion years old or so we're talking about time scales vastly longer than the current age of the universe but when they've gone Then in principle, we now think you could collect all that radiation in principle put it into some quantum computer pro carry out some operations on all that radiation and reconstruct the information about everything that fell in. But the implications of that that the mechanism by which that happens I would say is it's profoundly exciting because it really does seem to be giving us a glimpse of a deeper theory of gravity. The simple way to say it, it seems that space and time are not fundamental. So one view and I emphasize that that there are other views. We're at the cutting edge of research now, but one view is a field which has become known as emergent spacetime. So what does that mean? It means that space and time themselves are not fundamental. In in Einstein's picture, we assume there is such a thing as spacetime. this this four-dimensional surface manifold is the technical term but you assume it it's part of the model there are such things as space and time woven together into the fabric of the universe what they are Einstein has no nothing to say it seems that there is a deeper theory underlying what we think of as space and time which is a quantum theory and so the idea is that space and time themselves forms emerge from from what? From quantum entanglement from some kind of smaller parts or pieces. We don't know what those things are. We don't know the nature of them. But it does seem that there's a deeper underlying theory from which space and time emerge. That is what we call the quantum theory of gravity. But the key thing is the key thing to understand is we've been driven to that picture by asking very clearly defined precise questions about how black holes behave which goes all the way back to to well to Einstein and then to Steven Hawking and many others calculations through the 70s and 80s.

[00:38:17]Well, super massive black holes are fascinating things. So these are the black holes that we have images of radio telescope images. We have two images uh from a collaboration called the event horizon collaboration. One of a black hole was the first one they acquired of a black hole in a galaxy called M87 which is about 55 million lighty years away. This is a big galaxy something like a trillion suns. And in the center of that we've long suspected there was a black hole. Now we have an image of the black hole. It's a black hole that is so super massive. It's over six billion times the mass of our sun. A big black hole by any measure. We also have an image, by the way, of the the rather smaller super massive black hole at the center of the Milky Way galaxy, which is only about just over 4 million times the mass of our sun. So it's a little one, but still big. We think that virtually every galaxy has a super massive black hole at its heart. Virt caveat a little bit because there there a couple of papers have been released recently suggesting that there may be galaxies observed where there are no super massive black holes. There are lots of galaxies in the universe. Maybe there are exceptions, but it's fair to say that virtually every galaxy has a black hole at its center. That's an interesting observation because we don't know how those form. One of the reasons is we don't really fully understand how the first galaxies formed in the first place. It's current research, live research. One of the goals of the James Web Space Telescope is to observe the formation of the first stars and galaxies. It can do that because it can detect very faint, very long wavelength light, which is what you need to detect if you want to look far out into the universe and therefore far back in time towards the origin of the first stars and galaxies. So the JWST new instrument that's looking at that there also a series of new radio telescopes being built. the square kilometer array for example in Australia, South Africa which is a radio telescope array aimed at observing the formation of the first stars and galaxies. So this is live research. It's a fundamental question.

[00:40:37]How does structure form in the early universe? Not only are black holes interesting from a from a theoretical perspective, you know, allowing us to peer or uncover deep questions or ask deep questions about the structure of space and time itself. But they're also key to understanding how galaxies form and how structures form in the early universe.

[00:41:03]How many black holes do we know of? So direct observation the the in terms of radio telescopes we have a direct observation of two of them from the event horizon collaboration. The one in our galaxy the milky way and the one in the galaxy M87.

[00:41:21]We also have what I would call direct observation of black hole collisions.

[00:41:26]This is a completely different technology a different approach. It's called gravitational wave astronomy. So gravitational waves are ripples in the fabric of the universe. Let's describe them like that. Although I once heard Kip Thorne called them a storm in time, which I think is also a very beautiful idea. And so the idea is that what what is a ripple in space and time. They be passing through everywhere now. So through through you and me through the space in which we sit and they're ripples in space and time. So time speeds up and slows down a bit as these things go through. uh distances kind of shrink, can expand a little bit as well.

[00:42:06]So they're they're real distortions in space and time and they're caused by well actually pretty much everything that happens in the universe at some level, but they're very very faint and difficult to detect. The ripples we've been able to detect come from the collisions of black holes or the collisions of neutron stars. So these are highly energetic events in the universe and um the observatory is called LIGO Virgo. These are essentially laser beams, so-called laser interpherometers, about two and a half miles actually long, four kilometers long at right angles to each other. And they can detect little shifts in the distance between the mirrors between which the laser beams bounce far far smaller than the diameter of the nucleus of an atom. So tiny tiny ripples in the fabric of the universe. By observing those ripples, we can see the collisions of black holes. We tend to see the collisions of big ones and that's what's called a selection effect. Um the big ones are more energetic and so the gravitational waves are easier to detect. But the collisions we're seeing are between black holes that are for example around 30 times the mass of our sun. And the number of those collisions is I think it's fair to say larger than anyone would have imagined before the observations were made. So it seems there are quite a lot of these black holes floating around that are very massive indeed, 30, 40 times the mass of our sun and upwards. And I I think it's fair to say the formation of those things is a mystery as well. It's uh presumably that they're formed from the collapse of very massive stars, but uh that the number of them, I think it's fair to say, took everybody by surprise.

[00:44:00]So we've detected um I don't know what the current number is actually I'm speaking now what in June 2023 um there are collisions being detected reasonably often but it's tens of collisions maybe more actually but it's a we have multiple observations of collisions of black holes with black holes and indeed black holes and and neutron stars as well. So it's it's quite a few. So we're very certain now as certain as we can be that these things really exist. I said we have images and we have these wonderful observations of the collisions of them. Now how many black holes might there be out there? Um there'll be billions of them. Billions. So all massive stars. Stars that are um you know 3 4 5 6 7 10 times the mass of the sun. We there there are lots of stars that that are above the limit beyond which we understand that at the end of their lives when they run out of nuclear fuel they will collapse without limit.

[00:45:06]Nothing will stop the gravitational collapse and they will inextraably form black holes. So black holes are the natural we used to say end point in the lives of stars above what three four times the mass of our sun. We now know of course that ultimately those black holes will evaporate away again. But you know that really is an imprinciple statement in the far future. So the way to think about black holes is that any star more than let's say three or four times the mass of our sun and upwards will form a black hole when it when it runs out of nuclear fuel at the end of its life. The numbers are kind of debated, but let's say something like two trillion galaxies, large galaxies and smaller galaxies in the observable universe. So 2,000 billion galaxies. Pretty much every one of those will have a super massive black hole at its center as well. So, we're talking about not to steal Carl Sean's phrase, but billions and billions of black holes. Carl Sean said, "I never said that." So, I'm not stealing Carl Sean's phrase actually because he says that he never said billions and billions.

[00:46:20]Let me give you some glimpses into why black holes are so so fascinating and so perplexing and and wonderful. So if we go back right to the beginning of the work on black holes in the 1970s, Jacob Beckinstein, a colleague of Steven Hawings, actually one one of the first researchers to really begin working on black holes alongside greats like John Wheeler for example, Beckenstein noticed in a simple calculation which was initially pretty much a back of the envelope calculation actually um noticed that you can ask the question, you can answer the question, how much information can a black hole store?

[00:47:02]That's a strange thing to say because the model of a black hole is pure geometry, pure space time. So you might think, well, it it's not capable of storing any information. Now, how does something store information? You need some structure. You need atoms or some something that can store bits of information. Well, turns out that you can calculate that a black hole stores information and how much this is the fascinating thing. So it stores in bits the information content is equal to the surface area of the event horizon in square plank units.

[00:47:46]What's a plank unit? It's a fundamental distance in the universe that you can calculate by putting together things like the strength of gravity, planks constant, speed of light. It's the smallest distance often described as the smallest distance that we can talk about sensibly in physics as we understand it.

[00:48:06]The plank length, this is a bizarre result. Uh there's so many things that are bizarre about that. The questions it raises, what's storing the information?

[00:48:15]How is information stored? Why is the information content of a region of space equal to or why does it have anything at all to do with the surface area surrounding that region rather than the volume? If I asked you how much information can you store in your room, the room that you're sitting in now, just say it's a library. Then you would say, well, it's to do with how many books I can fit in the room or hard drives or whatever it is, right? is to do with the volume of the room, the space. But black holes seem to be telling us that there's something about the surface surrounding a region which is fundamental. This is the first glimpse I think of an idea called holography which now seems to be correct in a sense. So holography, what is that? It's the idea that there are different descriptions of our reality. There's one description which is the um the description that we're all comfortable with and familiar with I would say which is that we live in the space the three dimensions of space and time is a thing that ticks and Einstein told us that they're kind of mixed up but still you have this picture of space being this right the thing in which we exist. There's an equivalent description it seems. It's been proved by the way for a very specific model called ADSCft by a physicist called Maldina. Um there's an equivalent description which is of a theory that lives purely on the boundary of the space and it's absolutely equivalent. It's an absolute perfect description, a jewel theory of the description of the space itself in the interior of this region. That's called holography because that's if you think about what a hologram is, then at the very simplest level, it's a piece of film. But that piece of film contains all the information to make a three-dimensional image. So all the information about a threedimensional image is contained on a two-dimensional piece of film. It's basically a hologram. And it seems from our study of black holes and the hints go all the way back to the 1970s to Jacob Beckinstein's work that our universe is like that in a way that's not fully understood yet. The picture is that and and physicists I always like to joke when they don't really know or the language isn't present then physicists often say in some sense wave their hands. So in some sense there's an equivalent description of our reality which lives on a boundary surrounding it perfectly equivalent. You might ask the question well what's the real description of reality and the answer is we don't know but they're equivalent. So it's strongly suggestive that there's a let me say a deeper theory but at least a different theory of our experience of the world of space and time that does not have space and time in it. And that's that's one of the wonderful surprises uh that's really emerged from at least in part from the study of black holes in the attempt to answer the very well-posed questions that black holes pose. It's I should say that the that the work done by Maldescina the ADSCFT correspondent was p purely mathematical.

[00:51:43]So it wasn't framed in the study of black holes although the questions ultimately uh seem to be intimately related. So that's number one. So the study of black holes seems to be strongly suggesting that these ideas of holography holographic universe which came from a different region of physics actually from from trying to understand other things. those descriptions may be valid, may may be useful, maybe in some sense true. The other remarkable thing for me, I think in the study of black holes is an intimate relationship between black hole physics and quantum computing. This was wholly unexpected. I think it's fair to say the relationship comes by saying okay so there's a theory on a boundary a quantum theory which is a perfect description a dual description of the physics of the interior of a region of space you might say then well how is information encoded on this boundary how does that relate to the physics of the interior space and time and it seems that we're beginning to glimpse an answer at least in very simplified ified models and it seems that the information is stored on the boundary redundantly which means that you can lose a bit of it and still fully specify the physics of the interior. Redundant storage of information. Now if it goes to quantum computers, there's an engineering challenge in building quantum computers.

[00:53:20]And just to emphasize, we have these things. They're not theoretical. They exist in labs across the world. There's a challenge which is how to store information in the memory of the quantum computer safely, robustly because quantum computer memory is notoriously susceptible to any interference from the outside environment. If any of the environment in which the memory sits interacts with the memory in any way, then the information is destroyed. So it's a tremendous challenge and there are deep problems associated with the fact that you can't copy information in quantum mechanics which is basically the way that your iPhone or whatever it is stores information and prevents errors entering into the memory of the computers that we're all familiar with.

[00:54:11]It's basically copying information. You can't do that in quantum mechanics.

[00:54:16]Fundamental. It's called the no cloning theorem. engineers have had to develop very clever algorithms and ways of trying to store information in quantum computer memory and build the memory such that it's resilient to errors. And it turns out that the solutions that are being proposed and explored look like the solutions that nature itself uses in building space and time from the theory, the quantum theory that lives on the boundary. It's really strange. And I just emphasize, you're not meant to understand what I've just said because I don't understand what I've just said because nobody understands what I've just said. Right? We're catching glimpses of this theory. And that's where that the research is at the moment. It's why it's tremendously exciting. There's papers being published all the time that are digging more deeply. They're suggesting pictures of reality, physical pictures of what's happening. But there's no sense, and I emphasize it in which everybody agrees on these pictures. So I'm giving you an interpretation which and there will be other people who have different interpretations. But it does seem it does seem that whatever this quantum theory is that underlies our reality then there's some redundancy in the way the information is stored in that quantum theory. And it does seem that that's akin to or similar to the way that we will in the future encode information in the memory of quantum computers to protect them from errors called a quantum error correction code in the jargon. I find that fascinating.

[00:55:57]It It's tantalizing. It's It's a glimpse of Einstein said that if you look at nature really carefully and keep pulling at the intellectual threads and keep going and just keep delving down into into what nature seems to be trying to tell us, then if you're lucky and persistent, you can catch a glimpse of something deeply hidden. It's beautiful that, isn't it? a glimpse of something deeply hidden, which is the deep underlying structure of nature. And it does seem that we're beginning to glimpse something deeply hidden. From the study of black holes, the related surprisingly field of quantum computing and quantum information, we're glimpsing something deeply hidden, the deep underlying structure of reality itself.

[00:56:46]And I think it's very beautiful. No one really knows, I think it's fair to say, where this is going, but they're hints of something deeply hidden.

[00:56:56]Black holes may well um help us understand a different but related question, which is what happened at the beginning of the universe. So, I think black holes are at the moment the most interesting naturally occurring objects in the sense that they're helping us understand or forcing us into a deeper understanding of what space and time are. And my view is that if we're going to talk about the origin of the universe, even ask questions such as did the universe have a beginning in time, which we don't know the answer to, then it surely seems to me that we have to understand what space and time are before we can ask and have any chance of answering such questions. And black holes, it turns out, are the objects that we can see, that we can observe, that actually exist in nature that force us down that route to ask questions, sharp, well-defined questions actually about the nature of space and time themselves. It surely must be the case that as we uh get a deeper understanding of what the let's call it the fabric of the universe is then we can begin to get a rather more a rather deeper understanding or more insight into questions about the origin of the universe itself. Should it have one? I mean why do I keep saying should it have one? By the way, because you might be watching this saying, well, it's a big bang. We know there's a big bang. And that's true. We do know there was a thing called the big bang. What do we mean by that though? We mean that 13.8 billion years ago, the universe was very hot and very dense everywhere. The region of space that now forms the room in which you sit was very hot and very dense 13.8 billion years ago. We know that all the galaxies are receding from each other at the moment. And just put very simply, uh if we make the measurements of how they're receding and run time backwards, then you find that they're all on top of each other. 13.8 billion years ago. So, we know something interesting happened back then. We have a measurement to it. It's not up for debate. Often when people say, "Well, how how do you know about this big bang thing?" The vast amount of evidence that such a thing happened, the best evidence probably is you can see it. So, we see something called the cosmic microwave background radiation, which is the universe as it was about 380,000 years after the Big Bang. What we see there, we have a photograph of it is a universe that was very different from the one we see today. There were no stars, no planets, no galaxies. It's just a hot glowing mass of gas, primarily hydrogen and helium. And uh we have a photograph of that. So we know something interesting happened. But do we know that that was the origin of the universe? No, we don't. We have theories, strong physical pictures of the universe before the big bang in a very precise sense before the universe was hot and dense. Theories called inflation which say that space was still there and it was expanding very fast and then that fast expansion drew to a close and all the energy driving that expansion got dumped into space made particles and that's what we call the big bang the so-called hot big bang as we term it today.

[01:00:18]What happened to start inflation off?

[01:00:21]The answer is we don't know when did inflation start. We don't know. We have a minimum time that it needed to go on for, but we don't know what or even if that phase of the universe constitutes a beginning. There are so many reasons to study black holes. There's fascinating, beautiful things that we know to exist in the universe pose fundamental questions. But one of the reasons that we're really interested in them is that put it this way there singularities at the end of time.

[01:00:53]The Einstein description says that inside a black hole there's a singularity and locally at least in that region of the universe that constitutes the end of time. There's a singularity maybe at the beginning of time, the other end of time if you like that we're also interested in might be the you might call it the big bang singularity, the origin of the universe. And it seems to me to understand that singularity at the beginning beginning of time, the big bang singularity, then we need to understand the singularities at the end of time. And the great benefit is we can see those. So we can watch them bump into each other. We can see them at the centers of galaxies. We can make observations of them and we can do calculations.

[01:01:38]Understanding black holes, I think, will be the key to a deeper theory of our universe.