Nobel Prize for Black Holes - Sixty Symbols

Well, we're in Rady Nottingham, as you can see, and they've today announced the Nobel Prize for Physics. So in a nutshell, it's gone for black holes is the only way I could describe it. And in fact, the wording of the citation is kind of interesting. So let me just read it out. It says, the Nobel Prize in Physics 2020 was divided. One half awarded to Roger Penrose for the discovery that black hole formation is a robust prediction of the general theory of relativity. The other half jointly to Reinhard Genzel and Andrea Goetz for the discovery of a supermassive compact object at the centre of our galaxy. And you'll notice it sort of skates round saying anything about there being a massive black hole at the centre of our galaxy. Because technically, no one has really proved that that's what it is. It's just, you know, no one can think of anything else that it could possibly be at this point. The two astronomers, they've been looking at the black hole at the centre of our galaxy. Basically, what they were looking at, they managed to establish that there was a really compact object there, four million solar masses in basically a region the size of our solar system, or even less. And, you know, the remarkable astronomical observations. But the guy I want to talk about is the other half of the prize, which is Sir Roger Penrose, because I'm a theorist and he's a theorist. And so obviously we share a lot of interests and the work that he did. And the reason that Penrose got the prize was really, he basically sort of taught us something about the structure of black holes and what lurks inside them. And what lurks inside a black hole is this dreaded singularity. This is this region of sort of infinitely strong gravitational field, infinite space-time curvature, that horrible word infinity, that basically this horrible monster at the centre of a black hole exists. So I would say it fit, you know, that 1965 paper, that remarkable paper which showed that these singularities are for real. And it really shocked the world of physics. Everybody was stunned by it. No one saw it. They thought these singularities were just an artefact of symmetry. They didn't think they were real. Penrose showed that they were real. General relativity generally, you know, really did predict them. And then, you know, Hawking then took that on and worked with Penrose as well, and they showed that you could get these singularities, you know, in a sort of at the Big Bang, at the start of the universe, or maybe at the end of the universe. It wasn't just inside black holes. So the whole story sort of blossomed into something. And the other thing it tells you, these singularities tell you, is that general relativity is not a complete theory. Because it can't describe those singularities. It's outside, even though, even though it predicts they exist, and Penrose showed us that, you know, you follow your nose and follow general relativity, you're going to end up in this place with these dreaded singularities. Actually, it can't really describe it. And that's why we know we need more than general relativity. We need a quantum theory of gravity, whatever that might be, string theory, or whatever. So it also tells, it also sort of spelled the demise of general relativity. We know it's not the full story. It's just one of the other wonderful things about what Penrose did. This is a great piece of work. It's a fabulous piece of work. As I said, it's probably the most important paper in, you know, in gravity between Einstein's original papers and his in 1965. But you don't really see Nobel Prizes in the past given for these kind of theory papers. No one's got into a black hole and spotted a singularity that hasn't happened. And yet they've given him this prize. And I think it marks a bit of a change for theorists and potential Nobel Prize wins. And I think it's a great thing. I absolutely applaud it. So long may this continue. And maybe once again, Sir Roger's going to be a bit of a trailblazer for everybody else. Well, so it's really all about showing that there's a very massive object at the centre of the galaxy. And in principle, showing, measuring the mass of something showing that there is a massive object somewhere is reasonably straightforward in astronomy in that, for example, we know that the sun's at the centre of the solar system, not just because, you know, it glows in the sky, but actually because it holds all the planets in orbit around it. And so actually, even if the sun weren't giving out any light at all, we'd be able to say there's a massive object in the centre of the solar system. And actually, we'd even be able to say what its mass was just by studying the orbit of the Earth and the other planets around it. So in principle, you can sort of do that, make that kind of measurement for anything, which is gravitationally bound together. And in essence, that's what they've done for the centre of the Milky Way, in that they've been looking at the stars orbiting around the centre of the Milky Way and showing that the orbits they're following imply that there's a very massive object at the centre of the Milky Way. It sounds very simple the way I say it there. Of course, in practice, it's a huge amount more complicated. And in part, that's because so it's part in part is because you're looking at something which is pretty small, in that the whole of the region that they're looking at where those things are all orbiting around are all within an arc second or so. And that's the scale on which if you just take a picture of a star with a telescope from the ground, typically, the atmosphere blurs out the image to about an arc second. So actually, all those stars packed in with those central few arc seconds, if you just took a naive picture of the centre of the galaxy with a telescope, they will be blurred together, and you wouldn't actually be able to see anything at all and their motion. The other complication is that the centre of the Milky Way is a very difficult place to see. Because between us and the centre of the galaxy, there's a huge amount of muck. There's this stuff that astronomers refer to as dust, it's sort of this sooty smoky material, which obscures the view of the centre of the galaxy, which essentially means that very little of the light coming from the centre of the galaxy gets to us. If you look in the optical part of the spectrum, it's something like, you know, one photon in 10 billion emitted near the centre of the galaxy actually gets to us. So there's no chance of actually seeing it. The solution to both of these problems actually came along sort of in the 1990s, when astronomers started first making images, decent images in the infrared part of the spectrum. So it's sort of technology driven that we started getting the detectors that allow us to make images in the infrared part of the where previously we'd really only been able to make optical images. And the infrared has two big advantages. Firstly, that that muck is more or less transparent. So instead of sort of one photon in 10 billion getting through, it's something like one photon in five gets through. So still, a fair amount of it gets obscured by the dust between us in the centre of the galaxy. But actually, enough gets through that you can start making images. The other big advantage of infrared is it's more straightforward to take out those blurring effects of the atmosphere to sharpen up the images. And so a lot of the the breakthroughs in this area have come because adaptive optics, these techniques that astronomers use to take out the blurring effects of the atmosphere work well in the infrared part of the spectrum. So they've really been able to sharpen up those images. So instead of seeing those 20 or 30 stars near the centre of the galaxy, all blurred together into a single splodge, they really can make pictures of the individual stars. And that has enabled them to monitor those stars and actually literally see them orbiting around the centre of the Milky Way, apparently around nothing, there's nothing very much there. Well, it's a radio source there. So there's something there that we thought was a bit suspicious. And there's something that gives out radio waves. But there's not there's no obvious star there or anything bright there. And these stars near the centre of the galaxy are all following these nice elliptical orbits, just like the planets around the sun, whizzing around something which doesn't appear to be there. And now they've been able to observe them for long enough that some of those stars they've actually seen them go around their complete elliptical orbits. Now, that gives you a very strong constraint on Okay, I can actually measure I know what the pull of gravity has to be to be making things orbit around the centre of the galaxy at that speed. And that's how they come up with this number of a supermassive object in that central region of the Milky Way of a few million times the mass of the sun. So it's called a singularity, the black hole singularity, where Penrose entered the game was he wrote this paper in in in 1965, which basically shocked the world of physics. And basically what Penrose showed was that this singularity was likely to was there, okay, that it was predicted by general relativity. So what happened previously is people like Oppenheimer and Schneider had been looking at things like sort of collapsing, you know, spherically symmetric collapsing sort of clouds of dust and shells and things like that. And in these very symmetric scenarios, they were seeing as that those symmetric sort of distributions of matter collapsed in that indeed, you would form a black hole and at the centre of that black hole would be one of these dreaded singularities. And people were like, all right, we're not taking that seriously. That's it's just because you were considering a situation with an That's the only reason that weird singularity, that weird infinity, that madness is there. But what Penrose showed was it wasn't really an account of the symmetry, it would always be there, okay, that this was really an artefact of general relativity, that black holes do contain these singularities, okay, he proved these singularity theorems. And it was much more general, it didn't require symmetry. It was extremely general statement, all you had to say was basically, as long as I have effectively positive energy, if I'm thinking about matter that's made up of positive energy, then I'm guaranteed then it collapses to form black hole, I'm guaranteed to get one of these singularities, which is basically it's just an end of space or an end of time. It's one of those things that kind of crept up on the field in that, you know, back in the 1960s, people started discovering these things called quasars, which were these incredibly bright distant objects, which were varying on timescales of days, or at least, you know, weeks and months as well, which tells you to have to be very small, right, for something to vary very quickly. You know, if you actually had a whole galaxy's worth of stars, even if you found some way of switching them all off and on, because of the amount of time it takes light to travel just from one side of the galaxy to the other, you wouldn't see the whole thing switching on and off very quickly. So when you see something varying on timescales of say a few months, that tells you it can only be a few light months across. So these quasars were varying on very short timescales, which tells you they were very small. And they were also incredibly bright. And this combination of things people were casting around trying to figure out, could they be supermassive stars or various other ideas were thrown around. But the idea that won out is that had to be a very, very massive object, and material was falling into it. And you were seeing the kind of the energy that was liberated as the material fell into it. And that would both give you a lot of energy, because if the thing's very compact, the polar gravity is very strong. So the stuff falls in extremely fast, and you have a lot of energy, but it's all in a very small region. So it varies very quickly. So that was, you know, as I say, back in the 1960s, 90s, early 1970s, people were starting to think about it. In the 1970s, this radio source at the centre of the Milky Way, Sagittarius A star, which is where this black hole is located was found. And that got people thinking, well, maybe there's one in the middle of the Milky Way as well. With the advent of the Hubble Space Telescope in the 1990s, we were for the first time able to actually start resolving regions close enough to the centre of other galaxies to see actually, yes, stars are whizzing around extremely fast there as well. So it's one of those things that kind of crept up on us a bit at the time. But this is sort of, you know, this is the best measurement we have the ultimate measurement. And actually, at some level, you know, knowing it's there in our own galaxy tells us quite, you know, quite how commonplace this is that even an ordinary galaxy like ours has a massive black hole in the middle of it. What did Penrose actually show? He showed that that the space-time wasn't complete inside the black hole. And what he showed was it wasn't geodesically complete. Now, what's a geodesic? Geodesics are the sort of shortest paths in space-time. They're the paths that a particle will take if it's not accelerating through space-time, it will follow a shortest path. Now, assuming you were immortal, and you can, in principle, live for as long as time could exist, you would just follow geodesics forever, okay? You would just go on and on and on and on and on, right? What Penrose showed was if you fall into a black hole and you follow one of these geodesics, eventually the geodesic itself will grind to a halt. And think about what that really means. That means there's an end to either time or space. There's actually an end to time or space. So even though you might think you're immortal, nothing can destroy you. Well, the end of time can destroy you. So if you're falling into one of these black holes, you're following one of these geodesics, depending on the nature of the geodesic, you will start to feel very strong gravitational fields. You'll start to feel yourself being ripped apart by the tides of gravity. Literally, you know, the atoms in your body being torn apart. And then, you know, as you sort of beg for mercy, you know, even if you're capable of begging because your atoms are being torn apart, as you beg for mercy, what you'll find is, is that you'll be given that mercy because time itself, or possibly space, but in the case of, depends on the nature of the black hole, but time itself will end. Okay. So even though you think you're immortal, well, it doesn't matter because there is no more time. Time itself stops. And that's what Penrose showed, that these things are really lurking there inside black holes. We talked about these stars orbiting around very close to the centre of the Milky Way. They're still, you know, so there's one particular star, S2, which is the one that's gone closest to the centre. And that's one that they've now followed around a full orbit. And in fact, it got so close, you remember, if you think about elliptical orbits, like comets in the solar system, for example, when they're kind of in their outskirts of their orbits, they travel very slowly, and then they whiz past the sun and then back out again. It's just the same story here. The stars hang around. And then when they're close to the black hole, they whiz past it and then slow down when they're travelling away from it. The one that went closest to the, to the black hole at the centre of the Milky Way was travelling at about two and a half percent of the speed of light. So it was actually a decent fraction of the speed of light and got sufficiently close that there's all sorts of interesting things you can do with it. For example, it was sufficiently close to the black hole that the light escaping from it not only had to escape from the star but had to escape from the gravitational field of the black hole as well. And so they were able to measure that gravitational redshift, the energy that the light lost as it was escaping from the region of the black hole. But even in these cases, we're still, you know, we're still the distance, I think it was 100 or so astronomical units from the putative black hole in the centre of the Milky Way. So sort of 100 times the distance of the Earth to the sun, which is about 1000 or so times this thing called the Schwarzschild radius, which at some level is sort of the size of the black hole. It's that point of no return where even light can't escape. So the sort of the size of the black hole was 1000 times smaller than the scale that this star was probing. So we can't actually say we can say within 1000 times the size of a black hole was the mass of a black hole. But for it to actually be definitely a black hole, we have to be able to say, you know, within one times the size of the black hole, there's that much mass. But actually, we can only say it's within 1000 times that region. There is nothing else that anyone could think of that would pack that much mass into that smaller region that wasn't a black hole. But that's why they sort of hedged their bets that tiny bit in the citation by just saying there's this huge amount of mass in this very small region rather than saying there is a black hole there. I mean, the mass of whatever is there is incredibly accurately determined. I have to look up the number now. But it says, I think the best measurement of it is it's 4.15 plus or minus 0.01 million solar masses. So that's an incredible, you know, by astronomical standards, usually in astronomy, we're happy if we get something within a factor of two, right. So actually, you know, measuring something to that level of precision is incredible, that they know very accurately how much master is there, they just don't know for sure that it's all compressed within a region small enough that it has to be a black hole. So as I say, it almost certainly is there's nothing else that anyone could think of that would explain it. But they haven't in that sense, seen a black hole in the same way that for example, the event horizon telescope was able to actually image very close to a black hole itself. They're actually looking at a slightly larger region, which is telling you how much mass is there. And it's all very concentrated, but it doesn't in itself tell you for certain that there's a black hole there, they were very much in competition with each other. So there were two groups in the world. So gets working at UCLA, UCLA in California, was using the Keck telescopes in Hawaii, and Genzel was working in Germany and using the European Southern Observatory telescopes in Chile. So they really were sort of in competition. And it's one of those sort of healthy competitions. I don't think it was, you know, it wasn't that they weren't unpleasant towards each other. It's just one of those things. There was an exciting thing that sort of occurred to both of them at the same time, they both started working on it at the more or less the same time. And they've sort of both been competing to get the results out, which is both healthy, and that it's sort of competition, it drives us on to do things better. But also, it's sort of healthy to have that cross check, because if it was only one person in the world was claiming they'd found something, you know, you always think, you know, could they possibly have made a mistake somewhere along the line. So it's good to have two completely different groups using different instrumentation, and potentially different techniques for analysing their data, and so on, both arriving at essentially the same conclusion. We have one corner of the Penrose Institute, and we had the first sort of inaugural workshop at the Penrose Institute took place right here in Nottingham a couple of years ago. And he was here for an extended period and drank and we had a cup of coffee in the in the library there and talked for, you know, a long time about physics. He also gave us some lectures, which were fabulous. And the remarkable thing about it was this is a guy who's well into his 80s. And these lectures were going on for like two hours, and his stamina was astonishing. I first met him sort of about probably about 10-15 years ago now. And I was a postdoc in Oxford, and he was, he was actually retired by that point. But this idea he'd had in a sort of late 60s, early 70s, twisters, suddenly came back into fashion, and he came out of retirement to give us all some lectures on it. And what I remember about those lectures was, I mean, they were fabulous, but what I remember most about them was how good he was at drawing pictures. He would draw these pictures in the lectures, and they were just awesome. It's like what great, he's not just a great mathematician or great physicist, he's a great artist as well, so it seems. So that was one thing that stuck in my mind of those early days of meeting Roger Penrose. But obviously he's a legend of physics and maths, and it's been a privilege to have him in Nottingham, and it's great that he's won the Nobel Prize. One of them is the fact that as you tip the mirror through the night to look at different angles of the sky, gravity starts to affect the mirror and deform it ever so slightly. So what you do is you have something called active optics, which are, see all those little pistons there? There's 150 actuators, basically little pistons, that can push and pull.