安德魯·斯特羅明格：黑洞、量子引力與理論物理

Lex Fridman Podcast #359 – Andrew Strominger: Black Holes, Quantum Gravity, and Theoretical Physics Machine transcription by Qwen3-ASR 0.6B (MLX, local, Apache-2.0). May contain recognition errors. The following is a conversation with Andrew Struminger, theoretical physicist at Harvard, whose research seeks to shed light on the unification of fundamental laws of nature, the origin of the universe, and the quantum structure of black holes and event horizons. And now a quick few second mention of each sponsor. Check them out in the description. It's the best way to support this podcast. We got Eight Sleep for naps, Rocket Money for saving out subscriptions, Indeed for hiring excellent teams, and ExpressVPN for security and privacy on the internet. Choose wisely, my friends. And now on to the full ad reads. As always, no ads in the middle. I try to make this interesting, but if you skip them, please still check out our sponsors. I enjoy their stuff, maybe you will too. 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This is the lexfriedman podcast, supported. Please check out our sponsors in the description. And now dear friends, here is Andrew Struminger. You are part of the Harvard Black Hole Initiative, which has theoretical physicists, experimentalists, and even philosophers. So let me ask the big question: What is a black hole from a theoretical, from an experimental, maybe even from a philosophical perspective? So a black hole is defined theoretically as a region of space time from which light can never escape. Therefore, it's black. Now that's just the starting point. Many weird things follow from that basic definition, but that is that is the basic definition. What is light? That can't escape from a black hole. Well, light is you know the stuff that comes out of the sun, that stuff that goes into your eyes. Light is one of the the stuff that disappears when the lights go off, the stuff that appears when the lights come on. Of course, I could give you a mathematical definition, but or physical mathematical definition, but I think it's something that we will understand very intuitively. What is light? Black holes, on the other hand, we don't understand intuitively; they're very weird. And what are the questions is about black holes, which I think you were alluding to, is you know why doesn't light get out or how is it that there can be a region of space time from which light can escape? It definitely happens. We've seen those regions. We have spectacular pictures, especially in the last several years of those regions. Um, they're there. Fact, they're up in the sky, thousands or millions of them. We don't yet know how many. Um, but the proper explanation of why light doesn't escape from a black hole is still a matter of some debate. And one explanation which perhaps Einstein might have given is that light carries energy. You know it carries energy because you know we have photo cells and we can take the light from the sun and collect it, turn it into electricity. So there is energy in light. And anything that carries energy is subject to a gravitational pull. Gravity will pull at anything with energy. Now it turns out that the gravitational pull exerted by an object is proportional to its mass. And so if you get enough mass in a small enough region, you you can prevent light from escaping. And let me flesh that out a little more. If you are on the Earth and you are on a rocket ship leaving the surface of the Earth and if we ignore the friction from the air, if your rocket accelerates up to eleven kilometers per second, that's escape velocity. And it can if there were no friction it could just continue forever to the next galaxy. On the moon, which has less mass, it's only seven kilometers per second. So but going in the other direction, if you have enough mass in one place, the escape velocity can become the speed of light. If you shine light straight up away from the Earth, it doesn't have too much trouble. It's going way above the escape velocity. And but if you have enough mass there, even light can escape the escape velocity. And according to Einstein's theory of relativity, there is an absolute speed limit in the universe—the speed of light—and nothing makes any sense. Nothing could be self consistent if there were objects that could exceed light speed. And so in these very, very massive regions of space time, even light cannot escape. And the interesting thing is Einstein himself didn't think that these objects we call the black holes could exist. But let me actually linger on this. Yeah, that's incredibly interesting. There is a lot of interesting things here. First, the speed limit. How wild is it to you if you put yourself in the mind in the time of Einstein before him to come up with a speed limit of that there is a speed limit that and that speed limit is the speed of light. How difficult of an idea is that? Is it you know you said from a mathematical physics perspective everything just kind of falls into place but he wasn't. Perhaps maybe initially had the luxury to think mathematically. He had to come up with it intuitively. Yes, so like what how intuitive is this notion to you? Well, is it still crazy? No, no. So it's a very funny thing in physics. The best discoveries seem completely obvious in retrospect. Even my own discoveries, which of course are far lesser than Einstein's, but many of my papers, many of my collaborators get all confused. We'll try to understand something. We say we got to solve this problem. We'll get all confused. Finally, we'll solve it. We'll get it all together. And then we'll all of a sudden everything will fall into place. We'll explain it and then we'll look back at our discussions for the proceedings of months. And literally be unable to reconstruct how confused we were. And how we could ever have thought of it any other way. So not only can I not fathom how confused Einstein was before he when you know when he started thinking about the issues. I can't even reconstruct my own confusion from from two weeks ago. You know so the really beautiful ideas in physics have this very hard to get yourself back into the mindset. Of course, Einstein was confused about many many things. Doesn't matter if you are physicist. It's not how many things you got wrong. It's not the ratio of how many you got wrong to how many you got right. It's the number that you got right. So Einstein didn't believe black holes existed, even though he predicted them. And I went and I read that paper which he wrote. You know Einstein wrote down his field equations in nineteen fifteen. And Schwarzschild solved them and discovered the black hole solution three or four months later in very early nineteen sixteen. And twenty five years later, Einstein wrote a paper. So with twenty five years to think about what this solution means, wrote a paper in which he said that black holes didn't exist. And I am like, well, you know if one of my students in my general relativity course wrote this, you know I wouldn't pass them. You get a C minus. Oh, you wouldn't pass them. Okay, get a C minus. Okay, same thing with gravity waves. He didn't believe. Oh, he didn't believe in gravitational waves either. He went back and forth, but he wrote a paper and I think thirty four saying that gravity waves didn't exist because people were very confused about what a coordinate transformation is. And in fact, this confusion about what a coordinate transformation is has persisted. And we actually think we were on the edge of solving it. One hundred years later, one hundred years later, what is coordinate transformation as it was one hundred years ago to today. Let's imagine I want to draw a map with pictures of all the states and the mountains and then I want to draw the weather forecast what the temperatures are going to be all over the country. And I do that using one set of weather stations, and I number the weather stations, and you have some other set of weather stations, and you do the same thing. So the coordinates are the locations of the weather stations. They're how we describe where the things are. At the end of the day, we should draw the same map that is coordinate invariance. And if we're telling somebody, we're going to tell somebody at a real physical operation, we want you to stay as dry as possible on your drive from here to California. We should give them exactly the same route, no matter which weather stations we use or how we you know. It's a very trivial. It's it's the labeling of points is an artifact and not in the real physics. So it turns out that that's almost true, but but not quite. There is some subtleties to it. The statement that you should always have the same give it the same kind of trajectory the same kind of instructions, no matter the weather station. Yeah, yeah, there is some very delicate subtleties to that which began to be noticed in the in the fifties. It's mostly true, but when you have a space time with edges, it gets very tricky how you label the edges. In space time, in terms of space or in terms of time, in terms of everything—just space either one okay space or time—that gets very tricky. And Einstein didn't didn't didn't have it right. And in fact, he had an earlier version of general relativity in nineteen fourteen, which he was very excited about. Which was wrong? Gave it wasn't fully coordinate invariant. It was only partially coordinate invariant. It was wrong. It gave the wrong answer for bending light to the sun by a factor of two. There was an expedition sent out to measure it during World War One. They were captured before they could measure it, and that gave that gave Einstein four more years to clean to clean his act up by which time he he'd gotten it right. So it's a very it's a very tricky business, but once it's all laid out, it seems it's it's it's clear. Then why do you think Einstein didn't believe his own equations and didn't think that black holes are real? Why was that such a difficult idea for him? Well, something very interesting happens in Schwarzschild solution of the Einstein equation. I think his reasoning was ultimately wrong, but let me explain to you what it was. At the center of the black hole behind the horizon, in a region that nobody can see and live to tell about it, as a center of the black hole, there is a singularity, and if you pass the horizon, you go into the singularity, you get crushed, and that's the end of everything. Now the word singularity means that it just means that Einstein's equations break down. They become infinite. You write them down. You put them on the computer. When the computer hits that singularity, it crashes. Everything becomes infinite. There is two. So the equations are just no good there. Now that's actually not a bad thing. It's a really good thing, and let me explain why. So it's an odd thing that Maxwell's theory and Newton's theory never exhibit this phenomena. You write them down. You can solve them exactly. They're really Newton's theory of gravity. They're really very simple theories. You can solve them. Well, you can't solve the three body problem, but you can certainly solve a lot of things about them. Nevertheless, there was never any reason. Even though Maxwell and Newton perhaps fell for this trap, there were never any reason to think that these equations were exact. And every there is no equation. Well, there are some equations that we've written down that we still think are exact. Some people still think are exact. My view is that there is no exact equation. Everything is an approximation. Everything is an approximation. Are you trying to get as close as possible? Yeah. So you think you are saying objective truth doesn't exist in this world. The internet is going to be very. We could discuss that, but that's a different that's a different thing. We wouldn't say Newton's theory was wrong. It had very very small corrections, incredibly small corrections. It's actually a puzzle why they're so small. So if you watch the procession of Mercury's perihelion, this was the first indication of something going wrong. According to Newton's theory, Mercury has an elliptical orbit. The long part of it moves around. As other planets come by and perturb it and so on, and so this was measured by Leverrier in 1859, and he compared theory and experiment, and he found out that the perihelion process moves around the the sun. Once every two hundred and thirty three centuries instead of every two hundred and thirty one centuries. Now this is the wonderful thing about science. Why was this guy? I mean, you don't have any idea how much work this is. You know, but of course he made one of the greatest discoveries in the history of science without you know even knowing what what good it was going to be. So that's how small that that was the first sign that there was something wrong with Newton. Yeah. Now so the corrections to Newton's law are very very small, but they're definitely there. The corrections to electromagnetism. They're mostly the ones that we see are mostly coming from quantum effects. And so so the corrections are for Maxwell's equations is when you get super tiny and then the corrections for the for Newton's laws of gravity is when you get super big. That that that's when you require corrections. That's true, but I would phrase it as saying when it's super accurate. You know if you look at the Bohr atom, Maxwell electromagnetism is not a very good approximation to the force between the proton and the electron. The quantum mechanics. If you if you if you didn't have quantum mechanics, the electron would would spiral into the proton and the atom would collapse. It's quantum you know so that's a huge correction there. Sure. So every theory gets corrected as we learn more. Do just be no reason to suppose that it should be otherwise. How is this related to the singularity? Why the singularity? So when you hit the singularity, you know that you need some improvement to Einstein's theory of gravity. And that improvement, we understand what kind of things that improvement should involve. It should involve quantum mechanics, quantum effects become important there. It's a small thing. And we don't understand exactly what the theory is, but we know there is no reason to think you know Einstein's theory was invented to describe. Weakly curved things, the solar system and so on. It it's incredibly robust that we now see that it works very well near the horizons of. Around black holes and so on, so so it's a good thing that the theory drives itself. That it predicts its own demise. Newton's gravity had its demise. There were regimes in which it wasn't valid. Maxwell's electromagnetism had its demise. There was regimes in which quantum effects greatly modified the equations. But general relativity all on its own found. Drove a system which originally was fine would perversely wander off into a configuration in which Einstein's equations no longer applied. So to you the edges of the theory are wonderful. The failures of edges are wonderful because that keeps that keeps us in business. So one one of the things you said I think in your TED talk that the the fact that quantum mechanics and and relativity. Don't describe everything and then they clash is wonderful. I forget the adjective you used but it was something like this. So why is that? Why is that interesting? Do in that same way that there is contradictions that create discovery. There is no question in my mind. Of course many people would disagree with me that now is the most. Wonderful time to be a physicist. Why is that? So so people people look back at at um. It's a classical thing to say among physicists. I wish it were nineteen twenty. Quantum mechanics had been just understood. There was the periodic table. There was but in fact that was such a rich thing. Um that that well so that what a lot of exciting stuff happened around nineteen twenty. It took it took the whole it took a whole century. To sort out the new insights that we got. Especially adding some experimental stuff into the into the bunch actually making observations and adding all the experimental thing. All the computers also help with visualizations and all that kind of stuff. Yeah yeah yeah. It was a whole sort of wonderful century. I mean the seed of general relativity was the incompatibility of Maxwell's theory. Of the electromagnetic field with Newton's laws of gravity, they were incompatible because. If you look at Maxwell's theory, there is a contradiction. If anything goes faster than the speed of light. But Newton's theory of gravity, the gravitational field, the gravitational force is instantaneously transmitted across the entire universe. So you could you know if you had a a friend on you know in another galaxy with a very sensitive. Measuring device that could measure the gravitational field. They could just take this cup of coffee and move it up and down and Morse code and they could. Get the message instantaneously over another galaxy. That leads to all kinds of contradictions. It's not it's not self consistent. It was exactly in resolving those contradictions that Einstein came up with the general theory of of. Relativity and it's fascinating how this contradiction which seems like maybe it's kind of technical thing. Led to a whole new vision of the of the universe. Now let's not get fooled because. Lots of contradictions are technical things. We haven't set up the we run into other kinds of contradictions that are technical and they they don't seem to they we just we understood something wrong we made a mistake we set up equations in the wrong way we didn't translate the formalisms. As opposed to revealing some deep mystery that's yet to be uncovered. Yeah yeah and so we never we're never very sure which are which are the really important ones. But to you the difference between quantum mechanics and general relativity. The the tension the contradiction there seems to hint at some deeper deeper thing that's going to be discovered in the century. Yes because that one has been understood since the fifties. Polly was the first person to notice it and Hawking in the early seventies gave it a really much more visceral form. And people have been. Hurling themselves at it trying to reduce it to some technicality but nobody has succeeded. And the efforts to understand it have led to. All kinds of interesting relations between quantum systems and and applications to other fields and and so on. Well let's actually jump around. Okay we'll return to black holes. I have a million questions there but let's let's go into this unification. The battle against the contradictions and the tensions between the theories of physics. What is quantum gravity? Maybe what is the standard model of physics? What is quantum mechanics? What is general relativity? What is quantum gravity? What are all the different unification efforts? Okay so again five questions. Yeah, it's a theory that describes everything with astonishing accuracy. It's the most. Accurate theory in the history of human thought. Theory and experiment have been successfully compared to sixteen decimal place. We have that stenciled on the door where where I work you know it's that it's an amazing it's an amazing feat of the human mind. It describes the electromagnetic interaction, unifies the electromagnetic interaction with the so called weak interaction which. You need some good tools to even view the weak interaction, and then there is the strong interaction. Which binds the quarks into protons, and the forces between them are mediated by something called Yang Mills theory, which is a a beautiful mathematical. Generalization of electromagnetism, in which the analogs of the photons. In themselves carry charge, and so this the final piece of this. Of the standard model, everything in the standard model has been observed. Its properties have been measured. The final particle to be observed was the Higgs particle. Observed like a over a decade ago. The Higgs is already a decade ago. I think it is yeah. Wow time flies. But you better check me on that yeah. It's it's this true but so much fun has been happening. It's so much fun has been happening. And so that's all that's that's all pretty well understood. There are some things that might or might not around the edges of that you know dark matter neutrino masses. Some sort of fine points or things we haven't quite measured perfectly and so on but. It's largely a very complete complete theory and we don't expect. Anything very new conceptually in the completion of that. Anything contradictory but new because can't you? Anything contradictory. I'll have some wild questions for you on that front. But yeah anything that yeah because there is no gaps it's so accurate so precise in its predictions it's hard to imagine something. Yeah yeah yeah. And it was all based on something called let me not explain what it is let me just throw out the buzzword. Renormalizable quantum field theory they all fall in the category of renormalizable quantum field theory. I am going to throw it at a bar later to impress. Impress the girls. Good luck. Thank you. All right. So they all they all fall under that rubric. Gravity will not will not will not put that suit on. So the force of gravity cannot be tamed by the same renormalizable quantum field theory to which the all the other forces. So eagerly submitted. What is the effort of quantum gravity? What are the different efforts to to to have these two dance together effectively to try to unify. The standard model and and general relativity and you kind of model of gravity. Sort of the one fully consistent model that we have. That reconciles that it that sort of tames gravity and reconciles it with quantum mechanics. Is string theory and its cousins and we don't know what or if in any sense string theory describes the world. The physical world, but we do know that it is a consistent reconciliation of quantum mechanics and general relativity and moreover one. Which which is able to incorporate particles and forces like the ones we see around us. So. It hasn't been ruled out as an actual sort of unified theory of nature. But there also isn't a in my view some people would disagree with me. But there isn't a reasonable possibility that we would be able to do an experiment in the foreseeable future which would be sort of a yes or no. To to string theory. Okay so you've been there from the early days of string theory you've seen its developments what are some interesting developments what do you see as the also the future of string theory. And what is string theory? Well the basic idea which emerged in the early seventies. Was that if you you take the notion of a particle and you literally. Replace it by a little loop of string, that strings are sort of softer than than particles. What do you mean by softer? Well you know if you hit a particle if there were a particle on this table a big one and you hit it you might bruise yourself. But if there was a string on the table you would probably just push it around. And and the the source of the infinities in quantum field theories that would particles hit each other it's a little bit of a it's a little bit of a. A jarring effect and and I've never described it this way before but it's actually scientifically accurate but if you throw strings at each other it's a little more friendly. One thing I can't explain is how wonderfully precise well the mathematics is. That goes into describing string theory we don't just wave our hands and throw strings around and you know there are some very. Compelling mathematical equations that describe it now. What was realized in the early seventies is that if you replace particles by strings these infinities go away. And you get a consistent theory of gravity without the infinities. And that may sound a little trivial but at that point had already been. Fifteen years that people had been searching around for any kind of theory that could do this, and it was actually found kind of. By accident, and there are a lot of accidental discoveries in this subject. Now at the same time it was believed then that string theory was an interesting sort of toy model. For putting quantum mechanics and general relativity together on paper, but um. But that it couldn't describe some of the very idiosyncratic phenomena. That pertain to our own universe in particular the form of so called parity violation. Our world. Another term for the bar later tonight. Yeah, yeah. Parity violation. So, so if you go to the bar and. I already got the renormalizable quantity. And you look in the mirror across the bar. Yes. The universe that you see in the mirror. Is not identical. You would be able to tell if you show your your your your. The lady in the bar, a photograph that shows both the mirror and you. There is a difference. If she's smart enough, she'll be able to to tell which one is the real world. And which one is you? Now she would have to do some very precise measurements. And if the photograph was too grainy, it might not be possible. But a principle is possible. Why is this interesting? Why is it? Does this mean that there is some? Not perfect determinism or what does that mean? There is some uncertainty. No, it's a very interesting feature of the real world. That it isn't parity invariant, in string theory, it was thought could not tolerate that. And then it was learned. In the mid eighties, that not only could it tolerate that, but if you did things in the right way, you could. Construct a world out involving strings that reconciled quantum mechanics and general relativity, which looked more or less like the world that. We live in, and now that isn't to say that string theory predicted our world. It just meant that it was consistent that the hypothesis that string theory describes. Our world can't be ruled out from the get go, and it is also. The only proposal for a complete theory that would describe. Our world still nobody will believe it until there is some kind of direct experiment. And I don't even believe it myself. Sure, which is a good place to be mentally as a physicist, right? Always I mean Einstein didn't believe his own equations, right? With the black hole. Okay, well that when he was wrong about that. He was wrong about that. But you might be wrong too, right? So do you think string theory is dead? If you were to bet all your money on. Um. The future of string theory. I think it's a logical error to think that string theory is either right or wrong. Or dead or alive. What it is is a stepping stone. And an analogy I like to draw. Is Yang Mills theory, which I mentioned a few minutes ago in the context of standard model. Yang Mills theory was discovered by Yang and Mills in the fifties. And they thought that the symmetry of Yang and Mills. Theory described the relationship between the proton and the neutron. That's why they invented it. That turned out to be completely wrong. It does, however, describe everything else in the standard model. And it had a kind of inevitability. You know they had some of the right pieces, but not the other ones. Sure, they didn't have it quite in the right context. And it it had an inevitability to it, and it eventually sort of found its place. And it's also true of Einstein's theory of general relativity. You know he had the wrong version of it in 1914, and he was missing some pieces. And you wouldn't say that that his early version was right or wrong. He'd understood the equivalence principle. He'd understood spacetime curvature. He just didn't have everything. I mean technically you would have to say it was wrong. And technically you would have to say Yang and Mills were wrong. And I guess in that sense. I would believe just odds are. We always keep finding new wrinkles. Odds are we're going to find new wrinkles in string theory, and technically what we call string theory now isn't quite right. But. We're always going to be wrong, but hopefully a little bit less wrong every time. Exactly, and I I would you know bet the farm as they say. Do you have a farm? I you know I say that much more seriously because not only do I have a farm, but we just renovated it. So before I renovated, better get the farm. My wife has been five years renovating it before. You were much much looser with that statement, but now it really means something. Now it really means something. And and I would bet the farm on the. On the guess that 100 years from now string theory. Will be viewed as a stepping stone towards a greater understanding of. Of nature, and and it would I mean another thing that I didn't mention about string theory is of course we knew that it. Solved the infinity's problem, and then we later learned that it also solved Hawking's puzzle about. What's inside of a black hole? And you put in one assumption, you get five things out. Somehow you are doing something right, you know probably not everything, but you you you know there is some good signposts and there. There have been a lot of good signposts like that. It is also mathematical toolkit and you've used it you've used it with Comar and Vafa. Maybe we can sneak our way back from string theory into black holes. What was the idea that you and Comar and Vafa developed with the holographic principle and string theory? What were we able to discover through through this through string theory about black holes or. That connects us back to the reality of black holes. Yeah, so that is a very interesting story. I was interested in black holes before I was interested in string theory. I was sort of a reluctance. String theorists in the beginning, I thought I had to learn it because people were talking about it, but you know once I studied it, I I grew to love it. First I did it in a sort of beautiful way. These people say they've claimed quantum gravity. I ought to read their papers at least. And then the more I read them, the more interested I got, and I begin to see you know they. They phrased it in a very clumsy way. The description of string theory was was very clumsy and mathematically clumsy or just. It was all correct, but but mathematically clumsy. But it often happens. That in all kinds of branches of physics that. People start working on it really hard and they sort of dream about it and live it and breathe it and they begin to see inner relationships and. They see a beauty that is really there. They're not they're not deceived. They're really seeing something that exists, but. If you just kind of look at it, you know you can't you can't grasp it all in the beginning and. And so our understanding of string theory in. In 1985, was almost all about you know. Weekly coupled waves of strings colliding and so on. We didn't know how to describe. A big thing like a black hole in string theory. Of course we could show that strings in theory in some limit reproduced Einstein's theory of general relativity. And corrected it, but we couldn't do any better with black holes. Than before my work with Comar, we couldn't do any better than Einstein and Churchill had done. Now one of the puzzles. You know if you look at the. Hawking's headstone and also Boltzmann's headstone and you put them together, you get a formula. For they are really central equations in 20th century physics. I don't. Think there are many equations that made it to headstones. And they're really central equations and you put them together and you get a formula for the number of gigabytes in a black hole. Now in Schwarzs description. The black hole is literally a hole in space and there's no place to store the gigabytes. And it's not too hard to and this really was Wheeler and Beckenstein and. Wheeler Beckenstein and Hawking to come to the conclusion that. If there isn't a sense in which a black hole can store. Some large number of gigabytes that quantum mechanics and gravity can't be consistent. We got we got to go there a little bit. So so how is it possible? Won't say gigabytes that there's some information. So black holes can store information. How is this thing that sucks up all light and is supposed to basically be you know. Be super homogeneous and boring. How is that actually able to store information? Where does it store information on the inside on the surface. Where where's and what's information? I'm liking this ask five questions to see which one you actually answer. Oh okay. You say that I should try to memorize them and answer each one in order. Just answer. I don't know I don't know what I'm doing. I'm desperately. Desperately trying to figure it out as we go along here. So Einstein's black holes and Schwarzs black hole they can't store information. Stuff stuff goes in there and it just keeps flying and it goes to the singularity and it's gone. However, Einstein's theory is not exact. It has corrections, and string theory tells you what those. Corrections are and so you should be able to find some way of some alternate way of describing the black hole. That enables you. To understand where the gigabytes are stored, so what Hawking and Beckenstein really did was they showed that. Physics is inconsistent unless. A black hole can store a a number of gigabytes proportional to its area divided by four times. Newton's constant times Planck's constant. And that's another wild idea. You said area not volume. Exactly, and that's the holographic principle. The universe is so weird. And that's the holographic principle. That's called the holographic principle that it's it's the area. We're just jumping around. What is the holographic principle? What does that mean? Is there some kind of weird projection going on? What what the heck. Well I was just before I came here writing an introduction to a paper and the first sentence was. The as yet imprecisely defined holographic principle. Blah blah blah blah blah. So nobody knows exactly what it is. But roughly speaking, it says just what we were alluding