Andrew Strominger: Black Holes, Quantum Gravity, and Theoretical Physics | Lex Fridman Podcast #359

Andrew Strominger 0:00 A black hole is a mirror. And the way it's a mirror is if light, a photon, bounces off your face towards the black hole, it goes straight to the black hole, just falls in, you never see it again.

Lex Fridman 0:15 Mm-hmm.

Andrew Strominger 0:16 But if it just misses the black hole, it'll swing around the back and come back to you.

Lex Fridman 0:24 Yeah.

Andrew Strominger 0:25 And you see yourself from the photon that went around the back of the black hole. But not only can that happen, the black hole, the photon can swing around twice and come back. So you actually see an infinite number of copies of yourself.

Lex Fridman 0:46 The following is a conversation with Andrew Strominger, 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. This is the Lex Fridman Podcast. To support it, please check out our sponsors in the description. And now, dear friends, here's Andrew Strominger.

Lex Fridman 1:12 You are part of the Harvard Black Hole Initiative, which has theoretical physicists, experimentalists, and even philosophers. So, uh, let me ask the big question. What is a black hole from a theoretical, from an experimental, uh, maybe even from a philosophical perspective?

Andrew Strominger 1:30 So a black hole is defined, theoretically, as a region of spacetime from which light can never escape, therefore it's black. Now, that's just the starting point. Many weird things, uh, follow from that basic definition, but that is, that is the basic definition.

Lex Fridman 1:56 What is light-

Andrew Strominger 1:58 Well-

Lex Fridman 1:58 ... that can't escape from a black hole?

Andrew Strominger 2:00 Well, light is, uh, you know, the stuff that comes out of the sun, the stuff that goes into your eyes. Um, light is one of the, the stuff that disappears when the lights go off-

Lex Fridman 2:11 (laughs)

Andrew Strominger 2:11 ... the stuff that appears when the lights come on.

Lex Fridman 2:14 Yeah.

Andrew Strominger 2:14 Um, of course, I could give you a mathematical definition, but, um, or physical mathematical definition, but, uh, I think it's something that we, uh, all understand very intuitively, uh, what is light. Black holes, on the other hand, we don't understand intuitively. They're very weird. And one of 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 spacetime from which light can't 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. In 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 li- light doesn't escape from, uh, a black hole is still, uh, a matter of some debate. Um, and one explanation, which perhaps Einstein might have given, is that light carries energy. Um, you know it carries energy because, you know, we have, uh, photo cells and we can take the light from the sun and collect it, turn it into electricity. So there's 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 gravital, gravitational pull exerted by an object, uh, is proportional to its mass. And so if you get enough mass in a small enough region, um, you, you can, uh, prevent light from escaping. And let me flesh that out a little more. Um, if you're on the earth, uh, and you're on a rocket ship leaving the s- the surface of the earth, and if we ignore the friction from the air, um, if your rocket accelerates up to 11 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.

Lex Fridman 5:28 Mm-hmm.

Andrew Strominger 5:29 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, um, but if you have enough mass there, even light can't 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, uh, in these very, very massive regions of spacetime, even light cannot escape.

Lex Fridman 6:16 And the interesting thing is Einstein himself didn't think that, uh, these, uh, objects we call the black holes could exist. But let me actually linger on this-

Andrew Strominger 6:26 Yeah, that's incredibly interesting. Yeah.

Lex Fridman 6:27 There's a lot of interesting things here.

Andrew Strominger 6:29 Yeah.

Lex Fridman 6:29 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 l- 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- you know, you said from a mathema- uh, um, 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 counterintuitive is this notion to you?

Andrew Strominger 7:06 Well-

Lex Fridman 7:06 Is it still crazy? What-

Andrew Strominger 7:08 No, no. (laughs) So it's a very funny thing in physics. The best discoveries seem completely obvious in retrospect.

Lex Fridman 7:18 Yeah.

Andrew Strominger 7:19 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've got to solve this problem." We'll get all confused. Finally, we'll solve it, we'll get it all together, and, um, 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-

Lex Fridman 7:48 Mm-hmm.

Andrew Strominger 7:49 ... and literally be unable to reconstruct how confused we were-

Lex Fridman 7:55 Yeah.

Andrew Strominger 7:55 ... and how we could ever have thought of it any other way.

Lex Fridman 7:59 That's so fascinating.

Andrew Strominger 8:00 And 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. (laughs) Uh, 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. Um, doesn't matter if you're a 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 1915?

Lex Fridman 9:01 Mm-hmm.

Andrew Strominger 9:02 And Schwarzschild solved them and discovered the black hole solution, uh, three or four months later, in very early 1916. And, um, 25 years later, Einstein wrote a paper. So with 25 years to think about what this solution means-

Lex Fridman 9:20 Yeah.

Andrew Strominger 9:21 ... wrote a paper in which he said that black holes didn't exist. And I, I'm like, "Whoa." You know, if one of my students in my general relativity course wrote this, you know, I wouldn't pass them.

Lex Fridman 9:38 You get a- you get a C-? Oh, you wouldn't pass them. Okay. All right. (laughs)

Andrew Strominger 9:41 Yeah, you'd get a C-. Okay. Same thing with gravity waves. He didn't believe-

Lex Fridman 9:44 Oh, he didn't believe in gravitational waves either?

Andrew Strominger 9:46 He went back and forth, but he wrote a paper in, I think, '34 saying that gravity waves didn't exist because it, 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, we're on the edge of solving it 100 years later.

Lex Fridman 10:15 (laughs) Well, what-

Andrew Strominger 10:17 100 years later.

Lex Fridman 10:18 ... what- what is coordinate transformation as it was 100 years ago to today?

Andrew Strominger 10:23 Let's imagine I, uh, want to draw a map with pictures of all the states and the mountains, and then I wanna m- draw the weather forecast, what the temperatures are gonna be all over the country.

Lex Fridman 10:38 Mm-hmm.

Andrew Strominger 10:39 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.

Lex Fridman 10:58 Yeah.

Andrew Strominger 10:58 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- if we're telling somebody, uh, we- we- we're gonna tell somebody at a real physical operation, "We want you to stay as dry as possible on your drive from here to California," um, we should give them exactly the same route.

Lex Fridman 11:26 Sure.

Andrew Strominger 11:26 No matter which weather stations we use or how we, you know...

Lex Fridman 11:29 Mm-hmm.

Andrew Strominger 11:29 It's a very trivial, it's- it's the labeling of points is an artifact and not in the real physics.

Lex Fridman 11:37 Sure.

Andrew Strominger 11:38 So it turns out that that's almost true, but- but not quite. There's some subtleties to it.

Lex Fridman 11:49 The statement that you should always have the same, give- give at least the same kind of trajectory, the same kind of, uh, instructions, no matter the weather stations?

Andrew Strominger 11:57 Yeah. Yeah, there's some very delicate subtleties to that, which began to be noticed in the, in the '50s. It's mostly true, but when you have a spacetime with edges-It gets very tricky how you label the edges.

Lex Fridman 12:19 In spacetime, in terms of space or in terms of time, in terms of everything, just spacetime?

Andrew Strominger 12:22 Either one.

Lex Fridman 12:23 Okay.

Andrew Strominger 12:23 Space or time. That gets very tricky. And Einstein, uh, didn't, didn't, didn't have it right and it, in fact, he had an earlier version of general relativity in 1914 which he was very excited about, um, which was wrong. Um, 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 I. They were captured before they could measure it (laughs) and that gave, that gave Einstein four more years to, to clean it, to clean his act, act up, by which time he'd, 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, uh, it's, it's, it's clear.

Lex Fridman 13:29 Then why do you think Einstein didn't, uh, believe his own equations and didn't think that black holes are real? Why, why was that such a difficult idea for him?

Andrew Strominger 13:39 Well, something very interesting happens in Schwarzschild's solution of the Einstein equation. I think his reasoning was ultimately wrong, but let me explain to you, uh, what it was. Um, at the center of the black hole, behind the horizon, in a region that nobody can see and live to tell about it, is the center of the black hole, there's 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, um, it just means that Einstein's equations break down. They become infinite. You write them down, you put 'em on the computer. When the computer hits that singularity, it crashes. Everything becomes infinite. There's two... So the equations are just no good there.

Lex Fridman 14:52 Mm-hmm.

Andrew Strominger 14:54 Now, that's actually not a bad thing. It's a really good thing. And let me explain why. Um, 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, um, you can certainly solve a lot of things about them. Uh, 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. Um, and every e- there's no equation... Well, there's some equations that we've written down that we still think are exact, some people still think are exact. My view is that there's no exact equation.

Lex Fridman 16:17 Everything is an approximation.

Andrew Strominger 16:19 Everything is an approximation.

Lex Fridman 16:20 And you're trying to get as close as possible.

Andrew Strominger 16:23 Yeah.

Lex Fridman 16:23 So you're saying, you're saying objective truth doesn't exist in this world?

Andrew Strominger 16:27 No, I-

Lex Fridman 16:27 The, the internet's gonna be very mad at you.

Andrew Strominger 16:29 I, I, we could discuss that, but that's a different-

Lex Fridman 16:31 Okay.

Andrew Strominger 16:31 ... that's a different thing.

Lex Fridman 16:32 All right.

Andrew Strominger 16:33 Um, 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 Le Verrier in 1859, and he compared theory and experiment, and he found out that the perihelion precess moves around the f- the sun once every 233 centuries instead of every 231 centuries.

Lex Fridman 17:29 Mm-hmm.

Andrew Strominger 17:30 Okay, 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 it, what good it was gonna be.

Lex Fridman 17:49 Mm-hmm.

Andrew Strominger 17:53 So that's how small... That, that was the first sign that there was something wrong with Newton.

Lex Fridman 17:58 Yeah.

Andrew Strominger 18:00 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, um-

Lex Fridman 18:17 So, so the corrections are, for, uh, Maxwell's equations is when you get super tiny, and then the corrections for the-Um, for Newton's, uh, law of gravity is when you get super big. That- that- th- that's when you acquire corrections.

Andrew Strominger 18:34 That's true, but you- 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. Uh, the quantum mechanics, if- if you, if you, if you didn't have quantum mechanics, the electron would s- would spiral into the proton and the atom would collapse. It's quantum, you know, so that's a huge correction there.

Lex Fridman 19:04 Sure.

Andrew Strominger 19:05 So every theory gets corrected as we learn more. Uh, there'd just be no reason to suppose that it should be otherwise.

Lex Fridman 19:14 How is this really to the singularity? Why the singularity

Narrator 19:16 Yeah.

Andrew Strominger 19:17 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.

Lex Fridman 19:34 Mm-hmm.

Andrew Strominger 19:35 It should involve quantum mechanics, quantum effects become important there, it's a small thing. And, um, we don't understand exactly what the theory is, but we know there's 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, uh, 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.

Lex Fridman 20:26 Mm-hmm.

Andrew Strominger 20:26 Maxwell's electromagnetism had its demise. There was, uh, regimes in which quantum effects greatly modified the equations. But general relativity, all on its own, found, drew a- a system which originally was fine, would perversely wander off into a configuration in which Einstein's equations no longer applied.

Lex Fridman 21:00 So to you, the edges of the theory are wonderful. The failures of the-

Andrew Strominger 21:03 Edges are wonderful because that keeps, that keeps us in business.

Lex Fridman 21:09 So that one- one of the things you said I think in your TED Talk that, uh, the- the- the fact that quantum mechanics and, uh, and relativity don't describe everything and then they clash i- is wonderful. Right? I forget the adjective you used, but it was something like this. So why is that? Uh, why- why is that interesting? Do you, in that same way that there's-

Andrew Strominger 21:30 Oh, my God.

Lex Fridman 21:30 ... contradictions that-

Andrew Strominger 21:31 Oh, my God. It-

Lex Fridman 21:31 ... create discovery?

Andrew Strominger 21:32 There's no question in my mind-

Lex Fridman 21:34 Yeah.

Andrew Strominger 21:35 ... of course, many people would disagree with me, that now is the most wonderful time-

Lex Fridman 21:43 (laughs) .

Andrew Strominger 21:43 ... to be a physicist.

Lex Fridman 21:45 Why is that?

Andrew Strominger 21:45 So- so people- people look back-

Lex Fridman 21:48 Yeah.

Andrew Strominger 21:48 ... at- at, um, it's a classical thing to say among physicists, "I wish it were 1920."

Lex Fridman 21:58 Right.

Andrew Strominger 21:59 Quantum mechanics had been just understood, uh, there was the periodic table, there was, but in fact that was such a rich thing, um, that, uh, well, so that what, a lot of exciting stuff happened around 1920.

Lex Fridman 22:20 Sure.

Andrew Strominger 22:20 It took, it took the whole, it took a whole century to sort out the new insights that we got.

Lex Fridman 22:27 Especially adding some experimental stuff into the bu- into the bunch, actually making observations and creating all the data.

Andrew Strominger 22:33 Adding experimental thing, yeah.

Lex Fridman 22:34 Oh, the computers also help with visualizations and all that kind of stuff.

Andrew Strominger 22:37 Yeah, yeah, yeah. It was all 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's a contradiction if anything goes faster than the speed of light. But Newton's theory of gravity, the, uh, gravitational field, the gravitational force is instantaneously transmitted across the entire universe. So you could, you know, if you had a, uh, 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 in Morse code and they could-

Lex Fridman 23:43 Mm-hmm.

Andrew Strominger 23:44 ... 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 le- 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-You know, they, we just, we understood something wrong, we made a mistake, we set up our equations in the wrong way-

Lex Fridman 24:35 Oh, okay.

Andrew Strominger 24:35 ... we didn't translate the formalisms.

Lex Fridman 24:37 As opposed to revealing some deep mystery that's yet to be uncovered.

Andrew Strominger 24:40 Yeah. Yeah. Yeah. And so we never, we're never very sure which are, which are the really important ones.

Lex Fridman 24:47 But to you, the difference between quantum mechanics and general relativity seem, the, the tension, the contradiction there seems to hint at some deeper, deeper thing that's going to be discovered in this century.

Andrew Strominger 24:58 Yes. Because that one has been understood since the '50s. Pauli was the, uh, first person to notice it. And Hawking, in the early '70s, gave it a really much more visceral form. Um, 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, uh, all kinds of interesting relations between quantum systems and, and, uh, applications to other fields and, and so on.

Lex Fridman 25:42 Well, let's actually jump around.

Andrew Strominger 25:44 Okay.

Lex Fridman 25:44 So we'll return to black holes, I have a million questions there, but let's-

Andrew Strominger 25:47 Okay.

Lex Fridman 25:47 ... let, let's go into this unification, uh, 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's quantum gravity? Uh, what are all the different unification efforts?

Andrew Strominger 26:10 Okay. So-

Lex Fridman 26:11 Again, five questions.

Andrew Strominger 26:12 Yeah.

Lex Fridman 26:13 (laughs)

Andrew Strominger 26:14 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 16 decimal place. We have that stenciled on the door where, where I, where I work, you know? It's a, it's an amazing, it's an amazing feat of the human mind. It describes, um, 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's the strong interaction, uh, which binds the quarks into protons, and the forces between them are mediated by something called Yang-Mills Theory, which is a, a, a beautiful mathematical generalization of electromagnetism in which the analogs of the photons in, themselves carry charge. And, um, so this, uh, 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 over a decade ago.

Lex Fridman 27:56 Oh, Higgs was already a decade ago?

Andrew Strominger 27:58 I think it is, yeah.

Lex Fridman 27:59 Wow. Time flies when you're having fun.

Andrew Strominger 28:01 But you better check me on that. Yeah.

Lex Fridman 28:03 (laughs) It's, it's, hey, that's true, but so much fun has been happening.

Andrew Strominger 28:06 So much fun has been happening.

Lex Fridman 28:08 (sighs)

Andrew Strominger 28:08 And so that's all, um, 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, uh, complete theory, and we don't expect anything very new conceptually in the completion of that.

Lex Fridman 28:49 A- anything contradictory, by new. 'Cause can't you-

Andrew Strominger 28:53 Anything contradictory, yeah.

Lex Fridman 28:54 I, I'll have some wild questions, uh, for you-

Andrew Strominger 28:57 Yeah.

Lex Fridman 28:57 ... on that front. But yeah, anything that, yeah, 'cause there's no gaps. It's so accurate, it's so precise in its predictions, it's hard to imagine something

Narrator 29:04 coming through.

Andrew Strominger 29:05 Yeah. Yeah. Yeah. And it was all based on something called reno- let me not explain what it is, let me just throw out the buzzword.

Lex Fridman 29:13 Uh-huh. (laughs)

Andrew Strominger 29:14 Renormalizable quantum field theory. They all fall in the category of renormalizable quantum field theory.

Lex Fridman 29:22 I'm gonna throw that at a bar later to impress-

Andrew Strominger 29:25 (laughs)

Lex Fridman 29:26 ... impress the girls.

Andrew Strominger 29:27 (laughs) Good luck.

Lex Fridman 29:29 Thank you.

Andrew Strominger 29:31 (laughs)

Lex Fridman 29:31 (laughs) All right. So, uh-

Andrew Strominger 29:34 They, they all fa- they all fall under that rubric.

Lex Fridman 29:36 Mm-hmm.

Andrew Strominger 29:37 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.

Lex Fridman 29:56 What is the effort of quantum gravity? What are the different efforts to, um, to, to have these two dance together, uh, effectively, to try to unify, uh, the standard model and, um, and general relativity, any kind of model of gravity?

Andrew Strominger 30:13 Sort of the one fully, uh, consistent model that we have that, uh, reconciles the, it, it would, that sort of tames gravity and reconciles it with quantum mechanics, uh, 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, um, is a consistent reconciliation of quantum mechanics and general relativity, and moreover one which, um, which is able to incorporate particles and forces like the ones we s- 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- a reasonable, uh, possibility that we would be able to do an experiment in the foreseeable future which would be sort of a yes or no to, uh, to string theory.

Lex Fridman 31:40 Okay, so you've been there from the early days of string theory.

Andrew Strominger 31:42 Right.

Lex Fridman 31:42 You've seen its developments. What are some interesting developments? Uh, what do you see as the, also the future of string theory? And what is string theory?

Andrew Strominger 31:53 Well, the basic idea which emerged in the early '70s was that if you, uh, you take, uh, 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.

Lex Fridman 32:22 What do you mean by softer?

Andrew Strominger 32:24 Well, you know, if you hit a particle, (laughs) if there were a particle on this table, a big one, and you hit it-

Lex Fridman 32:31 Yeah.

Andrew Strominger 32:31 ... you might bruise yourself.

Lex Fridman 32:32 Sure.

Andrew Strominger 32:33 (laughs) But if there was a string on the table, you would probably just push it around.

Lex Fridman 32:37 Mm-hmm.

Andrew Strominger 32:38 And- and the- the source of the infinities in quantum field theories that when particles hit each other, it's a little bit of a, it's a little bit of a- a- a jarring effect. And- and, um, (laughs) , 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- there are some very, um, compelling mathematical equations that describe it. Now, what was realized in the early '70s is that if you replace particles by strings, these infinities go away and you get a, uh, consistent theory of gravity without the infinities.

Lex Fridman 33:39 Mm-hmm.

Andrew Strominger 33:40 And, um, that may sound a little trivial, but at that point it had already been 15 years that people had been searching around for any kind of theory that could do this. And it was actually found kind of, uh, by accident. And there are a lot of accidental discoveries, uh, 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 is-

Lex Fridman 34:39 Ooh, another term for the bar later tonight.

Andrew Strominger 34:41 Uh, yeah, yeah.

Lex Fridman 34:43 Parity violation. Okay.

Andrew Strominger 34:44 So- so if you go to the bar and... (laughs)

Lex Fridman 34:46 I already got the renormalizable quantum field theory.

Andrew Strominger 34:48 And you look in the mirror across the bar-

Lex Fridman 34:51 Yes.

Andrew Strominger 34:51 ... the universe that you see in the mirror is not identical, you would be able to tell, if you show your- your- your- your- your-

Lex Fridman 35:02 Which one?

Andrew Strominger 35:02 ... the lady in the bar-

Lex Fridman 35:04 Yeah.

Andrew Strominger 35:04 ... the ph- a photograph that shows both the mirror and you-

Lex Fridman 35:07 There's a difference.

Andrew Strominger 35:07 If she's smart enough, she'll be able to- to tell which one is the real world-

Lex Fridman 35:13 Ooh.

Andrew Strominger 35:13 ... and which one is you. Now, she would have to do some very precise measurements.

Lex Fridman 35:18 Yes. (laughs)

Andrew Strominger 35:18 (laughs)

Lex Fridman 35:19 Uh, I'm-

Andrew Strominger 35:19 And if the photograph was too grainy, it might not be possible. But in principle-

Lex Fridman 35:23 So wh-

Andrew Strominger 35:23 ... it's possible.

Lex Fridman 35:24 Why is this interesting? Why is it, does this mean that there is some imp- um, not perfect determinism? Or, uh, what does that mean? There's some uncertainty?

Andrew Strominger 35:32 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, um, then it was learned in the mid '80s that not only could it tolerate that, but if you did things in the right way, you could construct a world ou- involving strings that reconciled quantum mechanics and general r- 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 there, 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's some kind of direct experiment. And I don't even believe it myself.

Lex Fridman 36:53 Sure. Which is a good place to be mentally as a physicist, right? Always, I mean, Einstein didn't believe his own, uh, equations, right? With the black hole. Okay.

Andrew Strominger 37:01 Well, then when he was wrong about that. (laughs) He was wrong about that.

Lex Fridman 37:05 But you might be wrong too, right? Uh, so, uh, do you think string theory is dead, if you were to bet all your money on, um-

Andrew Strominger 37:13 No.

Lex Fridman 37:13 ... the future of string theory?

Andrew Strominger 37:15 I, I think it's a, 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, uh, 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 '50s, 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. (laughs) And it had a kind of inevitability. You know, they had some of the right pieces, but not the other ones.

Lex Fridman 38:18 Sure.

Andrew Strominger 38:19 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 gonna find new wrinkles in string theory and technically what we call string theory now isn't quite right, but-

Lex Fridman 39:11 We're always going to be wrong, but hopefully a little bit less wrong every time.

Andrew Strominger 39:16 Exactly. And I, I would, you know, bet the farm, as they say-

Lex Fridman 39:20 Do you have a farm?

Andrew Strominger 39:22 (laughs) I, you know, I say that much more seriously because not only do I have a farm, but we just renovated it. (laughs)

Lex Fridman 39:28 (laughs)

Andrew Strominger 39:29 So, so before I renovate-

Lex Fridman 39:31 A very serious statement.

Andrew Strominger 39:31 So before I renovate it, betting it, the farm, my wife and I spent five years renovating it before I... (laughs)

Lex Fridman 39:37 You were much, much looser with that statement, but now it really means something.

Andrew Strominger 39:40 Now, now, now it really means something. And, and, (laughs) I would bet the farm on the, um, on the, uh, 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 infinities problem, and then we later learned that it also solved Hawking's puzzle about what's inside of a black hole. And y- you put in one assumption, you get five things out, y- somehow, you're doing something right.

Lex Fridman 40:30 Mm-hmm.

Andrew Strominger 40:30 You know? Probably not everything, but y- you're, you're, you're, you know, there's some good signposts. And there, there have been a lot of good signposts like that.

Lex Fridman 40:38 It is also a mathematical toolkit, and you've used it.

Andrew Strominger 40:42 Yeah.

Lex Fridman 40:42 You've used it with Camran Vafa.

Lex Fridman 40:44 Uh, maybe we can sneak our way back from string theory into black holes.

Andrew Strominger 40:48 Yeah, yeah.

Lex Fridman 40:49 Um, what was the idea that you and Camran Vafa developed with the holographic principle in string theory? What were you able to discover through s- through this- through string theory about, uh, black holes? Or, um, that connects us back to the reality of black holes?

Andrew Strominger 41:05 Yeah. So that is a very interesting story. I was interested in black holes before I was interested in string theory. I was-

Lex Fridman 41:15 Sure.

Andrew Strominger 41:15 ... sort of a reluctant s- string theorist in the beginning. I thought I had to learn it 'cause 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 dutiful way. Th- these people say they've claimed quantum gravity, I ought to read their papers at least. And then, (laughs) the more I read them, the more interested I got, and I began to see, you know, they, they phrased it in a very clumsy way. The description of string theory was, was very clumsy and-

Lex Fridman 41:46 Mathematically clumsy or just the interpretation?

Andrew Strominger 41:48 Mathematically clumsy.

Lex Fridman 41:48 Okay, got it.

Andrew Strominger 41:50 It was all correct, but, but mathematically clumsy. But it often happens that in all kinds of branches of physics that, um, 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 interrelationships, 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, um... So our understanding of string theory in, uh, in 1985 was almost all about, uh, you know-...weakly coupled waves of strings colliding and so on. We didn't know how to describe a big thing like a black hole. And so, you know, in string theory. Of course, we could show that strings in s- 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, um... before my work with Cumrun, we couldn't do any better than Einstein and Schwarzschild had done. Now, um, one of the puzzles, um, you know, if you look at the, uh, Hawking's headstone and also Boltzmann's headstone and you put them together, you get a formula for their really central equations in 20th century physics. I don't think there are many equations that made it to headstones. (laughs)

Lex Fridman 43:51 (laughs)

Andrew Strominger 43:51 And, (laughs) 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 Schwarzschild's description, the black hole is literally a hole in space and there's no place to store the d- the gigabytes. And it's not too hard to... and this really was Wheeler and Bekenstein and... W- Wheeler, Bekenstein, 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 m- mechanics and gravity can't be consistent.

Lex Fridman 44:41 We gotta, we gotta go there a little bit. So, uh, so how is it possible? Uh, when we say gigabytes, so 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 s- super homogeneous and boring, how is that actually able to store information? Where does it store information? On the inside? On the surface?

Andrew Strominger 45:01 Yeah.

Lex Fridman 45:02 Uh, where? Where's-

Andrew Strominger 45:03 Yeah.

Lex Fridman 45:04 And what's information? I'm liking this ask five questions to see which one you actually answer.

Andrew Strominger 45:10 Oh, okay. So if you say that-

Lex Fridman 45:11 I'm gonna ask you a bunch of-

Andrew Strominger 45:11 ...I should try to memorize them and answer each one in order? Just answer them-

Lex Fridman 45:15 No, I don't know. I don't know what I'm doing.

Andrew Strominger 45:16 Oh, okay. (laughs)

Lex Fridman 45:16 I'm desperately, desperately, uh, trying to figure it out-

Andrew Strominger 45:20 Okay.

Lex Fridman 45:20 ...as we go along here.

Andrew Strominger 45:21 So, um, Einstein's black hole, the Schwarzschild's black hole, they can't store information. The stuff, stuff goes in there and it just keeps flying and it goes in a 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 Bekenstein really did was they showed that physics is inconsistent unless a black hole can store an... a number of gigabytes proportional to its area divided by 4 times Newton's constant times Planck's constant.

Lex Fridman 46:28 And that's another wild idea. You said area, not volume.

Andrew Strominger 46:32 Exactly. And that's the holographic principle.

Lex Fridman 46:35 The universe is so weird. (laughs)

Andrew Strominger 46:37 And that's the holographic principle.

Lex Fridman 46:38 That's called the holographic principle, that it's, it's the area. W- uh, we're just jumping around. What is the holographic principle? What does that mean? W- is there some kinda weird projection going on? What, what the heck, uh, have to solve it?

Andrew Strominger 46:50 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." (laughs)

Lex Fridman 47:03 (laughs)

Andrew Strominger 47:04 Blah, blah, blah, blah, blah.

Lex Fridman 47:05 Nice.

Andrew Strominger 47:05 So nobody knows exactly what it is. But roughly speaking, it says just what we were alluding to, that, um, really all the information that is in some volume of spacetime can be stored on the boundary of that region.

Lex Fridman 47:26 So this is not just about black holes, it's about any, any area of spacetime.

Andrew Strominger 47:29 Any area of space. However, we've made sense of the holographic principle for black holes. We've made sense of the holographic principle for something which could be called anti-de Sitter space, which could be thought of as a giant... as a black hole turned into a whole universe. And, um, we don't really understand how to talk about the holographic principle for either flat space, which we appear to live in, or asymptotically de Sitter space, which the astronomers tell us we actually live in as the universe continues to expand. So it's one of the, one of the huge problems in, uh, physics is to, you know, apply or even formulate the holographic principle for more realistic... well, black holes are realistic, we see them, but, um, yeah, in, in more general context. So-

Lex Fridman 48:38 Give a more-

Andrew Strominger 48:38 ...a more general statement of the holographic principle.

Lex Fridman 48:41 What's the difference between flat space and, uh, asymptotic de Sitter space? So flat space is just an approximation of, like, the world we live in. So, like, uh, uh, de Sitter space...... asymptotic, I wonder what that even means. Meaning, like, uh, asymptotic over what?

Andrew Strominger 48:57 Okay. So for thousands of years, you know, until the last half of the 20th, well, sorry, until the 20th century, um, we thought spacetime was flat.

Lex Fridman 49:10 Can you elaborate on flat? What, what do we mean by flat?

Andrew Strominger 49:15 Well, like the surface of this table-

Lex Fridman 49:18 Hmm.

Andrew Strominger 49:19 ... is, is flat. Let me just give an intuitive explanation. Surface of a table is flat, but the surface of a basketball is curved. So the universe itself could be flat, like the surface of a table, or it could be curved like a basketball, which actually has a positive curvature, and then there's another kind of curvature called a negative curvature. And curvature can be even weirder, because that kind of curvature I've just described is the curvature of space, but Einstein taught us that we really live in a spacetime continuum, so we can have curvature in a way that mixes up space and time. And that's kind of hard to visualize.

Lex Fridman 50:07 'Cause you have to step, what, a couple of dimensions up? So it's hard to...

Andrew Strominger 50:12 You have to step a couple... But even a, if you have flat space and it's expanding in time, you know, we could imagine we're sitting here, this room, good approximation it's flat, but imagine we suddenly start getting further and further apart.

Lex Fridman 50:30 Mm-hmm.

Andrew Strominger 50:31 Then space is flat, but it's expanding, which means that spacetime is curved.

Lex Fridman 50:39 Ultimately it's about spacetime. Okay, so, well, what's the, what's De Sitter and anti-De Sitter space?

Andrew Strominger 50:45 The three simplest spacetimes are flat spacetime, which we call Minkowski spacetime, and negatively curved spacetime, anti-De Sitter space, and positively curved spacetime, De Sitter space. And so astronomers, um, think that on large scales, even though for thousands of years we hadn't noticed it, beginning with Hubble, we started to notice that spacetime was curved. Space is expanding in time means that spacetime is curved.

Lex Fridman 51:24 Mm-hmm.

Andrew Strominger 51:26 And the nature of this curvature is affected by the matter in it, because matter itself causes the curvature of spacetime. But as it expands, the matter gets more and more diluted. And one might ask, when it's all diluted away, is spacetime still curved? And astronomers believe they've done precise enough measurements to determine this, and they believe that the answer is yes. The universe is now expanding. Eventually, all the uni- matter in it will be, uh, expanded away, but it will continue to expand because, uh, well, they would call it the dark energy. Einstein would call it a cosmological constant. In any case, it, the, the, in the far future, matter will be expanded away and we'll be left with empty De Sitter space.

Lex Fridman 52:27 Okay, so there's this cosmological, Einstein's cosmological constant that now hides this thing that we don't understand called dark energy. What's dark energy? What's your best guess at what this thing is? Why do we think it's there? It's because of the, it comes from the astronomers.

Andrew Strominger 52:44 Dark energy is synonymous with positive co- cosmological constant. And, um, uh, we think it's there because the astronomers have told us it's there. And, um, they, they know what they're doing.

Lex Fridman 53:03 And we don't know what the heck it is.

Andrew Strominger 53:04 Uh, it's a really, really hard measurement, but they know, they really know what they're doing, and we have no friggin' idea why it's there. Another big mystery. Another f- another reason it's fun to be a physicist. And if it is there, why should it be so small? Why should there be so little? Why should it have hid itself from us? Why shouldn't there enough, be enough of it to substantially cons- curve the space between us and the moon? Why did there have to be such a small amount that only the crazy best astronomers in the world could find it?

Lex Fridman 53:43 Well, can't the same thing be said about all, all of the constants? All of the... Can't that be said about gravity? Can't that be said about the speed of light?

Lex Fridman 53:53 Like, why is the speed of light so slow?

Andrew Strominger 53:56 So fast?

Lex Fridman 53:57 So slow. Relative to the size of the universe, can't it be faster? (laughs) Or no? Or is-

Andrew Strominger 54:04 Well, the speed of light is a funny one, because you could always choose units in which the speed of light is one. You know, we measure-

Lex Fridman 54:13 Sure.

Andrew Strominger 54:13 ... it in kilometers per second and it's 186,000, or miles per second, it's 186,000 miles per second. And but if we had used different units-

Lex Fridman 54:25 Yeah.

Andrew Strominger 54:26 ... then we could make it one. But you can make dimensionless ratios. So, um, you know, you could say, why is the timescale set by the expansion of the universe so large compared to the timescale of a human life, or-

Lex Fridman 54:43 Yeah.

Andrew Strominger 54:43 ... so large compared to the timescale for a neutron to decay? You know?

Lex Fridman 54:48 It's, yeah, yeah. I mean, ultimately-

Andrew Strominger 54:49 And-

Lex Fridman 54:49 ... the reference frame, the temporal reference frame here is a human life.

Andrew Strominger 54:53 Maybe. (laughs)

Lex Fridman 54:54 Isn't that the important thing for us, uh, descendants of apes? Isn't that a really important aspect of physics?Like, uh, because we kind of experience the world, we intuit the world-

Andrew Strominger 55:05 Yeah.

Lex Fridman 55:05 ... through the eyes of this, th- the, these biological organisms. I mean-

Andrew Strominger 55:10 Absolutely.

Lex Fridman 55:11 ... I guess mathematics helps you s- escape that for a time, but ultimately isn't that how you wonder about the world?

Andrew Strominger 55:18 Absolutely.

Lex Fridman 55:19 That, like, a human life-

Andrew Strominger 55:20 Yeah.

Lex Fridman 55:20 ... time is only 100 years? 'Cause if you think of everything, um, if you're able to think in, I don't know, in billions of years, uh, then maybe everything looks way different. Maybe universes, uh, are born and die and maybe all these, uh, physical phenomena become much more intuitive that we see at the grand scale of general relativity.

Andrew Strominger 55:44 Well, that is one of the, little off the track here, but that certainly is one of the nice things about being a physicist is you spend a lot of time thinking about, you know, insides of black holes and billions of years in the future, and, and it sort of, uh, gets you away from the day-to-day, uh, into, into another fantastic realm. Um, but I was answering your question about how there could be information in a black hole.

Lex Fridman 56:15 Yes.

Andrew Strominger 56:17 So Einstein only gave us an approximate description, and we now have a theory that corrects it, string theory. And now sort of was the moment of truth. Well, when we first discovered string theory, we knew, we knew from the get-go that string theory would correct what Einstein said, just like Einstein corrected what Newton said. Um, but we didn't understand it well enough to actually compute the correction, to compute how many gigabytes there were. And sometime in the early '90s, we began to understand the mathematics of string theory better and better. And it came to the point where it was clear that this was something we might be able to compute, and it was a kind of moment of truth for string theory because if it hadn't given the answer that Bekenstein and Hawking said it had to give for consistency, string theory itself would've been inconsistent and we wouldn't be doing this interview.

Lex Fridman 57:34 Mm-hmm. Well, (laughs) that's a very dramatic statement, but yes. Uh-

Andrew Strominger 57:40 (laughs) That's not the most, that's not the most dramatic thing to say. (laughs)

Lex Fridman 57:44 (laughs) I mean, I mean, but like, okay, that's very life and death. You mean like-

Andrew Strominger 57:48 (laughs)

Lex Fridman 57:49 ... that, that, that, uh, be- because string theory was central to your work at that time. Is, is that what you mean? (laughs)

Andrew Strominger 57:54 Well, string theory would've been inconsistent.

Lex Fridman 57:57 Yeah, okay. So, that'd be a-

Andrew Strominger 57:58 S- string theory would have been inconsistent and-

Lex Fridman 57:59 But those inconsistencies can give birth to other theories, like you said.

Andrew Strominger 58:03 The inconsistency... Right, something else could have happened.

Lex Fridman 58:05 Yes.

Andrew Strominger 58:05 Yeah. It would have been a major, a major, uh, um, change in the way we think about string theory if it... And it was a good thing that, you know, one supposition that the world is made of strings solves two problems, not, not one. It solves the infinity problem and it solved the Hawking's problem. And also the way that it did it was very, uh, was very beautiful. Um, it, it gave an alternate description. So alternate description thing, of things are, are, uh, are very common. I mean, we could, to take a simple example, this bottle of water here is 90% full. I could say it's 90% full. I could also say it's 10% empty. Those are obviously the same statement. And they're, it's trivial to see that they're the same. But there are many statements that can be made in mathematics and mathematical physics that are equivalent but might take years to understand that they're equivalent, and might take the invention or discovery of whole new fields of mathematics to prove they're equivalent. And this was one of those. We found an alternate description of the b- certain black holes in string theory which we could prove was equivalent, and it was a description of the black hole as a hologram that can be thought of, a holographic plate, uh, that could be thought of as sitting on the surface of the black hole. And the interior of the black hole itself sort of arises as a projection, uh, or the near horizon region of the black hole arises as a, as a projection of that holographic plate. So the two descriptions were the hologram, the three-dimensional image, and the holographic plate. And the hologram is what Einstein discovered, and the holographic plate is what we discovered. And this idea that you could describe things very, very concretely in string theory in these two different languages, of course, took off and was applied to many, uh, many different, many different contexts, uh, within string,

Andrew Strominger 1:00:40 string theory.

Lex Fridman 1:00:41 So you mentioned, uh, the infinity problem and the Hawking problem. Uh, which Hawking problem? The, the, that the black hole destroys information or that the, wh- wh- what, what, which Hawking problem are we talking about?

Andrew Strominger 1:00:51 Well, there's really two Hawking problems. They're very closely related. Um, one is how does the black hole store the information-

Lex Fridman 1:01:01 Yes.

Andrew Strominger 1:01:02 ... for which we have no answer. Uh-And, um, that is the one that we solved in some cases. So it's sort of like, um, you know, your, your smartphone. How does it store its 64 gigabytes? Well, you rip the cover off and you count the chips, and there's 64 of them, each with a gigabyte-

Lex Fridman 1:01:26 Mm-hmm.

Andrew Strominger 1:01:26 ...and you know there's 64 gigabytes. But that does not solve the problem of how you get information in and out of your smartphone. You have to understand a lot more about the WiFi, and the internet, and the, uh, cellular. And, and-

Lex Fridman 1:01:44 That's where Hawking radiation, this prediction, it starts to come into play?

Andrew Strominger 1:01:47 That's where Hawking radiation comes in. And that problem, of how the information gets in and out... You can't, you couldn't have explained how information gets in and out of an iPhone without first explaining how it's stored in the first place.

Lex Fridman 1:02:04 Yeah. So just to clarify, the storage is on the plate?

Andrew Strominger 1:02:08 Is on the plate.

Lex Fridman 1:02:09 On, on the, on the holographic plate, and then it projects somehow inside the-

Andrew Strominger 1:02:13 The, the bulk, the, the, the spacetime is the hologram.

Lex Fridman 1:02:18 The hologram. Man, I mean, d- do you have an intuitive... When you sit late at night and you stare out, out at the stars, do you have an intuitive understanding of what a holographic plate is?

Andrew Strominger 1:02:30 Um-

Lex Fridman 1:02:31 Like that there's two dimension, you know, projections that store information?

Andrew Strominger 1:02:37 How a black hole could store information on a holographic plate, I think we do understand, in, in great mathematical detail and also intuitively. And it's very much like an ordinary hologram, where you hol-, have a holographic plate and you sh-, it contains all the information. You shine a light through it and you get an image which looks three-dimensional.

Lex Fridman 1:03:05 Yeah, but why should there be a holographic plate? Like, wh-

Andrew Strominger 1:03:11 Why should there be?

Lex Fridman 1:03:12 Yeah. Why? (chuckles)

Andrew Strominger 1:03:14 That is the great thing about being a theoretical physicist, is anybody can very quickly stump you with a, going to the next level of whys.

Lex Fridman 1:03:26 Yeah, like-

Andrew Strominger 1:03:27 So, so if you're asking-

Lex Fridman 1:03:27 ... the whys have got you going, "I can just keep asking," yeah.

Andrew Strominger 1:03:29 Yeah, you could just keep asking and, and it won't take you very long to... So the trick in being a theore- uh, theoretical physics is finding the questions that you can answer.

Lex Fridman 1:03:41 Sure.

Andrew Strominger 1:03:42 So, so the questions that we think we might be able to answer now, and we've partially answered, is that, um, there is a holographic explanation for certain c- kinds of things in string theory.

Lex Fridman 1:03:59 Sure.

Andrew Strominger 1:04:00 We've answered that. Now, we'd like to take what we've learned, and that's what I've mostly been doing for the last 15, 20 years. I haven't really been working so much on string theory proper. I've been sort of taking the lessons that you, we learned in string theory and trying to apply them to the real world using only, assuming only what we know for sure about the real world.

Lex Fridman 1:04:28 Uh, so on this, uh, topic, you, you co-wrote, co-authored a paper with Stephen Hawking called Soft Hair on Black Holes.

Andrew Strominger 1:04:36 Yes.

Lex Fridman 1:04:36 That makes the argument against Hawking's original prediction that black holes destroy information. Can you explain this paper?

Andrew Strominger 1:04:43 Yes.

Lex Fridman 1:04:43 And the title? (laughs)

Andrew Strominger 1:04:44 (laughs) Okay, so, um, first of all, um, uh, the hair on black holes is a word that was coined by the greatest phrase master in the history of physics, John Wheeler. Invented the word black hole.

Lex Fridman 1:05:02 Mm-hmm.

Andrew Strominger 1:05:03 And he also said that, uh, he made the statement that black holes have no hair.

Lex Fridman 1:05:11 Mm-hmm.

Andrew Strominger 1:05:12 That is, every black hole in the universe, uh, is described just by its mass and spin. Uh, they ro-, they can also rotate, as was later shown by Kerr.

Lex Fridman 1:05:26 Mm-hmm.

Andrew Strominger 1:05:27 And, um, and this is very much unlike a star, right? Every star of the same mass is different in a multitude of different ways. Um, different chemical compositions, different motions of the individual molecules. Every star in the universe, even of the same mass, is different in many, many different ways. Black holes are all the same. And that means when you throw something... In Einstein's description of them.

Lex Fridman 1:06:05 Mm-hmm.

Andrew Strominger 1:06:06 Which we think must be corrected. And, um, if you throw something into a black hole, it gets sucked in, and if you s-, uh, th- throw in a red book or a blue book, the black hole gets a little bigger, but there's no way, within Einstein's theory, of telling, uh, how they're different. And that was one of the assumptions that Hawking made in his 1974, '75 papers in which he concluded that black holes destroy information. You can throw encyclopedias, thesis defenses, the Library of Congress.

Lex Fridman 1:06:54 It doesn't matter, it's going to, uh, behave exactly the same uniform way.

Andrew Strominger 1:06:58 Yeah. So what, what Hawking and I showed, and, uh, also Malcolm Perry, um-... is that, um, one has to be very careful about what happens at the boundary of the black hole. And this gets back to something I mentioned earlier about when two things which are related by a coordinate transformation are and are not equivalent. And, um, and what we showed is that they're very subtle imprints when you throw something into a black hole. There are very subtle imprints left on the horizon of the black hole which you can read off at least partially what went in. And, um, so this invalidates, uh, Stephen's original, uh, argument that the information is destroyed.

Lex Fridman 1:08:03 And that's the soft hair, those- those imprints.

Andrew Strominger 1:08:05 That's the soft hair, right. So, and soft is a word that is used in physics for things which have very low energy, and these things actually carry no energy. There are things in the universe which carry no energy.

Lex Fridman 1:08:20 You said, I think to, to Sean Carroll... Um, by the way, everyone should go check out Sean Carroll's Mindscape podcast, it's incredible. And Sean Carroll is an incredible person. I think you said there or maybe in a paper, I have a quote, you said that, "A soft particle is a particle that has zero energy," just like you said now. "And when the energy goes to zero, because the energy is proportionate to the wavelength, it's also spread over an infinitely large distance. If you like, it's spread over the whole universe. It somehow runs off (laughs) to the boundary. What we learned from that is that if you add a zero-energy particle to the vacuum, you get a new state. And so there are infinitely many vacua..." plural for vacuum-

Andrew Strominger 1:09:05 Right.

Lex Fridman 1:09:06 ... which can be thought of as being different from one another by the addition of soft photons or soft gravitons."

Andrew Strominger 1:09:13 Right.

Lex Fridman 1:09:13 Can you, uh, elaborate on this wild idea, "If you like, it spreads over the whole universe. When the energy goes to zero, because the energy is proportionate to the wavelength, it also spreads over an infinitely large distance. If you like, it's spread over the whole uni- it's spread over the whole universe." What, um, can, c- can you explain these soft gravitons and photons?

Andrew Strominger 1:09:37 Yeah. So the soft gravitons and photons, um, have been, uh, known about since the '60s. But exactly what we're supposed to do with them or how we're supposed to think about them, um, I- I think has been well-understood o- only recently. And in quantum mechanics, the energy of a particle is proportional to Planck's constant times its wavelength. So when the energy goes to zero, the wavelength gets, goes to infinity.

Lex Fridman 1:10:17 Mm-hmm.

Andrew Strominger 1:10:17 Now, if something has, uh, zero energy and it's spread all over the universe, in what sense is it actually there? That's-

Lex Fridman 1:10:30 Yeah.

Andrew Strominger 1:10:30 ... that's been the confusing thing, uh, to make a precise statement about when something is and isn't there. Now, the simplest way of seeing... So people might have taken the point of view that if it has zero energy and is spread all over the universe, it's not there, we can ignore it.

Lex Fridman 1:10:56 Mm-hmm.

Andrew Strominger 1:10:57 Um, but if you do this, you'll get into trouble. And one of the ways that you'll get into trouble is that even though it has zero energy, it doesn't have zero angular momentum. If it's a photon, it always has angular momentum one. If it's a graviton, it's, uh, angular momentum two. So you can't say that the state of the system with the zero-energy photon should be identified with the one without the zero-energy photon, that we can just ignore them, because then you will conclude that angular momentum is not conserved. And if angular momentum is not conserved, things won't be consistent. And, um, and of course you can have a lot of these things, and typically you do get a lot of them, and when you, you can actually do a calculation that shows that every time you scatter two particles, you create an infinite number of them.

Lex Fridman 1:12:04 Infinite number of the soft photons and gravitons?

Andrew Strominger 1:12:06 Of the zero-energy ones, yeah.

Lex Fridman 1:12:09 And so these are, and they're somehow everywhere, but they're-

Andrew Strominger 1:12:12 They're everywhere.

Lex Fridman 1:12:12 ... but they also contain information, or they're able to store information?

Andrew Strominger 1:12:16 And they're able to store information. They're able to store an arbitrary, large amount of information. So what we pointed out is... So what these things really do, one way of thinking of 'em, is they rush off to the edges of the universe.

Lex Fridman 1:12:31 (laughs)

Andrew Strominger 1:12:31 Spreading out all over the space is like saying they rush off to the energy, edge of the universe.

Lex Fridman 1:12:36 Right.

Andrew Strominger 1:12:37 And that includes, if the interior of the black hole is not considered part of the universe, that includes the edge of the black hole. So we need to set up our description of physics so that all the things that are conserved are still conserved in the way that we're describing them, and that will not be true if we ignore these things. We have to keep careful track of these things. And people had been sloppy about that, that. And we learned how to be very precise and careful about it.

Lex Fridman 1:13:11 And this, and when, once you're being precise, you can actually, uh, that makes, you can actually answer this kind of very problematic thing that Hawking suggested that black holes destroy information.

Andrew Strominger 1:13:21 Well, what we showed is that there's an error in the argument that, uh, all black holes are the same, because they hadn't kept track of these, uh, these very subtle things. And, um, whether or not this is the key error in the argument remains to be seen, or whether this is a technical point.

Lex Fridman 1:13:47 Yes. But it is an error.

Andrew Strominger 1:13:49 It is an error.

Lex Fridman 1:13:51 A- a- and, uh, Hawking obviously agreed with it.

Andrew Strominger 1:13:53 Hawking agreed with it, and he was sure that this was the, he was sure that this was, uh-

Lex Fridman 1:13:59 This was a critical error.

Andrew Strominger 1:14:00 ... that this was the critical error and that understanding this would, would, would, uh, would, would get us the whole story. And, and, and, and that could well be.

Lex Fridman 1:14:09 What was it like working with Stephen Hawking on this particular, uh, problem? Because it's kind of a whole journey, right?

Andrew Strominger 1:14:16 Well, you know, I, I, I, I love the guy. I, he's so passionate about physics. Uh, he just... yeah. His, his oneness with the problem and the, I mean, it's...

Lex Fridman 1:14:36 So his mind is all occupied by the world that's, uh-

Andrew Strominger 1:14:39 Yeah.

Lex Fridman 1:14:40 ... w- what, this world-

Andrew Strominger 1:14:40 And let me tell you, there's a lot of other things with his illness and with his celebrity and-

Lex Fridman 1:14:45 Yeah.

Andrew Strominger 1:14:46 ... a lot of other things.

Lex Fridman 1:14:49 A lot of distractions pulling at his-

Andrew Strominger 1:14:51 Yeah.

Lex Fridman 1:14:51 ... uh, at, at his mind, he's still there.

Andrew Strominger 1:14:54 That's right.

Lex Fridman 1:14:54 He's still right there.

Andrew Strominger 1:14:54 That's right. I remember him turning down tea with Lady Gaga so we could spend-

Lex Fridman 1:15:00 (laughs)

Andrew Strominger 1:15:00 ... another hour on our paper. (laughs)

Lex Fridman 1:15:04 (laughs) That, my friends, is dedication.

Andrew Strominger 1:15:08 (laughs)

Lex Fridman 1:15:08 Uh, what did you learn about physics, what did you learn about life, uh, from, uh, having worked, uh, with Stephen Hawking?

Andrew Strominger 1:15:15 Well, he was one of my great teachers. Of course, he's, he's older than me, and I was, I was reading his, um, his textbooks in, in, um, in graduate school. And, um, uh, you know, I learned a lot about, uh, relativity from him. I learned about passion for a problem. I, I learned about, um, not caring what other people think. You know, I mean, physics is an interesting culture. Even if you make a great discovery, like Hawking did, um, people don't believe everything you say.

Lex Fridman 1:15:58 Mm-hmm.

Andrew Strominger 1:15:58 In fact, uh, people love to disagree. (laughs) It's a, it's a, it's a, a culture that, uh, cherishes disagreement. And, and so, you know, he kept ahead with what he believed in, and sometimes he was right and sometimes he was wrong.

Lex Fridman 1:16:15 Do you feel pressure from the community? So, for example, with string theory, it was, it was very popular for a time. There's a bit of criticism, or it's less popular now. Do you feel the forces of the community as it moves in and out of different fields, or do you try to stay? Like how difficult is it to stay, uh, intellectually and, uh, mathematically independent from the community?

Andrew Strominger 1:16:39 Personally, uh, I'm lucky I'm well-equipped for that. I, when I started out in graduate school, the problem of quantum gravity was not considered interesting.

Lex Fridman 1:17:01 You still did it anyway.

Andrew Strominger 1:17:02 I still did it anyway. I, I'm, I'm a little bit of a contrarian, I guess.

Lex Fridman 1:17:06 Sure.

Andrew Strominger 1:17:06 And I think that has, has served me well. Um, and, uh, people are always sort of disagreeing with me. And they're usually right, but I'm right enough. (laughs)

Lex Fridman 1:17:22 And like you said, the contradiction ultimately paves the path of discovery.

Andrew Strominger 1:17:26 Yeah.

Lex Fridman 1:17:27 Uh, let me ask you, just on this tension, we've been dancing between physics and mathematics. Um, what to you is an interesting line you can draw between the two? Uh, you have done some very complicated mathematics in your life to explore the laws of nature. What's the difference between physics and mathematics to you?

Andrew Strominger 1:17:46 Well, um, I love math. I think my first love is, is physics, and the math that I've done, I've, I've done to, because it was needed.

Lex Fridman 1:17:59 Hmm. In service of physics.

Andrew Strominger 1:18:01 In, in service of physics, but then, of course, in the, in the heat of it, it has its own appeal and, uh-

Lex Fridman 1:18:09 (laughs) In the heat of it, I like it. (laughs) Sure.

Andrew Strominger 1:18:11 Uh, it has its ow- its, its own appeal, and I certainly enjoyed it. And ultimately, I would like to think, I wouldn't say I believe, but I would like, like to think that there's no difference between physics and mathematics, that all mathematics is realized in the physical world, and all physics has a firm mathematical basis, that, that they're really the same thing. I mean, why would there be math that had no physical manifestation? Uh, it seems a little odd, right? You have two kinds of math, some that are relevant to the real world and-

Lex Fridman 1:18:54 Well, they don't have to be contradictory. But you can have a n- can- can't you not have mathematical objects that are not at all connected to the physical world? So I mean, this is to the question of is math discovered or invented? So to you, math is, uh-

Andrew Strominger 1:19:07 Discovered.

Lex Fridman 1:19:08 ... discovered.

Andrew Strominger 1:19:08 Yeah.

Lex Fridman 1:19:08 And, and, and there's a deep linkage between the two.

Andrew Strominger 1:19:11 Yeah, yeah, yeah.

Lex Fridman 1:19:12 Uh, do you find it at all compelling these ideas of, like, something like Max Tegmark, where...... our universe is actually a fundamentally mathematical object, that math is, our universe is mathematical, fundamentally mathematical in nature.

Andrew Strominger 1:19:27 Uh, uh, my expertise as a phy- a physicist doesn't add anything to that, um-

Lex Fridman 1:19:36 (laughs)

Andrew Strominger 1:19:37 ... it, it's not really... You know, physics is, you know, I was all, once very interested in philosophy and, you know, physics. Physics, I like questions that can be answered, that it, it's not obvious what the answer is and that you can find a, an answer to the question and everybody will agree what the answer is and that there's a-

Lex Fridman 1:20:06 Yeah.

Andrew Strominger 1:20:06 ... an algorithm for, for getting there. Um, not that these other questions aren't interesting, um, and they don't somehow have a way of preventing- presenting themselves, but to me, the interesting thing is to, is, is motion in what we know, is learning more and understanding things that we didn't understand be- before. Things that seemed totally confusing, having them seem obvious, that's wonderful. So I think that's, those questions are there. I mean, I would even go further, you know, the whole multiverse, I don't, I don't think there's too much we, concrete we're ever gonna be able to say about it.

Lex Fridman 1:20:52 This, this is fascinating because you spend so much time on string theory, which w- is devoid from a connection to the physical world for a long time. Like, it, and not devoid, but it, it, it travels in a mathematical world that seems to be beautiful and consistent and seems to indicate, uh, that it could be a, a good model of the laws of nature. But there's, it's still traveling independently 'cause it's very difficult to experimentally verify.

Andrew Strominger 1:21:19 Well-

Lex Fridman 1:21:19 But there's a promise w- laden in it, in the same way multiverse or, uh, you could have a lot of kind of very far out there questions where your gut and instinct and intuition says that maybe in 50, 100, 200 years you'll be able to actually have strong e- uh, experimental validation, right?

Andrew Strominger 1:21:38 I think that with string theory, um, I don't think it's likely that we could measure it, but we could get lucky. In other words, just to take an example, about 10 or 20 years ago, it was thought that they had seen a string in the sky and it, that it was seen by, um, you know, uh, doubled stars that were gravitationally lensed around the gravitational field produced by some long string. There was a line of double lens star- Now the signal went away, okay? But people were hoping that they'd seen a string and it could be a fundamental string that had somehow gotten stretched and that would be some evidence for string theory.

Lex Fridman 1:22:25 Mm-hmm.

Andrew Strominger 1:22:26 There was also BICEP2, which it was a, the experiment was wrong, but it could have happened. It could have happened that we got lucky and this experiment was able to make direct measurements. Certainly would have been measurements of quantum gravity, if not string theory. So it's a logical, it's a very logical possibility-

Lex Fridman 1:22:51 Sure.

Andrew Strominger 1:22:51 ... that we could get experimental evidence from strings. That is a very different thing than saying, "Do this experiment, here's a billion dollars, and after you do it, we'll know whether or not strings are real." But I think it's a crucial difference. It's measurable in principle and we don't see how to get from here to there. If we see how to get from here to there, in my eyes, it's boring, right?

Lex Fridman 1:23:25 Mm-hmm.

Andrew Strominger 1:23:25 So when I was a graduate student, they knew how to measure the Higgs boson.

Lex Fridman 1:23:31 (laughs)

Andrew Strominger 1:23:32 Took 40 years, but they didn't... I, not to say that stuff is boring, I don't want to say that stuff is boring.

Lex Fridman 1:23:39 Yeah, yeah.

Andrew Strominger 1:23:40 But I, I, you know, y- you know, when Magellan set out, he didn't know he could get around the world.

Lex Fridman 1:23:46 (laughs)

Andrew Strominger 1:23:47 There was no map, you know?

Lex Fridman 1:23:49 Yeah.

Andrew Strominger 1:23:49 So I don't know how we're gonna, um, connect in a concrete way, um, all these ideas of string theory to the real world and... You know, when I started out in graduate school, I, I said, "What is the, what is the most interesting problem that there might be, the deepest, most interesting problem that there might be progress on in 60 years?" And I think it could be, you know, uh, that, you know, in another 30 years that maybe we'll learn that, uh, we have understood how black holes store information, you know? That doesn't seem wild, that, that we're able to abstract what we learned from string theory and show that it's operative and, and, and, and, you know, I mean, the Bose-Einstein condensate they did, you know, they, if you... When Bose and Einstein, uh, predicted it, I- when was that, the '30s maybe, early '30s, it took, they were, there were 20 orders of magnitude that were needed in order to, improvement in order to measure it, um, and they did, 50 years later. So, and then you couldn't have guessed how that had happened, i- i- h- how, how, how they could have gotten there. And it could happen that we, I don't think we're gonna, like, see the heterotic string spectrum at an accelerator, but, but it could be that... things come around and, and, uh, in an interesting way, and somehow it comes together. The fact that we can't see to the end isn't a reason not to do it, you know? We're just, you know, what did they do when they were trying to find the Pacific, right? They just, they took every route. They just tried everything. And that's what we're doing. And we're taking... and I'm taking the one that my nose tell- tells me is the best, you know? And other people are taking other ones, and that's good because we need every t- every person taking every route. And, you know, if somebody on another route, uh, finds something that looks really promising, you know, I'm gonna make a portage over the mountain and get on their stream, you know? So the fact that you don't see the experiment now isn't, to me, a reason to give up on what I view as the most fundamental paradox in 20th century, 20th, in present physics, 20, 21st century physics.

Lex Fridman 1:26:33 Absolutely. You can see that it's possible, you just don't know the way.

Lex Fridman 1:26:37 But that, that's what I mean why some of the philosophical questions could be formulated in a way that's explorable scientifically. So, uh, some of the stuff we've talked about but, you know, for example, uh, this topic that's become more okay to talk about, uh, which is the topic of consciousness. Uh, you know, to me as an artificial intelligence person, that's a very practically interesting topic. But there's also philosophers, uh, Sean Carroll loves to argue against them, but there's the philosophers that are panpsychist.

Andrew Strominger 1:27:07 I'm not against philosophers, it's just not as fun, I don't think.

Lex Fridman 1:27:10 It's not as fun, all right. (laughs)

Andrew Strominger 1:27:13 (laughs)

Lex Fridman 1:27:13 But they, they, uh, they start a little flame of a fire going that some of those flames, I think, eventually become physics. So eventually become something that we can really... Like, having them around is really important because you'll discover something by modeling and exploring black holes that's really weird. And having these ideas around, like the ideas of panpsychists, that consciousness could be a fundamental force of nature, just even having that crazy idea swimming around in the background could really spark something where, that you were missing something completely.

Andrew Strominger 1:27:52 Mm-hmm.

Lex Fridman 1:27:52 And it's just, that's where the philosophy done right, I think-

Andrew Strominger 1:27:57 Mm-hmm.

Lex Fridman 1:27:57 ... um, is very useful. That's where even the, you know, these thought experiments, which is very fun in the sort of the, the, the tech sci-fi world that we live in a simulation, uh, that, you know, taking a perspective of the universe as a, as a computer, as a, as a, as a computational system that processes information, which is a pretty intuitive notion, but you can, just even reframing it that way for yourself could really open up some different way of thinking. Uh-

Andrew Strominger 1:28:25 Could be.

Lex Fridman 1:28:26 And then you have, um, I don't know if you're familiar with Stephen Wolfram's work of, like, cellular automata and complexity.

Andrew Strominger 1:28:32 Yeah. I did a podcast with Stephen.

Lex Fridman 1:28:34 With Stephen? That's awesome.

Andrew Strominger 1:28:36 (laughs)

Lex Fridman 1:28:36 I mean, I, to me, forget physics, forget all that, uh, cellular automata make no sense. They're so beautiful. They're so-

Andrew Strominger 1:28:45 They are. They're really incredible.

Lex Fridman 1:28:45 Like that from simple rules you can create complexity. I, I just don't think, you know, he wrote a book, A New Kind of Science, um, (laughs) basically hinting at, which a lot of people have hinted at, is like we, we don't have a good way to talk about these objects. We don't, we can't figure out what is happening here. These simple-

Andrew Strominger 1:29:03 Yeah.

Lex Fridman 1:29:03 ... these trivial rules can create incredible complexity.

Andrew Strominger 1:29:06 Uh, he's totally right about that, yeah.

Lex Fridman 1:29:09 And I, and physicists, I guess, don't have, don't know what to do with that, don't know what to do with cellular automata. 'Cause you can describe the simple rules that govern the system or how complexity can emerge, like incredible complexity.

Andrew Strominger 1:29:24 Yeah. Of course, Wolfram's version of that is that physicists will never be able to describe it.

Lex Fridman 1:29:31 Right. Yeah, exactly. He tries to prove that it's impossible. (laughs) What do you make of that? What do you, what do you, uh, what, what do you make about the tension of being a physicist and potentially not being able to... It's like, uh, Freud or somebody that maybe, uh, Sigmund Freud, that maybe you'll never be able to actually describe the human psyche. Uh, is, is that a possibility for you, that you will never be able to get to the core fundamental description of, uh, the laws of nature?

Andrew Strominger 1:30:01 Yeah, so I had this conversation with Weinberg. (laughs)

Lex Fridman 1:30:06 (laughs) Yeah, how'd it go?

Andrew Strominger 1:30:10 So Weinberg has this book called Dreams of a Final Theory.

Lex Fridman 1:30:15 Yeah.

Andrew Strominger 1:30:16 And I had this conversation with him, I said, "Why do you think, um, there's ever gonna be a final theory? Why should there ever be a final theory? I mean, what, what does that mean? Do physics departments shut down, we've solved everything? Um, and, you know, what, is it, doesn't it seem that every time we answer some old questions, we'll, we'll just find new ones, and that it will just keep going on forever and ever?" And he said, "Well, that's what they used to say about the Nile. They were never gonna find the end."

Lex Fridman 1:30:48 (laughs)

Andrew Strominger 1:30:49 (laughs) Then one day they found it. (laughs)

Lex Fridman 1:30:53 Yeah.

Andrew Strominger 1:30:54 So I don't know. Um, string theory doesn't, string theory doesn't look like a candidate to me for a final theory. It-

Lex Fridman 1:31:06 As it stands now.

Andrew Strominger 1:31:08 It-

Lex Fridman 1:31:08 It doesn't get to the bottom of the well-

Andrew Strominger 1:31:09 It doesn't, it-

Lex Fridman 1:31:10 ... into the size, into the whole ƒ (inaudible) -

Andrew Strominger 1:31:11 Yeah, it seems to me that even if we kind of solved it and we've did experiments, there would, still would be more questions, like why are there four dimensions instead of six? It doesn't seem to have any- anything that, in it that would explain that, that you could, you can always hope, you know, that there's something that we don't know about string theory that will explain it. But-... but it, but it still doesn't look like it's gonna answer every question. And, um, why is there one time, not two? You know? Why is there... You know, it, it doesn't seem like it's... I don't even know what it would mean to answer every question.

Lex Fridman 1:31:49 Well, to, to answer every question, obviously, so wh- when you refer to the theory of everything, you'll be able to have a... If it exists, it would be a theory that allows you to predict precisely the, the behavior of objects in the universe and their, their movement, right? What, what about them? Their movement.

Andrew Strominger 1:32:11 Yeah.

Lex Fridman 1:32:12 Like, like precisely, no matter the object.

Andrew Strominger 1:32:14 Right. That's true. So, so that would be a really interesting state of affairs if we could predict everything, but not necessarily understand everything. So for example, let's just forget about gravity. I mean, we're not too far from that situation.

Lex Fridman 1:32:33 (laughs)

Andrew Strominger 1:32:34 If we forget about gravity, the standard model, in principle, given a big enough computer, predicts almost everything. But if you look at the standard model, it's kind of a laundry list, with neutrino masses and all that stuff. There, there are hundreds of free parameters. Where do they come from? Is there an organizing principle? Is there some further unification?

Lex Fridman 1:33:02 Sure.

Andrew Strominger 1:33:03 So, so being able to predict, uh, everything is not the only goal that physicists have.

Lex Fridman 1:33:10 Mm-hmm. So on the way to trying to predict, you're trying to understand. That's actually probably the goal, is to understand.

Andrew Strominger 1:33:16 Yeah. But, but, but, right, we're more interested in understanding than actually (laughs) than actually doing the predictions. But the predictions are more... Focusing on how to make predictions is a good way to improve your understanding, because you know you've understood it if you can do the predictions.

Lex Fridman 1:33:36 Yeah. One of, one of the interesting things that might come to a head with is ar- artificial intelligence. There's an increasing use of AI in, uh, in physics. We might live in a world where-

Andrew Strominger 1:33:49 Mm-hmm.

Lex Fridman 1:33:50 ... AI would be able to predict perfectly what's happening.

Andrew Strominger 1:33:52 Mm.

Lex Fridman 1:33:53 And so that will, as, as physicists, you'll have to come to (laughs) to, to the fact that you're actually not that interested in prediction. (laughs) I mean, it's very useful, but you're interested in really understanding the deep laws of nature versus-

Andrew Strominger 1:34:07 Yeah.

Lex Fridman 1:34:07 ... a perfect predictor.

Andrew Strominger 1:34:08 Yeah.

Lex Fridman 1:34:08 Like, uh, you wanna play chess-

Andrew Strominger 1:34:09 But even within AI-

Lex Fridman 1:34:11 Yeah.

Andrew Strominger 1:34:11 ... AI people are trying to understand what it is that the AI bots have learned, uh, in order to produce whatever they produce.

Lex Fridman 1:34:22 For sure, but you, you still don't understand deeply, especially because they're getting, you know, uh, especially language models, if you're paying attention, uh, the systems that are able to generate text, they're able to have conversations. ChatGPT is the recent manifestation of that. They're, they just seem to know everything. They're trained on the internet.

Andrew Strominger 1:34:40 Yeah.

Lex Fridman 1:34:40 They seem to be very, very good at, uh, something that looks like reasoning.

Andrew Strominger 1:34:46 Mm-hmm.

Lex Fridman 1:34:46 They're able to generate, uh, you can ask them questions, they can answer questions. It just feels like this thing is intelligent, right? Uh, and I could just see that being possible with physics. You ask any kind of physical question, and it'll be able to, very precise, about a particular star system or a particular black hole, and it'll say, "Well, these are the numbers you should..." It just, it'll perfectly predict. Uh, and then sure, you can understand, uh, how the neural network is... The architecture is structured. Actually, for most of them now, they're very simple. You can understand what data it's trained on, huge amount of data. You're giving it a huge amount of data from a very nice telescope or something. And then, but it, it seems to predict everything perfectly, you know? How a banana falls when you throw it. Uh, like, everything is perfectly predicted. You still don't have a deep understanding of what governs the whole thing. Um-

Andrew Strominger 1:35:37 Yeah.

Lex Fridman 1:35:37 ... and maybe you can ask it a question, it'll be some kind of Hitchhiker's Guide to the Galaxy type answer, um, that, you know, it's a funny world, uh, we live in. Of course, it's also possible that there's no such deep, simple governing laws of nature behind the whole thing. I mean, you, there's something in us humans-

Andrew Strominger 1:35:57 It's possible.

Lex Fridman 1:35:57 ... that wants it there to be.

Andrew Strominger 1:36:00 Yeah.

Lex Fridman 1:36:00 But doesn't have to be, right?

Andrew Strominger 1:36:01 Right.

Lex Fridman 1:36:02 I do... What, what's, where, where do you... Again, you're betting the f- you already bet the farm. But if, if you were to have-

Andrew Strominger 1:36:07 (laughs) .

Lex Fridman 1:36:07 ... a second farm, do you think there is a theory of everything that we might get at? So, um, simple laws that govern-

Andrew Strominger 1:36:13 (sighs)

Lex Fridman 1:36:13 ... the whole thing?

Andrew Strominger 1:36:15 I don't, I don't... Honestly, I, I don't know, but I'm pretty confident that if there is, we won't get to it in my lifetime. I don't think we're near it.

Lex Fridman 1:36:30 But doesn't it feel like there... Like, the fact that we have the laws we do, they're relatively simple already. That's kind of incredible. It's just there seems to be, there seems to be simple laws that govern things, right?

Andrew Strominger 1:36:42 By theory of everything, you mean theory, a theory of, uh, of everything, an, an algorithm to predict everything.

Lex Fridman 1:36:52 But a simple algorithm.

Andrew Strominger 1:36:54 A relatively simple algorithm to predict everything. So for me, it would be a sad day if we arrived at that without answering some deeper questions.

Lex Fridman 1:37:07 Sure, of course. It definitely is. But the question, yes. (laughs) But one of the questions before we arrive there we can ask, does such a destination even exist? So, because th- asking the question and the possible answers and the process of trying to answer that question is, in itself, super interesting. Is that, is, is it, is it even possible to get there, where there's an E equals MC squared type of... There's a function. Okay, you can have many parameters. Uh, but-... a finite number of parameter function that can predict a lot of things about our universe.

Andrew Strominger 1:37:42 Well, okay, but just to sort of throw one thing in, in order to answer every question, we would need a theory of the origin of the universe.

Lex Fridman 1:37:53 Right.

Andrew Strominger 1:37:55 And that is a huge, uh, task, right? So, and it, the fact that the universe seems to have a beginning defies everything we know and love, right?

Lex Fridman 1:38:09 (laughs)

Andrew Strominger 1:38:09 Because we, we, you know, one of the, one of the basic principles of physics is determinism, that the past follows from the f- the present follows from the past, the future follows from the present, so on. But if you have the origin of the universe, if you have a big bang, that means before that, there was nothing. And you can't have a theory in which something follows from nothing. So somehow-

Lex Fridman 1:38:38 It sounds like you don't like singularities.

Andrew Strominger 1:38:41 (laughs) Well-

Lex Fridman 1:38:42 I thought, for somebody that works with black holes, you would get used to them by now.

Andrew Strominger 1:38:45 No, no, I, I like this because it's, it's so hard to understand.

Lex Fridman 1:38:49 (laughs)

Andrew Strominger 1:38:50 I like it 'cause it's hard to understand.

Lex Fridman 1:38:52 (laughs)

Andrew Strominger 1:38:53 But, but it's really challenging us.

Lex Fridman 1:38:54 Yeah.

Andrew Strominger 1:38:54 It's not a s- I don't think we're close to solving that problem.

Lex Fridman 1:38:57 So even, uh-

Andrew Strominger 1:38:59 And string theory, string theory has basically had nothing, there's been almost nothing interesting said about that in the last many decades.

Lex Fridman 1:39:09 So string theory hasn't really looked at the big bang? It hasn't really tried to get the, see the origin?

Andrew Strominger 1:39:14 Not, not successfully. Not, not in, there, there aren't compelling papers that lots of people have read that, uh, people have taken it up and tried to go at it, but, but there aren't, there aren't, there aren't compelling... String theory doesn't seem to have a trick that, that, uh, helps us with that puzzle.

Lex Fridman 1:39:35 Do you think we'll be able to sneak up to the, the origin of the universe? Like reverse engineer it from experimental, from theoretical perspective? Like, okay, if, if we can, what-

Andrew Strominger 1:39:46 Well, it's-

Lex Fridman 1:39:46 ... what, what would be

Andrew Strominger 1:39:47 ... see, you're alr- ... the trajectory?

Lex Fridman 1:39:47 You've already gotten yourself in trouble. Okay.

Andrew Strominger 1:39:50 You see? 'Cause you used the word reverse engineer.

Lex Fridman 1:39:53 Sure.

Andrew Strominger 1:39:53 So if you're gonna reverse engineer, that means, you know, you, you... Forward engineering means that you take the present-

Lex Fridman 1:40:02 Yeah.

Andrew Strominger 1:40:02 ... and determine the future. Reverse engineering-

Lex Fridman 1:40:04 Mm-hmm.

Andrew Strominger 1:40:04 ... means that you take the present and determine the past.

Lex Fridman 1:40:07 Yeah.

Andrew Strominger 1:40:08 But-

Lex Fridman 1:40:08 E-estimate the past, but yes, sure. Uh-

Andrew Strominger 1:40:10 But, but, but if the past was nothing, how are you ever gonna reverse engineer to nothing?

Lex Fridman 1:40:17 Well, run up-

Andrew Strominger 1:40:17 That's hard to do.

Lex Fridman 1:40:18 ... run up against a nothing, right? Until they have mathematical models that break down nicely to where you can actually start to infer things. Do (laughs) do you think-

Andrew Strominger 1:40:30 Let's, let's work on it. (laughs)

Lex Fridman 1:40:32 No, but do you think that that's-

Andrew Strominger 1:40:33 I, I, it could, uh, maybe. Uh, but it is, people have tried to do things like that, but-

Lex Fridman 1:40:38 Yeah, and have not succeeded. But-

Andrew Strominger 1:40:39 ... it's not, it's not something that we, we, you know, we're getting A pluses in.

Lex Fridman 1:40:44 Sure. Let's pretend we live in a world where in 100 years we have an answer to that.

Andrew Strominger 1:40:51 Yeah.

Lex Fridman 1:40:51 What would that answer look like? Who, what department is that from? What fields left, led us there? What, uh, uh, not what fields, what set of ideas in theoretical physics? Um, is it experimental? Is it theoretical? Like, what can you imagine possibly, could have possibly lead us there? Is it through gravitational waves and some kind of observations there? Is it investigation of black holes? Is it simulation of universes? Is it, uh, maybe we start creating black holes somehow? I don't, I don't know. Um, maybe some kinda high energy physics type of experiments?

Andrew Strominger 1:41:30 Well, I have some late night ideas about that, that are-

Lex Fridman 1:41:34 (laughs)

Andrew Strominger 1:41:34 ... aren't really ready for prime time. (laughs)

Lex Fridman 1:41:36 Okay, sure. But you have some ideas, okay.

Andrew Strominger 1:41:38 Yeah, yeah. But, but the, but, um-

Lex Fridman 1:41:41 (laughs)

Andrew Strominger 1:41:41 ... and many people do. (laughs) It could be that some of the advances in quantum information theory are important in, in that they kind of go beyond taking quantum systems and just replicating themselves, but combining

Andrew Strominger 1:41:58 them with others.

Lex Fridman 1:41:59 Do you think, since, uh, you highlighted the issue with time and the origin of the universe, do you think time is fundamental or, or emergent?

Andrew Strominger 1:42:10 I, I think ultimately it has to be emergent.

Lex Fridman 1:42:13 Yeah, what does it mean for time to be emergent?

Andrew Strominger 1:42:18 Well-

Lex Fridman 1:42:19 There's a wide range.

Andrew Strominger 1:42:20 ... let's review what it means for space to be emergent.

Lex Fridman 1:42:23 Yes.

Andrew Strominger 1:42:24 What it means for space to be emergent is that, um, you, you have a holographic plate and you shine some light that's moving in space, and it produces an image which contains an extra spatial dimension, and time just goes along for the ride. So what we'd like to do, and indeed there is some rather concrete work in this direction, though again, I would say even within our stringing community, we're not getting A pluses on these efforts-

Lex Fridman 1:43:04 Mm-hmm.

Andrew Strominger 1:43:06 ... uh, but what we'd like to do is to see, um, examples in which the extra space-time dimension is time. In other words, usually what, what we understand very well mathematically is how to take systems, uh, in some number of space-time dimensions and rewrite them as a plate in fewer space dimensions. What we'd like to do is to take systems with-... one time and some number of space dimensions and to rewrite them as a system that had only space dimensions in it, had no time evolution. And there are some fairly concrete ideas about how to do that, but they're not, you know, u- universally accepted, even within the stringy community.

Lex Fridman 1:44:05 But isn't it wild to you-

Andrew Strominger 1:44:07 Yes.

Lex Fridman 1:44:08 ... for, to be emergent?

Andrew Strominger 1:44:10 Yes.

Lex Fridman 1:44:10 How do we intuit these kinds of ideas as human beings, for whom space and time seems as fundamental as-

Andrew Strominger 1:44:17 Well-

Lex Fridman 1:44:17 ... as apples and oranges?

Andrew Strominger 1:44:17 ... they're both illusions.

Lex Fridman 1:44:19 Okay.

Andrew Strominger 1:44:20 They're both illusions. Even time.

Lex Fridman 1:44:25 You co-authored a paper titled Photon Rings Around Warped Black Holes. First of all, whoever writes your paper titles (laughs) , you like, uh, the soft hair and the, and, and, the turn black hole and the big bang, you're very good at coming up with titles yourself. Anyway, you co-authored a paper title (laughs) , "Photon Rings around Warped Black Holes." In it you write, quote, "Recent work has identified a number of emergent symmetries related to the