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

Andrew Strominger is a theoretical physicist at Harvard. Please support this podcast by checking out our sponsors: - Eight Sleep: https://www.eightsleep.com/lex to get special savings - Rocket Money: https://rocketmoney.com/lex - Indeed: https://indeed.com/lex to get $75 credit - ExpressVPN: https://expressvpn.com/lexpod to get 3 months free EPISODE LINKS: Andrew's website: https://www.physics.harvard.edu/people/facpages/strominger Andrew's papers: Soft Hair on Black Holes: https://arxiv.org/abs/1601.00921 Photon Rings Around Warped Black Holes: https://arxiv.org/abs/2211.01674 PODCAST INFO: Podcast website: https://lexfridman.com/podcast Apple Podcasts: https://apple.co/2lwqZIr Spotify: https://spoti.fi/2nEwCF8 RSS: https://lexfridman.com/feed/podcast/ Full episodes playlist: https://www.youtube.com/playlist?list=PLrAXtmErZgOdP_8GztsuKi9nrraNbKKp4 Clips playlist: https://www.youtube.com/playlist?list=PLrAXtmErZgOeciFP3CBCIEElOJeitOr41 OUTLINE: 0:00 - Introduction 1:12 - Black holes 6:16 - Albert Einstein 25:44 - Quantum gravity 29:56 - String theory 40:44 - Holographic principle 48:41 - De Sitter space 53:53 - Speed of light 1:00:40 - Black hole information paradox 1:08:20 - Soft particles 1:17:27 - Physics vs mathematics 1:26:37 - Theory of everything 1:41:58 - Time 1:44:24 - Photon rings 2:00:05 - Thought experiments 2:08:26 - Aliens 2:14:04 - Nuclear weapons SOCIAL: - Twitter: https://twitter.com/lexfridman - LinkedIn: https://www.linkedin.com/in/lexfridman - Facebook: https://www.facebook.com/lexfridman - Instagram: https://www.instagram.com/lexfridman - Medium: https://medium.com/@lexfridman - Reddit: https://reddit.com/r/lexfridman - Support on Patreon: https://www.patreon.com/lexfridman

Andrew StromingerguestLex Fridmanhost
Feb 15, 20232h 19mWatch on YouTube ↗

EVERY SPOKEN WORD

  1. 0:00 – 1:12

    Introduction

    1. AS

      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.

    2. LF

      Mm-hmm.

    3. AS

      But if it just misses the black hole, it'll swing around the back and come back to you.

    4. LF

      Yeah.

    5. AS

      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.

    6. LF

      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.

  2. 1:12 – 6:16

    Black holes

    1. LF

      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?

    2. AS

      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.

    3. LF

      What is light-

    4. AS

      Well-

    5. LF

      ... that can't escape from a black hole?

    6. AS

      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-

    7. LF

      (laughs)

    8. AS

      ... the stuff that appears when the lights come on.

    9. LF

      Yeah.

    10. AS

      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.

    11. LF

      Mm-hmm.

    12. AS

      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.

  3. 6:16 – 25:44

    Albert Einstein

    1. AS

    2. LF

      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-

    3. AS

      Yeah, that's incredibly interesting. Yeah.

    4. LF

      There's a lot of interesting things here.

    5. AS

      Yeah.

    6. LF

      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?

    7. AS

      Well-

    8. LF

      Is it still crazy? What-

    9. AS

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

    10. LF

      Yeah.

    11. AS

      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-

    12. LF

      Mm-hmm.

    13. AS

      ... and literally be unable to reconstruct how confused we were-

    14. LF

      Yeah.

    15. AS

      ... and how we could ever have thought of it any other way.

    16. LF

      That's so fascinating.

    17. AS

      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?

    18. LF

      Mm-hmm.

    19. AS

      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-

    20. LF

      Yeah.

    21. AS

      ... 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.

    22. LF

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

    23. AS

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

    24. LF

      Oh, he didn't believe in gravitational waves either?

    25. AS

      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.

    26. LF

      (laughs) Well, what-

    27. AS

      100 years later.

    28. LF

      ... what- what is coordinate transformation as it was 100 years ago to today?

    29. AS

      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.

    30. LF

      Mm-hmm.

  4. 25:44 – 29:56

    Quantum gravity

    1. LF

    2. AS

      Okay.

    3. LF

      So we'll return to black holes, I have a million questions there, but let's-

    4. AS

      Okay.

    5. LF

      ... 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?

    6. AS

      Okay. So-

    7. LF

      Again, five questions.

    8. AS

      Yeah.

    9. LF

      (laughs)

    10. AS

      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.

    11. LF

      Oh, Higgs was already a decade ago?

    12. AS

      I think it is, yeah.

    13. LF

      Wow. Time flies when you're having fun.

    14. AS

      But you better check me on that. Yeah.

    15. LF

      (laughs) It's, it's, hey, that's true, but so much fun has been happening.

    16. AS

      So much fun has been happening.

    17. LF

      (sighs)

    18. AS

      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.

    19. LF

      A- anything contradictory, by new. 'Cause can't you-

    20. AS

      Anything contradictory, yeah.

    21. LF

      I, I'll have some wild questions, uh, for you-

    22. AS

      Yeah.

    23. LF

      ... 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

    24. NA

      coming through.

    25. AS

      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.

    26. LF

      Uh-huh. (laughs)

    27. AS

      Renormalizable quantum field theory. They all fall in the category of renormalizable quantum field theory.

    28. LF

      I'm gonna throw that at a bar later to impress-

    29. AS

      (laughs)

    30. LF

      ... impress the girls.

  5. 29:56 – 40:44

    String theory

    1. AS

    2. LF

      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?

    3. AS

      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.

    4. LF

      Okay, so you've been there from the early days of string theory.

    5. AS

      Right.

    6. LF

      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?

    7. AS

      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.

    8. LF

      What do you mean by softer?

    9. AS

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

    10. LF

      Yeah.

    11. AS

      ... you might bruise yourself.

    12. LF

      Sure.

    13. AS

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

    14. LF

      Mm-hmm.

    15. AS

      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.

    16. LF

      Mm-hmm.

    17. AS

      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-

    18. LF

      Ooh, another term for the bar later tonight.

    19. AS

      Uh, yeah, yeah.

    20. LF

      Parity violation. Okay.

    21. AS

      So- so if you go to the bar and... (laughs)

    22. LF

      I already got the renormalizable quantum field theory.

    23. AS

      And you look in the mirror across the bar-

    24. LF

      Yes.

    25. AS

      ... 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-

    26. LF

      Which one?

    27. AS

      ... the lady in the bar-

    28. LF

      Yeah.

    29. AS

      ... the ph- a photograph that shows both the mirror and you-

    30. LF

      There's a difference.

  6. 40:44 – 48:41

    Holographic principle

    1. LF

      Uh, maybe we can sneak our way back from string theory into black holes.

    2. AS

      Yeah, yeah.

    3. LF

      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?

    4. AS

      Yeah. So that is a very interesting story. I was interested in black holes before I was interested in string theory. I was-

    5. LF

      Sure.

    6. AS

      ... 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-

    7. LF

      Mathematically clumsy or just the interpretation?

    8. AS

      Mathematically clumsy.

    9. LF

      Okay, got it.

    10. AS

      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)

    11. LF

      (laughs)

    12. AS

      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.

    13. LF

      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?

    14. AS

      Yeah.

    15. LF

      Uh, where? Where's-

    16. AS

      Yeah.

    17. LF

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

    18. AS

      Oh, okay. So if you say that-

    19. LF

      I'm gonna ask you a bunch of-

    20. AS

      ...I should try to memorize them and answer each one in order? Just answer them-

    21. LF

      No, I don't know. I don't know what I'm doing.

    22. AS

      Oh, okay. (laughs)

    23. LF

      I'm desperately, desperately, uh, trying to figure it out-

    24. AS

      Okay.

    25. LF

      ...as we go along here.

    26. AS

      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.

    27. LF

      And that's another wild idea. You said area, not volume.

    28. AS

      Exactly. And that's the holographic principle.

    29. LF

      The universe is so weird. (laughs)

    30. AS

      And that's the holographic principle.

  7. 48:41 – 53:53

    De Sitter space

    1. AS

    2. LF

      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?

    3. AS

      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.

    4. LF

      Can you elaborate on flat? What, what do we mean by flat?

    5. AS

      Well, like the surface of this table-

    6. LF

      Hmm.

    7. AS

      ... 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.

    8. LF

      'Cause you have to step, what, a couple of dimensions up? So it's hard to...

    9. AS

      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.

    10. LF

      Mm-hmm.

    11. AS

      Then space is flat, but it's expanding, which means that spacetime is curved.

    12. LF

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

    13. AS

      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.

    14. LF

      Mm-hmm.

    15. AS

      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.

    16. LF

      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.

    17. AS

      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.

    18. LF

      And we don't know what the heck it is.

    19. AS

      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?

    20. LF

      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?

  8. 53:53 – 1:00:40

    Speed of light

    1. LF

      Like, why is the speed of light so slow?

    2. AS

      So fast?

    3. LF

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

    4. AS

      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-

    5. LF

      Sure.

    6. AS

      ... 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-

    7. LF

      Yeah.

    8. AS

      ... 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-

    9. LF

      Yeah.

    10. AS

      ... so large compared to the timescale for a neutron to decay? You know?

    11. LF

      It's, yeah, yeah. I mean, ultimately-

    12. AS

      And-

    13. LF

      ... the reference frame, the temporal reference frame here is a human life.

    14. AS

      Maybe. (laughs)

    15. LF

      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-

    16. AS

      Yeah.

    17. LF

      ... through the eyes of this, th- the, these biological organisms. I mean-

    18. AS

      Absolutely.

    19. LF

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

    20. AS

      Absolutely.

    21. LF

      That, like, a human life-

    22. AS

      Yeah.

    23. LF

      ... 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.

    24. AS

      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.

    25. LF

      Yes.

    26. AS

      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.

    27. LF

      Mm-hmm. Well, (laughs) that's a very dramatic statement, but yes. Uh-

    28. AS

      (laughs) That's not the most, that's not the most dramatic thing to say. (laughs)

    29. LF

      (laughs) I mean, I mean, but like, okay, that's very life and death. You mean like-

    30. AS

      (laughs)

  9. 1:00:40 – 1:08:20

    Black hole information paradox

    1. AS

      string theory.

    2. LF

      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?

    3. AS

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

    4. LF

      Yes.

    5. AS

      ... 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-

    6. LF

      Mm-hmm.

    7. AS

      ...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-

    8. LF

      That's where Hawking radiation, this prediction, it starts to come into play?

    9. AS

      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.

    10. LF

      Yeah. So just to clarify, the storage is on the plate?

    11. AS

      Is on the plate.

    12. LF

      On, on the, on the holographic plate, and then it projects somehow inside the-

    13. AS

      The, the bulk, the, the, the spacetime is the hologram.

    14. LF

      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?

    15. AS

      Um-

    16. LF

      Like that there's two dimension, you know, projections that store information?

    17. AS

      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.

    18. LF

      Yeah, but why should there be a holographic plate? Like, wh-

    19. AS

      Why should there be?

    20. LF

      Yeah. Why? (chuckles)

    21. AS

      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.

    22. LF

      Yeah, like-

    23. AS

      So, so if you're asking-

    24. LF

      ... the whys have got you going, "I can just keep asking," yeah.

    25. AS

      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.

    26. LF

      Sure.

    27. AS

      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.

    28. LF

      Sure.

    29. AS

      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.

    30. LF

      Uh, so on this, uh, topic, you, you co-wrote, co-authored a paper with Stephen Hawking called Soft Hair on Black Holes.

  10. 1:08:20 – 1:17:27

    Soft particles

    1. LF

      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-

    2. AS

      Right.

    3. LF

      ... which can be thought of as being different from one another by the addition of soft photons or soft gravitons."

    4. AS

      Right.

    5. LF

      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?

    6. AS

      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.

    7. LF

      Mm-hmm.

    8. AS

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

    9. LF

      Yeah.

    10. AS

      ... 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.

    11. LF

      Mm-hmm.

    12. AS

      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.

    13. LF

      Infinite number of the soft photons and gravitons?

    14. AS

      Of the zero-energy ones, yeah.

    15. LF

      And so these are, and they're somehow everywhere, but they're-

    16. AS

      They're everywhere.

    17. LF

      ... but they also contain information, or they're able to store information?

    18. AS

      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.

    19. LF

      (laughs)

    20. AS

      Spreading out all over the space is like saying they rush off to the energy, edge of the universe.

    21. LF

      Right.

    22. AS

      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.

    23. LF

      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.

    24. AS

      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.

    25. LF

      Yes. But it is an error.

    26. AS

      It is an error.

    27. LF

      A- a- and, uh, Hawking obviously agreed with it.

    28. AS

      Hawking agreed with it, and he was sure that this was the, he was sure that this was, uh-

    29. LF

      This was a critical error.

    30. AS

      ... 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.

  11. 1:17:27 – 1:26:37

    Physics vs mathematics

    1. AS

    2. LF

      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?

    3. AS

      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.

    4. LF

      Hmm. In service of physics.

    5. AS

      In, in service of physics, but then, of course, in the, in the heat of it, it has its own appeal and, uh-

    6. LF

      (laughs) In the heat of it, I like it. (laughs) Sure.

    7. AS

      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-

    8. LF

      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-

    9. AS

      Discovered.

    10. LF

      ... discovered.

    11. AS

      Yeah.

    12. LF

      And, and, and there's a deep linkage between the two.

    13. AS

      Yeah, yeah, yeah.

    14. LF

      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.

    15. AS

      Uh, uh, my expertise as a phy- a physicist doesn't add anything to that, um-

    16. LF

      (laughs)

    17. AS

      ... 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-

    18. LF

      Yeah.

    19. AS

      ... 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.

    20. LF

      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.

    21. AS

      Well-

    22. LF

      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?

    23. AS

      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.

    24. LF

      Mm-hmm.

    25. AS

      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-

    26. LF

      Sure.

    27. AS

      ... 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?

    28. LF

      Mm-hmm.

    29. AS

      So when I was a graduate student, they knew how to measure the Higgs boson.

    30. LF

      (laughs)

  12. 1:26:37 – 1:41:58

    Theory of everything

    1. LF

      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.

    2. AS

      I'm not against philosophers, it's just not as fun, I don't think.

    3. LF

      It's not as fun, all right. (laughs)

    4. AS

      (laughs)

    5. LF

      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.

    6. AS

      Mm-hmm.

    7. LF

      And it's just, that's where the philosophy done right, I think-

    8. AS

      Mm-hmm.

    9. LF

      ... 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-

    10. AS

      Could be.

    11. LF

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

    12. AS

      Yeah. I did a podcast with Stephen.

    13. LF

      With Stephen? That's awesome.

    14. AS

      (laughs)

    15. LF

      I mean, I, to me, forget physics, forget all that, uh, cellular automata make no sense. They're so beautiful. They're so-

    16. AS

      They are. They're really incredible.

    17. LF

      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-

    18. AS

      Yeah.

    19. LF

      ... these trivial rules can create incredible complexity.

    20. AS

      Uh, he's totally right about that, yeah.

    21. LF

      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.

    22. AS

      Yeah. Of course, Wolfram's version of that is that physicists will never be able to describe it.

    23. LF

      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?

    24. AS

      Yeah, so I had this conversation with Weinberg. (laughs)

    25. LF

      (laughs) Yeah, how'd it go?

    26. AS

      So Weinberg has this book called Dreams of a Final Theory.

    27. LF

      Yeah.

    28. AS

      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."

    29. LF

      (laughs)

    30. AS

      (laughs) Then one day they found it. (laughs)

  13. 1:41:58 – 1:44:24

    Time

    1. AS

      them with others.

    2. LF

      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?

    3. AS

      I, I think ultimately it has to be emergent.

    4. LF

      Yeah, what does it mean for time to be emergent?

    5. AS

      Well-

    6. LF

      There's a wide range.

    7. AS

      ... let's review what it means for space to be emergent.

    8. LF

      Yes.

    9. AS

      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-

    10. LF

      Mm-hmm.

    11. AS

      ... 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.

    12. LF

      But isn't it wild to you-

    13. AS

      Yes.

    14. LF

      ... for, to be emergent?

    15. AS

      Yes.

    16. LF

      How do we intuit these kinds of ideas as human beings, for whom space and time seems as fundamental as-

    17. AS

      Well-

    18. LF

      ... as apples and oranges?

    19. AS

      ... they're both illusions.

    20. LF

      Okay.

    21. AS

      They're both illusions. Even time.

  14. 1:44:24 – 2:00:05

    Photon rings

    1. AS

    2. LF

      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 intricate self-similar structure of the photon ring." So what are photon rings? What are some interesting characteristics of a photon ring?

Episode duration: 2:19:34

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