Leonard Susskind: Quantum Mechanics, String Theory and Black Holes | Lex Fridman Podcast #41

1. 0:00 – 15:00

   The following is a…

   1. LFLex Fridman0:00

      The following is a conversation with Leonard Susskind. He's a professor of theoretical physics at Stanford University, and founding director of Stanford Institute of Theoretical Physics. He is widely regarded as one of the fathers of string theory, and in general, as one of the greatest physicists of our time, both as a researcher and an educator. This is the Artificial Intelligence podcast. Perhaps you noticed that the people I've been speaking with are not just computer scientists, but philosophers, mathematicians, writers, psychologists, physicists, and soon, other disciplines. To me, AI is much bigger than deep learning, bigger than computing. It is our civilization's journey into understanding the human mind and creating echoes of it in the machine. If you enjoy the podcast, subscribe on YouTube, give it five stars on iTunes, support it on Patreon, or simply connect with me on Twitter @lexfridman, spelled F-R-I-D-M-A-N. And now, here's my conversation with Leonard Susskind. You worked and were friends with Richard Feynman. How has he influenced you and changed you as a physicist and thinker?

   2. LSLeonard Susskind1:10

      What I saw, I think what I saw was somebody who could do physics in this deeply intuitive way.

   3. LFLex Fridman1:17

      Mm-hmm.

   4. LSLeonard Susskind1:18

      His style was almost to close his eyes and visualize the phenomena that he was thinking about, and through visualization, outflank the mathematical, the highly mathematical and, um, very, very sophisticated technical arguments that people would use. I think that was also natural to me, but I saw somebody who was actually successful at it-

   5. LFLex Fridman1:43

      Hmm.

   6. LSLeonard Susskind1:44

      ... who could do physics in a way that, uh, that I regarded as simpler, more direct, more intuitive. And while I don't think he changed my way of thinking, I do think he validated it. He made me look at it and say, "Yeah, that's something, uh, you can do and get away with."

   7. LFLex Fridman2:06

      (laughs) .

   8. LSLeonard Susskind2:06

      Practically, you can get away with it.

   9. LFLex Fridman2:09

      So, do you find yourself, whether you're thinking about quantum mechanics or black holes or string theory, using intuition as a first step or a step throughout using visualization?

   10. LSLeonard Susskind2:22

       Yeah, very much so. Very much so. I tend not to think about the equations, I tend not to think about the symbols. I tend to th- try to visualize the phenomena themselves, and then when I get an insight that I think is valid, I might try to convert it to mathematics, but I'm not a math m- uh, I'm not a, a natural mathematician, or I'm good enough at it. I'm good enough at it, but I'm not a great mathematician. Um, so for me, the way of thinking about physics is first intuitive, first, um, visualization, uh, scribble a few equations maybe, but then try to convert it to mathematics. Experiences that other people are better at converting it to mathematics than I am.

   11. LFLex Fridman3:08

       And yet, you've worked with s- very counterintuitive ideas. So how do you-

   12. LSLeonard Susskind3:12

       But tha- no, that's true. That's true.

   13. LFLex Fridman3:13

       So how do you visualize something counterintuitive?

   14. LSLeonard Susskind3:15

       (laughs) . Okay, actually-

   15. LFLex Fridman3:15

       How do you dare?

   16. LSLeonard Susskind3:16

       ... by rewiring your brain in new ways.

   17. LFLex Fridman3:19

       Ha.

   18. LSLeonard Susskind3:20

       Yeah, quantum mechanics is not intuitive. Uh, very little of modern physics is intuitive. Intuitive, well, what does intuitive mean? It means the ability to think about it with basic classical physics, the physics that, uh, that, um, we evolved with, uh, throwing stones, uh, splashing water, whatever it happens to be. Uh, quantum physics, general relativity, quantum field theory are deeply unintuitive in that way, but, you know, after time and getting familiar with these things, you develop new intuitions.

   19. LFLex Fridman3:57

       Mm-hmm.

   20. LSLeonard Susskind3:57

       I always said you rewire. And i- it's to the point where me and many of my friends, I and many of my friends, can think more easily quantum mechanically than we can classically. We've gotten so used to it.

   21. LFLex Fridman4:13

       I mean, yes, our neural wiring-

   22. LSLeonard Susskind4:16

       Yeah.

   23. LFLex Fridman4:16

       ... in our brain is such that we understand rocks and stones and water and so on.

   24. LSLeonard Susskind4:20

       We sort of evolved for that.

   25. LFLex Fridman4:22

       Evolved for it.

   26. LSLeonard Susskind4:22

       Yeah.

   27. LFLex Fridman4:23

       Do you think it's possible to create a wiring of neuron-like state devices that more naturally understand, uh, quantum mechanics, understand wave function, understand these weird things that-

   28. LSLeonard Susskind4:38

       Well, I'm not sure. I think many of us have evolved the ability to, um, to think quantum mechanically to some extent, but that doesn't mean you can think like an electron. That doesn't mean a- another example. Forget for a minute quantum mechanics. Just visualizing four-dimensional space or five-dimensional space or six-dimensional space, I think we're fundamentally wired to visualize three dimensions. I can't even visualize two dimensions or one dimension without thinking about it as embedded in three dimensions.

   29. LFLex Fridman5:15

       Right.

   30. LSLeonard Susskind5:16

       If I want to visualize a line, I think of the line as being a line in three dimensions.
2. 15:00 – 30:00

   Mm-hmm. …

   1. LFLex Fridman15:00

      of space, if you just intuitively think about the space of algorithms that that unlocks for us. So there's a whole complexity theory around classical computers measuring the running time of things-

   2. LSLeonard Susskind15:12

      Mm-hmm.

   3. LFLex Fridman15:12

      ... and P, so on.

   4. LSLeonard Susskind15:13

      Mm-hmm.

   5. LFLex Fridman15:14

      What, what kind of algorithms, just intuitively, do you think is, it unlocks for us?

   6. LSLeonard Susskind15:18

      Okay, so we know that there are a handful of algorithms that can seriously beat quantum, or classical computers, and which can have exponentially more power.

   7. LFLex Fridman15:28

      Yeah.

   8. LSLeonard Susskind15:28

      This is a mathematical statement. Nobody's exhibited this in a laboratory. It's a mathematical statement. We know that's true, but it also seems more and more that the number of such things is very limited, only very, very special problems exhibit that much advantage for a quantum computer. Uh, of, of standard problems. To my mind, as far as I can tell, the great power of quantum computers will actually be to simulate quantum systems.

   9. LFLex Fridman15:58

      Mm-hmm.

   10. LSLeonard Susskind15:59

       If you're interested in a certain quantum system and it's too hard to simulate classically-

   11. LFLex Fridman16:05

       Mm-hmm.

   12. LSLeonard Susskind16:07

       ... you simply build a version of the same system. You build a version of it, you build a model of it that's actually functioning as the system, you run it, and then you do the same thing you would do with a quantum system, you make measurements on it.

   13. LFLex Fridman16:20

       Mm-hmm.

   14. LSLeonard Susskind16:20

       Quantum measurements on it. The advantages, you can run it much slower. Uh, y- you could say, why bother? Why not just use the real system?

   15. LFLex Fridman16:29

       (laughs) Right.

   16. LSLeonard Susskind16:29

       Why not just do experiments on the real system? Well, real systems are kinda limited. You can't change them, you can't manipulate them. Uh, you can't slow them down so that you can poke into them. Uh, you can't modify them in arbitrary kinds of ways to see what would happen if I, uh, if I change the system a little bit. So, I think that quantum computers will be extremely valuable in, in understanding quantum systems.

   17. LFLex Fridman17:01

       At the lowest level, the fundamental laws.

   18. LSLeonard Susskind17:03

       Yeah. They're actually satisfying the same laws as the systems that they're simulating.

   19. LFLex Fridman17:09

       That's right.

   20. LSLeonard Susskind17:09

       Okay, so on the one hand you have things like factoring.

   21. LFLex Fridman17:12

       Right.

   22. LSLeonard Susskind17:12

       Okay, factoring is the great, uh, uh, thing of quantum computers.

   23. LFLex Fridman17:17

       Yeah.

   24. LSLeonard Susskind17:17

       Factoring large numbers. That doesn't seem that much to do with quantum mechanics.

   25. LFLex Fridman17:22

       Right.

   26. LSLeonard Susskind17:22

       It, it seems to be almost a fluke, uh, that a, that a quantum computer can solve the factoring problem in a short time. So tho- and those problems seem to be extremely special, rare, and it's not clear to me that there's gonna be a lot of them.

   27. LFLex Fridman17:42

       Hmm.

   28. LSLeonard Susskind17:42

       On the other hand, there are a lot of quantum systems. There's chemistry, there's solid state physics, there's material science, there's quantum gravity, there's all kinds of quantum, quantum field theory, and some of these are actually turning out to be applied sciences as well as very fundamental sciences. So, we probably will run out of the ability to solve equations for these things. You know, solve equations by the standard methods of pencil and paper-

   29. LFLex Fridman18:11

       Yeah.

   30. LSLeonard Susskind18:11

       ... solve the equations by the method of classical computers.
3. 30:00 – 45:00

   So if we may,…

   1. LSLeonard Susskind30:00

      a day, sometimes 20 years. There are things which I thought we were very far from understanding, which practically in a snap of the fingers or a blink of the eye suddenly, uh, became understood, completely surprising to me. There are other things which I looked at and I said, "We're not gonna understand these things for 500 years," in particular, quantum gravity. The scale for that was 20 years, 25 years.... and we understand a lot, and we don't understand it completely now, by any means, but we... I thought it was 500 years to make any progress. It turned out to be very, very far from that. It turned out to be more like 20 or 25 years from the time when I thought it was 500 years.

   2. LFLex Fridman30:48

      So if we may, can we jump around, quantum gravity, some basic ideas in physics? What is the dream of string theory, mathematically? What is the hope? Where does it come from? What problem is it trying to solve?

   3. LSLeonard Susskind31:03

      I don't think the dream of string theory is any different than the dream of fundamental theoretical physics altogether.

   4. LFLex Fridman31:09

      Understanding a unified theory of everything.

   5. LSLeonard Susskind31:12

      Yeah. I, I don't like thinking of string theory as a subject unto itself-

   6. LFLex Fridman31:16

      Mm.

   7. LSLeonard Susskind31:17

      ... with people called string theorists who, uh, are the practitioners of this thing called string theory. I much prefer to think of them as theoretical physicists trying to answer deep, fundamental questions about nature, in particular gravity, in particular gravity and its connection with quantum mechanics-

   8. LFLex Fridman31:36

      Mm-hmm.

   9. LSLeonard Susskind31:37

      ... um, and who, at the present time, find string theory a useful tool, rather than saying there's a subject called string theorist. I don't like being referred to as a string theorist.

   10. LFLex Fridman31:48

       (laughs) Yes. But as a tool, is it useful to think about our nature in multiple dimensions as strings vibrating?

   11. LSLeonard Susskind31:57

       I believe it is useful. I'll tell you what the main use of it has been up till now.

   12. LFLex Fridman32:01

       Mm-hmm.

   13. LSLeonard Susskind32:02

       Well, it has had a number of main uses. Originally, string theory was invented, and I know there, I was there, I was, uh, right at the spot where it was being invented, uh, literally, and it was being invented to understand hadrons. Hadrons are sub-nuclear particles, protons, neutrons, mesons. And at that time, the late '60s, early '70s, it was clear from experiment that these particles called hadrons had, could vibrate, could rotate, could do all the things that a little, uh, closed string can do.

   14. LFLex Fridman32:39

       Mm-hmm.

   15. LSLeonard Susskind32:40

       And, uh, it was and is a valid and correct theory of these hadrons. It's been experimentally tested, and that is a done deal. It had a second life as a theory of gravity, the same basic mathematics except on a very, very much smaller distance scale. The objects of gravitation are 19 orders of magnitude smaller than a proton, but the same mathematics turned up. The same mathematics turned up. What has been its value? Its value is that it's mathematically rigorous in many ways and enabled us to, uh, to find, to find mathematical structures which have both quantum mechanics and gravity with rigor. We can test out ideas. We can test out ideas. We can't test them in a laboratory. The, the 19 orders of magnitude too small of things that we're interested in, but we can test them out mathematically and, and analyze their internal consistency.

   16. LFLex Fridman33:46

       Mm-hmm.

   17. LSLeonard Susskind33:47

       By now, 40 years ago, 35 years ago or so forth, people very, very m- much questioned the consistency between gravity and quantum mechanics. Stephen Hawking was very famous for it, rightly so. Now, nobody questions that consistency anymore. They don't because we have mathematically precise string theories which contain both gravity and quantum mechanics in a consistent way. So it's provided that, um, that certainty that quantum mechanics and gravity can co-exist. That's not a small thing. That's a very big-

   18. LFLex Fridman34:27

       It's a huge thing.

   19. LSLeonard Susskind34:28

       ... it's a huge thing.

   20. LFLex Fridman34:29

       Einstein would be proud.

   21. LSLeonard Susskind34:30

       Einstein, uh, he might be appalled, I don't know.

   22. LFLex Fridman34:32

       Appalled (laughs) . Yeah.

   23. LSLeonard Susskind34:32

       He didn't like quantum mechanics very much.

   24. LFLex Fridman34:34

       (laughs) Yeah.

   25. LSLeonard Susskind34:34

       But he would certainly be struck by it.

   26. LFLex Fridman34:36

       Yeah.

   27. LSLeonard Susskind34:37

       I think that may be, at this time, its biggest contribution to physics in illustrating almost definitively that quantum mechanics and gravity are very closely related and not inconsistent with each other.

   28. LFLex Fridman34:51

       Is there a possibility of something deeper, more profound, that still is, uh, consistent with string theory but is deeper that is to be found?

   29. LSLeonard Susskind35:03

       Well, you could ask the same thing about quantum mechanics. Is there something-

   30. LFLex Fridman35:05

       Exactly.
4. 45:00 – 57:14

   Mm-hmm. …

   1. LSLeonard Susskind45:00

      If you find a particle moving in a certain direction, let's not say a ma- a particle, a baseball. You stop it dead and then you simply reverse its motion. In principle, that's not too hard.

   2. LFLex Fridman45:12

      Mm-hmm.

   3. LSLeonard Susskind45:12

      And it'll go back along its, uh, trajectory in the backward direction.

   4. LFLex Fridman45:16

      Just running the program backwards.

   5. LSLeonard Susskind45:18

      Running the program backward.

   6. LFLex Fridman45:19

      Yeah.

   7. LSLeonard Susskind45:20

      Okay? If you have two baseballs colliding, well, you can do it, but, uh, you have to be very, very careful to get it just right.

   8. LFLex Fridman45:26

      Right. (laughs)

   9. LSLeonard Susskind45:28

      Uh, if you have 10 baseballs, really, really, or better yet, 10, uh, 10 billiard balls on an idealized frictionless billiard table.

   10. LFLex Fridman45:37

       Mm-hmm.

   11. LSLeonard Susskind45:38

       Okay, so you start the balls all in a triangle, right?

   12. LFLex Fridman45:40

       Yep.

   13. LSLeonard Susskind45:41

       And you whack 'em.

   14. LFLex Fridman45:42

       Yep.

   15. LSLeonard Susskind45:43

       Depending on the game you're playing, you either whack 'em or you're really careful, but, uh, but, uh, you whack 'em and they go flying off in all possible directions.

   16. LFLex Fridman45:50

       Yeah. Okay.

   17. LSLeonard Susskind45:51

       Try to reverse that. Try to reverse that. Imagine trying to take every billiard ball, stopping it dead at some, uh, at some point, and reversing its motion so that it was going in the opposite direction. If you did that with tremendous care, it would reassemble itself back into the triangle. Okay, that is a fact, and you can probably do it with two billiard balls, maybe with three billiard balls if you're really lucky. But what happens is as the system gets more and more complicated, you have to be more and more precise not to make the tiniest error, because the tiniest errors will get magnified-

   18. LFLex Fridman46:30

       Yeah.

   19. LSLeonard Susskind46:31

       ... and you'll simply not be able to do the reversal. So yeah, you could th- that, but I wouldn't call that time travel.

   20. LFLex Fridman46:38

       Yeah, that's something else.

   21. LSLeonard Susskind46:39

       Yeah.

   22. LFLex Fridman46:39

       But if we think, think of it, uh, and- and just made me think, if we think the unrolling of state that's happening-

   23. LSLeonard Susskind46:48

       Mm-hmm.

   24. LFLex Fridman46:48

       ... as a program, if, um, we look at the world, so the idea of, uh, looking at the world as a simulation, as a computer.

   25. LSLeonard Susskind46:57

       Right.

   26. LFLex Fridman46:58

       Uh, but it's not a computer, it's, uh, just a single program. Uh, a question arises that might be useful, how h- how hard is it to have a computer that runs the universe?

   27. LSLeonard Susskind47:11

       Okay, so there are mathematical universes that, uh, we know about. One of them is called anti-de Sitter space, where we... And it's quantum mechanics. But I think we could simulate it in a computer, in a quantum computer. Classical computer, all you can do is solve its equations, you can't make it work like a real system. If we could build a quantum computer, a big enough one, like a robust enough one, we could probably simulate a universe, uh, a small version of an anti-de Sitter universe. Anti-de Sitter is a kind of a cosmology.

   28. LFLex Fridman47:55

       Mm-hmm.

   29. LSLeonard Susskind47:57

       All right, so I think we know how to do that. The trouble is the universe that we live in is not the anti-de Sitter geometry, it's the de Sitter geometry, and we don't really understand its quantum mechanics at all. So at the present time, I would say we wouldn't have the vaguest idea how to simulate a universe similar to our own. No, I was gonna ask, could we, could we build in the laboratory a small version, a quantum mechanical v- version, collection of quantum computers entangled and, uh, and coupled together which would reproduce the, uh, the phenomena that go on in the universe, even on a small scale? Yes, if it were anti-de Sitter space. No, if it's de Sitter space.

   30. LFLex Fridman48:44

       Can you s- slightly describe, uh, de Sitter space and anti-de Sitter space?

Episode duration: 57:29