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

1. 0:00 – 2:09

   Feynman’s influence: intuition, visualization, and “getting away with it”

   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.

2. 2:09 – 6:54

   Rewiring intuition for quantum ideas and higher dimensions

   1. 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?

   2. 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.

   3. LFLex Fridman3:08

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

   4. LSLeonard Susskind3:12

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

   5. LFLex Fridman3:13

      So how do you visualize something counterintuitive?

   6. LSLeonard Susskind3:15

      (laughs) . Okay, actually-

   7. LFLex Fridman3:15

      How do you dare?

   8. LSLeonard Susskind3:16

      ... by rewiring your brain in new ways.

   9. LFLex Fridman3:19

      Ha.

   10. 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.

   11. LFLex Fridman3:57

       Mm-hmm.

   12. 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.

   13. LFLex Fridman4:13

       I mean, yes, our neural wiring-

   14. LSLeonard Susskind4:16

       Yeah.

   15. LFLex Fridman4:16

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

   16. LSLeonard Susskind4:20

       We sort of evolved for that.

   17. LFLex Fridman4:22

       Evolved for it.

   18. LSLeonard Susskind4:22

       Yeah.

   19. 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-

   20. 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.

   21. LFLex Fridman5:15

       Right.

   22. LSLeonard Susskind5:16

       If I want to visualize a line, I think of the line as being a line in three dimensions.

   23. LFLex Fridman5:21

       Right.

   24. LSLeonard Susskind5:23

       Or I think of the line as being a line on a piece of paper, with the piece of paper being in three dimensions. I never seem to be able to, in some abstract and pure way, visualize in my head the one dimension, the two dimension, the four dimension, the five dimensions, and I don't think that's ever gonna happen. The reason is, I think our neural wiring is just set up for that. On the other hand, we do learn ways to think about five, six, seven dimensions.

   25. LFLex Fridman5:51

       Right.

   26. LSLeonard Susskind5:52

       We learn ways, we learn s- mathematical ways, and we learn ways to visualize them, but they're different.

   27. LFLex Fridman5:59

       Mm-hmm.

   28. LSLeonard Susskind6:00

       And so, yeah, I think, uh, I think we do rewire ourselves. Whether we can ever completely rewire ourselves to be completely comfortable with these concepts, I doubt.

   29. LFLex Fridman6:11

       So that it's completely natural?

   30. LSLeonard Susskind6:13

       To, to where it's completely natural.
3. 6:54 – 8:08

   Ego in science: balancing arrogance and humility

   1. LFLex Fridman6:54

      Well, do you think ego is powerful or dangerous in science?

   2. LSLeonard Susskind7:00

      I think both, both, both. I think you have to have both arrogance and humility. You have to have the arrogance to say, "I can do this. Nature is difficult, nature is very, very hard. I'm smart enough, I can do it. I can win the battle with nature." On the other hand, I think you also have to have the humility to know that, um, you're very likely to be wrong on any given occasion.

   3. LFLex Fridman7:28

      Mm-hmm.

   4. LSLeonard Susskind7:29

      Everything you're thinking could suddenly change. Young people can come along and say things you won't understand, and you'll be lost and flabbergasted.

   5. LFLex Fridman7:39

      Yeah.

   6. LSLeonard Susskind7:39

      So I think it's a combination of both. You better recognize that you're very limited, and you better be able to say to yourself, "I'm not so limited that I can't win this battle with nature." It takes a special kind of, um, person who, uh, can manage both of those, I would say.

   7. LFLex Fridman7:59

      And I would say there's echoes of that in your own work, a little bit of ego, a little bit of, uh, outside of the box, humble thinking.

   8. LSLeonard Susskind8:07

      Well, I ho- (laughs) I hope so.

4. 8:08 – 11:17

   Outsider to insider: class background, academia, and belonging

   1. LFLex Fridman8:08

      (laughs) So was there a time where you comple- you felt, you looked at yourself and asked, "Am I completely wrong about this?"

   2. LSLeonard Susskind8:19

      Oh, yeah. About, uh, about the whole thing or about specific things?

   3. LFLex Fridman8:24

      The whole thing.

   4. LSLeonard Susskind8:24

      No, I, I-

   5. LFLex Fridman8:25

      (laughs) What do you... Wait, which whole thing?

   6. LSLeonard Susskind8:27

      Me, and me and my ability to do this thing.

   7. LFLex Fridman8:30

      Oh, those kinds of doubts.

   8. LSLeonard Susskind8:31

      Those kind of doubts, yeah.

   9. LFLex Fridman8:31

      Well, first of all, did you have those kinds of doubts?

   10. LSLeonard Susskind8:34

       No, I had different kind of doubts. I came from a very working class background, and I was uncomfortable in academia for, oh, for a long time. Um, but they weren't doubts about my ability or my, uh ... They were just the discomfort in being in an environment that, um, my family hadn't participated in, I knew nothing about as a young person. I didn't learn that there was such a thing called physics until I was almost 20 years old.

   11. LFLex Fridman9:02

       Yeah.

   12. LSLeonard Susskind9:04

       So I, uh, I did have certain kind of doubts, but not about my ability. I don't think I was too worried about whether I would succeed or not. I, I never f- I never felt this insecurity, "Am I ever gonna get a job?"

   13. LFLex Fridman9:22

       Hm.

   14. LSLeonard Susskind9:23

       That, uh, that had never occurred to me, that I wouldn't.

   15. LFLex Fridman9:27

       Maybe you could speak a little bit to this sense of, what is academia?

   16. LSLeonard Susskind9:31

       Huh.

   17. LFLex Fridman9:31

       Because I too feel a bit uncomfortable in it.

   18. LSLeonard Susskind9:35

       Mm-hmm.

   19. LFLex Fridman9:37

       I- there's something I can't put quite into words, what you have, that's not, doesn't... If we call it music, you play a different kind of music than a lot of academia. What, how have you, uh, joined this orchestra? How, uh, how do you think about it?

   20. LSLeonard Susskind9:54

       I don't know that I thought about it as much as I just, uh, felt it.

   21. LFLex Fridman9:58

       Yeah.

   22. LSLeonard Susskind9:59

       You know, thinking is one thing, feeling is another thing.

   23. LFLex Fridman10:02

       Right.

   24. LSLeonard Susskind10:02

       I felt like an outsider until a certain age, when I suddenly found myself the ultimate insider in academic physics.

   25. LFLex Fridman10:12

       Right.

   26. LSLeonard Susskind10:14

       And, uh-

   27. LFLex Fridman10:16

       (laughs)

   28. LSLeonard Susskind10:16

       ... it was a sharp transition, and I wa- I wasn't a young man. I was probably 50 years old.

   29. LFLex Fridman10:22

       So you were never quite... It was a phase transition. You were never quite-

   30. LSLeonard Susskind10:25

       Pretty much.
5. 11:17 – 12:13

   How Susskind develops ideas: solitude vs daily collaboration

   1. LFLex Fridman11:17

      So maybe, maybe a little bit with Feynman, but in general, how do you develop ideas? Do you work through ideas alone? Do you brainstorm with others?

   2. LSLeonard Susskind11:27

      Oh, both. Both, very definitely both. The ear- the younger time, I would, spent more time with myself. Now, because I'm at Stanford, because I'm, uh, because I have a lot of ex-students and ex-, you know, people who, uh, who are interested in the same thing I am, I spend a good deal of time almost on a daily basis interacting, brainstorming, as you said.

   3. LFLex Fridman11:58

      Mm-hmm.

   4. LSLeonard Susskind11:59

      It's, uh, it's a very important part. I spend less time probably completely self-focused in, uh, in, uh, in...... with a piece of paper and just sitting there staring at it.

6. 12:13 – 14:59

   Quantum computers: real quantum systems vs classical simulation limits

   1. LFLex Fridman12:13

      What are your hopes for quantum computers? So-

   2. LSLeonard Susskind12:17

      Yeah.

   3. LFLex Fridman12:17

      ... machines that are based on, that have some elements of, uh, leverage quantum mechanical ideas.

   4. LSLeonard Susskind12:24

      Yeah. It's not just leveraging quantum mechanical ideas. You can simulate quantum systems on a classical computer. Simulate them means solve the Schrodinger equation for them, or solve the equations of, uh, quantum mechanics on a computer, on a classical computer. But the classical computer is not doing, is not a quantum mechanical system itself. Of course, it is. Ev- Everything's made of quantum mechanics, but it's not functioning-

   5. LFLex Fridman12:53

      Some of it. Right.

   6. LSLeonard Susskind12:53

      ... it's not functioning as a quantum system. It's just solving equations. A quantum computer is truly a quantum system which is actually doing the things-

   7. LFLex Fridman13:04

      Mm-hmm.

   8. LSLeonard Susskind13:05

      ... that you're programming it to do. You want to program a, uh, quantum field theory? If you do it in classical physics, that program is not actually functioning in the computer as a quantum field theory.

   9. LFLex Fridman13:18

      Hmm.

   10. LSLeonard Susskind13:18

       It's just solving some equations. Uh, physically it's not doing the things that, uh, that the quantum system would do. A quantum computer is really a quantum mechanical system which is actually carrying out the quantum operations.

   11. LFLex Fridman13:33

       Mm-hmm.

   12. LSLeonard Susskind13:34

       You can measure it at the end. It intrinsically satisfies the uncertainty principle. It is limited in- in the same way that quantum systems are limited by uncertainty and so forth, and it really is a quantum system. That means that what you're- what you're doing when you program something for a quantum system is you're actually building a real version of the system.

   13. LFLex Fridman13:57

       Right.

   14. LSLeonard Susskind13:58

       The limits of a classical comput- classical computers are enormously limited when it comes to, um, to quantum systems. They're enormously limited because, you probably heard this before, but in order to store the amount of information that's in a quantum state of 400, uh, spins, that's not very many. 400, uh, I can put in my pocket. Uh, I can put, uh, 400 pennies in my pocket. To be able to simulate the quantum state of 400 elementary quantum systems, qubits we call them-

   15. LFLex Fridman14:36

       Mm-hmm.

   16. LSLeonard Susskind14:37

       ... to do that would take more information than can possibly be stored in the entire universe if it were packed so tightly that, uh, that you couldn't pack any more in.

   17. LFLex Fridman14:49

       Right. (laughs)

   18. LSLeonard Susskind14:50

       400 qubits. On the other hand, if your quantum computer is composed of 400 qubits, it can do everything 400 qubits can do.

7. 14:59 – 18:30

   What quantum advantage is for: simulation over ‘special’ algorithms like factoring

   1. LFLex Fridman14:59

      What kind 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.
8. 18:30 – 21:40

   Brains, macroscopic quantum phenomena, and black holes as “big” quantum systems

   1. LFLex Fridman18:30

      So, in finding out the things we want to know about very small systems, right? Uh, the... Is there something we can also find out about the macro level-

   2. LSLeonard Susskind18:42

      Well-

   3. LFLex Fridman18:42

      ... about, something about the function, forgive me, of our brain, uh, biological systems? The, the stuff that's about one meter in size versus, uh, much, much smaller.

   4. LSLeonard Susskind18:53

      Well, what all the excitement is about among the people that I interact with is understanding black holes.

   5. LFLex Fridman18:58

      Black holes.

   6. LSLeonard Susskind18:59

      Black holes are big things.

   7. LFLex Fridman19:01

      (laughs)

   8. LSLeonard Susskind19:01

      Uh, they have many, many degrees of freedom. There is another kind of quantum system that is big. It's a large quantum computer, and one of the things we've learned is that the physics of large quantum computers is in some ways similar to the physics of large quantum black holes, and we're using that relationship. Now you asked... You didn't ask about quantum computers as systems, you didn't ask about, uh, black holes. You asked about brains.

   9. LFLex Fridman19:30

      Yeah, about stuff that's in the middle of the two.

   10. LSLeonard Susskind19:32

       It's different.

   11. LFLex Fridman19:34

       So, b- black holes are, uh, there's something fundamental about black holes that feels to be very different than a brain.

   12. LSLeonard Susskind19:42

       Yes. (laughs) And they also function in a very quantum mechanical way.

   13. LFLex Fridman19:46

       Right.

   14. LSLeonard Susskind19:46

       Okay. It is, f- first of all, unclear to me, but of course it's unclear to me. I'm not a, I'm not a neuroscientist. I have, I don't even have very many friends who are neuroscientists.

   15. LFLex Fridman19:59

       (laughs)

   16. LSLeonard Susskind20:00

       I would like to have more friends who are neuroscientists. I just don't run into them very often. Among the few neuroscientists I've ever talked about, about this, they are pretty convinced that the brain functions classically.

   17. LFLex Fridman20:16

       Hm.

   18. LSLeonard Susskind20:16

       That it is not intrinsically a quantum mechanical system, or it doesn't make use of the, of the special features, entanglement, coherent superposition. Are they right? I don't know. I, I sort of hope they're wrong, just because I, I like the romantic idea that the brain is a quantum system.

   19. LFLex Fridman20:34

       Yeah.

   20. LSLeonard Susskind20:35

       But I, I think the, I think probably not. The other thing, big systems can be composed of lots of little systems, okay? Materials, the materials that, uh, that we work with and so forth are, can be large systems, you know, a large piece of material, but they're ba- they're made out of quantum systems. Now, one of the things that's been happening over the last good number of years is we're discovering materials and quantum systems which function much more quantum mechanically than, uh, than we imagined. Topological insulators, this kind of thing, that kind of thing. Those are macroscopic systems, but they... Or just superconductors, superconductors-

   21. LFLex Fridman21:19

       Mm-hmm.

   22. LSLeonard Susskind21:19

       ... uh, have a lot of quantum mechanics in them. You can have a large chunk of superconductor. So it's a big piece of material. On the other hand, it's functioning and its properties depend very, very strongly on quantum mechanics, and to analyze them, you need the tools of quantum mechanics.

   23. LFLex Fridman21:37

       If we can go onto black holes.

   24. LSLeonard Susskind21:39

       Mm-hmm.

9. 21:40 – 27:22

   Universe as information processing, consciousness, and why introspection misleads

   1. LFLex Fridman21:40

      (laughs) And, uh, looking at the universe as, uh, information processing system, as a computer-

   2. LSLeonard Susskind21:45

      Mm-hmm.

   3. LFLex Fridman21:45

      ... as a giant computer.

   4. LSLeonard Susskind21:46

      It's a giant computer.

   5. LFLex Fridman21:48

      What's the power of thinking of the universe as an information processing system, and what is pers- perhaps its use besides the mathematical use of discussing black holes, uh, and your famous debates and ideas around that, to human beings or life in general as information processing systems?

   6. LSLeonard Susskind22:08

      Well, all systems are information processing systems. You poke them. They change a little bit. They evolve. All systems are information processing systems.

   7. LFLex Fridman22:18

      So there's no extra magic to us humans? It certainly feels, consciousness, intelligence feels like magic.

   8. LSLeonard Susskind22:26

      It sure does.

   9. LFLex Fridman22:26

      Where does it emerge from? If we look at it as information processing, what are the emergent phenomena that, uh, come from viewing the world as an information processing system?

   10. LSLeonard Susskind22:39

       Here is what I think, and my thoughts are not worth much in this. If you ask me about physics, my thoughts may be worth something.

   11. LFLex Fridman22:46

       Yes.

   12. LSLeonard Susskind22:46

       If you ask by, me about this, I'm not sure my thoughts are worth anything. But, uh, as I said earlier, I think when we do introspection, when we imagine doing introspection and try to figure out what it is when we do when we're thinking, I think we, I think we get it wrong.

   13. LFLex Fridman23:03

       Mm-hmm.

   14. LSLeonard Susskind23:03

       I'm pretty sure we get it wrong. Everything I've heard about the way the brain functions is so counterintuitive. For example, you have neurons which detect vertical lines. You have different neurons which detect lines at 45 degrees.

   15. LFLex Fridman23:17

       Mm-hmm.

   16. LSLeonard Susskind23:17

       You have different neurons... I never imagined that there were whole circuits which were devoted to vertical lines in my brain.

   17. LFLex Fridman23:24

       (laughs) Yeah.

   18. LSLeonard Susskind23:25

       It doesn't seem to be the way my brain works. My brain seems to work if I put my finger up vertically or if I put it horizontally or if I put it this way or that way, it seems to me it's the same, uh, the same circuits that are... It's not the way it works. Uh, the way the brain is compartmentalized seems to be very, very different than what I would have imagined if I were just doing, um, psychological introspection about how things work. My conclusion is that we won't get it right that way.

   19. LFLex Fridman23:55

       Hm.

   20. LSLeonard Susskind23:56

       That, um, how will we get it right? I think maybe computer scientists will get it right eventually. I don't think they're anywhere as near it. I don't even think they're thinking about it. But by computer... Eventually, we will build machines, perhaps, which are complicated enough and partly engineered, partly evolved, maybe evolved by machine learning and so forth. This machine learning is very interesting.

   21. LFLex Fridman24:23

       Mm-hmm.

   22. LSLeonard Susskind24:23

       By machine learning, we will evolve systems, and we may start to discover mechanisms-... that, uh, that have implications for how we think and for what, wh- what this consciousness thing is all about, and we'll be able to do experiments on them and perhaps answer questions that we can't possibly answer, uh, by, uh, by introspection.

   23. LFLex Fridman24:49

       So, that's, that's a really interesting point. You've, uh, in many cases, if we look at even the string theory, when you first think about a system, it seems really complicated, like the human brain. And through some basic reasoning, e- and trying to discover fundamental low-level behavior of the system, you find out that it's actually much simpler. Do you, one, have you-

   24. LSLeonard Susskind25:11

       Yeah.

   25. LFLex Fridman25:12

       ... is that generally the process? And two, do you have that also hope for biological systems as well, for all the kinds of stuff we're studying at the human level?

   26. LSLeonard Susskind25:21

       Of course, physics always begins by trying to find the simplest version of something and analyze it. Yeah, I mean, there are lots of examples where, uh, physics has, um, taken very complicated systems, analyzed them, and found simplicity in them, for sure. I, I said superconductors before, it's an obvious one. A superconductor seems like a monstrously complicated thing with, uh, all sorts of crazy, uh, electrical properties, magnetic properties, and so forth. And when it finally is boiled down to its simplest elements, it's a very simple quantum mechanical phenomenon called spontaneous symmetry breaking. And, uh, which we, in other contexts, we learned about and we're very familiar with. So yeah, uh, I mean, yes, we do take complicated things, make them simple. But what we don't want to do is take things which are intrinsically complicated and fool ourselves into thinking, uh, that we can make them simple. We don't wanna make... Uh, I don't know who said this, but we don't want to make them simpler than they really are.

   27. LFLex Fridman26:24

       Right.

   28. LSLeonard Susskind26:25

       Okay? Is the brain a thing which ultimately functions by some simple rules, or is it just complicated?

   29. LFLex Fridman26:35

       In terms of artificial intelligence, nobody really knows, uh, what are the limits of our current approaches. You mentioned machine learning.

   30. LSLeonard Susskind26:42

       Yeah.
10. 27:22 – 30:48

    Physics meets machine learning: why it works, and tensor networks as a bridge

    1. LSLeonard Susskind27:22

       I wish I knew, but there's a particular reason why I wish I knew. I have a, uh, second job. I consult for Google.

    2. LFLex Fridman27:32

       Ah.

    3. LSLeonard Susskind27:32

       Not for Google, for Google X. I am the senior academic advisor to, uh, to a group of machine learning physicists at, uh... Now, that sounds crazy because I know nothing about the subject. I know very little about the subject. On the other hand, I'm good at giving advice.

    4. LFLex Fridman27:52

       Yeah.

    5. LSLeonard Susskind27:52

       So, I give them advice on things. Anyway, I see these young physicists who are approaching the machine learning problem. There is a m- there, there is a real machine learning problem, namely, why does it work as well as it does, right? It, uh, nobody really seems to understand why it is capable of doing the kind of generalizations that it does and so forth. And, um, there are three g- groups of people who have thought about this. There are the engineers. The engineers are incredibly smart, but they tend not to think as hard about why the thing is working as much as they do how to use it, obviously. They provided a lot of data, and it is they who've demonstrated that machine learning can work much better than you had any right to expect. The machine learning systems are systems that the system's not too different than the kind of systems that physicists study. There's not all that much difference between, uh, quantum c- uh, in, in the structure of the mathematics. Physically, yes, but in the structure of the mathematics, between a, uh, tensor network designed to, uh, describe a, uh, quantum system on the one hand and the kind of networks that are used in machine learning. So, uh, th- there are more and more, I think, young physicists are being drawn to this field of machine learning, some very, very good ones. I work with a number of very good ones, not on machine learning, but, uh, on having lunch.

    6. LFLex Fridman29:20

       On having lunch? (laughs)

    7. LSLeonard Susskind29:21

       Right.

    8. LFLex Fridman29:22

       Yeah.

    9. LSLeonard Susskind29:22

       Uh, and I can tell you, they are s- super smart.

    10. LFLex Fridman29:27

        Yes.

    11. LSLeonard Susskind29:27

        They don't seem to be, um, so arrogant about their physics backgrounds that they think they can do things that nobody else can do.

    12. LFLex Fridman29:35

        Yeah.

    13. LSLeonard Susskind29:35

        But the physics way of thinking, I think, will add, will add great value to, um, or will bring value to the machine learning. I, I believe it will, and I think it already has.

    14. LFLex Fridman29:48

        At what time scale do you think predicting the future becomes useless, in your long experience in being surprised at new discoveries?

    15. LSLeonard Susskind29:59

        Sometimes 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.

11. 30:48 – 34:51

    String theory’s purpose: quantum gravity, consistency, and tools (not a tribe)

    1. 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?

    2. LSLeonard Susskind31:03

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

    3. LFLex Fridman31:09

       Understanding a unified theory of everything.

    4. LSLeonard Susskind31:12

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

    5. LFLex Fridman31:16

       Mm.

    6. 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-

    7. LFLex Fridman31:36

       Mm-hmm.

    8. 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.

    9. LFLex Fridman31:48

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

    10. LSLeonard Susskind31:57

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

    11. LFLex Fridman32:01

        Mm-hmm.

    12. 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.

    13. LFLex Fridman32:39

        Mm-hmm.

    14. 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.

    15. LFLex Fridman33:46

        Mm-hmm.

    16. 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-

    17. LFLex Fridman34:27

        It's a huge thing.

    18. LSLeonard Susskind34:28

        ... it's a huge thing.

    19. LFLex Fridman34:29

        Einstein would be proud.

    20. LSLeonard Susskind34:30

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

    21. LFLex Fridman34:32

        Appalled (laughs) . Yeah.

    22. LSLeonard Susskind34:32

        He didn't like quantum mechanics very much.

    23. LFLex Fridman34:34

        (laughs) Yeah.

    24. LSLeonard Susskind34:34

        But he would certainly be struck by it.

    25. LFLex Fridman34:36

        Yeah.

    26. 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.

12. 34:51 – 39:26

    Deeper reality? determinism vs quantum mechanics, free will, and the observer as entanglement

    1. 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?

    2. LSLeonard Susskind35:03

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

    3. LFLex Fridman35:05

       Exactly.

    4. LSLeonard Susskind35:05

       ... yeah, yeah. I think string theory is just an example of a quantum mechanical system that contains both gravitation and, uh, and quantum mechanics.

    5. LFLex Fridman35:15

       Right.

    6. LSLeonard Susskind35:16

       So is there something underlying quantum mechanics?

    7. LFLex Fridman35:19

       Perhaps something deterministic.

    8. LSLeonard Susskind35:21

       Perhaps something deterministic. My friend, uh, Gerard 't Hooft, whose name you may know-

    9. LFLex Fridman35:27

       Mm-hmm.

    10. LSLeonard Susskind35:27

        ... he's a very famous physicist, Dutch, not as famous as he should be, but, uh, but, uh-

    11. LFLex Fridman35:33

        Hard to spell his name, so maybe-

    12. LSLeonard Susskind35:34

        It's hard, it's hard to say his name.

    13. LFLex Fridman35:36

        Yeah.

    14. LSLeonard Susskind35:36

        No, it's easy to spell his name. Apostrophe, he's the only person I know who's name begins with an apostrophe.

    15. LFLex Fridman35:42

        (laughs)

    16. LSLeonard Susskind35:42

        And he's one of my heroes in physics, and he's a little younger than me, but he's nevertheless one of my heroes. 't Hooft believes that there is some substructure to the world which is classical in character, de- deterministic in character, which somehow, by some mechanism that he has a hard time spelling out, emerges as quantum mechanics. I don't.

    17. LFLex Fridman36:11

        The wave function is somehow emergent.

    18. LSLeonard Susskind36:13

        The wave function and the wh- not just the wave function, but the whole mech and the whole thing that goes with quantum mechanics, uncertainty, entanglement, all these things are emergent.

    19. LFLex Fridman36:24

        So you think quantum mechanics is the bottom of the well? Is, is the, uh-

    20. LSLeonard Susskind36:28

        Well, here, I think is, uh-uh, here, I think is where you have to be humble. Here's where humility comes. I don't think anybody should say anything is, uh, the bottom of the well at this time.As I think we c- I think we can reasonably say... or I can reasonably say, when I look into the well, I can't see past quantum mechanics. I don't see any reason for there to be anything beyond quantum mechanics. I think Etuf does ask very interesting and deep questions. I don't like his answers. (laughs)

    21. LFLex Fridman37:02

        (laughs) Well-

    22. LSLeonard Susskind37:02

        Um...

    23. LFLex Fridman37:02

        ... again, let me ask, if we look at the deepest nature of reality, w- whether it's deterministic or when observed as probabilistic, um, what does that mean for, uh, our human level of ideas of free will? Is there any connection whatsoever from this perception, perhaps illusion of free will that we have-

    24. LSLeonard Susskind37:25

        Mm-hmm.

    25. LFLex Fridman37:25

        ... and the fundamental nature of reality?

    26. LSLeonard Susskind37:27

        The only thing I can say is I am, I am puzzled by that as much as you are.

    27. LFLex Fridman37:32

        The illusion of it, the-

    28. LSLeonard Susskind37:33

        The illusion-

    29. LFLex Fridman37:34

        ... illusion.

    30. LSLeonard Susskind37:35

        ... of consciousness, the illusion of free will, the illusion of self. Uh-
13. 39:26 – 46:39

    Time’s arrow, entropy, and reversing trajectories without “time travel”

    1. LFLex Fridman39:26

       So, the arrow of time, are you comfortable with that arrow? (laughs) Do you think time is an emergent phenomena or is it fundamental to nature?

    2. LSLeonard Susskind39:35

       That is a big question in physics right now.

    3. LFLex Fridman39:37

       Okay.

    4. LSLeonard Susskind39:37

       All the physics that we do, or at least that the people that I am comfortable talking to-

    5. LFLex Fridman39:44

       (laughs)

    6. LSLeonard Susskind39:44

       ... my, my, my friends-

    7. LFLex Fridman39:47

       Yeah, yeah.

    8. LSLeonard Susskind39:47

       ... my friends, w- well, we all ask the same question that you just asked.

    9. LFLex Fridman39:51

       Yeah.

    10. LSLeonard Susskind39:52

        Space, we have a pretty good idea is emergent, and it emerges out of t- entanglement and other, uh, and other things. Time always seems to be built into our equations as just what Newton pretty much would've thought, Newton modified a little bit by Einstein, would've called time. And in... mostly in our equations, it is not emergent. Time and physics is completely symmetric-

    11. LFLex Fridman40:21

        Right, symmetric.

    12. LSLeonard Susskind40:21

        ... forward and backward.

    13. LFLex Fridman40:23

        So, (laughs) so you don't really need to think about the arrow of time for most physical phenomena?

    14. LSLeonard Susskind40:30

        For mo- most microscopic phenomena, no.

    15. LFLex Fridman40:32

        All right.

    16. LSLeonard Susskind40:33

        It's only when the phenomena involves systems which are big enough for thermodynamics to become important-

    17. LFLex Fridman40:38

        Right.

    18. LSLeonard Susskind40:38

        ... for entropy to become important. For small sub- a small system, entropy is not a good concept. Entropy is something which, which emerges out of large numbers. It's a probabilistic idea, it's a statistical idea, and it's a thermodynamic idea. Thermodynamics requires lots and lots and lots of little substructures, okay? So, it's not until you emerge at the thermodynamic level that there's an arrow of time. Do we understand it? Yeah, I think, I think we understand better than most people think they under- most people say they think we understand it. Yeah, I think we understand it. It's a st- a statistical idea.

    19. LFLex Fridman41:23

        The... you mean, like, second law of thermodynamics, uh, entropy-

    20. LSLeonard Susskind41:26

        Yeah.

    21. LFLex Fridman41:26

        ... and so on?

    22. LSLeonard Susskind41:27

        Yeah.

    23. LFLex Fridman41:27

        So, would you-

    24. LSLeonard Susskind41:28

        Take a pack of cards and you fling it in the air, and you, and you look what happens to it.

    25. LFLex Fridman41:31

        Yeah, but-

    26. LSLeonard Susskind41:31

        It gets random.

    27. LFLex Fridman41:32

        ... we understand-

    28. LSLeonard Susskind41:32

        It doesn't s- it doesn't go from random to simple. It goes from simple to random.

    29. LFLex Fridman41:38

        But do you think it ever breaks down?

    30. LSLeonard Susskind41:41

        What I think c- you can do is in a laboratory setting, you can take a system which is somewhere intermediate between being small and being large-
14. 46:39 – 54:07

    Simulating universes: AdS vs dS, cosmology mysteries, infinity, and black hole observations

    1. 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-

    2. LSLeonard Susskind46:48

       Mm-hmm.

    3. 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.

    4. LSLeonard Susskind46:57

       Right.

    5. 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?

    6. 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.

    7. LFLex Fridman47:55

       Mm-hmm.

    8. 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.

    9. LFLex Fridman48:44

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

    10. LSLeonard Susskind48:48

        Yeah.

    11. LFLex Fridman48:49

        What are the geometric properties of, of-

    12. LSLeonard Susskind48:51

        They differ, they differ by the, the sign of a single constant called the cosmological constant.

    13. LFLex Fridman48:56

        Mm-hmm.

    14. LSLeonard Susskind48:58

        One of them is negatively curved, the other is positively curved. The anti-de Sitter space, which is the negatively curved one, you can think of as an isolated system in a box with reflecting walls.

    15. LFLex Fridman49:14

        Mm-hmm.

    16. LSLeonard Susskind49:14

        You could think of it as a system, a qu- quantum mechanical system isolated in an isolated environment. De Sitter space is the one we really live in, and that's the one that's exponentially expanding.

    17. LFLex Fridman49:26

        Mm-hmm.

    18. LSLeonard Susskind49:26

        Exponential expansion, dark energy, whatever we want to call it, and we don't understand that mathematically.

    19. LFLex Fridman49:35

        Do we understand, uh-

    20. LSLeonard Susskind49:37

        Not everybody would agree with me, but I don't understand. Well, uh, they would agree with me, they definitely would agree with me that I don't understand it.

    21. LFLex Fridman49:43

        (laughs) What about, is there an understanding of the, the birth, the origin-

    22. LSLeonard Susskind49:49

        No.

    23. LFLex Fridman49:49

        ... the Big Bang? So-

    24. LSLeonard Susskind49:50

        No, no.

    25. LFLex Fridman49:50

        ... does one come with the other?

    26. LSLeonard Susskind49:51

        There's theories, there are theories. My favorite is the one called eternal inflation.

    27. LFLex Fridman49:59

        The infinity can be on both sides, on one of the sides, on none of the sides. So what, uh, what's-

    28. LSLeonard Susskind50:03

        I like-

    29. LFLex Fridman50:04

        ... eternal infinity?

    30. LSLeonard Susskind50:05

        Okay. Infinity on both sides.
15. 54:07 – 57:14

    What science may (and may not) answer: AI evolution and the ‘G-word’ question

    1. LFLex Fridman54:07

       (laughs) What kind of questions can science not currently answer, but you hope might be able to soon?

    2. LSLeonard Susskind54:13

       Well, you, you've already addressed them. What is consciousness, for example?

    3. LFLex Fridman54:17

       You think sci- that's within the reach of science?

    4. LSLeonard Susskind54:19

       I think it's somewhat within the reach of science, but I think, now I think it's in the hands of the computer scientists and the neuroscientists.

    5. LFLex Fridman54:27

       Not a physicist?

    6. LSLeonard Susskind54:28

       Perhaps in the-

    7. LFLex Fridman54:29

       With the help.

    8. LSLeonard Susskind54:29

       Perhaps at some point, but I think physicists will try to s- uh, simplify it down to something that they can use their methods, and maybe they're not appropriate. Maybe we, we, maybe we simply need to do more machine learning on bigger scales, evolve machines, machines not only that learn but evolve their own architecture, as a process of learning, evolve an architecture. Not under our control, only partially under our control, but under the control of machine learning, uh...

    9. LFLex Fridman54:59

       Yeah.

    10. LSLeonard Susskind55:00

        I'll tell you another thing that I find awesome.You know this Google thing that they taught the computers how to play chess?

    11. LFLex Fridman55:07

        Yeah, yeah.

    12. LSLeonard Susskind55:08

        Okay. They taught the computers how to play chess, not by teaching them how to play chess, but just having them play against each other.

    13. LFLex Fridman55:14

        Against each other. So-

    14. LSLeonard Susskind55:15

        Against each other. This is a form of evolution. These machines evolved. They evolved an intelligence.

    15. LFLex Fridman55:23

        Mm-hmm.

    16. LSLeonard Susskind55:25

        They evolved an intelligence without anybody telling them how to do it. They were not engineered. They just played against each other and got better and better and better.

    17. LFLex Fridman55:36

        Better.

    18. LSLeonard Susskind55:37

        That makes me think that machines can evolve intelligence. What exact kind of intelligence, I don't know. But in understanding that better and better, maybe we'll get better clues as to what, uh, goes on in our own intelligence.

    19. LFLex Fridman55:52

        What life and intelligence is.

    20. LSLeonard Susskind55:54

        Yeah.

    21. LFLex Fridman55:55

        Last question. What kind of questions can science not currently answer and may never be able to answer?

    22. LSLeonard Susskind56:01

        Yeah. Is there an intelligence out there that underlies the whole thing? You can call them with the G word if you want. Uh, I can say, uh, are we a computer simulation with a purpose? Is there an agent, an intelligent agent, that underlies, uh, or is responsible for the whole thing? Does that intelligent agent satisfy the laws of physics? Does it satisfy the laws of quantum mechanics? Is it made of atoms and molecules?

    23. LFLex Fridman56:34

        Mm-hmm.

    24. LSLeonard Susskind56:34

        Yeah, there's a lot of questions, and I don't see... It seems, it seems to be a real question.

    25. LFLex Fridman56:42

        It's an answerable question if they're-

    26. LSLeonard Susskind56:43

        Well, I don't know if it's answerable. Do questions have to be answerable to be real? Some philosophers would say that a question is not a question unless it's answerable. This question doesn't seem to me answerable by any known method, but it seems to me real.

    27. LFLex Fridman57:05

        There's no better place to end. Leonard, thank you so much for talking today.

    28. LSLeonard Susskind57:08

        Okay, good.

Episode duration: 57:29