Leonard Susskind: Quantum Mechanics, String Theory, and Black Holes

Leonard Susskind is a professor of theoretical physics at Stanford University, and founding director of the Stanford Institute for 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 conversation is part of the Artificial Intelligence podcast. If you would like to get more information about this podcast go to https://lexfridman.com/ai or connect with @lexfridman on Twitter https://twitter.com/lexfridman, LinkedIn https://www.linkedin.com/in/lexfridman/, Facebook https://www.facebook.com/lexfridman, Medium https://medium.com/@lexfridman, or YouTube https://www.youtube.com/lexfridman where you can watch the video versions of these conversations. If you enjoy the podcast, please rate it 5 stars on iTunes or support it on Patreon https://www.patreon.com/lexfridman. Here’s the outline with timestamps for this episode (on some players you can click on the timestamp to jump to that point in the episode): 00:00 – Introduction 01:02 – Richard Feynman 02:09 – Visualization and intuition 06:45 – Ego in Science 09:27 – Academia 11:18 – Developing ideas 12:12 – Quantum computers 21:37 – Universe as an information processing system 26:35 – Machine learning 29:47 – Predicting the future 30:48 – String theory 37:03 – Free will 39:26 – Arrow of time 46:39 – Universe as a computer 49:45 – Big bang 50:50 – Infinity 51:35 – First image of a black hole 54:08 – Questions within the reach of science 55:55 – Questions out of reach of science

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Lex Fridman?

  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.

• 0:45Lex Fridman?

  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 at LexFriedman, spelled F-R-I-D-M-A-N. And now, here's my conversation with Leonard Susskind.

• 1:19Lex Fridman?

  You worked and were friends with Richard Feynman. How has he influenced you and changed you as a physicist and thinker?

• 1:27Speaker 01

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

• 2:07Speaker 01

  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 you can do and get away with. Practically, you can get away with it.

• 2:25Lex Fridman?

  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 step throughout, using visualization?

• 2:38Speaker 01

  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 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-- I'm not a natural mathematician, or I'm good enough at it. I'm good enough at it, but I'm not a great mathematician. So for me, the way of thinking about physics is first intuitive, first visualization, scribble a few equations maybe, but then try to convert it to mathematics. Experience is that other people are better at converting it to mathematics than I am.

• 3:24Lex Fridman?

  And yet, you've worked with very counterintuitive ideas.

• 3:28Speaker 01

  No, that's true, that's true.

• 3:30Lex Fridman?

  So how do you visualize something counterintuitive? How do you dare?

• 3:33Speaker 01

  By rewiring your brain in new ways. Yeah. Quantum mechanics is not intuitive. Very little of modern physics is intuitive. Intuitive, or what does intuitive mean? It means the ability to think about it with basic classical physics, the physics that we evolved with throwing stones, splashing water, whatever it happens to be. 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. I always said you rewire.

• 4:16Speaker 01

  And it's to the point where me and many of my friends, I and many of my friends--

• 4:24Speaker 01

  Can think more easily quantum mechanically than we can classically. We've gotten so used to it.

• 4:30Lex Fridman?

  I mean, yes, our neural wiring in our brain is such that we understand rocks and stones and water and so on.

• 4:37Speaker 01

  We're sort of evolved for that.

• 4:38Lex Fridman?

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

• 4:54Speaker 01

  Well, I'm not sure. I think many of us have evolved the ability to think quantum mechanically to some extent, but that doesn't mean you can think like an electron.

• 5:07Speaker 01

  That doesn't mean, 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.

• 5:32Lex Fridman?

  Right.

• 5:32Speaker 01

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

• 5:38Lex Fridman?

  Right.

• 5:39Speaker 01

  Or I think of the line as being a line on a piece of paper with a 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.

• 6:03Speaker 01

  On the other hand, we do learn ways to think about five, six, seven dimensions. We learn ways, we learn mathematical ways, and we learn ways to visualize them, but they're different. And so, yeah, I think, I think we do rewire ourselves. Whether we can ever completely rewire ourselves to be completely comfortable with these concepts, I doubt.

• 6:28Lex Fridman?

  So that it's completely natural?

• 6:30Speaker 01

  To where it's completely natural.

• 6:31Lex Fridman?

  So I'm sure there's somewhat, you could argue, creatures that live in a two dimensional space.

• 6:39Speaker 01

  Yeah, maybe there are.

• 6:41Lex Fridman?

  And while it's romanticizing the notion of, of course, we're all living, as far as we know, in three dimensional space, but how do, how do those creatures imagine 3D space?

• 6:52Speaker 01

  Well, probably the way we imagine 4D, by using some mathematics and some equations and some, some tricks.

• 7:01Lex Fridman?

  Okay, so jumping back to Feynman just for a second, he had a little bit of an ego.

• 7:10Speaker 01

  Yes.

• 7:11Lex Fridman?

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

• 7:16Speaker 01

  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 you're very likely to be wrong on any given occasion. Everything you're thinking could suddenly change.

• 7:50Speaker 01

  Young people can come along and say things you won't understand, and you'll be lost and flabbergasted. 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 person who can manage both of those, I would say.

• 8:16Lex Fridman?

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

• 8:25Speaker 01

  I hope so.

• 8:26Lex Fridman?

  So was there a time?

• 8:29Lex Fridman?

  Where you complete, you felt, you looked at yourself and asked, am I completely wrong about this?

• 8:36Speaker 01

  Oh, yeah, about, about the whole thing or about specific things?

• 8:40Lex Fridman?

  The whole thing. What do you, wait, which whole thing?

• 8:43Speaker 01

  Me and me and my ability to do this thing.

• 8:46Lex Fridman?

  Oh, those kinds of doubts. First of all, did you have those kinds of doubts?

• 8:50Speaker 01

  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.

• 9:21Speaker 01

  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.

• 9:34Speaker 01

  I never, I never felt this insecurity. Am I ever gonna get a job? That, uh, that had never occurred to me that I wouldn't.

• 9:43Lex Fridman?

  Maybe you could speak a little bit to this sense of what is academia? Because I too feel a bit uncomfortable in it.

• 9:54Lex Fridman?

  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?

• 10:11Speaker 01

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

• 10:15Lex Fridman?

  Yeah.

• 10:15Speaker 01

  You know, thinking is one thing, feeling is another thing. I felt like an outsider until a certain age when I suddenly found myself the ultimate insider in academic physics.

• 10:31Speaker 01

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

• 10:39Lex Fridman?

  You were never quite, it was a phase transition. You were never quite

• 10:42Speaker 01

  Pretty much.

• 10:43Lex Fridman?

  in the middle.

• 10:44Speaker 01

  Yeah, that's right. I wasn't. I always felt a little bit of an outsider. In the beginning, a lot an outsider.

• 10:54Speaker 01

  My way of thinking was different. My approach to mathematics was different. But also, my social background that I came from was different. Now, these days, half the young people I meet, their parents are professors.

• 11:09Lex Fridman?

  Right.

• 11:10Speaker 01

  Right. That was not my case. So, yeah. But then, all of a sudden, at some point, I found myself at the, very much, the center of, maybe not the only one at the center, but certainly one of the people in the center of a certain kind of physics. And all that went away. I mean, it went away in a flash.

• 11:34Lex Fridman?

  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?

• 11:44Speaker 01

  Oh, both. Both, very definitely both. The younger a time, I spent more time with myself. Now, because I'm at Stanford, because I'm, because I have a lot of ex-students and, you know, people 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. It's a, it's a very important part. I spend less time, probably, completely self-focused than, than with a piece of paper and just sitting there staring at it.

• 12:29Lex Fridman?

  What are your hopes for quantum computers? So, machines that are based on, that have some elements of leveraged quantum mechanical ideas?

• 12:41Speaker 01

  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 quantum mechanics on a computer, on a classical computer. But the classical computer is not doing, is not a quantum mechanical system itself. Oh, of course it is. Everything's made of quantum mechanics, but it's not functioning. It's not functioning as a quantum system. It's just solving equations. The quantum computer is truly a quantum system, which is actually doing the things that you're programming it to do.

• 13:24Speaker 01

  You want to program a quantum field theory? If you do it in classical physics, that program is not actually functioning in the computer as a quantum field theory. It's just solving some equations. Physically, it's not doing the things that the quantum system would do. The quantum computer is really a quantum mechanical system, which is actually carrying out the quantum operations. You can measure it at the end. It intrinsically satisfies the uncertainty principle. It is limited in the same way that quantum systems are limited by uncertainty and so forth. And it really is a quantum system.

• 14:06Speaker 01

  That means that what you're doing when you program something for a quantum system is you're actually building a real version of the system. The limits of a classical computer. Classical computers are enormously limited when it comes to the quantum systems. They're enormously limited because, you've probably heard this before, but in order to store the amount of information that's in a quantum state of 400 spins, that's not very many, 400 I can put in my pocket, 400 pennies in my pocket.

• 14:44Speaker 01

  To be able to simulate the quantum state of 400 elementary quantum systems, qubits we call them, to do that would take more information than can possibly be stored in the entire universe if it were packed so tightly that you couldn't pack anymore in.

• 15:06Lex Fridman?

  Right.

• 15:07Speaker 01

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

• 15:16Lex Fridman?

  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, NP, so on. What kind of algorithms, just intuitively, do you think it unlocks for us?

• 15:35Speaker 01

  OK, so we know that there are a handful of algorithms that can seriously beat classical computers and which can have exponentially more power. This is a mathematical statement. Nobody's exhibited this in the 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, 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. If you're interested in a certain quantum system

• 16:19Speaker 01

  and it's too hard to simulate classically, 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 to the quantum system. You make measurements on it, quantum measurements on it. The advantage is you can run it much slower. You could say, why bother? Why not just use the real system? Why not just do experiments on the real system? Well, real systems are kind of limited. You can't change them. You can't manipulate them. You can't slow them down so that you can poke into them. You can't modify them in arbitrary kinds of ways to see what would happen

• 17:02Speaker 01

  if I changed the system a little bit. So I think that quantum computers...

• 17:10Speaker 01

  will be extremely valuable in

• 17:14Speaker 01

  in understanding quantum systems.

• 17:17Lex Fridman?

  At the lowest level, the fundamental laws.

• 17:20Speaker 01

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

• 17:25Lex Fridman?

  That's right.

• 17:26Speaker 01

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

• 17:33Speaker 01

  thing of quantum computers, factoring large numbers. That doesn't seem that much to do with quantum mechanics.

• 17:38Lex Fridman?

  Right.

• 17:39Speaker 01

  It's it seems to be almost a fluke

• 17:42Speaker 01

  that that a quantum computer can solve the factoring problem in a short time. So those and those problems seem to be extremely special, rare. And it's not clear to me that there's going to be a lot of them. On the other hand, there are a lot of quantum systems, 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.

• 18:28Speaker 01

  Solve the equations by the method of classical computers. And so what we'll do is we'll build versions of these systems. Run them and run them under controlled circumstances where we can change them, manipulate them. Make measurements on them and find out all the things we want to know.

• 18:47Lex Fridman?

  So in finding out the things we want to know about very small systems, right, now the is there something we can also find out about the macro level? About something about the function, forgive me, of our brain. Biological systems, the the stuff that's about one meter in size versus much, much smaller.

• 19:10Speaker 01

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

• 19:15Lex Fridman?

  Black holes.

• 19:16Speaker 01

  Black holes are big things. They are 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 or systems, you didn't ask about the black holes, you asked about brains.

• 19:46Lex Fridman?

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

• 19:49Speaker 01

  It's different.

• 19:49Lex Fridman?

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

• 19:59Speaker 01

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

• 20:02Lex Fridman?

  Right.

• 20:03Speaker 01

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

• 20:45Speaker 01

  I, I sort of hope they're wrong, just because I, I like the romantic idea that the brain is a quantum system.

• 20:51Lex Fridman?

  Yeah.

• 20:52Speaker 01

  But I think, I think probably not. The other thing, big systems can be composed of lots of little systems. Okay? Materials, the materials that, that we work with and so forth are. Can be large systems, a large piece of material, but 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 we imagine. Topological insulators, this kind of thing, that kind of thing. Those are macroscopic systems, but they're just superconductors.

• 21:34Speaker 01

  Superconductors 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, its functioning and its properties depend very, very strongly on quantum mechanics. And to analyze them, you need the tools of quantum mechanics.

• 21:54Lex Fridman?

  If we can go on to black holes and looking at the universe as an information processing system, as a computer, as a giant computer.

• 22:03Speaker 01

  It's a giant computer.

• 22:05Lex Fridman?

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

• 22:25Speaker 01

  All systems are information processing systems. You poke them, they change a little bit, they evolve. All systems are information processing.

• 22:35Lex Fridman?

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

• 22:42Speaker 01

  It sure does.

• 22:43Lex Fridman?

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

• 22:56Speaker 01

  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.

• 23:03Lex Fridman?

  Yes.

• 23:03Speaker 01

  If you ask me about this, I'm not sure my thoughts are worth anything. But 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 and we're thinking, I think we get it wrong. 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 forty five degrees. You have different neurons. I never imagined that there were whole circuits which were devoted to vertical lines in my brain.

• 23:41Lex Fridman?

  Yeah.

• 23:42Speaker 01

  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.

• 24:12Lex Fridman?

  Hmm.

• 24:12Speaker 01

  But, um, how will we get it right? I think maybe computer scientists will get it right eventually. I don't think that anywhere is 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. 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, what this consciousness thing is all about.

• 24:56Speaker 01

  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.

• 25:06Lex Fridman?

  So that's, that's a really interesting point. You've, in many cases, if you 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 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?

• 25:28Speaker 01

  Yeah.

• 25:28Lex Fridman?

  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?

• 25:38Speaker 01

  Well, 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 physics has, um. Take in very complicated systems, analyze them and found simplicity in them for sure. I said superconductors before, it's an obvious one. A superconductor seems like a monstrously complicated thing with all sorts of crazy 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 which we, in other contexts, we learned about and we're very familiar with.

• 26:23Speaker 01

  So yeah, 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 that we can make them simple. We don't want to make, I don't know who said this, but we don't want to make them simpler than they really are.

• 26:41Lex Fridman?

  Right.

• 26:42Speaker 01

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

• 26:52Lex Fridman?

  In terms of artificial intelligence, nobody really knows what are the limits of our current approaches, you mentioned machine learning. How do we create human level intelligence? It seems that there's a lot of very smart physicists who perhaps oversimplify the nature of intelligence and think of it as information processing. And therefore, there doesn't seem to be any theoretical reason why we can't artificially create human level or superhuman level intelligence. In fact, the reasoning goes, if you create human level intelligence, the same approach you just used to create human level intelligence should allow you to create superhuman level intelligence very easily, exponentially.

• 27:32Lex Fridman?

  So what do you think that way of thinking that comes from physicists is all about?

• 27:38Speaker 01

  I wish I knew, but there's a particular reason why I wish I knew.

• 27:44Speaker 01

  I have a second job. I consult for Google. Ah. Not for Google, for Google X. I am the senior academic advisor to a group of machine learning physicists.

• 28:00Speaker 01

  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. Yeah. So I give them advice on things. Anyway, I see these young physicists who are approaching the machine learning problem. There is a real machine learning problem, namely, why does it work as well as it does? Right. Nobody really seems to understand why it is capable of doing the kind of generalizations that it does and so forth. And there are three 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.

• 28:46Speaker 01

  They've provided a lot of data. And it is they who demonstrated that machine learning can work much better than you had any right to expect. The machine learning systems are systems. The system's not too different than the kind of systems that physicists study. There's not all that much difference between quantum, in the structure of mathematics. Physically, yes, but in the structure of the mathematics, between a tensor network designed to describe a quantum system on the one hand and the kind of networks that are used in machine learning. So there are more and more, I think, young physicists are being drawn to this field of machine learning, some very, very good ones.

• 29:31Speaker 01

  I work with a number of very good ones, not on machine learning, but on having lunch. On having lunch? Right. Yeah. And I can tell you, they are super smart. They don't seem to be so arrogant about their physics backgrounds that they think they can do things that nobody else can do. But the physics way of thinking, I think, will add, will add great value to, will bring value to the machine learning. I believe it will. And I think it already has.

• 30:04Lex Fridman?

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

• 30:16Speaker 01

  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 became understood, completely surprising to me.

• 30:38Speaker 01

  There are other things which I looked at and I said, "We're not going to 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.

• 31:05Lex Fridman?

  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?

• 31:20Speaker 01

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

• 31:26Lex Fridman?

  Understanding a unified theory of everything.

• 31:28Speaker 01

  Yeah. I don't like thinking of string theory as a subject unto itself with people called string theorists who 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. And who, at the present time, find string theory a useful tool.

• 32:00Speaker 01

  Rather than saying there's this subject called string theorists, I don't like being referred to as a string theorist.

• 32:05Lex Fridman?

  Yes, but as a tool, is it useful to think about our nature in multiple dimensions, the strings vibrating?

• 32:14Speaker 01

  I believe it is useful. I'll tell you what the main use of it has been up till now. Well, it has had a number of main uses. Originally, string theory was invented, and I know that I was there. I was right at the spot where it was being invented, 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 closed string can do.

• 32:57Speaker 01

  And 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.

• 33:19Speaker 01

  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 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 the laboratory. They're 19 orders of magnitude too small, the things that we're interested in. But we can test them out mathematically and analyze their internal consistency.

• 34:04Speaker 01

  By now, 40 years ago, 35 years ago, or so forth, people very, very much questioned the consistency between gravity and quantum mechanics. Stephen Hawking was very famous for it, rightly so.

• 34:22Speaker 01

  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 certainty that quantum mechanics and gravity can coexist. That's not a small thing.

• 34:44Lex Fridman?

  It's a huge thing.

• 34:45Speaker 01

  It's a huge thing.

• 34:45Lex Fridman?

  Einstein would be proud.

• 34:47Speaker 01

  Einstein, he might be appalled, I don't know. He didn't like quantum mechanics very much. But he would certainly be struck by it.

• 34:52Lex Fridman?

  Yeah.

• 34:54Speaker 01

  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.

• 35:07Lex Fridman?

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

• 35:19Speaker 01

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

• 35:22Lex Fridman?

  Exactly.

• 35:22Speaker 01

  Yeah, yeah. I think string theory is just an example of a quantum mechanical system that contains both gravitation and quantum mechanics. So is there something underlying quantum mechanics?

• 35:36Lex Fridman?

  Perhaps something deterministic.

• 35:38Speaker 01

  Perhaps something deterministic. My friend Gerard Ituff, whose name you may know, he's a very famous physicist, Dutch. Not as famous as he should be, but...

• 35:50Lex Fridman?

  Hard to spell his name.

• 35:51Speaker 01

  It's hard to say his name. No, it's easy to spell his name. Apostrophe. He's the only person I know whose name begins with an apostrophe. And he's one of my heroes in physics. He's a little younger than me, but he's nevertheless one of my heroes. Ituff believes that there is some sub-structure to the world which is classical in character, deterministic in character, which somehow, by some mechanism that he has a hard time spelling out, emerges as quantum mechanics.

• 36:27Speaker 01

  I don't.

• 36:28Lex Fridman?

  The wave function is somehow emergent.

• 36:30Speaker 01

  The wave function and the whole, 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.

• 36:40Lex Fridman?

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

• 36:45Speaker 01

  Well, here I think is, here I think is where you have to be humble. Here's where humility comes. I don't think anybody should say anything is the bottom of the well at this time. As I think we, 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 Etouff has asked very interesting and deep questions. I don't like his answers.

• 37:18Lex Fridman?

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

• 37:44Speaker 01

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

• 37:49Lex Fridman?

  The illusion of it, the.

• 37:50Speaker 01

  The illusion of consciousness, the illusion of free will, the illusion of self.

• 37:58Lex Fridman?

  Does that connect to?

• 38:00Speaker 01

  How can a physical system do that? And, uh, and I am as puzzled as anybody.

• 38:05Lex Fridman?

  There's echoes of it in the observer effect.

• 38:08Speaker 01

  Yeah.

• 38:08Lex Fridman?

  So do you understand what it means to be an observer?

• 38:12Speaker 01

  I understand it at a technical level. An observer is a system with enough degrees of freedom that it can record information and which can become entangled with the thing that it's measuring. Entanglement is the key.

• 38:25Speaker 01

  When a system, which we call an apparatus or an observer, same thing, interacts with the system that it's observing, it doesn't just look at it, it becomes physically entangled with it. And it's that entanglement which we call an observation or a measurement. Now, does that satisfy me personally as an observer?

• 38:49Speaker 01

  Yes and no. I find it very satisfying that we have a mathematical representation of what it means to observe a system.

• 38:57Lex Fridman?

  You are observing stuff right now.

• 38:59Speaker 01

  Yeah.

• 38:59Lex Fridman?

  The conscious level.

• 39:00Speaker 01

  Right.

• 39:01Lex Fridman?

  You think there's echoes of that kind of entanglement in our macro scale?

• 39:06Speaker 01

  Yes, absolutely, for sure. We're entangled with, quantum mechanically entangled with everything in this room. If we weren't, then we would just, well, we wouldn't be observing it. But on the other hand, you can ask, do I really, am I really comfortable with it? And I'm uncomfortable with it in the same way that I can never get comfortable with five dimensions. My, my brain isn't wired for it.

• 39:35Lex Fridman?

  Are you comfortable with four dimensions?

• 39:38Speaker 01

  A little bit more because I can always imagine the fourth dimension is time.

• 39:43Lex Fridman?

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

• 39:51Speaker 01

  That is a big question in physics right now. All the physics that we do, or at least the people that I am comfortable with talking to, my, my friends.

• 40:04Lex Fridman?

  Yeah.

• 40:04Speaker 01

  My friends. We all ask the same question that you just asked. Space, we have a pretty good idea is emergent and it emerges out of entanglement and other, and other things. Time always seems to be built into our equations as just what Newton pretty much would have thought. Newton, modified a little bit by Einstein, would have called time. And in, mostly in our equations, it is not emergent. Time in physics is completely symmetric, forward and backward.

• 40:40Lex Fridman?

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

• 40:47Speaker 01

  For most microscopic phenomena, no. It's only when the phenomena involves systems which are big enough for thermodynamics to become important, for entropy to become important. For a small, a small system, entropy is not a good concept.

• 41:04Speaker 01

  Entropy is something 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. 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 we understand better than most people think they understand. Most people say they think we understand it. Yeah, I think we understand it. It's a statistical idea.

• 41:40Lex Fridman?

  You mean like second law of thermodynamics, entropy and so on?

• 41:44Speaker 01

  Yeah, yeah. Take a pack of cards and you fling it in the air and you look what happens to it. It gets random.

• 41:49Lex Fridman?

  We understand it.

• 41:50Speaker 01

  It doesn't go from random to simple. It goes from simple to random.

• 41:55Lex Fridman?

  But do you think it ever breaks down?

• 41:58Speaker 01

  What I think you can do is in a laboratory setting, you can take a system which is somewhere intermediate between being small and being large and make it go backward. A thing which looks like it only wants to go forward because of statistical, mechanical reasons, because of the second law. You can very, very carefully manipulate it to make it run backward. I don't think you can take an egg, Humpty Dumpty, who fell on the floor and reverse that. But you can, in a very controlled situation, you can take systems which appear to be evolving statistically toward randomness,

• 42:42Speaker 01

  stop them, reverse them, and make them go back.

• 42:46Lex Fridman?

  What's the intuition behind that? How do we do that? How do we reverse it? You're saying a closed system.

• 42:52Speaker 01

  Yeah, pretty much closed system, yes.

• 42:55Lex Fridman?

  Did you just say that time travel is possible?

• 42:58Speaker 01

  No, I didn't say time travel is possible. I said you can make a system go backward.

• 43:02Lex Fridman?

  In time.

• 43:03Speaker 01

  You can make it go backward. You can make it reverse its steps. You can make it reverse its trajectory.

• 43:08Lex Fridman?

  Yeah. How do we do it? What's the intuition there? Does it have... Is it just a fluke? Thing that we can do at a small scale in the lab that doesn't have.

• 43:18Speaker 01

  Well, what I'm saying is you can do it on a little bit better than a small scale. You can certainly do it with a simple, small system. Small systems don't have any sense of the arrow of time. Atoms, atoms are, uh, no sense of an arrow of time. They're completely reversible. It's only when you have, you know, the second law of thermodynamics is the law of large numbers.

• 43:45Lex Fridman?

  So you can break the law because it's not.

• 43:47Speaker 01

  You can break it, you can break it, but it's hard. It requires great care. The bigger the system is, the more care, the more, the harder it is. You have to overcome what's called chaos. And that's hard. And it requires more and more precision. For 10 particles, you might be able to do it with a, with a some effort. For 100 particles, it's really hard. For a thousand or a million particles, forget it. But not for any fundamental reason, just because it's technologically too hard to make the system go backward.

• 44:25Lex Fridman?

  So, so no, no time travel for, for engineering reasons?

• 44:30Speaker 01

  Uh, no, no, no, no. What is time travel? Time travel, time travel to the future? That's easy. You just close your eyes, go to sleep and you wake up in the future.

• 44:40Lex Fridman?

  Yeah, yeah. A good nap gets you there, yeah.

• 44:42Speaker 01

  A good nap gets you there, right.

• 44:44Lex Fridman?

  But in reversing the second law of thermodynamics, going backward in time. For anything that's human scale is a very difficult engineering effort.

• 44:56Speaker 01

  I, I wouldn't call that time travel because it gets too mixed up with what the science fiction calls time travel. This is just the ability to reverse a system. You, you take the system and you reverse the direction of motion of every molecule in it. That, you can do it with one molecule. If you find a particle moving in a certain direction, let's not say a particle, a baseball. You stop it dead and then you simply reverse its motion. In principle, that's not too hard.

• 45:29Lex Fridman?

  Mm hmm.

• 45:29Speaker 01

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

• 45:33Lex Fridman?

  Just running the program backwards.

• 45:35Speaker 01

  Running the program backward.

• 45:36Lex Fridman?

  Yeah.

• 45:36Speaker 01

  Okay, if you have two baseballs colliding, well, you can do it. But you have to be very, very careful to get it just right.

• 45:45Speaker 01

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

• 45:54Lex Fridman?

  Mm hmm.

• 45:55Speaker 01

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

• 45:57Lex Fridman?

  Yep.

• 45:58Speaker 01

  And you whack them.

• 45:59Lex Fridman?

  Yep.

• 46:00Speaker 01

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

• 46:07Lex Fridman?

  Yeah.

• 46:08Speaker 01

  Okay, try to reverse that.

• 46:11Speaker 01

  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.

• 46:47Lex Fridman?

  Yeah.

• 46:47Speaker 01

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

• 46:55Lex Fridman?

  Yeah, that's something else. But if you think, think of it, it just made me think, if you think the unrolling of state. That's happening as a program. If, um, we look at the world, silly idea of, uh, looking at the world as a simulation, as a computer.

• 47:14Speaker 01

  Right.

• 47:15Lex Fridman?

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

• 47:28Speaker 01

  Okay, so. There are mathematical universes.

• 47:35Speaker 01

  That we know about, one of them is called anti-de Sitter space, where we and it's quantum mechanics. Well, 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 the real system. If we could build a quantum computer, a big enough one, a robust enough one, we could probably simulate a universe, a small version of an anti-de Sitter universe. Anti-de Sitter is a kind of a cosmology. All right. So I think we know how to do that.

• 48:16Speaker 01

  The trouble is, the universe that we live in is not the anti-de Sitter geometry. It's the D 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, we could ask, could we, could we build in the laboratory, a small version, a quantum mechanical version, the collection of quantum computers entangled and coupled together, which would reproduce 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 the Sitter space.

• 49:01Lex Fridman?

  Can you slightly describe the Sitter space and anti-de Sitter space?

• 49:05Speaker 01

  Yeah.

• 49:06Lex Fridman?

  What are the geometric properties of?

• 49:08Speaker 01

  They differ, they differ by the sign of a single constant called the cosmological constant. 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. You could think of it as a system of quantum mechanical system, isolated, in an isolated environment. The Sitter space is the one we really live in, and that's the one that's exponentially expanding. Exponential expansion, dark energy, whatever we want to call it. And we don't understand that mathematically.

• 49:52Lex Fridman?

  Do we understand?

• 49:53Speaker 01

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

• 50:01Lex Fridman?

  What about, is there an understanding of the, the birth, the origin?

• 50:06Speaker 01

  No.

• 50:06Lex Fridman?

  The Bing Bang?

• 50:07Speaker 01

  No, no, no. There's theories, there are theories.

• 50:12Speaker 01

  My favorite is the one called eternal inflation.

• 50:15Lex Fridman?

  The infinity can be on both sides, on one of the sides, and on other sides. So what's eternal infinity?

• 50:22Speaker 01

  Okay.

• 50:26Speaker 01

  Infinity on both sides.

• 50:29Lex Fridman?

  Oh, boy.

• 50:30Speaker 01

  Yeah, yeah, that's.

• 50:32Lex Fridman?

  Why is that your favorite? Because it's the, the most just mind blowing?

• 50:38Speaker 01

  No.

• 50:38Lex Fridman?

  Because we want a beginning.

• 50:39Speaker 01

  No, why do we want a beginning?

• 50:42Lex Fridman?

  I.

• 50:43Speaker 01

  In practice, there was a beginning, of course. In practice, there was a, a beginning. But could it have been um, a, a random fluctuation in an otherwise infinite time? Maybe.

• 50:58Speaker 01

  In any case, the, the eternal inflation theory, I think if correctly understood, would be infinite in both directions.

• 51:07Lex Fridman?

  How do you think about infinity?

• 51:09Speaker 01

  Oh, God.

• 51:10Lex Fridman?

  So, okay, of course, you can think about mathematically.

• 51:14Speaker 01

  I just finished this, I just finished this discussion with my friend Sergey Brin.

• 51:17Lex Fridman?

  Yes.

• 51:18Speaker 01

  How do you think about infinity? I say, well, Sergey Brin is infinitely rich.

• 51:24Lex Fridman?

  Uh, how do you test that hypothesis? Okay, uh, that is such a good line, right?

• 51:32Lex Fridman?

  Yeah, so there's no, there's really no way to visualize some of these things, like.

• 51:36Speaker 01

  Yeah, no, this is a very good question. Does physics have any, uh, is, does infinity have any place in physics?

• 51:44Lex Fridman?

  Right.

• 51:44Speaker 01

  Right, and all I can say is very good question.

• 51:52Lex Fridman?

  So what do you think of the recent first image of a black hole visualized from the IVA Horizon Telescope?

• 51:59Speaker 01

  It's an incredible triumph of science in itself. The fact that there are black holes which collide is not a surprise, and they seem to work exactly the way they're supposed to work. Will we learn a great deal from it? I don't know. I can't, I, I, we might, but the kind of things we'll learn won't really be about black holes. Why there are black holes in nature of that particular mass scale and why they're so common may tell us something about the structure, evolution of structure in the universe. But I don't think it's going to tell us anything new about black holes. But it's a triumph in the sense that you go back 100 years and it was a continuous

• 52:43Speaker 01

  development, general relativity, the discovery of black holes, LIGO, the incredible technology that went into LIGO. It is something that I never would have believed was going to happen, you know, 30, 40 years ago. And I, I think it's a magnificent structure, magnificent thing, this evolution of general relativity, LIGO, high precision, ability to measure things on a scale of 10 to the minus 21. So.

• 53:23Lex Fridman?

  So you're just astonished that we, this path took us to this picture. Is it different? You know, you've thought a lot about black holes. Is it, how did you visualize them in, in your mind? And is the picture different than you visualized it?

• 53:43Speaker 01

  No, no. It's simply confirmed. You know, it's a magnificent triumph to have confirmed a direct observation that Einstein's theory of gravity at the level of black hole collisions actually works.

• 53:59Speaker 01

  Is awesome. It is really awesome. You know, I know some of the people who are involved in that. They're just ordinary people.

• 54:06Lex Fridman?

  Yeah.

• 54:06Speaker 01

  And the idea that they could carry this out. I just, I'm shocked.

• 54:13Lex Fridman?

  Yeah. Just these little homo sapiens.

• 54:16Speaker 01

  Yeah, just these little monkeys.

• 54:17Lex Fridman?

  Yeah, got together.

• 54:18Speaker 01

  Right.

• 54:19Lex Fridman?

  And took a picture of.

• 54:21Speaker 01

  Slightly advanced livers, I think.

• 54:25Lex Fridman?

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

• 54:30Speaker 01

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

• 54:33Lex Fridman?

  You think that's within the reach of science?

• 54:36Speaker 01

  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.

• 54:43Lex Fridman?

  Not a physicist.

• 54:45Speaker 01

  Perhaps.

• 54:45Lex Fridman?

  With the help.

• 54:46Speaker 01

  Perhaps at some point, but I think physicists will try to simplify it down to something that they can use their methods and maybe they're not appropriate. Maybe 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 a machine learning. I'll tell you another thing that I find awesome. You know, this Google thing that they taught the computers how to play chess?

• 55:24Lex Fridman?

  Yeah, yeah.

• 55:24Speaker 01

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

• 55:31Lex Fridman?

  Against each other, self play.

• 55:32Speaker 01

  Against each other. This is a form of evolution. These machines evolved. They evolved an intelligence. 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. 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 goes on in our own intelligence.

• 56:09Lex Fridman?

  Well, life and intelligence is.

• 56:11Speaker 01

  Yeah.

• 56:12Lex Fridman?

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

• 56:18Speaker 01

  Yeah.

• 56:22Speaker 01

  Is there an intelligence out there that's underlies the whole thing? You can call them with the G word if you want. I can say, are we a computer simulation with a purpose?

• 56:35Speaker 01

  Is there an agent, an intelligent agent, that underlies 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? Yeah, there's a lot of questions. And I don't see, it seems to me a real question.

• 56:58Lex Fridman?

  It's an answerable question.

• 57:00Speaker 01

  Well, I don't know if it's answerable. The questions have to be answerable to be real.

• 57:06Speaker 01

  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.

• 57:21Lex Fridman?

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

• 57:25Speaker 01

  Okay, good.