Lex Fridman PodcastLeonard Susskind: Quantum Mechanics, String Theory and Black Holes | Lex Fridman Podcast #41
EVERY SPOKEN WORD
90 min read · 18,120 words- 0:00 – 1:02
Introduction
- LFLex 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. 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.
- 1:02 – 2:09
Richard Feynman
- LFLex Fridman
You worked and were friends with Richard Feynman. How has he influenced you and changed you as a physicist and thinker?
- LSLeonard Susskind
What I saw, I think what I saw was somebody who could do physics in this deeply intuitive way.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
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-
- LFLex Fridman
Hmm.
- LSLeonard Susskind
... 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."
- LFLex Fridman
(laughs) .
- LSLeonard Susskind
Practically, you can get away with it.
- 2:09 – 6:45
Visualization and intuition
- LFLex 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 a step throughout using visualization?
- LSLeonard Susskind
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.
- LFLex Fridman
And yet, you've worked with s- very counterintuitive ideas. So how do you-
- LSLeonard Susskind
But tha- no, that's true. That's true.
- LFLex Fridman
So how do you visualize something counterintuitive?
- LSLeonard Susskind
(laughs) . Okay, actually-
- LFLex Fridman
How do you dare?
- LSLeonard Susskind
... by rewiring your brain in new ways.
- LFLex Fridman
Ha.
- LSLeonard Susskind
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.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
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.
- LFLex Fridman
I mean, yes, our neural wiring-
- LSLeonard Susskind
Yeah.
- LFLex Fridman
... in our brain is such that we understand rocks and stones and water and so on.
- LSLeonard Susskind
We sort of evolved for that.
- LFLex Fridman
Evolved for it.
- LSLeonard Susskind
Yeah.
- LFLex Fridman
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-
- LSLeonard Susskind
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.
- LFLex Fridman
Right.
- LSLeonard Susskind
If I want to visualize a line, I think of the line as being a line in three dimensions.
- LFLex Fridman
Right.
- LSLeonard Susskind
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.
- LFLex Fridman
Right.
- LSLeonard Susskind
We learn ways, we learn s- mathematical ways, and we learn ways to visualize them, but they're different.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
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.
- LFLex Fridman
So that it's completely natural?
- LSLeonard Susskind
To, to where it's completely natural.
- 6:45 – 9:27
Ego in Science
- LFLex Fridman
jumping back to, uh, Feynman just for a second, um, he had a, a little bit of an ego.
- LSLeonard Susskind
(laughs)
- LFLex Fridman
And-
- LSLeonard Susskind
Yes.
- LFLex Fridman
Well, do you think ego is powerful or dangerous in science?
- LSLeonard Susskind
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.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
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.
- LFLex Fridman
Yeah.
- LSLeonard Susskind
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.
- LFLex Fridman
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.
- LSLeonard Susskind
Well, I ho- (laughs) I hope so.
- LFLex Fridman
(laughs) So was there a time where you comple- you felt, you looked at yourself and asked, "Am I completely wrong about this?"
- LSLeonard Susskind
Oh, yeah. About, uh, about the whole thing or about specific things?
- LFLex Fridman
The whole thing.
- LSLeonard Susskind
No, I, I-
- LFLex Fridman
(laughs) What do you... Wait, which whole thing?
- LSLeonard Susskind
Me, and me and my ability to do this thing.
- LFLex Fridman
Oh, those kinds of doubts.
- LSLeonard Susskind
Those kind of doubts, yeah.
- LFLex Fridman
Well, first of all, did you have those kinds of doubts?
- LSLeonard Susskind
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.
- LFLex Fridman
Yeah.
- LSLeonard Susskind
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?"
- LFLex Fridman
Hm.
- LSLeonard Susskind
That, uh, that had never occurred to me, that I wouldn't.
- 9:27 – 11:18
Academia
- LSLeonard Susskind
- LFLex Fridman
Maybe you could speak a little bit to this sense of, what is academia?
- LSLeonard Susskind
Huh.
- LFLex Fridman
Because I too feel a bit uncomfortable in it.
- LSLeonard Susskind
Mm-hmm.
- LFLex Fridman
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?
- LSLeonard Susskind
I don't know that I thought about it as much as I just, uh, felt it.
- LFLex Fridman
Yeah.
- LSLeonard Susskind
You know, thinking is one thing, feeling is another thing.
- LFLex Fridman
Right.
- LSLeonard Susskind
I felt like an outsider until a certain age, when I suddenly found myself the ultimate insider in academic physics.
- LFLex Fridman
Right.
- LSLeonard Susskind
And, uh-
- LFLex Fridman
(laughs)
- LSLeonard Susskind
... it was a sharp transition, and I wa- I wasn't a young man. I was probably 50 years old.
- LFLex Fridman
So you were never quite... It was a phase transition. You were never quite-
- LSLeonard Susskind
Pretty much.
- LFLex Fridman
... in the middle.
- LSLeonard Susskind
Yeah.
- LFLex Fridman
(laughs)
- LSLeonard Susskind
That, uh, that's right, I wasn't.
- LFLex Fridman
(laughs)
- LSLeonard Susskind
I always felt a little bit of an outsider. In the beginning, a lot an outsider. My way of thinking was different. My approach to mathematics was different. But also, uh, this, my social background that I came from was different. You know, these days, uh, half the young people I meet, their parents were professors.
- LFLex Fridman
Right.
- LSLeonard Susskind
Right. Uh, 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.
- LFLex Fridman
So
- 11:18 – 12:12
Developing ideas
- LFLex Fridman
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?
- LSLeonard Susskind
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.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
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.
- 12:12 – 21:37
Quantum computers
- LFLex Fridman
What are your hopes for quantum computers? So-
- LSLeonard Susskind
Yeah.
- LFLex Fridman
... machines that are based on, that have some elements of, uh, leverage quantum mechanical ideas.
- LSLeonard Susskind
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-
- LFLex Fridman
Some of it. Right.
- LSLeonard Susskind
... 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-
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
... 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.
- LFLex Fridman
Hmm.
- LSLeonard Susskind
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.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
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.
- LFLex Fridman
Right.
- LSLeonard Susskind
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-
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
... 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.
- LFLex Fridman
Right. (laughs)
- LSLeonard Susskind
400 qubits. On the other hand, if your quantum computer is composed of 400 qubits, it can do everything 400 qubits can do.
- LFLex 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-
- LSLeonard Susskind
Mm-hmm.
- LFLex Fridman
... and P, so on.
- LSLeonard Susskind
Mm-hmm.
- LFLex Fridman
What, what kind of algorithms, just intuitively, do you think is, it unlocks for us?
- LSLeonard Susskind
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.
- LFLex Fridman
Yeah.
- LSLeonard Susskind
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.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
If you're interested in a certain quantum system and it's too hard to simulate classically-
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
... 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.
- 21:37 – 26:35
Universe as an information processing system
- LFLex Fridman
If we can go onto black holes.
- LSLeonard Susskind
Mm-hmm.
- LFLex Fridman
(laughs) And, uh, looking at the universe as, uh, information processing system, as a computer-
- LSLeonard Susskind
Mm-hmm.
- LFLex Fridman
... as a giant computer.
- LSLeonard Susskind
It's a giant computer.
- LFLex Fridman
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?
- LSLeonard Susskind
Well, all systems are information processing systems. You poke them. They change a little bit. They evolve. All systems are information processing systems.
- LFLex Fridman
So there's no extra magic to us humans? It certainly feels, consciousness, intelligence feels like magic.
- LSLeonard Susskind
It sure does.
- LFLex Fridman
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?
- LSLeonard Susskind
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.
- LFLex Fridman
Yes.
- LSLeonard Susskind
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.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
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.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
You have different neurons... I never imagined that there were whole circuits which were devoted to vertical lines in my brain.
- LFLex Fridman
(laughs) Yeah.
- LSLeonard Susskind
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.
- LFLex Fridman
Hm.
- LSLeonard Susskind
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.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
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.
- LFLex Fridman
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-
- LSLeonard Susskind
Yeah.
- LFLex 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?
- LSLeonard Susskind
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.
- LFLex Fridman
Right.
- LSLeonard Susskind
Okay? Is the brain a thing which ultimately functions by some simple rules, or is it just complicated?
- 26:35 – 29:47
Machine learning
- LSLeonard Susskind
- LFLex Fridman
In terms of artificial intelligence, nobody really knows, uh, what are the limits of our current approaches. You mentioned machine learning.
- LSLeonard Susskind
Yeah.
- LFLex Fridman
How do we create human-level intelligence? It seems that there's a lot of very smart physicists-
- LSLeonard Susskind
Yeah.
- LFLex Fridman
... 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 w- uh, h- 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. So, what do you think that way of thinking that comes from physicists is all about?
- LSLeonard Susskind
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.
- LFLex Fridman
Ah.
- LSLeonard Susskind
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.
- LFLex Fridman
Yeah.
- LSLeonard Susskind
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.
- LFLex Fridman
On having lunch? (laughs)
- LSLeonard Susskind
Right.
- LFLex Fridman
Yeah.
- LSLeonard Susskind
Uh, and I can tell you, they are s- super smart.
- LFLex Fridman
Yes.
- LSLeonard Susskind
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.
- LFLex Fridman
Yeah.
- LSLeonard Susskind
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.
- 29:47 – 30:48
Predicting the future
- LSLeonard Susskind
- LFLex Fridman
At what time scale do you think predicting the future becomes useless, in your long experience in being surprised at new discoveries?
- LSLeonard Susskind
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.
- 30:48 – 37:03
String theory
- LFLex 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?
- LSLeonard Susskind
I don't think the dream of string theory is any different than the dream of fundamental theoretical physics altogether.
- LFLex Fridman
Understanding a unified theory of everything.
- LSLeonard Susskind
Yeah. I, I don't like thinking of string theory as a subject unto itself-
- LFLex Fridman
Mm.
- LSLeonard Susskind
... 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-
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
... 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.
- LFLex Fridman
(laughs) Yes. But as a tool, is it useful to think about our nature in multiple dimensions as strings vibrating?
- LSLeonard Susskind
I believe it is useful. I'll tell you what the main use of it has been up till now.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
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.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
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.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
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-
- LFLex Fridman
It's a huge thing.
- LSLeonard Susskind
... it's a huge thing.
- LFLex Fridman
Einstein would be proud.
- LSLeonard Susskind
Einstein, uh, he might be appalled, I don't know.
- LFLex Fridman
Appalled (laughs) . Yeah.
- LSLeonard Susskind
He didn't like quantum mechanics very much.
- LFLex Fridman
(laughs) Yeah.
- LSLeonard Susskind
But he would certainly be struck by it.
- LFLex Fridman
Yeah.
- LSLeonard Susskind
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.
- LFLex Fridman
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?
- LSLeonard Susskind
Well, you could ask the same thing about quantum mechanics. Is there something-
- LFLex Fridman
Exactly.
- LSLeonard Susskind
... yeah, yeah. I think string theory is just an example of a quantum mechanical system that contains both gravitation and, uh, and quantum mechanics.
- 37:03 – 39:26
Free will
- LFLex Fridman
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-
- LSLeonard Susskind
Mm-hmm.
- LFLex Fridman
... and the fundamental nature of reality?
- LSLeonard Susskind
The only thing I can say is I am, I am puzzled by that as much as you are.
- LFLex Fridman
The illusion of it, the-
- LSLeonard Susskind
The illusion-
- LFLex Fridman
... illusion.
- LSLeonard Susskind
... of consciousness, the illusion of free will, the illusion of self. Uh-
- LFLex Fridman
Does that connect to...
- LSLeonard Susskind
How can a physical system do that? And, uh, and I am as puzzled as anybody.
- LFLex Fridman
There's echoes of it in the observer effect.
- LSLeonard Susskind
Yeah.
- LFLex Fridman
So, do you understand what it means to be an observer?
- LSLeonard Susskind
I understand it at a technical level. An observer is a system with enough degrees of freedom that it can r-record information and which can become entangled with the thing that it's measuring. Entanglement is the key. When a system, which we call an apparatus or an observer, same thing, interacts with a system that it's observing-
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
... it doesn't just look at it. It becomes physically entangled-
- LFLex Fridman
Entangled.
- LSLeonard Susskind
... with it. And it's that entanglement which we call an observation or mea- or measurement. Now, does that satisfy me personally as an observer?
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
Yes and no. I find it very satisfying that we have a mathematical representation of what it means to observe a system.
- LFLex Fridman
You are observing stuff right now.
- LSLeonard Susskind
Yeah.
- LFLex Fridman
At the conscious level.
- LSLeonard Susskind
Right.
- LFLex Fridman
Is... Do you think there's echoes of, uh, that kind of entanglement in our macro scale?
- LSLeonard Susskind
Yes, absolutely. For sure. We're, we're entangled with ev- quantum mechanically entangled with everything in this room. If we weren't, uh, then it 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?
- LFLex Fridman
(laughs)
- LSLeonard Susskind
And I'm uncomfortable with it in the same way that I can never get comfortable with five dimensions.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
My, my brain isn't wired for it.
- 39:26 – 46:39
Arrow of time
- LSLeonard Susskind
- LFLex Fridman
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?
- LSLeonard Susskind
That is a big question in physics right now.
- LFLex Fridman
Okay.
- LSLeonard Susskind
All the physics that we do, or at least that the people that I am comfortable talking to-
- LFLex Fridman
(laughs)
- LSLeonard Susskind
... my, my, my friends-
- LFLex Fridman
Yeah, yeah.
- LSLeonard Susskind
... my friends, w- well, we all ask the same question that you just asked.
- LFLex Fridman
Yeah.
- LSLeonard Susskind
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-
- LFLex Fridman
Right, symmetric.
- LSLeonard Susskind
... forward and backward.
- LFLex Fridman
So, (laughs) so you don't really need to think about the arrow of time for most physical phenomena?
- LSLeonard Susskind
For mo- most microscopic phenomena, no.
- LFLex Fridman
All right.
- LSLeonard Susskind
It's only when the phenomena involves systems which are big enough for thermodynamics to become important-
- LFLex Fridman
Right.
- LSLeonard Susskind
... 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.
- LFLex Fridman
The... you mean, like, second law of thermodynamics, uh, entropy-
- LSLeonard Susskind
Yeah.
- LFLex Fridman
... and so on?
- LSLeonard Susskind
Yeah.
- LFLex Fridman
So, would you-
- LSLeonard Susskind
Take a pack of cards and you fling it in the air, and you, and you look what happens to it.
- LFLex Fridman
Yeah, but-
- LSLeonard Susskind
It gets random.
- LFLex Fridman
... we understand-
- LSLeonard Susskind
It doesn't s- it doesn't go from random to simple. It goes from simple to random.
- LFLex Fridman
But do you think it ever breaks down?
- 46:39 – 49:45
Universe as a computer
- LFLex Fridman
else.
- LSLeonard Susskind
Yeah.
- LFLex Fridman
But if we think, think of it, uh, and- and just made me think, if we think the unrolling of state that's happening-
- LSLeonard Susskind
Mm-hmm.
- LFLex Fridman
... 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.
- LSLeonard Susskind
Right.
- LFLex Fridman
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?
- LSLeonard Susskind
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.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
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.
- LFLex Fridman
Can you s- slightly describe, uh, de Sitter space and anti-de Sitter space?
- LSLeonard Susskind
Yeah.
- LFLex Fridman
What are the geometric properties of, of-
- LSLeonard Susskind
They differ, they differ by the, the sign of a single constant called the cosmological constant.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
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.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
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.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
Exponential expansion, dark energy, whatever we want to call it, and we don't understand that mathematically.
- LFLex Fridman
Do we understand, uh-
- LSLeonard Susskind
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.
- LFLex Fridman
(laughs) What about,
- 49:45 – 50:50
Big bang
- LFLex Fridman
is there an understanding of the, the birth, the origin-
- LSLeonard Susskind
No.
- LFLex Fridman
... the Big Bang? So-
- LSLeonard Susskind
No, no.
- LFLex Fridman
... does one come with the other?
- LSLeonard Susskind
There's theories, there are theories. My favorite is the one called eternal inflation.
- LFLex Fridman
The infinity can be on both sides, on one of the sides, on none of the sides. So what, uh, what's-
- LSLeonard Susskind
I like-
- LFLex Fridman
... eternal infinity?
- LSLeonard Susskind
Okay. Infinity on both sides.
- LFLex Fridman
Oh, boy.
- LSLeonard Susskind
Yeah, yeah, that, that's-
- LFLex Fridman
Why is that your favorite? 'Cause it's the, the most just mind-blowing?
- LSLeonard Susskind
No.
- LFLex Fridman
'Cause we want a beginning.
- LSLeonard Susskind
No. Why do we want a beginning?
- LFLex Fridman
I, uh-
- LSLeonard Susskind
In practice, there was a beginning, of course. In practice, there was a beginning. But could it have been, um, a, uh, a random fluctuation in an otherwise infinite, uh, time? Maybe. In any case, the, uh, the eternal inflation theory, I think if correctly understood, would be infinite in both directions.
- 50:50 – 51:35
Infinity
- LSLeonard Susskind
- LFLex Fridman
How do you think about infinity?
- LSLeonard Susskind
Oh, God.
- LFLex Fridman
So, uh-
- LSLeonard Susskind
God.
- LFLex Fridman
Okay, of course you can think about it mathematically, but-
- LSLeonard Susskind
I just finished this, I just finished this discussion with my friend Sergey Brin.
- LFLex Fridman
Yes.
- LSLeonard Susskind
"How do you think about infinity?" I say, "Well, Sergey Brin is infinitely rich."
- LFLex Fridman
(laughs) Uh, how do you test that hypothesis? Okay, uh... (laughs) That's such a good line.
- LSLeonard Susskind
(laughs)
- LFLex Fridman
All right, uh, yeah, so there's no, there's really no way to visualize some of these things. Like, like-
- LSLeonard Susskind
Infini- Yeah, no, this is a very good question. Does physics have any, uh, is, does infinity have any place in physics?
- LFLex Fridman
Right.
- LSLeonard Susskind
Right. And all I can say is, very good question.
- LFLex Fridman
(laughs)
- 51:35 – 54:08
First image of a black hole
- LFLex Fridman
So, uh, what do you think of, uh, the recent first image of a black hole visualized from the Event Horizon Telescope?
- LSLeonard Susskind
It's, it's, it's an incredible triumph of science. In itself, the fact that there are black holes which collide is not a surprise.
- LFLex Fridman
Right.
- LSLeonard Susskind
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, 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, um, the structure, evolution of structure in the universe.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
But I don't think it's gonna tell us anything new about black holes. But it's a triumph in the sense that, uh, you go back 100 years and it was a continuous 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 gonna happen, you know, 30, 40 years ago. And, um, I, I think it's a magnificent, uh, structure, magnificent thing, this evolution of general relativity-
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
... LIGO, high precision ability to measure things on a scale of 10 to the minus 21. So-
- LFLex Fridman
(laughs) So you're just in awe-
- LSLeonard Susskind
Astonishing.
- LFLex Fridman
... that we-
- LSLeonard Susskind
Just in awe.
- LFLex Fridman
... that this path-
- LSLeonard Susskind
Just complete awe.
- LFLex Fridman
... took us to this-
- LSLeonard Susskind
Right.
- LFLex Fridman
... 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 realized it?
- LSLeonard Susskind
No, no, it simply confirmed... You know, it's a magnificent triumph to have confirmed-
- LFLex Fridman
Confirmed.
- LSLeonard Susskind
... by direct observation-
- LFLex Fridman
Yeah.
- LSLeonard Susskind
... that Einstein's theory of gravity at the level of black hole collisions actually works is awesome.
- LFLex Fridman
(laughs)
- LSLeonard Susskind
It is really awesome. You know, I know some of the people who were involved in that. They're just ordinary people.
- LFLex Fridman
Yeah.
- LSLeonard Susskind
And the idea that they could carry this out-
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
... I, I just don't, I'm shocked.
- LFLex Fridman
Yeah. Just these little homo sapiens-
- LSLeonard Susskind
Yeah, just these little, uh, monkeys.
- 54:08 – 55:55
Questions within the reach of science
- LFLex Fridman
What kind of questions can science not currently answer, but you hope might be able to soon?
- LSLeonard Susskind
Well, you, you've already addressed them. What is consciousness, for example?
- LFLex Fridman
You think sci- that's within the reach of science?
- LSLeonard Susskind
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.
- LFLex Fridman
Not a physicist?
- LSLeonard Susskind
Perhaps in the-
- LFLex Fridman
With the help.
- LSLeonard Susskind
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...
- LFLex Fridman
Yeah.
- LSLeonard Susskind
I'll tell you another thing that I find awesome.You know this Google thing that they taught the computers how to play chess?
- LFLex Fridman
Yeah, yeah.
- LSLeonard Susskind
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.
- LFLex Fridman
Against each other. So-
- LSLeonard Susskind
Against each other. This is a form of evolution. These machines evolved. They evolved an intelligence.
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
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.
- LFLex Fridman
Better.
- LSLeonard Susskind
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.
- LFLex Fridman
What life and intelligence is.
- LSLeonard Susskind
Yeah.
- 55:55 – 57:14
Questions out of reach of science
- LSLeonard Susskind
- LFLex Fridman
Last question. What kind of questions can science not currently answer and may never be able to answer?
- LSLeonard Susskind
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?
- LFLex Fridman
Mm-hmm.
- LSLeonard Susskind
Yeah, there's a lot of questions, and I don't see... It seems, it seems to be a real question.
- LFLex Fridman
It's an answerable question if they're-
- LSLeonard Susskind
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.
- LFLex Fridman
There's no better place to end. Leonard, thank you so much for talking today.
- LSLeonard Susskind
Okay, good.
Episode duration: 57:29
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