back to indexLeonard Susskind: Quantum Mechanics, String Theory and Black Holes | Lex Fridman Podcast #41
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The following is a conversation with Leonard Susskind.
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He's a professor of theoretical physics
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at Stanford University and founding director
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of Stanford Institute of Theoretical Physics.
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He is widely regarded as one of the fathers
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of string theory and in general,
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as one of the greatest physicists of our time,
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both as a researcher and an educator.
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This is the Artificial Intelligence Podcast.
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Perhaps you noticed that the people I've been speaking with
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are not just computer scientists,
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but philosophers, mathematicians, writers,
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psychologists, physicists, and soon other disciplines.
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To me, AI is much bigger than deep learning,
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bigger than computing.
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It is our civilization's journey
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into understanding the human mind
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and creating echoes of it in the machine.
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If you enjoy the podcast, subscribe on YouTube,
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give it five stars on iTunes, support it on Patreon,
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or simply connect with me on Twitter
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at Lex Friedman, spelled F R I D M A M.
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And now, here's my conversation with Leonard Susskind.
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You worked and were friends with Richard Feynman.
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How has he influenced you, changed you
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as a physicist and thinker?
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What I saw, I think what I saw was somebody
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who could do physics in this deeply intuitive way.
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His style was almost to close his eyes
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and visualize the phenomena that he was thinking about.
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And through visualization, outflank the mathematical,
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the highly mathematical and very, very sophisticated
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technical arguments that people would use.
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I think that was also natural to me,
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but I saw somebody who was actually successful at it,
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who could do physics in a way that I regarded
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as simpler, more direct, more intuitive.
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And while I don't think he changed my way of thinking,
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I do think he validated it.
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He made me look at it and say, yeah,
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that's something you can do and get away with.
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Practically didn't get away with it.
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So do you find yourself, whether you're thinking
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about quantum mechanics or black holes
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or string theory, using intuition as a first step
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or step throughout using visualization?
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Yeah, very much so, very much so.
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I tend not to think about the equations.
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I tend not to think about the symbols.
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I tend to try to visualize the phenomena themselves.
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And then when I get an insight that I think is valid,
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I might try to convert it to mathematics,
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but I'm not a natural mathematician.
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I'm good enough at it.
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I'm good enough at it, but I'm not a great mathematician.
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So for me, the way of thinking about physics
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is first intuitive, first visualization,
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scribble a few equations maybe,
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but then try to convert it to mathematics.
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Experience is that other people are better
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at converting it to mathematics than I am.
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And yet you've worked with very counterintuitive ideas.
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You can visualize something counterintuitive.
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By rewiring your brain in new ways.
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Yeah, quantum mechanics is not intuitive.
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Very little of modern physics is intuitive.
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Intuitive, what does intuitive mean?
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It means the ability to think about it
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with basic classical physics,
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the physics that we evolved with throwing stones,
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or splashing water, whatever it happens to be.
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Quantum physics, general relativity,
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quantum field theory are deeply unintuitive in that way.
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But after time and getting familiar with these things,
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you develop new intuitions.
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I always said you rewire.
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And it's to the point where me and many of my friends,
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I and many of my friends,
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can think more easily quantum mechanically
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than we can classically.
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We've gotten so used to it.
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I mean, yes, our neural wiring in our brain
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is such that we understand rocks and stones and water
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We sort of evolved for that.
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Do you think it's possible to create a wiring
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of neuron like state devices that more naturally
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understand quantum mechanics, understand wave function,
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understand these weird things?
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Well, I'm not sure.
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I think many of us have evolved the ability
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to think quantum mechanically to some extent.
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But that doesn't mean you can think like an electron.
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That doesn't mean another example.
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Forget for a minute quantum mechanics.
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Just visualizing four dimensional space
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or five dimensional space or six dimensional space,
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I think we're fundamentally wired
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to visualize three dimensions.
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I can't even visualize two dimensions or one dimension
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without thinking about it as embedded in three dimensions.
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If I wanna visualize a line,
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I think of the line as being a line in three dimensions.
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Or I think of the line as being a line on a piece of paper
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with a piece of paper being in three dimensions.
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I never seem to be able to, in some abstract and pure way,
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visualize in my head the one dimension,
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the two dimension, the four dimension, the five dimensions.
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And I don't think that's ever gonna happen.
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The reason is I think our neural wiring
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is just set up for that.
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On the other hand, we do learn ways
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to think about five, six, seven dimensions.
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We learn ways, we learn mathematical ways,
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and we learn ways to visualize them, but they're different.
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And so yeah, I think we do rewire ourselves.
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Whether we can ever completely rewire ourselves
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to be completely comfortable with these concepts, I doubt.
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So that it's completely natural.
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To where it's completely natural.
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So I'm sure there's somewhat, you could argue,
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creatures that live in a two dimensional space.
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Yeah, maybe there are.
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And while it's romanticizing the notion of curse,
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we're all living, as far as we know,
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in three dimensional space.
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But how do those creatures imagine 3D space?
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Well, probably the way we imagine 4D,
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by using some mathematics and some equations
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Okay, so jumping back to Feynman just for a second.
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He had a little bit of an ego.
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Why, do you think ego is powerful or dangerous in science?
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I think both, both, both.
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I think you have to have both arrogance and humility.
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You have to have the arrogance to say, I can do this.
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Nature is difficult, nature is very, very hard.
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I'm smart enough, I can do it.
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I can win the battle with nature.
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On the other hand, I think you also have to have
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the humility to know that you're very likely
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to be wrong on any given occasion.
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Everything you're thinking could suddenly change.
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Young people can come along and say things
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you won't understand and you'll be lost and flabbergasted.
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So I think it's a combination of both.
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You better recognize that you're very limited,
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and you better be able to say to yourself,
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I'm not so limited that I can't win this battle with nature.
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It takes a special kind of person
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who can manage both of those, I would say.
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And I would say there's echoes of that in your own work,
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a little bit of ego, a little bit of outside of the box,
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So was there a time where you felt,
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you looked at yourself and asked,
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am I completely wrong about this?
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Oh yeah, about the whole thing or about specific things?
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Wait, which whole thing?
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Me and me and my ability to do this thing.
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Oh, those kinds of doubts.
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First of all, did you have those kinds of doubts?
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No, I had different kind of doubts.
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I came from a very working class background
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and I was uncomfortable in academia for,
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oh, for a long time.
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But they weren't doubts about my ability or my,
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they were just the discomfort in being in an environment
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that my family hadn't participated in,
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I knew nothing about as a young person.
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I didn't learn that there was such a thing called physics
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until I was almost 20 years old.
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Yeah, so I did have certain kind of doubts,
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but not about my ability.
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I don't think I was too worried
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about whether I would succeed or not.
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I never felt this insecurity, am I ever gonna get a job?
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That had never occurred to me that I wouldn't.
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Maybe you could speak a little bit to this sense
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of what is academia.
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Because I too feel a bit uncomfortable in it.
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There's something I can't put quite into words
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what you have that's not, doesn't, if we call it music,
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you play a different kind of music than a lot of academia.
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How have you joined this orchestra?
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How do you think about it?
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I don't know that I thought about it
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as much as I just felt it.
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Thinking is one thing, feeling is another thing.
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I felt like an outsider until a certain age
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when I suddenly found myself the ultimate insider
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in academic physics.
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And that was a sharp transition, and I wasn't a young man.
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I was probably 50 years old.
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So you were never quite, it was a phase transition,
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you were never quite in the middle.
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Yeah, that's right, I wasn't.
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I always felt a little bit of an outsider.
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In the beginning, a lot an outsider.
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My way of thinking was different,
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my approach to mathematics was different,
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but also my social background
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that I came from was different.
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Now these days, half the young people I meet,
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they're parents or professors.
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That was not my case.
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But then all of a sudden, at some point,
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I found myself at very much the center of,
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maybe not the only one at the center,
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but certainly one of the people in the center
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of a certain kind of physics.
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And all that went away, it went away in a flash.
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So maybe a little bit with Feynman,
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but in general, how do you develop ideas?
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Do you work through ideas alone?
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Do you brainstorm with others?
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Oh, both, both, very definitely both.
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The younger time, I spent more time with myself.
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Now, because I'm at Stanford,
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because I have a lot of ex students
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and people who are interested in the same thing I am,
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I spend a good deal of time, almost on a daily basis,
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interacting, brainstorming, as you said.
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It's a very important part.
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I spend less time probably completely self focused
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than with a piece of paper
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and just sitting there staring at it.
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What are your hopes for quantum computers?
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So machines that are based on,
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that have some elements of leverage quantum mechanical ideas.
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Yeah, it's not just leveraging quantum mechanical ideas.
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You can simulate quantum systems on a classical computer.
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Simulate them means solve the Schrodinger equation for them
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or solve the equations of quantum mechanics
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or solve the equations of quantum mechanics
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on a computer, on a classical computer.
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But the classical computer is not doing,
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is not a quantum mechanical system itself.
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Everything's made of quantum mechanics,
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but it's not functioning.
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It's not functioning as a quantum system.
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It's just solving equations.
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The quantum computer is truly a quantum system
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which is actually doing the things
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that you're programming it to do.
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You want to program a quantum field theory.
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If you do it in classical physics,
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that program is not actually functioning in the computer
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as a quantum field theory.
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It's just solving some equations.
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Physically, it's not doing the things
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that the quantum system would do.
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The quantum computer is really a quantum mechanical system
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which is actually carrying out the quantum operations.
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You can measure it at the end.
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It intrinsically satisfies the uncertainty principle.
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It is limited in the same way that quantum systems
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are limited by uncertainty and so forth.
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And it really is a quantum system.
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That means that what you're doing
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when you program something for a quantum system
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is you're actually building a real version of the system.
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The limits of a classical computer,
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classical computers are enormously limited
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when it comes to the quantum systems.
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They're enormously limited
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because you've probably heard this before,
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but in order to store the amount of information
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that's in a quantum state of 400 spins,
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that's not very many, 400 I can put in my pocket,
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I can put 400 pennies in my pocket.
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To be able to simulate the quantum state
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of 400 elementary quantum systems, qubits we call them,
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to do that would take more information
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than can possibly be stored in the entire universe
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if it were packed so tightly
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that you couldn't pack any more in.
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On the other hand, if your quantum computer
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is composed of 400 qubits,
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it can do everything 400 qubits can do.
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What kind of space, if you just intuitively think
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about the space of algorithms that that unlocks for us,
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so there's a whole complexity theory
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around classical computers,
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measuring the running time of things,
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and P, so on, what kind of algorithms
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just intuitively do you think it unlocks for us?
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Okay, so we know that there are a handful of algorithms
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that can seriously beat classical computers
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and which can have exponentially more power.
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This is a mathematical statement.
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Nobody's exhibited this in the laboratory.
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It's a mathematical statement.
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We know that's true, but it also seems more and more
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that the number of such things is very limited.
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Only very, very special problems
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exhibit that much advantage for a quantum computer,
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of standard problems.
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To my mind, as far as I can tell,
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the great power of quantum computers
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will actually be to simulate quantum systems.
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If you're interested in a certain quantum system
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and it's too hard to simulate classically,
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you simply build a version of the same system.
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You build a version of it.
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You build a model of it
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that's actually functioning as the system.
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You run it, and then you do the same thing
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you would do to the quantum system.
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You make measurements on it, quantum measurements on it.
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The advantage is you can run it much slower.
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You could say, why bother?
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Why not just use the real system?
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Why not just do experiments on the real system?
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Well, real systems are kind of limited.
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You can't change them.
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You can't manipulate them.
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You can't slow them down so that you can poke into them.
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You can't modify them in arbitrary kinds of ways
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to see what would happen if I change the system a little bit.
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I think that quantum computers will be extremely valuable
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in understanding quantum systems.
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At the lowest level of the fundamental laws.
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They're actually satisfying the same laws
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as the systems that they're simulating.
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Okay, so on the one hand, you have things like factoring.
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Factoring is the great thing of quantum computers.
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Factoring large numbers, that doesn't seem that much
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to do with quantum mechanics.
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It seems to be almost a fluke that a quantum computer
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can solve the factoring problem in a short time.
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And those problems seem to be extremely special, rare,
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and it's not clear to me
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that there's gonna be a lot of them.
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On the other hand, there are a lot of quantum systems.
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Chemistry, there's solid state physics,
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there's material science, there's quantum gravity,
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there's all kinds of quantum field theory.
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And some of these are actually turning out
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to be applied sciences,
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as well as very fundamental sciences.
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So we probably will run out of the ability
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to solve equations for these things.
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Solve equations by the standard methods of pencil and paper.
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Solve the equations by the method of classical computers.
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And so what we'll do is we'll build versions
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of these systems, run them,
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and run them under controlled circumstances
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where we can change them, manipulate them,
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make measurements on them,
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and find out all the things we wanna know.
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So in finding out the things we wanna know
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about very small systems, is there something
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that we can also find out about the macro level,
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about something about the function, forgive me,
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of our brain, biological systems,
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the stuff that's about one meter in size
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versus much, much smaller?
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Well, what all the excitement is about
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among the people that I interact with
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is understanding black holes.
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Black holes are big things.
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They are many, many degrees of freedom.
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There is another kind of quantum system that is big.
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It's a large quantum computer.
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And one of the things we've learned
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is that the physics of large quantum computers
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is in some ways similar to the physics
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of large quantum black holes.
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And we're using that relationship.
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Now you asked, you didn't ask about quantum computers
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or systems, you didn't ask about black holes,
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you asked about brains.
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Yeah, about stuff that's in the middle of the two.
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So black holes are,
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there's something fundamental about black holes
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that feels to be very different than a brain.
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And they also function in a very quantum mechanical way.
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It is, first of all, unclear to me,
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but of course it's unclear to me.
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I'm not a neuroscientist.
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I have, I don't even have very many friends
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who are neuroscientists.
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I would like to have more friends who are neuroscientists.
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I just don't run into them very often.
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Among the few neuroscientists
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I've ever talked about about this,
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they are pretty convinced
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that the brain functions classically,
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that it is not intrinsically a quantum mechanical system
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or it doesn't make use of the special features,
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entanglement, coherence, superposition.
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I sort of hope they're wrong
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just because I like the romantic idea
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that the brain is a quantum system.
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But I think probably not.
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big systems can be composed of lots of little systems.
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Materials, the materials that we work with and so forth
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are, can be large systems, a large piece of material,
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but they're made out of quantum systems.
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Now, one of the things that's been happening
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over the last good number of years
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is we're discovering materials and quantum systems,
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which function much more quantum mechanically
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Topological insulators, this kind of thing,
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that kind of thing.
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Those are macroscopic systems,
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but they're just superconductors.
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Superconductors have a lot of quantum mechanics in them.
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You can have a large chunk of superconductor.
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So it's a big piece of material.
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On the other hand, it's functioning and its properties
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depend very, very strongly on quantum mechanics.
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And to analyze them, you need the tools of quantum mechanics.
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If we can go on to black holes
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and looking at the universe
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as a information processing system,
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as a computer, as a giant computer.
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It's a giant computer.
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What's the power of thinking of the universe
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as an information processing system?
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Or what is perhaps its use
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besides the mathematical use of discussing black holes
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and your famous debates and ideas around that
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or life in general as information processing systems?
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Well, all systems are information processing systems.
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You poke them, they change a little bit, they evolve.
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All systems are information processing systems.
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So there's no extra magic to us humans?
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It certainly feels, consciousness intelligence
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Where does it emerge from?
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If we look at information processing,
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what are the emergent phenomena
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that come from viewing the world
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as an information processing system?
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Here is what I think.
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My thoughts are not worth much in this.
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If you ask me about physics,
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my thoughts may be worth something.
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If you ask me about this,
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I'm not sure my thoughts are worth anything.
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But as I said earlier,
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I think when we do introspection,
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when we imagine doing introspection
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and try to figure out what it is
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when we do when we're thinking,
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I think we get it wrong.
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I'm pretty sure we get it wrong.
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Everything I've heard about the way the brain functions
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is so counterintuitive.
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For example, you have neurons which detect vertical lines.
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You have different neurons
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which detect lines at 45 degrees.
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You have different neurons.
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I never imagined that there were whole circuits
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which were devoted to vertical lines in my brain.
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Doesn't seem to be the way my brain works.
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My brain seems to work if I put my finger up vertically
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or if I put it horizontally
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or if I put it this way or that way.
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It seems to me it's the same circuits.
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It's not the way it works.
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The way the brain is compartmentalized
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seems to be very, very different
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than what I would have imagined
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if I were just doing psychological introspection
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about how things work.
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My conclusion is that we won't get it right that way,
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that how will we get it right?
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I think maybe computer scientists will get it right eventually.
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I don't think there are any ways near it.
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I don't even think they're thinking about it,
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but eventually we will build machines perhaps
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which are complicated enough
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and partly engineered, partly evolved,
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maybe evolved by machine learning and so forth.
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This machine learning is very interesting.
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By machine learning, we will evolve systems
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and we may start to discover mechanisms
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that have implications for how we think
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and for what this consciousness thing is all about
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and we'll be able to do experiments on them
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and perhaps answer questions
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that we can't possibly answer by introspection.
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So that's a really interesting point.
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In many cases, if you look at even a string theory,
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when you first think about a system,
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it seems really complicated, like the human brain,
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and through some basic reasoning
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and trying to discover fundamental low level behavior
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of the system, you find out that it's actually much simpler.
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Do you, one, have you, is that generally the process
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and two, do you have that also hope
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for biological systems as well,
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for all the kinds of stuff we're studying at the human level?
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Of course, physics always begins
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by trying to find the simplest version of something
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Yeah, I mean, there are lots of examples
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where physics has taken very complicated systems,
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analyzed them and found simplicity in them for sure.
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I said superconductors before, it's an obvious one.
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A superconductor seems like a monstrously complicated thing
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with all sorts of crazy electrical properties,
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magnetic properties and so forth.
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And when it finally is boiled down
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to its simplest elements,
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it's a very simple quantum mechanical phenomenon
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called spontaneous symmetry breaking,
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and which we, in other contexts, we learned about
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and we're very familiar with.
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So yeah, I mean, yes, we do take complicated things,
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make them simple, but what we don't want to do
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is take things which are intrinsically complicated
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and fool ourselves into thinking
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that we can make them simple.
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We don't want to make, I don't know who said this,
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but we don't want to make them simpler
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than they really are, okay?
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Is the brain a thing which ultimately functions
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by some simple rules or is it just complicated?
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In terms of artificial intelligence,
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nobody really knows what are the limits
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of our current approaches, you mentioned machine learning.
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How do we create human level intelligence?
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It seems that there's a lot of very smart physicists
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who perhaps oversimplify the nature of intelligence
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and think of it as information processing,
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and therefore there doesn't seem to be
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any theoretical reason why we can't artificially create
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human level or superhuman level intelligence.
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In fact, the reasoning goes,
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if you create human level intelligence,
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the same approach you just used
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to create human level intelligence
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should allow you to create superhuman level intelligence
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very easily, exponentially.
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So what do you think that way of thinking
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that comes from physicists is all about?
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I wish I knew, but there's a particular reason
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why I wish I knew.
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I have a second job.
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I consult for Google, not for Google, for Google X.
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I am the senior academic advisor
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to a group of machine learning physicists.
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Now that sounds crazy because I know nothing
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about the subject.
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I know very little about the subject.
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On the other hand, I'm good at giving advice,
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so I give them advice on things.
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Anyway, I see these young physicists
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who are approaching the machine learning problem.
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There is a real machine learning problem.
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Namely, why does it work as well as it does?
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Nobody really seems to understand
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why it is capable of doing the kind of generalizations
link |
that it does and so forth.
link |
And there are three groups of people
link |
who have thought about this.
link |
There are the engineers.
link |
The engineers are incredibly smart,
link |
but they tend not to think as hard
link |
about why the thing is working
link |
as much as they do how to use it.
link |
Obviously, they provided a lot of data,
link |
and it is they who demonstrated
link |
that machine learning can work much better
link |
than you have any right to expect.
link |
The machine learning systems are systems.
link |
The system's not too different
link |
than the kind of systems that physicists study.
link |
There's not all that much difference
link |
between quantum, in the structure of mathematics,
link |
physically, yes, but in the structure of mathematics,
link |
between a tensor network designed
link |
to describe a quantum system on the one hand
link |
and the kind of networks that are used in machine learning.
link |
So there are more and more, I think,
link |
young physicists are being drawn
link |
to this field of machine learning,
link |
some very, very good ones.
link |
I work with a number of very good ones,
link |
not on machine learning, but on having lunch.
link |
And I can tell you they are super smart.
link |
They don't seem to be so arrogant
link |
about their physics backgrounds
link |
that they think they can do things that nobody else can do.
link |
But the physics way of thinking, I think,
link |
will add great value to,
link |
or will bring value to the machine learning.
link |
I believe it will.
link |
And I think it already has.
link |
At what time scale do you think
link |
predicting the future becomes useless
link |
in your long experience
link |
and being surprised at new discoveries?
link |
Well, sometimes a day, sometimes 20 years.
link |
There are things which I thought
link |
we were very far from understanding,
link |
which practically in a snap of the fingers
link |
or a blink of the eye suddenly became understood,
link |
completely surprising to me.
link |
There are other things which I looked at and I said,
link |
we're not gonna understand these things for 500 years,
link |
in particular quantum gravity.
link |
The scale for that was 20 years, 25 years.
link |
And we understand a lot
link |
and we don't understand it completely now by any means,
link |
but I thought it was 500 years to make any progress.
link |
It turned out to be very, very far from that.
link |
It turned out to be more like 20 or 25 years
link |
from the time when I thought it was 500 years.
link |
So if we may, can we jump around quantum gravity,
link |
some basic ideas in physics?
link |
What is the dream of string theory mathematically?
link |
Where does it come from?
link |
What problem is it trying to solve?
link |
I don't think the dream of string theory
link |
is any different than the dream
link |
of fundamental theoretical physics altogether.
link |
Understanding a unified theory of everything.
link |
I don't like thinking of string theory
link |
as a subject unto itself
link |
with people called string theorists
link |
who are the practitioners
link |
of this thing called string theory.
link |
I much prefer to think of them as theoretical physicists
link |
trying to answer deep fundamental questions about nature,
link |
in particular gravity,
link |
in particular gravity and its connection
link |
with quantum mechanics,
link |
and who at the present time find string theory
link |
a useful tool rather than saying
link |
there's a subject called string theorists.
link |
I don't like being referred to as a string theorist.
link |
Yes, but as a tool, is it useful to think about our nature
link |
in multiple dimensions, the strings vibrating?
link |
I believe it is useful.
link |
I'll tell you what the main use of it has been up till now.
link |
Well, it has had a number of main uses.
link |
Originally, string theory was invented,
link |
and I know that I was there.
link |
I was right at the spot
link |
where it was being invented literally,
link |
and it was being invented to understand hadrons.
link |
Hadrons are subnuclear particles,
link |
protons, neutrons, mesons,
link |
and at that time, the late 60s, early 70s,
link |
it was clear from experiment
link |
that these particles called hadrons could vibrate,
link |
could rotate, could do all the things
link |
that a little closed string can do,
link |
and it was and is a valid and correct theory of these hadrons.
link |
It's been experimentally tested, and that is a done deal.
link |
It had a second life as a theory of gravity,
link |
the same basic mathematics,
link |
except on a very, very much smaller distance scale.
link |
The objects of gravitation are 19 orders of magnitude
link |
or orders of magnitude smaller than a proton,
link |
but the same mathematics turned up.
link |
The same mathematics turned up.
link |
What has been its value?
link |
Its value is that it's mathematically rigorous in many ways
link |
and enabled us to find mathematical structures
link |
which have both quantum mechanics and gravity.
link |
With rigor, we can test out ideas.
link |
We can test out ideas.
link |
We can't test them in the laboratory.
link |
They're 19 orders of magnitude too small
link |
are things that we're interested in,
link |
but we can test them out mathematically
link |
and analyze their internal consistency.
link |
By now, 40 years ago, 35 years ago, and so forth,
link |
people very, very much questioned the consistency
link |
between gravity and quantum mechanics.
link |
Stephen Hawking was very famous for it, rightly so.
link |
Now, nobody questions that consistency anymore.
link |
They don't because we have mathematically precise
link |
string theories which contain both gravity
link |
and quantum mechanics in a consistent way.
link |
So it's provided that certainty that quantum mechanics
link |
and gravity can coexist.
link |
That's not a small thing.
link |
It's a very big thing.
link |
It's a huge thing.
link |
Einstein would be proud.
link |
Einstein, he might be appalled.
link |
He didn't like it.
link |
He didn't like it.
link |
He might not be appalled, I don't know.
link |
He didn't like quantum mechanics very much,
link |
but he would certainly be struck by it.
link |
I think that may be, at this time,
link |
its biggest contribution to physics
link |
in illustrating almost definitively
link |
that quantum mechanics and gravity
link |
are very closely related
link |
and not inconsistent with each other.
link |
Is there a possibility of something deeper,
link |
more profound that still is consistent with string theory
link |
but is deeper, that is to be found?
link |
Well, you could ask the same thing about quantum mechanics.
link |
Is there something?
link |
I think string theory is just an example
link |
of a quantum mechanical system
link |
that contains both gravitation and quantum mechanics.
link |
So is there something underlying quantum mechanics?
link |
Perhaps something deterministic.
link |
Perhaps something deterministic.
link |
My friend, Ferad Etouf, whose name you may know,
link |
he's a very famous physicist.
link |
Dutch, not as famous as he should be, but...
link |
Hard to spell his name.
link |
It's hard to say his name.
link |
No, it's easy to spell his name.
link |
Apostrophe, he's the only person I know
link |
whose name begins with an apostrophe.
link |
And he's one of my heroes in physics.
link |
He's a little younger than me,
link |
but he's nevertheless one of my heroes.
link |
Etouf believes that there is some substructure to the world
link |
which is classical in character,
link |
deterministic in character,
link |
which somehow by some mechanism
link |
that he has a hard time spelling out
link |
emerges as quantum mechanics.
link |
The wave function is somehow emergent.
link |
The wave function, not just the wave function,
link |
but the whole thing that goes with quantum mechanics,
link |
uncertainty, entanglement, all these things,
link |
are emergent. So you think quantum mechanics
link |
is the bottom of the well?
link |
Here I think is where you have to be humble.
link |
Here's where humility comes.
link |
I don't think anybody should say anything
link |
is the bottom of the well at this time.
link |
I think we can reasonably say,
link |
I can reasonably say when I look into the well,
link |
I can't see past quantum mechanics.
link |
I can't see past quantum mechanics.
link |
I don't see any reason for there to be anything
link |
beyond quantum mechanics.
link |
I think Etouf has asked very interesting
link |
and deep questions.
link |
I don't like his answers.
link |
Well, again, let me ask,
link |
if we look at the deepest nature of reality
link |
with whether it's deterministic
link |
or when observed as probabilistic,
link |
what does that mean for our human level
link |
of ideas of free will?
link |
Is there any connection whatsoever
link |
from this perception, perhaps illusion of free will
link |
that we have and the fundamental nature of reality?
link |
The only thing I can say is I am puzzled by that
link |
as much as you are.
link |
The illusion of it.
link |
The illusion of consciousness,
link |
the illusion of free will, the illusion of self.
link |
Does that connect to?
link |
How can a physical system do that?
link |
And I am as puzzled as anybody.
link |
There's echoes of it in the observer effect.
link |
So do you understand what it means to be an observer?
link |
I understand it at a technical level.
link |
An observer is a system with enough degrees of freedom
link |
that it can record information
link |
and which can become entangled
link |
with the thing that it's measuring.
link |
Entanglement is the key.
link |
When a system which we call an apparatus or an observer,
link |
same thing, interacts with the system
link |
that it's observing, it doesn't just look at it.
link |
It becomes physically entangled with it.
link |
And it's that entanglement which we call an observation
link |
Now, does that satisfy me personally as an observer?
link |
I find it very satisfying
link |
that we have a mathematical representation
link |
of what it means to observe a system.
link |
You are observing stuff right now, the conscious level.
link |
Do you think there's echoes of that kind of entanglement
link |
in our macro scale?
link |
Yes, absolutely, for sure.
link |
We're entangled with,
link |
quantum mechanically entangled with everything in this room.
link |
If we weren't, then it would just,
link |
well, we wouldn't be observing it.
link |
But on the other hand, you can ask,
link |
do I really, am I really comfortable with it?
link |
And I'm uncomfortable with it in the same way
link |
that I can never get comfortable with five dimensions.
link |
My brain isn't wired for it.
link |
Are you comfortable with four dimensions?
link |
A little bit more,
link |
because I can always imagine the fourth dimension is time.
link |
So the arrow of time, are you comfortable with that arrow?
link |
Do you think time is an emergent phenomena
link |
or is it fundamental to nature?
link |
That is a big question in physics right now.
link |
All the physics that we do,
link |
or at least that the people that I am comfortable
link |
with talking to, my friends, my friends.
link |
No, we all ask the same question that you just asked.
link |
Space, we have a pretty good idea is emergent
link |
and it emerges out of entanglement and other things.
link |
Time always seems to be built into our equations
link |
as just what Newton pretty much would have thought.
link |
Newton, modified a little bit by Einstein,
link |
would have called time.
link |
And mostly in our equations, it is not emergent.
link |
Time in physics is completely symmetric,
link |
forward and backward.
link |
Right, it's symmetric.
link |
So you don't really need to think about the arrow of time
link |
for most physical phenomena.
link |
For most microscopic phenomena, no.
link |
It's only when the phenomena involve systems
link |
which are big enough for thermodynamics to become important,
link |
for entropy to become important.
link |
For a small system, entropy is not a good concept.
link |
Entropy is something which emerges out of large numbers.
link |
It's a probabilistic idea or it's a statistical idea
link |
and it's a thermodynamic idea.
link |
Thermodynamics requires lots and lots
link |
and lots of little substructures, okay?
link |
So it's not until you emerge at the thermodynamic level
link |
that there's an arrow of time.
link |
Do we understand it?
link |
Yeah, I think we understand better
link |
than most people think they have.
link |
Most people say they think we understand it.
link |
Yeah, I think we understand it.
link |
It's a statistical idea.
link |
You mean like second law of thermodynamics,
link |
entropy and so on?
link |
Yeah, take a pack of cards and you fling it in the air
link |
and you look what happens to it, it gets random.
link |
It doesn't go from random to simple.
link |
It goes from simple to random.
link |
But do you think it ever breaks down?
link |
What I think you can do is in a laboratory setting,
link |
you can take a system which is somewhere intermediate
link |
between being small and being large
link |
and make it go backward.
link |
A thing which looks like it only wants to go forward
link |
because of statistical mechanical reasons,
link |
because of the second law,
link |
you can very, very carefully manipulate it
link |
to make it run backward.
link |
I don't think you can take an egg, a Humpty Dumpty
link |
who fell on the floor and reverse that.
link |
But you can, in a very controlled situation,
link |
you can take systems which appear to be evolving
link |
statistically toward randomness,
link |
stop them, reverse them, and make them go back.
link |
What's the intuition behind that?
link |
How do we do that?
link |
How do we reverse it?
link |
You're saying a closed system.
link |
Yeah, pretty much closed system, yes.
link |
Did you just say that time travel is possible?
link |
No, I didn't say time travel is possible.
link |
I said you can make a system go backward.
link |
You can make it go back.
link |
You can make it reverse its steps.
link |
You can make it reverse its trajectory.
link |
What's the intuition there?
link |
Does it have, is it just a fluke thing
link |
that we can do at a small scale in the lab
link |
that doesn't have?
link |
Well, what I'm saying is you can do it
link |
a little bit better than a small scale.
link |
You can certainly do it with a simple, small system.
link |
Small systems don't have any sense of the arrow of time.
link |
Atoms, atoms are no sense of an arrow of time.
link |
They're completely reversible.
link |
It's only when you have, you know,
link |
the second law of thermodynamics
link |
is the law of large numbers.
link |
So you can break the law because it's not
link |
a deterministic law. You can break it,
link |
you can break it, but it's hard.
link |
It requires great care.
link |
The bigger the system is, the more care,
link |
the more, the harder it is.
link |
You have to overcome what's called chaos.
link |
And it requires more and more precision.
link |
For 10 particles, you might be able to do it
link |
For a hundred particles, it's really hard.
link |
For a thousand or a million particles, forget it,
link |
but not for any fundamental reason,
link |
just because it's technologically too hard
link |
to make the system go backward.
link |
So, no time travel for engineering reasons.
link |
Oh, no, no, no, no.
link |
What is time travel?
link |
Time travel to the future?
link |
You just close your eyes, go to sleep,
link |
and you wake up in the future.
link |
Yeah, yeah, a good nap gets you there, yeah.
link |
A good nap gets you there, right.
link |
But reversing the second law of thermodynamics,
link |
going backward in time for anything that's human scale
link |
is a very difficult engineering effort.
link |
I wouldn't call that time travel
link |
because it gets too mixed up
link |
with what science fiction calls time travel.
link |
This is just the ability to reverse a system.
link |
You take the system and you reverse the direction
link |
of motion of every molecule in it.
link |
That, you can do it with one molecule.
link |
If you find a particle moving in a certain direction,
link |
let's not say a particle, a baseball,
link |
you stop it dead and then you simply reverse its motion.
link |
In principle, that's not too hard.
link |
And it'll go back along its trajectory
link |
in the backward direction.
link |
Just running the program backwards.
link |
Running the program backward.
link |
If you have two baseballs colliding,
link |
well, you can do it,
link |
but you have to be very, very careful to get it just right.
link |
If you have 10 baseballs, really, really, better yet,
link |
10 billiard balls on an idealized,
link |
frictionless billiard table.
link |
Okay, so you start the balls all on a triangle, right?
link |
And you whack them.
link |
Depending on the game you're playing,
link |
you either whack them or you're really careful,
link |
but you whack them.
link |
And they go flying off in all possible directions.
link |
Okay, try to reverse that.
link |
Try to reverse that.
link |
Imagine trying to take every billiard ball,
link |
stopping it dead at some point,
link |
and reversing its motion
link |
so that it was going in the opposite direction.
link |
If you did that with tremendous care,
link |
it would reassemble itself back into the triangle.
link |
Okay, that is a fact.
link |
And you can probably do it with two billiard balls,
link |
maybe with three billiard balls if you're really lucky.
link |
But what happens is as the system
link |
gets more and more complicated,
link |
you have to be more and more precise
link |
not to make the tiniest error,
link |
because the tiniest errors will get magnified
link |
and you'll simply not be able to do the reversal.
link |
So yeah, but I wouldn't call that time travel.
link |
Yeah, that's something else.
link |
But if you think of it, it just made me think,
link |
if you think the unrolling of state
link |
that's happening as a program,
link |
if we look at the world,
link |
silly idea of looking at the world as a simulation,
link |
But it's not a computer, it's just a single program.
link |
A question arises that might be useful.
link |
How hard is it to have a computer that runs the universe?
link |
Okay, so there are mathematical universes
link |
that we know about.
link |
One of them is called anti de Sitter space,
link |
where we, and it's quantum mechanics,
link |
I think we could simulate it in a computer,
link |
in a quantum computer.
link |
Classical computer, all you can do is solve its equations.
link |
You can't make it work like the real system.
link |
If we could build a quantum computer, a big enough one,
link |
a robust enough one, we could probably simulate a universe,
link |
a small version of an anti de Sitter universe.
link |
Anti de Sitter is a kind of cosmology.
link |
So I think we know how to do that.
link |
The trouble is the universe that we live in
link |
is not the anti de Sitter geometry,
link |
it's the de Sitter geometry.
link |
And we don't really understand its quantum mechanics at all.
link |
So at the present time,
link |
I would say we wouldn't have the vaguest idea
link |
how to simulate a universe similar to our own.
link |
No, we can ask, could we build in the laboratory
link |
a small version, a quantum mechanical version,
link |
the collection of quantum computers
link |
and tangled and coupled together,
link |
which would reproduce the phenomena that go on in the universe,
link |
even on a small scale.
link |
Yes, if it were anti de Sitter space,
link |
no, if it's de Sitter space.
link |
Can you slightly describe de Sitter space
link |
and anti de Sitter space?
link |
What are the geometric properties of?
link |
They differ by the sign of a single constant
link |
called the cosmological constant.
link |
One of them is negatively curved,
link |
the other is positively curved.
link |
Anti de Sitter space, which is the negatively curved one,
link |
you can think of as an isolated system
link |
in a box with reflecting walls.
link |
You could think of it as a system
link |
of quantum mechanical system isolated
link |
in an isolated environment.
link |
De Sitter space is the one we really live in.
link |
And that's the one that's exponentially expanding,
link |
exponential expansion, dark energy,
link |
whatever we wanna call it.
link |
And we don't understand that mathematically.
link |
Not everybody would agree with me,
link |
but I don't understand.
link |
They would agree with me,
link |
they definitely would agree with me
link |
that I don't understand it.
link |
What about, is there an understanding of the birth,
link |
the origin, the big bang?
link |
So there's one problem with the other.
link |
No, no, there's theories.
link |
There are theories.
link |
My favorite is the one called eternal inflation.
link |
The infinity can be on both sides,
link |
on one of the sides and none of the sides.
link |
So what's eternal infinity?
link |
Infinity on both sides.
link |
Yeah, yeah, that's.
link |
Why is that your favorite?
link |
Because it's the most just mind blowing?
link |
Because we want a beginning.
link |
No, why do we want a beginning?
link |
In practice there was a beginning, of course.
link |
In practice there was a beginning.
link |
But could it have been a random fluctuation
link |
in an otherwise infinite time?
link |
In any case, the eternal inflation theory,
link |
I think if correctly understood,
link |
would be infinite in both directions.
link |
How do you think about infinity?
link |
So, okay, of course you can think about it mathematically.
link |
I just finished this discussion with my friend Sergei Brin.
link |
How do you think about infinity?
link |
I say, well, Sergei Brin is infinitely rich.
link |
How do you test that hypothesis?
link |
Yeah, so there's really no way
link |
to visualize some of these things.
link |
Yeah, no, this is a very good question.
link |
Does physics have any,
link |
does infinity have any place in physics?
link |
Right, and all I can say is very good question.
link |
So what do you think of the recent first image
link |
of a black hole visualized from the Horizon Telescope?
link |
It's an incredible triumph of science.
link |
In itself, the fact that there are black holes
link |
which collide is not a surprise.
link |
And they seem to work exactly
link |
the way they're supposed to work.
link |
Will we learn a great deal from it?
link |
I don't know, we might.
link |
But the kind of things we'll learn
link |
won't really be about black holes.
link |
Why there are black holes in nature
link |
of that particular mass scale and why they're so common
link |
may tell us something about the structure,
link |
evolution of structure in the universe.
link |
But I don't think it's gonna tell us
link |
anything new about black holes.
link |
But it's a triumph in the sense
link |
that you go back 100 years
link |
and it was a continuous development,
link |
general relativity, the discovery of black holes,
link |
LIGO, the incredible technology that went into LIGO.
link |
It is something that I never would have believed
link |
was gonna happen 30, 40 years ago.
link |
And I think it's a magnificent structure,
link |
magnificent thing, this evolution of general relativity,
link |
LIGO, high precision, ability to measure things
link |
on a scale of 10 to the minus 21.
link |
So you're just in awe that this path
link |
took us to this picture.
link |
You've thought a lot about black holes.
link |
How did you visualize them in your mind?
link |
And is the picture different than you've visualized it?
link |
No, it's simply confirmed.
link |
It's a magnificent triumph to have confirmed
link |
a direct observation that Einstein's theory of gravity
link |
at the level of black hole collisions actually works
link |
is awesome, it is really awesome.
link |
I know some of the people who are involved in that.
link |
They're just ordinary people.
link |
And the idea that they could carry this out,
link |
I just, I'm shocked.
link |
Yeah, just these little homo sapiens?
link |
Yeah, just these little monkeys.
link |
Yeah, got together and took a picture of...
link |
Slightly advanced limer's, I think.
link |
What kind of questions can science not currently answer
link |
but you hope might be able to soon?
link |
Well, you've already addressed them.
link |
What is consciousness, for example?
link |
You think that's within the reach of science?
link |
I think it's somewhat within the reach of science,
link |
but I think that now I think it's in the hands
link |
of the computer scientists and the neuroscientists.
link |
Not a physicist, with the help.
link |
Perhaps at some point, but I think physicists
link |
will try to simplify it down to something
link |
that they can use their methods
link |
and maybe they're not appropriate.
link |
Maybe we simply need to do more machine learning
link |
on bigger scales, evolve machines.
link |
Machines not only that learn
link |
but evolve their own architecture.
link |
As a process of learning, evolve in architecture.
link |
Not under our control, only partially under our control,
link |
but under the control of machine learning.
link |
I'll tell you another thing that I find awesome.
link |
You know this Google thing that they taught
link |
the computers how to play chess?
link |
Okay, they taught the computers how to play chess,
link |
not by teaching them how to play chess,
link |
but just having them play against each other.
link |
Against each other, self play.
link |
Against each other, this is a form of evolution.
link |
These machines evolved, they evolved in intelligence.
link |
They evolved in intelligence
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without anybody telling them how to do it.
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They were not engineered, they just played
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against each other and got better and better and better.
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That makes me think that machines can evolve intelligence.
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What exact kind of intelligence, I don't know.
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But in understanding that better and better,
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maybe we'll get better clues as to what goes on
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in our own intelligence.
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What life in intelligence is.
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Last question, what kind of questions can science
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not currently answer and may never be able to answer?
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Is there an intelligence out there
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that's underlies the whole thing?
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You can call them with a G word if you want.
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I can say, are we a computer simulation with a purpose?
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Is there an agent, an intelligent agent
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that underlies or is responsible for the whole thing?
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Does that intelligent agent satisfy the laws of physics?
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Does it satisfy the laws of quantum mechanics?
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Is it made of atoms and molecules?
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Yeah, there's a lot of questions.
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And I don't see, it seems to me a real question.
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It's an answerable question.
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Well, I don't know if it's answerable.
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The questions have to be answerable to be real.
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Some philosophers would say that a question
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is not a question unless it's answerable.
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This question doesn't seem to me answerable
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by any known method, but it seems to me real.
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There's no better place to end.
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Leonard, thank you so much for talking today.