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Sean Carroll: Quantum Mechanics and the Many-Worlds Interpretation | Lex Fridman Podcast #47


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The following is a conversation with Sean Carroll, Part 2, the second time we've spoken
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on the podcast.
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You can get the link to the first time in the description.
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This time we focus on quantum mechanics and the many worlds interpretation that he details
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elegantly in his new book titled Something Deeply Hidden.
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I own and enjoy both the eBook and audiobook versions of it.
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Listening to Sean read about entanglement, complementarity, and the emergence of space
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time reminds me of Bob Ross teaching the world how to paint on his old television show.
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If you don't know who Bob Ross is, you're truly missing out.
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Look him up.
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He'll make you fall in love with painting.
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Sean Carroll is the Bob Ross of theoretical physics.
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He's the author of several popular books, a host of a great podcast called Mindscape,
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and is a theoretical physicist at Caltech and the Santa Fe Institute, specializing in
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quantum mechanics, arrow of time, cosmology, and gravitation.
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This is the Artificial Intelligence Podcast.
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If you enjoy it, subscribe on YouTube, give it five stars on iTunes, support it on Patreon,
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or simply connect with me on Twitter at Lex Friedman, spelled F R I D M A N.
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And now here's my conversation with Sean Carroll.
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Isaac Newton developed what we now call classical mechanics that you describe very nicely in
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your new book, as you do with a lot of basic concepts in physics.
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So with classical mechanics, I can throw a rock and can predict the trajectory of that
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rock's flight.
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But if we could put ourselves back into Newton's time, his theories work to predict things,
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but as I understand, he himself thought that they were, their interpretations of those
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predictions were absurd.
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Perhaps he just said it for religious reasons and so on, but in particular, sort of a world
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of interaction without contact, so action at a distance.
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It didn't make sense to him on a sort of a human interpretation level.
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Does it make sense to you that things can affect other things at a distance?
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It does, but that was one of Newton's worries.
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You're actually right in a slightly different way about the religious worries.
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He was smart enough, this is off the topic but still fascinating, Newton almost invented
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chaos theory as soon as he invented classical mechanics.
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He realized that in the solar system, so he was able to explain how planets move around
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the Sun, but typically you would describe the orbit of the Earth ignoring the effects
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of Jupiter and Saturn and so forth, just doing the Earth and the Sun.
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He kind of knew, even though he couldn't do the math, that if you included the effects
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of Jupiter and Saturn and the other planets, the solar system would be unstable, like the
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orbits of the planets would get out of whack.
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So he thought that God would intervene occasionally to sort of move the planets back into orbit,
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which is the only way you could explain how they were there presumably forever.
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But the worries about classical mechanics were a little bit different, the worry about
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gravity in particular.
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It wasn't a worry about classical mechanics, it was a worry about gravity.
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How in the world does the Earth know that there's something called the Sun, 93 million
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miles away, that is exerting gravitational force on it?
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And he literally said, you know, I leave that for future generations to think about because
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I don't know what the answer is.
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And in fact, people under emphasized this, but future generations figured it out.
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Pierre Simone Laplace in circa 1800 showed that you could rewrite Newtonian gravity as
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a field theory.
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So instead of just talking about the force due to gravity, you can talk about the gravitational
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field or the gravitational potential field, and then there's no action at a distance.
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It's exactly the same theory empirically, it makes exactly the same predictions.
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But what's happening is instead of the Sun just reaching out across the void, there is
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a gravitational field in between the Sun and the Earth that obeys an equation, Laplace's
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equation, cleverly enough, and that tells us exactly what the field does.
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So even in Newtonian gravity, you don't need action at a distance.
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Now what many people say is that Einstein solved this problem because he invented general
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relativity.
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And in general relativity, there's certainly a field in between the Earth and the Sun.
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But also there's the speed of light as a limit.
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In Laplace's theory, which was exactly Newton's theory, just in a different mathematical language,
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there could still be instantaneous action across the universe, whereas in general relativity,
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if you shake something here, its gravitational impulse radiates out at the speed of light
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and we call that a gravitational wave and we can detect those.
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So but I really, it rubs me the wrong way to think that we should presume the answer
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should look one way or the other.
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Like if it turned out that there was action at a distance in physics and that was the
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best way to describe things, then I would do it that way.
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It's actually a very deep question because when we don't know what the right laws of
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physics are, when we're guessing at them, when we're hypothesizing at what they might
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be, we are often guided by our intuitions about what they should be.
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I mean, Einstein famously was very guided by his intuitions and he did not like the
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idea of action at a distance.
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We don't know whether he was right or not.
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It depends on your interpretation of quantum mechanics and it depends on even how you talk
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about quantum mechanics within any one interpretation.
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So if you see every force as a field or any other interpretation of action at a distance,
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just stepping back to sort of caveman thinking, like do you really, can you really sort of
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understand what it means for a force to be a field that's everywhere?
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So if you look at gravity, like what do you think about?
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I think so.
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Is this something that you've been conditioned by society to think that, to map the fact
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that science is extremely well predictive of something to believing that you actually
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understand it?
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Like you can intuitively, the degree that human beings can understand anything that
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you actually understand it.
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Or are you just trusting the beauty and the power of the predictive power of science?
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That depends on what you mean by this idea of truly understanding something, right?
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You know, I mean, can I truly understand Fermat's last theorem?
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You know, it's easy to state it, but do I really appreciate what it means for incredibly
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large numbers, right?
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I think yes, I think I do understand it, but like if you want to just push people on well,
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but your intuition doesn't go to the places where Andrew Wiles needed to go to prove Fermat's
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last theorem, then I can say fine, but I still think I understand the theorem.
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And likewise, I think that I do have a pretty good intuitive understanding of fields pervading
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space time, whether it's the gravitational field or the electromagnetic field or whatever,
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the Higgs field.
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Of course, one's intuition gets worse and worse as you get trickier in the quantum field
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theory and all sorts of new phenomena that come up in quantum field theory.
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So our intuitions aren't perfect, but I think it's also okay to say that our intuitions
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get trained, right?
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Like, you know, I have different intuitions now than I had when I was a baby.
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That's okay.
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That's not, an intuition is not necessarily intrinsic to who we are.
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We can train it a little bit.
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So that's where I'm going to bring in Noam Chomsky for a second, who thinks that our
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cognitive abilities are sort of evolved through time, and so they're biologically constrained.
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And so there's a clear limit, as he puts it, to our cognitive abilities, and it's a very
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harsh limit.
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But you actually kind of said something interesting in nature versus nurture thing here, is we
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can train our intuitions to sort of build up the cognitive muscles to be able to understand
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some of these tricky concepts.
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So do you think there's limits to our understanding that's deeply rooted, hardcoded into our biology
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that we can't overcome?
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There could be limits to things like our ability to visualize, okay?
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But when someone like Ed Witten proves a theorem about, you know, 100 dimensional mathematical
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spaces, he's not visualizing it.
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He's doing the math.
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That doesn't stop him from understanding the result.
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I think, and I would love to understand this better, but my rough feeling, which is not
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very educated, is that, you know, there's some threshold that one crosses in abstraction
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when one becomes kind of like a Turing machine, right?
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One has the ability to contain in one's brain logical, formal, symbolic structures and manipulate
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them.
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And that's a leap that we can make as human beings that dogs and cats haven't made.
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And once you get there, I'm not sure that there are any limits to our ability to understand
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the scientific world at all.
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Maybe there are.
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There's certainly limits in our ability to calculate things, right?
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You know, people are not very good at taking cube roots of million digit numbers in their
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head.
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But that's not an element of understanding.
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It's certainly not a limit in principle.
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So of course, as a human, you would say there doesn't feel to be limits to our understanding.
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But sort of, have you thought that the universe is actually a lot simpler than it appears
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to us?
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And we just will never be able to, like, it's outside of our, okay.
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So us, our cognitive abilities combined with our mathematical prowess and whatever kind
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of experimental simulation devices we can put together, is there limits to that?
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Is it possible there's limits to that?
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Well, of course it's possible that there are limits to that.
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Is there any good reason to think that we're anywhere close to the limits is a harder question.
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Look, imagine asking this question 500 years ago to the world's greatest thinkers, right?
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Like are we approaching the limits of our ability to understand the natural world?
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And by definition, there are questions about the natural world that are most interesting
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to us that are the ones we don't quite yet understand, right?
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So there's always, we're always faced with these puzzles we don't yet know.
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And I don't know what they would have said 500 years ago, but they didn't even know about
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classical mechanics, much less quantum mechanics.
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So we know that they were nowhere close to how well they could do, right?
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They could do enormously better than they were doing at the time.
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I see no reason why the same thing isn't true for us today.
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So of all the worries that keep me awake at night, the human mind's inability to rationally
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comprehend the world is low on the list.
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Well put.
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So one interesting philosophical point that quantum mechanics bring up is the, that you
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talk about the distinction between the world as it is and the world as we observe it.
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So staying at the human level for a second, how big is the gap between what our perception
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system allows us to see and the world as it is outside our mind's eye sort of, sort of
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not at the quantum mechanical level, but as just our, these particular tools we have,
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which is the few senses and cognitive abilities to process those senses.
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Well, that last phrase, having the cognitive abilities to process them carries a lot, right?
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I mean, there is our sort of intuitive understanding of the world.
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You don't need to teach people about gravity for them to know that apples fall from trees,
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right?
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That's something that we figure out pretty quickly.
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Project permanence, things like that, the three dimensionality of space, even if we
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don't have the mathematical language to say that, we kind of know that it's true.
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On the other hand, no one opens their eyes and sees atoms, right?
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Or molecules or cells for that matter, forget about quantum mechanics.
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So but we got there, we got to understanding that there are atoms and cells using the combination
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of our senses and our cognitive capacities.
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So adding the ability of our cognitive capacities to our senses is adding an enormous amount
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and I don't think it is a hard and fast boundary.
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You know, if you believe in cells, if you believe that we understand those, then there's
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no reason you believe we can't believe in quantum mechanics just as well.
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What to you is the most beautiful idea in physics?
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Conservation of momentum.
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Can you elaborate?
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Yeah.
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So if you were Aristotle, when Aristotle wrote his book on physics, he made the following
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very obvious point.
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We're on video here, right?
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So people can see this.
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Yeah.
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So if I push the bottle, let me cover this bottle so we do not have a mess, but okay.
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So I push the bottle, it moves, and if I stop pushing, it stops moving.
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And this kind of thing is repeated a large number of times all over the place.
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If you don't keep pushing things, they stop moving.
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This is an indisputably true fact about our everyday environment, okay?
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And for Aristotle, this blew up into a whole picture of the world in which things had natures
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and teleologies, and they had places they wanted to be, and when you were pushing them,
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you were moving them away from where they wanted to be, and they would return and stuff
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like that.
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And it took a thousand years or 1500 years for people to say, actually, if it weren't
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for things like dissipation and air resistance and friction and so forth, the natural thing
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is for things to move forever in a straight line, there's a constant velocity, right?
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Conservation of momentum.
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And the reason why I think that's the most beautiful idea in physics is because it shifts
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us from a view of natures and teleology to a view of patterns in the world.
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So when you were Aristotle, you needed to talk a vocabulary of why is this happening,
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what's the purpose of it, what's the cause, etc., because, you know, it's nature does
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or does not want to do that, whereas once you believe in conservation of momentum, things
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just happen.
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They just follow the pattern.
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You give me, you have Laplace's demon, ultimately, right?
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You give me the state of the world today, I can predict what it's going to do in the
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future, I can predict where it was in the past.
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It's impersonal, and it's also instantaneous.
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It's not directed toward any future goals, it's just doing what it does given the current
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state of the universe.
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I think even more than either classical mechanics or quantum mechanics, that is the profound
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deep insight that gets modern science off the ground.
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You don't need natures and purposes and goals, you just need some patterns.
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So it's the first moment in our understanding of the way the universe works where you branch
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from the intuitive physical space to kind of the space of ideas.
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And also the other point you said, which is, conveniently, most of the interesting ideas
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are acting in the moment.
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You don't need to know the history of time or the future.
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And of course, this took a long time to get there, right?
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I mean, the conservation of momentum itself took hundreds of years.
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It's weird, because like, someone would say something interesting, and then the next interesting
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thing would be said like 150 or 200 years later, right?
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They weren't even talking to each other, they were reading each other's books.
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And probably the first person to directly say that in outer space, in the vacuum, a
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projectile would move at a constant velocity was Avicenna, Ibn Sina in the Persian Golden
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Age, circa 1000.
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And he didn't like the idea.
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He used that, just like Schrodinger used Schrodinger's cat to say, surely you don't believe that,
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right?
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Ibn Sina was saying, surely you don't believe there really is a vacuum, because if there
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was a really vacuum, things could keep moving forever, right?
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But still, he got right the idea that there was this conservation of something impetus
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or mile, he would call it.
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And that's 500 years, 600 years before classical mechanics and Isaac Newton.
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So Galileo played a big role in this, but he didn't exactly get it right.
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And so it just takes a long time for this to sink in, because it is so against our everyday
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experience.
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Do you think it was a big leap, a brave or a difficult leap of sort of math and science
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to be able to say that momentum is conserved?
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I do.
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You know, I think it's an example of human reason in action.
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You know, even Aristotle knew that his theory had issues, because you could fire an arrow
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and it would go a long way before it stopped.
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So if his theory was things just automatically stop, what's going on?
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And he had this elaborate story.
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I don't know if you've heard the story, but the arrow would push the air in front of it
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away and the molecules of air would run around to the back of the arrow and push it again.
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And anyone reading this is going like, really, that's what you thought?
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But it was that kind of thought experiment that ultimately got people to say like, actually,
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no, if it weren't for the air molecules at all, the arrow would just go on by itself.
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And it's always this give and take between thought and experience, back and forth, right?
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Theory and experiment, we would say today.
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Another big question that I think comes up, certainly with quantum mechanics, is what's
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the difference between math and physics to you?
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To me, you know, very, very roughly, math is about the logical structure of all possible
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worlds and physics is about our actual world.
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And it just feels like our actual world is a gray area when you start talking about interpretations
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of quantum mechanics, or no?
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I'm certainly using the word world in the broadest sense, all of reality.
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So I think that reality is specific.
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I don't think that there's every possible thing going on in reality.
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I think that there are rules, whether it's the Schrodinger equation or whatever.
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So I think that there's a sensible notion of the set of all possible worlds and we live
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in one of them.
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The world that we're talking about might be a multiverse, might be many worlds of quantum
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mechanics, might be much bigger than the world of our everyday experience, but it's still
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one physically contiguous world in some sense.
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But so if you look at the overlap of math and physics, it feels like when physics tries
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to reach for understanding of our world, it uses the tools of math to sort of reach beyond
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the limit of our current understanding.
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What do you make of that process of sort of using math to, so you start maybe with intuition
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or you might start with the math and then build up an intuition or, but this kind of
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reaching into the darkness, into the mystery of the world with math.
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Well, I think I would put it a little bit differently.
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I think we have theories, theories of the physical world, which we then extrapolate
link |
00:18:57.900
and ask, you know, what do we conclude if we take these seriously well beyond where
link |
00:19:02.380
we've actually tested them?
link |
00:19:03.700
It is separately true that math is really, really useful when we construct physical theories
link |
00:19:09.180
and you know, famously Eugene Wigner asked about the unreasonable success of mathematics
link |
00:19:13.060
and physics.
link |
00:19:14.060
I think that's a little bit wrong because anything that could happen, any other theory
link |
00:19:20.140
of physics that wasn't the real world, but some other world, you could always describe
link |
00:19:24.700
it mathematically.
link |
00:19:25.700
It's just that it might be a mess.
link |
00:19:28.180
The surprising thing is not that math works, but that the math is so simple and easy that
link |
00:19:33.660
you can write it down on a t shirt, right?
link |
00:19:35.660
I mean, that's what is amazing.
link |
00:19:37.640
That's an enormous compression of information that seems to be valid in the real world.
link |
00:19:44.020
So that's an interesting fact about our world, which maybe we could hope to explain or just
link |
00:19:48.340
take as a brute fact.
link |
00:19:49.620
I don't know.
link |
00:19:50.880
But once you have that, you know, there's this indelible relationship between math and
link |
00:19:56.100
physics, but philosophically I do want to separate them.
link |
00:19:59.380
What we extrapolate, we don't extrapolate math because there's a whole bunch of wrong
link |
00:20:03.740
math, you know, that doesn't apply to our world, right?
link |
00:20:06.060
We extrapolate the physical theory that we best think explains our world.
link |
00:20:09.980
Again, an unanswerable question.
link |
00:20:12.700
Why do you think our world is so easily compressible into beautiful equations?
link |
00:20:19.780
Yeah.
link |
00:20:20.780
I mean, like I just hinted at, I don't know if there's an answer to that question.
link |
00:20:23.940
There could be.
link |
00:20:24.940
What would an answer look like?
link |
00:20:26.460
Well, an answer could look like if you showed that there was something about our world that
link |
00:20:31.380
maximizes something.
link |
00:20:33.260
You know, the mean of the simplicity and the powerfulness of the laws of physics or, you
link |
00:20:40.100
know, maybe we're just generic.
link |
00:20:41.920
Maybe in the set of all possible worlds, this is what the world would look like, right?
link |
00:20:44.980
Like I don't really know.
link |
00:20:46.500
I tend to think not.
link |
00:20:48.340
I tend to think that there is something specific and rock bottom about the facts of our world
link |
00:20:54.220
that don't have further explanation.
link |
00:20:56.060
Like the fact of the world exists at all.
link |
00:20:58.260
And furthermore, the specific laws of physics that we have.
link |
00:21:00.780
I think that in some sense, we're just going to, at some level, we're going to say, and
link |
00:21:04.660
that's how it is.
link |
00:21:05.660
And, you know, we can't explain anything more.
link |
00:21:07.340
I don't know how, if we're anywhere close to that right now, but that seems plausible
link |
00:21:11.580
to me.
link |
00:21:12.580
And speaking of rock bottom, one of the things sort of your book kind of reminded me or revealed
link |
00:21:16.780
to me is that what's fundamental and what's emergent, it just feels like I don't even
link |
00:21:23.340
know anymore what's fundamental in physics, if there's anything.
link |
00:21:28.420
It feels like everything, especially with quantum mechanics, is revealing to us is that
link |
00:21:33.980
most interesting things that I would, as a limited human would think are fundamental
link |
00:21:41.460
can actually be explained as emergent from the more deeper laws.
link |
00:21:48.980
I mean, we don't know, of course.
link |
00:21:51.140
You had to get that on the table.
link |
00:21:52.480
We don't know what is fundamental.
link |
00:21:54.620
We do have reasons to say that certain things are more fundamental than others, right?
link |
00:22:00.480
Atoms and molecules are more fundamental than cells and organs.
link |
00:22:03.980
Quantum fields are more fundamental than atoms and molecules.
link |
00:22:07.600
We don't know if that ever bottoms out.
link |
00:22:09.780
I do think that there's sensible ways to think about this.
link |
00:22:13.860
If you describe something like this table as a table, it has a height and a width and
link |
00:22:18.140
it's made of a certain material and it has a certain solidity and weight and so forth.
link |
00:22:22.580
That's a very useful description as far as it goes.
link |
00:22:24.940
There's a whole other description of this table in terms of a whole collection of atoms
link |
00:22:29.380
strung together in certain ways.
link |
00:22:31.640
The language of the atoms is more comprehensive than the language of the table.
link |
00:22:36.340
You could break apart the table, smash it to pieces, still talk about it as atoms, but
link |
00:22:41.100
you could no longer talk about it as a table, right?
link |
00:22:43.940
So I think that this comprehensiveness, the domain of validity of a theory gets broader
link |
00:22:48.380
and broader as the theory gets more and more fundamental.
link |
00:22:52.900
So what do you think Newton would say?
link |
00:22:57.620
Maybe right in the book review, if you read your latest book on quantum mechanics, something
link |
00:23:02.140
deeply hidden.
link |
00:23:03.140
It would take a long time for him to think that any of this was making any sense.
link |
00:23:08.140
You catch him up pretty quick in the beginning.
link |
00:23:10.260
Yeah.
link |
00:23:11.260
You give him a shout out in the beginning.
link |
00:23:13.220
That's right.
link |
00:23:14.220
He is the man.
link |
00:23:15.220
I'm happy to say that Newton was the greatest scientist who ever lived.
link |
00:23:19.460
He invented calculus in his spare time, which would have made him the greatest mathematician
link |
00:23:22.500
just all by himself, all by that one thing.
link |
00:23:25.620
But of course, it's funny because Newton was in some sense still a pre modern thinker.
link |
00:23:33.540
Rocky Kolb, who is a cosmologist at the University of Chicago said that Galileo, even though
link |
00:23:39.480
he came before Newton, was a more modern thinker than Newton was.
link |
00:23:43.740
If you got Galileo and brought him to the present day, it would take him six months
link |
00:23:46.660
to catch up and then he'd be in your office telling you why your most recent paper was
link |
00:23:49.580
wrong.
link |
00:23:50.740
Whereas Newton just thought in this kind of more mystical way.
link |
00:23:55.440
He wrote a lot more about the Bible and alchemy than he ever did about physics, but he was
link |
00:24:01.920
also more brilliant than anybody else and way more mathematically astute than Galileo.
link |
00:24:06.260
So I really don't know.
link |
00:24:08.420
He might have, he might just, yeah, say like, give me the textbooks, leave me alone for
link |
00:24:12.060
a few months and then be caught up.
link |
00:24:15.200
But he might have had mental blocks against seeing the world in this way.
link |
00:24:19.900
I really don't know.
link |
00:24:20.900
Or perhaps find an interesting mystical interpretation of quantum mechanics.
link |
00:24:24.780
Very possible.
link |
00:24:25.780
Yeah.
link |
00:24:26.780
Is there any other scientists or philosophers through history that you would like to know
link |
00:24:31.500
their opinion of your book?
link |
00:24:33.580
That's a, that's a good question.
link |
00:24:36.420
I mean, Einstein is the obvious one, right?
link |
00:24:38.500
We all, I mean, he was not that long ago, but I even speculated at the end of my book
link |
00:24:42.100
about what his opinion would be.
link |
00:24:44.140
I am curious as to, you know, what about older philosophers like Hume or Kant, right?
link |
00:24:50.140
Like what would they have thought?
link |
00:24:51.140
Or Aristotle, you know, what would they have thought about modern physics?
link |
00:24:55.840
Because they do in philosophy, your predilections end up playing a much bigger role in your
link |
00:25:01.980
ultimate conclusions because you're not as tied down by what the data is in physics.
link |
00:25:07.180
You know, physics is lucky because we can't stray too far off the reservation as long
link |
00:25:11.060
as we're trying to explain the world that we actually see in our telescopes and microscopes.
link |
00:25:15.820
But it's just not fair to play that game because the people we're thinking about didn't know
link |
00:25:20.900
a whole bunch of things that we know, right?
link |
00:25:23.580
Like we lived through a lot that they didn't live through.
link |
00:25:26.380
So by the time we got them caught up, they'd be different people.
link |
00:25:32.700
So let me ask a bunch of basic questions.
link |
00:25:35.940
I think it would be interesting, useful for people who are not familiar, but even for
link |
00:25:40.220
people who are extremely well familiar.
link |
00:25:42.940
Let's start with what is quantum mechanics?
link |
00:25:47.260
Quantum mechanics is the paradigm of physics that came into being in the early part of
link |
00:25:51.820
the 20th century that replaced classical mechanics, and it replaced classical mechanics in a weird
link |
00:25:58.380
way that we're still coming to terms with.
link |
00:26:00.380
So in classical mechanics, you have an object, it has a location, it has a velocity, and
link |
00:26:05.580
if you know the location and velocity of everything in the world, you can say what everything's
link |
00:26:08.660
going to do.
link |
00:26:10.420
Quantum mechanics has an aspect of it that is kind of on the same lines.
link |
00:26:15.660
There's something called the quantum state or the wave function.
link |
00:26:19.340
And there's an equation governing what the quantum state does.
link |
00:26:22.260
So it's very much like classical mechanics.
link |
00:26:23.980
The wave function is different.
link |
00:26:25.660
It's sort of a wave.
link |
00:26:26.980
It's a vector in a huge dimensional vector space rather than a position and a velocity,
link |
00:26:31.380
but okay, that's a detail.
link |
00:26:33.060
The equation is the Schrodinger equation, not Newton's laws, but okay, again, a detail.
link |
00:26:37.700
Where quantum mechanics really becomes weird and different is that there's a whole other
link |
00:26:41.380
set of rules in our textbook formulation of quantum mechanics in addition to saying that
link |
00:26:46.820
there's a quantum state and it evolves in time.
link |
00:26:49.320
And all these new rules have to do with what happens when you look at the system, when
link |
00:26:52.820
you observe it, when you measure it.
link |
00:26:55.100
In classical mechanics, there were no rules about observing.
link |
00:26:58.420
You just look at it and you see what's going on.
link |
00:27:00.260
That was it, right?
link |
00:27:01.580
In quantum mechanics, the way we teach it, there's something profoundly fundamental about
link |
00:27:06.940
the act of measurement or observation, and the system dramatically changes its state.
link |
00:27:12.260
Even though it has a wave function, like the electron in an atom is not orbiting in a circle,
link |
00:27:16.780
it's sort of spread out in a cloud, when you look at it, you don't see that cloud.
link |
00:27:21.460
When you look at it, it looks like a particle with a location.
link |
00:27:24.840
So it dramatically changes its state right away, and the effects of that change can be
link |
00:27:29.380
instantly seen in what the electron does next.
link |
00:27:32.120
So again, we need to be careful because we don't agree on what quantum mechanics says.
link |
00:27:39.060
That's why I need to say like in the textbook view, et cetera, right?
link |
00:27:41.660
But in the textbook view, quantum mechanics, unlike any other theory of physics, gives
link |
00:27:48.540
a fundamental role to the act of measurement.
link |
00:27:51.060
So maybe even more basic, what is an atom and what is an electron?
link |
00:27:56.980
Sure.
link |
00:27:57.980
This all came together in a few years around the turn of the last century, right?
link |
00:28:01.780
Around the year 1900.
link |
00:28:05.180
Atoms predated then, of course, the word atom goes back to the ancient Greeks, but it was
link |
00:28:09.220
the chemists in the 1800s that really first got experimental evidence for atoms.
link |
00:28:15.300
They realized that there were two different types of tin oxide.
link |
00:28:21.020
And in these two different types of tin oxide, there was exactly twice as much oxygen in
link |
00:28:25.300
one type as the other.
link |
00:28:27.180
And like, why is that?
link |
00:28:28.940
Why is it never 1.5 times as much, right?
link |
00:28:31.700
And so Dalton said, well, it's because there are tin atoms and oxygen atoms, and one form
link |
00:28:38.280
of tin oxide is one atom of tin and one atom of oxygen, and the other is one atom of tin
link |
00:28:42.940
and two atoms of oxygen.
link |
00:28:44.820
And on the basis of this, you know, a speculation, a theory, right, a hypothesis, but then on
link |
00:28:49.220
the basis of that, you make other predictions, and the chemists became quickly convinced
link |
00:28:52.900
that atoms were real.
link |
00:28:54.540
The physicists took a lot longer to catch on, but eventually they did.
link |
00:28:58.880
And I mean, Boltzmann, who believed in atoms, had a really tough time his whole life because
link |
00:29:04.380
he worked in Germany where atoms were not popular.
link |
00:29:07.220
They were popular in England, but not in Germany.
link |
00:29:09.800
And there, in general, the idea of atoms is, it's the most, the smallest building block
link |
00:29:15.160
of the universe for them.
link |
00:29:17.080
That's the kind of how they thought it was.
link |
00:29:18.080
That was the Greek idea, but the chemists in the 1800s jumped the gun a little bit.
link |
00:29:22.760
So these days, an atom is the smallest building block of a chemical element, right?
link |
00:29:27.860
Hydrogen, tin, oxygen, carbon, whatever, but we know that atoms can be broken up further
link |
00:29:33.180
than that.
link |
00:29:34.180
That's what physicists discovered in the early 1900s, Rutherford, especially, and his colleagues.
link |
00:29:40.620
So the atom that we think about now, the cartoon, is that picture you've always seen of a little
link |
00:29:46.740
nucleus and then electrons orbiting it like a little solar system.
link |
00:29:50.260
And we now know the nucleus is made of protons and neutrons.
link |
00:29:53.180
So the weight of the atom, the mass, is almost all in its nucleus.
link |
00:29:58.640
Protons and neutrons are something like 1800 times as heavy as electrons are.
link |
00:30:03.880
Protons are much lighter, but because they're lighter, they give all the life to the atoms.
link |
00:30:09.020
So when atoms get together, combine chemically, when electricity flows through a system, it's
link |
00:30:13.740
all the electrons that are doing all the work.
link |
00:30:16.900
And where quantum mechanics steps in, as you mentioned, with the position of velocity with
link |
00:30:21.300
classical mechanics and quantum mechanics is modeling the behavior of the electron.
link |
00:30:26.900
I mean, you can model the behavior of anything, but the electron, because that's where the
link |
00:30:30.620
fun is.
link |
00:30:31.900
The electron was the biggest challenge right from the start.
link |
00:30:34.580
Yeah.
link |
00:30:35.580
So what's a wave function?
link |
00:30:36.580
You said it's an interesting detail, but in any interpretation, what is the wave function
link |
00:30:43.540
in quantum mechanics?
link |
00:30:44.540
Well, you know, we had this idea from Rutherford that atoms look like little solar systems,
link |
00:30:50.260
but people very quickly realize that can't possibly be right because if an electron is
link |
00:30:54.760
orbiting in a circle, it will give off light.
link |
00:30:57.640
All the light that we have in this room comes from electrons zooming up and down and wiggling.
link |
00:31:01.540
That's what electromagnetic waves are.
link |
00:31:03.260
And you can calculate how long would it take for the electron just to spiral into the nucleus?
link |
00:31:07.660
And the answer is 10 to the minus 11 seconds, okay, 100 billionth of a second.
link |
00:31:12.180
So that's not right.
link |
00:31:14.380
Meanwhile, people had realized that light, which we understood from the 1800s was a wave,
link |
00:31:21.300
had properties that were similar to that of particles, right?
link |
00:31:23.940
This is Einstein and Planck and stuff like that.
link |
00:31:26.740
So if something that we agree was a wave had particle like properties, then maybe something
link |
00:31:34.020
we think is a particle, the electron has wave like properties, right?
link |
00:31:38.540
And so a bunch of people eventually came to the conclusion, don't think about the electron
link |
00:31:42.900
as a little point particle orbiting like a solar system.
link |
00:31:47.180
Think of it as a wave that is spread out.
link |
00:31:49.780
They cleverly gave this the name the wave function, which is the dopiest name in the
link |
00:31:53.100
world for one of the most profound things in the universe.
link |
00:31:57.420
There's literally a number at every point in space, which is the value of the electron's
link |
00:32:03.100
wave function at that point.
link |
00:32:04.980
And there's only one wave function.
link |
00:32:07.580
Yeah, they eventually figured that out.
link |
00:32:09.600
That took longer.
link |
00:32:10.960
But when you have two electrons, you do not have a wave function for electron one and
link |
00:32:15.220
a wave function for electron two.
link |
00:32:17.140
You have one combined wave function for both of them.
link |
00:32:20.420
And indeed, as you say, there's only one wave function for the entire universe at once.
link |
00:32:26.980
And that's where this beautiful dance, can you say what is entanglement?
link |
00:32:32.620
It seems one of the most fundamental ideas of quantum mechanics.
link |
00:32:35.580
Well, let's temporarily buy into the textbook interpretation of quantum mechanics.
link |
00:32:39.580
And what that says is that this wave function, so it's very small outside the atom, very
link |
00:32:43.820
big in the atom, basically the wave function, you take it and you square it, you square
link |
00:32:48.940
the number that gives you the probability of observing the system at that location.
link |
00:32:54.220
So if you say that for two electrons, there's only one wave function, and that wave function
link |
00:32:58.700
gives you the probability of observing both electrons at once doing something, okay?
link |
00:33:03.060
So maybe the electron can be here or here, here, here, and the other electron can also
link |
00:33:07.220
be there.
link |
00:33:08.300
But we have a wave function set up where we don't know where either electron is going
link |
00:33:12.600
to be seen.
link |
00:33:14.100
But we know they'll both be seen in the same place, okay?
link |
00:33:17.800
So we don't know exactly what we're going to see for either electron, but there's entanglement
link |
00:33:22.100
between the two of them.
link |
00:33:23.740
There's a sort of conditional statement.
link |
00:33:25.460
If we see one in one location, then we know the other one's going to be doing a certain
link |
00:33:29.440
thing.
link |
00:33:30.440
So that's a feature of quantum mechanics that is nowhere to be found in classical mechanics.
link |
00:33:34.220
In classical mechanics, there's no way I can say, well, I don't know where either one of
link |
00:33:37.660
these particles is, but if I know, if I find out where this one is, then I know where the
link |
00:33:40.700
other one is.
link |
00:33:41.700
That just never happens.
link |
00:33:42.700
They're truly separate.
link |
00:33:43.700
I don't know, it feels like, if you think of a wave function like as a dance floor,
link |
00:33:47.980
it seems like entanglement is strongest between things that are dancing together closest.
link |
00:33:53.860
So there's a closeness that's important.
link |
00:33:56.420
Well, that's another step.
link |
00:33:58.340
We have to be careful here because in principle, if you're talking about the entanglement of
link |
00:34:02.660
two electrons, for example, they can be totally entangled or totally unentangled no matter
link |
00:34:08.860
where they are in the universe.
link |
00:34:10.180
There's no relationship between the amount of entanglement and the distance between two
link |
00:34:15.180
electrons.
link |
00:34:16.180
But we now know that the reality of our best way of understanding the world is through
link |
00:34:21.120
quantum fields, not through particles.
link |
00:34:23.860
So even the electron, not just gravity and electromagnetism, but even the electron and
link |
00:34:28.540
the quarks and so forth are really vibrations in quantum fields.
link |
00:34:33.160
So even empty space is full of vibrating quantum fields.
link |
00:34:38.340
And those quantum fields in empty space are entangled with each other in exactly the way
link |
00:34:42.840
you just said.
link |
00:34:43.840
If they're nearby, if you have like two vibrating quantum fields that are nearby, then they'll
link |
00:34:47.020
be highly entangled.
link |
00:34:48.580
If they're far away, they will not be entangled.
link |
00:34:50.540
So what do quantum fields in a vacuum look like?
link |
00:34:52.880
Empty space?
link |
00:34:53.880
Just like empty space.
link |
00:34:54.880
It's as empty as it can be.
link |
00:34:56.600
But there's still a field.
link |
00:34:57.980
It's just, what does nothing look like?
link |
00:35:02.420
Just like right here, this location in space, there's a gravitational field, which I can
link |
00:35:06.340
detect by dropping something.
link |
00:35:07.980
Yes.
link |
00:35:08.980
I don't see it, but there it is.
link |
00:35:12.860
So we got a little bit of an idea of entanglement.
link |
00:35:17.860
Now, what is Hilbert space and Euclidean space?
link |
00:35:23.980
Yeah, you know, I think that people are very welcome to go through their lives not knowing
link |
00:35:28.180
what Hilbert space is.
link |
00:35:29.360
But if you dig into a little bit more into quantum mechanics, it becomes necessary.
link |
00:35:33.580
You know, the English language was invented long before quantum mechanics, or various
link |
00:35:39.420
forms of higher mathematics were invented.
link |
00:35:41.260
So we use the word space to mean different things.
link |
00:35:45.020
Of course, most of us think of space as this three dimensional world in which we live,
link |
00:35:48.700
right?
link |
00:35:49.700
I mean, some of us just think of it as outer space.
link |
00:35:51.420
Okay, but space around us gives us the three dimensional location of things and objects.
link |
00:35:56.420
But mathematicians use any generic abstract collection of elements as a space, okay?
link |
00:36:05.820
A space of possibilities, you know, momentum space, etc.
link |
00:36:09.940
So Hilbert space is the space of all possible quantum wave functions, either for the universe
link |
00:36:14.480
or for some specific system.
link |
00:36:16.740
And it could be an infinite dimensional space, or it could be just really, really large dimensional
link |
00:36:21.300
but finite.
link |
00:36:22.300
We don't know because we don't know the final theory of everything.
link |
00:36:24.960
But this abstract Hilbert space is really, really, really big and has no immediate connection
link |
00:36:29.540
to the three dimensional space in which we live.
link |
00:36:31.780
What do dimensions in Hilbert space mean?
link |
00:36:35.220
You know, it's just a way of mathematically representing how much information is contained
link |
00:36:40.020
in the state of the system.
link |
00:36:41.660
How many numbers do you have to give me to specify what the thing is doing?
link |
00:36:45.720
So in classical mechanics, I give you the location of something by giving you three
link |
00:36:51.020
numbers, right?
link |
00:36:52.020
Up, down, left, X, Y, Z coordinates.
link |
00:36:54.340
But then I might want to give you its entire state, physical state, which means both its
link |
00:37:00.100
position and also its velocity.
link |
00:37:02.680
The velocity also has three components.
link |
00:37:04.520
So its state lives in something called phase space, which is six dimensional, three dimensions
link |
00:37:09.860
of position, three dimensions of velocity.
link |
00:37:12.540
And then if it also has an orientation in space, that's another three dimensions and
link |
00:37:16.200
so forth.
link |
00:37:17.200
So as you describe more and more information about the system, you have an abstract mathematical
link |
00:37:22.620
space that has more and more numbers that you need to give.
link |
00:37:26.460
And each one of those numbers corresponds to a dimension in that space.
link |
00:37:29.840
So in terms of the amount of information, what is entropy?
link |
00:37:34.760
This mystical word that's overused in math and physics, but has a very specific meaning
link |
00:37:40.740
in this context.
link |
00:37:41.940
Sadly, it has more than one very specific meeting.
link |
00:37:44.300
This is the reason why it is hard.
link |
00:37:46.660
Entropy means different things even to different physicists.
link |
00:37:49.200
But one way of thinking about it is a measure of how much we don't know about the state
link |
00:37:54.120
of a system.
link |
00:37:55.160
So if I have a bottle of water molecules, and I know that, OK, there's a certain number
link |
00:38:00.540
of water molecules.
link |
00:38:01.540
I could weigh it and figure out.
link |
00:38:02.940
I know the volume of it, and I know the temperature and pressure and things like that.
link |
00:38:06.740
I certainly don't know the exact position and velocity of every water molecule.
link |
00:38:12.280
So there's a certain amount of information I know, a certain amount that I don't know
link |
00:38:15.600
that is part of the complete state of the system.
link |
00:38:18.540
And that's what the entropy characterizes, how much unknown information there is, the
link |
00:38:23.340
difference between what I do know about the system and its full exact microscopic state.
link |
00:38:28.300
So when we try to describe a quantum mechanical system, is it infinite or finite but very
link |
00:38:36.660
large?
link |
00:38:37.660
Yeah, we don't know.
link |
00:38:38.660
That depends on the system.
link |
00:38:39.660
You know, it's easy to mathematically write down a system that would have a potentially
link |
00:38:44.300
infinite entropy, an infinite dimensional Hilbert space.
link |
00:38:47.480
So let's go back a little bit.
link |
00:38:50.100
We said that the Hilbert space was the space in which quantum wave functions lived for
link |
00:38:54.340
different systems that will be different sizes.
link |
00:38:57.220
They could be infinite or finite.
link |
00:38:58.420
So that's the number of numbers, the number of pieces of information you could potentially
link |
00:39:03.380
give me about the system.
link |
00:39:04.620
So the bigger Hilbert space is, the bigger the entropy of that system could be, depending
link |
00:39:11.100
on what I know about it.
link |
00:39:12.100
If I don't know anything about it, then it has a huge entropy, right, but only up to
link |
00:39:16.260
the size of its Hilbert space.
link |
00:39:18.160
So we don't know in the real physical world whether or not, you know, this region of space
link |
00:39:23.860
that contains that water bottle has potentially an infinite entropy or just a finite entropy.
link |
00:39:29.140
We have different arguments on different sides.
link |
00:39:31.580
So if it's infinite, how do you think about infinity?
link |
00:39:35.640
Is this something you can, your cognitive abilities are able to process or is it just
link |
00:39:42.020
a mathematical tool?
link |
00:39:44.060
It's somewhere in between, right?
link |
00:39:45.060
I mean, we can say things about it.
link |
00:39:46.420
We can use mathematical tools to manipulate infinity very, very accurately.
link |
00:39:51.220
We can define what we mean.
link |
00:39:52.980
You know, for any number n, there's a number bigger than it.
link |
00:39:56.180
So there's no biggest number, right?
link |
00:39:58.180
So there's something called the total number of all numbers.
link |
00:40:00.580
It's infinite.
link |
00:40:01.640
But it is hard to wrap your brain around that, and I think that gives people pause because
link |
00:40:07.180
we talk about infinity as if it's a number, but it has plenty of properties that real
link |
00:40:11.380
numbers don't have.
link |
00:40:12.380
You know, if you multiply infinity by two, you get infinity again, right?
link |
00:40:15.820
That's a little bit different than what we're used to.
link |
00:40:18.500
Okay.
link |
00:40:19.500
But are you comfortable with the idea that in thinking of what the real world actually
link |
00:40:25.780
is that infinity could be part of that world?
link |
00:40:28.780
Are you comfortable that a world in some dimension, in some aspect?
link |
00:40:31.380
I'm comfortable with lots of things.
link |
00:40:33.020
I mean, you know, I don't want my level of comfort to affect what I think about the world.
link |
00:40:40.740
You know, I'm pretty open minded about what the world could be at the fundamental level.
link |
00:40:44.020
Yeah, but infinity is a tricky one.
link |
00:40:47.900
It's not almost a question of comfort.
link |
00:40:50.460
It's a question of, is it an overreach of our intuition?
link |
00:40:56.420
Sort of, it could be a convenient, almost like when you add a constant to an equation
link |
00:41:01.180
just because it'll help, it just feels like it's useful to at least be able to imagine
link |
00:41:06.860
a concept, not directly, but in some kind of way that this feels like it's a description
link |
00:41:13.380
of the real world.
link |
00:41:14.380
Think of it this way.
link |
00:41:15.940
There's only three numbers that are simple.
link |
00:41:19.420
There's zero, there's one, and there's infinity.
link |
00:41:24.380
A number like 318 is just bizarre.
link |
00:41:29.580
You need a lot of bits to give me what that number is.
link |
00:41:33.220
But zero and one and infinity, like once you have 300 things, you might as well have infinity
link |
00:41:36.660
things, right?
link |
00:41:37.660
Otherwise, you have to say when to stop making the things, right?
link |
00:41:40.060
So there's a sense in which infinity is a very natural number of things to exist.
link |
00:41:44.860
I was never comfortable with infinity because it's just such a, it was too good to be true.
link |
00:41:50.420
Because in math, it just helps make things work out.
link |
00:41:55.780
When things get very large, close to infinity, things seem to work out nicely.
link |
00:42:02.800
It's kind of like, because my deepest passion is probably psychology.
link |
00:42:07.740
And I'm uncomfortable how in the average, the beauty of how much we vary is lost.
link |
00:42:18.660
In that same kind of sense, infinity seems like a convenient way to erase the details.
link |
00:42:24.340
But the thing about infinity is it seems to pop up whether we like it or not, right?
link |
00:42:29.320
Like you're trying to be a computer scientist, you ask yourself, well, how long will it take
link |
00:42:33.020
this program to run?
link |
00:42:34.540
And you realize, well, for some of them, the answer is infinitely long.
link |
00:42:37.440
It's not because you tried to get there.
link |
00:42:39.540
You wrote a five line computer program, it doesn't halt.
link |
00:42:43.420
So coming back to the textbook definition of quantum mechanics, this idea that I don't
link |
00:42:48.220
think we talked about, can you, this one of the most interesting philosophical points,
link |
00:42:55.300
we talked at the human level, but at the physics level, that at least the textbook definition
link |
00:43:02.100
of quantum mechanics separates what is observed and what is real.
link |
00:43:07.660
One, how does that make you feel?
link |
00:43:13.100
And two, what does it then mean to observe something and why is it different than what
link |
00:43:19.020
is real?
link |
00:43:20.020
Yeah, you know, my personal feeling, such as it is, is that things like measurement
link |
00:43:27.700
and observers and stuff like that are not going to play a fundamental role in the ultimate
link |
00:43:32.100
laws of physics.
link |
00:43:33.100
But my feeling that way is because so far, that's where all the evidence has been pointing.
link |
00:43:38.500
I could be wrong.
link |
00:43:39.960
And there's certainly a sense in which it would be infinitely cool if somehow observation
link |
00:43:45.920
or mental cogitation did play a fundamental role in the nature of reality.
link |
00:43:52.180
But I don't think so.
link |
00:43:53.180
And again, I don't see any evidence for it.
link |
00:43:54.580
So I'm not spending a lot of time worrying about that possibility.
link |
00:43:58.280
So what do you do about the fact that in the textbook interpretation of quantum mechanics,
link |
00:44:02.020
this idea of measurement or looking at things seems to play an important role?
link |
00:44:07.860
Well, you come up with better interpretations of quantum mechanics and there are several
link |
00:44:11.780
alternatives.
link |
00:44:12.780
My favorite is the many worlds interpretation, which says two things.
link |
00:44:17.180
Number one, you, the observer, are just a quantum system like anything else.
link |
00:44:22.180
There's nothing special about you.
link |
00:44:23.660
Don't get so proud of yourself, you know, you're just a bunch of atoms.
link |
00:44:27.260
You have a wave function, you obey the Schrodinger equation like everything else.
link |
00:44:31.300
And number two, when you think you're measuring something or observing something, what's really
link |
00:44:35.940
happening is you're becoming entangled with that thing.
link |
00:44:40.480
So when you think there's a wave function for the electron, it's all spread out.
link |
00:44:43.820
But you look at it and you only see it in one location.
link |
00:44:46.660
What's really happening is that there's still the wave function for the electron in all
link |
00:44:50.060
those locations.
link |
00:44:51.060
But now it's entangled with the wave function of you in the following way.
link |
00:44:55.960
There's part of the wave function that says the electron was here and you think you saw
link |
00:44:59.340
it there.
link |
00:45:00.340
The electron was there and you think you saw it there.
link |
00:45:02.380
The electron was over there and you think you saw it there, etc.
link |
00:45:05.620
So in all of those different parts of the wave function, once they come into being,
link |
00:45:10.260
no longer talk to each other.
link |
00:45:11.600
They no longer interact or influence each other.
link |
00:45:13.820
It's as if they are separate worlds.
link |
00:45:16.380
So this was the invention of Hugh Everett III, who was a graduate student at Princeton
link |
00:45:21.620
in the 1950s.
link |
00:45:22.620
And he said, basically, look, you don't need all these extra rules about looking at things.
link |
00:45:28.600
Just listen to what the Schrodinger equation is telling you.
link |
00:45:31.020
It's telling you that you have a wave function, that you become entangled, and that the different
link |
00:45:35.300
versions of you no longer talk to each other.
link |
00:45:37.820
So just accept it.
link |
00:45:39.420
It's just he did therapy more than anything else.
link |
00:45:41.540
He said, like, it's okay.
link |
00:45:42.980
You don't need all these extra rules.
link |
00:45:45.060
All you need to do is believe the Schrodinger equation.
link |
00:45:47.200
The cost is there's a whole bunch of extra worlds out there.
link |
00:45:50.140
So are the worlds being created whether there's an observer or not?
link |
00:45:57.340
The worlds are created any time a quantum system that's in a superposition becomes entangled
link |
00:46:01.520
with the outside world.
link |
00:46:04.020
What's the outside world?
link |
00:46:06.140
It depends.
link |
00:46:07.140
Let's back up.
link |
00:46:08.700
Whatever it really says, what his theory is, is there's a wave function of the universe
link |
00:46:13.980
and it obeys the Schrodinger equation all the time.
link |
00:46:17.240
That's it.
link |
00:46:18.240
That's the full theory right there.
link |
00:46:20.580
The question, all of the work is how in the world do you map that theory onto reality,
link |
00:46:27.140
onto what we observe?
link |
00:46:29.280
So part of it is carving up the wave function into these separate worlds, saying, look,
link |
00:46:33.740
it describes a whole bunch of things that don't interact with each other.
link |
00:46:36.180
Let's call them separate worlds.
link |
00:46:38.120
Another part is distinguishing between systems and their environments.
link |
00:46:41.860
The environment is basically all the degrees of freedom, all the things going on in the
link |
00:46:45.660
world that you don't keep track of.
link |
00:46:48.100
So again, in the bottle of water, I might keep track of the total amount of water and
link |
00:46:52.980
the volume.
link |
00:46:53.980
I don't keep track of the individual positions and velocities.
link |
00:46:57.140
I don't keep track of all the photons or the air molecules in this room.
link |
00:47:00.700
So that's the outside world.
link |
00:47:02.120
The outside world is all the parts of the universe that you're not keeping track of
link |
00:47:06.180
when you're asking about the behavior of subsystem of it.
link |
00:47:10.840
So how many worlds are there?
link |
00:47:14.220
Yeah, we don't know that one either.
link |
00:47:17.100
There could be an infinite number.
link |
00:47:18.860
There could be only a finite number, but it's a big number one way or the other.
link |
00:47:21.660
It's just a very, very big number.
link |
00:47:23.860
In one of the talks, somebody asked, well, if it's finite.
link |
00:47:32.020
So actually I'm not sure exactly the logic you used to derive this, but is there going
link |
00:47:38.700
to be overlap, a duplicate world that you return to?
link |
00:47:47.540
So you've mentioned, and I'd love if you can elaborate on sort of idea that it's possible
link |
00:47:52.020
that there's some kind of equilibrium that these splitting worlds arrive at and then
link |
00:47:56.780
maybe over time, maybe somehow connected to entropy, you get a large number of worlds
link |
00:48:03.700
that are very similar to each other.
link |
00:48:05.260
Yeah.
link |
00:48:06.260
So this question of whether or not Hilbert space is finite or infinite dimensional is
link |
00:48:11.860
actually secretly connected to gravity and cosmology.
link |
00:48:16.420
This is the part that we're still struggling to understand right now, but we discovered
link |
00:48:19.460
back in 1998 that our universe is accelerating and what that means if it continues, which
link |
00:48:25.220
we think it probably will, but we're not sure.
link |
00:48:26.940
But if it does, that means there's a horizon around us.
link |
00:48:31.340
Because the universe is not only expanding, but expanding faster and faster, things can
link |
00:48:34.820
get so far away from us that from our perspective, it looks like they're moving away faster in
link |
00:48:40.220
the speed of light.
link |
00:48:41.220
We will never see them again.
link |
00:48:42.580
So there's literally a horizon around us and that horizon approaches some fixed distance
link |
00:48:47.400
away from us.
link |
00:48:48.780
And you can then argue that within that horizon, there's only a finite number of things that
link |
00:48:53.260
can possibly happen, the finite dimensional Hilbert space.
link |
00:48:55.980
In fact, we even have a guess for what the dimensionality is.
link |
00:48:59.500
It's 10 to the power of 10 to the power of 122.
link |
00:49:04.540
That's a very large number.
link |
00:49:05.540
Yes.
link |
00:49:06.540
Just to compare, the age of the universe is something like 10 to the 14 seconds, 10 to
link |
00:49:11.340
the 17 or 18 seconds maybe.
link |
00:49:13.340
The number of particles in the universe is 10 to the 88th.
link |
00:49:16.300
But the number of dimensions of Hilbert space is 10 to the 10 to the 122.
link |
00:49:21.320
So that's just crazy big.
link |
00:49:23.340
If that story is right, that in our observable horizon, there's only a finite dimensional
link |
00:49:28.020
Hilbert space, then this idea of branching of the wave function of the universe into
link |
00:49:32.620
multiple distinct separate branches has to reach a limit at some time.
link |
00:49:37.780
Once you branch that many times, you've run out of room in Hilbert space.
link |
00:49:41.780
And roughly speaking, that corresponds to the universe just expanding and emptying out
link |
00:49:46.620
and cooling off and entering a phase where it's just empty space, literally forever.
link |
00:49:53.380
What's the difference between splitting and copying, do you think?
link |
00:49:58.900
In terms of, a lot of this is an interpretation that helps us sort of model the world.
link |
00:50:09.220
So perhaps shouldn't be thought of as like, you know, philosophically or metaphysically.
link |
00:50:16.860
But in even at the physics level, do you see a difference between generating new copies
link |
00:50:24.180
of the world or splitting?
link |
00:50:26.900
I think it's better to think of in quantum mechanics in many worlds, the universe splits
link |
00:50:31.700
rather than new copies, because people otherwise worry about things like energy conservation.
link |
00:50:36.540
And no one who understands quantum mechanics worries about energy conservation, because
link |
00:50:40.340
the equation is perfectly clear.
link |
00:50:42.340
But if all you know is that someone told you the universe duplicates, then you have a reasonable
link |
00:50:45.720
worry about where all the energy for that came from.
link |
00:50:48.660
So a pre existing universe splitting into two skinnier universes is a better way of
link |
00:50:53.620
thinking about it.
link |
00:50:54.620
And mathematically, it's just like, you know, if you draw an x and y axis, and you draw
link |
00:50:58.380
a vector of length one, 45 degree angle, you know that you can write that vector of length
link |
00:51:04.540
one as the sum of two vectors pointing along x and y of length one over the square root
link |
00:51:10.420
of two.
link |
00:51:11.420
Okay, so I write one arrow as the sum of two arrows.
link |
00:51:14.940
But there's a conservation of arrowness, right?
link |
00:51:17.020
Like there's now two arrows, but the length is the same, I just I'm describing it in a
link |
00:51:20.940
different way.
link |
00:51:21.940
And that's exactly what happens when the universe branches, the the wave function of the universe
link |
00:51:25.700
is a big old vector.
link |
00:51:27.380
So to somebody who brings up a question of saying, doesn't this violate the conservation
link |
00:51:34.020
of energy?
link |
00:51:35.420
Can you give further elaboration?
link |
00:51:38.060
Right?
link |
00:51:39.060
So let's just be super duper perfectly clear.
link |
00:51:42.100
There's zero question about whether or not many worlds violates conservation of energy.
link |
00:51:46.700
Yes, it does not.
link |
00:51:47.700
Great.
link |
00:51:48.700
And I say this definitively, because there are other questions that I think there's answers
link |
00:51:51.980
to, but they're legitimate questions, right about, you know, where does probability come
link |
00:51:55.620
from and things like that, this conservation of energy question, we know the answer to
link |
00:51:59.900
it.
link |
00:52:00.900
And the answer to it is that energy is conserved.
link |
00:52:03.020
All of the effort goes into how best to translate what the equation unambiguously says into
link |
00:52:09.220
plain English, right?
link |
00:52:11.040
So this idea that there's a universe that has that that the universe comes equipped
link |
00:52:14.480
with a thickness, and it sort of divides up into thinner pieces, but the total amount
link |
00:52:18.700
of universe is is conserved over time, is a reasonably good way of putting English words
link |
00:52:25.360
to the underlying mathematics.
link |
00:52:27.380
So one of my favorite things about many worlds is, I mean, I love that there's something
link |
00:52:33.220
controversial in science.
link |
00:52:35.400
And for some reason, it makes people actually not like upset, but just get excited.
link |
00:52:41.780
Why do you think it is a controversial idea?
link |
00:52:45.960
So there's a lot of, it's actually one of the cleanest ways to think about quantum mechanics.
link |
00:52:52.380
So why do you think there's a discomfort a little bit among certain people?
link |
00:52:57.380
Well, I draw the distinction in my book between two different kinds of simplicity in a physical
link |
00:53:02.740
theory.
link |
00:53:03.740
There's simplicity in the theory itself, right?
link |
00:53:06.300
How we describe what's going on according to the theory by its own rights.
link |
00:53:10.220
But then, you know, theory is just some sort of abstract mathematical formalism, you have
link |
00:53:13.480
to map it onto the world somehow, right?
link |
00:53:16.860
And sometimes, like for Newtonian physics, it's pretty obvious, like, okay, here is a
link |
00:53:23.000
bottle and has a center of mass and things like that.
link |
00:53:26.260
Sometimes it's a little bit harder with general relativity, curvature of space time is a little
link |
00:53:30.660
bit harder to grasp.
link |
00:53:33.180
quantum mechanics is very hard to map what you're the language you're talking in a wave
link |
00:53:37.620
functions and things like that on to reality.
link |
00:53:40.460
And many worlds is the version of quantum mechanics where it is hardest to map on the
link |
00:53:45.260
underlying formalism to reality.
link |
00:53:47.860
So that's where the lack of simplicity comes in, not in the theory, but in how we use the
link |
00:53:53.260
theory to map on to reality.
link |
00:53:54.660
In fact, all of the work in sort of elaborating many worlds quantum mechanics is in the this
link |
00:54:01.740
effort to map it on to the world that we see.
link |
00:54:04.420
So it's perfectly legitimate to be bugged by that, right?
link |
00:54:08.340
To say like, well, no, that's just too far away from my experience, I am therefore intrinsically
link |
00:54:15.320
skeptical of it.
link |
00:54:16.580
Of course, you should give up on that skepticism if there are no alternatives.
link |
00:54:19.620
And this theory always keeps working, then eventually you should overcome your skepticism.
link |
00:54:23.280
But right now there are alternatives that are that, you know, people work to make alternatives
link |
00:54:28.060
that are by their nature closer to what we observe directly.
link |
00:54:31.780
Can you describe the alternatives?
link |
00:54:33.180
I don't think we touched on it, sort of the Copenhagen interpretation and the many worlds.
link |
00:54:40.480
Maybe there's a difference between the Everettian many worlds and many worlds as it is now,
link |
00:54:47.260
like has the idea sort of developed and so on.
link |
00:54:50.020
And just in general, what is the space of promising contenders?
link |
00:54:54.060
We have democratic debates now, there's a bunch of candidates.
link |
00:54:57.060
12 candidates on stage.
link |
00:54:59.540
What are the quantum mechanical candidates on stage for the debate?
link |
00:55:02.480
So if you had a debate between quantum mechanical contenders, there'd be no problem getting
link |
00:55:08.540
12 people up there on stage, but there would still be only three front runners.
link |
00:55:14.220
And right now the front runners would be Everett, hidden variable theories are another one.
link |
00:55:19.460
So the hidden variable theories say that the wave function is real, but there's something
link |
00:55:24.180
in addition to the wave function.
link |
00:55:25.820
The wave function is not everything, it's part of reality, but it's not everything.
link |
00:55:29.440
What else is there?
link |
00:55:30.880
We're not sure, but in the simplest version of the theory, there are literally particles.
link |
00:55:36.020
So many worlds says that quantum systems are sometimes are wave like in some ways and particle
link |
00:55:43.100
like in another because they really, really are waves, but under certain observational
link |
00:55:48.060
circumstances they look like particles.
link |
00:55:50.460
Whereas hidden variable says they look like waves and particles because there are both
link |
00:55:54.980
waves and particles involved in the dynamics.
link |
00:55:58.760
And that's easy to do if your particles are just non relativistic Newtonian particles
link |
00:56:03.780
moving around.
link |
00:56:04.780
They get pushed around by the wave function roughly.
link |
00:56:07.740
It becomes much harder when you take quantum field theory or quantum gravity into account.
link |
00:56:13.300
The other big contender are spontaneous collapse theories.
link |
00:56:17.820
So in the conventional textbook interpretation, we say when you look at a quantum system,
link |
00:56:22.580
its wave function collapses and you see it in one location, a spontaneous collapse theory
link |
00:56:27.260
says that every particle has a chance per second of having its wave function spontaneously
link |
00:56:35.340
collapse.
link |
00:56:36.460
The chance is very small for a typical particle, it will take hundreds of millions of years
link |
00:56:39.980
before it happens even once, but in a table or some macroscopic object, there are way
link |
00:56:44.380
more than a hundred million particles and they're all entangled with each other.
link |
00:56:48.140
So when one of them collapses, it brings everything else along with it.
link |
00:56:52.860
There's a slight variation of this.
link |
00:56:54.260
That's a spontaneous collapse theory.
link |
00:56:55.620
There are also induced collapse theories like Roger Penrose thinks that when the gravitational
link |
00:57:00.260
difference between two parts of the wave function becomes too large, the wave function collapses
link |
00:57:05.500
automatically.
link |
00:57:06.860
So those are basically in my mind, the three big alternatives, many worlds, which is just
link |
00:57:11.700
there's a wave function and always obeys the Schrodinger equation, hidden variables.
link |
00:57:16.060
There's a wave function that always obeys the Schrodinger equation, but there are also
link |
00:57:18.820
new variables or collapse theories, which the wave function sometimes obeys the Schrodinger
link |
00:57:24.420
equation and sometimes it collapses.
link |
00:57:26.220
So you can see that the alternatives are more complicated in their formalism than many worlds
link |
00:57:31.380
is, but they are closer to our experience.
link |
00:57:34.380
So just this moment of collapse, do you think of it as a wave function, fundamentally sort
link |
00:57:42.220
of a probabilistic description of the world and this collapse sort of reducing that part
link |
00:57:49.140
of the world into something deterministic, where again, you can now describe the position
link |
00:57:53.740
and the velocity in this simple classical model?
link |
00:57:56.980
Well there is...
link |
00:57:57.980
Is that how you think about collapse?
link |
00:57:58.980
There is a fourth category, there's a fourth contender, there's a mayor Pete of quantum
link |
00:58:03.140
mechanical interpretations, which are called epistemic interpretations.
link |
00:58:08.140
And what they say is all the wave function is, is a way of making predictions for experimental
link |
00:58:13.220
outcomes.
link |
00:58:14.220
It's not mapping onto an element of reality in any real sense.
link |
00:58:18.780
And in fact, two different people might have two different wave functions for the same
link |
00:58:22.300
physical system because they know different things about it, right?
link |
00:58:25.340
The wave function is really just a prediction mechanism.
link |
00:58:28.100
And then the problem with those epistemic interpretations is if you say, okay, but it's
link |
00:58:32.980
predicting about what, like what is the thing that is being predicted?
link |
00:58:37.980
And they say, no, no, no, that's not what we're here for.
link |
00:58:41.460
We're just here to tell you what the observational outcomes are going to be.
link |
00:58:44.500
But the other, the other interpretations kind of think that the wave function is real.
link |
00:58:49.100
Yes, that's right.
link |
00:58:50.760
So that's an ontic interpretation of the wave function, ontology being the study of what
link |
00:58:55.980
is real, what exists, as opposed to an epistemic interpretation of the wave function, epistemology
link |
00:59:01.060
being the study of what we know.
link |
00:59:02.060
That would actually just love to see that debate on stage.
link |
00:59:06.580
There was a version of it on stage at the world science festival a few years ago that
link |
00:59:10.500
you can look up online.
link |
00:59:11.500
On YouTube?
link |
00:59:12.500
Yep.
link |
00:59:13.500
It's on YouTube.
link |
00:59:14.500
Okay, awesome.
link |
00:59:15.500
I'll link it and watch it.
link |
00:59:16.500
Who won?
link |
00:59:17.500
I won.
link |
00:59:18.500
I don't know, there was no vote, there was no vote, but those there's Brian Green was
link |
00:59:24.700
the moderator and David Albert stood up for a spontaneous collapse and Shelley Goldstein
link |
00:59:29.420
was there for hidden variables and RĂ¼diger Schock was there for epistemic approaches.
link |
00:59:34.460
Why do you, I think you mentioned it, but just to elaborate, why do you find many worlds
link |
00:59:38.980
so compelling?
link |
00:59:39.980
Well, there's two reasons actually.
link |
00:59:43.460
One is, like I said, it is the simplest, right?
link |
00:59:45.540
It's like the most bare bones, austere, pure version of quantum mechanics.
link |
00:59:49.960
And I am someone who is very willing to put a lot of work into mapping the formalism onto
link |
00:59:55.380
reality.
link |
00:59:56.380
I'm less willing to complicate the formalism itself.
link |
00:59:59.100
But the other big reason is that there's something called modern physics with quantum fields
link |
01:00:04.180
and quantum gravity and holography and space time doing things like that.
link |
01:00:09.280
And when you take any of the other versions of quantum theory, they bring along classical
link |
01:00:14.540
baggage, all of the other versions of quantum mechanics, prejudice or privilege some version
link |
01:00:21.820
of classical reality like locations in space, okay?
link |
01:00:26.020
And I think that that's a barrier to doing better at understanding the theory of everything
link |
01:00:31.220
and understanding quantum gravity and the emergence of space time.
link |
01:00:34.460
Whenever if you change your theory from, you know, here's a harmonic oscillator, oh, there's
link |
01:00:38.660
a spin, here's an electromagnetic field, in hidden variable theories or dynamical collapse
link |
01:00:43.640
theories.
link |
01:00:44.640
You have to start from scratch.
link |
01:00:45.640
You have to say like, well, what are the hidden variables for this theory or how does it collapse
link |
01:00:48.220
or whatever?
link |
01:00:49.220
Whereas many worlds is plug and play.
link |
01:00:50.900
You tell me the theory and I can give you as many worlds version.
link |
01:00:53.900
So when we have a situation like we have with gravity and space time, where the classical
link |
01:00:58.900
description seems to break down in a dramatic way, then I think you should start from the
link |
01:01:04.100
most quantum theory that you have, which is really many worlds.
link |
01:01:07.160
So start with the quantum theory and try to build up a model of space time, the emergence
link |
01:01:14.940
of space time.
link |
01:01:15.940
That's it.
link |
01:01:16.940
Okay.
link |
01:01:17.940
So I thought space time was fundamental.
link |
01:01:21.020
Yeah, I know.
link |
01:01:22.360
So this sort of dream that Einstein had that everybody had and everybody has of, you know,
link |
01:01:28.860
the theory of everything.
link |
01:01:30.660
So how do we build up from many worlds from quantum mechanics, a model of space time model
link |
01:01:37.460
of gravity?
link |
01:01:38.460
Well, yeah, I mean, let me first mention very quickly why we think it's necessary.
link |
01:01:42.580
You know, we've had gravity in the form that Einstein bequeathed it to us for over a hundred
link |
01:01:47.420
years now, like 1915 or 1916, he put general relativity in the final form.
link |
01:01:52.600
So gravity is the curvature of space time and there's a field that pervades all the
link |
01:01:57.020
universe that tells us how curved space time is.
link |
01:02:00.100
And that's a fundamentally classical.
link |
01:02:01.980
That's totally classical.
link |
01:02:02.980
Right.
link |
01:02:03.980
Exactly.
link |
01:02:04.980
But we also have a formalism, an algorithm for taking a classical theory and quantizing
link |
01:02:09.980
it.
link |
01:02:10.980
This is how we get quantum electrodynamics, for example.
link |
01:02:13.900
And it could be tricky.
link |
01:02:14.900
I mean, you think you're quantizing something, so that means taking a classical theory and
link |
01:02:19.500
promoting it to a quantum mechanical theory.
link |
01:02:22.400
But you can run into problems.
link |
01:02:23.940
So they ran into problems and they did that with electromagnetism, namely that certain
link |
01:02:27.460
quantities were infinity and you don't like infinity, right?
link |
01:02:30.300
So Feynman and Tominaga and Schwinger won the Nobel Prize for teaching us how to deal
link |
01:02:35.200
with the infinities.
link |
01:02:36.200
And then Ken Wilson won another Nobel Prize for saying you shouldn't have been worried
link |
01:02:39.320
about those infinities after all.
link |
01:02:41.380
But still, that was the, it's always the thought that that's how you will make a good quantum
link |
01:02:45.400
theory.
link |
01:02:46.400
You'll start with a classical theory and quantize it.
link |
01:02:47.860
So if we have a classical theory, general relativity, we can quantize it or we can try
link |
01:02:51.760
to, but we run into even bigger problems with gravity than we ran into with electromagnetism.
link |
01:02:57.780
And so far, those problems are insurmountable.
link |
01:03:00.060
We've not been able to get a successful theory of gravity, quantum gravity, by starting with
link |
01:03:04.980
classical general relativity and quantizing it.
link |
01:03:08.020
And there's evidence that, there's a good reason why this is true, that whatever the
link |
01:03:12.700
quantum theory of gravity is, it's not a field theory.
link |
01:03:17.020
It's something that has weird nonlocal features built into it somehow that we don't understand.
link |
01:03:22.300
We get this idea from black holes and Hawking radiation and information conservation and
link |
01:03:27.540
a whole bunch of other ideas I talk about in the book.
link |
01:03:30.180
So if that's true, if the fundamental theory isn't even local in the sense that an ordinary
link |
01:03:34.580
quantum field theory would be, then we just don't know where to start in terms of getting
link |
01:03:39.060
a classical precursor and quantizing it.
link |
01:03:42.400
So the only sensible thing, or at least the next obvious sensible thing to me would be
link |
01:03:46.220
to say, okay, let's just start intrinsically quantum and work backwards, see if we can
link |
01:03:50.020
find a classical limit.
link |
01:03:51.020
So the idea of locality, the fact that locality is not fundamental to the nature of our existence,
link |
01:04:03.900
I guess in that sense, modeling everything as a field makes sense to me.
link |
01:04:07.460
Stuff that's close by interacts, stuff that's far away doesn't.
link |
01:04:12.300
So what's locality and why is it not fundamental?
link |
01:04:15.580
And how is that even possible?
link |
01:04:16.580
Yeah.
link |
01:04:17.580
I mean, locality is the answer to the question that Isaac Newton was worried about back in
link |
01:04:21.260
the beginning of our conversation, right?
link |
01:04:22.580
I mean, how can the earth know what the gravitational field of the sun is?
link |
01:04:27.300
And the answer as spelled out by Laplace and Einstein and others is that there's a field
link |
01:04:31.340
in between.
link |
01:04:32.340
And the way a field works is that what's happening to the field at this point in space only depends
link |
01:04:38.660
directly on what's happening at points right next to it.
link |
01:04:41.580
But what's happening at those points depends on what's happening right next to those, right?
link |
01:04:45.180
And so you can build up an influence across space through only local interactions.
link |
01:04:50.620
That's what locality means.
link |
01:04:51.860
What happens here is only affected by what's happening right next to it.
link |
01:04:54.780
That's locality.
link |
01:04:57.000
The idea of locality is built into every field theory, including general relativity as a
link |
01:05:01.460
classical theory.
link |
01:05:03.100
It seems to break down when we talk about black holes and, you know, Hawking taught
link |
01:05:07.340
us in the 1970s that black holes radiate, they give off, they eventually evaporate away.
link |
01:05:12.520
They're not completely black once we take quantum mechanics into account.
link |
01:05:17.180
And we think, we don't know for sure, but most of us think that if you make a black
link |
01:05:22.540
hole out of certain stuff, then like Laplace's demon taught us, you should be able to predict
link |
01:05:28.380
what that black hole will turn into if it's just obeying the Schrodinger equation.
link |
01:05:32.660
And if that's true, there are good arguments that can't happen while preserving locality
link |
01:05:37.820
at the same time.
link |
01:05:38.820
It's just that the information seems to be spread out nonlocally in interesting ways.
link |
01:05:44.180
And people should, you talk about holography with the Leonard Susskind on your Mindscape
link |
01:05:49.540
podcast.
link |
01:05:50.540
Oh yes, I have a podcast.
link |
01:05:51.540
I didn't even mention that.
link |
01:05:52.540
This is terrible.
link |
01:05:53.540
No, I'm going to, I'm going to ask you questions about that too, and I've been not shutting
link |
01:05:57.340
up about it.
link |
01:05:58.340
It's my favorite science podcast.
link |
01:05:59.340
So, or not, it's a, it's not even a science podcast.
link |
01:06:02.900
It's like, it's a scientist doing a podcast.
link |
01:06:06.140
That's right.
link |
01:06:07.140
That's what it is.
link |
01:06:08.140
Yeah.
link |
01:06:09.140
Anyway.
link |
01:06:10.140
Yeah.
link |
01:06:11.140
So holography is this idea when you have a black hole and black hole is a region of space
link |
01:06:14.580
inside of which gravity is so strong that you can't escape.
link |
01:06:17.580
And there's this weird feature of black holes that, again, it's totally a thought experiment
link |
01:06:21.980
feature because we haven't gone and probed any yet.
link |
01:06:24.520
But there seems to be one way of thinking about what happens inside a black hole as
link |
01:06:29.960
seen by an observer who's falling in, which is actually pretty normal.
link |
01:06:33.660
Like everything looks pretty normal until you hit the singularity and you die.
link |
01:06:37.020
But from the point of view of the outside observer, it seems like all the information
link |
01:06:41.380
that fell in is actually smeared over the horizon in a nonlocal way.
link |
01:06:47.460
And that's puzzling and that's, so holography because that's a two dimensional surface that
link |
01:06:51.660
is encapsulating the whole three dimensional thing inside, right?
link |
01:06:55.420
Still trying to deal with that.
link |
01:06:56.420
Still trying to figure out how to get there.
link |
01:06:58.100
But it's an indication that we need to think a little bit more subtly when we quantize
link |
01:07:01.340
gravity.
link |
01:07:02.340
And because you can describe everything that's going on in the three dimensional space by
link |
01:07:07.460
looking at the two dimensional projection of it, it means that locality doesn't, it's
link |
01:07:13.900
not necessary.
link |
01:07:14.900
Well, it means that somehow it's only a good approximation.
link |
01:07:18.900
It's not really what's going on.
link |
01:07:20.380
How are we supposed to feel about that?
link |
01:07:22.020
We're supposed to feel liberated.
link |
01:07:25.700
You know, space is just a good approximation and this was always going to be true once
link |
01:07:29.680
you started quantizing gravity.
link |
01:07:31.680
So we're just beginning now to face up to the dramatic implications of quantizing gravity.
link |
01:07:38.820
Is there other weird stuff that happens to quantum mechanics in black hole?
link |
01:07:43.780
I don't think that anything weird has happened with quantum mechanics.
link |
01:07:47.020
I think weird things happen with space time.
link |
01:07:48.620
I mean, that's what it is.
link |
01:07:49.620
Like quantum mechanics is still just quantum mechanics, but our ordinary notions of space
link |
01:07:53.680
time don't really quite work.
link |
01:07:57.620
And there's a principle that goes hand in hand with holography called complementarity,
link |
01:08:03.940
which says that there's no one unique way to describe what's going on inside a black
link |
01:08:10.540
hole.
link |
01:08:11.540
Different observers will have different descriptions, both of which are accurate, but sound completely
link |
01:08:17.220
incompatible with each other.
link |
01:08:18.640
So depends on how you look at it.
link |
01:08:20.900
The word complementarity in this context is borrowed from Niels Bohr, who points out you
link |
01:08:25.060
can measure the position or you can measure the momentum.
link |
01:08:28.260
You can't measure both at the same time in quantum mechanics.
link |
01:08:30.900
So a couple of questions on many worlds.
link |
01:08:34.900
How does many worlds help us understand our particular branch of reality?
link |
01:08:40.740
So okay, that's fine and good that is everything is splitting, but we're just traveling down
link |
01:08:45.100
a single branch of it.
link |
01:08:46.220
So how does it help us understand our little unique branch?
link |
01:08:50.780
Yeah, I mean, that's a great question.
link |
01:08:52.840
But that's the point is that we didn't invent many worlds because we thought it was cool
link |
01:08:56.260
to have a whole bunch of worlds, right?
link |
01:08:57.820
We invented it because we were trying to account for what we observe here in our world.
link |
01:09:02.700
And what we observe here in our world are wave functions collapsing, okay?
link |
01:09:07.500
We do have a position, a situation where the electron seems to be spread out.
link |
01:09:11.500
But then when we look at it, we don't see it spread out.
link |
01:09:13.380
We see it located somewhere.
link |
01:09:15.100
So what's going on?
link |
01:09:16.160
That's the measurement problem of quantum mechanics.
link |
01:09:17.880
That's what we have to face up to.
link |
01:09:19.400
So many worlds is just a proposed solution to that problem.
link |
01:09:22.900
And the answer is nothing special is happening.
link |
01:09:25.660
It's still just the Schrodinger equation, but you have a wave function too.
link |
01:09:30.080
And that's a different answer than would be given in hidden variables or dynamical collapse
link |
01:09:34.380
theories or whatever.
link |
01:09:36.020
So the entire point of many worlds is to explain what we observe, but it tries to explain what
link |
01:09:42.100
we already have observed, right?
link |
01:09:44.280
It's not trying to be different from what we've observed because that would be something
link |
01:09:48.380
other than quantum mechanics.
link |
01:09:50.240
But you know, the idea that there's worlds that we didn't observe that keep branching
link |
01:09:54.220
off is kind of, it's stimulating to the imagination.
link |
01:10:00.000
So is it possible to hop from, you mentioned the branches are independent.
link |
01:10:08.040
Is it possible to hop from one to the other?
link |
01:10:10.260
No.
link |
01:10:11.260
So it's a physical limit.
link |
01:10:14.180
The theory says it's impossible.
link |
01:10:16.260
There's already a copy of you in the other world, don't worry.
link |
01:10:18.620
Yes.
link |
01:10:19.620
Leave them alone.
link |
01:10:20.620
No, but there's a fear of missing out, FOMO, that I feel like immediately start to wonder
link |
01:10:28.780
if that other copy is having more or less fun.
link |
01:10:32.900
Well, the downside to many worlds is that you're missing out on an enormous amount.
link |
01:10:38.620
And that's always what it's going to be like.
link |
01:10:40.460
And I mean, there's a certain stage of acceptance in that.
link |
01:10:44.380
In terms of rewinding, do you think we can rewind the system back, sort of the nice thing
link |
01:10:49.980
about many worlds, I guess, is it really emphasizes the, maybe you can correct me, but the deterministic
link |
01:10:58.800
nature of a branch and it feels like it could be rewound back.
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01:11:05.700
Is it, do you see it as something that could be perfectly rewound back, rewinding back?
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01:11:11.900
Yeah.
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01:11:12.900
If you're at a fancy French restaurant and there's a nice linen white tablecloth and
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01:11:17.060
you have your glass of Bordeaux and you knock it over and the wine spills across the tablecloth.
link |
01:11:22.620
If the world were classical, okay, it would be possible that if you just lifted the wine
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01:11:28.440
glass up, you'd be lucky enough that every molecule of wine would hop back into the glass,
link |
01:11:33.020
right?
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01:11:34.020
But guess what?
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01:11:35.020
It's not going to happen in the real world.
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01:11:37.100
And the quantum wave function is exactly the same way.
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01:11:40.020
It is possible in principle to rewind everything if you start from perfect knowledge of the
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01:11:44.540
entire wave function of the universe.
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01:11:46.820
In practice, it's never going to happen.
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01:11:49.320
So time travel, not possible.
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01:11:53.020
Nope.
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01:11:54.020
At least quantum mechanics has no help.
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01:11:57.760
What about memory?
link |
01:12:00.540
Does the universe have a memory of itself where we could, in, in, so not time travel,
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01:12:07.720
but peek back in time and do a little like replay?
link |
01:12:13.100
Well, it's exactly the same in quantum mechanics as classical mechanics.
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01:12:18.100
So whatever you want to say about that, you know, the fundamental laws of physics in either
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01:12:22.540
many worlds, quantum mechanics or Newtonian physics conserve information.
link |
01:12:28.100
So if you have all the information about the quantum state of the world right now, your
link |
01:12:32.620
Laplace is demon like in your knowledge and calculational capacity, you can wind the clock
link |
01:12:37.500
backward.
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01:12:38.500
But none of us is.
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01:12:39.500
Right?
link |
01:12:40.500
And, you know, so in practice you can never do that.
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01:12:42.380
You can do experiments over and over again, starting from the same initial conditions
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01:12:46.020
for small systems.
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01:12:47.860
But once things get to be large, Avogadro's number of particles, right?
link |
01:12:51.780
Bigger than a cell, no chance.
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01:12:54.960
We we've talked a little bit about arrow of time last time, but in many worlds that there
link |
01:13:00.780
is a kind of implied arrow of time, right?
link |
01:13:07.520
So you've talked about the arrow of time that has to do with the second law of thermodynamics.
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01:13:14.480
That's the arrow of time that's emergent or fundamental.
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01:13:18.340
We don't know, I guess.
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01:13:19.340
No, it's emergent.
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01:13:20.340
Is that, does everyone agree on that?
link |
01:13:23.340
Well, nobody agrees with everything.
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01:13:25.380
They should.
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01:13:26.380
They should.
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01:13:28.300
So that arrow of time, is that different than the arrow of time that's implied by many worlds?
link |
01:13:34.000
It's not different, actually, no.
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01:13:35.420
In both cases, you have fundamental laws of physics that are completely reversible.
link |
01:13:40.940
If you give me the state of the universe at one moment in time, I can run the clock forward
link |
01:13:45.040
or backward equally well.
link |
01:13:46.360
There's no arrow of time built into the laws of physics at the most fundamental level.
link |
01:13:51.360
But what we do have are special initial conditions 14 billion years ago near the Big Bang.
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01:13:57.920
In thermodynamics, those special initial conditions take the form of things were low entropy and
link |
01:14:03.520
entropy has been increasing ever since, making the universe more disorganized and chaotic
link |
01:14:08.480
and that's the arrow of time.
link |
01:14:10.540
In quantum mechanics, the special initial conditions take the form of there was only
link |
01:14:15.080
one branch of the wave function and the universe has been branching more and more ever since.
link |
01:14:20.140
Okay, so if time is emergent, so it seems like our human cognitive capacity likes to
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01:14:28.540
take things that are emergent and assume and feel like they're fundamental.
link |
01:14:33.200
So what, so if time is emergent and locality, like is space emergent?
link |
01:14:42.600
Yes.
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01:14:43.600
Okay.
link |
01:14:44.600
But I didn't say time was emergent, I said the arrow of time was emergent.
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01:14:46.820
Those are different.
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01:14:47.820
What's the difference between the arrow of time and time?
link |
01:14:52.560
Are you using arrow of time to simply mean this, they're synonymous with the second law
link |
01:14:56.920
of thermodynamics?
link |
01:14:57.920
No, but the arrow of time is the difference between the past and future.
link |
01:15:01.520
So there's space, but there's no arrow of space.
link |
01:15:04.500
You don't feel that space has to have an arrow, right?
link |
01:15:06.920
You could live in thermodynamic equilibrium, there'd be no arrow of time, but there'd still
link |
01:15:10.700
be time.
link |
01:15:11.700
There'd still be a difference between now and the future or whatever.
link |
01:15:14.740
So if nothing changes, there's still time.
link |
01:15:19.000
Well things could even change, like if the whole universe consisted of the earth going
link |
01:15:23.760
around the sun, it would just go in circles or ellipses, right?
link |
01:15:29.440
Things would change, but it's not increasing entropy, there's no arrow.
link |
01:15:32.560
If you took a movie of that and I played you the movie backward, you would never know.
link |
01:15:38.900
So the arrow of time can theoretically point in the other direction for briefly.
link |
01:15:45.680
To the extent that it points in different directions, it's not a very good arrow.
link |
01:15:49.000
I mean, the arrow of time in the macroscopic world is so powerful that there's just no
link |
01:15:53.520
chance of going back.
link |
01:15:55.000
When you get down to tiny systems with only three or four moving parts, then entropy can
link |
01:15:58.680
fluctuate up and down.
link |
01:16:00.480
What does it mean for space to be an emergent phenomenon?
link |
01:16:03.440
It means that the fundamental description of the world does not include the word space.
link |
01:16:07.280
It'll be something like a vector in Hilbert space, right, and you have to say, well why
link |
01:16:10.920
is there a good approximate description which involves three dimensional space and stuff
link |
01:16:16.380
inside it?
link |
01:16:17.380
Okay, so time and space are emergent.
link |
01:16:21.200
We kind of mentioned in the beginning, can you elaborate, what do you feel hope is fundamental
link |
01:16:28.620
in our universe?
link |
01:16:30.400
A wave function living in Hilbert space.
link |
01:16:32.160
A wave function in Hilbert space that we can't intellectualize or visualize really.
link |
01:16:36.960
We can't visualize it, we can intellectualize it very easily.
link |
01:16:40.160
Like how do you think about?
link |
01:16:42.240
It's a vector in a 10 to the 10 to the 122 dimensional vector space.
link |
01:16:45.560
It's a complex vector, unit norm, it evolves according to the Schrodinger equation.
link |
01:16:50.920
Got it.
link |
01:16:52.420
When you put it that way.
link |
01:16:53.880
What's so hard, really?
link |
01:16:56.880
It's like, yep, quantum computers, there's some excitement, actually a lot of excitement
link |
01:17:04.440
with people that it will allow us to simulate quantum mechanical systems.
link |
01:17:10.640
What kind of questions do you about quantum mechanics, about the things we've been talking
link |
01:17:14.680
about, do you think, do you hope we can answer through quantum simulation?
link |
01:17:21.720
Well I think that there are, there's a whole fascinating frontier of things you can do
link |
01:17:26.340
with quantum computers.
link |
01:17:27.640
Both sort of practical things with cryptography or money, privacy eavesdropping, sorting things,
link |
01:17:37.400
simulating quantum systems, right?
link |
01:17:40.280
So it's a broader question maybe even outside of quantum computers.
link |
01:17:44.800
Some of the theories that we've been talking about, what's your hope, what's most promising
link |
01:17:49.960
to test these theories?
link |
01:17:52.160
What are kind of experiments we can conduct, whether in simulation or in the physical world
link |
01:17:57.620
that would validate or disprove or expand these theories?
link |
01:18:03.160
Well I think for, there's two parts of that question.
link |
01:18:06.840
One is many worlds and the other one is sort of emergent space time.
link |
01:18:10.300
For many worlds, you know, there are experiments ongoing to test whether or not wave functions
link |
01:18:14.280
spontaneously collapse.
link |
01:18:17.480
And if they do, then that rules out many worlds and that would be falsified.
link |
01:18:21.660
If there are hidden variables, there's a theorem that seems to indicate that the predictions
link |
01:18:27.400
will always be the same as many worlds.
link |
01:18:29.800
I'm a little skeptical of this theorem.
link |
01:18:31.240
I'm not complete.
link |
01:18:32.240
I haven't internalized it.
link |
01:18:33.240
I haven't made it in part of my intuitive view of the world yet, so there might be loopholes
link |
01:18:36.680
to that theorem.
link |
01:18:37.680
I'm not sure about that.
link |
01:18:38.680
Part of me thinks that there should be different experimental predictions if there are hidden
link |
01:18:42.080
variables, but I'm not sure.
link |
01:18:45.180
But otherwise, it's just quantum mechanics all the way down.
link |
01:18:47.320
And so there's this cottage industry in science journalism of writing breathless articles
link |
01:18:53.120
that say, you know, quantum mechanics shown to be more astonishing than ever before thought.
link |
01:18:57.840
And really, it's the same quantum mechanics we've been doing since 1926.
link |
01:19:01.840
Whereas with the emergent space time stuff, we know a lot less about what the theory is.
link |
01:19:06.000
It's in a very primitive state.
link |
01:19:07.760
We don't even really have a safely written down, respectable, honest theory yet.
link |
01:19:13.880
So there could very well be experimental predictions we just don't know about yet.
link |
01:19:17.460
That is one of the things that we're trying to figure out.
link |
01:19:20.200
Yeah, for emergent space time, you need really big stuff, right?
link |
01:19:24.320
Well, or really fast stuff, or really energetic stuff.
link |
01:19:27.800
We don't know.
link |
01:19:28.800
That's the thing.
link |
01:19:29.800
You know, so there could be violations of the speed of light if you have emergent space
link |
01:19:34.440
time.
link |
01:19:35.860
Not going faster than the speed of light, but the speed of light could be different
link |
01:19:39.240
for light of different wavelengths, right?
link |
01:19:42.320
That would be a dramatic violation of physics as we know it, but it could be possible.
link |
01:19:46.840
Or not.
link |
01:19:47.840
I mean, it's not an absolute prediction.
link |
01:19:48.840
That's the problem.
link |
01:19:49.840
The theories are just not well developed enough yet to say.
link |
01:19:54.080
Is there anything that quantum mechanics can teach us about human nature or the human mind?
link |
01:20:01.120
If you think about sort of consciousness and these kinds of topics, is there...
link |
01:20:06.820
It's certainly excessively used, as you point out.
link |
01:20:10.320
The word quantum is used for everything besides quantum mechanics.
link |
01:20:15.520
But in more seriousness, is there something that goes to the human level and can help
link |
01:20:23.600
us understand our mind?
link |
01:20:27.280
Not really is the short answer, you know.
link |
01:20:29.680
Minds are pretty classical.
link |
01:20:31.400
I don't think.
link |
01:20:32.920
We don't know this for sure, but I don't think that phenomena like entanglement are crucial
link |
01:20:36.600
to how the human mind works.
link |
01:20:38.440
What about consciousness?
link |
01:20:40.160
So you mentioned, I think early on in the conversation, you said it would be unlikely,
link |
01:20:48.920
but incredible if sort of the observer is somehow a fundamental part.
link |
01:20:54.680
So observer, not to romanticize the notion, but seems interlinked to the idea of consciousness.
link |
01:21:01.900
So if consciousness, as the panpsychists believe, is fundamental to the universe, is that possible?
link |
01:21:09.040
Is that weight...
link |
01:21:10.040
I mean, every...
link |
01:21:11.040
Everything's possible.
link |
01:21:12.040
Just like Joe Rogan likes to say, it's entirely possible.
link |
01:21:16.920
But okay.
link |
01:21:17.920
But is it on a spectrum of crazy out there?
link |
01:21:22.960
How the statistically speaking, how often do you ponder the possibility that consciousness
link |
01:21:28.600
is fundamental or the observer is fundamental to...
link |
01:21:32.120
Personally don't at all.
link |
01:21:33.120
There are people who do.
link |
01:21:34.120
I'm a thorough physicalist when it comes to consciousness.
link |
01:21:37.160
I do not think that there are any separate mental states or mental properties.
link |
01:21:41.800
I think they're all emergent, just like space time is and space time is hard enough to understand.
link |
01:21:46.000
So the fact that we don't yet understand consciousness is not at all surprising to me.
link |
01:21:51.320
You, as we mentioned, have an amazing podcast called Mindscape.
link |
01:21:55.280
It's as I said, one of my favorite podcasts sort of both for your explanation of physics,
link |
01:22:03.680
which a lot of people love, and when you venture out into things that are beyond your expertise,
link |
01:22:10.520
but it's just a really smart person exploring even questions like morality, for example.
link |
01:22:18.960
It's very interesting.
link |
01:22:19.960
I think you did a solo episode and so on.
link |
01:22:21.920
I mean, there's a lot of really interesting conversations that you have.
link |
01:22:28.200
What are some from memory, amazing conversations that pop to mind that you've had?
link |
01:22:34.680
What did you learn from them?
link |
01:22:36.320
Something that maybe changed your mind or just inspired you or just what did this whole
link |
01:22:40.360
experience of having conversations, what stands out to you?
link |
01:22:45.160
It's an unfair question.
link |
01:22:46.160
Totally unfair.
link |
01:22:47.160
That's okay.
link |
01:22:48.160
That's all right.
link |
01:22:49.160
You know, it's often the ones I feel like the ones I do on physics and closely related
link |
01:22:54.000
science or even philosophy ones are like, I know this stuff and I'm helping people learn
link |
01:23:01.160
about it.
link |
01:23:02.160
But I learn more from the ones that have nothing to do with physics or philosophy, right?
link |
01:23:05.900
So talking to Wynton Marsalis about jazz or talking to a Master Sommelier about wine,
link |
01:23:12.160
talking to Will Wilkinson about partisan polarization and the urban rural divide, talking to psychologists
link |
01:23:18.760
like Carol Tavris about cognitive dissonance and how those things work.
link |
01:23:26.320
Scott Derrickson who is the director of the movie Dr. Strange, I had a wonderful conversation
link |
01:23:31.160
with him where we went through the mechanics of making a blockbuster superhero movie, right?
link |
01:23:36.680
And he's also not a naturalist, he's an evangelical Christian so we talked about the nature of
link |
01:23:41.440
reality there.
link |
01:23:42.440
I want to have a couple more, you know, discussions with highly educated theists who know the
link |
01:23:51.360
theology really well but I haven't quite arranged those yet.
link |
01:23:56.600
I would love to hear that.
link |
01:23:57.600
I mean that's, how comfortable are you venturing into questions of religion?
link |
01:24:02.520
Oh, I'm totally comfortable doing it.
link |
01:24:04.960
You know, I did talk with Alan Lightman who is also an atheist but he, you know, he is
link |
01:24:10.320
trying to rescue the sort of spiritual side of things for atheism and I did talk to very
link |
01:24:18.600
vocal atheists like Alex Rosenberg so I need to talk to some, I've talked to some religious
link |
01:24:23.280
believers but I need to talk to more.
link |
01:24:26.360
How have you changed through having all these conversations?
link |
01:24:31.320
You know, part of the motivation was I had a long stack of books that I hadn't read and
link |
01:24:35.360
I couldn't find time to read them and I figured if I interviewed their authors, forced me
link |
01:24:38.880
to read them, right, and that has totally worked by the way.
link |
01:24:42.080
Now I'm annoyed that people write such long books.
link |
01:24:45.680
I think I'm still very much learning how to be a good interviewer.
link |
01:24:49.360
I think that's a skill that, you know, I think I have good questions but, you know, there's
link |
01:24:54.520
the give and take that is still I think I can be better at.
link |
01:24:58.920
Like I want to offer something to the conversation but not too much, right?
link |
01:25:02.480
I've had conversations where I barely talked at all and I have conversations where I talked
link |
01:25:06.000
half the time and I think there's a happy medium in between there.
link |
01:25:09.000
So I think I remember listening to, without mentioning names, some of your conversations
link |
01:25:13.720
where I wish you would have disagreed more.
link |
01:25:16.480
As a listener, it's more fun sometimes.
link |
01:25:20.400
Well, that's a very good question because, you know, everyone has an attitude toward
link |
01:25:24.960
that.
link |
01:25:25.960
Like some people are really there to basically give their point of view and their guest is
link |
01:25:32.720
supposed to, you know, respond accordingly.
link |
01:25:35.480
I want to sort of get my view on the record but I don't want to dwell on it when I'm talking
link |
01:25:41.400
to someone like David Chalmers who I disagree with a lot.
link |
01:25:44.120
You know, I want to say like, here's why I disagree with you but, you know, we're here
link |
01:25:48.860
to listen to you.
link |
01:25:49.980
Like I have an episode every week and you're only on once a week, right?
link |
01:25:53.240
So I have an upcoming podcast episode with Philip Goff who is a much more dedicated pan
link |
01:26:00.760
psychist and so there we really get into it.
link |
01:26:02.640
I think that I probably have disagreed with him more on that episode than I ever have
link |
01:26:06.680
with another podcast guest but that's what he wanted so it worked very well.
link |
01:26:10.240
Yeah, yeah.
link |
01:26:11.240
That kind of debate structure is beautiful when it's done right.
link |
01:26:15.880
Like when you're, when you can detect that the intent is that you have fundamental respect
link |
01:26:21.400
for the person.
link |
01:26:22.400
Yeah.
link |
01:26:23.400
That, and that's, for some reason, it's super fun to listen to when two really smart people
link |
01:26:29.040
are just arguing and sometimes lose their shit a little bit if I may say so.
link |
01:26:32.920
Well, there's a fine line because I have zero interest in bringing, I mean, like, I mean,
link |
01:26:39.160
maybe you implied this, I have zero interest in bringing on people for whom I don't have
link |
01:26:43.280
any intellectual respect.
link |
01:26:44.600
Like I constantly get requests like, you know, bring on a flat earther or whatever and really
link |
01:26:49.120
slap them down or a creationist, like I have zero interest.
link |
01:26:52.800
I'm happy to bring on, you know, a religious person, a believer, but I want someone who's
link |
01:26:56.800
smart and can act in good faith and can talk, not a charlatan or a lunatic, right?
link |
01:27:02.160
So I will only, I will happily bring on people with whom I disagree, but only people from
link |
01:27:08.160
whom I think the audience can learn something interesting.
link |
01:27:10.320
So let me ask, the idea of charlatan is an interesting idea.
link |
01:27:15.740
You might be more educated on this topic than me, but there's, there's folks, for example,
link |
01:27:22.920
who argue various aspects of evolution sort of try to approach and say that evolution
link |
01:27:31.280
sort of our current theory of evolution has many holes in it, has many flaws.
link |
01:27:37.440
And they argue that I think like Cambridge, Cambrian explosion, which is like a huge added
link |
01:27:46.820
variability of species, doesn't make sense under our current description of evolution
link |
01:27:52.160
and theory of evolution sort of, if you had to, were to have the conversation with people
link |
01:27:57.520
like that, how do you know that they're the difference in outside the box thinkers and
link |
01:28:05.720
people who are fundamentally unscientific and even bordering on charlatans?
link |
01:28:13.080
That's a great question.
link |
01:28:15.080
And you know, the further you get away from my expertise, the harder it is for me to really
link |
01:28:19.200
judge exactly those things.
link |
01:28:21.040
And, you know, yeah, I don't have a satisfying answer for that one because I think the example
link |
01:28:25.520
you use of someone who, you know, believes in the basic structure of natural selection,
link |
01:28:30.840
but thinks that, you know, this particular thing cannot be understood in the terms of
link |
01:28:35.240
our current understanding of Darwinism.
link |
01:28:38.360
That's a perfect edge case where it's hard to tell, right?
link |
01:28:41.560
And I would have, I would try to talk to people who I do respect and who do know things and
link |
01:28:45.680
I would have to, you know, given that I'm a physicist, I know that physicists will sometimes
link |
01:28:50.920
be too dismissive of alternative points of view.
link |
01:28:53.480
I have to take into account that biologists can also be too dismissive of alternative points
link |
01:28:57.320
of view.
link |
01:28:58.320
So, yeah, that's a tricky one.
link |
01:29:00.760
Have you gotten heat yet?
link |
01:29:02.040
I get heat all the time.
link |
01:29:03.040
Like there's always something, I mean, it's hilarious because I do have, I try very hard
link |
01:29:09.000
not to like have the same topic several times in a row.
link |
01:29:12.080
I did have like two climate change episodes, but they were from very different perspectives,
link |
01:29:16.320
but I like to mix it up.
link |
01:29:17.320
That's the whole, that's why I'm having fun.
link |
01:29:19.040
And every time I do an episode, someone says, oh, the person you should really get on to
link |
01:29:22.720
talk about exactly that is this other person.
link |
01:29:24.600
I'm like, well, I don't, but I did that now.
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01:29:26.080
I don't want to do that anymore.
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01:29:28.080
Well, I hope you keep doing it.
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01:29:30.880
You're inspiring millions of people, your books, your podcasts.
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01:29:34.240
Sean, it's an honor to talk to you.
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01:29:36.240
Thank you so much.
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01:29:37.240
Thank you very much, Lex.