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The following is a conversation with Sean Carroll, Part 2, the second time we've spoken

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This time we focus on quantum mechanics and the many worlds interpretation that he details

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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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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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If you enjoy it, subscribe on YouTube, give it five stars on iTunes, support it on Patreon,

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Isaac Newton developed what we now call classical mechanics that you describe very nicely in

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So with classical mechanics, I can throw a rock and can predict the trajectory of that

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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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Perhaps he just said it for religious reasons and so on, but in particular, sort of a world

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He was smart enough, this is off the topic but still fascinating, Newton almost invented

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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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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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So he thought that God would intervene occasionally to sort of move the planets back into orbit,

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But the worries about classical mechanics were a little bit different, the worry about

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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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And he literally said, you know, I leave that for future generations to think about because

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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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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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Now what many people say is that Einstein solved this problem because he invented general

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And in general relativity, there's certainly a field in between the Earth and the Sun.

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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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So but I really, it rubs me the wrong way to think that we should presume the answer

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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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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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I mean, Einstein famously was very guided by his intuitions and he did not like the

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It depends on your interpretation of quantum mechanics and it depends on even how you talk

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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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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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Like you can intuitively, the degree that human beings can understand anything that

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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, it's easy to state it, but do I really appreciate what it means for incredibly

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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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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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Of course, one's intuition gets worse and worse as you get trickier in the quantum field

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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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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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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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So do you think there's limits to our understanding that's deeply rooted, hardcoded into our biology

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But when someone like Ed Witten proves a theorem about, you know, 100 dimensional mathematical

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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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One has the ability to contain in one's brain logical, formal, symbolic structures and manipulate

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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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You know, people are not very good at taking cube roots of million digit numbers in their

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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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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 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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And I don't know what they would have said 500 years ago, but they didn't even know about

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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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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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Well, that last phrase, having the cognitive abilities to process them carries a lot, right?

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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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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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So but we got there, we got to understanding that there are atoms and cells using the combination

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So adding the ability of our cognitive capacities to our senses is adding an enormous amount

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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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So if you were Aristotle, when Aristotle wrote his book on physics, he made the following

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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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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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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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And the reason why I think that's the most beautiful idea in physics is because it shifts

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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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You give me the state of the world today, I can predict what it's going to do in the

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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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I think even more than either classical mechanics or quantum mechanics, that is the profound

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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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And also the other point you said, which is, conveniently, most of the interesting ideas

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It's weird, because like, someone would say something interesting, and then the next interesting

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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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He used that, just like Schrodinger used Schrodinger's cat to say, surely you don't believe that,

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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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But still, he got right the idea that there was this conservation of something impetus

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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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Do you think it was a big leap, a brave or a difficult leap of sort of math and science

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You know, even Aristotle knew that his theory had issues, because you could fire an arrow

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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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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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Another big question that I think comes up, certainly with quantum mechanics, is what's

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To me, you know, very, very roughly, math is about the logical structure of all possible

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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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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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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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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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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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I think we have theories, theories of the physical world, which we then extrapolate

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and ask, you know, what do we conclude if we take these seriously well beyond where

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It is separately true that math is really, really useful when we construct physical theories

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and you know, famously Eugene Wigner asked about the unreasonable success of mathematics

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I think that's a little bit wrong because anything that could happen, any other theory

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of physics that wasn't the real world, but some other world, you could always describe

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The surprising thing is not that math works, but that the math is so simple and easy that

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That's an enormous compression of information that seems to be valid in the real world.

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So that's an interesting fact about our world, which maybe we could hope to explain or just

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But once you have that, you know, there's this indelible relationship between math and

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What we extrapolate, we don't extrapolate math because there's a whole bunch of wrong

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I mean, like I just hinted at, I don't know if there's an answer to that question.

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Well, an answer could look like if you showed that there was something about our world that

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You know, the mean of the simplicity and the powerfulness of the laws of physics or, you

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Maybe in the set of all possible worlds, this is what the world would look like, right?

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I tend to think that there is something specific and rock bottom about the facts of our world

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I think that in some sense, we're just going to, at some level, we're going to say, and

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I don't know how, if we're anywhere close to that right now, but that seems plausible

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And speaking of rock bottom, one of the things sort of your book kind of reminded me or revealed

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to me is that what's fundamental and what's emergent, it just feels like I don't even

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It feels like everything, especially with quantum mechanics, is revealing to us is that

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most interesting things that I would, as a limited human would think are fundamental

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We do have reasons to say that certain things are more fundamental than others, right?

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If you describe something like this table as a table, it has a height and a width and

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it's made of a certain material and it has a certain solidity and weight and so forth.

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There's a whole other description of this table in terms of a whole collection of atoms

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You could break apart the table, smash it to pieces, still talk about it as atoms, but

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So I think that this comprehensiveness, the domain of validity of a theory gets broader

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Maybe right in the book review, if you read your latest book on quantum mechanics, something

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It would take a long time for him to think that any of this was making any sense.

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He invented calculus in his spare time, which would have made him the greatest mathematician

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But of course, it's funny because Newton was in some sense still a pre modern thinker.

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Rocky Kolb, who is a cosmologist at the University of Chicago said that Galileo, even though

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If you got Galileo and brought him to the present day, it would take him six months

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to catch up and then he'd be in your office telling you why your most recent paper was

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He wrote a lot more about the Bible and alchemy than he ever did about physics, but he was

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also more brilliant than anybody else and way more mathematically astute than Galileo.

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He might have, he might just, yeah, say like, give me the textbooks, leave me alone for

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Is there any other scientists or philosophers through history that you would like to know

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We all, I mean, he was not that long ago, but I even speculated at the end of my book

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I am curious as to, you know, what about older philosophers like Hume or Kant, right?

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Because they do in philosophy, your predilections end up playing a much bigger role in your

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ultimate conclusions because you're not as tied down by what the data is in physics.

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You know, physics is lucky because we can't stray too far off the reservation as long

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as we're trying to explain the world that we actually see in our telescopes and microscopes.

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But it's just not fair to play that game because the people we're thinking about didn't know

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I think it would be interesting, useful for people who are not familiar, but even for

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Quantum mechanics is the paradigm of physics that came into being in the early part of

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the 20th century that replaced classical mechanics, and it replaced classical mechanics in a weird

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So in classical mechanics, you have an object, it has a location, it has a velocity, and

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if you know the location and velocity of everything in the world, you can say what everything's

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It's a vector in a huge dimensional vector space rather than a position and a velocity,

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The equation is the Schrodinger equation, not Newton's laws, but okay, again, a detail.

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Where quantum mechanics really becomes weird and different is that there's a whole other

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set of rules in our textbook formulation of quantum mechanics in addition to saying that

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And all these new rules have to do with what happens when you look at the system, when

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In quantum mechanics, the way we teach it, there's something profoundly fundamental about

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the act of measurement or observation, and the system dramatically changes its state.

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Even though it has a wave function, like the electron in an atom is not orbiting in a circle,

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it's sort of spread out in a cloud, when you look at it, you don't see that cloud.

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So it dramatically changes its state right away, and the effects of that change can be

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So again, we need to be careful because we don't agree on what quantum mechanics says.

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But in the textbook view, quantum mechanics, unlike any other theory of physics, gives

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This all came together in a few years around the turn of the last century, right?

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Atoms predated then, of course, the word atom goes back to the ancient Greeks, but it was

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the chemists in the 1800s that really first got experimental evidence for atoms.

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And in these two different types of tin oxide, there was exactly twice as much oxygen in

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And so Dalton said, well, it's because there are tin atoms and oxygen atoms, and one form

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of tin oxide is one atom of tin and one atom of oxygen, and the other is one atom of tin

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And on the basis of this, you know, a speculation, a theory, right, a hypothesis, but then on

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the basis of that, you make other predictions, and the chemists became quickly convinced

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And I mean, Boltzmann, who believed in atoms, had a really tough time his whole life because

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And there, in general, the idea of atoms is, it's the most, the smallest building block

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That was the Greek idea, but the chemists in the 1800s jumped the gun a little bit.

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So these days, an atom is the smallest building block of a chemical element, right?

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Hydrogen, tin, oxygen, carbon, whatever, but we know that atoms can be broken up further

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That's what physicists discovered in the early 1900s, Rutherford, especially, and his colleagues.

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So the atom that we think about now, the cartoon, is that picture you've always seen of a little

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Protons are much lighter, but because they're lighter, they give all the life to the atoms.

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So when atoms get together, combine chemically, when electricity flows through a system, it's

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And where quantum mechanics steps in, as you mentioned, with the position of velocity with

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classical mechanics and quantum mechanics is modeling the behavior of the electron.

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I mean, you can model the behavior of anything, but the electron, because that's where the

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You said it's an interesting detail, but in any interpretation, what is the wave function

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Well, you know, we had this idea from Rutherford that atoms look like little solar systems,

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but people very quickly realize that can't possibly be right because if an electron is

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All the light that we have in this room comes from electrons zooming up and down and wiggling.

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And you can calculate how long would it take for the electron just to spiral into the nucleus?

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Meanwhile, people had realized that light, which we understood from the 1800s was a wave,

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So if something that we agree was a wave had particle like properties, then maybe something

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And so a bunch of people eventually came to the conclusion, don't think about the electron

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They cleverly gave this the name the wave function, which is the dopiest name in the

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There's literally a number at every point in space, which is the value of the electron's

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But when you have two electrons, you do not have a wave function for electron one and

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And indeed, as you say, there's only one wave function for the entire universe at once.

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Well, let's temporarily buy into the textbook interpretation of quantum mechanics.

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And what that says is that this wave function, so it's very small outside the atom, very

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big in the atom, basically the wave function, you take it and you square it, you square

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the number that gives you the probability of observing the system at that location.

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So if you say that for two electrons, there's only one wave function, and that wave function

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gives you the probability of observing both electrons at once doing something, okay?

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So maybe the electron can be here or here, here, here, and the other electron can also

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But we have a wave function set up where we don't know where either electron is going

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So we don't know exactly what we're going to see for either electron, but there's entanglement

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If we see one in one location, then we know the other one's going to be doing a certain

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So that's a feature of quantum mechanics that is nowhere to be found in classical mechanics.

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In classical mechanics, there's no way I can say, well, I don't know where either one of

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these particles is, but if I know, if I find out where this one is, then I know where the

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I don't know, it feels like, if you think of a wave function like as a dance floor,

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it seems like entanglement is strongest between things that are dancing together closest.

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We have to be careful here because in principle, if you're talking about the entanglement of

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two electrons, for example, they can be totally entangled or totally unentangled no matter

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There's no relationship between the amount of entanglement and the distance between two

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But we now know that the reality of our best way of understanding the world is through

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So even the electron, not just gravity and electromagnetism, but even the electron and

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And those quantum fields in empty space are entangled with each other in exactly the way

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If they're nearby, if you have like two vibrating quantum fields that are nearby, then they'll

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Just like right here, this location in space, there's a gravitational field, which I can

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Yeah, you know, I think that people are very welcome to go through their lives not knowing

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But if you dig into a little bit more into quantum mechanics, it becomes necessary.

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You know, the English language was invented long before quantum mechanics, or various

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Of course, most of us think of space as this three dimensional world in which we live,

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Okay, but space around us gives us the three dimensional location of things and objects.

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But mathematicians use any generic abstract collection of elements as a space, okay?

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So Hilbert space is the space of all possible quantum wave functions, either for the universe

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And it could be an infinite dimensional space, or it could be just really, really large dimensional

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But this abstract Hilbert space is really, really, really big and has no immediate connection

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You know, it's just a way of mathematically representing how much information is contained

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So in classical mechanics, I give you the location of something by giving you three

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But then I might want to give you its entire state, physical state, which means both its

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So its state lives in something called phase space, which is six dimensional, three dimensions

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And then if it also has an orientation in space, that's another three dimensions and

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So as you describe more and more information about the system, you have an abstract mathematical

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This mystical word that's overused in math and physics, but has a very specific meaning

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But one way of thinking about it is a measure of how much we don't know about the state

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So if I have a bottle of water molecules, and I know that, OK, there's a certain number

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I know the volume of it, and I know the temperature and pressure and things like that.

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So there's a certain amount of information I know, a certain amount that I don't know

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And that's what the entropy characterizes, how much unknown information there is, the

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difference between what I do know about the system and its full exact microscopic state.

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So when we try to describe a quantum mechanical system, is it infinite or finite but very

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You know, it's easy to mathematically write down a system that would have a potentially

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We said that the Hilbert space was the space in which quantum wave functions lived for

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So that's the number of numbers, the number of pieces of information you could potentially

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So the bigger Hilbert space is, the bigger the entropy of that system could be, depending

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If I don't know anything about it, then it has a huge entropy, right, but only up to

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So we don't know in the real physical world whether or not, you know, this region of space

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that contains that water bottle has potentially an infinite entropy or just a finite entropy.

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Is this something you can, your cognitive abilities are able to process or is it just

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But it is hard to wrap your brain around that, and I think that gives people pause because

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we talk about infinity as if it's a number, but it has plenty of properties that real

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But are you comfortable with the idea that in thinking of what the real world actually

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I mean, you know, I don't want my level of comfort to affect what I think about the world.

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You know, I'm pretty open minded about what the world could be at the fundamental level.

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Sort of, it could be a convenient, almost like when you add a constant to an equation

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just because it'll help, it just feels like it's useful to at least be able to imagine

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a concept, not directly, but in some kind of way that this feels like it's a description

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But zero and one and infinity, like once you have 300 things, you might as well have infinity

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So there's a sense in which infinity is a very natural number of things to exist.

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I was never comfortable with infinity because it's just such a, it was too good to be true.

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And I'm uncomfortable how in the average, the beauty of how much we vary is lost.

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In that same kind of sense, infinity seems like a convenient way to erase the details.

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But the thing about infinity is it seems to pop up whether we like it or not, right?

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Like you're trying to be a computer scientist, you ask yourself, well, how long will it take

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So coming back to the textbook definition of quantum mechanics, this idea that I don't

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think we talked about, can you, this one of the most interesting philosophical points,

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we talked at the human level, but at the physics level, that at least the textbook definition

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And two, what does it then mean to observe something and why is it different than what

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Yeah, you know, my personal feeling, such as it is, is that things like measurement

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and observers and stuff like that are not going to play a fundamental role in the ultimate

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But my feeling that way is because so far, that's where all the evidence has been pointing.

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And there's certainly a sense in which it would be infinitely cool if somehow observation

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So what do you do about the fact that in the textbook interpretation of quantum mechanics,

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Well, you come up with better interpretations of quantum mechanics and there are several

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You have a wave function, you obey the Schrodinger equation like everything else.

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And number two, when you think you're measuring something or observing something, what's really

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So when you think there's a wave function for the electron, it's all spread out.

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What's really happening is that there's still the wave function for the electron in all

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There's part of the wave function that says the electron was here and you think you saw

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So in all of those different parts of the wave function, once they come into being,

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So this was the invention of Hugh Everett III, who was a graduate student at Princeton

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And he said, basically, look, you don't need all these extra rules about looking at things.

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It's telling you that you have a wave function, that you become entangled, and that the different

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The worlds are created any time a quantum system that's in a superposition becomes entangled

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Whatever it really says, what his theory is, is there's a wave function of the universe

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The question, all of the work is how in the world do you map that theory onto reality,

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So part of it is carving up the wave function into these separate worlds, saying, look,

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The environment is basically all the degrees of freedom, all the things going on in the

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So again, in the bottle of water, I might keep track of the total amount of water and

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The outside world is all the parts of the universe that you're not keeping track of

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There could be only a finite number, but it's a big number one way or the other.

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So actually I'm not sure exactly the logic you used to derive this, but is there going

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So you've mentioned, and I'd love if you can elaborate on sort of idea that it's possible

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that there's some kind of equilibrium that these splitting worlds arrive at and then

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maybe over time, maybe somehow connected to entropy, you get a large number of worlds

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So this question of whether or not Hilbert space is finite or infinite dimensional is

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This is the part that we're still struggling to understand right now, but we discovered

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back in 1998 that our universe is accelerating and what that means if it continues, which

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Because the universe is not only expanding, but expanding faster and faster, things can

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get so far away from us that from our perspective, it looks like they're moving away faster in

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So there's literally a horizon around us and that horizon approaches some fixed distance

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And you can then argue that within that horizon, there's only a finite number of things that

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Just to compare, the age of the universe is something like 10 to the 14 seconds, 10 to

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If that story is right, that in our observable horizon, there's only a finite dimensional

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Hilbert space, then this idea of branching of the wave function of the universe into

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And roughly speaking, that corresponds to the universe just expanding and emptying out

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and cooling off and entering a phase where it's just empty space, literally forever.

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In terms of, a lot of this is an interpretation that helps us sort of model the world.

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So perhaps shouldn't be thought of as like, you know, philosophically or metaphysically.

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But in even at the physics level, do you see a difference between generating new copies

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I think it's better to think of in quantum mechanics in many worlds, the universe splits

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rather than new copies, because people otherwise worry about things like energy conservation.

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And no one who understands quantum mechanics worries about energy conservation, because

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But if all you know is that someone told you the universe duplicates, then you have a reasonable

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So a pre existing universe splitting into two skinnier universes is a better way of

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And mathematically, it's just like, you know, if you draw an x and y axis, and you draw

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a vector of length one, 45 degree angle, you know that you can write that vector of length

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one as the sum of two vectors pointing along x and y of length one over the square root

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Like there's now two arrows, but the length is the same, I just I'm describing it in a

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And that's exactly what happens when the universe branches, the the wave function of the universe

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So to somebody who brings up a question of saying, doesn't this violate the conservation

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There's zero question about whether or not many worlds violates conservation of energy.

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And I say this definitively, because there are other questions that I think there's answers

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to, but they're legitimate questions, right about, you know, where does probability come

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from and things like that, this conservation of energy question, we know the answer to

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All of the effort goes into how best to translate what the equation unambiguously says into

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So this idea that there's a universe that has that that the universe comes equipped

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with a thickness, and it sort of divides up into thinner pieces, but the total amount

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of universe is is conserved over time, is a reasonably good way of putting English words

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So one of my favorite things about many worlds is, I mean, I love that there's something

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And for some reason, it makes people actually not like upset, but just get excited.

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So there's a lot of, it's actually one of the cleanest ways to think about quantum mechanics.

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Well, I draw the distinction in my book between two different kinds of simplicity in a physical

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But then, you know, theory is just some sort of abstract mathematical formalism, you have

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And sometimes, like for Newtonian physics, it's pretty obvious, like, okay, here is a

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Sometimes it's a little bit harder with general relativity, curvature of space time is a little

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quantum mechanics is very hard to map what you're the language you're talking in a wave

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And many worlds is the version of quantum mechanics where it is hardest to map on the

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So that's where the lack of simplicity comes in, not in the theory, but in how we use the

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In fact, all of the work in sort of elaborating many worlds quantum mechanics is in the this

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To say like, well, no, that's just too far away from my experience, I am therefore intrinsically

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And this theory always keeps working, then eventually you should overcome your skepticism.

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But right now there are alternatives that are that, you know, people work to make alternatives

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I don't think we touched on it, sort of the Copenhagen interpretation and the many worlds.

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Maybe there's a difference between the Everettian many worlds and many worlds as it is now,

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So if you had a debate between quantum mechanical contenders, there'd be no problem getting

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And right now the front runners would be Everett, hidden variable theories are another one.

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So the hidden variable theories say that the wave function is real, but there's something

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The wave function is not everything, it's part of reality, but it's not everything.

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We're not sure, but in the simplest version of the theory, there are literally particles.

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So many worlds says that quantum systems are sometimes are wave like in some ways and particle

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like in another because they really, really are waves, but under certain observational

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Whereas hidden variable says they look like waves and particles because there are both

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And that's easy to do if your particles are just non relativistic Newtonian particles

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It becomes much harder when you take quantum field theory or quantum gravity into account.

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So in the conventional textbook interpretation, we say when you look at a quantum system,

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its wave function collapses and you see it in one location, a spontaneous collapse theory

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says that every particle has a chance per second of having its wave function spontaneously

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The chance is very small for a typical particle, it will take hundreds of millions of years

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before it happens even once, but in a table or some macroscopic object, there are way

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more than a hundred million particles and they're all entangled with each other.

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There are also induced collapse theories like Roger Penrose thinks that when the gravitational

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difference between two parts of the wave function becomes too large, the wave function collapses

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So those are basically in my mind, the three big alternatives, many worlds, which is just

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there's a wave function and always obeys the Schrodinger equation, hidden variables.

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There's a wave function that always obeys the Schrodinger equation, but there are also

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new variables or collapse theories, which the wave function sometimes obeys the Schrodinger

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So you can see that the alternatives are more complicated in their formalism than many worlds

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So just this moment of collapse, do you think of it as a wave function, fundamentally sort

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of a probabilistic description of the world and this collapse sort of reducing that part

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of the world into something deterministic, where again, you can now describe the position

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There is a fourth category, there's a fourth contender, there's a mayor Pete of quantum

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And what they say is all the wave function is, is a way of making predictions for experimental

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And in fact, two different people might have two different wave functions for the same

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And then the problem with those epistemic interpretations is if you say, okay, but it's

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But the other, the other interpretations kind of think that the wave function is real.

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So that's an ontic interpretation of the wave function, ontology being the study of what

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is real, what exists, as opposed to an epistemic interpretation of the wave function, epistemology

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There was a version of it on stage at the world science festival a few years ago that

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I don't know, there was no vote, there was no vote, but those there's Brian Green was

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the moderator and David Albert stood up for a spontaneous collapse and Shelley Goldstein

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was there for hidden variables and RÃ¼diger Schock was there for epistemic approaches.

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Why do you, I think you mentioned it, but just to elaborate, why do you find many worlds

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And I am someone who is very willing to put a lot of work into mapping the formalism onto

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But the other big reason is that there's something called modern physics with quantum fields

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And when you take any of the other versions of quantum theory, they bring along classical

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baggage, all of the other versions of quantum mechanics, prejudice or privilege some version

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And I think that that's a barrier to doing better at understanding the theory of everything

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Whenever if you change your theory from, you know, here's a harmonic oscillator, oh, there's

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a spin, here's an electromagnetic field, in hidden variable theories or dynamical collapse

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You have to say like, well, what are the hidden variables for this theory or how does it collapse

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So when we have a situation like we have with gravity and space time, where the classical

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description seems to break down in a dramatic way, then I think you should start from the

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So start with the quantum theory and try to build up a model of space time, the emergence

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So this sort of dream that Einstein had that everybody had and everybody has of, you know,

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So how do we build up from many worlds from quantum mechanics, a model of space time model

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Well, yeah, I mean, let me first mention very quickly why we think it's necessary.

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You know, we've had gravity in the form that Einstein bequeathed it to us for over a hundred

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So gravity is the curvature of space time and there's a field that pervades all the

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But we also have a formalism, an algorithm for taking a classical theory and quantizing

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I mean, you think you're quantizing something, so that means taking a classical theory and

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So they ran into problems and they did that with electromagnetism, namely that certain

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So Feynman and Tominaga and Schwinger won the Nobel Prize for teaching us how to deal

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And then Ken Wilson won another Nobel Prize for saying you shouldn't have been worried

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But still, that was the, it's always the thought that that's how you will make a good quantum

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So if we have a classical theory, general relativity, we can quantize it or we can try

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to, but we run into even bigger problems with gravity than we ran into with electromagnetism.

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We've not been able to get a successful theory of gravity, quantum gravity, by starting with

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And there's evidence that, there's a good reason why this is true, that whatever the

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It's something that has weird nonlocal features built into it somehow that we don't understand.

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We get this idea from black holes and Hawking radiation and information conservation and

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So if that's true, if the fundamental theory isn't even local in the sense that an ordinary

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quantum field theory would be, then we just don't know where to start in terms of getting

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So the only sensible thing, or at least the next obvious sensible thing to me would be

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to say, okay, let's just start intrinsically quantum and work backwards, see if we can

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So the idea of locality, the fact that locality is not fundamental to the nature of our existence,

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I mean, locality is the answer to the question that Isaac Newton was worried about back in

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And the answer as spelled out by Laplace and Einstein and others is that there's a field

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And the way a field works is that what's happening to the field at this point in space only depends

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But what's happening at those points depends on what's happening right next to those, right?

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And so you can build up an influence across space through only local interactions.

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The idea of locality is built into every field theory, including general relativity as a

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It seems to break down when we talk about black holes and, you know, Hawking taught

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us in the 1970s that black holes radiate, they give off, they eventually evaporate away.

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And we think, we don't know for sure, but most of us think that if you make a black

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hole out of certain stuff, then like Laplace's demon taught us, you should be able to predict

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what that black hole will turn into if it's just obeying the Schrodinger equation.

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And if that's true, there are good arguments that can't happen while preserving locality

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It's just that the information seems to be spread out nonlocally in interesting ways.

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And people should, you talk about holography with the Leonard Susskind on your Mindscape

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No, I'm going to, I'm going to ask you questions about that too, and I've been not shutting

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So holography is this idea when you have a black hole and black hole is a region of space

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And there's this weird feature of black holes that, again, it's totally a thought experiment

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But there seems to be one way of thinking about what happens inside a black hole as

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But from the point of view of the outside observer, it seems like all the information

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And that's puzzling and that's, so holography because that's a two dimensional surface that

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But it's an indication that we need to think a little bit more subtly when we quantize

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And because you can describe everything that's going on in the three dimensional space by

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looking at the two dimensional projection of it, it means that locality doesn't, it's

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You know, space is just a good approximation and this was always going to be true once

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So we're just beginning now to face up to the dramatic implications of quantizing gravity.

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Like quantum mechanics is still just quantum mechanics, but our ordinary notions of space

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And there's a principle that goes hand in hand with holography called complementarity,

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which says that there's no one unique way to describe what's going on inside a black

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Different observers will have different descriptions, both of which are accurate, but sound completely

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The word complementarity in this context is borrowed from Niels Bohr, who points out you

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So okay, that's fine and good that is everything is splitting, but we're just traveling down

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But that's the point is that we didn't invent many worlds because we thought it was cool

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We invented it because we were trying to account for what we observe here in our world.

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And that's a different answer than would be given in hidden variables or dynamical collapse

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So the entire point of many worlds is to explain what we observe, but it tries to explain what

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It's not trying to be different from what we've observed because that would be something

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But you know, the idea that there's worlds that we didn't observe that keep branching

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No, but there's a fear of missing out, FOMO, that I feel like immediately start to wonder

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Well, the downside to many worlds is that you're missing out on an enormous amount.

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In terms of rewinding, do you think we can rewind the system back, sort of the nice thing

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about many worlds, I guess, is it really emphasizes the, maybe you can correct me, but the deterministic

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Is it, do you see it as something that could be perfectly rewound back, rewinding back?

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If you're at a fancy French restaurant and there's a nice linen white tablecloth and

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you have your glass of Bordeaux and you knock it over and the wine spills across the tablecloth.

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If the world were classical, okay, it would be possible that if you just lifted the wine

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glass up, you'd be lucky enough that every molecule of wine would hop back into the glass,

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It is possible in principle to rewind everything if you start from perfect knowledge of the

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Does the universe have a memory of itself where we could, in, in, so not time travel,

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So whatever you want to say about that, you know, the fundamental laws of physics in either

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So if you have all the information about the quantum state of the world right now, your

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Laplace is demon like in your knowledge and calculational capacity, you can wind the clock

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You can do experiments over and over again, starting from the same initial conditions

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We we've talked a little bit about arrow of time last time, but in many worlds that there

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So you've talked about the arrow of time that has to do with the second law of thermodynamics.

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So that arrow of time, is that different than the arrow of time that's implied by many worlds?

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In both cases, you have fundamental laws of physics that are completely reversible.

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If you give me the state of the universe at one moment in time, I can run the clock forward

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There's no arrow of time built into the laws of physics at the most fundamental level.

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But what we do have are special initial conditions 14 billion years ago near the Big Bang.

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In thermodynamics, those special initial conditions take the form of things were low entropy and

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entropy has been increasing ever since, making the universe more disorganized and chaotic

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In quantum mechanics, the special initial conditions take the form of there was only

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one branch of the wave function and the universe has been branching more and more ever since.

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Okay, so if time is emergent, so it seems like our human cognitive capacity likes to

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Are you using arrow of time to simply mean this, they're synonymous with the second law

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You could live in thermodynamic equilibrium, there'd be no arrow of time, but there'd still

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Well things could even change, like if the whole universe consisted of the earth going

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If you took a movie of that and I played you the movie backward, you would never know.

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So the arrow of time can theoretically point in the other direction for briefly.

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To the extent that it points in different directions, it's not a very good arrow.

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I mean, the arrow of time in the macroscopic world is so powerful that there's just no

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When you get down to tiny systems with only three or four moving parts, then entropy can

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It means that the fundamental description of the world does not include the word space.

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It'll be something like a vector in Hilbert space, right, and you have to say, well why

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is there a good approximate description which involves three dimensional space and stuff

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We kind of mentioned in the beginning, can you elaborate, what do you feel hope is fundamental

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A wave function in Hilbert space that we can't intellectualize or visualize really.

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It's a complex vector, unit norm, it evolves according to the Schrodinger equation.

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It's like, yep, quantum computers, there's some excitement, actually a lot of excitement

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What kind of questions do you about quantum mechanics, about the things we've been talking

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Well I think that there are, there's a whole fascinating frontier of things you can do

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Both sort of practical things with cryptography or money, privacy eavesdropping, sorting things,

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Some of the theories that we've been talking about, what's your hope, what's most promising

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What are kind of experiments we can conduct, whether in simulation or in the physical world

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For many worlds, you know, there are experiments ongoing to test whether or not wave functions

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If there are hidden variables, there's a theorem that seems to indicate that the predictions

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I haven't made it in part of my intuitive view of the world yet, so there might be loopholes

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Part of me thinks that there should be different experimental predictions if there are hidden

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And so there's this cottage industry in science journalism of writing breathless articles

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that say, you know, quantum mechanics shown to be more astonishing than ever before thought.

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Whereas with the emergent space time stuff, we know a lot less about what the theory is.

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We don't even really have a safely written down, respectable, honest theory yet.

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So there could very well be experimental predictions we just don't know about yet.

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You know, so there could be violations of the speed of light if you have emergent space

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Not going faster than the speed of light, but the speed of light could be different

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That would be a dramatic violation of physics as we know it, but it could be possible.

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Is there anything that quantum mechanics can teach us about human nature or the human mind?

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But in more seriousness, is there something that goes to the human level and can help

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We don't know this for sure, but I don't think that phenomena like entanglement are crucial

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So you mentioned, I think early on in the conversation, you said it would be unlikely,

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So observer, not to romanticize the notion, but seems interlinked to the idea of consciousness.

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So if consciousness, as the panpsychists believe, is fundamental to the universe, is that possible?

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How the statistically speaking, how often do you ponder the possibility that consciousness

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I think they're all emergent, just like space time is and space time is hard enough to understand.

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So the fact that we don't yet understand consciousness is not at all surprising to me.

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It's as I said, one of my favorite podcasts sort of both for your explanation of physics,

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which a lot of people love, and when you venture out into things that are beyond your expertise,

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but it's just a really smart person exploring even questions like morality, for example.

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What are some from memory, amazing conversations that pop to mind that you've had?

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Something that maybe changed your mind or just inspired you or just what did this whole

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You know, it's often the ones I feel like the ones I do on physics and closely related

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science or even philosophy ones are like, I know this stuff and I'm helping people learn

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But I learn more from the ones that have nothing to do with physics or philosophy, right?

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So talking to Wynton Marsalis about jazz or talking to a Master Sommelier about wine,

link |

talking to Will Wilkinson about partisan polarization and the urban rural divide, talking to psychologists

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Scott Derrickson who is the director of the movie Dr. Strange, I had a wonderful conversation

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with him where we went through the mechanics of making a blockbuster superhero movie, right?

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And he's also not a naturalist, he's an evangelical Christian so we talked about the nature of

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I want to have a couple more, you know, discussions with highly educated theists who know the

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You know, I did talk with Alan Lightman who is also an atheist but he, you know, he is

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trying to rescue the sort of spiritual side of things for atheism and I did talk to very

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vocal atheists like Alex Rosenberg so I need to talk to some, I've talked to some religious

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You know, part of the motivation was I had a long stack of books that I hadn't read and

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I couldn't find time to read them and I figured if I interviewed their authors, forced me

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I think that's a skill that, you know, I think I have good questions but, you know, there's

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I've had conversations where I barely talked at all and I have conversations where I talked

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So I think I remember listening to, without mentioning names, some of your conversations

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Well, that's a very good question because, you know, everyone has an attitude toward

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Like some people are really there to basically give their point of view and their guest is

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I want to sort of get my view on the record but I don't want to dwell on it when I'm talking

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You know, I want to say like, here's why I disagree with you but, you know, we're here

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So I have an upcoming podcast episode with Philip Goff who is a much more dedicated pan

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I think that I probably have disagreed with him more on that episode than I ever have

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Like when you're, when you can detect that the intent is that you have fundamental respect

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That, and that's, for some reason, it's super fun to listen to when two really smart people

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Well, there's a fine line because I have zero interest in bringing, I mean, like, I mean,

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maybe you implied this, I have zero interest in bringing on people for whom I don't have

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Like I constantly get requests like, you know, bring on a flat earther or whatever and really

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I'm happy to bring on, you know, a religious person, a believer, but I want someone who's

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smart and can act in good faith and can talk, not a charlatan or a lunatic, right?

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So I will only, I will happily bring on people with whom I disagree, but only people from

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You might be more educated on this topic than me, but there's, there's folks, for example,

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who argue various aspects of evolution sort of try to approach and say that evolution

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And they argue that I think like Cambridge, Cambrian explosion, which is like a huge added

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variability of species, doesn't make sense under our current description of evolution

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and theory of evolution sort of, if you had to, were to have the conversation with people

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like that, how do you know that they're the difference in outside the box thinkers and

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And you know, the further you get away from my expertise, the harder it is for me to really

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And, you know, yeah, I don't have a satisfying answer for that one because I think the example

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you use of someone who, you know, believes in the basic structure of natural selection,

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but thinks that, you know, this particular thing cannot be understood in the terms of

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And I would have, I would try to talk to people who I do respect and who do know things and

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I would have to, you know, given that I'm a physicist, I know that physicists will sometimes

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I have to take into account that biologists can also be too dismissive of alternative points

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Like there's always something, I mean, it's hilarious because I do have, I try very hard

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I did have like two climate change episodes, but they were from very different perspectives,

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And every time I do an episode, someone says, oh, the person you should really get on to