Transcript
00:00:00 The following is a conversation with Sean Carroll, Part 2, the second time we’ve spoken
00:00:05 on the podcast.
00:00:06 You can get the link to the first time in the description.
00:00:10 This time we focus on quantum mechanics and the many worlds interpretation that he details
00:00:15 elegantly in his new book titled Something Deeply Hidden.
00:00:19 I own and enjoy both the eBook and audiobook versions of it.
00:00:24 Listening to Sean read about entanglement, complementarity, and the emergence of space
00:00:29 time reminds me of Bob Ross teaching the world how to paint on his old television show.
00:00:35 If you don’t know who Bob Ross is, you’re truly missing out.
00:00:39 Look him up.
00:00:40 He’ll make you fall in love with painting.
00:00:42 Sean Carroll is the Bob Ross of theoretical physics.
00:00:48 He’s the author of several popular books, a host of a great podcast called Mindscape,
00:00:53 and is a theoretical physicist at Caltech and the Santa Fe Institute, specializing in
00:00:59 quantum mechanics, arrow of time, cosmology, and gravitation.
00:01:04 This is the Artificial Intelligence Podcast.
00:01:07 If you enjoy it, subscribe on YouTube, give it five stars on iTunes, support it on Patreon,
00:01:12 or simply connect with me on Twitter at Lex Friedman, spelled F R I D M A N.
00:01:18 And now here’s my conversation with Sean Carroll.
00:01:23 Isaac Newton developed what we now call classical mechanics that you describe very nicely in
00:01:28 your new book, as you do with a lot of basic concepts in physics.
00:01:32 So with classical mechanics, I can throw a rock and can predict the trajectory of that
00:01:39 rock’s flight.
00:01:41 But if we could put ourselves back into Newton’s time, his theories work to predict things,
00:01:47 but as I understand, he himself thought that they were, their interpretations of those
00:01:52 predictions were absurd.
00:01:56 Perhaps he just said it for religious reasons and so on, but in particular, sort of a world
00:02:01 of interaction without contact, so action at a distance.
00:02:05 It didn’t make sense to him on a sort of a human interpretation level.
00:02:09 Does it make sense to you that things can affect other things at a distance?
00:02:16 It does, but that was one of Newton’s worries.
00:02:20 You’re actually right in a slightly different way about the religious worries.
00:02:24 He was smart enough, this is off the topic but still fascinating, Newton almost invented
00:02:29 chaos theory as soon as he invented classical mechanics.
00:02:33 He realized that in the solar system, so he was able to explain how planets move around
00:02:37 the Sun, but typically you would describe the orbit of the Earth ignoring the effects
00:02:43 of Jupiter and Saturn and so forth, just doing the Earth and the Sun.
00:02:46 He kind of knew, even though he couldn’t do the math, that if you included the effects
00:02:50 of Jupiter and Saturn and the other planets, the solar system would be unstable, like the
00:02:54 orbits of the planets would get out of whack.
00:02:57 So he thought that God would intervene occasionally to sort of move the planets back into orbit,
00:03:01 which is the only way you could explain how they were there presumably forever.
00:03:05 But the worries about classical mechanics were a little bit different, the worry about
00:03:08 gravity in particular.
00:03:09 It wasn’t a worry about classical mechanics, it was a worry about gravity.
00:03:12 How in the world does the Earth know that there’s something called the Sun, 93 million
00:03:16 miles away, that is exerting gravitational force on it?
00:03:20 And he literally said, you know, I leave that for future generations to think about because
00:03:24 I don’t know what the answer is.
00:03:26 And in fact, people under emphasized this, but future generations figured it out.
00:03:31 Pierre Simone Laplace in circa 1800 showed that you could rewrite Newtonian gravity as
00:03:38 a field theory.
00:03:39 So instead of just talking about the force due to gravity, you can talk about the gravitational
00:03:43 field or the gravitational potential field, and then there’s no action at a distance.
00:03:48 It’s exactly the same theory empirically, it makes exactly the same predictions.
00:03:52 But what’s happening is instead of the Sun just reaching out across the void, there is
00:03:55 a gravitational field in between the Sun and the Earth that obeys an equation, Laplace’s
00:04:01 equation, cleverly enough, and that tells us exactly what the field does.
00:04:05 So even in Newtonian gravity, you don’t need action at a distance.
00:04:09 Now what many people say is that Einstein solved this problem because he invented general
00:04:13 relativity.
00:04:14 And in general relativity, there’s certainly a field in between the Earth and the Sun.
00:04:19 But also there’s the speed of light as a limit.
00:04:21 In Laplace’s theory, which was exactly Newton’s theory, just in a different mathematical language,
00:04:26 there could still be instantaneous action across the universe, whereas in general relativity,
00:04:31 if you shake something here, its gravitational impulse radiates out at the speed of light
00:04:36 and we call that a gravitational wave and we can detect those.
00:04:39 So but I really, it rubs me the wrong way to think that we should presume the answer
00:04:45 should look one way or the other.
00:04:47 Like if it turned out that there was action at a distance in physics and that was the
00:04:51 best way to describe things, then I would do it that way.
00:04:54 It’s actually a very deep question because when we don’t know what the right laws of
00:04:58 physics are, when we’re guessing at them, when we’re hypothesizing at what they might
00:05:03 be, we are often guided by our intuitions about what they should be.
00:05:07 I mean, Einstein famously was very guided by his intuitions and he did not like the
00:05:11 idea of action at a distance.
00:05:15 We don’t know whether he was right or not.
00:05:17 It depends on your interpretation of quantum mechanics and it depends on even how you talk
00:05:21 about quantum mechanics within any one interpretation.
00:05:24 So if you see every force as a field or any other interpretation of action at a distance,
00:05:32 just stepping back to sort of caveman thinking, like do you really, can you really sort of
00:05:40 understand what it means for a force to be a field that’s everywhere?
00:05:45 So if you look at gravity, like what do you think about?
00:05:47 I think so.
00:05:48 Is this something that you’ve been conditioned by society to think that, to map the fact
00:05:58 that science is extremely well predictive of something to believing that you actually
00:06:04 understand it?
00:06:06 Like you can intuitively, the degree that human beings can understand anything that
00:06:12 you actually understand it.
00:06:13 Or are you just trusting the beauty and the power of the predictive power of science?
00:06:19 That depends on what you mean by this idea of truly understanding something, right?
00:06:25 You know, I mean, can I truly understand Fermat’s last theorem?
00:06:29 You know, it’s easy to state it, but do I really appreciate what it means for incredibly
00:06:34 large numbers, right?
00:06:37 I think yes, I think I do understand it, but like if you want to just push people on well,
00:06:41 but your intuition doesn’t go to the places where Andrew Wiles needed to go to prove Fermat’s
00:06:46 last theorem, then I can say fine, but I still think I understand the theorem.
00:06:50 And likewise, I think that I do have a pretty good intuitive understanding of fields pervading
00:06:56 space time, whether it’s the gravitational field or the electromagnetic field or whatever,
00:07:00 the Higgs field.
00:07:03 Of course, one’s intuition gets worse and worse as you get trickier in the quantum field
00:07:08 theory and all sorts of new phenomena that come up in quantum field theory.
00:07:12 So our intuitions aren’t perfect, but I think it’s also okay to say that our intuitions
00:07:17 get trained, right?
00:07:18 Like, you know, I have different intuitions now than I had when I was a baby.
00:07:21 That’s okay.
00:07:22 That’s not, an intuition is not necessarily intrinsic to who we are.
00:07:26 We can train it a little bit.
00:07:27 So that’s where I’m going to bring in Noam Chomsky for a second, who thinks that our
00:07:33 cognitive abilities are sort of evolved through time, and so they’re biologically constrained.
00:07:40 And so there’s a clear limit, as he puts it, to our cognitive abilities, and it’s a very
00:07:45 harsh limit.
00:07:47 But you actually kind of said something interesting in nature versus nurture thing here, is we
00:07:52 can train our intuitions to sort of build up the cognitive muscles to be able to understand
00:07:58 some of these tricky concepts.
00:07:59 So do you think there’s limits to our understanding that’s deeply rooted, hardcoded into our biology
00:08:07 that we can’t overcome?
00:08:09 There could be limits to things like our ability to visualize, okay?
00:08:15 But when someone like Ed Witten proves a theorem about, you know, 100 dimensional mathematical
00:08:20 spaces, he’s not visualizing it.
00:08:22 He’s doing the math.
00:08:23 That doesn’t stop him from understanding the result.
00:08:27 I think, and I would love to understand this better, but my rough feeling, which is not
00:08:32 very educated, is that, you know, there’s some threshold that one crosses in abstraction
00:08:37 when one becomes kind of like a Turing machine, right?
00:08:41 One has the ability to contain in one’s brain logical, formal, symbolic structures and manipulate
00:08:48 them.
00:08:49 And that’s a leap that we can make as human beings that dogs and cats haven’t made.
00:08:55 And once you get there, I’m not sure that there are any limits to our ability to understand
00:09:00 the scientific world at all.
00:09:02 Maybe there are.
00:09:03 There’s certainly limits in our ability to calculate things, right?
00:09:07 You know, people are not very good at taking cube roots of million digit numbers in their
00:09:10 head.
00:09:11 But that’s not an element of understanding.
00:09:14 It’s certainly not a limit in principle.
00:09:15 So of course, as a human, you would say there doesn’t feel to be limits to our understanding.
00:09:21 But sort of, have you thought that the universe is actually a lot simpler than it appears
00:09:30 to us?
00:09:31 And we just will never be able to, like, it’s outside of our, okay.
00:09:36 So us, our cognitive abilities combined with our mathematical prowess and whatever kind
00:09:43 of experimental simulation devices we can put together, is there limits to that?
00:09:50 Is it possible there’s limits to that?
00:09:52 Well, of course it’s possible that there are limits to that.
00:09:57 Is there any good reason to think that we’re anywhere close to the limits is a harder question.
00:10:02 Look, imagine asking this question 500 years ago to the world’s greatest thinkers, right?
00:10:07 Like are we approaching the limits of our ability to understand the natural world?
00:10:12 And by definition, there are questions about the natural world that are most interesting
00:10:17 to us that are the ones we don’t quite yet understand, right?
00:10:19 So there’s always, we’re always faced with these puzzles we don’t yet know.
00:10:23 And I don’t know what they would have said 500 years ago, but they didn’t even know about
00:10:26 classical mechanics, much less quantum mechanics.
00:10:28 So we know that they were nowhere close to how well they could do, right?
00:10:33 They could do enormously better than they were doing at the time.
00:10:36 I see no reason why the same thing isn’t true for us today.
00:10:39 So of all the worries that keep me awake at night, the human mind’s inability to rationally
00:10:44 comprehend the world is low on the list.
00:10:47 Well put.
00:10:49 So one interesting philosophical point that quantum mechanics bring up is the, that you
00:10:54 talk about the distinction between the world as it is and the world as we observe it.
00:11:02 So staying at the human level for a second, how big is the gap between what our perception
00:11:08 system allows us to see and the world as it is outside our mind’s eye sort of, sort of
00:11:15 not at the quantum mechanical level, but as just our, these particular tools we have,
00:11:21 which is the few senses and cognitive abilities to process those senses.
00:11:26 Well, that last phrase, having the cognitive abilities to process them carries a lot, right?
00:11:31 I mean, there is our sort of intuitive understanding of the world.
00:11:36 You don’t need to teach people about gravity for them to know that apples fall from trees,
00:11:41 right?
00:11:42 That’s something that we figure out pretty quickly.
00:11:43 Project permanence, things like that, the three dimensionality of space, even if we
00:11:47 don’t have the mathematical language to say that, we kind of know that it’s true.
00:11:52 On the other hand, no one opens their eyes and sees atoms, right?
00:11:56 Or molecules or cells for that matter, forget about quantum mechanics.
00:12:00 So but we got there, we got to understanding that there are atoms and cells using the combination
00:12:07 of our senses and our cognitive capacities.
00:12:11 So adding the ability of our cognitive capacities to our senses is adding an enormous amount
00:12:16 and I don’t think it is a hard and fast boundary.
00:12:19 You know, if you believe in cells, if you believe that we understand those, then there’s
00:12:23 no reason you believe we can’t believe in quantum mechanics just as well.
00:12:29 What to you is the most beautiful idea in physics?
00:12:36 Conservation of momentum.
00:12:38 Can you elaborate?
00:12:39 Yeah.
00:12:40 So if you were Aristotle, when Aristotle wrote his book on physics, he made the following
00:12:44 very obvious point.
00:12:45 We’re on video here, right?
00:12:46 So people can see this.
00:12:47 Yeah.
00:12:48 So if I push the bottle, let me cover this bottle so we do not have a mess, but okay.
00:12:51 So I push the bottle, it moves, and if I stop pushing, it stops moving.
00:12:56 And this kind of thing is repeated a large number of times all over the place.
00:13:00 If you don’t keep pushing things, they stop moving.
00:13:03 This is an indisputably true fact about our everyday environment, okay?
00:13:09 And for Aristotle, this blew up into a whole picture of the world in which things had natures
00:13:15 and teleologies, and they had places they wanted to be, and when you were pushing them,
00:13:19 you were moving them away from where they wanted to be, and they would return and stuff
00:13:23 like that.
00:13:24 And it took a thousand years or 1500 years for people to say, actually, if it weren’t
00:13:31 for things like dissipation and air resistance and friction and so forth, the natural thing
00:13:37 is for things to move forever in a straight line, there’s a constant velocity, right?
00:13:42 Conservation of momentum.
00:13:44 And the reason why I think that’s the most beautiful idea in physics is because it shifts
00:13:51 us from a view of natures and teleology to a view of patterns in the world.
00:13:58 So when you were Aristotle, you needed to talk a vocabulary of why is this happening,
00:14:05 what’s the purpose of it, what’s the cause, etc., because, you know, it’s nature does
00:14:08 or does not want to do that, whereas once you believe in conservation of momentum, things
00:14:12 just happen.
00:14:13 They just follow the pattern.
00:14:15 You give me, you have Laplace’s demon, ultimately, right?
00:14:17 You give me the state of the world today, I can predict what it’s going to do in the
00:14:21 future, I can predict where it was in the past.
00:14:23 It’s impersonal, and it’s also instantaneous.
00:14:26 It’s not directed toward any future goals, it’s just doing what it does given the current
00:14:30 state of the universe.
00:14:32 I think even more than either classical mechanics or quantum mechanics, that is the profound
00:14:37 deep insight that gets modern science off the ground.
00:14:40 You don’t need natures and purposes and goals, you just need some patterns.
00:14:45 So it’s the first moment in our understanding of the way the universe works where you branch
00:14:51 from the intuitive physical space to kind of the space of ideas.
00:14:57 And also the other point you said, which is, conveniently, most of the interesting ideas
00:15:03 are acting in the moment.
00:15:05 You don’t need to know the history of time or the future.
00:15:09 And of course, this took a long time to get there, right?
00:15:12 I mean, the conservation of momentum itself took hundreds of years.
00:15:16 It’s weird, because like, someone would say something interesting, and then the next interesting
00:15:19 thing would be said like 150 or 200 years later, right?
00:15:22 They weren’t even talking to each other, they were reading each other’s books.
00:15:25 And probably the first person to directly say that in outer space, in the vacuum, a
00:15:30 projectile would move at a constant velocity was Avicenna, Ibn Sina in the Persian Golden
00:15:35 Age, circa 1000.
00:15:38 And he didn’t like the idea.
00:15:39 He used that, just like Schrodinger used Schrodinger’s cat to say, surely you don’t believe that,
00:15:44 right?
00:15:45 Ibn Sina was saying, surely you don’t believe there really is a vacuum, because if there
00:15:48 was a really vacuum, things could keep moving forever, right?
00:15:52 But still, he got right the idea that there was this conservation of something impetus
00:15:56 or mile, he would call it.
00:15:58 And that’s 500 years, 600 years before classical mechanics and Isaac Newton.
00:16:03 So Galileo played a big role in this, but he didn’t exactly get it right.
00:16:07 And so it just takes a long time for this to sink in, because it is so against our everyday
00:16:12 experience.
00:16:13 Do you think it was a big leap, a brave or a difficult leap of sort of math and science
00:16:21 to be able to say that momentum is conserved?
00:16:25 I do.
00:16:26 You know, I think it’s an example of human reason in action.
00:16:31 You know, even Aristotle knew that his theory had issues, because you could fire an arrow
00:16:36 and it would go a long way before it stopped.
00:16:38 So if his theory was things just automatically stop, what’s going on?
00:16:42 And he had this elaborate story.
00:16:43 I don’t know if you’ve heard the story, but the arrow would push the air in front of it
00:16:48 away and the molecules of air would run around to the back of the arrow and push it again.
00:16:53 And anyone reading this is going like, really, that’s what you thought?
00:16:56 But it was that kind of thought experiment that ultimately got people to say like, actually,
00:17:00 no, if it weren’t for the air molecules at all, the arrow would just go on by itself.
00:17:04 And it’s always this give and take between thought and experience, back and forth, right?
00:17:09 Theory and experiment, we would say today.
00:17:13 Another big question that I think comes up, certainly with quantum mechanics, is what’s
00:17:20 the difference between math and physics to you?
00:17:25 To me, you know, very, very roughly, math is about the logical structure of all possible
00:17:30 worlds and physics is about our actual world.
00:17:35 And it just feels like our actual world is a gray area when you start talking about interpretations
00:17:40 of quantum mechanics, or no?
00:17:43 I’m certainly using the word world in the broadest sense, all of reality.
00:17:47 So I think that reality is specific.
00:17:50 I don’t think that there’s every possible thing going on in reality.
00:17:54 I think that there are rules, whether it’s the Schrodinger equation or whatever.
00:17:58 So I think that there’s a sensible notion of the set of all possible worlds and we live
00:18:03 in one of them.
00:18:04 The world that we’re talking about might be a multiverse, might be many worlds of quantum
00:18:07 mechanics, might be much bigger than the world of our everyday experience, but it’s still
00:18:10 one physically contiguous world in some sense.
00:18:15 But so if you look at the overlap of math and physics, it feels like when physics tries
00:18:24 to reach for understanding of our world, it uses the tools of math to sort of reach beyond
00:18:30 the limit of our current understanding.
00:18:34 What do you make of that process of sort of using math to, so you start maybe with intuition
00:18:41 or you might start with the math and then build up an intuition or, but this kind of
00:18:45 reaching into the darkness, into the mystery of the world with math.
00:18:49 Well, I think I would put it a little bit differently.
00:18:51 I think we have theories, theories of the physical world, which we then extrapolate
00:18:57 and ask, you know, what do we conclude if we take these seriously well beyond where
00:19:02 we’ve actually tested them?
00:19:03 It is separately true that math is really, really useful when we construct physical theories
00:19:09 and you know, famously Eugene Wigner asked about the unreasonable success of mathematics
00:19:13 and physics.
00:19:14 I think that’s a little bit wrong because anything that could happen, any other theory
00:19:20 of physics that wasn’t the real world, but some other world, you could always describe
00:19:24 it mathematically.
00:19:25 It’s just that it might be a mess.
00:19:28 The surprising thing is not that math works, but that the math is so simple and easy that
00:19:33 you can write it down on a t shirt, right?
00:19:35 I mean, that’s what is amazing.
00:19:37 That’s an enormous compression of information that seems to be valid in the real world.
00:19:44 So that’s an interesting fact about our world, which maybe we could hope to explain or just
00:19:48 take as a brute fact.
00:19:49 I don’t know.
00:19:50 But once you have that, you know, there’s this indelible relationship between math and
00:19:56 physics, but philosophically I do want to separate them.
00:19:59 What we extrapolate, we don’t extrapolate math because there’s a whole bunch of wrong
00:20:03 math, you know, that doesn’t apply to our world, right?
00:20:06 We extrapolate the physical theory that we best think explains our world.
00:20:09 Again, an unanswerable question.
00:20:12 Why do you think our world is so easily compressible into beautiful equations?
00:20:19 Yeah.
00:20:20 I mean, like I just hinted at, I don’t know if there’s an answer to that question.
00:20:23 There could be.
00:20:24 What would an answer look like?
00:20:26 Well, an answer could look like if you showed that there was something about our world that
00:20:31 maximizes something.
00:20:33 You know, the mean of the simplicity and the powerfulness of the laws of physics or, you
00:20:40 know, maybe we’re just generic.
00:20:41 Maybe in the set of all possible worlds, this is what the world would look like, right?
00:20:44 Like I don’t really know.
00:20:46 I tend to think not.
00:20:48 I tend to think that there is something specific and rock bottom about the facts of our world
00:20:54 that don’t have further explanation.
00:20:56 Like the fact of the world exists at all.
00:20:58 And furthermore, the specific laws of physics that we have.
00:21:00 I think that in some sense, we’re just going to, at some level, we’re going to say, and
00:21:04 that’s how it is.
00:21:05 And, you know, we can’t explain anything more.
00:21:07 I don’t know how, if we’re anywhere close to that right now, but that seems plausible
00:21:11 to me.
00:21:12 And speaking of rock bottom, one of the things sort of your book kind of reminded me or revealed
00:21:16 to me is that what’s fundamental and what’s emergent, it just feels like I don’t even
00:21:23 know anymore what’s fundamental in physics, if there’s anything.
00:21:28 It feels like everything, especially with quantum mechanics, is revealing to us is that
00:21:33 most interesting things that I would, as a limited human would think are fundamental
00:21:41 can actually be explained as emergent from the more deeper laws.
00:21:48 I mean, we don’t know, of course.
00:21:51 You had to get that on the table.
00:21:52 We don’t know what is fundamental.
00:21:54 We do have reasons to say that certain things are more fundamental than others, right?
00:22:00 Atoms and molecules are more fundamental than cells and organs.
00:22:03 Quantum fields are more fundamental than atoms and molecules.
00:22:07 We don’t know if that ever bottoms out.
00:22:09 I do think that there’s sensible ways to think about this.
00:22:13 If you describe something like this table as a table, it has a height and a width and
00:22:18 it’s made of a certain material and it has a certain solidity and weight and so forth.
00:22:22 That’s a very useful description as far as it goes.
00:22:24 There’s a whole other description of this table in terms of a whole collection of atoms
00:22:29 strung together in certain ways.
00:22:31 The language of the atoms is more comprehensive than the language of the table.
00:22:36 You could break apart the table, smash it to pieces, still talk about it as atoms, but
00:22:41 you could no longer talk about it as a table, right?
00:22:43 So I think that this comprehensiveness, the domain of validity of a theory gets broader
00:22:48 and broader as the theory gets more and more fundamental.
00:22:52 So what do you think Newton would say?
00:22:57 Maybe right in the book review, if you read your latest book on quantum mechanics, something
00:23:02 deeply hidden.
00:23:03 It would take a long time for him to think that any of this was making any sense.
00:23:08 You catch him up pretty quick in the beginning.
00:23:10 Yeah.
00:23:11 You give him a shout out in the beginning.
00:23:13 That’s right.
00:23:14 He is the man.
00:23:15 I’m happy to say that Newton was the greatest scientist who ever lived.
00:23:19 He invented calculus in his spare time, which would have made him the greatest mathematician
00:23:22 just all by himself, all by that one thing.
00:23:25 But of course, it’s funny because Newton was in some sense still a pre modern thinker.
00:23:33 Rocky Kolb, who is a cosmologist at the University of Chicago said that Galileo, even though
00:23:39 he came before Newton, was a more modern thinker than Newton was.
00:23:43 If you got Galileo and brought him to the present day, it would take him six months
00:23:46 to catch up and then he’d be in your office telling you why your most recent paper was
00:23:49 wrong.
00:23:50 Whereas Newton just thought in this kind of more mystical way.
00:23:55 He wrote a lot more about the Bible and alchemy than he ever did about physics, but he was
00:24:01 also more brilliant than anybody else and way more mathematically astute than Galileo.
00:24:06 So I really don’t know.
00:24:08 He might have, he might just, yeah, say like, give me the textbooks, leave me alone for
00:24:12 a few months and then be caught up.
00:24:15 But he might have had mental blocks against seeing the world in this way.
00:24:19 I really don’t know.
00:24:20 Or perhaps find an interesting mystical interpretation of quantum mechanics.
00:24:24 Very possible.
00:24:25 Yeah.
00:24:26 Is there any other scientists or philosophers through history that you would like to know
00:24:31 their opinion of your book?
00:24:33 That’s a, that’s a good question.
00:24:36 I mean, Einstein is the obvious one, right?
00:24:38 We all, I mean, he was not that long ago, but I even speculated at the end of my book
00:24:42 about what his opinion would be.
00:24:44 I am curious as to, you know, what about older philosophers like Hume or Kant, right?
00:24:50 Like what would they have thought?
00:24:51 Or Aristotle, you know, what would they have thought about modern physics?
00:24:55 Because they do in philosophy, your predilections end up playing a much bigger role in your
00:25:01 ultimate conclusions because you’re not as tied down by what the data is in physics.
00:25:07 You know, physics is lucky because we can’t stray too far off the reservation as long
00:25:11 as we’re trying to explain the world that we actually see in our telescopes and microscopes.
00:25:15 But it’s just not fair to play that game because the people we’re thinking about didn’t know
00:25:20 a whole bunch of things that we know, right?
00:25:23 Like we lived through a lot that they didn’t live through.
00:25:26 So by the time we got them caught up, they’d be different people.
00:25:32 So let me ask a bunch of basic questions.
00:25:35 I think it would be interesting, useful for people who are not familiar, but even for
00:25:40 people who are extremely well familiar.
00:25:42 Let’s start with what is quantum mechanics?
00:25:47 Quantum mechanics is the paradigm of physics that came into being in the early part of
00:25:51 the 20th century that replaced classical mechanics, and it replaced classical mechanics in a weird
00:25:58 way that we’re still coming to terms with.
00:26:00 So in classical mechanics, you have an object, it has a location, it has a velocity, and
00:26:05 if you know the location and velocity of everything in the world, you can say what everything’s
00:26:08 going to do.
00:26:10 Quantum mechanics has an aspect of it that is kind of on the same lines.
00:26:15 There’s something called the quantum state or the wave function.
00:26:19 And there’s an equation governing what the quantum state does.
00:26:22 So it’s very much like classical mechanics.
00:26:23 The wave function is different.
00:26:25 It’s sort of a wave.
00:26:26 It’s a vector in a huge dimensional vector space rather than a position and a velocity,
00:26:31 but okay, that’s a detail.
00:26:33 The equation is the Schrodinger equation, not Newton’s laws, but okay, again, a detail.
00:26:37 Where quantum mechanics really becomes weird and different is that there’s a whole other
00:26:41 set of rules in our textbook formulation of quantum mechanics in addition to saying that
00:26:46 there’s a quantum state and it evolves in time.
00:26:49 And all these new rules have to do with what happens when you look at the system, when
00:26:52 you observe it, when you measure it.
00:26:55 In classical mechanics, there were no rules about observing.
00:26:58 You just look at it and you see what’s going on.
00:27:00 That was it, right?
00:27:01 In quantum mechanics, the way we teach it, there’s something profoundly fundamental about
00:27:06 the act of measurement or observation, and the system dramatically changes its state.
00:27:12 Even though it has a wave function, like the electron in an atom is not orbiting in a circle,
00:27:16 it’s sort of spread out in a cloud, when you look at it, you don’t see that cloud.
00:27:21 When you look at it, it looks like a particle with a location.
00:27:24 So it dramatically changes its state right away, and the effects of that change can be
00:27:29 instantly seen in what the electron does next.
00:27:32 So again, we need to be careful because we don’t agree on what quantum mechanics says.
00:27:39 That’s why I need to say like in the textbook view, et cetera, right?
00:27:41 But in the textbook view, quantum mechanics, unlike any other theory of physics, gives
00:27:48 a fundamental role to the act of measurement.
00:27:51 So maybe even more basic, what is an atom and what is an electron?
00:27:56 Sure.
00:27:57 This all came together in a few years around the turn of the last century, right?
00:28:01 Around the year 1900.
00:28:05 Atoms predated then, of course, the word atom goes back to the ancient Greeks, but it was
00:28:09 the chemists in the 1800s that really first got experimental evidence for atoms.
00:28:15 They realized that there were two different types of tin oxide.
00:28:21 And in these two different types of tin oxide, there was exactly twice as much oxygen in
00:28:25 one type as the other.
00:28:27 And like, why is that?
00:28:28 Why is it never 1.5 times as much, right?
00:28:31 And so Dalton said, well, it’s because there are tin atoms and oxygen atoms, and one form
00:28:38 of tin oxide is one atom of tin and one atom of oxygen, and the other is one atom of tin
00:28:42 and two atoms of oxygen.
00:28:44 And on the basis of this, you know, a speculation, a theory, right, a hypothesis, but then on
00:28:49 the basis of that, you make other predictions, and the chemists became quickly convinced
00:28:52 that atoms were real.
00:28:54 The physicists took a lot longer to catch on, but eventually they did.
00:28:58 And I mean, Boltzmann, who believed in atoms, had a really tough time his whole life because
00:29:04 he worked in Germany where atoms were not popular.
00:29:07 They were popular in England, but not in Germany.
00:29:09 And there, in general, the idea of atoms is, it’s the most, the smallest building block
00:29:15 of the universe for them.
00:29:17 That’s the kind of how they thought it was.
00:29:18 That was the Greek idea, but the chemists in the 1800s jumped the gun a little bit.
00:29:22 So these days, an atom is the smallest building block of a chemical element, right?
00:29:27 Hydrogen, tin, oxygen, carbon, whatever, but we know that atoms can be broken up further
00:29:33 than that.
00:29:34 That’s what physicists discovered in the early 1900s, Rutherford, especially, and his colleagues.
00:29:40 So the atom that we think about now, the cartoon, is that picture you’ve always seen of a little
00:29:46 nucleus and then electrons orbiting it like a little solar system.
00:29:50 And we now know the nucleus is made of protons and neutrons.
00:29:53 So the weight of the atom, the mass, is almost all in its nucleus.
00:29:58 Protons and neutrons are something like 1800 times as heavy as electrons are.
00:30:03 Protons are much lighter, but because they’re lighter, they give all the life to the atoms.
00:30:09 So when atoms get together, combine chemically, when electricity flows through a system, it’s
00:30:13 all the electrons that are doing all the work.
00:30:16 And where quantum mechanics steps in, as you mentioned, with the position of velocity with
00:30:21 classical mechanics and quantum mechanics is modeling the behavior of the electron.
00:30:26 I mean, you can model the behavior of anything, but the electron, because that’s where the
00:30:30 fun is.
00:30:31 The electron was the biggest challenge right from the start.
00:30:34 Yeah.
00:30:35 So what’s a wave function?
00:30:36 You said it’s an interesting detail, but in any interpretation, what is the wave function
00:30:43 in quantum mechanics?
00:30:44 Well, you know, we had this idea from Rutherford that atoms look like little solar systems,
00:30:50 but people very quickly realize that can’t possibly be right because if an electron is
00:30:54 orbiting in a circle, it will give off light.
00:30:57 All the light that we have in this room comes from electrons zooming up and down and wiggling.
00:31:01 That’s what electromagnetic waves are.
00:31:03 And you can calculate how long would it take for the electron just to spiral into the nucleus?
00:31:07 And the answer is 10 to the minus 11 seconds, okay, 100 billionth of a second.
00:31:12 So that’s not right.
00:31:14 Meanwhile, people had realized that light, which we understood from the 1800s was a wave,
00:31:21 had properties that were similar to that of particles, right?
00:31:23 This is Einstein and Planck and stuff like that.
00:31:26 So if something that we agree was a wave had particle like properties, then maybe something
00:31:34 we think is a particle, the electron has wave like properties, right?
00:31:38 And so a bunch of people eventually came to the conclusion, don’t think about the electron
00:31:42 as a little point particle orbiting like a solar system.
00:31:47 Think of it as a wave that is spread out.
00:31:49 They cleverly gave this the name the wave function, which is the dopiest name in the
00:31:53 world for one of the most profound things in the universe.
00:31:57 There’s literally a number at every point in space, which is the value of the electron’s
00:32:03 wave function at that point.
00:32:04 And there’s only one wave function.
00:32:07 Yeah, they eventually figured that out.
00:32:09 That took longer.
00:32:10 But when you have two electrons, you do not have a wave function for electron one and
00:32:15 a wave function for electron two.
00:32:17 You have one combined wave function for both of them.
00:32:20 And indeed, as you say, there’s only one wave function for the entire universe at once.
00:32:26 And that’s where this beautiful dance, can you say what is entanglement?
00:32:32 It seems one of the most fundamental ideas of quantum mechanics.
00:32:35 Well, let’s temporarily buy into the textbook interpretation of quantum mechanics.
00:32:39 And what that says is that this wave function, so it’s very small outside the atom, very
00:32:43 big in the atom, basically the wave function, you take it and you square it, you square
00:32:48 the number that gives you the probability of observing the system at that location.
00:32:54 So if you say that for two electrons, there’s only one wave function, and that wave function
00:32:58 gives you the probability of observing both electrons at once doing something, okay?
00:33:03 So maybe the electron can be here or here, here, here, and the other electron can also
00:33:07 be there.
00:33:08 But we have a wave function set up where we don’t know where either electron is going
00:33:12 to be seen.
00:33:14 But we know they’ll both be seen in the same place, okay?
00:33:17 So we don’t know exactly what we’re going to see for either electron, but there’s entanglement
00:33:22 between the two of them.
00:33:23 There’s a sort of conditional statement.
00:33:25 If we see one in one location, then we know the other one’s going to be doing a certain
00:33:29 thing.
00:33:30 So that’s a feature of quantum mechanics that is nowhere to be found in classical mechanics.
00:33:34 In classical mechanics, there’s no way I can say, well, I don’t know where either one of
00:33:37 these particles is, but if I know, if I find out where this one is, then I know where the
00:33:40 other one is.
00:33:41 That just never happens.
00:33:42 They’re truly separate.
00:33:43 I don’t know, it feels like, if you think of a wave function like as a dance floor,
00:33:47 it seems like entanglement is strongest between things that are dancing together closest.
00:33:53 So there’s a closeness that’s important.
00:33:56 Well, that’s another step.
00:33:58 We have to be careful here because in principle, if you’re talking about the entanglement of
00:34:02 two electrons, for example, they can be totally entangled or totally unentangled no matter
00:34:08 where they are in the universe.
00:34:10 There’s no relationship between the amount of entanglement and the distance between two
00:34:15 electrons.
00:34:16 But we now know that the reality of our best way of understanding the world is through
00:34:21 quantum fields, not through particles.
00:34:23 So even the electron, not just gravity and electromagnetism, but even the electron and
00:34:28 the quarks and so forth are really vibrations in quantum fields.
00:34:33 So even empty space is full of vibrating quantum fields.
00:34:38 And those quantum fields in empty space are entangled with each other in exactly the way
00:34:42 you just said.
00:34:43 If they’re nearby, if you have like two vibrating quantum fields that are nearby, then they’ll
00:34:47 be highly entangled.
00:34:48 If they’re far away, they will not be entangled.
00:34:50 So what do quantum fields in a vacuum look like?
00:34:52 Empty space?
00:34:53 Just like empty space.
00:34:54 It’s as empty as it can be.
00:34:56 But there’s still a field.
00:34:57 It’s just, what does nothing look like?
00:35:02 Just like right here, this location in space, there’s a gravitational field, which I can
00:35:06 detect by dropping something.
00:35:07 Yes.
00:35:08 I don’t see it, but there it is.
00:35:12 So we got a little bit of an idea of entanglement.
00:35:17 Now, what is Hilbert space and Euclidean space?
00:35:23 Yeah, you know, I think that people are very welcome to go through their lives not knowing
00:35:28 what Hilbert space is.
00:35:29 But if you dig into a little bit more into quantum mechanics, it becomes necessary.
00:35:33 You know, the English language was invented long before quantum mechanics, or various
00:35:39 forms of higher mathematics were invented.
00:35:41 So we use the word space to mean different things.
00:35:45 Of course, most of us think of space as this three dimensional world in which we live,
00:35:48 right?
00:35:49 I mean, some of us just think of it as outer space.
00:35:51 Okay, but space around us gives us the three dimensional location of things and objects.
00:35:56 But mathematicians use any generic abstract collection of elements as a space, okay?
00:36:05 A space of possibilities, you know, momentum space, etc.
00:36:09 So Hilbert space is the space of all possible quantum wave functions, either for the universe
00:36:14 or for some specific system.
00:36:16 And it could be an infinite dimensional space, or it could be just really, really large dimensional
00:36:21 but finite.
00:36:22 We don’t know because we don’t know the final theory of everything.
00:36:24 But this abstract Hilbert space is really, really, really big and has no immediate connection
00:36:29 to the three dimensional space in which we live.
00:36:31 What do dimensions in Hilbert space mean?
00:36:35 You know, it’s just a way of mathematically representing how much information is contained
00:36:40 in the state of the system.
00:36:41 How many numbers do you have to give me to specify what the thing is doing?
00:36:45 So in classical mechanics, I give you the location of something by giving you three
00:36:51 numbers, right?
00:36:52 Up, down, left, X, Y, Z coordinates.
00:36:54 But then I might want to give you its entire state, physical state, which means both its
00:37:00 position and also its velocity.
00:37:02 The velocity also has three components.
00:37:04 So its state lives in something called phase space, which is six dimensional, three dimensions
00:37:09 of position, three dimensions of velocity.
00:37:12 And then if it also has an orientation in space, that’s another three dimensions and
00:37:16 so forth.
00:37:17 So as you describe more and more information about the system, you have an abstract mathematical
00:37:22 space that has more and more numbers that you need to give.
00:37:26 And each one of those numbers corresponds to a dimension in that space.
00:37:29 So in terms of the amount of information, what is entropy?
00:37:34 This mystical word that’s overused in math and physics, but has a very specific meaning
00:37:40 in this context.
00:37:41 Sadly, it has more than one very specific meeting.
00:37:44 This is the reason why it is hard.
00:37:46 Entropy means different things even to different physicists.
00:37:49 But one way of thinking about it is a measure of how much we don’t know about the state
00:37:54 of a system.
00:37:55 So if I have a bottle of water molecules, and I know that, OK, there’s a certain number
00:38:00 of water molecules.
00:38:01 I could weigh it and figure out.
00:38:02 I know the volume of it, and I know the temperature and pressure and things like that.
00:38:06 I certainly don’t know the exact position and velocity of every water molecule.
00:38:12 So there’s a certain amount of information I know, a certain amount that I don’t know
00:38:15 that is part of the complete state of the system.
00:38:18 And that’s what the entropy characterizes, how much unknown information there is, the
00:38:23 difference between what I do know about the system and its full exact microscopic state.
00:38:28 So when we try to describe a quantum mechanical system, is it infinite or finite but very
00:38:36 large?
00:38:37 Yeah, we don’t know.
00:38:38 That depends on the system.
00:38:39 You know, it’s easy to mathematically write down a system that would have a potentially
00:38:44 infinite entropy, an infinite dimensional Hilbert space.
00:38:47 So let’s go back a little bit.
00:38:50 We said that the Hilbert space was the space in which quantum wave functions lived for
00:38:54 different systems that will be different sizes.
00:38:57 They could be infinite or finite.
00:38:58 So that’s the number of numbers, the number of pieces of information you could potentially
00:39:03 give me about the system.
00:39:04 So the bigger Hilbert space is, the bigger the entropy of that system could be, depending
00:39:11 on what I know about it.
00:39:12 If I don’t know anything about it, then it has a huge entropy, right, but only up to
00:39:16 the size of its Hilbert space.
00:39:18 So we don’t know in the real physical world whether or not, you know, this region of space
00:39:23 that contains that water bottle has potentially an infinite entropy or just a finite entropy.
00:39:29 We have different arguments on different sides.
00:39:31 So if it’s infinite, how do you think about infinity?
00:39:35 Is this something you can, your cognitive abilities are able to process or is it just
00:39:42 a mathematical tool?
00:39:44 It’s somewhere in between, right?
00:39:45 I mean, we can say things about it.
00:39:46 We can use mathematical tools to manipulate infinity very, very accurately.
00:39:51 We can define what we mean.
00:39:52 You know, for any number n, there’s a number bigger than it.
00:39:56 So there’s no biggest number, right?
00:39:58 So there’s something called the total number of all numbers.
00:40:00 It’s infinite.
00:40:01 But it is hard to wrap your brain around that, and I think that gives people pause because
00:40:07 we talk about infinity as if it’s a number, but it has plenty of properties that real
00:40:11 numbers don’t have.
00:40:12 You know, if you multiply infinity by two, you get infinity again, right?
00:40:15 That’s a little bit different than what we’re used to.
00:40:18 Okay.
00:40:19 But are you comfortable with the idea that in thinking of what the real world actually
00:40:25 is that infinity could be part of that world?
00:40:28 Are you comfortable that a world in some dimension, in some aspect?
00:40:31 I’m comfortable with lots of things.
00:40:33 I mean, you know, I don’t want my level of comfort to affect what I think about the world.
00:40:40 You know, I’m pretty open minded about what the world could be at the fundamental level.
00:40:44 Yeah, but infinity is a tricky one.
00:40:47 It’s not almost a question of comfort.
00:40:50 It’s a question of, is it an overreach of our intuition?
00:40:56 Sort of, it could be a convenient, almost like when you add a constant to an equation
00:41:01 just because it’ll help, it just feels like it’s useful to at least be able to imagine
00:41:06 a concept, not directly, but in some kind of way that this feels like it’s a description
00:41:13 of the real world.
00:41:14 Think of it this way.
00:41:15 There’s only three numbers that are simple.
00:41:19 There’s zero, there’s one, and there’s infinity.
00:41:24 A number like 318 is just bizarre.
00:41:29 You need a lot of bits to give me what that number is.
00:41:33 But zero and one and infinity, like once you have 300 things, you might as well have infinity
00:41:36 things, right?
00:41:37 Otherwise, you have to say when to stop making the things, right?
00:41:40 So there’s a sense in which infinity is a very natural number of things to exist.
00:41:44 I was never comfortable with infinity because it’s just such a, it was too good to be true.
00:41:50 Because in math, it just helps make things work out.
00:41:55 When things get very large, close to infinity, things seem to work out nicely.
00:42:02 It’s kind of like, because my deepest passion is probably psychology.
00:42:07 And I’m uncomfortable how in the average, the beauty of how much we vary is lost.
00:42:18 In that same kind of sense, infinity seems like a convenient way to erase the details.
00:42:24 But the thing about infinity is it seems to pop up whether we like it or not, right?
00:42:29 Like you’re trying to be a computer scientist, you ask yourself, well, how long will it take
00:42:33 this program to run?
00:42:34 And you realize, well, for some of them, the answer is infinitely long.
00:42:37 It’s not because you tried to get there.
00:42:39 You wrote a five line computer program, it doesn’t halt.
00:42:43 So coming back to the textbook definition of quantum mechanics, this idea that I don’t
00:42:48 think we talked about, can you, this one of the most interesting philosophical points,
00:42:55 we talked at the human level, but at the physics level, that at least the textbook definition
00:43:02 of quantum mechanics separates what is observed and what is real.
00:43:07 One, how does that make you feel?
00:43:13 And two, what does it then mean to observe something and why is it different than what
00:43:19 is real?
00:43:20 Yeah, you know, my personal feeling, such as it is, is that things like measurement
00:43:27 and observers and stuff like that are not going to play a fundamental role in the ultimate
00:43:32 laws of physics.
00:43:33 But my feeling that way is because so far, that’s where all the evidence has been pointing.
00:43:38 I could be wrong.
00:43:39 And there’s certainly a sense in which it would be infinitely cool if somehow observation
00:43:45 or mental cogitation did play a fundamental role in the nature of reality.
00:43:52 But I don’t think so.
00:43:53 And again, I don’t see any evidence for it.
00:43:54 So I’m not spending a lot of time worrying about that possibility.
00:43:58 So what do you do about the fact that in the textbook interpretation of quantum mechanics,
00:44:02 this idea of measurement or looking at things seems to play an important role?
00:44:07 Well, you come up with better interpretations of quantum mechanics and there are several
00:44:11 alternatives.
00:44:12 My favorite is the many worlds interpretation, which says two things.
00:44:17 Number one, you, the observer, are just a quantum system like anything else.
00:44:22 There’s nothing special about you.
00:44:23 Don’t get so proud of yourself, you know, you’re just a bunch of atoms.
00:44:27 You have a wave function, you obey the Schrodinger equation like everything else.
00:44:31 And number two, when you think you’re measuring something or observing something, what’s really
00:44:35 happening is you’re becoming entangled with that thing.
00:44:40 So when you think there’s a wave function for the electron, it’s all spread out.
00:44:43 But you look at it and you only see it in one location.
00:44:46 What’s really happening is that there’s still the wave function for the electron in all
00:44:50 those locations.
00:44:51 But now it’s entangled with the wave function of you in the following way.
00:44:55 There’s part of the wave function that says the electron was here and you think you saw
00:44:59 it there.
00:45:00 The electron was there and you think you saw it there.
00:45:02 The electron was over there and you think you saw it there, etc.
00:45:05 So in all of those different parts of the wave function, once they come into being,
00:45:10 no longer talk to each other.
00:45:11 They no longer interact or influence each other.
00:45:13 It’s as if they are separate worlds.
00:45:16 So this was the invention of Hugh Everett III, who was a graduate student at Princeton
00:45:21 in the 1950s.
00:45:22 And he said, basically, look, you don’t need all these extra rules about looking at things.
00:45:28 Just listen to what the Schrodinger equation is telling you.
00:45:31 It’s telling you that you have a wave function, that you become entangled, and that the different
00:45:35 versions of you no longer talk to each other.
00:45:37 So just accept it.
00:45:39 It’s just he did therapy more than anything else.
00:45:41 He said, like, it’s okay.
00:45:42 You don’t need all these extra rules.
00:45:45 All you need to do is believe the Schrodinger equation.
00:45:47 The cost is there’s a whole bunch of extra worlds out there.
00:45:50 So are the worlds being created whether there’s an observer or not?
00:45:57 The worlds are created any time a quantum system that’s in a superposition becomes entangled
00:46:01 with the outside world.
00:46:04 What’s the outside world?
00:46:06 It depends.
00:46:07 Let’s back up.
00:46:08 Whatever it really says, what his theory is, is there’s a wave function of the universe
00:46:13 and it obeys the Schrodinger equation all the time.
00:46:17 That’s it.
00:46:18 That’s the full theory right there.
00:46:20 The question, all of the work is how in the world do you map that theory onto reality,
00:46:27 onto what we observe?
00:46:29 So part of it is carving up the wave function into these separate worlds, saying, look,
00:46:33 it describes a whole bunch of things that don’t interact with each other.
00:46:36 Let’s call them separate worlds.
00:46:38 Another part is distinguishing between systems and their environments.
00:46:41 The environment is basically all the degrees of freedom, all the things going on in the
00:46:45 world that you don’t keep track of.
00:46:48 So again, in the bottle of water, I might keep track of the total amount of water and
00:46:52 the volume.
00:46:53 I don’t keep track of the individual positions and velocities.
00:46:57 I don’t keep track of all the photons or the air molecules in this room.
00:47:00 So that’s the outside world.
00:47:02 The outside world is all the parts of the universe that you’re not keeping track of
00:47:06 when you’re asking about the behavior of subsystem of it.
00:47:10 So how many worlds are there?
00:47:14 Yeah, we don’t know that one either.
00:47:17 There could be an infinite number.
00:47:18 There could be only a finite number, but it’s a big number one way or the other.
00:47:21 It’s just a very, very big number.
00:47:23 In one of the talks, somebody asked, well, if it’s finite.
00:47:32 So actually I’m not sure exactly the logic you used to derive this, but is there going
00:47:38 to be overlap, a duplicate world that you return to?
00:47:47 So you’ve mentioned, and I’d love if you can elaborate on sort of idea that it’s possible
00:47:52 that there’s some kind of equilibrium that these splitting worlds arrive at and then
00:47:56 maybe over time, maybe somehow connected to entropy, you get a large number of worlds
00:48:03 that are very similar to each other.
00:48:05 Yeah.
00:48:06 So this question of whether or not Hilbert space is finite or infinite dimensional is
00:48:11 actually secretly connected to gravity and cosmology.
00:48:16 This is the part that we’re still struggling to understand right now, but we discovered
00:48:19 back in 1998 that our universe is accelerating and what that means if it continues, which
00:48:25 we think it probably will, but we’re not sure.
00:48:26 But if it does, that means there’s a horizon around us.
00:48:31 Because the universe is not only expanding, but expanding faster and faster, things can
00:48:34 get so far away from us that from our perspective, it looks like they’re moving away faster in
00:48:40 the speed of light.
00:48:41 We will never see them again.
00:48:42 So there’s literally a horizon around us and that horizon approaches some fixed distance
00:48:47 away from us.
00:48:48 And you can then argue that within that horizon, there’s only a finite number of things that
00:48:53 can possibly happen, the finite dimensional Hilbert space.
00:48:55 In fact, we even have a guess for what the dimensionality is.
00:48:59 It’s 10 to the power of 10 to the power of 122.
00:49:04 That’s a very large number.
00:49:05 Yes.
00:49:06 Just to compare, the age of the universe is something like 10 to the 14 seconds, 10 to
00:49:11 the 17 or 18 seconds maybe.
00:49:13 The number of particles in the universe is 10 to the 88th.
00:49:16 But the number of dimensions of Hilbert space is 10 to the 10 to the 122.
00:49:21 So that’s just crazy big.
00:49:23 If that story is right, that in our observable horizon, there’s only a finite dimensional
00:49:28 Hilbert space, then this idea of branching of the wave function of the universe into
00:49:32 multiple distinct separate branches has to reach a limit at some time.
00:49:37 Once you branch that many times, you’ve run out of room in Hilbert space.
00:49:41 And roughly speaking, that corresponds to the universe just expanding and emptying out
00:49:46 and cooling off and entering a phase where it’s just empty space, literally forever.
00:49:53 What’s the difference between splitting and copying, do you think?
00:49:58 In terms of, a lot of this is an interpretation that helps us sort of model the world.
00:50:09 So perhaps shouldn’t be thought of as like, you know, philosophically or metaphysically.
00:50:16 But in even at the physics level, do you see a difference between generating new copies
00:50:24 of the world or splitting?
00:50:26 I think it’s better to think of in quantum mechanics in many worlds, the universe splits
00:50:31 rather than new copies, because people otherwise worry about things like energy conservation.
00:50:36 And no one who understands quantum mechanics worries about energy conservation, because
00:50:40 the equation is perfectly clear.
00:50:42 But if all you know is that someone told you the universe duplicates, then you have a reasonable
00:50:45 worry about where all the energy for that came from.
00:50:48 So a pre existing universe splitting into two skinnier universes is a better way of
00:50:53 thinking about it.
00:50:54 And mathematically, it’s just like, you know, if you draw an x and y axis, and you draw
00:50:58 a vector of length one, 45 degree angle, you know that you can write that vector of length
00:51:04 one as the sum of two vectors pointing along x and y of length one over the square root
00:51:10 of two.
00:51:11 Okay, so I write one arrow as the sum of two arrows.
00:51:14 But there’s a conservation of arrowness, right?
00:51:17 Like there’s now two arrows, but the length is the same, I just I’m describing it in a
00:51:20 different way.
00:51:21 And that’s exactly what happens when the universe branches, the the wave function of the universe
00:51:25 is a big old vector.
00:51:27 So to somebody who brings up a question of saying, doesn’t this violate the conservation
00:51:34 of energy?
00:51:35 Can you give further elaboration?
00:51:38 Right?
00:51:39 So let’s just be super duper perfectly clear.
00:51:42 There’s zero question about whether or not many worlds violates conservation of energy.
00:51:46 Yes, it does not.
00:51:47 Great.
00:51:48 And I say this definitively, because there are other questions that I think there’s answers
00:51:51 to, but they’re legitimate questions, right about, you know, where does probability come
00:51:55 from and things like that, this conservation of energy question, we know the answer to
00:51:59 it.
00:52:00 And the answer to it is that energy is conserved.
00:52:03 All of the effort goes into how best to translate what the equation unambiguously says into
00:52:09 plain English, right?
00:52:11 So this idea that there’s a universe that has that that the universe comes equipped
00:52:14 with a thickness, and it sort of divides up into thinner pieces, but the total amount
00:52:18 of universe is is conserved over time, is a reasonably good way of putting English words
00:52:25 to the underlying mathematics.
00:52:27 So one of my favorite things about many worlds is, I mean, I love that there’s something
00:52:33 controversial in science.
00:52:35 And for some reason, it makes people actually not like upset, but just get excited.
00:52:41 Why do you think it is a controversial idea?
00:52:45 So there’s a lot of, it’s actually one of the cleanest ways to think about quantum mechanics.
00:52:52 So why do you think there’s a discomfort a little bit among certain people?
00:52:57 Well, I draw the distinction in my book between two different kinds of simplicity in a physical
00:53:02 theory.
00:53:03 There’s simplicity in the theory itself, right?
00:53:06 How we describe what’s going on according to the theory by its own rights.
00:53:10 But then, you know, theory is just some sort of abstract mathematical formalism, you have
00:53:13 to map it onto the world somehow, right?
00:53:16 And sometimes, like for Newtonian physics, it’s pretty obvious, like, okay, here is a
00:53:23 bottle and has a center of mass and things like that.
00:53:26 Sometimes it’s a little bit harder with general relativity, curvature of space time is a little
00:53:30 bit harder to grasp.
00:53:33 quantum mechanics is very hard to map what you’re the language you’re talking in a wave
00:53:37 functions and things like that on to reality.
00:53:40 And many worlds is the version of quantum mechanics where it is hardest to map on the
00:53:45 underlying formalism to reality.
00:53:47 So that’s where the lack of simplicity comes in, not in the theory, but in how we use the
00:53:53 theory to map on to reality.
00:53:54 In fact, all of the work in sort of elaborating many worlds quantum mechanics is in the this
00:54:01 effort to map it on to the world that we see.
00:54:04 So it’s perfectly legitimate to be bugged by that, right?
00:54:08 To say like, well, no, that’s just too far away from my experience, I am therefore intrinsically
00:54:15 skeptical of it.
00:54:16 Of course, you should give up on that skepticism if there are no alternatives.
00:54:19 And this theory always keeps working, then eventually you should overcome your skepticism.
00:54:23 But right now there are alternatives that are that, you know, people work to make alternatives
00:54:28 that are by their nature closer to what we observe directly.
00:54:31 Can you describe the alternatives?
00:54:33 I don’t think we touched on it, sort of the Copenhagen interpretation and the many worlds.
00:54:40 Maybe there’s a difference between the Everettian many worlds and many worlds as it is now,
00:54:47 like has the idea sort of developed and so on.
00:54:50 And just in general, what is the space of promising contenders?
00:54:54 We have democratic debates now, there’s a bunch of candidates.
00:54:57 12 candidates on stage.
00:54:59 What are the quantum mechanical candidates on stage for the debate?
00:55:02 So if you had a debate between quantum mechanical contenders, there’d be no problem getting
00:55:08 12 people up there on stage, but there would still be only three front runners.
00:55:14 And right now the front runners would be Everett, hidden variable theories are another one.
00:55:19 So the hidden variable theories say that the wave function is real, but there’s something
00:55:24 in addition to the wave function.
00:55:25 The wave function is not everything, it’s part of reality, but it’s not everything.
00:55:29 What else is there?
00:55:30 We’re not sure, but in the simplest version of the theory, there are literally particles.
00:55:36 So many worlds says that quantum systems are sometimes are wave like in some ways and particle
00:55:43 like in another because they really, really are waves, but under certain observational
00:55:48 circumstances they look like particles.
00:55:50 Whereas hidden variable says they look like waves and particles because there are both
00:55:54 waves and particles involved in the dynamics.
00:55:58 And that’s easy to do if your particles are just non relativistic Newtonian particles
00:56:03 moving around.
00:56:04 They get pushed around by the wave function roughly.
00:56:07 It becomes much harder when you take quantum field theory or quantum gravity into account.
00:56:13 The other big contender are spontaneous collapse theories.
00:56:17 So in the conventional textbook interpretation, we say when you look at a quantum system,
00:56:22 its wave function collapses and you see it in one location, a spontaneous collapse theory
00:56:27 says that every particle has a chance per second of having its wave function spontaneously
00:56:35 collapse.
00:56:36 The chance is very small for a typical particle, it will take hundreds of millions of years
00:56:39 before it happens even once, but in a table or some macroscopic object, there are way
00:56:44 more than a hundred million particles and they’re all entangled with each other.
00:56:48 So when one of them collapses, it brings everything else along with it.
00:56:52 There’s a slight variation of this.
00:56:54 That’s a spontaneous collapse theory.
00:56:55 There are also induced collapse theories like Roger Penrose thinks that when the gravitational
00:57:00 difference between two parts of the wave function becomes too large, the wave function collapses
00:57:05 automatically.
00:57:06 So those are basically in my mind, the three big alternatives, many worlds, which is just
00:57:11 there’s a wave function and always obeys the Schrodinger equation, hidden variables.
00:57:16 There’s a wave function that always obeys the Schrodinger equation, but there are also
00:57:18 new variables or collapse theories, which the wave function sometimes obeys the Schrodinger
00:57:24 equation and sometimes it collapses.
00:57:26 So you can see that the alternatives are more complicated in their formalism than many worlds
00:57:31 is, but they are closer to our experience.
00:57:34 So just this moment of collapse, do you think of it as a wave function, fundamentally sort
00:57:42 of a probabilistic description of the world and this collapse sort of reducing that part
00:57:49 of the world into something deterministic, where again, you can now describe the position
00:57:53 and the velocity in this simple classical model?
00:57:56 Well there is…
00:57:57 Is that how you think about collapse?
00:57:58 There is a fourth category, there’s a fourth contender, there’s a mayor Pete of quantum
00:58:03 mechanical interpretations, which are called epistemic interpretations.
00:58:08 And what they say is all the wave function is, is a way of making predictions for experimental
00:58:13 outcomes.
00:58:14 It’s not mapping onto an element of reality in any real sense.
00:58:18 And in fact, two different people might have two different wave functions for the same
00:58:22 physical system because they know different things about it, right?
00:58:25 The wave function is really just a prediction mechanism.
00:58:28 And then the problem with those epistemic interpretations is if you say, okay, but it’s
00:58:32 predicting about what, like what is the thing that is being predicted?
00:58:37 And they say, no, no, no, that’s not what we’re here for.
00:58:41 We’re just here to tell you what the observational outcomes are going to be.
00:58:44 But the other, the other interpretations kind of think that the wave function is real.
00:58:49 Yes, that’s right.
00:58:50 So that’s an ontic interpretation of the wave function, ontology being the study of what
00:58:55 is real, what exists, as opposed to an epistemic interpretation of the wave function, epistemology
00:59:01 being the study of what we know.
00:59:02 That would actually just love to see that debate on stage.
00:59:06 There was a version of it on stage at the world science festival a few years ago that
00:59:10 you can look up online.
00:59:11 On YouTube?
00:59:12 Yep.
00:59:13 It’s on YouTube.
00:59:14 Okay, awesome.
00:59:15 I’ll link it and watch it.
00:59:16 Who won?
00:59:17 I won.
00:59:18 I don’t know, there was no vote, there was no vote, but those there’s Brian Green was
00:59:24 the moderator and David Albert stood up for a spontaneous collapse and Shelley Goldstein
00:59:29 was there for hidden variables and Rüdiger Schock was there for epistemic approaches.
00:59:34 Why do you, I think you mentioned it, but just to elaborate, why do you find many worlds
00:59:38 so compelling?
00:59:39 Well, there’s two reasons actually.
00:59:43 One is, like I said, it is the simplest, right?
00:59:45 It’s like the most bare bones, austere, pure version of quantum mechanics.
00:59:49 And I am someone who is very willing to put a lot of work into mapping the formalism onto
00:59:55 reality.
00:59:56 I’m less willing to complicate the formalism itself.
00:59:59 But the other big reason is that there’s something called modern physics with quantum fields
01:00:04 and quantum gravity and holography and space time doing things like that.
01:00:09 And when you take any of the other versions of quantum theory, they bring along classical
01:00:14 baggage, all of the other versions of quantum mechanics, prejudice or privilege some version
01:00:21 of classical reality like locations in space, okay?
01:00:26 And I think that that’s a barrier to doing better at understanding the theory of everything
01:00:31 and understanding quantum gravity and the emergence of space time.
01:00:34 Whenever if you change your theory from, you know, here’s a harmonic oscillator, oh, there’s
01:00:38 a spin, here’s an electromagnetic field, in hidden variable theories or dynamical collapse
01:00:43 theories.
01:00:44 You have to start from scratch.
01:00:45 You have to say like, well, what are the hidden variables for this theory or how does it collapse
01:00:48 or whatever?
01:00:49 Whereas many worlds is plug and play.
01:00:50 You tell me the theory and I can give you as many worlds version.
01:00:53 So when we have a situation like we have with gravity and space time, where the classical
01:00:58 description seems to break down in a dramatic way, then I think you should start from the
01:01:04 most quantum theory that you have, which is really many worlds.
01:01:07 So start with the quantum theory and try to build up a model of space time, the emergence
01:01:14 of space time.
01:01:15 That’s it.
01:01:16 Okay.
01:01:17 So I thought space time was fundamental.
01:01:21 Yeah, I know.
01:01:22 So this sort of dream that Einstein had that everybody had and everybody has of, you know,
01:01:28 the theory of everything.
01:01:30 So how do we build up from many worlds from quantum mechanics, a model of space time model
01:01:37 of gravity?
01:01:38 Well, yeah, I mean, let me first mention very quickly why we think it’s necessary.
01:01:42 You know, we’ve had gravity in the form that Einstein bequeathed it to us for over a hundred
01:01:47 years now, like 1915 or 1916, he put general relativity in the final form.
01:01:52 So gravity is the curvature of space time and there’s a field that pervades all the
01:01:57 universe that tells us how curved space time is.
01:02:00 And that’s a fundamentally classical.
01:02:01 That’s totally classical.
01:02:02 Right.
01:02:03 Exactly.
01:02:04 But we also have a formalism, an algorithm for taking a classical theory and quantizing
01:02:09 it.
01:02:10 This is how we get quantum electrodynamics, for example.
01:02:13 And it could be tricky.
01:02:14 I mean, you think you’re quantizing something, so that means taking a classical theory and
01:02:19 promoting it to a quantum mechanical theory.
01:02:22 But you can run into problems.
01:02:23 So they ran into problems and they did that with electromagnetism, namely that certain
01:02:27 quantities were infinity and you don’t like infinity, right?
01:02:30 So Feynman and Tominaga and Schwinger won the Nobel Prize for teaching us how to deal
01:02:35 with the infinities.
01:02:36 And then Ken Wilson won another Nobel Prize for saying you shouldn’t have been worried
01:02:39 about those infinities after all.
01:02:41 But still, that was the, it’s always the thought that that’s how you will make a good quantum
01:02:45 theory.
01:02:46 You’ll start with a classical theory and quantize it.
01:02:47 So if we have a classical theory, general relativity, we can quantize it or we can try
01:02:51 to, but we run into even bigger problems with gravity than we ran into with electromagnetism.
01:02:57 And so far, those problems are insurmountable.
01:03:00 We’ve not been able to get a successful theory of gravity, quantum gravity, by starting with
01:03:04 classical general relativity and quantizing it.
01:03:08 And there’s evidence that, there’s a good reason why this is true, that whatever the
01:03:12 quantum theory of gravity is, it’s not a field theory.
01:03:17 It’s something that has weird nonlocal features built into it somehow that we don’t understand.
01:03:22 We get this idea from black holes and Hawking radiation and information conservation and
01:03:27 a whole bunch of other ideas I talk about in the book.
01:03:30 So if that’s true, if the fundamental theory isn’t even local in the sense that an ordinary
01:03:34 quantum field theory would be, then we just don’t know where to start in terms of getting
01:03:39 a classical precursor and quantizing it.
01:03:42 So the only sensible thing, or at least the next obvious sensible thing to me would be
01:03:46 to say, okay, let’s just start intrinsically quantum and work backwards, see if we can
01:03:50 find a classical limit.
01:03:51 So the idea of locality, the fact that locality is not fundamental to the nature of our existence,
01:04:03 I guess in that sense, modeling everything as a field makes sense to me.
01:04:07 Stuff that’s close by interacts, stuff that’s far away doesn’t.
01:04:12 So what’s locality and why is it not fundamental?
01:04:15 And how is that even possible?
01:04:16 Yeah.
01:04:17 I mean, locality is the answer to the question that Isaac Newton was worried about back in
01:04:21 the beginning of our conversation, right?
01:04:22 I mean, how can the earth know what the gravitational field of the sun is?
01:04:27 And the answer as spelled out by Laplace and Einstein and others is that there’s a field
01:04:31 in between.
01:04:32 And the way a field works is that what’s happening to the field at this point in space only depends
01:04:38 directly on what’s happening at points right next to it.
01:04:41 But what’s happening at those points depends on what’s happening right next to those, right?
01:04:45 And so you can build up an influence across space through only local interactions.
01:04:50 That’s what locality means.
01:04:51 What happens here is only affected by what’s happening right next to it.
01:04:54 That’s locality.
01:04:57 The idea of locality is built into every field theory, including general relativity as a
01:05:01 classical theory.
01:05:03 It seems to break down when we talk about black holes and, you know, Hawking taught
01:05:07 us in the 1970s that black holes radiate, they give off, they eventually evaporate away.
01:05:12 They’re not completely black once we take quantum mechanics into account.
01:05:17 And we think, we don’t know for sure, but most of us think that if you make a black
01:05:22 hole out of certain stuff, then like Laplace’s demon taught us, you should be able to predict
01:05:28 what that black hole will turn into if it’s just obeying the Schrodinger equation.
01:05:32 And if that’s true, there are good arguments that can’t happen while preserving locality
01:05:37 at the same time.
01:05:38 It’s just that the information seems to be spread out nonlocally in interesting ways.
01:05:44 And people should, you talk about holography with the Leonard Susskind on your Mindscape
01:05:49 podcast.
01:05:50 Oh yes, I have a podcast.
01:05:51 I didn’t even mention that.
01:05:52 This is terrible.
01:05:53 No, I’m going to, I’m going to ask you questions about that too, and I’ve been not shutting
01:05:57 up about it.
01:05:58 It’s my favorite science podcast.
01:05:59 So, or not, it’s a, it’s not even a science podcast.
01:06:02 It’s like, it’s a scientist doing a podcast.
01:06:06 That’s right.
01:06:07 That’s what it is.
01:06:08 Yeah.
01:06:09 Anyway.
01:06:10 Yeah.
01:06:11 So holography is this idea when you have a black hole and black hole is a region of space
01:06:14 inside of which gravity is so strong that you can’t escape.
01:06:17 And there’s this weird feature of black holes that, again, it’s totally a thought experiment
01:06:21 feature because we haven’t gone and probed any yet.
01:06:24 But there seems to be one way of thinking about what happens inside a black hole as
01:06:29 seen by an observer who’s falling in, which is actually pretty normal.
01:06:33 Like everything looks pretty normal until you hit the singularity and you die.
01:06:37 But from the point of view of the outside observer, it seems like all the information
01:06:41 that fell in is actually smeared over the horizon in a nonlocal way.
01:06:47 And that’s puzzling and that’s, so holography because that’s a two dimensional surface that
01:06:51 is encapsulating the whole three dimensional thing inside, right?
01:06:55 Still trying to deal with that.
01:06:56 Still trying to figure out how to get there.
01:06:58 But it’s an indication that we need to think a little bit more subtly when we quantize
01:07:01 gravity.
01:07:02 And because you can describe everything that’s going on in the three dimensional space by
01:07:07 looking at the two dimensional projection of it, it means that locality doesn’t, it’s
01:07:13 not necessary.
01:07:14 Well, it means that somehow it’s only a good approximation.
01:07:18 It’s not really what’s going on.
01:07:20 How are we supposed to feel about that?
01:07:22 We’re supposed to feel liberated.
01:07:25 You know, space is just a good approximation and this was always going to be true once
01:07:29 you started quantizing gravity.
01:07:31 So we’re just beginning now to face up to the dramatic implications of quantizing gravity.
01:07:38 Is there other weird stuff that happens to quantum mechanics in black hole?
01:07:43 I don’t think that anything weird has happened with quantum mechanics.
01:07:47 I think weird things happen with space time.
01:07:48 I mean, that’s what it is.
01:07:49 Like quantum mechanics is still just quantum mechanics, but our ordinary notions of space
01:07:53 time don’t really quite work.
01:07:57 And there’s a principle that goes hand in hand with holography called complementarity,
01:08:03 which says that there’s no one unique way to describe what’s going on inside a black
01:08:10 hole.
01:08:11 Different observers will have different descriptions, both of which are accurate, but sound completely
01:08:17 incompatible with each other.
01:08:18 So depends on how you look at it.
01:08:20 The word complementarity in this context is borrowed from Niels Bohr, who points out you
01:08:25 can measure the position or you can measure the momentum.
01:08:28 You can’t measure both at the same time in quantum mechanics.
01:08:30 So a couple of questions on many worlds.
01:08:34 How does many worlds help us understand our particular branch of reality?
01:08:40 So okay, that’s fine and good that is everything is splitting, but we’re just traveling down
01:08:45 a single branch of it.
01:08:46 So how does it help us understand our little unique branch?
01:08:50 Yeah, I mean, that’s a great question.
01:08:52 But that’s the point is that we didn’t invent many worlds because we thought it was cool
01:08:56 to have a whole bunch of worlds, right?
01:08:57 We invented it because we were trying to account for what we observe here in our world.
01:09:02 And what we observe here in our world are wave functions collapsing, okay?
01:09:07 We do have a position, a situation where the electron seems to be spread out.
01:09:11 But then when we look at it, we don’t see it spread out.
01:09:13 We see it located somewhere.
01:09:15 So what’s going on?
01:09:16 That’s the measurement problem of quantum mechanics.
01:09:17 That’s what we have to face up to.
01:09:19 So many worlds is just a proposed solution to that problem.
01:09:22 And the answer is nothing special is happening.
01:09:25 It’s still just the Schrodinger equation, but you have a wave function too.
01:09:30 And that’s a different answer than would be given in hidden variables or dynamical collapse
01:09:34 theories or whatever.
01:09:36 So the entire point of many worlds is to explain what we observe, but it tries to explain what
01:09:42 we already have observed, right?
01:09:44 It’s not trying to be different from what we’ve observed because that would be something
01:09:48 other than quantum mechanics.
01:09:50 But you know, the idea that there’s worlds that we didn’t observe that keep branching
01:09:54 off is kind of, it’s stimulating to the imagination.
01:10:00 So is it possible to hop from, you mentioned the branches are independent.
01:10:08 Is it possible to hop from one to the other?
01:10:10 No.
01:10:11 So it’s a physical limit.
01:10:14 The theory says it’s impossible.
01:10:16 There’s already a copy of you in the other world, don’t worry.
01:10:18 Yes.
01:10:19 Leave them alone.
01:10:20 No, but there’s a fear of missing out, FOMO, that I feel like immediately start to wonder
01:10:28 if that other copy is having more or less fun.
01:10:32 Well, the downside to many worlds is that you’re missing out on an enormous amount.
01:10:38 And that’s always what it’s going to be like.
01:10:40 And I mean, there’s a certain stage of acceptance in that.
01:10:44 In terms of rewinding, do you think we can rewind the system back, sort of the nice thing
01:10:49 about many worlds, I guess, is it really emphasizes the, maybe you can correct me, but the deterministic
01:10:58 nature of a branch and it feels like it could be rewound back.
01:11:05 Is it, do you see it as something that could be perfectly rewound back, rewinding back?
01:11:11 Yeah.
01:11:12 If you’re at a fancy French restaurant and there’s a nice linen white tablecloth and
01:11:17 you have your glass of Bordeaux and you knock it over and the wine spills across the tablecloth.
01:11:22 If the world were classical, okay, it would be possible that if you just lifted the wine
01:11:28 glass up, you’d be lucky enough that every molecule of wine would hop back into the glass,
01:11:33 right?
01:11:34 But guess what?
01:11:35 It’s not going to happen in the real world.
01:11:37 And the quantum wave function is exactly the same way.
01:11:40 It is possible in principle to rewind everything if you start from perfect knowledge of the
01:11:44 entire wave function of the universe.
01:11:46 In practice, it’s never going to happen.
01:11:49 So time travel, not possible.
01:11:53 Nope.
01:11:54 At least quantum mechanics has no help.
01:11:57 What about memory?
01:12:00 Does the universe have a memory of itself where we could, in, in, so not time travel,
01:12:07 but peek back in time and do a little like replay?
01:12:13 Well, it’s exactly the same in quantum mechanics as classical mechanics.
01:12:18 So whatever you want to say about that, you know, the fundamental laws of physics in either
01:12:22 many worlds, quantum mechanics or Newtonian physics conserve information.
01:12:28 So if you have all the information about the quantum state of the world right now, your
01:12:32 Laplace is demon like in your knowledge and calculational capacity, you can wind the clock
01:12:37 backward.
01:12:38 But none of us is.
01:12:39 Right?
01:12:40 And, you know, so in practice you can never do that.
01:12:42 You can do experiments over and over again, starting from the same initial conditions
01:12:46 for small systems.
01:12:47 But once things get to be large, Avogadro’s number of particles, right?
01:12:51 Bigger than a cell, no chance.
01:12:54 We we’ve talked a little bit about arrow of time last time, but in many worlds that there
01:13:00 is a kind of implied arrow of time, right?
01:13:07 So you’ve talked about the arrow of time that has to do with the second law of thermodynamics.
01:13:14 That’s the arrow of time that’s emergent or fundamental.
01:13:18 We don’t know, I guess.
01:13:19 No, it’s emergent.
01:13:20 Is that, does everyone agree on that?
01:13:23 Well, nobody agrees with everything.
01:13:25 They should.
01:13:26 They should.
01:13:28 So that arrow of time, is that different than the arrow of time that’s implied by many worlds?
01:13:34 It’s not different, actually, no.
01:13:35 In both cases, you have fundamental laws of physics that are completely reversible.
01:13:40 If you give me the state of the universe at one moment in time, I can run the clock forward
01:13:45 or backward equally well.
01:13:46 There’s no arrow of time built into the laws of physics at the most fundamental level.
01:13:51 But what we do have are special initial conditions 14 billion years ago near the Big Bang.
01:13:57 In thermodynamics, those special initial conditions take the form of things were low entropy and
01:14:03 entropy has been increasing ever since, making the universe more disorganized and chaotic
01:14:08 and that’s the arrow of time.
01:14:10 In quantum mechanics, the special initial conditions take the form of there was only
01:14:15 one branch of the wave function and the universe has been branching more and more ever since.
01:14:20 Okay, so if time is emergent, so it seems like our human cognitive capacity likes to
01:14:28 take things that are emergent and assume and feel like they’re fundamental.
01:14:33 So what, so if time is emergent and locality, like is space emergent?
01:14:42 Yes.
01:14:43 Okay.
01:14:44 But I didn’t say time was emergent, I said the arrow of time was emergent.
01:14:46 Those are different.
01:14:47 What’s the difference between the arrow of time and time?
01:14:52 Are you using arrow of time to simply mean this, they’re synonymous with the second law
01:14:56 of thermodynamics?
01:14:57 No, but the arrow of time is the difference between the past and future.
01:15:01 So there’s space, but there’s no arrow of space.
01:15:04 You don’t feel that space has to have an arrow, right?
01:15:06 You could live in thermodynamic equilibrium, there’d be no arrow of time, but there’d still
01:15:10 be time.
01:15:11 There’d still be a difference between now and the future or whatever.
01:15:14 So if nothing changes, there’s still time.
01:15:19 Well things could even change, like if the whole universe consisted of the earth going
01:15:23 around the sun, it would just go in circles or ellipses, right?
01:15:29 Things would change, but it’s not increasing entropy, there’s no arrow.
01:15:32 If you took a movie of that and I played you the movie backward, you would never know.
01:15:38 So the arrow of time can theoretically point in the other direction for briefly.
01:15:45 To the extent that it points in different directions, it’s not a very good arrow.
01:15:49 I mean, the arrow of time in the macroscopic world is so powerful that there’s just no
01:15:53 chance of going back.
01:15:55 When you get down to tiny systems with only three or four moving parts, then entropy can
01:15:58 fluctuate up and down.
01:16:00 What does it mean for space to be an emergent phenomenon?
01:16:03 It means that the fundamental description of the world does not include the word space.
01:16:07 It’ll be something like a vector in Hilbert space, right, and you have to say, well why
01:16:10 is there a good approximate description which involves three dimensional space and stuff
01:16:16 inside it?
01:16:17 Okay, so time and space are emergent.
01:16:21 We kind of mentioned in the beginning, can you elaborate, what do you feel hope is fundamental
01:16:28 in our universe?
01:16:30 A wave function living in Hilbert space.
01:16:32 A wave function in Hilbert space that we can’t intellectualize or visualize really.
01:16:36 We can’t visualize it, we can intellectualize it very easily.
01:16:40 Like how do you think about?
01:16:42 It’s a vector in a 10 to the 10 to the 122 dimensional vector space.
01:16:45 It’s a complex vector, unit norm, it evolves according to the Schrodinger equation.
01:16:50 Got it.
01:16:52 When you put it that way.
01:16:53 What’s so hard, really?
01:16:56 It’s like, yep, quantum computers, there’s some excitement, actually a lot of excitement
01:17:04 with people that it will allow us to simulate quantum mechanical systems.
01:17:10 What kind of questions do you about quantum mechanics, about the things we’ve been talking
01:17:14 about, do you think, do you hope we can answer through quantum simulation?
01:17:21 Well I think that there are, there’s a whole fascinating frontier of things you can do
01:17:26 with quantum computers.
01:17:27 Both sort of practical things with cryptography or money, privacy eavesdropping, sorting things,
01:17:37 simulating quantum systems, right?
01:17:40 So it’s a broader question maybe even outside of quantum computers.
01:17:44 Some of the theories that we’ve been talking about, what’s your hope, what’s most promising
01:17:49 to test these theories?
01:17:52 What are kind of experiments we can conduct, whether in simulation or in the physical world
01:17:57 that would validate or disprove or expand these theories?
01:18:03 Well I think for, there’s two parts of that question.
01:18:06 One is many worlds and the other one is sort of emergent space time.
01:18:10 For many worlds, you know, there are experiments ongoing to test whether or not wave functions
01:18:14 spontaneously collapse.
01:18:17 And if they do, then that rules out many worlds and that would be falsified.
01:18:21 If there are hidden variables, there’s a theorem that seems to indicate that the predictions
01:18:27 will always be the same as many worlds.
01:18:29 I’m a little skeptical of this theorem.
01:18:31 I’m not complete.
01:18:32 I haven’t internalized it.
01:18:33 I haven’t made it in part of my intuitive view of the world yet, so there might be loopholes
01:18:36 to that theorem.
01:18:37 I’m not sure about that.
01:18:38 Part of me thinks that there should be different experimental predictions if there are hidden
01:18:42 variables, but I’m not sure.
01:18:45 But otherwise, it’s just quantum mechanics all the way down.
01:18:47 And so there’s this cottage industry in science journalism of writing breathless articles
01:18:53 that say, you know, quantum mechanics shown to be more astonishing than ever before thought.
01:18:57 And really, it’s the same quantum mechanics we’ve been doing since 1926.
01:19:01 Whereas with the emergent space time stuff, we know a lot less about what the theory is.
01:19:06 It’s in a very primitive state.
01:19:07 We don’t even really have a safely written down, respectable, honest theory yet.
01:19:13 So there could very well be experimental predictions we just don’t know about yet.
01:19:17 That is one of the things that we’re trying to figure out.
01:19:20 Yeah, for emergent space time, you need really big stuff, right?
01:19:24 Well, or really fast stuff, or really energetic stuff.
01:19:27 We don’t know.
01:19:28 That’s the thing.
01:19:29 You know, so there could be violations of the speed of light if you have emergent space
01:19:34 time.
01:19:35 Not going faster than the speed of light, but the speed of light could be different
01:19:39 for light of different wavelengths, right?
01:19:42 That would be a dramatic violation of physics as we know it, but it could be possible.
01:19:46 Or not.
01:19:47 I mean, it’s not an absolute prediction.
01:19:48 That’s the problem.
01:19:49 The theories are just not well developed enough yet to say.
01:19:54 Is there anything that quantum mechanics can teach us about human nature or the human mind?
01:20:01 If you think about sort of consciousness and these kinds of topics, is there…
01:20:06 It’s certainly excessively used, as you point out.
01:20:10 The word quantum is used for everything besides quantum mechanics.
01:20:15 But in more seriousness, is there something that goes to the human level and can help
01:20:23 us understand our mind?
01:20:27 Not really is the short answer, you know.
01:20:29 Minds are pretty classical.
01:20:31 I don’t think.
01:20:32 We don’t know this for sure, but I don’t think that phenomena like entanglement are crucial
01:20:36 to how the human mind works.
01:20:38 What about consciousness?
01:20:40 So you mentioned, I think early on in the conversation, you said it would be unlikely,
01:20:48 but incredible if sort of the observer is somehow a fundamental part.
01:20:54 So observer, not to romanticize the notion, but seems interlinked to the idea of consciousness.
01:21:01 So if consciousness, as the panpsychists believe, is fundamental to the universe, is that possible?
01:21:09 Is that weight…
01:21:10 I mean, every…
01:21:11 Everything’s possible.
01:21:12 Just like Joe Rogan likes to say, it’s entirely possible.
01:21:16 But okay.
01:21:17 But is it on a spectrum of crazy out there?
01:21:22 How the statistically speaking, how often do you ponder the possibility that consciousness
01:21:28 is fundamental or the observer is fundamental to…
01:21:32 Personally don’t at all.
01:21:33 There are people who do.
01:21:34 I’m a thorough physicalist when it comes to consciousness.
01:21:37 I do not think that there are any separate mental states or mental properties.
01:21:41 I think they’re all emergent, just like space time is and space time is hard enough to understand.
01:21:46 So the fact that we don’t yet understand consciousness is not at all surprising to me.
01:21:51 You, as we mentioned, have an amazing podcast called Mindscape.
01:21:55 It’s as I said, one of my favorite podcasts sort of both for your explanation of physics,
01:22:03 which a lot of people love, and when you venture out into things that are beyond your expertise,
01:22:10 but it’s just a really smart person exploring even questions like morality, for example.
01:22:18 It’s very interesting.
01:22:19 I think you did a solo episode and so on.
01:22:21 I mean, there’s a lot of really interesting conversations that you have.
01:22:28 What are some from memory, amazing conversations that pop to mind that you’ve had?
01:22:34 What did you learn from them?
01:22:36 Something that maybe changed your mind or just inspired you or just what did this whole
01:22:40 experience of having conversations, what stands out to you?
01:22:45 It’s an unfair question.
01:22:46 Totally unfair.
01:22:47 That’s okay.
01:22:48 That’s all right.
01:22:49 You know, it’s often the ones I feel like the ones I do on physics and closely related
01:22:54 science or even philosophy ones are like, I know this stuff and I’m helping people learn
01:23:01 about it.
01:23:02 But I learn more from the ones that have nothing to do with physics or philosophy, right?
01:23:05 So talking to Wynton Marsalis about jazz or talking to a Master Sommelier about wine,
01:23:12 talking to Will Wilkinson about partisan polarization and the urban rural divide, talking to psychologists
01:23:18 like Carol Tavris about cognitive dissonance and how those things work.
01:23:26 Scott Derrickson who is the director of the movie Dr. Strange, I had a wonderful conversation
01:23:31 with him where we went through the mechanics of making a blockbuster superhero movie, right?
01:23:36 And he’s also not a naturalist, he’s an evangelical Christian so we talked about the nature of
01:23:41 reality there.
01:23:42 I want to have a couple more, you know, discussions with highly educated theists who know the
01:23:51 theology really well but I haven’t quite arranged those yet.
01:23:56 I would love to hear that.
01:23:57 I mean that’s, how comfortable are you venturing into questions of religion?
01:24:02 Oh, I’m totally comfortable doing it.
01:24:04 You know, I did talk with Alan Lightman who is also an atheist but he, you know, he is
01:24:10 trying to rescue the sort of spiritual side of things for atheism and I did talk to very
01:24:18 vocal atheists like Alex Rosenberg so I need to talk to some, I’ve talked to some religious
01:24:23 believers but I need to talk to more.
01:24:26 How have you changed through having all these conversations?
01:24:31 You know, part of the motivation was I had a long stack of books that I hadn’t read and
01:24:35 I couldn’t find time to read them and I figured if I interviewed their authors, forced me
01:24:38 to read them, right, and that has totally worked by the way.
01:24:42 Now I’m annoyed that people write such long books.
01:24:45 I think I’m still very much learning how to be a good interviewer.
01:24:49 I think that’s a skill that, you know, I think I have good questions but, you know, there’s
01:24:54 the give and take that is still I think I can be better at.
01:24:58 Like I want to offer something to the conversation but not too much, right?
01:25:02 I’ve had conversations where I barely talked at all and I have conversations where I talked
01:25:06 half the time and I think there’s a happy medium in between there.
01:25:09 So I think I remember listening to, without mentioning names, some of your conversations
01:25:13 where I wish you would have disagreed more.
01:25:16 As a listener, it’s more fun sometimes.
01:25:20 Well, that’s a very good question because, you know, everyone has an attitude toward
01:25:24 that.
01:25:25 Like some people are really there to basically give their point of view and their guest is
01:25:32 supposed to, you know, respond accordingly.
01:25:35 I want to sort of get my view on the record but I don’t want to dwell on it when I’m talking
01:25:41 to someone like David Chalmers who I disagree with a lot.
01:25:44 You know, I want to say like, here’s why I disagree with you but, you know, we’re here
01:25:48 to listen to you.
01:25:49 Like I have an episode every week and you’re only on once a week, right?
01:25:53 So I have an upcoming podcast episode with Philip Goff who is a much more dedicated pan
01:26:00 psychist and so there we really get into it.
01:26:02 I think that I probably have disagreed with him more on that episode than I ever have
01:26:06 with another podcast guest but that’s what he wanted so it worked very well.
01:26:10 Yeah, yeah.
01:26:11 That kind of debate structure is beautiful when it’s done right.
01:26:15 Like when you’re, when you can detect that the intent is that you have fundamental respect
01:26:21 for the person.
01:26:22 Yeah.
01:26:23 That, and that’s, for some reason, it’s super fun to listen to when two really smart people
01:26:29 are just arguing and sometimes lose their shit a little bit if I may say so.
01:26:32 Well, there’s a fine line because I have zero interest in bringing, I mean, like, I mean,
01:26:39 maybe you implied this, I have zero interest in bringing on people for whom I don’t have
01:26:43 any intellectual respect.
01:26:44 Like I constantly get requests like, you know, bring on a flat earther or whatever and really
01:26:49 slap them down or a creationist, like I have zero interest.
01:26:52 I’m happy to bring on, you know, a religious person, a believer, but I want someone who’s
01:26:56 smart and can act in good faith and can talk, not a charlatan or a lunatic, right?
01:27:02 So I will only, I will happily bring on people with whom I disagree, but only people from
01:27:08 whom I think the audience can learn something interesting.
01:27:10 So let me ask, the idea of charlatan is an interesting idea.
01:27:15 You might be more educated on this topic than me, but there’s, there’s folks, for example,
01:27:22 who argue various aspects of evolution sort of try to approach and say that evolution
01:27:31 sort of our current theory of evolution has many holes in it, has many flaws.
01:27:37 And they argue that I think like Cambridge, Cambrian explosion, which is like a huge added
01:27:46 variability of species, doesn’t make sense under our current description of evolution
01:27:52 and theory of evolution sort of, if you had to, were to have the conversation with people
01:27:57 like that, how do you know that they’re the difference in outside the box thinkers and
01:28:05 people who are fundamentally unscientific and even bordering on charlatans?
01:28:13 That’s a great question.
01:28:15 And you know, the further you get away from my expertise, the harder it is for me to really
01:28:19 judge exactly those things.
01:28:21 And, you know, yeah, I don’t have a satisfying answer for that one because I think the example
01:28:25 you use of someone who, you know, believes in the basic structure of natural selection,
01:28:30 but thinks that, you know, this particular thing cannot be understood in the terms of
01:28:35 our current understanding of Darwinism.
01:28:38 That’s a perfect edge case where it’s hard to tell, right?
01:28:41 And I would have, I would try to talk to people who I do respect and who do know things and
01:28:45 I would have to, you know, given that I’m a physicist, I know that physicists will sometimes
01:28:50 be too dismissive of alternative points of view.
01:28:53 I have to take into account that biologists can also be too dismissive of alternative points
01:28:57 of view.
01:28:58 So, yeah, that’s a tricky one.
01:29:00 Have you gotten heat yet?
01:29:02 I get heat all the time.
01:29:03 Like there’s always something, I mean, it’s hilarious because I do have, I try very hard
01:29:09 not to like have the same topic several times in a row.
01:29:12 I did have like two climate change episodes, but they were from very different perspectives,
01:29:16 but I like to mix it up.
01:29:17 That’s the whole, that’s why I’m having fun.
01:29:19 And every time I do an episode, someone says, oh, the person you should really get on to
01:29:22 talk about exactly that is this other person.
01:29:24 I’m like, well, I don’t, but I did that now.
01:29:26 I don’t want to do that anymore.
01:29:28 Well, I hope you keep doing it.
01:29:30 You’re inspiring millions of people, your books, your podcasts.
01:29:34 Sean, it’s an honor to talk to you.
01:29:36 Thank you so much.
01:29:37 Thank you very much, Lex.