Sean Carroll: Quantum Mechanics and the Many-Worlds Interpretation

00:00:00.000 | The following is a conversation with Sean Carroll, part two, the second time we've

00:00:04.640 | spoken on the podcast. You can get the link to the first time in the description.

00:00:09.120 | This time we focus on quantum mechanics and the many worlds interpretation that he details

00:00:15.280 | elegantly in his new book titled Something Deeply Hidden. I own and enjoy both the ebook

00:00:21.840 | and audiobook versions of it. Listening to Sean read about entanglement, complementarity,

00:00:28.240 | and the emergence of space-time reminds me of Bob Ross teaching the world how to paint

00:00:33.200 | on his old television show. If you don't know who Bob Ross is, you're truly missing out. Look him up,

00:00:40.320 | he'll make you fall in love with painting. Sean Carroll is the Bob Ross of theoretical physics.

00:00:47.200 | He's the author of several popular books, a host of a great podcast called Mindscape,

00:00:53.120 | and is a theoretical physicist at Caltech and the Santa Fe Institute, specializing in quantum

00:00:59.680 | mechanics, arrow of time, cosmology, and gravitation. This is the Artificial Intelligence Podcast.

00:01:06.800 | If you enjoy it, subscribe on YouTube, give it five stars on iTunes, support it on Patreon,

00:01:12.480 | or simply connect with me on Twitter @LexFriedman, spelled F-R-I-D-M-A-N.

00:01:18.320 | And now, here's my conversation with Sean Carroll. Isaac Newton developed what we now call

00:01:25.840 | classical mechanics that you describe very nicely in your new book, as you do with a lot of basic

00:01:30.560 | concepts in physics. So, with classical mechanics, I can throw a rock and can predict the trajectory

00:01:38.640 | of that rock's flight. But if we could put ourselves back into Newton's time, his theories

00:01:45.440 | work to predict things, but as I understand, he himself thought that their interpretations

00:01:52.080 | of those predictions were absurd. Perhaps he just said it for religious reasons and so on.

00:01:58.800 | But in particular, sort of a world of interaction without contact, so action at a distance,

00:02:05.360 | it didn't make sense to him on a sort of a human interpretation level. Does it make sense to you

00:02:11.600 | that things can affect other things at a distance?

00:02:14.960 | >> It does, but you know, so that was one of Newton's worries. You're actually right

00:02:20.720 | in a slightly different way about the religious worries. He was smart enough, this is off the

00:02:26.560 | topic but still fascinating, Newton almost invented chaos theory as soon as he invented

00:02:31.600 | classical mechanics. He realized that in the solar system, so he was able to explain how planets move

00:02:37.280 | around the sun, but typically you would describe the orbit of the Earth ignoring the effects of

00:02:43.120 | Jupiter and Saturn and so forth, just doing the Earth and the sun. He kind of knew, even though

00:02:48.240 | he couldn't do the math, that if you included the effects of Jupiter and Saturn, the other planets,

00:02:52.400 | the solar system would be unstable, like the orbits of the planets would get out of whack.

00:02:56.800 | So, he thought that God would intervene occasionally to sort of move the planets

00:03:00.240 | back into orbit, which is the only way you could explain how they were there presumably forever.

00:03:05.200 | But the worries about classical mechanics were a little bit different, the worry about gravity in

00:03:08.960 | particular. It wasn't a worry about classical mechanics, it was a worry about gravity.

00:03:12.160 | How in the world does the Earth know that there's something called the sun 93 million miles away

00:03:17.600 | that is exerting a gravitational force on it? And he literally said, "I leave that for future

00:03:22.960 | generations to think about because I don't know what the answer is." And in fact, people

00:03:28.080 | underemphasized this, but future generations figured it out. Pierre-Simone Laplace in circa

00:03:33.840 | 1800 showed that you could rewrite Newtonian gravity as a field theory. So, instead of just

00:03:39.920 | talking about the force due to gravity, you can talk about the gravitational field or the

00:03:44.320 | gravitational potential field. And then there's no action at a distance. It's exactly the same

00:03:48.960 | theory empirically, it makes exactly the same predictions. But what's happening is instead of

00:03:53.280 | the sun just reaching out across the void, there is a gravitational field in between the sun and

00:03:58.480 | the Earth that obeys an equation, Laplace's equation, cleverly enough. And that tells us

00:04:04.400 | exactly what the field does. So, even in Newtonian gravity, you don't need action at a distance.

00:04:09.680 | Now, what many people say is that Einstein solved this problem because he invented general relativity.

00:04:14.960 | And in general relativity, there's certainly a field in between the Earth and the sun. But also,

00:04:19.760 | there's the speed of light as a limit. In Laplace's theory, which was exactly Newton's theory, just in

00:04:24.560 | a different mathematical language, there could still be instantaneous action across the universe.

00:04:29.520 | Whereas in general relativity, if you shake something here, its gravitational impulse

00:04:34.640 | radiates out at the speed of light. And we call that a gravitational wave and we can detect those.

00:04:38.240 | It rubs me the wrong way to think that we should presume the answer should look one way or the

00:04:46.560 | other. Like if it turned out that there was action at a distance in physics, and that was the best

00:04:51.920 | way to describe things, then I would do it that way. It's actually a very deep question because

00:04:57.040 | when we don't know what the right laws of physics are, when we're guessing at them,

00:05:01.280 | when we're hypothesizing at what they might be, we are often guided by our intuitions about what

00:05:06.480 | they should be. I mean, Einstein famously was very guided by his intuitions. And he did not

00:05:11.440 | like the idea of action at a distance. We don't know whether he was right or not. It depends on

00:05:17.840 | your interpretation of quantum mechanics and it depends on even how you talk about quantum mechanics

00:05:22.640 | within any one interpretation. - So if you see every force as a field or any other interpretation

00:05:28.480 | of action at a distance, it's just stepping back to sort of caveman thinking. Like, do you really,

00:05:38.400 | can you really sort of understand what it means for a force to be a field that's everywhere?

00:05:45.200 | So if you look at gravity, like what do you think about-- - I think so.

00:05:48.400 | - Is this something that you've been conditioned by society to think that,

00:05:55.520 | to map the fact that science is extremely well predictive of something to believing that you

00:06:03.040 | actually understand it? Like you can intuitively, the degree that human beings can understand

00:06:11.360 | anything that you actually understand it. Are you just trusting the beauty and the power of the

00:06:16.800 | predictive power of science? - That depends on what you mean by this idea of truly understanding

00:06:24.000 | something, right? I mean, can I truly understand Fermat's last theorem? It's easy to state it,

00:06:30.400 | but do I really appreciate what it means for incredibly large numbers? I think yes,

00:06:37.920 | I think I do understand it. But like, if you want to just push people on, well, but your intuition

00:06:42.160 | doesn't go to the places where Andrew Wiles needed to go to prove Fermat's last theorem,

00:06:47.040 | then I can say fine, but I still think I understand the theorem. And likewise,

00:06:51.120 | I think that I do have a pretty good intuitive understanding of fields pervading space-time,

00:06:56.720 | whether it's the gravitational field or the electromagnetic field or whatever, the Higgs

00:07:00.960 | field. Of course, one's intuition gets worse and worse as you get trickier in the quantum field

00:07:08.080 | theory and all sorts of new phenomena that come up in quantum field theory. So our intuitions

00:07:13.520 | aren't perfect, but I think it's also okay to say that our intuitions get trained, right? Like,

00:07:18.320 | you know, I have different intuitions now than I had when I was a baby. That's okay. That's not,

00:07:22.400 | an intuition is not necessarily intrinsic to who we are. We can train it a little bit.

00:07:27.040 | - So that's where I'm gonna bring in Noam Chomsky for a second, who thinks that our

00:07:33.040 | cognitive abilities are sort of evolved through time. And so they're biologically constrained.

00:07:39.440 | And so there's a clear limit as he puts it to our cognitive abilities, and it's a very harsh limit.

00:07:46.320 | But you actually kind of said something interesting in nature versus nurture

00:07:51.520 | thing here is we can train our intuitions to sort of build up the cognitive muscles to be

00:07:57.360 | able to understand some of these tricky concepts. Do you think there's limits to our understanding

00:08:02.880 | that's deeply rooted, hard-coded into our biology that we can't overcome?

00:08:08.400 | - There could be limits to things like our ability to visualize, okay? But when someone

00:08:16.080 | like Ed Witten proves a theorem about, you know, 100 dimensional mathematical spaces,

00:08:21.200 | he's not visualizing it, he's doing the math. That doesn't stop him from understanding the result.

00:08:26.720 | I think, and I would love to understand this better, but my rough feeling,

00:08:31.120 | which is not very educated, is that, you know, there's some threshold that one crosses

00:08:36.800 | in abstraction when one becomes kind of like a Turing machine, right? One has the ability to

00:08:43.200 | contain in one's brain logical, formal, symbolic structures and manipulate them.

00:08:48.960 | And that's a leap that we can make as human beings that dogs and cats haven't made. And

00:08:55.360 | once you get there, I'm not sure that there are any limits to our ability to understand the

00:09:00.480 | scientific world at all. Maybe there are. There's certainly limits on our ability to calculate

00:09:06.080 | things, right? You know, people are not very good at taking cube roots of million digit numbers in

00:09:10.560 | their head, but that's not an element of understanding. It's certainly not a limit

00:09:14.880 | of principle. - So of course, as a human, you would say there doesn't feel to be limits to

00:09:19.840 | our understanding, but sort of, have you thought that the universe is actually a lot simpler than

00:09:29.760 | it appears to us and we just will never be able to, like it's outside of our, okay. So us, our

00:09:37.120 | cognitive abilities combined with our mathematical prowess and whatever kind of experimental

00:09:44.000 | simulation devices we can put together, is there limits to that? Is it possible there's limits to

00:09:52.160 | that? - Well, of course it's possible that there are limits to that. Is there any good reason to

00:09:58.560 | think that we're anywhere close to the limits is a harder question. Look, imagine asking this

00:10:04.240 | question 500 years ago to the world's greatest thinkers, right? Like are we approaching the

00:10:08.880 | limits of our ability to understand the natural world? And by definition, there are questions

00:10:14.640 | about the natural world that are most interesting to us that are the ones we don't quite yet

00:10:18.960 | understand, right? So there's always, we're always faced with these puzzles we don't yet know.

00:10:23.040 | And I don't know what they would have said 500 years ago, but they didn't even know about

00:10:26.160 | classical mechanics, much less quantum mechanics. So we know that they were nowhere close to how

00:10:31.680 | well they could do, right? They could do enormously better than they were doing at the time.

00:10:36.240 | I see no reason why the same thing isn't true for us today. So of all the worries that keep me awake

00:10:41.120 | at night, the human mind's inability to rationally comprehend the world is low on the list.

00:10:46.640 | - Well put. So one interesting philosophical point that quantum mechanics bring up is that

00:10:54.080 | you talk about the distinction between the world as it is and the world as we observe it.

00:11:01.840 | So staying at the human level for a second, how big is the gap between what our perception system

00:11:08.560 | allows us to see and the world as it is outside our mind's eye, sort of? Not at the quantum

00:11:16.160 | mechanical level, but as just these particular tools we have, which is the few senses and

00:11:23.360 | cognitive abilities to process those senses. - Well, that last phrase, having the cognitive

00:11:28.720 | abilities to process them, carries a lot, right? I mean, there is our sort of intuitive

00:11:34.560 | understanding of the world. You don't need to teach people about gravity for them to know

00:11:39.520 | that apples fall from trees, right? That's something that we figure out pretty quickly.

00:11:43.520 | Object permanence, things like that. The three-dimensionality of space, even if we don't

00:11:47.520 | have the mathematical language to say that, we kind of know that it's true. On the other hand,

00:11:53.040 | no one opens their eyes and sees atoms, right? Or molecules, or cells for that matter. Forget

00:11:58.480 | about quantum mechanics. But we got there. We got to understanding that there are atoms and cells

00:12:05.520 | using the combination of our senses and our cognitive capacities. So adding the ability

00:12:12.960 | of our cognitive capacities to our senses is adding an enormous amount. And I don't think

00:12:17.360 | it is a hard and fast boundary. If you believe in cells, if you believe that we understand those,

00:12:23.120 | then there's no reason you believe we can't believe in quantum mechanics just as well.

00:12:27.440 | - What to you is the most beautiful idea in physics?

00:12:32.240 | - Conservation of momentum. - Can you elaborate?

00:12:38.960 | - Yeah. If you were Aristotle, when Aristotle wrote his book on physics, he made the following

00:12:44.160 | very obvious point. We're on video here, right? So people can see this. So if I push the bottle,

00:12:48.640 | let me cover this bottle so we do not have a mess, but okay. So I push the bottle, it moves,

00:12:53.040 | and if I stop pushing, it stops moving. And this kind of thing is repeated a large number of times

00:12:59.600 | all over the place. If you don't keep pushing things, they stop moving. This is an indisputably

00:13:05.200 | true fact about our everyday environment, okay? And for Aristotle, this blew up into a whole

00:13:12.000 | picture of the world in which things had natures and teleologies, and they had places they wanted

00:13:17.920 | to be, and when you were pushing them, you were moving them away from where they wanted to be,

00:13:21.680 | and they would return and stuff like that. And it took 1,000 years or 1,500 years for people to

00:13:27.040 | say, "Actually, if it weren't for things like dissipation and air resistance and friction

00:13:34.480 | and so forth, the natural thing is for things to move forever in a straight line,

00:13:40.800 | there's a constant velocity, right? Conservation of momentum." And the reason why I think that's

00:13:47.680 | the most beautiful idea in physics is because it shifts us from a view of natures and teleology

00:13:55.040 | to a view of patterns in the world. So when you were Aristotle, you needed to talk a vocabulary of

00:14:03.520 | why is this happening, what's the purpose of it, what's the cause, et cetera, because it's nature

00:14:08.080 | does or does not want to do that. Whereas once you believe in conservation of momentum, things just

00:14:12.400 | happen. They just follow the pattern. You have Laplace's demon ultimately, right? You give me

00:14:18.400 | the state of the world today, I can predict what it's going to do in the future. I can predict

00:14:21.840 | where it was in the past. It's impersonal, and it's also instantaneous. It's not directed toward

00:14:27.680 | any future goals. It's just doing what it does given the current state of the universe. I think

00:14:32.560 | even more than either classical mechanics or quantum mechanics, that is the profound deep

00:14:37.360 | insight that gets modern science off the ground. You don't need natures and purposes and goals.

00:14:43.680 | You just need some patterns. >> So it's the first moment in our

00:14:48.240 | understanding of the way the universe works where you branch from the intuitive physical space to

00:14:55.600 | kind of the space of ideas. And also the other point you said, which is conveniently,

00:15:01.440 | most of the interesting ideas are acting in the moment. You don't need to know the history

00:15:07.440 | of time or the future. >> And of course, this took a long time to

00:15:11.280 | get there, right? I mean, the conservation of momentum itself took hundreds of years.

00:15:15.200 | It's weird because someone would say something interesting, and then the next interesting thing

00:15:19.200 | would be said like 150 or 200 years later, right? They weren't even talking to each other,

00:15:23.040 | they were just reading each other's books. And probably the first person to directly say that

00:15:27.680 | in outer space, in the vacuum, a projectile would move at a constant velocity was Avicenna,

00:15:34.160 | Ibn Sina in the Persian Golden Age, circa 1000. And he didn't like the idea. He used that,

00:15:40.480 | just like Schrodinger used Schrodinger's cat to say, surely you don't believe that, right?

00:15:44.240 | Ibn Sina was saying, surely you don't believe there really is a vacuum because if there was

00:15:48.720 | a really vacuum, things could keep moving forever, right? But still he got right the idea that there

00:15:54.000 | was this conservation of something, impetus or mile, he would call it. And that's 600 years before

00:16:01.280 | classical mechanics and Isaac Newton. So Galileo played a big role in this, but he didn't exactly

00:16:06.080 | get it right. And so it just takes a long time for this to sink in because it is so

00:16:11.040 | against our everyday experience. >> Do you think it was a big leap,

00:16:14.720 | a brave or a difficult leap of math and science to be able to say that momentum is conserved?

00:16:25.040 | >> I do. I think it's a example of human reason in action. Even Aristotle knew that his theory

00:16:33.120 | had issues because you could fire an arrow and it would go a long way before it stopped.

00:16:38.480 | So if his theory was things just automatically stop, what's going on? And he had this elaborate

00:16:43.440 | story. I don't know if you've heard this story, but the arrow would push the air in front of it

00:16:48.400 | away and the molecules of air would run around to the back of the arrow and push it again.

00:16:52.400 | And anyone reading this is going like, really, that's what you thought? But it was that kind

00:16:57.920 | of thought experiment that ultimately got people to say like, actually, no, if it weren't for the

00:17:01.200 | air molecules at all, the arrow would just go on by itself. And it's always this give and take

00:17:06.480 | between thought and experience back and forth, right? Theory and experiment, we would say today.

00:17:13.200 | >> Another big question that I think comes up certainly with quantum mechanics is what's the

00:17:20.560 | difference between math and physics to you? >> To me, very, very roughly. Math is about

00:17:28.560 | the logical structure of all possible worlds and physics is about our actual world.

00:17:32.400 | >> And it just feels like our actual world is a gray area when you start talking about

00:17:39.920 | interpretations of quantum mechanics or no. >> I'm certainly using the word world in the

00:17:45.280 | broadest sense, all of reality. So I think that reality is specific. I don't think that there's

00:17:51.920 | every possible thing going on in reality. I think that there are rules, whether it's the

00:17:55.920 | Schrodinger equation or whatever. So I think that there's a sensible notion of the set of

00:18:01.520 | all possible worlds and we live in one of them. The world that we're talking about might be a

00:18:05.840 | multiverse, might be many worlds of quantum mechanics, might be much bigger than the world

00:18:09.040 | of our everyday experience, but it's still one physically contiguous world in some sense.

00:18:13.840 | >> So if you look at the overlap of math and physics,

00:18:19.520 | it feels like when physics tries to reach for understanding of our world,

00:18:26.640 | it uses the tools of math to sort of reach beyond the limit of our current understanding.

00:18:34.080 | What do you make of that process of sort of using math to, so you start maybe with intuition or you

00:18:41.600 | might start with the math and then build up an intuition, but this kind of reaching into the

00:18:46.160 | darkness, into the mystery of the world with math? >> Well, I think I would put it a little

00:18:51.200 | bit differently. I think we have theories, theories of the physical world, which we then

00:18:57.200 | extrapolate and ask, what do we conclude if we take these seriously well beyond where we've

00:19:02.560 | actually tested them? It is separately true that math is really, really useful when we construct

00:19:08.000 | physical theories. And famously, Eugene Wigner asked about the unreasonable success of mathematics

00:19:13.040 | and physics. I think that's a little bit wrong because anything that could happen,

00:19:19.040 | any other theory of physics that wasn't the real world, but some other world,

00:19:23.360 | you could always describe it mathematically. It's just that it might be a mess.

00:19:28.000 | The surprising thing is not that math works, but that the math is so simple and easy that you can

00:19:33.840 | write it down on a t-shirt, right? I mean, that's what is amazing. That's an enormous

00:19:38.720 | compression of information that seems to be valid in the real world. So that's an interesting fact

00:19:45.360 | about our world, which maybe we could hope to explain or just take as a brute fact, I don't know.

00:19:50.000 | But once you have that, there's this indelible relationship between math and physics. But

00:19:57.200 | philosophically, I do want to separate them. We don't extrapolate math because there's a whole

00:20:02.720 | bunch of wrong math that doesn't apply to our world, right? We extrapolate the physical theory

00:20:07.680 | that we best think explains our world. >> Again, an unanswerable question.

00:20:12.480 | Why do you think our world is so easily compressible into beautiful equations?

00:20:19.920 | >> Yeah, I mean, like I just hinted at, I don't know if there's an answer to that question.

00:20:23.760 | There could be. >> What would an answer look like?

00:20:26.160 | >> Well, an answer could look like if you showed that there was something about our world

00:20:31.120 | that maximized something, the mean of the simplicity and the powerfulness of the laws

00:20:38.480 | of physics. Or maybe we're just generic. Maybe in the set of all possible worlds,

00:20:43.120 | this is what the world would look like, right? I don't really know. I tend to think not. I tend

00:20:48.640 | to think that there is something specific and rock bottom about the facts of our world that don't

00:20:54.560 | have further explanation. Like the fact that the world exists at all, and furthermore, the specific

00:20:59.440 | laws of physics that we have. I think that in some sense, we're just going to, at some level,

00:21:03.520 | we're going to say, and that's how it is. And we can't explain anything more. I don't know how,

00:21:08.160 | if we're anywhere close to that right now, but that seems plausible to me.

00:21:11.760 | >> And speaking of rock bottom, one of the things, sort of your book kind of reminded me or revealed

00:21:16.720 | to me is that what's fundamental and what's emergent, it just feels like I don't even know

00:21:23.440 | anymore what's fundamental in physics, if there's anything. It feels like everything, especially

00:21:30.320 | with quantum mechanics is revealing to us is that most interesting things that I would, as a limited

00:21:37.680 | human would think are fundamental, can actually be explained as emergent from the more deeper laws.

00:21:48.400 | >> I mean, we don't know, of course. You had to get that on the table. We don't know what

00:21:53.200 | is fundamental. We do have reason to say that certain things are more fundamental than others.

00:21:58.400 | Atoms and molecules are more fundamental than cells and organs. Quantum fields are more

00:22:04.480 | fundamental than atoms and molecules. We don't know if that ever bottoms out. I do think that

00:22:11.200 | there's sensible ways to think about this. If you describe something like this table as a table,

00:22:16.560 | it has a height and a width, and it's made of a certain material, and it has a certain solidity

00:22:20.400 | and weight and so forth. That's a very useful description as far as it goes. There's a whole

00:22:25.280 | another description of this table in terms of a whole collection of atoms strung together in

00:22:30.000 | certain ways. The language of the atoms is more comprehensive than the language of the table.

00:22:36.160 | You could break apart the table, smash it to pieces, still talk about it as atoms, but you

00:22:41.040 | could no longer talk about it as a table. So I think of this comprehensiveness, the domain of

00:22:46.080 | validity of a theory gets broader and broader as the theory gets more and more fundamental.

00:22:50.960 | >> So what do you think Newton would say, maybe right in a book review, if you read your latest

00:23:00.080 | book on quantum mechanics, something deeply hidden? >> It would take a long time for him to

00:23:05.040 | think that any of this was making any sense. >> You catch him up pretty quick in the beginning.

00:23:09.760 | >> Yeah. >> You give him a shout out in the beginning.

00:23:12.480 | >> That's right. I mean, he is the man. I'm happy to say that Newton was the greatest scientist who

00:23:17.200 | ever lived. I mean, he invented calculus in his spare time, which would have made him the greatest

00:23:21.840 | mathematician just all by himself, right? All by that one thing. But of course, it's funny because

00:23:28.560 | Newton was in some sense still a pre-modern thinker. Rocky Kolb, who is a cosmologist at

00:23:35.280 | the University of Chicago said that Galileo, even though he came before Newton, was a more modern

00:23:41.920 | thinker than Newton was. If you got Galileo and brought him to the present day, it would take him

00:23:46.160 | six months to catch up and then he'd be in your office telling you why your most recent paper was

00:23:49.520 | wrong. Whereas Newton just thought in this kind of more mystical way. He wrote a lot more about

00:23:57.200 | the Bible and alchemy than he ever did about physics. But he's also more brilliant than

00:24:02.560 | anybody else and way more mathematically astute than Galileo. So I really don't know. He might

00:24:09.600 | just, yeah, say like, give me the textbooks, leave me alone for a few months and then be caught up.

00:24:13.920 | Or he might have had mental blocks against seeing the world in this way. I really don't know.

00:24:20.400 | - Or perhaps find an interesting mystical interpretation of quantum mechanics.

00:24:24.400 | - Very possible, yeah.

00:24:25.440 | - Is there any other scientists or philosophers through history

00:24:29.040 | that you would like to know their opinion of your book?

00:24:33.040 | - That's a good question. I mean, Einstein is the obvious one, right? I mean,

00:24:38.960 | he was not that long ago, but I even speculate at the end of my book about what his opinion would be.

00:24:43.040 | I am curious as to what about older philosophers like Hume or Kant, right? What would they have

00:24:50.560 | thought? Or Aristotle, what would they have thought about modern physics? Because they do

00:24:56.240 | in philosophy, your predilections end up playing a much bigger role in your ultimate conclusions

00:25:02.880 | because you're not as tied down by what the data is. In physics, physics is lucky because we can't

00:25:08.960 | stray too far off the reservation as long as we're trying to explain the world that we actually see

00:25:13.200 | in our telescopes and microscopes. But it's just not fair to play that game because the people we're

00:25:19.600 | thinking about didn't know a whole bunch of things that we know, right? We lived through a lot that

00:25:25.040 | they didn't live through. So by the time we got them caught up, they'd be different people.

00:25:32.560 | - So let me ask a bunch of basic questions. I think it would be interesting, useful for people

00:25:38.400 | who are not familiar, but even for people who are extremely well familiar. Let's start with,

00:25:43.520 | what is quantum mechanics? - Quantum mechanics is the paradigm of physics that came into being in

00:25:51.200 | the early part of the 20th century that replaced classical mechanics. And it replaced classical

00:25:57.200 | mechanics in a weird way that we're still coming to terms with. So in classical mechanics,

00:26:01.680 | you have an object, it has a location, it has a velocity. And if you know the location and velocity

00:26:06.640 | of everything in the world, you can say what everything's going to do. Quantum mechanics

00:26:10.880 | has an aspect of it that is kind of on the same lines. There's something called the quantum state

00:26:16.960 | or the wave function. And there's an equation governing what the quantum state does. So it's

00:26:22.400 | very much like classical mechanics. The wave function is different. It's sort of a wave.

00:26:26.720 | It's a vector in a huge dimensional vector space rather than a position and a velocity. But okay,

00:26:31.520 | that's a detail. And the equation is the Schrodinger equation, not Newton's laws, but okay,

00:26:35.920 | again, a detail. Where quantum mechanics really becomes weird and different is that there's a

00:26:40.720 | whole nother set of rules in our textbook formulation of quantum mechanics, in addition

00:26:46.080 | to saying that there's a quantum state and it evolves in time. And all these new rules have

00:26:50.320 | to do with what happens when you look at the system, when you observe it, when you measure it.

00:26:54.800 | In classical mechanics, there were no rules about observing. You just look at it and you see what's

00:26:59.360 | going on. That was it, right? In quantum mechanics, the way we teach it, there's something profoundly

00:27:05.840 | fundamental about the act of measurement or observation. And the system dramatically changes

00:27:11.040 | its state, even though it has a wave function, like the electron in an atom is not orbiting in

00:27:16.320 | a circle, it's sort of spread out in a cloud. When you look at it, you don't see that cloud.

00:27:21.200 | When you look at it, it looks like a particle with a location. So it dramatically changes its

00:27:26.160 | state right away. And the effects of that change can be instantly seen in what the electron does

00:27:30.960 | next. So that's the, again, we need to be careful because we don't agree on what quantum mechanics

00:27:38.000 | says. So that's why I need to say like in the textbook view, et cetera, right? But in the

00:27:41.840 | textbook view, quantum mechanics, unlike any other theory of physics, gives a fundamental role to the

00:27:49.600 | act of measurement. >> So maybe even more basic, what is an atom and what is an electron?

00:27:56.560 | >> Sure. This all came together in a few years around the turn of the last century,

00:28:01.280 | right? Around the year 1900. Atoms predated then, of course, the word atom goes back to

00:28:07.840 | the ancient Greeks, but it was the chemists in the 1800s that really first got experimental

00:28:13.440 | evidence for atoms. They realized that there were two different types of tin oxide. And in these two

00:28:21.440 | different types of tin oxide, there was exactly twice as much oxygen in one type as the other.

00:28:26.880 | And like, why is that? Why is it never 1.5 times as much, right? And so Dalton said, well, it's

00:28:33.840 | because there are tin atoms and oxygen atoms. And one form of tin oxide is one atom of tin and one

00:28:40.400 | atom of oxygen, and the other is one atom of tin and two atoms of oxygen. And on the basis of this,

00:28:45.600 | so this is a speculation, a theory, right? A hypothesis. But then on the basis of that,

00:28:49.680 | you make other predictions. And the chemists became quickly convinced that atoms were real.

00:28:53.760 | The physicists took a lot longer to catch on, but eventually they did. And I mean, Boltzmann,

00:28:59.600 | who believed in atoms, had a really tough time his whole life because he worked in Germany,

00:29:05.760 | where atoms were not popular. They were popular in England, but not in Germany.

00:29:09.360 | - And there, in general, the idea of atoms is it's the smallest building block of the universe

00:29:16.000 | for them. That's the kind of how they thought of it.

00:29:18.160 | - That was the Greek idea, but the chemists in the 1800s jumped the gun a little bit. So these days,

00:29:23.600 | an atom is the smallest building block of a chemical element, right? Hydrogen, tin, oxygen,

00:29:29.440 | carbon, whatever. But we know that atoms can be broken up further than that. And that's what

00:29:34.000 | physicists discovered in the early 1900s, Rutherford especially, and his colleagues. So

00:29:41.040 | the atom that we think about now, the cartoon, is that picture you've always seen of a little nucleus

00:29:47.360 | and then electrons orbiting it like a little solar system. And we now know the nucleus is made of

00:29:51.680 | protons and neutrons. So the weight of the atom, the mass, is almost all in its nucleus. Protons

00:29:58.480 | and neutrons are something like 1800 times as heavy as electrons are. Electrons are much lighter,

00:30:04.480 | but because they're lighter, they give all the life to the atoms. So when atoms get together,

00:30:10.080 | combine chemically, when electricity flows through a system, it's all the electrons that are doing

00:30:14.880 | all the work. - And where quantum mechanics steps in,

00:30:18.720 | as you mentioned, with the position of velocity, with classical mechanics, and quantum mechanics

00:30:23.520 | is modeling the behavior of the electron. I mean, you can model the behavior of anything,

00:30:28.960 | but the electron, because that's where the fun is. - The electron was the biggest challenge,

00:30:33.360 | right from the start, yeah. - So what's the wave function? You said

00:30:36.960 | it's an interesting detail, but in any interpretation, what is the wave function

00:30:43.520 | in quantum mechanics? - Well, we had this idea from Rutherford

00:30:47.120 | that atoms look like little solar systems. But people very quickly realized that can't possibly

00:30:52.880 | be right, because if an electron is orbiting in a circle, it will give off light. All the light that

00:30:58.000 | we have in this room comes from electrons zooming up and down and wiggling, and that's what

00:31:01.600 | electromagnetic waves are. And you can calculate how long would it take for the electron just to

00:31:06.320 | spiral into the nucleus, and the answer is 10 to the minus 11 seconds, okay? 100 billionth of a

00:31:11.360 | second. So that's not right. Meanwhile, people had realized that light, which we understood from the

00:31:18.720 | 1800s was a wave, had properties that were similar to that of particles, right? This is Einstein and

00:31:24.880 | Planck and stuff like that. So if something that we agree was a wave had particle-like properties,

00:31:32.000 | then maybe something we think is a particle, the electron, has wave-like properties, right?

00:31:37.600 | And so a bunch of people eventually came to the conclusion, don't think about the electron as a

00:31:42.960 | little point particle orbiting like a solar system. Think of it as a wave that is spread out.

00:31:49.600 | They cleverly gave this the name the wave function, which is the dopiest name in the world

00:31:53.360 | for one of the most profound things in the universe. There's literally a number at every

00:31:59.840 | point in space, which is the value of the electron's wave function at that point.

00:32:04.720 | >>ANDREW: And there's only one wave function. >>BD: Yeah, they eventually figured that out.

00:32:09.360 | That took longer. But when you have two electrons, you do not have a wave function for electron one

00:32:15.040 | and a wave function for electron two. You have one combined wave function for both of them. And

00:32:20.400 | indeed, as you say, there's only one wave function for the entire universe at once.

00:32:24.800 | >>ANDREW: And that's where this beautiful dance, can you say what is entanglement?

00:32:31.520 | It seems one of the most fundamental ideas of quantum mechanics.

00:32:35.360 | >>BD: Well, let's temporarily buy into the textbook interpretation of quantum mechanics.

00:32:39.520 | And what that says is that this wave function, so it's very small outside the atom, very big

00:32:44.080 | in the atom. Basically the wave function, you take it and you square it, you square the number,

00:32:49.600 | that gives you the probability of observing the system at that location. So if you say that for

00:32:55.120 | two electrons, there's only one wave function, and that wave function gives you the probability

00:32:59.440 | of observing both electrons at once doing something. So maybe the electron can be here

00:33:04.880 | or here, here or here, and the other electron can also be there. But we have a wave function

00:33:09.360 | set up where we don't know where either electron is going to be seen, but we know they'll both be

00:33:14.880 | seen in the same place. So we don't know exactly what we're going to see for either electron,

00:33:20.800 | but there's entanglement between the two of them. There's a conditional statement. If we see one in

00:33:26.320 | one location, then we know the other one's going to be doing a certain thing. So that's a feature

00:33:30.960 | of quantum mechanics that is nowhere to be found in classical mechanics. In classical mechanics,

00:33:35.360 | there's no way I can say, "Well, I don't know where either one of these particles is, but if I

00:33:39.200 | find out where this one is, then I know where the other one is." That just never happens. They're

00:33:42.320 | truly separate. >> And in general, it feels like if you think of a wave function like as a dance floor,

00:33:47.760 | it seems like entanglement is strongest between things that are dancing together closest. So

00:33:53.840 | there's a closeness that's important. >> Well, that's another step. We have to be

00:33:58.880 | careful here because in principle, if you're talking about the entanglement of two electrons,

00:34:03.360 | for example, they can be totally entangled or totally unentangled no matter where they are in

00:34:09.440 | the universe. There's no relationship between the amount of entanglement and the distance between

00:34:14.400 | two electrons. But we now know that the reality of our best way of understanding the world is

00:34:20.880 | through quantum fields, not through particles. So even the electron, not just gravity and

00:34:25.920 | electromagnetism, but even the electron and the quarks and so forth are really vibrations in

00:34:31.360 | quantum fields. So even empty space is full of vibrating quantum fields. And those quantum

00:34:39.120 | fields in empty space are entangled with each other in exactly the way you just said. If they're

00:34:43.840 | nearby, if you have two vibrating quantum fields that are nearby, then they will be highly entangled.

00:34:48.320 | If they're far away, they will not be entangled. >> So what do quantum fields in a vacuum look like,

00:34:52.560 | empty space? >> Just like empty space,

00:34:54.800 | it's as empty as it can be. >> But there's still a field.

00:34:57.440 | What does nothing look like? >> Just like right here,

00:35:03.280 | this location in space, there's a gravitational field which I can detect by dropping something.

00:35:07.760 | I don't see it, but there it is. >> So we got a little bit of an idea of

00:35:15.680 | entanglement. Now, what is Hilbert space and Euclidean space?

00:35:23.760 | >> Yeah, I think that people are very welcome to go through their lives not knowing what Hilbert

00:35:28.560 | space is, but if you want to dig into a little bit more into quantum mechanics, it becomes necessary.

00:35:33.440 | You know, the English language was invented long before quantum mechanics or various forms of

00:35:39.680 | higher mathematics were invented. So we use the word space to mean different things. Of course,

00:35:45.440 | most of us think of space as this three-dimensional world in which we live, right? I mean, some of us

00:35:49.360 | just think of it as outer space. But space around us, it gives us the three-dimensional location of

00:35:55.360 | things and objects. But mathematicians use any generic abstract collection of elements as a

00:36:04.080 | space, a space of possibilities, momentum space, et cetera. So Hilbert space is the space of all

00:36:11.680 | possible quantum wave functions, either for the universe or for some specific system. And it could

00:36:16.960 | be an infinite dimensional space, or it could be just really, really large dimensional but finite.

00:36:21.760 | We don't know because we don't know the final theory of everything. But this abstract Hilbert

00:36:25.920 | space is really, really, really big and has no immediate connection to the three-dimensional

00:36:30.240 | space in which we live. >> What do dimensions in Hilbert space mean?

00:36:34.560 | >> You know, it's just a way of mathematically representing how much information is contained

00:36:39.920 | in the state of the system. How many numbers do you have to give me to specify what the thing is

00:36:45.200 | doing? So in classical mechanics, I give you the location of something by giving you three numbers,

00:36:51.120 | right? Up, down, left, X, Y, Z coordinates. But then I might want to give you its entire

00:36:57.200 | state, physical state, which means both its position and also its velocity.

00:37:02.400 | The velocity also has three components. So its state lives in something called phase space,

00:37:07.840 | which is six-dimensional, three dimensions of position, three dimensions of velocity.

00:37:11.680 | And then if it also has an orientation in space, that's another three dimensions and so forth. So

00:37:16.880 | as you describe more and more information about the system, you have an abstract mathematical space

00:37:23.440 | that has more and more numbers that you need to give. And each one of those numbers corresponds

00:37:27.680 | to a dimension in that space. >> So in terms of the amount of

00:37:31.520 | information, what is entropy? This mystical word that's overused in math and physics,

00:37:38.880 | but has a very specific meaning in this context. >> Sadly, it has more than one very specific

00:37:43.840 | meaning. This is the reason why it is hard. Entropy means different things even to different

00:37:48.240 | physicists. But one way of thinking about it is a measure of how much we don't know

00:37:53.440 | about the state of a system, right? So if I have a bottle of water molecules, and I know that,

00:37:59.520 | okay, there's a certain number of water molecules, I could weigh it, right, and figure out, I know

00:38:03.040 | the volume of it, and I know the temperature and pressure and things like that. I certainly don't

00:38:07.840 | know the exact position and velocity of every water molecule, right? So there's a certain amount

00:38:12.800 | of information I know, certain amount that I don't know that is part of the complete state of the

00:38:17.600 | system. And that's what the entropy characterizes, how much unknown information there is, the

00:38:23.120 | difference between what I do know about the system and its full exact microscopic state.

00:38:27.040 | >> So when we try to describe a quantum mechanical system, is it infinite or finite but very large?

00:38:36.640 | >> Yeah, we don't know. That depends on the system. You know, it's easy to mathematically

00:38:41.200 | write down a system that would have a potentially infinite entropy, an infinite dimensional Hilbert

00:38:46.720 | space. So let's go back a little bit. We said that the Hilbert space was the space in which

00:38:52.560 | quantum wave functions lived. For different systems, that will be different sizes. They

00:38:57.120 | could be infinite or finite. So that's the number of numbers, the number of pieces of information

00:39:02.640 | you could potentially give me about the system. So the bigger Hilbert space is, the bigger the

00:39:07.920 | entropy of that system could be, depending on what I know about it. If I don't know anything

00:39:12.400 | about it, then, you know, it has a huge entropy, right? But only up to the size of its Hilbert

00:39:17.040 | space. So we don't know in the real physical world whether or not, you know, this region of space

00:39:23.920 | that contains that water bottle has potentially an infinite entropy or just a finite entropy. We have

00:39:29.520 | different arguments on different sides. >> So if it's infinite, how do you think about

00:39:34.000 | infinity? Is this something you can, your cognitive abilities are able to process,

00:39:41.200 | or is it just a mathematical tool? >> It's somewhere in between, right? I mean,

00:39:45.120 | we can say things about it. We can use mathematical tools to manipulate infinity very,

00:39:50.400 | very accurately. We can define what we mean. You know, for any number n, there's a number bigger

00:39:55.360 | than it. So there's no biggest number, right? So there's something called the total number of all

00:39:59.760 | numbers, and it's infinite. But it is hard to wrap your brain around that. And I think that gives

00:40:04.480 | people pause because we talk about infinity as if it's a number, but it has plenty of properties

00:40:10.960 | that real numbers don't have. You know, if you multiply infinity by two, you get infinity again,

00:40:15.120 | right? That's a little bit different than what we're used to.

00:40:17.520 | >> Okay, but are you comfortable with the idea that in thinking of what the real world actually

00:40:25.840 | is, that infinity could be part of that world? Are you comfortable that a world in some dimension--

00:40:30.720 | >> I'm comfortable with lots of things. I mean, you know, I don't want my level of comfort to

00:40:37.360 | affect what I think about the world. You know, I'm pretty open-minded about what the world could

00:40:42.320 | be at a fundamental level. >> Yeah, but infinity is a tricky one. It's

00:40:47.840 | not almost a question of comfort. It's a question of is it an overreach of our intuition? Sort of,

00:40:57.600 | it could be a convenient, almost like when you add a constant to an equation just because it'll help.

00:41:03.520 | It just feels like it's useful to at least be able to imagine a concept, not directly,

00:41:08.560 | but in some kind of way that this feels like it's a description of the real world.

00:41:14.080 | >> Think of it this way. There's only three numbers that are simple. There's zero,

00:41:20.720 | there's one, and there's infinity. A number like 318 is just bizarre. You need a lot of bits to

00:41:30.960 | give me what that number is. But zero and one and infinity, like once you have 300 things,

00:41:35.680 | you might as well have infinity things, right? Otherwise, you have to say when to stop making

00:41:38.960 | the things, right? So there's a sense in which infinity is a very natural number of things to

00:41:44.240 | exist. >> I was never comfortable with infinity because it's just such a-- It was too good to be

00:41:49.280 | true because in math, it just helps make things work out. When things get very large, close to

00:41:59.600 | infinity, things seem to work out nicely. It's kind of like, because my deepest passion is

00:42:05.840 | probably psychology. And I'm uncomfortable how in the average, the beauty of how much we vary

00:42:17.360 | is lost. In that same kind of sense, infinity seems like a convenient way to erase the details.

00:42:23.600 | >> But the thing about infinity is it seems to pop up whether we like it or not, right? Like

00:42:29.360 | you're trying to be a computer scientist. You ask yourself, well, how long will it take this

00:42:33.120 | program to run? And you realize, well, for some of them, the answer is infinitely long. It's not

00:42:37.520 | because you tried to get there. You wrote a five-line computer program. It doesn't halt.

00:42:42.320 | >> So coming back to the textbook definition of quantum mechanics, this idea that I don't think

00:42:48.320 | we talked about, can you-- This one of the most interesting philosophical points we talked at

00:42:55.600 | the human level, but at the physics level, that at least the textbook definition of quantum mechanics

00:43:03.600 | separates what is observed and what is real. One, how does that make you feel? And two,

00:43:13.360 | what does it then mean to observe something, and why is it different than what is real?

00:43:19.440 | >> Yeah. My personal feeling, such as it is, is that things like measurement and observers and

00:43:28.640 | stuff like that are not going to play a fundamental role in the ultimate laws of physics. But my

00:43:33.440 | feeling that way is because so far, that's where all the evidence has been pointing. I could be

00:43:39.200 | wrong, and there's certainly a sense in which it would be infinitely cool if somehow observation

00:43:45.840 | or mental cogitation did play a fundamental role in the nature of reality. But I don't think so,

00:43:52.880 | and again, I don't see any evidence for it, so I'm not spending a lot of time worrying about

00:43:56.240 | that possibility. So what do you do about the fact that in the textbook interpretation of quantum

00:44:01.200 | mechanics, this idea of measurement or looking at things seems to play an important role?

00:44:07.680 | Well, you come up with better interpretations of quantum mechanics, and there are several

00:44:11.600 | alternatives. My favorite is the many worlds interpretation, which says two things. Number one,

00:44:17.440 | you, the observer, are just a quantum system like anything else. There's nothing special about you.

00:44:23.360 | Don't get so proud of yourself. You're just a bunch of atoms. You have a wave function. You

00:44:28.560 | obey the Schrodinger equation like everything else. And number two, when you think you're

00:44:33.360 | measuring something or observing something, what's really happening is you're becoming entangled

00:44:38.960 | with that thing. So when you think there's a wave function for the electron, it's all spread out,

00:44:43.600 | but you look at it and you only see it in one location. What's really happening is that there's

00:44:48.160 | still the wave function for the electron in all those locations, but now it's entangled

00:44:52.400 | with the wave function of you in the following way. There's part of the wave function that says

00:44:57.120 | the electron was here and you think you saw it there. The electron was there and you think you

00:45:01.760 | saw it there. The electron was over there and you think you saw it there, et cetera.

00:45:06.000 | And all of those different parts of the wave function, once they come into being,

00:45:09.360 | no longer talk to each other. They no longer interact or influence each other. It's as if

00:45:14.160 | they are separate worlds. So this was the invention of Hugh Everett III, who was a

00:45:19.600 | graduate student at Princeton in the 1950s. And he said, basically, look, you don't need all these

00:45:25.920 | extra rules about looking at things. Just listen to what the Schrodinger equation is telling you.

00:45:30.800 | It's telling you that you have a wave function, that you become entangled, and that the different

00:45:35.200 | versions of you no longer talk to each other. So just accept it. He did therapy more than anything

00:45:40.880 | else. He said, it's okay. You don't need all these extra rules. All you need to do is believe

00:45:45.760 | the Schrodinger equation. The cost is there's a whole bunch of extra worlds out there.

00:45:49.520 | >> So the world's being created whether there's an observer or not.

00:45:56.080 | >> The world's created anytime a quantum system that's in a superposition becomes

00:46:00.880 | entangled with the outside world. >> What's the outside world?

00:46:04.720 | >> It depends. Let's back up. Whatever it really says, what his theory is, is there's a wave

00:46:11.920 | function of the universe, and it obeys the Schrodinger equation all the time. That's it.

00:46:17.360 | That's the full theory right there. The question, all of the work is how in the world do you map

00:46:24.960 | that theory onto reality, onto what we observe? So part of it is carving up the wave function

00:46:31.840 | into these separate worlds, saying, look, it describes a whole bunch of things that don't

00:46:35.200 | interact with each other. Let's call them separate worlds. Another part is distinguishing between

00:46:39.760 | systems and their environments. And the environment is basically all the degrees of freedom, all the

00:46:44.720 | things going on in the world that you don't keep track of. So again, in the bottle of water,

00:46:50.240 | I might keep track of the total amount of water and the volume. I don't keep track of the individual

00:46:55.600 | positions and velocities. I don't keep track of all the photons or the air molecules in this room.

00:47:00.480 | So that's the outside world. The outside world is all the parts of the universe that you're not

00:47:05.360 | keeping track of when you're asking about the behavior of some subsystem of it.

00:47:08.800 | So how many worlds are there?

00:47:13.760 | Yeah, we don't know that one either. There could be an infinite number. There could be only a

00:47:19.200 | finite number, but it's a big number one way or the other.

00:47:21.760 | It's just a very, very big number. In one of the talks, somebody asked, well, if it's finite,

00:47:31.200 | so actually I'm not sure exactly the logic you used to derive this, but is there going to be

00:47:38.880 | overlap, a duplicate world that you return to? So you've mentioned, and I'd love if you can

00:47:49.360 | elaborate on sort of idea that it's possible that there's some kind of equilibrium that these

00:47:54.640 | splitting worlds arrive at. And then maybe over time, maybe somehow connected to entropy,

00:48:00.880 | you get a large number of worlds that are very similar to each other.

00:48:04.880 | Yeah. So this question of whether or not Hilbert space is finite or infinite dimensional is

00:48:11.840 | actually secretly connected to gravity and cosmology. This is the part that we're still

00:48:17.360 | struggling to understand right now, but we discovered back in 1998 that our universe is

00:48:21.360 | accelerating. And what that means if it continues, which we think it probably will,

00:48:25.920 | but we're not sure, but if it does, that means there's a horizon around us. Because the universe

00:48:31.680 | not only expanding, but expanding faster and faster, things can get so far away from us

00:48:36.400 | that from our perspective, it looks like they're moving away faster than the speed of light.

00:48:40.800 | We will never see them again. So there's literally a horizon around us and that horizon

00:48:45.360 | approaches some fixed distance away from us. And you can then argue that within that horizon,

00:48:51.600 | there's only a finite number of things that can possibly happen, the finite dimensional

00:48:55.200 | Hilbert space. In fact, we even have a guess for what the dimensionality is. It's 10 to the power

00:49:01.040 | of 10 to the power of 122. That's a very large number. Just to compare it, the age of the

00:49:07.760 | universe is something like 10 to the 14 seconds, 10 to the 17 or 18 seconds maybe. The number of

00:49:13.600 | particles in the universe is 10 to the 88th, but the number of dimensions of Hilbert space is 10

00:49:18.960 | to the 10 to the 122. So that's just crazy big. If that story is right, that in our observable

00:49:26.000 | horizon, there's only a finite dimensional Hilbert space, then this idea of branching

00:49:31.200 | of the wave function of the universe into multiple distinct separate branches has to reach a limit

00:49:36.880 | at some time. Once you branch that many times, you've run out of room in Hilbert space.

00:49:41.520 | And roughly speaking, that corresponds to the universe just expanding and emptying out and

00:49:46.800 | cooling off and entering a phase where it's just empty space literally forever.

00:49:51.760 | >> What's the difference between splitting and copying, do you think? A lot of this is

00:50:00.640 | an interpretation that helps us sort of model the world. So perhaps shouldn't be

00:50:10.720 | thought of as like, philosophically or metaphysically, but even at the physics level,

00:50:19.760 | do you see a difference between sort of generating new copies of the world or splitting?

00:50:26.080 | >> I think it's better to think of in quantum mechanics in many worlds, the universe splits

00:50:31.600 | rather than new copies because people otherwise worry about things like energy conservation.

00:50:35.760 | And no one who understands quantum mechanics worries about energy conservation because the

00:50:40.320 | equation is perfectly clear. But if all you know is that someone told you the universe duplicates,

00:50:44.800 | then you have a reasonable worry about where all the energy for that came from. So a pre-existing

00:50:50.080 | universe splitting into two skinnier universes is a better way of thinking about it. And

00:50:54.480 | mathematically, it's just like, if you draw an X and Y axis and you draw a vector of length one,

00:50:59.600 | a 45 degree angle, you know that you can write that vector of length one as the sum of two vectors

00:51:07.200 | pointing along X and Y of length one over the square root of two. So I write one arrow as the

00:51:13.200 | sum of two arrows, but there's a conservation of arrow-ness, right? Like there's now two arrows,

00:51:18.080 | but the length is the same. I just am describing it in a different way. And that's exactly what

00:51:22.720 | happens when the universe branches. The wave function of the universe is a big old vector.

00:51:26.640 | >> So to somebody who brings up a question of saying, doesn't this violate the conservation

00:51:33.920 | of energy? Can you give further elaboration? >> Right. So let's just be super duper perfectly

00:51:41.120 | clear. There's zero question about whether or not many worlds violate conservation of energy.

00:51:46.720 | It does not. And I say this definitively because there are other questions that I think there's

00:51:51.360 | answers to, but they're legitimate questions, right? About where does probability come from

00:51:55.760 | and things like that. This conservation of energy question, we know the answer to it. And the answer

00:52:00.640 | to it is that energy is conserved. All of the effort goes into how best to translate what the

00:52:06.480 | equation unambiguously says into plain English, right? So this idea that the universe comes

00:52:14.080 | equipped with a thickness and it sort of divides up into thinner pieces, but the total amount of

00:52:18.720 | universe is conserved over time is a reasonably good way of putting English words to the underlying

00:52:25.920 | mathematics. >> So one of my favorite things about many worlds is, I mean, I love that there's

00:52:32.800 | something controversial in science. And for some reason it makes people actually not like upset,

00:52:40.000 | but just get excited. Why do you think it is a controversial idea? So there's a lot of,

00:52:47.360 | it's actually one of the cleanest ways to think about quantum mechanics. So why do you think

00:52:53.280 | there's a discomfort a little bit amongst certain people? >> Well, I draw the distinction in my book

00:52:59.280 | between two different kinds of simplicity in a physical theory. There's simplicity

00:53:04.000 | in the theory itself, right? How we describe what's going on according to the theory by its own

00:53:09.120 | rights. But then, you know, theory is just some sort of abstract mathematical formalism. You have

00:53:13.440 | to map it onto the world somehow, right? And sometimes like for Newtonian physics, it's pretty

00:53:20.960 | obvious, like, okay, here is a bottle and it has a center of mass and things like that. Sometimes

00:53:26.160 | it's a little bit harder with general relativity, curvature of space-time is a little bit harder to

00:53:31.840 | grasp. Quantum mechanics is very hard to map what the language you're talking in of wave functions

00:53:38.000 | and things like that onto reality. And many worlds is the version of quantum mechanics where it is

00:53:43.360 | hardest to map on the underlying formalism to reality. So that's where the lack of simplicity

00:53:50.320 | comes in, not in the theory, but in how we use the theory to map onto reality. And in fact,

00:53:55.280 | all of the work in sort of elaborating many worlds quantum mechanics is in this effort to map it on

00:54:02.720 | to the world that we see. So it's perfectly legitimate to be bugged by that, right? To say,

00:54:08.560 | like, well, no, that's just too far away from my experience. I am therefore intrinsically skeptical

00:54:15.840 | of it. Of course, you should give up on that skepticism if there are no alternatives and this

00:54:19.680 | theory always keeps working, then eventually you should overcome your skepticism. But right now,

00:54:23.600 | there are alternatives that are, that, you know, people work to make alternatives that are by their

00:54:29.040 | nature closer to what we observe directly. - Can you describe the alternatives? I don't

00:54:33.360 | think we touched on it. So the Copenhagen interpretation and the many worlds, maybe

00:54:40.400 | there's a difference between the Everettian many worlds and many worlds as it is now, like has the

00:54:47.760 | idea sort of developed and so on. And just in general, what is the space of promising contenders?

00:54:54.160 | We have democratic debates now, there's a bunch of candidates. - 12 candidates.

00:54:57.840 | - 12 candidates on stage. What are the quantum mechanical candidates on stage for the debate?

00:55:02.160 | - So if you had a debate between quantum mechanical contenders, there'd be no problem

00:55:08.000 | getting 12 people up there on stage, but there would still be only three front runners.

00:55:13.760 | And right now the front runners would be Everett. Hidden variable theories are another one. So the

00:55:19.360 | hidden variable theories say that the wave function is real, but there's something in

00:55:24.240 | addition to the wave function. The wave function is not everything, it's part of reality, but it's

00:55:28.320 | not everything. What else is there? We're not sure, but in the simplest version of the theory,

00:55:34.160 | there are literally particles. So many worlds says that quantum systems are sometimes are

00:55:40.960 | wave-like in some ways and particle-like in another because they really, really are waves,

00:55:45.920 | but under certain observational circumstances, they look like particles. Whereas hidden variable

00:55:51.920 | says they look like waves and particles because there are both waves and particles involved in

00:55:56.880 | the dynamics. And that's easy to do if your particles are just non-relativistic Newtonian

00:56:03.280 | particles moving around, they get pushed around by the wave function roughly. It becomes much

00:56:08.160 | harder when you take quantum field theory or quantum gravity into account. The other big

00:56:13.680 | contender are spontaneous collapse theories. So in the conventional textbook interpretation, we say

00:56:20.560 | when you look at a quantum system, its wave function collapses and you see it in one location.

00:56:25.040 | A spontaneous collapse theory says that every particle has a chance per second of having its

00:56:34.080 | wave function spontaneously collapse. The chance is very small for a typical particle will take

00:56:38.800 | hundreds of millions of years before it happens even once, but in a table or some macroscopic

00:56:43.440 | object, there are way more than a hundred million particles and they're all entangled with each

00:56:47.760 | other. So when one of them collapses, it brings everything else along with it. There's a slight

00:56:53.200 | variation of this. That's a spontaneous collapse theory. There are also induced collapse theories

00:56:57.120 | like Roger Penrose thinks that when the gravitational difference between two parts of the

00:57:01.440 | wave function becomes too large, the wave function collapses automatically. So those are basically,

00:57:08.160 | in my mind, the three big alternatives. Many worlds, which is just there's a wave function

00:57:12.400 | and always obeys the Schrodinger equation. Hidden variables, there's a wave function that always

00:57:16.880 | obeys the Schrodinger equation, but there are also new variables. Or collapse theories, which

00:57:21.760 | the wave function sometimes obeys the Schrodinger equation and sometimes it collapses. So you can

00:57:26.720 | see that the alternatives are more complicated in their formalism than many worlds is, but they are

00:57:32.080 | closer to our experience. - So just this moment of collapse, do you think of it as a wave function,

00:57:40.240 | fundamentally, sort of a probabilistic description of the world? And is collapse sort of reducing

00:57:48.640 | that part of the world into something deterministic? Where again, you can now describe the position and

00:57:53.760 | the velocity in this simple classical model? Is that how you think about collapse?

00:57:58.560 | - There is a fourth category, there's a fourth contender, there's a Mayor Pete of

00:58:02.720 | quantum mechanical interpretations, which are called epistemic interpretations. And what they

00:58:08.400 | say is all the wave function is, is a way of making predictions for experimental outcomes. It's not

00:58:14.400 | mapping onto an element of reality in any real sense. And in fact, two different people might

00:58:20.400 | have two different wave functions for the same physical system because they know different things

00:58:24.000 | about it, right? The wave function is really just a prediction mechanism. And then the problem with

00:58:28.640 | those epistemic interpretations is if you say, okay, but it's predicting about what? Like what

00:58:35.760 | is the thing that is being predicted? And they say, no, no, no, that's not what we're here for.

00:58:41.200 | We're just here to tell you what the observational outcomes are going to be.

00:58:44.160 | - But the other interpretations kind of think that the wave function is real.

00:58:47.920 | - Yes, that's right. So that's an ontic interpretation of the wave function,

00:58:54.400 | ontology being the study of what is real, what exists, as opposed to an epistemic

00:58:58.800 | interpretation of the wave function, epistemology being the study of what we know.

00:59:01.920 | - I would actually just love to see that debate on stage.

00:59:05.520 | - There was a version of it on stage at the World Science Festival a few years ago

00:59:09.520 | that you can look up online.

00:59:11.040 | - On YouTube?

00:59:11.760 | - Yep, it's on YouTube.

00:59:12.960 | - Okay, awesome. I'll link it and watch it. Who won?

00:59:16.640 | - I won. I don't know, there was no vote. There was no vote. But Brian Green was the moderator,

00:59:25.360 | and David Albert stood up for spontaneous collapse, and Shelley Goldstein was there for

00:59:30.480 | hidden variables, and Rüdiger Schock was there for epistemic approaches.

00:59:33.520 | - Why do you, I think you mentioned it, but just to elaborate,

00:59:37.360 | why do you find many worlds so compelling?

00:59:40.560 | - Well, there's two reasons, actually. One is, like I said, it is the simplest, right? It's

00:59:45.440 | like the most bare-bones, austere, pure version of quantum mechanics. And I am someone who is very

00:59:52.080 | willing to put a lot of work into mapping the formalism onto reality. I'm less willing to

00:59:56.560 | complicate the formalism itself. But the other big reason is that there's something called modern

01:00:01.600 | physics, with quantum fields and quantum gravity and holography and space-time, doing things like

01:00:08.000 | that. And when you take any of the other versions of quantum theory, they bring along classical

01:00:14.480 | baggage. All of the other versions of quantum mechanics prejudice or privilege some version

01:00:21.760 | of classical reality, like locations in space, okay? And I think that that's a barrier to doing

01:00:29.440 | better at understanding the theory of everything, understanding quantum gravity and the emergence

01:00:32.800 | of space-time. Whenever, if you change your theory from, you know, here's a harmonic oscillator,

01:00:38.160 | oh, there's a spin, here's an electromagnetic field, in hidden variable theories or dynamical

01:00:43.200 | collapse theories, you have to start from scratch. You have to say like, well, what are the hidden

01:00:46.240 | variables for this theory or how does it collapse or whatever? Whereas many worlds is plug and play.

01:00:50.720 | You tell me the theory and I can give you as many worlds version. So when we have a situation like

01:00:55.280 | we have with gravity and space-time, where the classical description seems to break down in a

01:01:00.720 | dramatic way, then I think you should start from the most quantum theory that you have,

01:01:05.600 | which is really many worlds. >> So start with the quantum theory and try to

01:01:10.880 | build up a model of space-time, the emergence of space-time.

01:01:15.760 | >> That's right. >> Okay, so I thought space-time was

01:01:20.080 | fundamental. >> Yeah, I know.

01:01:22.080 | >> So this sort of dream that Einstein had, that everybody had and everybody has of, you know,

01:01:28.720 | the theory of everything. So how do we build up from many worlds, from quantum mechanics,

01:01:34.400 | a model of space-time, a model of gravity? >> Well, yeah, I mean, let me first mention

01:01:40.400 | very quickly why we think it's necessary. You know, we've had gravity in the form that Einstein

01:01:45.920 | bequeathed it to us for over a hundred years now, like 1915 or 1916, he put general relativity in

01:01:51.120 | the final form. So gravity is the curvature of space-time and there's a field that pervades

01:01:56.640 | all the universe that tells us how curved space-time is.

01:01:59.680 | >> And that's a fundamentally classical. >> That's totally classical, right, exactly.

01:02:03.920 | But we also have a formalism, an algorithm for taking a classical theory and quantizing it.

01:02:10.160 | >> This is how we get quantum electrodynamics, for example. And it could be tricky. I mean,

01:02:14.880 | you think you're quantizing something, so that means taking a classical theory and promoting

01:02:19.840 | it to a quantum mechanical theory, but you can run into problems. So they ran into problems and

01:02:24.960 | they did that with electromagnetism, namely that certain quantities were infinity and you don't

01:02:29.040 | like infinity, right? So Feynman and Tomonaga and Schwinger won the Nobel Prize for teaching us how

01:02:34.880 | to deal with the infinities. And then Ken Wilson won another Nobel Prize for saying you shouldn't

01:02:38.800 | have been worried about those infinities after all. But still, it's always the thought that that's

01:02:44.080 | how you will make a good quantum theory. You'll start with a classical theory and quantize it.

01:02:47.600 | So if we have a classical theory of general relativity, we can quantize it or we can try to,

01:02:52.080 | but we run into even bigger problems with gravity than we ran into with electromagnetism. And so

01:02:57.920 | far those problems are insurmountable. We have not been able to get a successful theory of

01:03:02.160 | quantum gravity by starting with classical general relativity and quantizing it.

01:03:07.760 | And there's evidence that there's a good reason why this is true that whatever the quantum theory

01:03:13.440 | of gravity is, it's not a field theory. It's something that has weird non-local features

01:03:19.600 | built into it somehow that we don't understand. And we get this idea from black holes and Hawking

01:03:25.200 | radiation and information conservation and a whole bunch of other ideas I talk about in the book.

01:03:29.200 | So if that's true, if the fundamental theory isn't even local in the sense that an ordinary

01:03:34.560 | quantum field theory would be, then we just don't know where to start in terms of getting a classical

01:03:40.080 | precursor and quantizing it. So the only sensible thing, or at least the next obvious sensible thing

01:03:45.520 | to me would be to say, okay, let's just start intrinsically quantum and work backwards. See

01:03:49.680 | if we can find a classical limit. So the idea of locality, the fact that locality is not fundamental

01:03:58.240 | to the nature of our existence, I guess in that sense, modeling everything as a field makes sense

01:04:06.720 | to me. Stuff that's close by interacts, stuff that's far away doesn't. So what's locality and

01:04:13.680 | why is it not fundamental? And how is that even possible? - Yeah, I mean, locality is the answer

01:04:18.480 | to the question that Isaac Newton was worried about back at the beginning of our conversation,

01:04:22.240 | right? I mean, how can the earth know what the gravitational field of the sun is? And the answer

01:04:28.320 | as spelled out by Laplace and Einstein and others is that there's a field in between.

01:04:32.320 | And the way a field works is that what's happening to the field at this point in space

01:04:37.360 | only depends directly on what's happening at points right next to it. But what's happening

01:04:42.400 | at those points depends on what's happening right next to those, right? And so you can build up an

01:04:46.240 | influence across space through only local interactions. That's what locality means.

01:04:51.600 | What happens here is only affected by what's happening right next to it. That's locality.

01:04:56.800 | The idea of locality is built into every field theory, including general relativity as a

01:05:01.440 | classical theory. It seems to break down when we talk about black holes. And Hawking taught us in

01:05:07.440 | the 1970s that black holes radiate, they give off, they will eventually evaporate away. They're not

01:05:12.560 | completely black once we take quantum mechanics into account. And we think, we don't know for

01:05:18.560 | sure, but most of us think that if you make a black hole out of certain stuff, then like Laplace's

01:05:26.080 | demon taught us, you should be able to predict what that black hole will turn into if it's just

01:05:30.560 | obeying the Schrodinger equation. And if that's true, there are good arguments that can't happen

01:05:36.400 | while preserving locality at the same time. It's just that the information seems to be spread out

01:05:41.520 | non-locally in interesting ways. >> And people should, you talk about holography

01:05:46.560 | with Leonard Susskind on your Mindscape podcast. >> Oh yes, I have a podcast. I didn't even mention

01:05:51.600 | that. This is terrible. >> No, I'm going to ask you questions about

01:05:55.040 | that too. And I've been not shutting up about it. It's my favorite science podcast. Or not,

01:06:00.080 | it's not even a science podcast. It's like a scientist doing a podcast.

01:06:05.920 | >> That's right. That's what it is. Absolutely, yes.

01:06:07.840 | >> Yeah. Anyway. >> Yeah, so holography is this idea when

01:06:11.120 | you have a black hole. And black hole is a region of space inside of which gravity is so strong that

01:06:16.080 | you can't escape. And there's this weird feature of black holes that, again, is totally a thought

01:06:21.200 | experiment feature because we haven't gone and probed any yet. But there seems to be one way

01:06:26.400 | of thinking about what happens inside a black hole as seen by an observer who's falling in,

01:06:32.240 | which is actually pretty normal. Everything looks pretty normal until you hit the singularity and

01:06:35.680 | you die. But from the point of view of the outside observer, it seems like all the information that

01:06:41.520 | fell in is actually smeared over the horizon in a non-local way. And that's puzzling.

01:06:48.640 | And that's so holography because that's a two-dimensional surface that is encapsulating

01:06:52.320 | the whole three-dimensional thing inside, right? Still trying to deal with that, still trying to

01:06:56.480 | figure out how to get there. But it's an indication that we need to think a little bit more subtly

01:07:00.640 | when we quantize gravity. >> So because you can describe

01:07:03.600 | everything that's going on in three-dimensional space by looking at the two-dimensional

01:07:08.640 | projection of it, it means that locality is not necessary.

01:07:14.640 | >> Well, it means that somehow it's only a good approximation. It's not really what's going on.

01:07:19.920 | >> How are we supposed to feel about that? >> You're supposed to feel liberated.

01:07:23.680 | You know, space is just a good approximation. And this was always going to be true once you

01:07:29.680 | started quantizing gravity. So we're just beginning now to face up to the dramatic

01:07:36.400 | implications of quantizing gravity. >> Is there other weird stuff that

01:07:40.480 | happens to quantum mechanics in black hole? >> I don't think that anything weird has happened

01:07:46.160 | with quantum mechanics. I think weird things happen with space-time. I mean, that's what it is.

01:07:49.440 | >> That's right. >> Like quantum mechanics is still just

01:07:50.720 | quantum mechanics. But our ordinary notions of space-time don't really quite work. And

01:07:57.360 | there's a principle that goes hand-in-hand with holography called complementarity,

01:08:03.680 | which says that there's no one unique way to describe what's going on inside a black hole.

01:08:10.800 | Different observers will have different descriptions, both of which are accurate,

01:08:14.320 | but sound completely incompatible with each other. So it depends on how you look at it.

01:08:20.080 | The word complementarity in this context is borrowed from Niels Bohr, who points out you

01:08:24.960 | can measure the position or you can measure the momentum. You can't measure both at the same time

01:08:29.680 | in quantum mechanics. >> So a couple of questions on many worlds.

01:08:33.920 | How does many worlds help us understand our particular branch of reality? So, okay,

01:08:41.040 | that's fine and good, that everything is splitting, but we're just traveling down a

01:08:45.120 | single branch of it. So how does it help us understand our little unique branch?

01:08:50.560 | >> Yeah, I mean, that's a great question. But that's the point, is that we didn't invent many

01:08:55.040 | worlds because we thought it was cool to have a whole bunch of worlds, right? We invented it

01:08:58.240 | because we were trying to account for what we observe here in our world. And what we observe

01:09:04.000 | here in our world are wave functions collapsing, okay? We do have a situation where the electron

01:09:10.320 | seems to be spread out, but then when we look at it, we don't see it spread out. We see it located

01:09:14.080 | somewhere. So what's going on? That's the measurement problem of quantum mechanics.

01:09:17.600 | That's what we have to face up to. So many worlds is just a proposed solution to that problem. And

01:09:22.880 | the answer is nothing special is happening. It's still just the Schrodinger equation, but you have

01:09:28.080 | a wave function too. And that's a different answer than would be given in hidden variables

01:09:33.200 | or dynamical collapse theories or whatever. So the entire point of many worlds is to explain

01:09:39.200 | what we observe, but it tries to explain what we already have observed, right? It's not trying to

01:09:44.880 | be different from what we've observed because that would be something other than quantum mechanics.

01:09:49.440 | >> But the idea that there's worlds that we didn't observe that keep branching off is kind of,

01:09:55.920 | it's stimulating to the imagination. So is it possible to hop from, you mentioned the branches

01:10:05.360 | are independent. Is it possible to hop from one to the other? So it's a physical limit.

01:10:12.320 | The theory says it's impossible. >> There's already a copy of you in

01:10:17.040 | the other world, don't worry. >> Yes.

01:10:18.640 | >> Then leave them alone. >> No, but there's a fear of missing out,

01:10:24.240 | FOMO. >> Yes.

01:10:25.120 | >> That I feel like immediately start to wonder if that other copy is having more or less fun.

01:10:32.640 | >> Yeah, well, the downside to many worlds is that you're missing out on an enormous amount.

01:10:36.880 | And that's always what it's going to be like. >> And I mean, there's a certain stage of

01:10:42.480 | acceptance in that. >> Yes.

01:10:43.520 | >> In terms of rewinding, you think we can rewind the system back. The nice thing about many worlds,

01:10:51.440 | I guess, is it really emphasizes the, maybe you can correct me, but the deterministic

01:10:58.800 | nature of a branch. And it feels like it could be rewound back. Do you see as something that

01:11:07.840 | could be perfectly rewinded back? >> If you're at a fancy French restaurant,

01:11:14.560 | and there's a nice linen white tablecloth, and you have your glass of Bordeaux, and you knock it over,

01:11:19.520 | and the wine spills across the tablecloth. If the world were classical, okay, it would be possible

01:11:26.880 | that if you just lifted the wine glass up, you'd be lucky enough that every molecule of wine would

01:11:31.520 | hop back into the glass, right? But guess what? It's not going to happen in the real world.

01:11:35.840 | And the quantum wave function is exactly the same way. It is possible in principle to rewind

01:11:42.080 | everything if you start from perfect knowledge of the entire wave function of the universe.

01:11:46.560 | In practice, it's never going to happen. >> So time travel, not possible?

01:11:52.640 | >> Nope. At least quantum mechanics has no help. >> What about memory? Does the universe have a

01:12:01.280 | memory of itself where we could, so not time travel, but peek back in time and do a little

01:12:10.800 | like replay? >> Well, it's exactly the same in quantum mechanics as classical mechanics. So

01:12:18.080 | whatever you want to say about that. The fundamental laws of physics in either many

01:12:22.720 | worlds, quantum mechanics or Newtonian physics, conserve information. So if you have all the

01:12:29.600 | information about the quantum state of the world right now, your Laplace's demon-like in your

01:12:34.000 | knowledge and calculational capacity, you can wind the clock backward. But none of us is, right? And

01:12:40.400 | so in practice, you can never do that. You can do experiments over and over again, starting from the

01:12:44.800 | same initial conditions for small systems. But once things get to be large, Avogadro's number

01:12:50.480 | of particles, right? Bigger than a cell, no chance. >> We talked a little bit about error of time last

01:12:57.760 | time, but in many worlds that there is a kind of implied error of time, right? So you've talked

01:13:08.320 | about the error of time that has to do with the second law of thermodynamics. That's the error

01:13:14.800 | of time that's emergent or fundamental. We don't know, I guess. >> No, it's emergent. >> Does

01:13:22.160 | everyone agree on that? Well, nobody agrees with everything. >> They should. >> They should.

01:13:28.080 | So that error of time, is that different than the error of time that's implied by many worlds?

01:13:33.280 | >> It's not different, actually, no. In both cases, you have fundamental laws of physics that

01:13:39.120 | are completely reversible. If you give me the state of the universe at one moment in time,

01:13:43.520 | I can run the clock forward or backward equally well. There's no arrow of time built into the

01:13:48.400 | laws of physics at the most fundamental level. But what we do have are special initial conditions

01:13:55.040 | 14 billion years ago near the Big Bang. In thermodynamics, those special initial conditions

01:14:00.320 | take the form of things were low entropy, and entropy has been increasing ever since,

01:14:05.440 | making the universe more disorganized and chaotic, and that's the arrow of time.

01:14:09.520 | In quantum mechanics, these special initial conditions take the form of there was only

01:14:15.040 | one branch of the wave function, and the universe has been branching more and more ever since.

01:14:19.200 | >> Okay, so if time is emergent, so it seems like our human cognitive capacity likes to take things

01:14:28.960 | that are emergent and assume and feel like they're fundamental. So if time is emergent,

01:14:36.080 | and locality, is space emergent? >> Yes.

01:14:42.720 | >> Okay. >> But I didn't say time was emergent,

01:14:44.880 | I said the arrow of time was emergent. Those are different.

01:14:47.200 | >> What's the difference between the arrow of time and time? Are you using arrow of time to

01:14:54.080 | simply mean they're synonymous with the second law of thermodynamics?

01:14:57.760 | >> No, but the arrow of time is the difference between the past and future. So there's space,

01:15:03.120 | but there's no arrow of space. You don't feel that space has to have an arrow, right? You could live

01:15:07.440 | in thermodynamic equilibrium. There'd be no arrow of time, but there'd still be time. There'd still

01:15:11.760 | be a difference between now and the future or whatever. >> Oh, so okay. So if nothing changes,

01:15:16.960 | there's still time. >> Well, things could even change.

01:15:20.240 | If the whole universe consisted of the Earth going around the Sun, it would just go in circles,

01:15:27.040 | or ellipses, right? >> That's another reason.

01:15:28.960 | >> Things would change, but it's not increasing entropy. There's no arrow. If you took a movie

01:15:33.360 | of that and I played you the movie backward, you would never know.

01:15:36.240 | >> So the arrow of time can theoretically point in the other direction for brief, briefly.

01:15:45.040 | >> To the extent that it points in different directions, it's not a very good arrow. I mean,

01:15:49.280 | the arrow of time in the macroscopic world is so powerful that there's just no chance of going

01:15:54.080 | back. When you get down to tiny systems with only three or four moving parts, then entropy can

01:15:58.640 | fluctuate up and down. >> What does it mean for space

01:16:01.280 | to be an emergent phenomena? >> It means that the fundamental

01:16:04.400 | description of the world does not include the word space. It'll be something like a vector

01:16:08.640 | in Hilbert space, right? And you have to say, well, why is there a good approximate description

01:16:13.360 | which involves three-dimensional space and stuff inside it?

01:16:16.880 | >> Okay. So time and space are emergent. We kind of mentioned in the beginning,

01:16:23.120 | can you elaborate, what do you feel hope is fundamental in our universe?

01:16:29.840 | >> A wave function living in Hilbert space. >> A wave function in Hilbert space that we

01:16:34.400 | can't intellectualize or visualize really. >> We can't visualize it. We can intellectualize

01:16:38.800 | it very easily. >> Like, well, how do you think about?

01:16:41.200 | >> It's a vector in a 10 to the 10 to the 122-dimensional vector space. It's a complex

01:16:46.560 | vector, unit norm. It evolves according to the Schrodinger equation.

01:16:49.760 | >> Got it. When you put it that way. >> What's so hard, really?

01:16:54.560 | >> It's like, yep. Quantum computers. There's some excitement, actually a lot of excitement

01:17:04.400 | with people that it will allow us to simulate quantum mechanical systems.

01:17:08.960 | What kind of questions do you, about quantum mechanics, about the things we've been talking

01:17:14.640 | about, do you think, do you hope we can answer through quantum simulation?

01:17:19.520 | >> Well, I think that there's a whole fascinating frontier of things you can do with quantum

01:17:26.800 | computers, both sort of practical things with cryptography or money, privacy eavesdropping,

01:17:33.440 | sorting things, simulating quantum systems, right?

01:17:39.040 | >> So, it's a broader question, maybe even outside of quantum computers. Some of the

01:17:45.200 | theories that we've been talking about, what's your hope? What's most promising to test these

01:17:50.800 | theories? What are kind of experiments we can conduct, whether in simulation or in the

01:17:56.880 | physical world, that would validate or disprove or expand these theories?

01:18:02.560 | >> Well, I think for, there's two parts of that question. One is many worlds and the

01:18:07.680 | other one is sort of emergent space time. For many worlds, there are experiments ongoing

01:18:12.640 | to test whether or not wave functions spontaneously collapse. And if they do, then that rules

01:18:18.480 | out many worlds and that would be falsified. If there are hidden variables, there's a theorem

01:18:24.320 | that seems to indicate that the predictions will always be the same as many worlds. I'm

01:18:29.840 | a little skeptical of this theorem. I'm not completely, I haven't internalized it. I haven't

01:18:32.960 | made it in part of my intuitive view of the world yet. So, there might be loopholes to

01:18:36.720 | that theorem. I'm not sure about that. Part of me thinks that there should be different

01:18:40.400 | experimental predictions if there are hidden variables, but I'm not sure.

01:18:43.280 | But otherwise, it's just quantum mechanics all the way down. And so, there's this

01:18:48.160 | cottage industry in science journalism of writing breathless articles that say,

01:18:54.000 | quantum mechanics shown to be more astonishing than ever before thought. And really, it's the

01:18:58.320 | same quantum mechanics we've been doing since 1926. Whereas with the emergent space time stuff,

01:19:03.520 | we know a lot less about what the theory is. It's in a very primitive state. We don't even

01:19:08.640 | really have a safely written down, respectable, honest theory yet. So, there could very well be

01:19:15.040 | experimental predictions we just don't know about yet. That is one of the things that we're trying

01:19:18.800 | to figure out. But for emergent space time, you need really big stuff, right?

01:19:24.320 | Well, or really fast stuff or really energetic stuff. We don't know. That's the thing. So,

01:19:29.920 | there could be violations of the speed of light if you have emergent space time. Not going faster

01:19:36.720 | than the speed of light, but the speed of light could be different for light of different

01:19:39.920 | wavelengths, right? That would be a dramatic violation of physics as we know it, but it could

01:19:45.280 | be possible. Or not. I mean, it's not an absolute prediction. That's the problem. The theories are

01:19:50.160 | just not well-developed enough yet to say. >> Is there anything that quantum mechanics

01:19:57.040 | can teach us about human nature or the human mind? Do you think about sort of consciousness

01:20:03.600 | and these kinds of topics? It's certainly excessively used, as you point out. The word

01:20:10.560 | quantum is used for everything besides quantum mechanics. But in more seriousness, is there

01:20:17.680 | something that goes to the human level and can help us understand our mind?

01:20:25.680 | >> Not really, is the short answer. Minds are pretty classical, I don't think. We don't know

01:20:33.040 | this for sure, but I don't think that phenomena like entanglement are crucial to how the human

01:20:37.200 | mind works. >> What about consciousness? So, you mentioned, I think, early on in the conversation,

01:20:42.880 | you said it would be unlikely but incredible if sort of the observer is somehow a fundamental part.

01:20:53.360 | So, observer, not to romanticize the notion, but seems interlinked to the idea of consciousness.

01:21:01.040 | So, if consciousness is, as the panpsychics believe, is fundamental to the universe,

01:21:07.280 | is that possible? Is that weight? I mean, everything's possible.

01:21:11.840 | >> Just like Joe Rogan likes to say, it's entirely possible. But, okay, but is it

01:21:18.560 | on a spectrum of crazy out there? Statistically speaking, how often do you ponder the possibility

01:21:27.680 | that consciousness is fundamental or the observer is fundamental to-

01:21:31.520 | >> I personally don't at all. There are people who do. I'm a thorough physicalist when it comes

01:21:36.320 | to consciousness. I do not think that there are any separate mental states or mental properties.

01:21:41.680 | I think they're all emergent, just like space-time is. And space-time is hard enough to understand.

01:21:45.840 | So, the fact that we don't yet understand consciousness is not at all surprising to me.

01:21:50.320 | >> You, as we mentioned, have an amazing podcast called Mindscape. It's, as I said,

01:21:56.000 | one of my favorite podcasts, sort of both for your explanation of physics, which a lot of people love,

01:22:05.280 | and when you venture out into things that are beyond your expertise. But it's just a really

01:22:11.200 | smart person exploring even questions like morality, for example. It's very interesting.

01:22:19.840 | I think you did a solo episode and so on. I mean, there's a lot of really interesting

01:22:23.840 | conversations that you have. What are some, from memory, amazing conversations that pop to mind

01:22:32.640 | that you've had? What did you learn from them? Something that maybe changed your mind or just

01:22:38.000 | inspired you? Or just, through this whole experience of having conversations, what stands out to you?

01:22:43.360 | >> It's an unfair question. >> It's totally unfair, but that's okay.

01:22:47.360 | That's all right. It's often the ones, I feel like the ones I do on physics and closely related

01:22:54.000 | science, or even philosophy ones, are like, I know this stuff and I'm helping people learn about it.

01:23:01.920 | But I learn more from the ones that have nothing to do with physics or philosophy, right? So,

01:23:06.320 | talking to Wynton Marsalis about jazz, or talking to Master Sommelier about wine,

01:23:11.120 | talking to Will Wilkinson about partisan polarization and the urban-rural divide,

01:23:16.320 | talking to psychologists like Harold Tavris about cognitive dissonance and how those things work.

01:23:24.880 | Scott Derrickson, who is the director of the movie Doctor Strange, I had a wonderful conversation

01:23:31.040 | with him where we went through the mechanics of making a blockbuster superhero movie, right?

01:23:36.320 | And he's also not a naturalist, he's an evangelical Christian, so we talked about

01:23:40.880 | the nature of reality there. I want to have a couple more discussions with

01:23:46.640 | highly educated theists who know the theology really well, but I haven't quite arranged those

01:23:55.680 | yet. >> I would love to hear that. I mean,

01:23:57.600 | how comfortable are you venturing into questions of religion?

01:24:02.320 | >> Oh, I'm totally comfortable doing it. I did talk with Alan Lightman, who is also

01:24:06.960 | an atheist, but he is trying to rescue the sort of spiritual side of things for atheism. And I did

01:24:16.320 | talk to very vocal atheists like Alex Rosenberg. So, I've talked to some religious believers,

01:24:23.600 | but I need to talk to more. >> How have you changed

01:24:27.360 | through having all these conversations? >> You know, part of the motivation was I had

01:24:33.040 | a long stack of books that I hadn't read and I couldn't find time to read them, and I figured

01:24:36.960 | if I interviewed their authors, it forced me to read them, right? And that has totally worked,

01:24:41.360 | by the way. Now I'm annoyed that people write such long books. I think I'm still very much

01:24:47.520 | learning how to be a good interviewer. I think that's a skill that I think I have good questions,

01:24:53.280 | but there's the give and take that is still, I think, I can be better at. I want to offer

01:24:59.680 | something to the conversation, but not too much, right? I've had conversations where I barely

01:25:04.160 | talked at all, and I've had conversations where I talked half the time, and I think there's a

01:25:07.200 | happy medium in between there. >> So, I think I remember listening to,

01:25:10.240 | without mentioning names, some of your conversations where I wish you would have

01:25:14.560 | disagreed more. As a listener, it's more fun sometimes. >> Well, that's a very good question,

01:25:22.400 | because everyone has an attitude toward that. Some people are really there to basically give

01:25:29.200 | their point of view, and their guest is supposed to respond accordingly. I want to get my view on

01:25:37.760 | the record, but I don't want to dwell on it when I'm talking to someone like David Chalmers, who I

01:25:42.800 | disagree with a lot. I want to say, "Here's why I disagree with you, but we're here to listen to

01:25:49.440 | you." I have an episode every week, and you're only on once a week, right? So, I have an upcoming

01:25:54.720 | podcast episode with Philip Goff, who is a much more dedicated panpsychist. So, there we really

01:26:02.080 | get into it. I think that I probably have disagreed with him more on that episode than I ever have

01:26:06.560 | with another podcast guest, but that's what he wanted, so it worked very well.

01:26:09.920 | >> Yeah, yeah. That kind of debate structure is beautiful when it's done right. When you can

01:26:17.360 | detect that the intent is that you have fundamental respect for the person.

01:26:21.920 | For some reason, it's super fun to listen to when two really smart people are just arguing,

01:26:30.000 | and sometimes lose their shit a little bit, if I may say so.

01:26:32.400 | >> Well, there's a fine line, because I have zero interest in bringing... I mean, maybe you

01:26:39.680 | implied this. I have zero interest in bringing on people for whom I don't have any intellectual

01:26:43.840 | respect. I constantly get requests like, "Bring on a flat earther or whatever, and really slap

01:26:49.360 | them down," or a creationist. I have zero interest. I'm happy to bring on a religious person,

01:26:55.520 | a believer, but I want someone who's smart and can act in good faith and can talk, not a

01:27:00.000 | charlatan or a lunatic, right? So, I will happily bring on people with whom I disagree,

01:27:06.800 | but only people from whom I think the audience can learn something interesting.

01:27:10.000 | >> So, let me ask. The idea of charlatan is an interesting idea. You might be more educated on

01:27:17.120 | this topic than me, but there's folks, for example, who argue various aspects of evolution,

01:27:26.320 | sort of try to approach and say that evolution, sort of our current theory of evolution,

01:27:34.720 | has many holes in it, has many flaws. And they argue that, I think, like Cambrian explosion,

01:27:42.880 | which is like a huge added variability of species, doesn't make sense under our current description

01:27:51.520 | of evolution, theory of evolution. Sort of, if you were to have the conversation with people

01:27:57.440 | like that, how do you know that they're... the difference between outside the box thinkers

01:28:04.240 | and people who are fundamentally unscientific and even bordering on charlatans?

01:28:12.800 | >> That's a great question. And the further you get away from my expertise, the harder it is for

01:28:17.680 | me to really judge exactly those things. And I don't have a satisfying answer for that one,

01:28:23.840 | because I think the example you use of someone who believes in the basic structure of natural

01:28:29.520 | selection, but thinks that this particular thing cannot be understood in terms of our current

01:28:35.200 | understanding of Darwinism, that's a perfect edge case where it's hard to tell, right? And I would

01:28:42.240 | try to talk to people who I do respect and who do know things. And I would have to, given that I'm

01:28:47.120 | a physicist, I know that physicists will sometimes be too dismissive of alternative points of view.

01:28:53.040 | I have to take into account that biologists can also be too dismissive of alternative points of

01:28:56.960 | view. So yeah, that's a tricky one. >> Have you gotten heat yet?

01:29:01.760 | >> I get heat all the time. There's always something... I mean, it's hilarious because

01:29:05.760 | I try very hard not to have the same topic several times in a row. I did have two climate change

01:29:14.000 | episodes, but they were from very different perspectives. But I like to mix it up. That's

01:29:17.120 | the whole point. That's why I'm having fun. And every time I do an episode, someone says, "Oh,

01:29:21.040 | the person you should really get on to talk about exactly that is this other person." I'm like,

01:29:24.640 | "Well, I don't... But I did that now. I don't want to do that anymore."

01:29:26.880 | >> Well, I hope you keep doing it. You're inspiring millions of people with your books,

01:29:33.200 | your podcast. Sean, it's an honor to talk to you. Thank you so much.

01:29:36.560 | >> Thanks very much, Lex.

01:29:38.080 | [END]

01:29:38.800 | >> Thanks, Lex.

01:29:39.760 | [END]

01:29:39.840 | >> Thanks, Lex.

01:29:40.340 | [END]

01:29:41.300 | >> Thanks, Lex.

01:29:41.800 | [END]

01:29:42.840 | >> Thanks, Lex.

01:29:43.340 | [END]

01:29:43.420 | >> Thanks, Lex.

01:29:43.920 | [END]

01:29:43.920 | >> Thanks, Lex.

01:29:44.420 | [END]

01:29:44.420 | >> Thanks, Lex.

01:29:44.920 | [END]

01:29:44.920 | >> Thanks, Lex.

01:29:45.420 | [END]

01:29:45.420 | >> Thanks, Lex.

01:29:45.920 | [END]

01:29:45.920 | >> Thanks, Lex.

01:29:46.420 | [END]

01:29:46.420 | >> Thanks, Lex.

01:29:46.920 | [END]

01:29:46.920 | >> Thanks, Lex.

01:29:47.420 | [END]

01:29:47.420 | >> Thanks, Lex.

01:29:47.920 | [END]

01:29:47.920 | >> Thanks, Lex.

01:29:48.420 | [END]

01:29:48.420 | >> Thanks, Lex.

01:29:48.920 | [END]

01:29:49.420 | >> Thanks, Lex.

01:29:49.920 | [END]

01:29:49.920 | >> Thanks, Lex.

01:29:50.420 | [END]

01:29:50.420 | >> Thanks, Lex.

01:29:50.920 | [END]

01:29:51.420 | >> Thanks, Lex.

01:29:51.920 | [END]

01:29:51.920 | >> Thanks, Lex.

01:29:52.420 | [END]

01:29:52.420 | >> Thanks, Lex.

01:29:52.920 | [END]

01:29:52.920 | [BLANK_AUDIO]

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

Chapters