#428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens

0:00:00 The following is a conversation with Sean Carroll, his third time in this podcast.
0:00:05 He is a theoretical physicist at John Hopkins, host of the Mindscape podcast that I personally
0:00:11 love and highly recommend, and author of many books, including the most recent book series called
0:00:17 The Biggest Ideas in the Universe. The first book of which is titled Space, Time, and Motion,
0:00:23 and it’s on the topic of general relativity. And the second, coming out on May 14th,
0:00:29 so you should definitely pre-order it, is titled Quanta and Fields, and that one is on the topic
0:00:35 of quantum mechanics. Sean is a legit active theoretical physicist, and at the same time,
0:00:41 is one of the greatest communicators of physics ever. I highly encourage you listen to his podcast,
0:00:48 read his books, and pre-order the new book to support his work. This was, as always,
0:00:54 a big honor and a pleasure for me. And now, a quick few second mention of his sponsor. Check
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0:10:43 And now, dear friends, here’s Sean Carroll.
0:10:58 In book one of the series, The Biggest Ideas in the Universe, called Spacetime Motion,
0:11:08 you take on classical mechanics, general relativity by taking on the main equation
0:11:15 of general relativity and making it accessible, easy to understand. So maybe at the high level,
0:11:21 what is general relativity? What’s a good way to start to try to explain it?
0:11:26 Probably the best way to start to try to explain it is special relativity, which came first, 1905.
0:11:33 It was the culmination of many decades of people putting things together, but it was Einstein
0:11:38 in 1905. In fact, it wasn’t even Einstein. I should give more credit to Minkowski in 1907.
0:11:45 So Einstein in 1905 figured out that you could get rid of the aether, the idea of a rest frame for
0:11:51 the universe, and all the equations of physics would make sense, with the speed of light being
0:11:57 a maximum. But then it was Minkowski, who used to be Einstein’s professor in 1907,
0:12:02 who realized the most elegant way of thinking about this idea of Einstein’s was to blend
0:12:08 space and time together into spacetime, to really imagine that there is no hard and fast division
0:12:16 of the four-dimensional world in which we live into space and time separately.
0:12:20 Einstein was at first dismissive of this. He thought it was just like, oh, the mathematicians
0:12:25 are over formalizing again. But then he later realized that if spacetime is a thing, it can have
0:12:33 properties. And in particular, it can have a geometry. It can be curved from place to place.
0:12:38 And that was what let him solve the problem of gravity. He had previously been trying to fit in
0:12:44 what we knew about gravity from Newtonian mechanics, the inverse square law of gravity,
0:12:50 to his new relativistic theory. It didn’t work. So the final leap was to say, gravity is the
0:12:56 curvature of spacetime. And that statement is basically general relativity.
0:13:01 And the tension with Minkowski was, he was a mathematician.
0:13:05 Yes.
0:13:05 So it’s the tension between physics and mathematics. In fact, in your lecture about this equation,
0:13:11 one of them, you say that Einstein is a better physicist than he gets credit for.
0:13:18 Yep. I know, that’s hard. That’s a little bit of a joke there, right?
0:13:23 Yeah.
0:13:23 Because we all give Einstein a lot of credit. But then we also, partly based on fact, but
0:13:29 partly to make ourselves feel better, tell ourselves a story about how later in life,
0:13:33 Einstein couldn’t keep up. There were younger people doing quantum mechanics and quantum field
0:13:38 theory and particle physics. And he was just sort of unable to really philosophically get over his
0:13:44 objections to that. And I think that that story about the latter part is completely wrong,
0:13:50 like almost 180 degrees wrong. I think that Einstein understood quantum mechanics as well
0:13:56 as anyone, at least up through the 1930s. I think that his philosophical objections to it
0:14:01 are correct. So he should actually have been taken much more seriously about that.
0:14:06 And what he did, what he achieved in trying to think these problems through is to really
0:14:13 basically understand the idea of quantum entanglement, which is kind of important these
0:14:18 days when it comes to understanding quantum mechanics. Now, it’s true that in the 40s and 50s,
0:14:23 he placed his efforts in hopes for unifying electricity and magnetism with gravity that
0:14:29 didn’t really work out very well. All of us try things that don’t work out. I don’t hold that
0:14:35 against him. But in terms of IQ points, in terms of trying to be a clear thinking physicist,
0:14:40 he was really, really great. What does greatness look like for a physicist? So how difficult is it
0:14:46 to take the leap from special relativity to general relativity? How difficult is it to imagine that
0:14:53 to consider space time together and to imagine that there’s a curvature to this whole thing?
0:15:00 Yeah, that’s a great question. I think that if you want to make the case for Einstein’s greatness,
0:15:06 which is not hard to do, there’s two things you point out. One is in 1905, his famous miracle year,
0:15:13 he writes three different papers on three wildly different subjects, all of which
0:15:21 would make you famous just for writing that one paper. Special relativity is one of them.
0:15:27 Brownian motion is another one, which is just the little vibrations of tiny little dust specks
0:15:34 in the air. But who cares about that? What matters is it proves the existence of atoms.
0:15:39 He explains Brownian motion by imagining their molecules in the air and driving their properties.
0:15:43 Brilliant. And then he basically starts the world on the road to quantum mechanics with his paper on
0:15:50 which, again, is given a boring label of the photoelectric effect. What it really was is he
0:15:55 invented photons. He showed that light should be thought of as particles as well as waves.
0:16:01 And he did all three of those very different things in one year. Okay. But the other thing
0:16:06 that gets him genius status is, like you say, general relativity. So this takes 10 years from
0:16:11 1905 to 1915. He wasn’t only doing general relativity. He was working on other things. He
0:16:15 wrote, he invented a refrigerator. He did various interesting things. And he wasn’t even the only
0:16:21 one working on the problem. There are other people who suggested relativistic theories of gravity,
0:16:26 but he really applied himself to it. And I think as your question suggests, the solution was not
0:16:35 a matter of turning a crank. It was something fundamentally creative. In his own telling of
0:16:43 the story, his greatest moment, his happiest moment was when he realized that if the way that we
0:16:49 would say it in modern terms, if you were in a rocket ship accelerating at 1G,
0:16:55 at 1 acceleration due to gravity, if the rocket ship were very quiet, you wouldn’t be able to
0:17:00 know the difference between being in a rocket ship and being on the surface of the earth.
0:17:04 Gravity is sort of not detectable or at least not distinguishable from acceleration. So number
0:17:11 one, that’s a pretty clever thing to think. But number two, if you or I had had that thought,
0:17:15 we would have gone, huh, we’re pretty clever. He reasons from there to say, okay, if gravity is
0:17:21 not detectable, then it can’t be like an ordinary force, right? The electromagnetic force is detectable.
0:17:28 We can put charged particles around positively charged particles and negatively charged particles
0:17:32 respond differently to an electric field or to a magnetic field. He realizes that what his thought
0:17:38 experiment showed or at least suggested is that gravity isn’t like that. Everything responds in
0:17:44 the same way to gravity. How could that be the case? And then this other leap he makes is, oh,
0:17:50 it’s because it’s the curvature of space time, right? It’s a feature of space time. It’s not a
0:17:54 force on top of it. And the feature that it is is curvature. And then finally, he says, okay,
0:17:59 clearly, I’m going to need the mathematical tools necessary to describe curvature. I don’t know them.
0:18:07 So I will learn them. And they didn’t have MOOCs or AI helpers back in those days. He had to
0:18:13 sit down and read the math papers. And he taught himself differential geometry
0:18:17 and invented general relativity. What about the step of including time
0:18:20 as just another dimension? So combining space and time. Is that a simple mathematical leap,
0:18:27 as Minkowski suggested? It’s certainly not simple, actually. It’s a profound insight. That’s why I
0:18:35 said I think we should give Minkowski more credit than we do. He’s the one who really put the finishing
0:18:41 touches on special relativity. Again, many people had talked about how things change when you move
0:18:49 close to the speed of light, what Maxwell’s equations of electromagnetism predict and so forth,
0:18:55 what their symmetries are. So people like Lorentz and Fitzgerald and Poincare, there’s a story that
0:19:00 goes there. And in the usual telling, Einstein sort of puts the capstone on it. He’s the one who says,
0:19:06 “All of this makes much more sense if there just is no ether. It is undetectable. We don’t know
0:19:11 how fast. Everything is relative, thus the name relativity.” But he didn’t take the actual final
0:19:16 step, which was to realize that the underlying structure that he had invented is best thought
0:19:22 of as unifying space and time together. I honestly don’t know what was going through Minkowski’s
0:19:27 mind when he thought that. I’m not sure if he was so mathematically adept that it was just
0:19:34 clear to him, or he was really struggling it and he did trial and error for a while. I’m not sure.
0:19:39 Do you, for him or for Einstein, visualize the four-dimensional space, try to play with the
0:19:44 idea of time as just another dimension? Oh yeah, all the time. I mean, we, of course, make our
0:19:50 lives easy by ignoring two of the dimensions of space. So instead of four-dimensional space time,
0:19:55 we just draw pictures of one dimension of space, one dimension of time, the so-called space-time
0:20:01 diagram. But maybe this is lurking underneath your question, but even the best physicists
0:20:08 will draw a vertical axis and a horizontal axis, and they’ll go space, time. But deep down, that’s
0:20:15 wrong because you’re sort of preferring one direction of space and one direction of time,
0:20:21 and it’s really the whole two-dimensional thing that is space-time. The more legitimate thing to
0:20:27 draw on that picture are rays of light, are light cones. From every point, there is a fixed
0:20:33 direction at which the speed of light would represent, and that is actually inherent in
0:20:39 the structure. The division into space and time is something that’s easy for us human beings.
0:20:44 What is the difference between space and time from the perspective of general relativity?
0:20:50 It’s the difference between x and y when you draw axes on a piece of paper.
0:20:54 So there’s really no difference. There’s almost no difference. There’s one difference that is
0:20:59 kind of important, which is the following. If you have a curve in space, I’m going to draw it
0:21:05 horizontally because that’s usually what we do in space-time diagrams. If you have a curve in space,
0:21:09 you’ve heard the motto before that the shortest distance between two points is a straight line.
0:21:14 If you have a curve in time, which is, by the way, literally all of our lives, we all evolve
0:21:20 in time. So you can start with one event in space-time and another event in space-time.
0:21:24 What Minkowski points out is that the time you measure along your trajectory in the universe
0:21:32 is precisely analogous to the distance you travel on a curve through space. And by precisely, I mean,
0:21:39 it is also true that the actual distance you travel through depends on your path,
0:21:45 right? You can go a straight line, shortest distance, you can curve a line to be longer.
0:21:49 The time you measure in space-time, the literal time that takes off in your clock,
0:21:53 also depends on your path. But it depends on it the other way so that the longest time
0:22:00 between two points is a straight line. And if you zig back and forth in space-time,
0:22:04 you take less and less time to go from point A to point B.
0:22:07 How do we make sense of that? The difference between the observed reality and the
0:22:16 objective reality are underneath it. Or is objective reality a silly notion given
0:22:21 general relativity? I’m a huge believer in objective reality. I think the objective reality
0:22:25 of objective is real. But I do think that people kind of are a little overly casual about the
0:22:34 relationship between what we observe and objective reality in the following sense.
0:22:39 Of course, in order to explain the world, our starting point and our ending point is our
0:22:46 observations, our experimental input, the phenomena we experience and see around us in the world.
0:22:52 But in between, there’s a theory. There’s a mathematical formalization of our ideas about
0:23:00 what is going on. And if a theory fits the data and is very simple and makes sense in its own
0:23:08 terms, then we say that the theory is right. And that means that we should attribute some
0:23:14 reality to the entities that play an important role in that theory, at least provisionally
0:23:20 until we come up with a better theory down the road. I think a nice way to test the difference
0:23:24 between objective reality and the observed reality is what happens at the edge of the horizon of a
0:23:31 black hole. So technically, as you get closer to that horizon, time stands still. Yes and no.
0:23:40 It depends on exactly how careful we’re being. So here’s a bunch of things I think are correct.
0:23:48 If you imagine there is a black hole space time, so like the whole solution Einstein’s equation,
0:23:55 and you treat you and me as what we call test particles, so we don’t have any gravitational
0:24:01 fields ourselves. We just move around in the gravitational field. And that’s obviously an
0:24:05 approximation. But let’s imagine that. And you stand outside the black hole and I fall in.
0:24:12 And as I’m falling in, I’m waving to you because I’m going into the black hole, you will see me
0:24:18 move more and more slowly. And also the light from me is redshifted. So I kind of look embarrassed
0:24:25 because I’m falling into a black hole. And there is a limit. There is a last moment
0:24:30 that light will be emitted from me from your perspective forever. Okay, now you don’t literally
0:24:37 see it because I’m emitting photons more and more slowly, right? Because from your point of view.
0:24:44 So it’s not like I’m equally bright. I basically fade from view in that picture. Okay, so that’s
0:24:50 one approximation. The other approximation is I do have a gravitational field of my own. And
0:24:56 therefore, as I approach the black hole, the black hole doesn’t just sit there and let me pass
0:25:01 through, it kind of moves out to eat me up because its net energy mass is going to be mine plus its.
0:25:09 But roughly speaking, yes, I think so. I don’t like to go to the dramatic extremes because that’s
0:25:14 where the approximations break down. But if you see something falling into a black hole,
0:25:17 you see its clock ticking more and more slowly. How do we know it fell in? We don’t.
0:25:24 I mean, how would we? Because it’s always possible that right at the last minute,
0:25:28 it had a change of heart and starts accelerating away, right? If you don’t see it, pass in. You
0:25:33 don’t know. And let’s point out that as smart as Einstein was, he never figured out black holes.
0:25:38 And he could have. It’s kind of embarrassing. It took decades for people thinking about
0:25:43 general relativity to understand that there are such things as black holes. Because basically,
0:25:49 Einstein comes up with general relativity in 1915. Two years later, Schwarzschild, Carl Schwarzschild,
0:25:56 derives the solution to Einstein’s equation that represents a black hole, the Schwarzschild
0:26:03 solution. No one recognized it for what it was until the 50s, David Finkelstein and other people.
0:26:09 And that’s just one of these examples of physicists not being as clever as they should have been.
0:26:13 Well, that’s the singularity. That’s kind of the edge of the theory, the limit. So it’s understandable
0:26:19 that’s difficult to imagine the limit of things. It is absolutely hard to imagine. And a black hole
0:26:25 is very different in many ways from what we’re used to. On the other hand, I mean, the real reason,
0:26:30 of course, is that between 1915 and 1955, there’s a bunch of other things that are really interesting
0:26:36 going on in physics, all particle physics and quantum field theory. So many of the greatest minds
0:26:41 were focused on that. But still, if the universe hands you a solution to general relativity in terms
0:26:46 of curved spacetime, and it’s kind of mysterious, certain features of it, I would put some effort
0:26:51 in trying to figure it out. So how does a black hole work? Put yourself in the shoes of Einstein
0:26:57 and take general relativity to its natural conclusion about these massive things.
0:27:01 It’s best to think of a black hole as not an object so much as a region of spacetime, okay?
0:27:09 It’s a region with the property, at least in classical general relativity, quantum mechanics
0:27:13 makes everything harder. But let’s imagine we’re being classical for the moment. It’s a region
0:27:18 of spacetime with the property that if you enter, you can’t leave. Literally, the equivalent of
0:27:24 escaping a black hole would be moving faster than the speed of light. They’re both precisely
0:27:29 equally difficult. You’d have to move faster than the speed of light to escape from the black hole.
0:27:32 So once you’re in, that’s fine. You know, in principle, you don’t even notice when you
0:27:38 cross the event horizon, as we call it. The event horizon is that point of no return,
0:27:42 where once you’re inside, you can’t leave. But meanwhile, the spacetime is sort of collapsing
0:27:48 around you to ultimately a singularity in your future, which means that the gravitational forces
0:27:55 are so strong, they tear your body apart, and you will die in a finite amount of time. The time it
0:28:01 takes, if the black hole is about the mass of the sun to go from the event horizon to the singularity,
0:28:08 takes about one millionth of a second. And what happens to you if you fall into the black hole?
0:28:14 If we think of an object as information, that information gets destroyed.
0:28:19 Well, you’ve raised a crucially difficult point. So that’s why I keep needing to distinguish between
0:28:28 black holes according to Einstein’s theory of general relativity, which is book one of
0:28:33 spacetime and geometry, which is perfectly classical. And then come the 1970s, we start
0:28:40 asking about quantum mechanics and what happens in quantum mechanics. According to classical
0:28:44 general relativity, the information that makes up you, when you fall into the black hole, is lost
0:28:50 to the outside world. It’s there, it’s inside the black hole, but we can’t get it anymore.
0:28:55 In the 1970s, Stephen Hawking comes along and points out that black holes radiate.
0:29:01 They give off photons and other particles to the universe around them. And as they radiate,
0:29:07 they lose mass and eventually they evaporate, they disappear. So once that happens, I can no longer
0:29:14 say the information about you or a book that I threw in the black hole or whatever is still
0:29:19 there is hidden behind the black hole, because the black hole has gone away. So either that
0:29:24 information is destroyed, like you said, or it is somehow transferred to the radiation
0:29:29 that is coming out to the Hawking radiation. A large majority of people who think about this
0:29:35 believe that the information is somehow transferred to the radiation and information is conserved.
0:29:40 That is a feature both of general relativity by itself and of quantum mechanics by itself.
0:29:46 So when you put them together, that should still be a feature. We don’t know that for sure. There
0:29:50 are people who have doubted it, including Stephen Hawking for a long time. But that’s what most
0:29:55 people think. And so what we’re trying to do now in a topic which has generated many,
0:30:01 many hundreds of papers called the black hole information loss puzzle is figure out how to
0:30:06 get the information from you or the book into the radiation that is escaping the black hole.
0:30:11 Is there any way to observe Hawking radiation to a degree where you can start getting insight?
0:30:18 Or is this all just in the space of theory right now?
0:30:21 Right now, we are nowhere close to observing Hawking radiation. Here’s the sad fact.
0:30:26 The larger the black hole is, the lower its temperature is. So a small black hole,
0:30:34 like a microscopically small black hole, might be very visible. It’s given off light.
0:30:38 But something like the black hole in the center of our galaxy,
0:30:40 three million times the mass of the sun or something like that, Sagittarius A star,
0:30:45 that is so cold and low temperature that its radiation will never be observable.
0:30:52 Black holes are hard to make. We don’t have any nearby. The ones we have out there in the
0:30:56 universe are very, very faint. So there’s no immediate hope for detecting Hawking radiation.
0:31:00 Allegedly, we don’t have any nearby.
0:31:02 As far as we know, we don’t have any nearby.
0:31:04 Could tiny ones be hard to detect?
0:31:06 Absolutely.
0:31:06 Somewhere at the edges of the solar system maybe?
0:31:08 So you don’t want them to be too tiny or they’re exploding, right? They’re very bright and then
0:31:14 they would be visible. But there’s an absolutely regime where black holes are large enough not to
0:31:19 be visible because the larger ones are fainter, right? Not giving off radiation,
0:31:22 but small enough to not be detected through their gravitational effect.
0:31:25 Yeah.
0:31:25 Psychologically, just emotionally, how do you feel about black holes? They scare you?
0:31:30 I love them. I love black holes. But the universe weirdly makes it hard to make a black hole,
0:31:36 right? Because you really need to squeeze an enormous amount of matter and energy into a
0:31:41 very, very small region of space. So we know how to make stellar black holes,
0:31:47 a supermassive star can collapse to make a black hole. We know we also have these supermassive
0:31:52 black holes at the center of galaxies. We’re a little unclear where they came from. I mean,
0:31:58 maybe stellar black holes that got together and combined. But that’s one of the
0:32:04 exciting things about new data from the James Webb Space Telescope is that
0:32:09 quite large black holes seem to exist relatively early in the history of the universe.
0:32:14 So it was already difficult to figure out where they came from. Now it’s an even tougher puzzle.
0:32:18 So these supermassive black holes were formed somewhere early on in the universe.
0:32:23 I mean, that’s a feature, not a bug, right? That we don’t have too many of them. Otherwise,
0:32:27 we wouldn’t have the time or the space to form the little pockets of complexity that we call humans.
0:32:36 I think that’s fair. Yeah. It’s always interesting when something is difficult,
0:32:42 but happens anyway, right? I mean, the probability of making a black hole could have been zero.
0:32:47 It could have been one. But it’s this interesting number in between, which is kind of fun.
0:32:51 Are there more intelligent alien civilization than there are supermassive black holes?
0:32:55 Yeah. I have no idea. But I think your intuition is right that it would have been easy for there
0:33:04 to be lots of civilizations and then we would have noticed them already. And we haven’t. So
0:33:09 absolutely the simplest explanation for why we haven’t is that they’re not there.
0:33:13 Yeah. I just think it’s so easy to make them though. So I understand that’s the simplest
0:33:19 explanation. How easy is it to make life or eukaryotic life or multicellular life?
0:33:26 It seems like life finds a way. Intelligent alien civilizations, sure. Maybe there is
0:33:32 somewhere along that chain a really, really hard leap. But once you start life, once you get the
0:33:39 origin of life, it seems like life just finds a way everywhere in every condition. They just
0:33:44 figures it out. I mean, I get it. I get exactly what you’re thinking. I think it’s a perfectly
0:33:49 reasonable attitude to have before you confront the data. I would not have expected it to be
0:33:56 special in any way. I would have expected there to be plenty of very noticeable
0:33:59 extraterrestrial civilizations out there. But even if life finds a way, even if we buy
0:34:07 everything you say, how long does it take for life to find a way? What if it typically
0:34:12 takes 100 billion years? Then we’d be alone. So it’s a time thing. So to you, really, there’s
0:34:19 most likely there’s no alien civilizations out there. I just, I can’t see it. I believe there’s
0:34:25 a ton of them and there’s another explanation why we can’t see them. I don’t believe that very
0:34:29 strongly. Look, I’m not going to place a lot of bets here. I would not, I’m both pretty up in the
0:34:35 air about whether or not life itself is all over the place. It’s possible when we visit
0:34:40 other worlds, other solar systems, there’s very tiny microscopic life ubiquitous, but none of it
0:34:47 has reached some complex form. It’s also possible there’s just, there isn’t any. It’s also possible
0:34:54 that there are intelligent civilizations that have better things to do than knock on our doors.
0:34:58 So I think we should be very humble about these things we know so little about.
0:35:02 And it’s also possible there’s a great filter where there’s something fundamental about
0:35:06 once a civilization develops complex enough technology, that technology is more
0:35:12 statistically likely to destroy everybody versus to continue being creative.
0:35:18 That is absolutely possible. I’m actually putting less credence on that one just because you need
0:35:23 to happen every single time, right? If even one, I mean, this goes back to von Neumann pointing,
0:35:29 John von Neumann pointed out that you don’t need to send the aliens around the galaxy.
0:35:34 You can build self-reproducing probes and send them around the galaxy.
0:35:39 And you might think, well, the galaxy is very big. It’s really not. It’s some tens of thousands
0:35:44 of light years across and billions of years old. So you don’t need to move at a high fraction
0:35:51 the speed of light to fill the galaxy. So if you were an intelligent alien civilization,
0:35:57 the dictator of one, you would just send out a lot of probes, self-reproducing probes.
0:36:00 100%. And just spread out.
0:36:03 Yes. And what you should do, so if you want the optimistic spin, here’s the optimistic spin.
0:36:07 People looking for intelligent life elsewhere often tune in with their radio telescopes,
0:36:13 right? At least we did before Arecibo was decommissioned. That’s not a very promising way
0:36:21 to find intelligent life elsewhere because why in the world would a super intelligent alien
0:36:25 civilization waste all of its energy by beaming it in random directions into the sky? For one thing,
0:36:32 it just passes you by, right? So if we’re here on Earth, we’ve only been listening to radio waves
0:36:38 for a hundred or a couple hundred years, okay? So if intelligent alien civilization exists for
0:36:45 a billion years, they have to pinpoint exactly the right time to send us this signal. It is
0:36:51 much, much more efficient to send probes and to park, to go to the other solar systems,
0:36:58 just sit there and wait for an intelligent civilization to arise in that solar system.
0:37:04 This is kind of the 2001 monolith hypothesis, right? I would be less surprised to find
0:37:12 sort of quiescent alien artifact in our solar system than I would to catch a radio signal
0:37:20 from an intelligent civilization. So you’re a sucker for in-person conversations versus remote?
0:37:26 I just want to integrate over time. A probe can just sit there and wait, whereas a radio wave goes
0:37:33 right by you. How hard is it for an alien civilization, again, you’re the dictator of one,
0:37:39 to figure out a probe that is most likely to find a common language with whatever it finds?
0:37:47 Couldn’t it be like the elected leader of a democratic alien civilization?
0:37:54 I think we would figure out that language thing pretty quickly. I mean, maybe not
0:37:59 as quickly as we do when different human tribes find each other, because obviously there’s a lot
0:38:05 of commonalities in humanity. But there is logic and math, and there is the physical world. You
0:38:11 can point to a rock and go rock, right? I don’t think it would take that long. I know that arrival,
0:38:18 the movie, based on a Ted Chang story, suggested that the way that aliens communicate is going to be
0:38:25 fundamentally different, but also they had recognition and other things I don’t believe in.
0:38:30 So I think that if we actually find aliens, that will not be our long-term problem.
0:38:37 So one of the places you’re affiliated with is Santa Fe, and they approach the question of
0:38:41 complexity in many different ways and ask the question in many different ways of what is life,
0:38:46 thinking broadly. So you would be able to find it. You show up, approach shows up to a planet,
0:38:55 we’ll see a thing, and be like, yeah, that’s a living thing.
0:38:59 Well, again, if it’s intelligent and technologically advanced, the more short-term question of if we
0:39:09 get some spectroscopic data from an exoplanet, so we know a little bit about what is in its atmosphere,
0:39:16 how can we judge whether or not that atmosphere is giving us a signature of life existing?
0:39:21 That’s a very hard question that people are debating about. I mean, one very simple-minded,
0:39:26 but perhaps interesting approach is to say small molecules don’t tell you anything because even if
0:39:34 life could make them, something else could also make them, but long molecules, that’s the kind
0:39:38 of thing that life would produce. So signs of complexity. I don’t know. I just have this
0:39:45 nervous feeling that we won’t be able to detect. We’ll show up to a planet,
0:39:50 there’s a bunch of liquid on it. We take a swim in the liquid and we won’t be able to see the
0:39:57 intelligence in it, whether that intelligence looks like something like ants or we’ll see
0:40:06 movement, perhaps strange movement, but we won’t be able to see the intelligence in it or communicate
0:40:15 with it. I guess if we have nearly infinite amount of time to play with different ideas,
0:40:20 we might be able to. I think I’m in favor of this kind of humility, this intellectual humility
0:40:26 that we won’t know because we should be prepared for surprises, but I do always keep coming back
0:40:32 to the idea that we all live in the same physical universe. Let’s put it this way. The development
0:40:40 of our intelligence has certainly been connected to our ability to manipulate the physical world
0:40:47 around us. I would guess without 100% credence by any means, but my guess would be that any
0:40:55 advanced kind of life would also have that capability. Both dolphins and octopuses are
0:41:03 potential counter examples to that, but I think in the details, there would be enough similarities
0:41:09 that we would recognize it. I don’t know how we got on this topic, but I think it was from
0:41:13 supermassive black holes. If we return to black holes and talk about the holographic principle
0:41:19 more broadly, you have a recent paper on the topic. You’ve been thinking about the topic in terms of
0:41:25 rigorous research perspective and just as a popular book writer. What is the holographic
0:41:33 principle? Well, it goes back to this question that we were talking about with the information
0:41:38 and how it gets out. In quantum mechanics, certainly, arguably even before quantum mechanics
0:41:45 comes along in classical statistical mechanics, there’s a relationship between information
0:41:51 and entropy. Entropy is my favorite thing to talk about that I’ve written books about. We’ll
0:41:56 continue to write books about. Hawking tells us that black holes have entropy and it’s a finite
0:42:02 amount of entropy. It’s not an infinite amount, but the belief is, and now we’re already getting
0:42:07 quite speculative, the belief is that the entropy of a black hole is the largest amount of entropy
0:42:15 that you can have in a region of spacetime. It’s the most densely packed that entropy can be.
0:42:21 What that means is there’s a maximum amount of information that you can fit into that region
0:42:26 of space and you call it a black hole. Interestingly, you might expect if I have a box,
0:42:31 and I’m going to put information in it. I don’t tell you how I’m going to put the information in,
0:42:37 but I ask, how does the information I can put in scale with the size of the box?
0:42:42 You might think, well, it goes as the volume of the box because the information takes up some
0:42:47 volume and I can only fit in a certain amount. That is what you might guess for the black hole,
0:42:52 but it’s not what the answer is. The answer is that the maximum information, as reflected in the
0:42:57 black hole entropy, scales as the area of the black holes event horizon, not the volume inside.
0:43:06 So people thought about that in both deep and superficial ways for a long time,
0:43:11 and they proposed what we now call the holographic principle that the way that spacetime and quantum
0:43:17 gravity convey information or hold information is not different bits or qubits for quantum
0:43:26 information at every point in spacetime. It is something holographic, which means it’s embedded
0:43:33 in or located in or can be thought of as pertaining to one dimension less of the three dimensions of
0:43:41 space that we live in. So in the case of the black hole, the event horizon is two-dimensional,
0:43:45 embedded in a three-dimensional universe, and the holographic principle would say
0:43:48 all of the information contained in the black hole can be thought of as living on the event
0:43:53 horizon rather than in the interior of the black hole. I need to say one more thing about that,
0:44:00 which is that this was an idea. The idea I just told you was the original holographic principle
0:44:04 put forward by people like Gerard Tuft and Leonard Susskind, a super famous
0:44:08 physicist. Leonard Susskind was on my podcast and gave a great talk. He’s very good at explaining
0:44:15 these things. My escape podcast, everybody can listen. That’s right, yes. And you don’t just
0:44:20 have physicists on. I don’t. I love my escape. Oh, thank you very much. Curiosity driven. Yeah,
0:44:26 ideas. Exploration of ideas. Yeah. But anyway, what I was trying to get at was Susskind and also
0:44:31 at Tuft were a little vague. They were a little hand wavy about holography and what it meant,
0:44:36 where holography, the idea that information is sort of encoded on a boundary really came into
0:44:42 its own was with Juan Maldesena in the 1990s and the ADS CFD correspondence, which we don’t have
0:44:50 to get into that into any detail, but it’s a whole full blown theory of it’s two different
0:44:56 theories. One theory in n dimensions of space time without gravity and another theory in n plus
0:45:03 one dimensions of space time with gravity. And the idea is that this n dimensional theory is,
0:45:08 you know, casting a hologram into the n plus one dimensional universe to make it look like it has
0:45:14 gravity. And that’s holography with a vengeance. And that’s that’s an enormous source of interest
0:45:22 for theoretical physicists these days. How should we picture what impact that has?
0:45:27 The fact that you can store all the information you could think of as all the information that goes
0:45:33 into a black hole can be stored at the event horizon. Yeah, I mean, it’s a good question.
0:45:39 One of the things that quantum field theory indirectly suggests is that there’s not that
0:45:46 much information in you and me compared to the volume of space time we take up. As far as quantum
0:45:52 field theory is concerned, you and I are mostly empty space. And so we are not information dense,
0:45:59 right, the density of information in us or in a book or a CD or whatever computer RAM is indeed
0:46:07 encoded by volume, like there’s different bits located at different points in space.
0:46:11 But that density of information is super duper low. So we’re just like the speed of light or just
0:46:16 like the big bang. For the information in a black hole, we are far away in our everyday experience
0:46:22 from the regime where these questions become relevant. So it’s very far away from our intuition.
0:46:27 We don’t really know how to think about these things. We can do the math, but we don’t feel it
0:46:31 in our bones. So you can just write off that weird stuff happens in a black hole. Well, we’d like to
0:46:36 do better, but we’re trying. I mean, that’s why we have an information loss puzzle because we haven’t
0:46:41 completely solved it. So here’s just one thing to keep in mind. Once space time becomes flexible,
0:46:50 which it does according to general relativity, and you have quantum mechanics, which has fluctuations
0:46:56 and virtual particles and things like that, the very idea of a location in space time becomes a
0:47:01 little bit fuzzy, right? Because it’s flexible and quantum mechanics says you can’t even pin it down.
0:47:06 So information can propagate in ways that you might not have expected. And that’s easy to say,
0:47:13 and it’s true, but we haven’t yet come up with the right way to talk about it that is perfectly rigorous.
0:47:18 But it’s crazy how dense with information a black hole is. And then plus like quantum mechanics
0:47:24 starts to come into play. So, you know, you almost want to romanticize the kind of an
0:47:29 interesting computation type things that are going on inside the black hole.
0:47:32 You do, you do. But I will point out one other thing. It’s information dense, but it’s also very,
0:47:39 very high entropy. So a black hole is kind of like a very, very, very specific random number,
0:47:46 right? It takes a lot of digits to specify it, but the digits don’t tell you anything. They don’t
0:47:52 give you anything useful to work on. So it takes a lot of information, but it’s not of a form that
0:47:58 we can learn a lot from. But hypothetically, I guess, as you mentioned, the information might be
0:48:06 preserved. The information that goes into a black hole, it doesn’t get destroyed. So what does that
0:48:11 mean when the entropy is really high? Well, the black hole, I said that the black hole is the
0:48:16 highest density of information, but it’s not the highest amount of information because the black
0:48:22 hole can evaporate. And when it evaporates, and people have done the equations for this,
0:48:27 when it evaporates, the entropy that it turns into is actually higher than the entropy of the
0:48:32 black hole was, which is good because entropy is supposed to go up. But it’s much more dilute,
0:48:37 right? It’s spread across a huge volume of space time. So in principle, all that you made the
0:48:44 black hole out of, the information that it took is still there, we think, in that information,
0:48:50 but it’s scattered to the forewinds. We just talked about the event horizon of a black hole.
0:48:54 What’s on the inside? What’s at the center of it? No one’s been there.
0:48:58 So, again, this is a theoretical prediction. But I’ll say one super crucial feature of the
0:49:05 black holes that we know and love, the kind that Schwarzschild first invented. There’s a
0:49:10 singularity, but it’s not at the middle of the black hole. Remember, space and time are parts
0:49:16 of two different parts of one unified spacetime. The location of the singularity in the black hole
0:49:22 is not the middle of space, but our future. It is a moment of time. It is like a big crunch.
0:49:29 The big bang was an expansion from a singularity in the past. Big crunch probably doesn’t exist,
0:49:34 but if it did, it would be a collapse to a singularity in the future. That’s what the
0:49:38 interiors of black holes are like. You can be fine in the interior, but things are becoming more
0:49:44 and more crowded. Space time is becoming more and more warped, and eventually you hit a limit,
0:49:48 and that’s the singularity in your future. I wonder what time is like on the inside of a
0:49:53 black hole. Time always ticks by at one second per second. That’s all it can ever do. Time can
0:49:59 tick by differently for different people, and so you have things like the twin paradox, where two
0:50:04 people initially are the same age. One goes off near the speed of light and comes back. Now they’re
0:50:09 not. You can even work out that the one who goes out and comes back will be younger because they
0:50:14 did not take the shortest distance path. Locally, as far as you and your wristwatch are concerned,
0:50:21 time is not funny. Your neurological signals in your brain and your heartbeat and your wristwatch,
0:50:30 whatever is happening to them, is happening to all of them at the same time, so time always
0:50:35 seems to be ticking along at the same rate. If you fall into a black hole and then I’m an observer
0:50:41 just watching it, and then you come out once it evaporates a million years later, I guess you’ll
0:50:51 be exactly the same age. Have you aged at all? You would be converted into photons. You would not
0:50:57 be you anymore. Right. So it’s not at all possible that information is preserved exactly as it went
0:51:03 in. It depends on what you mean by preserved. It’s there in the microscopic configuration of the
0:51:08 universe. It’s exactly as if I took a regular book, made a paper, and I burned it. The laws of
0:51:15 physics say that all the information in the book is still there in the heat and light and ashes.
0:51:20 You’re never going to get it, it’s a matter of practice, but in principle, it’s still there.
0:51:24 But what about the age of things from the observer perspective, from outside the black hole?
0:51:29 From outside the black hole doesn’t matter because they’re inside the black hole.
0:51:35 No. Okay. There’s no way to escape the black hole except to let it evaporate.
0:51:41 To let it evaporate. But also, by the way, just in relativity, special relativity,
0:51:46 forget about general relativity, it’s enormously tempting to say, “Okay, here’s what’s happening
0:51:52 to me right now. I want to know what’s happening far away right now.” The whole point of relativity
0:51:58 is to say there’s no such thing as right now when you’re far away. And that is doubly true for what’s
0:52:04 inside a black hole. So you’re tempted to say, “Well, how fast is their clock ticking, or how old
0:52:09 are they now?” Not allowed to say that according to relativity. Because space and time are treated
0:52:15 the same and so it doesn’t even make sense. What happens to time in the holographic principle?
0:52:20 As far as we know, nothing dramatic happens. We’re not anywhere close to being confident that we
0:52:28 know what’s going on here yet. So there are good unanswered questions about whether time is fundamental,
0:52:34 whether time is emergent, whether it has something to do with quantum entanglement,
0:52:38 whether time really exists at all, different theories, different proponents of different
0:52:44 things. But there’s nothing specifically about holography that would make us change our opinions
0:52:50 about time, whatever they happen to be. But holography is fundamentally about
0:52:53 the question of space? It really is. Yeah. Okay, so time is just like a-
0:52:58 Time just goes along for the ride, as far as we know. So all the questions about time is just
0:53:01 almost like separate questions, whether it’s emergent and all that kind of stuff. Yeah. I mean,
0:53:05 that might be a reflection of our ignorance right now, but yes. If we figure out a lot,
0:53:11 millions of years from now about black holes, how surprised would you be if they travel back
0:53:16 in time and told you everything you want to know about black holes? How much
0:53:20 do you think there is still to know? And how mind-blowing would it be?
0:53:29 It does depend on what they would say. I think that there are colleagues of mine
0:53:35 who think that we’re pretty close to figuring out how information gets out of black holes,
0:53:41 how to quantize gravity, things like that. I’m more skeptical that we are pretty close. I think
0:53:46 that there’s room for a bunch of surprises to come. So in that sense, I suspect I would be surprised.
0:53:53 The biggest and most interesting surprise to me would be if quantum mechanics itself
0:53:59 were somehow superseded by something better. As far as I know, there’s no empirical evidence-based
0:54:07 reason to think that quantum mechanics is not 100% correct. But it might not be. That’s always
0:54:13 possible. And there are, again, respectable friends of mine who speculate about it. So
0:54:20 that’s the first thing I’d want to know. The black hole would be the most clear illustration.
0:54:27 If there’s something, it would show up there. Maybe. The point is that black holes are mysterious
0:54:33 for various reasons. So, yeah, if our best theory of the universe is wrong, that might help explain
0:54:38 why. Do you think it’s possible we’ll find something interesting like black holes sometimes
0:54:45 create new universes? Or black holes are a kind of portal through space-time to another place or
0:54:51 something like this? And then our whole conception of what is the fabric of space-time changes
0:54:57 completely because black holes is like Swiss cheese type of situation. Yeah, that would be
0:55:04 less surprising to me because I’ve already written papers about that. We don’t have, again, strong
0:55:11 reason to think that the interior black hole leads to another universe. But it is possible. And it’s
0:55:16 also very possible that that’s true for some black holes and not others. This is stuff we don’t
0:55:21 know. It’s easy to ask questions we don’t know the answer to. The problem is the questions that are
0:55:25 easy to ask that we don’t know the answer to are super hard to answer. Because these objects are
0:55:30 very difficult to test and to explore. The regimes are just very far away. So either literally far
0:55:34 away in space, but also in energy or mass or time or whatever. You’ve published a paper on the
0:55:41 holographic principle or that involves the holographic principle. What can you explain
0:55:44 the details of that? Yeah, I’m always interested in, since my first published paper, taking these
0:55:52 wild speculative ideas and trying to test them against data. And the problem is when you’re
0:55:57 dealing with wild speculative ideas, they’re usually not well-defined enough to make a prediction,
0:56:04 right? Like it’s kind of, I know it’s going to happen in some cases. I don’t know what’s going
0:56:07 to happen in other cases. So we did the following thing. As I’ve already mentioned, the holographic
0:56:14 principle, which is meant to reflect the information contained in black holes, seems to be telling us
0:56:20 that information, there’s less information, less stuff that can go on than you might naively expect.
0:56:28 So let’s upgrade naively expect to predict using quantum field theory. Quantum field theory is
0:56:34 our best theory of fundamental physics right now. Unlike this holographic black hole stuff,
0:56:39 quantum field theory is entirely local. In every point of space, something can go on and then you
0:56:45 add up all the different points in space, okay? Not holographic at all. So there’s a mismatch
0:56:50 between the expectation for what is happening, even in empty space in quantum field theory,
0:56:54 versus what the holographic principle would predict. How do you reconcile these two things?
0:57:00 So there’s one way of doing it that had been suggested previously, which is to say that
0:57:05 in the quantum field theory way of talking, it implies there’s a whole bunch more states,
0:57:12 a whole bunch more ways the system could be than there really are. And just I’ll do a little bit
0:57:19 of math, just because there might be some people in the audience who like the math. If I draw two
0:57:25 axes on a two-dimensional geometry, like the surface of the table, right, you know that the
0:57:31 whole point of it being two-dimensional is I can draw two vectors that are perpendicular to each
0:57:35 other. I can’t draw three vectors that are all perpendicular to each other, right? They need
0:57:40 to overlap a little bit. That’s true for any numbers of dimensions. But I can ask, okay,
0:57:46 how much do they have to overlap? If I try to put more vectors into a vector space
0:57:52 than the dimensionality of the vector space, can I make them almost perpendicular to each other?
0:57:59 And the mathematical answer is, as the number of dimensions gets very, very large, you can fit a
0:58:05 huge extra number of vectors in that are almost perpendicular to each other. So in this case,
0:58:12 what we’re suggesting is the number of things that can happen in a region of space is correctly
0:58:19 described by holography. It is somewhat over counted by quantum field theory. But that’s because
0:58:26 the quantum field theory states are not exactly perpendicular to each other. I should have mentioned
0:58:32 that in quantum mechanics, states are given by vectors in some huge dimensional vector space,
0:58:36 very, very, very, very large dimensional vector space. So maybe the quantum field theory states
0:58:42 are not quite perpendicular to each other. If that is true, that’s a speculation already,
0:58:48 but if that’s true, how would you know? What is the experimental deviation? And it would have been
0:58:54 completely respectable if we had gone through and made some guesses and found that there is no
0:58:59 noticeable experimental difference because, again, these things are in regimes very, very far away.
0:59:05 We stuck our necks out. We made some very, very specific guesses as to how this weird overlap
0:59:13 of states would show up in the equations of motion for particles like neutrinos. And then
0:59:20 we made predictions on how the neutrinos would behave on the basis of those wild guesses. And
0:59:26 then we compared them with data. And what we found is we’re pretty close, but haven’t yet
0:59:33 reached the detectability of the effect that we are predicting. In other words, basically,
0:59:39 one way of saying what we predict is, if a neutrino, and there’s reasons why it’s neutrinos,
0:59:43 we can go into it if you want, but it’s not that interesting. The neutrino comes to us from across
0:59:47 the universe from some galaxy very, very far away. There is a probability as it’s traveling
0:59:53 that it will dissolve into other neutrinos because they’re not really perpendicular to each other
0:59:58 as vectors, as they would ordinarily be in quantum field theory. And that means that if you look
1:00:03 at neutrinos coming from far enough away with high enough energies, they should disappear.
1:00:10 Like if you see a whole bunch of nearby neutrinos, but then further away, you should see fewer.
1:00:16 And there is an experiment called Icecube, which is this amazing testament to the ingenuity of
1:00:24 human beings, where they go to Antarctica, and they drill holes, and they put photo detectors
1:00:31 on a string a mile deep in these holes. And they basically use all of the ice in a cube.
1:00:39 I don’t know whether it’s a mile or not, but it’s like a kilometer or something like that,
1:00:42 some big region. That much ice is their detector. And they’re looking for flashes when a cosmic
1:00:49 ray or neutrino or whatever hits ice molecule, water molecule in the ice.
1:00:55 With flashes in the ice. They’re looking for flashes.
1:00:59 But isn’t there some craze? What does the detector of that look like?
1:01:02 It’s a bunch of strings, many, many, many strings with 360-degree photo detectors.
1:01:10 That’s really cool.
1:01:13 It’s extremely cool. And they’ve done amazing work, and they find neutrinos.
1:01:18 They’re looking for neutrinos.
1:01:19 Yeah. So the whole point is most cosmic rays are protons. Because why? Because protons exist,
1:01:26 and they’re massive enough that you can accelerate them to very high energies.
1:01:31 So high-energy cosmic rays tend to be protons.
1:01:34 They also tend to hit the Earth’s atmosphere and decay into other particles.
1:01:38 So neutrinos, on the other hand, punch right through, at least usually, to a great extent.
1:01:44 So not just Antarctica, but the whole Earth.
1:01:46 Occasionally, a neutrino will interact with a particle here on Earth.
1:01:52 And a neutrino is going through your body all the time, from the Sun, from the universe, etc.
1:01:56 And so if you’re patient enough, and you have a big enough part of the Antarctic ice sheet to look at,
1:02:02 it’s the nice thing about ice is it’s transparent.
1:02:05 So you’ve built, yourself, nature has built you a neutrino detector.
1:02:09 So why ice cube does?
1:02:10 Why ice? So is it just because of the low noise, and you get to watch this thing, and it’s…
1:02:16 It’s much more dense than air, but it’s transparent.
1:02:21 So you have much more dense, so higher probability, and then it’s transparency,
1:02:25 and then it’s also in the middle of nowhere.
1:02:27 That’s all you need. There’s not that much ice, right?
1:02:30 Yeah. So there’s more ice in Antarctica than anywhere else.
1:02:33 Right. So anyway, you can go and you can get a plot from the ice cube experiment,
1:02:39 how many neutrinos there are that they’ve detected with very high energies.
1:02:43 And we predict, in our weird little holographic guessing game, that there should be a cut-off.
1:02:49 You should see neutrinos as you get to higher and higher energies, and then they should disappear.
1:02:54 If you look at the data, their data gives out exactly where our cut-off is.
1:03:00 That doesn’t mean that our cut-off is right.
1:03:02 It means they lose the ability to do the experiment exactly where we predict the cut-off should be.
1:03:06 Oh, boy. Okay.
1:03:08 But why is there a limit?
1:03:12 Oh, just because there are fewer and fewer high-energy neutrinos.
1:03:15 So there’s a spectrum, and it goes down, but what we’re plotting here is
1:03:20 the number of neutrinos versus energy, it’s fading away, and they just get very, very few.
1:03:25 And you need the high-energy neutrinos for your prediction.
1:03:29 Our effect is a little bit bigger for higher energies, yeah.
1:03:32 And that effect has to do with this almost perpendicular thing.
1:03:35 And let me just mention the name of Oliver Friedrich, who was a postdoc who led this.
1:03:39 He deserves the credit for doing this. I was a co-author and a collaborator.
1:03:42 I did some work, but he really gets the lines here.
1:03:45 Thank you, Oliver. Thank you for pushing this wild science forward.
1:03:49 Just to speak to that, the meta process of it, how do you approach asking these big questions
1:03:57 and trying to formulate as a paper, as an experiment that could make a prediction,
1:04:02 all that kind of stuff? What’s your process?
1:04:04 There’s a very interesting thing that happens once you’re a theoretical physicist,
1:04:08 once you become trained. You’re a graduate student, you’ve written some papers and whatever.
1:04:12 Suddenly, you are the world’s expert in a really infinitesimally tiny area of knowledge, right?
1:04:17 And you know not that much about other areas.
1:04:20 There’s an overwhelming temptation to just drill deep, right?
1:04:24 Just keep doing basically the thing that you started doing.
1:04:26 But maybe the thing you started doing is not the most interesting thing,
1:04:31 to the world or to you or whatever.
1:04:34 So you need to separately develop the capability of stepping back and going,
1:04:38 okay, now that I can write papers in that area,
1:04:42 now that I’m sort of trained enough in the general procedure,
1:04:46 what is the best match between my interests, my abilities and what is actually interesting?
1:04:52 And honestly, I’ve not been very good at that over my career.
1:04:57 You know, I have, my process traditionally was I was working in this general area of
1:05:04 particle physics, field theory, general relativity, cosmology.
1:05:09 And I would sort of try to take things other people were talking about
1:05:15 and ask myself whether or not it really fit together.
1:05:18 Like my two, so I guess I have three papers that I’ve ever written
1:05:23 that have done super well in terms of getting cited and things like that.
1:05:28 One was my first ever paper that I get very little credit for,
1:05:31 that was my advisor and his collaborator, you know, set that up.
1:05:35 The other two were basically my idea.
1:05:36 One was right after we discovered that the universe was accelerating.
1:05:41 So in 1998, observations show that not only is universe expanding,
1:05:44 but it’s expanding faster and faster.
1:05:46 So that’s attributed to either Einstein’s cosmological constant
1:05:51 or some more complicated form of dark energy,
1:05:53 some mysterious thing that fills the universe.
1:05:56 And people were throwing around ideas about this dark energy stuff,
1:05:59 what could it be and so forth.
1:06:00 Most of the people throwing around these ideas were cosmologists.
1:06:04 They work on cosmology.
1:06:05 They think about the universe all at once.
1:06:08 I, you know, since I like to talk to people in different areas,
1:06:13 I was sort of more familiar than average
1:06:16 with what a respectable working particle physicist
1:06:19 would think about these things.
1:06:21 And what I immediately thought was, you know,
1:06:23 you guys are throwing around these theories.
1:06:25 These theories are wildly unnatural.
1:06:27 They’re super finely tuned.
1:06:28 Like any particle physicist would just be embarrassed to be talking about this.
1:06:31 But rather than just scoffing at them,
1:06:36 I sat down and asked myself, okay, is there a respectable version?
1:06:40 Is there a way to keep the particle physicist happy,
1:06:43 but also make the universe accelerate?
1:06:45 And I realized that there is some very specific set of models
1:06:49 that is that is relatively natural.
1:06:51 And guess what?
1:06:53 You can make a new experimental prediction on the basis of those.
1:06:56 And so I did that.
1:06:57 People were very happy about that.
1:06:59 What was the thing that would make physicists happy?
1:07:01 That would make sense of this fragile thing that people call dark energy.
1:07:08 So the fact that dark energy pervades the whole universe and is slowly changing,
1:07:15 that should immediately set off alarm bells.
1:07:17 Because particle physics is a story of length scales and time scales
1:07:23 that are generally, guess what, small, right?
1:07:26 Particles are small.
1:07:27 They vibrate quickly.
1:07:29 And you’re telling me now, I have a new field.
1:07:31 And its typical rate of change is once every billion years, right?
1:07:35 Like that’s just not natural.
1:07:37 And indeed, you can formalize that and say, you know, look,
1:07:42 even if you wrote down a particle that evolved slowly over billions of years,
1:07:47 if you let it interact with other particles at all, that would make it move faster.
1:07:54 Its dynamics would be faster.
1:07:55 Its mass would be higher, et cetera, et cetera.
1:07:57 So there’s a whole story.
1:07:58 Things need to be robust and they all talk to each other in quantum field theory.
1:08:01 So how do you stop that from happening?
1:08:03 And the answer is symmetry.
1:08:04 You can impose a symmetry that protects your new field from talking to any other fields.
1:08:11 Okay.
1:08:12 And this is good for two reasons.
1:08:14 Number one, it can keep the dynamics slow.
1:08:17 So if you just, you can’t tell me why it’s slow.
1:08:19 You just made that up, but at least it can protect it from speeding up
1:08:23 because it’s not talking to any other particles.
1:08:25 And the other is it makes it harder to detect.
1:08:27 Naively, experiments looking for fifth forces or time changes of fundamental
1:08:36 constants of nature like the charge of the electron, these experiments
1:08:40 should have been able to detect these dark energy fields.
1:08:44 And I was able to propose a way to stop that from happening.
1:08:47 The detection.
1:08:48 The detection, yeah, because a symmetry could stop it from interacting
1:08:52 with all these other fields and therefore makes it harder to detect.
1:08:55 And just by luck, I realized, because it was actually based on my first ever paper,
1:08:59 there’s one loophole.
1:09:01 If you impose these symmetries, so you protect the dark energy field
1:09:06 from interacting with any other fields, there’s one interaction that is still
1:09:11 allowed that you can’t rule out.
1:09:12 And it is a very specific interaction between your dark energy field and photons,
1:09:18 which are very common.
1:09:20 And it has the following effect.
1:09:22 As a photon travels through the dark energy, the photon has a polarization
1:09:26 up, down, left, right, whatever it happens to be.
1:09:30 And as it travels through the dark energy, that photon will rotate its polarization.
1:09:33 This is called birefringence.
1:09:35 And you can kind of run the numbers and say, you know, you can’t make a very
1:09:40 precise prediction because we’re just making up this model.
1:09:43 But if you want to roughly fit the data, you can predict how much
1:09:47 polarization rotation there should be a couple of degrees.
1:09:50 Okay, not that much.
1:09:52 So that’s very hard to detect.
1:09:54 People have been trying to do it.
1:09:56 Right now, literally, we’re on the edge of either being able to detect it
1:10:01 or rule it out using the cosmic microwave background.
1:10:04 And there is just, you know, truth in advertising.
1:10:07 There is a claim on the market that it’s been detected, that it’s there.
1:10:12 It’s not very statistically significant.
1:10:15 If I were to bet, I think it would probably go away.
1:10:19 It’s a very hard thing to observe.
1:10:21 But maybe as you get better and better data, cleaner and cleaner analysis,
1:10:26 it will persist and we will have directly detected the dark energy.
1:10:29 So if we just take this tangent of dark energy, people will sometimes bring up dark
1:10:36 energy and dark matter as an example why physicists have lost it, lost their mind.
1:10:44 We’re just going to say that there is this field that permeates everything.
1:10:49 It’s unlike any other field and it’s invisible.
1:10:51 And it helps us work out some of the math.
1:10:55 How do you respond to that kind of suggestion?
1:10:59 Well, two ways.
1:11:00 One way is those people would have had to say the same thing
1:11:04 when we discovered the planet Neptune.
1:11:05 Because it’s exactly analogous where we have a very good theory, in that case,
1:11:12 Newtonian gravity in the solar system.
1:11:14 We made predictions.
1:11:15 The predictions were slightly off for the motion of the outer planets.
1:11:19 You found that you could explain that motion by positing something very simple.
1:11:24 One more planet in a very, very particular place.
1:11:28 And you went and looked for it and there it was.
1:11:29 That was the first successful example of finding dark matter in the universe.
1:11:34 It’s a matter that we can’t see.
1:11:36 Neptune was dark.
1:11:36 There’s a difference between dark matter and dark energy.
1:11:40 Dark matter, as far as we are hypothesizing it, is a particle of some sort.
1:11:46 It’s just a particle that interacts with us very weakly.
1:11:49 So we know how much of it there is.
1:11:50 We know more or less where it is.
1:11:52 We know some of its properties.
1:11:54 We don’t know specifically what it is.
1:11:56 But it’s not anything fundamentally mysterious.
1:12:01 It’s a particle.
1:12:02 Dark energy is a different story.
1:12:04 So dark energy is indeed uniformly spread throughout space
1:12:09 and has this very weird property that it doesn’t seem to evolve,
1:12:13 as far as we can tell.
1:12:14 It’s the same amount of energy in every cubic centimeter of space
1:12:18 from moment to moment in time.
1:12:19 That’s why far and away the leading candidate for dark energy
1:12:23 is Einstein’s cosmological constant.
1:12:25 The cosmological constant is strictly constant, 100% constant.
1:12:29 The data say it had better be 98% constant or better.
1:12:33 So 100% constant works, right?
1:12:35 And it’s also very robust.
1:12:37 It’s just there.
1:12:38 It’s not doing anything.
1:12:39 It doesn’t interact with any other particles.
1:12:41 It makes perfect sense.
1:12:42 Probably the dark energy is the cosmological constant.
1:12:44 The dark matter, super important to emphasize here.
1:12:48 You know, it was hypothesized at first in the 70s and 80s,
1:12:53 mostly to explain the rotation of galaxies.
1:12:57 Today, the evidence for dark matter is both much better
1:13:04 than it was in the 1980s and from different sources.
1:13:07 It is mostly from observations of the cosmic background
1:13:10 radiation or of large scale structure.
1:13:12 So we have multiple independent lines of evidence,
1:13:17 also gravitational lensing and things like that,
1:13:19 many, many pieces of evidence that say that dark matter is there.
1:13:22 And also that say that the effects of dark matter are different
1:13:27 than if we modified gravity.
1:13:30 So that was my first answer to your question is dark matter,
1:13:34 we have a lot of evidence for.
1:13:35 But the other one is, of course, we would love it
1:13:39 if it weren’t dark matter.
1:13:41 Our vested interest is 100% aligned with it being
1:13:45 something more cool and interesting than dark matter
1:13:48 because dark matter is just a particle.
1:13:50 That’s the most boring thing in the world.
1:13:51 And it’s non-uniformly distributed to space, dark matter.
1:13:55 Absolutely, yeah.
1:13:56 And so this–
1:13:56 You can even see maps of it that we’ve constructed
1:13:58 from gravitational lensing.
1:14:00 It’s a verifiable sort of clumps of dark matter
1:14:03 in the galaxy that explains stuff.
1:14:04 Bigger than the galaxy, sadly.
1:14:06 Like, we think that in the galaxy, dark matter is lumpy,
1:14:09 but it’s just weaker.
1:14:12 Its effects are weaker.
1:14:13 But over the scale of large-scale structure
1:14:15 and clusters of galaxies and things like that,
1:14:17 yes, we can show you where the dark matter is.
1:14:19 Could there be a super cool explanation for dark matter
1:14:23 that would be interesting
1:14:24 as opposed to just another particle that sits there in clumps?
1:14:27 The super cool explanation would be modifying gravity
1:14:31 rather than inventing a new particle.
1:14:34 Sadly, that doesn’t really work.
1:14:36 We’ve tried.
1:14:36 I’ve tried.
1:14:37 That’s my third paper that was very successful.
1:14:40 I tried to unify dark matter and dark energy together.
1:14:44 That was my idea.
1:14:46 That was my aspiration, not even an idea.
1:14:48 I tried to do it.
1:14:49 It failed even before we wrote the paper.
1:14:51 I realized that my idea did not help.
1:14:54 It helps.
1:14:54 It could possibly explain away the dark energy,
1:14:58 but it would not explain away the dark matter.
1:15:00 And so I thought it was not that interesting, actually.
1:15:03 And then two different collaborators of mine said,
1:15:05 “Is anyone thought of this idea?”
1:15:06 They thought of exactly the same idea,
1:15:08 completely independently of me.
1:15:10 I said, “Well, if three different people found the same idea,
1:15:13 maybe it is interesting.”
1:15:14 And so we wrote the paper.
1:15:15 And yeah, it was very interesting.
1:15:17 People are very interested in it.
1:15:18 Can you describe this paper a little bit?
1:15:20 It’s fascinating how much of a thing there is dark energy
1:15:25 and dark matter, and we don’t quite understand it.
1:15:26 So what was your dive into exploring how to unify the two?
1:15:31 Here is what we know about dark matter and dark energy.
1:15:34 They become important in regimes
1:15:39 where gravity is very, very, very weak.
1:15:42 That’s kind of the opposite from what you would expect
1:15:47 if you actually were modifying gravity.
1:15:49 Like there’s a rule of thumb in quantum field theory, et cetera,
1:15:52 that new effects show up when the effects are strong.
1:15:56 We understand weak fields.
1:15:58 We don’t understand strong fields.
1:15:59 But okay, maybe this is different, right?
1:16:02 So what do I mean by when gravity is weak?
1:16:05 The dark energy shows up late in the history of the universe.
1:16:08 Early in the history of the universe,
1:16:10 the dark energy is irrelevant.
1:16:11 But remember, the density of dark energy stays constant.
1:16:15 The density of matter and radiation go down.
1:16:18 So at early times, the dark energy was completely irrelevant
1:16:21 compared to matter and radiation.
1:16:23 At late times, it becomes important.
1:16:25 That’s also when the universe is dilute
1:16:27 and gravity is relatively weak.
1:16:29 Now think about galaxies, okay?
1:16:31 A galaxy is more dense in the middle,
1:16:33 less dense on the outside.
1:16:35 And there is a phenomenological fact about galaxies
1:16:37 that in the interior of galaxies, you don’t need dark matter.
1:16:40 That’s not so surprising because the density of stars and gas
1:16:44 is very high there and the dark matter is just subdominant.
1:16:47 But there’s generally a radius inside of which
1:16:52 you don’t need dark matter to fit the data,
1:16:54 outside of which you do need dark matter to fit the data.
1:16:56 So that’s again when gravity is weak, right?
1:17:00 So I asked myself, of course, we know in field theory,
1:17:05 new effects should show up when fields are strong, not weak.
1:17:08 But let’s throw that out of the window.
1:17:10 Can I write down a theory where gravity alters when it is weak?
1:17:16 And we’ve already said what gravity is.
1:17:19 What is gravity?
1:17:19 It’s the curvature of spacetime.
1:17:21 So there are mathematical quantities
1:17:24 that measure the curvature of spacetime.
1:17:27 And generally, you would say like I have an understanding
1:17:30 Einstein’s equation, which I explained to the readers in the book,
1:17:33 relates the curvature of spacetime to matter and energy.
1:17:37 The more matter and energy, the more curvature.
1:17:40 So I’m saying, what if you add a new term in there
1:17:42 that says the less matter and energy, the more curvature?
1:17:47 No reason to do that except to fit the data, right?
1:17:52 So I tried to unify the need for dark matter and the need for dark energy.
1:17:57 That would be really cool if that was the case.
1:17:59 Super cool, right?
1:18:00 It’d be the best.
1:18:00 It would be great.
1:18:01 But it didn’t work.
1:18:02 But it’d be really interesting if gravity did something funky
1:18:07 when there’s not much of it.
1:18:09 Almost like at the edges of it, it gets noisy.
1:18:12 That was exactly the hope.
1:18:14 But the great thing about physics is there are equations, right?
1:18:20 I mean, you can come up with the words and you can wave your hands,
1:18:23 but then you got to write down the equations and I did.
1:18:26 And I figured out that it could help with the dark energy,
1:18:29 the acceleration universe.
1:18:30 It doesn’t help with dark matter at all.
1:18:32 It just sucks at the scale of galaxies and scale of solar systems.
1:18:38 The physics is kind of boring.
1:18:42 Yeah, it does.
1:18:43 I agree.
1:18:43 And again, that’s why it is a little bit, I tear my hair out
1:18:48 when people who are not physicists think, you know, accused physicists,
1:18:53 like you say, of sort of losing the plot because they need dark matter and dark energy.
1:18:58 I don’t want dark matter and dark energy.
1:19:00 I want something much cooler than that.
1:19:02 I’ve tried.
1:19:03 But you got to listen to the equations and to the data.
1:19:06 You mentioned three papers.
1:19:08 Your first ever, your first awesome paper ever.
1:19:12 And your second awesome paper ever.
1:19:14 Of course, you wrote many papers, so you’re being very harsh on the others.
1:19:19 Well, by the way, this is not awesomeness.
1:19:21 This is impact.
1:19:22 Impact.
1:19:23 Right.
1:19:24 There’s no correlation between awesomeness and impact.
1:19:27 Some of my best papers fell without a stone.
1:19:29 Tree falls in the forest.
1:19:31 Yeah.
1:19:31 The first paper was called Limits on a Lorentz and Parity Violating
1:19:35 Modification of Electromagnetism or Electrodynamics.
1:19:38 So we figured out how to violate Lorentz invariance,
1:19:41 which is the symmetry underlying relativity.
1:19:44 And the important thing is we figured out a way to do it
1:19:47 that didn’t violate anything else and was experimentally testable.
1:19:51 So people love that.
1:19:53 The second paper was called Quintessence and the Rest of the World.
1:19:57 So Quintessence is this dynamical dark energy field.
1:20:01 The rest of the world is because I was talking about
1:20:03 how the Quintessence field would interact with other particles and fields
1:20:06 and how to avoid the interactions you don’t want.
1:20:09 And the third paper was called Is Cosmic Speed Up
1:20:15 Due to Gravitational Physics?
1:20:18 Something like that.
1:20:19 So you see the common theme.
1:20:21 I’m taking what we know, the standard model of particle physics,
1:20:24 general relativity, tweaking them in some way and then trying to fit the data.
1:20:28 And trying to make it so it’s experimentally validated.
1:20:31 Ideally, yes.
1:20:32 That’s right.
1:20:33 That’s the goal.
1:20:33 You wrote the book Something Deeply Hidden on the Mysteries of Quantum Mechanics
1:20:38 and a new book coming out soon, part of that biggest ideas in the universe series
1:20:43 we mentioned called Quanta and Fields.
1:20:46 So that’s focusing on quantum mechanics.
1:20:50 Big question first.
1:20:51 Biggest ideas in the universe.
1:20:53 What to you is most beautiful or perhaps most mysterious about quantum mechanics?
1:21:00 Quantum mechanics is a harder one.
1:21:02 You know, I wrote a textbook on general relativity and I started it by saying
1:21:06 general relativity is most beautiful physical theory ever invented.
1:21:10 And I will stand by that.
1:21:11 It is less fundamental than quantum mechanics,
1:21:15 but quantum mechanics is a little more mysterious.
1:21:17 So it’s a little bit cludgy right now.
1:21:20 You know, if you think about how we teach quantum mechanics to our students,
1:21:25 the Copenhagen interpretation, it’s a god-awful mess.
1:21:28 Like no one’s going to accuse that of being very beautiful.
1:21:31 I’m a fan of the many worlds interpretation of quantum mechanics,
1:21:34 and that is very beautiful in the sense that fewer ingredients,
1:21:39 just one equation, and it could cover everything in the world.
1:21:44 It depends what you mean by beauty, but I think that the answer to your question is,
1:21:47 quantum mechanics can start with extraordinarily austere, tiny ingredients,
1:21:54 and in principle lead to the world, right?
1:21:57 That boggles my mind.
1:22:00 It’s a much more comprehensive.
1:22:02 General relativity is about gravity, and that’s great.
1:22:04 Quantum mechanics is about everything, and seems to be up to the task.
1:22:09 And so I don’t know, is that beauty or not, but it’s certainly impressive.
1:22:12 So both for the theory, the predictive power of the theory,
1:22:14 and the fact that the theory describes tiny things creating everything we see around us.
1:22:19 It’s a monist theory in classical mechanics.
1:22:25 I have a particle here, particle there.
1:22:27 I describe them separately.
1:22:29 I can tell you what this particle is doing, what that particle is doing.
1:22:31 In quantum mechanics, we have entanglement, right?
1:22:33 As Einstein pointed out to us in 1935.
1:22:36 And what that means is there is a single state for these two particles.
1:22:42 There’s not one state for this particle, one state for the other particle.
1:22:45 And indeed, there’s a single state for the whole universe,
1:22:48 called the wave function of the universe, if you want to call it that.
1:22:50 And it obeys one equation and is our job then to sort of chop it up,
1:22:58 to carve it up, to figure out how to get tables and chairs and things like that out of it.
1:23:02 You mentioned the many worlds interpretation, and it is in fact beautiful.
1:23:07 But it’s one of your more controversial things you stand behind.
1:23:12 You’ve probably gotten a bunch of flak for it.
1:23:14 I’m a big boy.
1:23:15 I can take it.
1:23:16 Well, can you first explain it, and then maybe speak to the flak you may have gotten?
1:23:20 Sure.
1:23:21 You know, the classic experiment to explain quantum mechanics to people
1:23:26 is called the Stern-Gerlach experiment.
1:23:29 You’re measuring the spin of a particle, okay?
1:23:32 And in quantum mechanics, the spin is, you know, it’s just a spin.
1:23:37 It’s a rate at which something is rotating around in a very down-to-earth sense.
1:23:40 The difference being is that it’s quantized.
1:23:42 So for something like a single electron or a single neutron,
1:23:46 it’s either spinning clockwise or counterclockwise.
1:23:49 Those are the only two, let’s put it this way,
1:23:51 those are the only two measurement outcomes you will ever get.
1:23:55 There’s no, it’s spinning faster or slower.
1:23:56 It’s either spinning one direction or the other.
1:23:58 That’s it, two choices, okay?
1:24:01 According to the rules of quantum mechanics, I can set up an electron, let’s say,
1:24:05 in a state where it is neither purely clockwise or counterclockwise,
1:24:11 but a superposition of both.
1:24:13 And that’s not just because we don’t know the answer.
1:24:16 It’s because it truly is both until we measure it.
1:24:19 And then when we measure it, we see one or the other.
1:24:22 So this is the fundamental mystery of quantum mechanics,
1:24:24 is that how we describe the system, when we’re not looking at it,
1:24:26 is different from what we see when we look at it.
1:24:29 So what we teach our students in the Copenhagen way of thinking,
1:24:32 is that the act of measuring the spin of the electron
1:24:37 causes a radical change in the physical state.
1:24:41 It spontaneously collapses from being a superposition
1:24:45 of clockwise and counterclockwise to being one or the other.
1:24:49 And you can tell me the probability that that happens,
1:24:51 but that’s all you can tell me.
1:24:53 And I can’t be very specific about when it happens,
1:24:56 what caused it to happen, why it’s happening, none of that.
1:24:59 That’s all called the measurement problem of quantum mechanics.
1:25:02 So many worlds just says, look, I just told you a minute ago
1:25:08 that there’s only one wave function for the whole universe.
1:25:10 And that means that you can’t take too seriously
1:25:15 just describing the electron.
1:25:16 You have to include everything else in the universe.
1:25:18 In particular, you clearly have to interact with the electron
1:25:22 in order to measure it.
1:25:23 So whatever is interacting with the electron
1:25:26 should be included in the wave function that you’re describing.
1:25:30 And look, maybe it’s just you.
1:25:31 Maybe your eyeballs are able to perceive it.
1:25:33 But okay, I’m going to include you in the wave function.
1:25:36 And if you do that, let’s be, since you have
1:25:40 a very sophisticated listenership,
1:25:42 I’ll be a little bit more careful than average.
1:25:44 What does it mean to measure the spin of the electron?
1:25:47 We don’t need to go into details,
1:25:49 but we want the following thing to be true.
1:25:52 If the electron were in a state that was 100% spinning clockwise,
1:25:57 then we want the measurement to tell us it was spinning clockwise.
1:26:02 We want your brain to go, yes,
1:26:03 the electron was spinning clockwise, right?
1:26:05 Likewise, if it was 100% counterclockwise,
1:26:08 we want to see that, to measure that.
1:26:11 The rules of quantum mechanics,
1:26:13 the Schrödinger equation of quantum mechanics,
1:26:15 is 100% clear that if you want to measure it clockwise
1:26:19 when it’s clockwise, and measure it counterclockwise
1:26:22 when it’s counterclockwise,
1:26:23 then when it starts out in a superposition,
1:26:27 what will happen is that you and the electron
1:26:31 will entangle with each other.
1:26:34 And by that, I mean that the state of the universe
1:26:37 evolves into part saying the electron was spinning clockwise
1:26:41 and I saw it clockwise.
1:26:42 And part of the state is it’s in a superposition with
1:26:46 the part that says the electron was spinning counterclockwise
1:26:48 and I saw it counterclockwise.
1:26:50 Everyone agrees with this, entirely uncontroversial,
1:26:54 straightforward consequence of the Schrödinger equation.
1:26:57 And then Niels Bohr would say,
1:27:00 “And then part of that wave function disappears.”
1:27:02 And we’re in the other part.
1:27:04 And you can’t predict which part it will be,
1:27:06 only the probability.
1:27:06 Hugh Everett, who was a graduate student in the 1950s,
1:27:10 who was thinking about this, says, “I have a better idea.
1:27:12 Part of the wave function does not magically disappear.
1:27:16 It stays there.”
1:27:17 The reason why that idea, Everett’s idea,
1:27:20 that the whole wave function always sticks around
1:27:22 and just obeys the Schrödinger equation,
1:27:24 was not thought of years before,
1:27:26 is because naively you look at it and you go,
1:27:30 “Okay, this is predicting that I will be in a superposition.
1:27:34 That I will be in a superposition of having seen
1:27:39 the electron be clockwise and having seen it be counterclockwise.”
1:27:43 No experimenter has ever felt like they were in a superposition.
1:27:46 You always see an outcome, okay?
1:27:48 Everett’s move, which was kind of genius,
1:27:52 was to say the problem is not the Schrödinger equation.
1:27:55 The problem is you have misidentified yourself
1:27:58 in the Schrödinger equation.
1:28:00 You have said, “Oh, look, there’s a person who saw counterclockwise.
1:28:04 There’s a person who saw clockwise.
1:28:06 I should be in that superposition of both.”
1:28:10 And Everett says, “No, no, no, you’re not.”
1:28:12 Because the part of the wave function
1:28:15 in which the spin was clockwise, once that exists,
1:28:19 it is completely unaffected by the part of the wave function
1:28:23 that says the spin was counterclockwise.
1:28:26 They are apart from each other.
1:28:28 They are uninteracting.
1:28:30 They have no influence.
1:28:31 What happens in one part has no influence in the other part.
1:28:34 So Everett says the simple resolution
1:28:36 is to identify yourself as either the one who saw spin clockwise
1:28:42 or the one who saw spin counterclockwise.
1:28:45 There are now two people.
1:28:46 Once you’ve done that experiment,
1:28:48 the Schrödinger equation doesn’t have to be messed with.
1:28:51 All you have to do is locate yourself correctly
1:28:53 in the wave function.
1:28:55 That’s many worlds.
1:28:55 The number of worlds is very, very, very, very big.
1:29:02 Where do those worlds fit?
1:29:05 Where do they go?
1:29:07 The short answer is the worlds don’t exist in space.
1:29:14 Space exists separately in each world.
1:29:18 So there’s a technical answer to your question,
1:29:21 which is Hilbert space,
1:29:22 the space of all possible quantum mechanical states.
1:29:24 But physically, we want to put these worlds somewhere.
1:29:28 That’s just a wrong intuition that we have.
1:29:32 There is no such thing as the physical spatial location
1:29:35 of the worlds because space is inside the worlds.
1:29:38 One of the properties of this interpretation
1:29:40 is that you can’t travel from one world to the other.
1:29:43 That’s right.
1:29:44 Which kind of makes you feel that they’re existing separately.
1:29:52 They are existing separately and simultaneously.
1:29:54 And simultaneously.
1:29:55 Without locations in space.
1:29:57 Without locations in space.
1:29:58 How is it possible to visualize them existing
1:30:02 without a location in space?
1:30:03 The real answer to that, the honest answer is the equations predicted.
1:30:10 If you can’t visualize it, so much worse for you.
1:30:14 The equations are crystal clear about what they’re predicting.
1:30:16 Is there a way to get to the closer to understanding
1:30:20 and visualizing the weirdness of the implications of this?
1:30:23 You know, I don’t think it’s that hard.
1:30:25 It wasn’t that hard for me.
1:30:27 I don’t mind the idea that when I make a quantum mechanical measurement,
1:30:33 there is later on in the universe multiple descendants
1:30:38 of my present self who got different answers
1:30:40 for that measurement.
1:30:41 I can’t interact with them.
1:30:43 Hilbert space, the spaceball quantum wave functions
1:30:47 was always big enough to include all of them.
1:30:49 I’m going to worry about the parts of the universe I can observe.
1:30:55 So let’s put it this way.
1:30:57 Many worlds comes about by taking the Schrodinger equation seriously.
1:31:02 The Schrodinger equation was invented to fit the data,
1:31:05 to fit the spectrum of different atoms
1:31:07 and different emission and absorption experiments.
1:31:10 And it’s perfectly legitimate to say,
1:31:14 well, okay, you’re taking the Schrodinger equation.
1:31:17 You’re extrapolating it.
1:31:18 You’re trusting it, believing it beyond what we can observe.
1:31:24 I don’t want to do that, right?
1:31:26 That’s perfectly legit.
1:31:27 Except, okay, then what do you believe?
1:31:30 Come up with a better theory.
1:31:33 You’re saying you don’t believe the Schrodinger equation.
1:31:36 Tell me the equation that you believe in, turns out,
1:31:39 and people have done that, turns out it’s super hard
1:31:42 to do that in a legitimate way that fits the data.
1:31:45 And many worlds is a really clean, absolutely the most austere, clean,
1:31:50 no extra baggage theory of quantum mechanics.
1:31:53 So if it, in fact, is correct, isn’t it the weirdest thing of anything we know?
1:32:03 Yes, in fact, let me put it this way.
1:32:06 The single best reason in my mind to be skeptical about many worlds
1:32:12 is not because it doesn’t make sense or it doesn’t fit the data
1:32:16 or I don’t know where the worlds are going or whatever.
1:32:19 It’s because to make that extrapolation,
1:32:23 to take seriously the equation that we know is correct in other regimes,
1:32:26 requires new philosophy, requires a new way of thinking about identity,
1:32:33 about probability, about prediction, a whole bunch of things.
1:32:36 It’s work to do that philosophy and I’ve been doing it and others have done it
1:32:41 and I think it’s very, very doable, but it’s not straightforward.
1:32:46 It’s not a simple extrapolation from what we already know.
1:32:50 It’s a grand extrapolation very far away.
1:32:52 And if you just wanted to be sort of methodologically conservative
1:32:57 and say that’s a step too far, I don’t want to buy it,
1:33:00 I’m sympathetic to that.
1:33:02 I think that you’re just wimping out.
1:33:04 I think that you should have more courage, but I get the impulse.
1:33:08 And there is under many worlds an arrow of time where if you rewind it back,
1:33:17 there’s going to be one initial state.
1:33:22 That’s right.
1:33:22 All of quantum mechanics, all different versions require a kind of arrow of time.
1:33:26 It might be different in every kind.
1:33:28 But the quantum measurement process is irreversible.
1:33:33 You can measure something, it collapses, you can’t go backwards.
1:33:36 If someone tells you the outcome, if I say I have measured it in electron,
1:33:40 it’s been as clockwise.
1:33:41 And they say, what was it before I measured it?
1:33:44 You know there was some part of it that was clockwise, but you don’t know how much, right?
1:33:48 And many worlds is no different.
1:33:50 But the nice thing is that the kind of arrow of time you need in many worlds
1:33:55 is exactly the kind of arrow of time you need anyway.
1:33:58 For entropy and thermodynamics and so forth.
1:34:00 You need a simple low entropy initial state.
1:34:03 That’s what you need in both cases.
1:34:05 So if you actually look at under many worlds into the entire history of the universe,
1:34:10 correct me if I’m wrong, but it looks very deterministic.
1:34:14 Yes.
1:34:15 In each moment, does the moment contain the memory of the entire history of the universe?
1:34:20 To you, does the moment contain the memory of everything that preceded it?
1:34:26 As far as we know, so according to many worlds, the wave function of the universe,
1:34:32 all the branches of the universe at once, all the worlds, it does contain all the information.
1:34:36 Calling a memory is a little bit dangerous because it’s not the same kind of memory
1:34:44 that you and I have in our brains because our memories rely on the arrow of time.
1:34:48 And the whole point of the Schrodinger equation or Newton’s laws is they don’t have an arrow of
1:34:55 time built in.
1:34:56 They’re reversible.
1:34:58 The state of the universe not only remembers where it came from,
1:35:02 but also determines where it’s going to go in a way that our memories don’t do that.
1:35:05 But our memories, you can do replay.
1:35:08 Can you do this?
1:35:09 We can, but the act of forming a memory increases the entropy of the universe.
1:35:13 It is an irreversible process also, right?
1:35:16 You can walk on a beach and leave your footprints there.
1:35:20 That’s a record of your passing.
1:35:23 It will eventually be erased by the ever-increasing entropy of the universe.
1:35:27 But you can imperfectly replay it.
1:35:29 I guess, can we return travel back in time imperfectly?
1:35:34 Oh, it depends on the level of precision you’re trying to ask that question.
1:35:39 The universe contains the information about where the universe was,
1:35:45 but you and I don’t.
1:35:46 We’re nowhere close.
1:35:47 Is what computationally very costly to try to consult the universe?
1:35:54 Well, it depends on, again, exactly what you’re asking.
1:35:56 Like, there are some simple questions.
1:35:58 Like, what was the temperature of the universe 30 seconds after the Big Bang?
1:36:03 We can answer that, right?
1:36:06 That’s kind of amazing that we can answer that to pretty high precision.
1:36:10 But if you want to know where every atom was, then no.
1:36:13 What do you is the Big Bang?
1:36:17 Why did it– why?
1:36:20 Why did it happen?
1:36:21 We have no idea.
1:36:22 I think that that’s a super important question that I can imagine making progress on.
1:36:28 But right now, I’m more or less maximally uncertain about what the answer is.
1:36:33 Do you think black holes will help?
1:36:34 No.
1:36:34 Potentially?
1:36:35 Not that much.
1:36:36 Quantum gravity will help.
1:36:38 And maybe black holes will help us figure out quantum gravity.
1:36:41 So indirectly, yes.
1:36:43 But we have the situation where general relativity, Einstein’s theory, unambiguously predicts there
1:36:49 was a singularity in the past.
1:36:51 There was a moment of time when the universe had infinite curvature, infinite energy,
1:36:57 infinite expansion rate, the whole bit.
1:36:59 That’s just a fancy way of saying the theory has broken down.
1:37:03 And classical general relativity is not up to the task of what’s saying what really happened
1:37:08 at that moment.
1:37:09 So it is completely possible there was, in some sense, a moment of time before which there were
1:37:15 no other moments.
1:37:16 And that would be the Big Bang, even if it’s not a classical general relativity kind of thing,
1:37:20 even if quantum mechanics is involved, maybe that’s what happened.
1:37:23 It’s also completely possible there was time before that, space and time, and they evolved
1:37:29 into our hot Big Bang by some procedure that we don’t really understand.
1:37:33 And if time and space are emergent, then the before even starts getting real weird.
1:37:38 Well, I think that if there is a first moment of time, that would be very good evidence or
1:37:45 that would fit hand and glove with the idea that time is emergent.
1:37:48 If time is fundamental, then it tends to go forever, because it’s fundamental.
1:37:52 Well, yeah, I mean, the general formulation of this question is what’s outside of it?
1:37:57 Well, what’s outside of our universe?
1:37:58 So in time and in space, I know it’s a pothead question, Sean.
1:38:03 I understand.
1:38:04 I apologize.
1:38:06 That’s my life.
1:38:07 My life is asking pothead questions.
1:38:09 Some of them, the answer is that’s not the right way to think about it.
1:38:12 Okay.
1:38:12 But is it possible to think at all about what’s outside our universe?
1:38:17 It’s absolutely legit to ask questions, but you have to be comfortable with the possibility
1:38:23 that the answer is there’s no such thing as outside our universe.
1:38:26 That’s absolutely on the table.
1:38:27 In fact, that is the simplest, most likely to be correct answer that we know of.
1:38:32 But it’s the only thing in the universe that wouldn’t have an outside.
1:38:38 Yeah, if the universe is the totality of everything, it would not have an outside.
1:38:43 It’s so weird to think that there’s not an outside.
1:38:46 We want there to be a creator, a creative force that led to this, an outside.
1:38:56 Like, this is our town, and then there’s a bigger world, and there’s always a bigger world.
1:39:01 Because that is our experience.
1:39:02 That’s the world we grew up in, right?
1:39:05 The universe doesn’t need to obey those rules.
1:39:08 Such a weird thing.
1:39:10 When I was a kid, that used to keep me up at night.
1:39:13 Like, what if the universe had not existed?
1:39:14 Right, and it feels like a lot of pressure that this is the only universe.
1:39:22 And we’re here, one of the few intelligent civilizations, maybe the only one.
1:39:30 It’s the old theories that were at the center of everything.
1:39:32 It just feels suspicious.
1:39:34 That’s why many worlds is kind of exciting to me, because it’s humbling in all the right kinds of ways.
1:39:40 It feels like infinity is the way this whole thing runs.
1:39:45 There’s one pitfall that I’ll just mention, because there’s a move that is made in these
1:39:51 theoretical edges of cosmology that I think is a little bit mistaken, which is to say,
1:39:56 I’m going to think about the universe on the basis of imagining that I am a typical observer.
1:40:02 This is called the principle of typicality, or the principle of mediocrity,
1:40:07 or even the Copernican principle.
1:40:08 Nothing special about me.
1:40:10 I’m just typical in the universe.
1:40:12 But then you draw some conclusions from this.
1:40:14 And what you end up realizing is you’ve been hilariously presumptuous.
1:40:20 Because by saying I’m a typical observer in the universe,
1:40:22 you’re saying typical observers in the universe are like me.
1:40:25 And that is completely unjustified by anything.
1:40:28 So I’m not telling you what the right way to do it is.
1:40:32 But these kinds of questions that are not quite grounded in experimental verification
1:40:37 or falsification are ones you have to be very careful about.
1:40:42 That to me is one of the most interesting questions.
1:40:44 There’s different ways to approach it, but what’s outside of this?
1:40:49 How did the big mess start?
1:40:51 How do we get something from nothing?
1:40:54 That’s always the thing you’re sneaking up to.
1:40:56 When you’re studying all of these questions, you’re always thinking,
1:41:01 that’s where the black hole and the unifying, getting quantum gravity,
1:41:04 all this kind of stuff.
1:41:04 You’re always sneaking up to that question.
1:41:06 Where did all of this come from?
1:41:10 And I think that’s probably an answerable question.
1:41:14 Right?
1:41:16 No.
1:41:18 It doesn’t have to be.
1:41:20 So you think there could be a turtle at the bottom of this
1:41:22 that refuses to reveal its identity?
1:41:26 Yes.
1:41:26 I think that specifically the question,
1:41:30 why is there something rather than nothing?
1:41:32 Yeah.
1:41:32 Does not have the kind of answer that we would ordinarily attribute to why questions?
1:41:38 Because typical why questions are embedded in the universe.
1:41:45 And when we answer them, we take advantage of the features of the universe that we know and love.
1:41:49 But the universe itself, as far as we know, is not embedded in anything
1:41:52 bigger or stronger, and therefore it can just be.
1:41:56 Do you think it’s possible this whole place is simulated?
1:41:59 Sure.
1:42:01 It’s a really interesting, dark, twisted video game that we’re all existing in.
1:42:06 You know, my own podcast listeners,
1:42:08 mindscape listeners tease me because they know from my AMA episodes
1:42:12 that if you ever start a question by asking,
1:42:16 do you think it’s possible that the answer is going to be yes?
1:42:20 That might not be the answer that you care about.
1:42:24 But it’s possible, sure, as long as you’re not, you know,
1:42:26 adding two even numbers together and getting an odd number.
1:42:30 When you say it’s possible, there’s a mathematically yes,
1:42:33 and then there’s more of like intuitive.
1:42:35 Yeah.
1:42:36 You want to know whether it’s plausible.
1:42:37 You want to know is there a reasonable non-zero credence to attach to this?
1:42:42 I don’t think that there’s any philosophical knockout objection to the simulation hypothesis.
1:42:50 I also think that there’s absolutely no reason to take it seriously.
1:42:53 Do you think humans will try to create one?
1:42:56 I guess that that’s how I always think about it.
1:42:59 You know, I see what, I’ve spent quite a bit of time
1:43:02 over the past few years and a lot more recently in virtual worlds,
1:43:09 and just am always captivated by the possibility of creating
1:43:14 higher and higher resolution worlds.
1:43:15 And as we’ll talk a little bit about artificial intelligence,
1:43:19 sort of the advancement on the Sora front,
1:43:21 you can automatically generate those worlds,
1:43:25 and the possibility of existing in those automatically generated worlds is pretty exciting,
1:43:31 as long as there’s a consistent physics quantum mechanics and general relativity
1:43:35 that governs the generation of those worlds.
1:43:37 Yep. So it just seems like humans will for sure try to create this.
1:43:43 Yeah, I think they will create better and better simulations.
1:43:46 I think the philosopher David Chalmers has done what I consider to be a good job of arguing
1:43:51 that we should treat things that happen in virtual reality and in simulated realities
1:43:55 as just as real as the reality that we experience.
1:43:59 I also think that as a practical matter, people will realize how much harder it is
1:44:04 to simulate a realistic world than we naively believe.
1:44:08 So this is not a my lifetime kind of worry.
1:44:10 Yeah, the practical matter of going from a sort of a prototype that’s impressive
1:44:15 to a thing that governs everything.
1:44:17 Similar question on this front is in AGI.
1:44:21 Yeah, you said that we’re very far away from AGI.
1:44:25 I want to eliminate the phrase AGI.
1:44:30 So basically, when you’re analyzing large language models and seeing how far they from
1:44:36 whatever AGI is, and we could talk about different notions of intelligence that we
1:44:40 were not as close as kind of some people in public view are talking about.
1:44:48 So what’s your intuition behind that?
1:44:49 My intuition is basically that artificial intelligence is different than human intelligence.
1:44:56 And so the mistake that is being made by focusing on AGI among those who do
1:45:01 is an artificial agent that as we can make them now or in the near future might be way better
1:45:09 than human beings at some things, way worse than human beings at other things.
1:45:14 And rather than trying to ask how close is it to being a human-like intelligent,
1:45:19 we should appreciate it for what its capabilities are.
1:45:22 And that will both be more accurate and help us put it to work and protect us from the dangers
1:45:28 better rather than always anthropomorphizing it.
1:45:30 I think the underlying idea there under the definition of AGI is that
1:45:36 the capabilities are extremely impressive.
1:45:42 That’s not a precise stage.
1:45:44 No, I get that.
1:45:45 I completely agree.
1:45:46 And then the underlying question where a lot of the debate is how impressive is it?
1:45:52 What are the limits of large-language models?
1:45:54 Can they really do things like common sense reasoning?
1:45:58 How much do they really understand about the world?
1:46:01 Or are they just fancy mimicry machines?
1:46:03 And where do you fall on that?
1:46:07 That’s to the limits of large-language models.
1:46:11 I don’t think that there are many limits in principle.
1:46:15 I’m a physicalist about consciousness and awareness and things like that.
1:46:20 I see no obstacle to, in principle, building an artificial machine that is indistinguishable
1:46:26 in thought and cognition from a human being.
1:46:28 But we’re not trying to do that, right?
1:46:32 What a large-language model is trying to do is to predict text.
1:46:35 That’s what it does.
1:46:37 And it is leveraging the fact that we human beings, for very good evolutionary biology reasons,
1:46:46 attribute intentionality and intelligence and agency to things that act like human beings.
1:46:52 As I was driving here to get to this podcast space, I was using Google Maps.
1:46:59 And Google Maps was talking to me.
1:47:01 But I wanted to stop to get a cup of coffee.
1:47:04 So I didn’t do what Google Maps told me to do.
1:47:07 I went around a block that it didn’t like.
1:47:09 And so it gets annoyed, right?
1:47:12 It says, like, no, why are you doing it?
1:47:14 It doesn’t say exactly in this.
1:47:15 But you know what I mean?
1:47:16 It’s like, no, turn left, turn left, and you turn right.
1:47:18 It is impossible as a human being not to feel a little bit sad that Google Maps is getting mad at you.
1:47:25 It’s not.
1:47:27 It’s not even trying to.
1:47:28 It’s not a large-language model.
1:47:29 It’s not no aspirations to intentionality.
1:47:32 But we attribute that all the time.
1:47:35 Dan Dennett, the philosopher, wrote a very influential paper on the intentional stance.
1:47:41 The fact that it’s the most natural thing in the world for we human beings to attribute
1:47:46 more intentionality to artificial things than are really there.
1:47:51 Which is not to say it can’t be really there.
1:47:52 But if you’re trying to be rational and clear thinking about this,
1:47:57 the first step is to recognize our huge bias towards attributing things below the surface
1:48:04 to systems that are able to, at the surface level, act human.
1:48:10 So if that huge bias of intentionality is there in the data, in the human data,
1:48:15 in the vast landscape of human data, that AI models, large-language models and video models
1:48:22 in the future are trained on, don’t you think that that intentionality will emerge
1:48:29 as fundamental to the behavior of these systems naturally?
1:48:32 Well, I don’t think it will happen naturally.
1:48:35 I think it could happen.
1:48:36 Again, I’m not against the principle.
1:48:38 But again, the way that large-language models came to be and what they’re optimized for
1:48:47 is wildly different than the way that human beings came to be and what they’re optimized for.
1:48:52 So I think we’re missing a chance to be much more clear-headed about what large-language models
1:49:01 are by judging them against human beings, again, both in positive ways and negative ways.
1:49:05 Well, I think sort of to push back on what they’re optimized for is different to describe
1:49:09 how they’re trained versus what they’re optimized for.
1:49:12 So their train is a very trivial way of predicting text tokens.
1:49:16 But you can describe what they’re optimized for and what the actual task in hand is,
1:49:21 is to construct a world model, meaning an understanding of the world.
1:49:26 And that’s where it starts getting closer to what humans are kind of doing.
1:49:30 We’re just, in the case of large-language models, know how the sausage is made.
1:49:34 And we don’t know how it’s made for us humans.
1:49:37 But they’re not optimized for that.
1:49:38 They’re optimized to sound human.
1:49:40 That’s the fine-tuning.
1:49:41 But the actual training is optimized for understanding, creating a compressed representation
1:49:50 of all the stuff that humans have created on the internet.
1:49:53 And the hope is that that gives you a deep understanding of the world.
1:49:58 Yeah.
1:49:59 So that’s why I think that there’s a set of hugely interesting questions to be asked
1:50:04 about the ways in which large-language models actually do represent the world.
1:50:09 Because what is clear is that they’re very good at acting human.
1:50:13 The open question in my mind is, is the easiest, most efficient, best way to act human
1:50:20 to do the same things that human beings do?
1:50:24 Or are there other ways?
1:50:25 And I think that’s an open question.
1:50:27 I just heard a talk by Melanie Mitchell at Santa Fe Institute, an artificial intelligence researcher.
1:50:32 And she told two stories about two different papers.
1:50:35 One that someone else wrote and one that her group is following up on.
1:50:38 And they were modeling Othello.
1:50:40 Othello, the game was a little rectangular board, white and black squares.
1:50:44 So the experiment was the following.
1:50:46 They fed neural network the moves that were being made in the most symbolic form, like E5.
1:50:55 Just means that, okay, you put a token down E5.
1:50:57 So it gives a long string.
1:50:58 It does this for millions of games, real legitimate games.
1:51:02 And then it asks the question, the paper asks the question, okay,
1:51:05 you’ve trained it to tell what would be a legitimate next move from not a legitimate next move.
1:51:12 Did it in its brain, in its little large language model brain?
1:51:17 I don’t even know if it’s technically a large language model, but a deep learning network.
1:51:20 Did it come up with a representation of the Othello board?
1:51:23 Well, how do you know?
1:51:25 And so they construct a little probe network that they insert and you ask it,
1:51:29 what is it doing right at this moment, right?
1:51:32 And the answer is that the little probe network can ask, would this be legitimate?
1:51:38 Or is this token white or black or whatever?
1:51:40 Things that in practice would amount to, it’s invented the Othello board.
1:51:46 And it found that the probe got the right answer, not 100% of the time, but more than by chance,
1:51:55 substantially more than by chance.
1:51:58 So they said, there’s some tentative evidence that this neural network has discovered the
1:52:04 Othello board just out of data, raw data, right?
1:52:07 But then Melanie’s group asked the question, okay, are you sure that that understanding
1:52:13 of the Othello board wasn’t built into your probe?
1:52:17 And what they found was like, at least half of the improvement was built into the probe,
1:52:22 you know, not all of it, right?
1:52:24 And look, a Othello board is way simpler than the world.
1:52:29 So that’s why I just think it’s an open question, whether or not the,
1:52:37 I mean, it would be remarkable either way to learn that large language models that are good
1:52:43 at doing what we train them to do are good because they’ve built the same kind of model
1:52:48 of the world that we have in our minds, or that they’re good despite not having that model.
1:52:53 Either one of these is an amazing thing.
1:52:55 I just don’t think the data are clear on which one is true.
1:52:57 I think I have some sort of intellectual humility about the whole thing because I was
1:53:02 humbled by several stages in the machine learning development over the past 20 years.
1:53:07 And I would just would never have predicted that LLMs, the way they’re trained
1:53:16 on the scale of data they’re trained would be as impressive as they are.
1:53:20 And there that’s where intellectual humility steps in where my intuition would say something
1:53:26 like with Melanie where you need to be able to have very sort of concrete common sense reasoning,
1:53:32 symbolic reasoning type things in a system in order for it to be very intelligent.
1:53:38 But here you’re, I’m so impressed by what it’s capable to do train on the next token prediction,
1:53:45 essentially. That’s, I just, my conception of the nature of intelligence is just completely,
1:53:53 not completely, but humbled, I should say.
1:53:57 Look, and I think that’s perfectly fair. I also was, I will say pleasantly, I don’t know
1:54:03 whether it’s pleasantly or unpleasantly, but factually surprised by the recent rate of progress.
1:54:07 Clearly some kind of phase transition percolation has happened, right? And the improvement has
1:54:12 been remarkable, absolutely amazing. That I have no arguments with. I’m, that doesn’t yet
1:54:20 tell me the mechanism by which that improvement happened. Constructing a model much like a
1:54:26 human being would have is clearly one possible mechanism, but part of the intellectual humility
1:54:31 is to say maybe there are others. I was chatting with the CEO of Anthropic,
1:54:35 Dario Mede, so behind Claude and that company, but a lot of, a lot of the AI companies are
1:54:43 really focused on expanding the scale of compute. Sort of, if we assume that AI is not data limited,
1:54:51 but is compute limited, you can make the system much more intelligent by using more compute.
1:54:59 So let me ask you on the, almost on the physics level, do you think physics can help
1:55:06 expand the scale of compute and maybe the scale of energy required to make that compute happen?
1:55:11 Yeah, 100%. I think this is like one of the biggest things that physics can help with. And
1:55:17 it’s an obvious kind of low hanging fruit situation where the heat generation, the
1:55:25 inefficiency, the waste of existing high level computers is nowhere near the efficiency of
1:55:33 our brains. It’s hilariously worse. And we kind of haven’t tried to optimize that hard on that
1:55:39 frontier. I mean, your laptop heats up when you’re sitting on your lap, right? It doesn’t need to,
1:55:43 your brain doesn’t heat up like, like that. So clearly, there exists in the world of physics
1:55:49 the capability of doing these computations with much less waste heat being generated. And I look
1:55:55 forward to people doing that. Yeah. Are you excited for the possibility of nuclear fusion?
1:55:59 I am cautiously optimistic, excited to be too strong. I mean, it’d be great, right? But if we
1:56:06 really tried solar power, it would also be great. So I think Ilias the discover said this, that the
1:56:13 future of humanity on earth will be just the entire surface of earth is covered in solar panels
1:56:20 and data centers. Why would you waste the surface of the earth with solar panels? Put them in space.
1:56:25 Sure. You can go in space. Yeah. Space is bigger than the earth. Yeah. Just solar panels everywhere.
1:56:30 Yeah. I like it. We already have fusion. It’s called the sun. Yeah, that’s true. And there’s
1:56:37 probably more and more efficient ways of catching that energy. Sending it down is the hard part.
1:56:43 Absolutely. But that’s an engineering problem. Yeah. So I just wonder where data centers,
1:56:50 the compute centers can expand to. If that’s the future, if AI is as effective as a promise,
1:56:56 as it possibly could be, then the scale of computation will keep increasing.
1:57:01 And perhaps it’s a race between efficiency and scale. There are constraints, right? You know,
1:57:07 there’s a certain amount of energy, a certain amount of damage we can do to the environment
1:57:10 before it is not worth it anymore. So yeah, I think that’s a new question. In fact, it’s kind
1:57:15 of frustrating because we get better and better at doing things efficiently, but we invent more
1:57:21 things we want to do faster than we get good at doing them efficiently. So we’re continuing to make
1:57:26 things worse in various ways. I mean, that’s the dance of humanity where we’re constantly creating
1:57:32 better, better technologies that are potentially causing a lot more harm. And that includes for
1:57:37 weapons, includes AI used as weapons, that includes nuclear weapons, of course, which is surprising
1:57:42 to me that we haven’t destroyed human civilization yet given how many nuclear warheads are out there.
1:57:49 Look, I’m with you. Between nuclear and bio weapons, it is a little bit surprising that we
1:57:56 haven’t caused enormous devastation. Of course, we did drop two atomic bombs on Japan, but compared
1:58:00 to what could have happened or could happen tomorrow, it could be much worse. Yeah.
1:58:06 It does seem like there’s an underlying, speaking of quantum fields, there’s like a
1:58:11 field of goodness within the human heart that in some kind of game-theoretic way,
1:58:20 we create really powerful things that could destroy each other. And there’s greed and ego and all this
1:58:24 kind of power-hungry dictators that are at play here in all the geopolitical landscape. But we
1:58:31 somehow always don’t go too far. Yeah, but that’s exactly what you would say right before we went
1:58:36 too far. Right before we went too far. And that’s why we don’t see aliens. So you’re, like I mentioned,
1:58:44 associated with Santa Fe Institute. I just would love to take a stroll down the landscape of ideas
1:58:51 explored there. So they look at complexity in all kinds of ways. What do you think about
1:58:57 the emergence of complexity from simple things interacting simply?
1:59:00 I think it’s a fascinating topic. I mean, that’s why I’m thinking about these things these days,
1:59:05 rather than the papers that I was describing to you before. All of those papers I described to you
1:59:10 before are guesses. Like, what if the laws of physics are different in the following way? And
1:59:15 then you can work out the consequences. At some point in my life, I said, what is the chance I’m
1:59:19 going to guess right? You know, Einstein guessed right, Steven Weinberg guessed right, but there’s
1:59:23 a very small number of times that people guessed right. Whereas with this emergence of complexity
1:59:29 from simplicity, I really do think that we haven’t understood the basics yet. I think we’re still
1:59:35 kind of pre-paradigmatic. There have been some spectacular discoveries. People like Jeffrey West
1:59:41 at Santa Fe and others have really given us true insights into important systems. But still,
1:59:47 there’s a lot of the basics I think are not understood. And so, searching for the general
1:59:52 principles is what I like to do. And I think it’s absolutely possible that, I mean, to be a little
1:59:57 bit more substantive than that, I think this is kind of a cliche. I think the key is information.
2:00:03 And I think that what we see through the history of the universe, as you go from simple to more
2:00:09 and more complex, is really subsystems of the universe figuring out how to use information
2:00:16 to do whatever, to survive, or to thrive, or to reproduce. I mean, that’s the sort of fuel,
2:00:21 the leverage, the resource that we have for a while anyway, until the heat death, but that’s
2:00:27 where the complexity is really driven by. Yeah, but the mechanism of it, what, I mean,
2:00:31 you mentioned Jeffrey West, what are interesting in clings of progress in this realm? And what are
2:00:36 systems that interest you in terms of information? So, I mean, for me, just as a fan of complexity,
2:00:43 just even looking at simple cellular automata is always just a fascinating way to illustrate
2:00:48 the emergence of complexity. So, for those of the listeners who don’t know, viewers,
2:00:53 cellular automata come from imagining a very simple configuration. For example, a set of
2:01:01 ones and zeros along a line. And then you met a rule that says, okay, I’m going to evolve this
2:01:07 in time. And generally, the simplest ones start with just each block of three ones and zeros have
2:01:14 a rule that they will deterministically go to either one or zero. And you can actually classify
2:01:19 all the different possibilities, a small number of possible cellular automata of that form.
2:01:23 And what was discovered by various people, including Stephen Wolfram, is some of these
2:01:29 cellular automata have the feature that you start from almost nothing, like 00001, 0000.
2:01:36 And you let it rip, and it becomes wildly complex, okay? So, this is very provocative,
2:01:43 very interesting. It’s also not how physics works at all. Because as we said, physics
2:01:50 conserves information. You can go forward or backwards. These cellular automata do not.
2:01:55 They’re not reversible in any sense. You’ve built in an arrow of time. You have a starting point,
2:02:00 and then you evolve. So, what I’m interested in is seeing how in the real world, with the
2:02:06 real laws of physics and underlying reversibility, but macroscopic irreversibility from entropy in
2:02:12 the arrow of time, et cetera, how does that lead to complexity? I think that that’s an
2:02:16 answerable question. I don’t think that cellular automata are really helping us in that one.
2:02:20 So, what is in that, what is the landscape of entropy in the universe look like?
2:02:26 Well, entropy is hard to localize. It’s a property of systems, not of parts of systems,
2:02:33 right? Having said that, we can do approximate answers to the question. The answer is black
2:02:39 holes are huge in entropy. Most, let’s put it this way. The whole observable universe that we’re in
2:02:47 had a certain amount of entropy before stars and planets and black holes started to form.
2:02:54 10 to the 88th, I can even tell you the number, okay? The single black hole at the center of
2:02:59 our galaxy has entropy, 10 to the 90. Single black holes at our galaxy has more entropy than
2:03:05 the whole universe used to have not too long ago. So, most of the entropy in the universe
2:03:11 today is in the form of black holes. Okay, that’s fascinating, first of all. But second of all,
2:03:16 if we take black holes away, what are the different interesting perturbations in entropy
2:03:22 across space? Where do we earthlings fit into that? The interesting thing to me is that
2:03:30 if you start with a system that is isolated from the rest of the universe, and you start it
2:03:36 at low entropy, there’s almost a theorem that says if you’re very, very, very low entropy,
2:03:42 then the system looks pretty simple. Because there’s low entropy means there’s only a small
2:03:47 number of ways that you can rearrange the parts to look like that. So, if there’s not that many
2:03:52 ways, the answer is going to look simple. But there’s also almost a theorem that says when
2:03:56 you’re at maximum entropy, the system is going to look simple because it’s all smeared out. If
2:04:01 it had like interesting structure, then it would be complicated, right? So, entropy in this isolated
2:04:07 system only goes up. That’s the second law of thermodynamics. But complexity starts low, goes
2:04:14 up, and then goes down again. Sometimes, people mistakenly think that complexity or life or
2:04:22 whatever is fighting against the second law of thermodynamics, fighting against the increase
2:04:27 of entropy. That is precisely the wrong way to think about it. We are surfers riding the wave
2:04:33 of increasing entropy. We rely on increasing entropy to survive. That is part of what makes
2:04:40 us special. This table maintains its stability mechanically, by which I mean there’s molecules,
2:04:47 they have forces on each other, and it holds up. You and I aren’t like that. We maintain our
2:04:54 stability dynamically by ingesting food, fuel, food and water and air and so forth, burning it,
2:05:03 increasing its entropy. We are non-equilibrium quasi-study state systems. We are using the fuel
2:05:09 the universe gives us in the form of low entropy energy to maintain our stability.
2:05:15 I just wonder what that mechanism of surfing looks like.
2:05:18 First of all, I have one question to ask. Do you think it’s possible to have a kind of science
2:05:25 of complexity where you have very precise ways or clearly defined ways of measuring complexity?
2:05:33 I think it is, and I think we don’t. It’s possible to have it. I don’t think we yet have it.
2:05:40 In part because complexity is not a unit valent thing. There’s different ideas that go under
2:05:45 the rubric of complexity. One version is just homologoral of complexity. If you have a configuration
2:05:52 or a string of numbers or whatever, can you compress it so that you have a small program
2:05:58 that will output that? That’s homologoral of complexity. But that’s the complexity of a
2:06:02 string of numbers. It’s not like the complexity of a problem. Computational complexity,
2:06:09 the traveling salesman problem or factoring large numbers. That’s a whole different kind of question
2:06:13 that is also about complexity. We don’t have a unified view of it. Do you think it’s possible
2:06:20 to have a complexity of a physical system in the same way we do entropy? Yeah.
2:06:25 You think that’s a Sean Carroll paper or what? We’re working on various things. The glib thing
2:06:33 that I’m trying to work on right now with a student is complexogenesis. How does complexity come to
2:06:38 be if all the universe is doing is moving from low entropy to high entropy? It’s a sexy name.
2:06:43 That’s a good name. Yeah, I like the name. I just got to write the paper. Sometimes a name
2:06:48 arose by any other name. In which context the birth of complexity are you most interested in?
2:06:58 Well, I think it comes in stages. I’m, again, a physicist. So biologists studying evolution
2:07:08 will talk about how complexity evolves all the time, the complexity of the genome, the complexity
2:07:12 of our physiology. But they take for granted that life already existed and entropy is increasing
2:07:20 and so forth. I want to go back to the beginning and say the early universe was simple and low
2:07:25 entropy and entropy increases with time and the universe sort of differentiates and becomes more
2:07:30 complex. But that statement, which is indisputably true, has different meanings because complexity
2:07:38 has different meanings. So sort of the most basic primal version of complexity is what you might
2:07:44 think of as configurational complexity. That’s what Kamal Grove gets at. How much information do
2:07:50 you need to specify the configuration of the system? Then there’s a whole other step where
2:07:56 subsystems of the universe start burning fuel, right? So in many ways, a planet and a star
2:08:03 are not that different in configurational complexity. They’re both spheres.
2:08:08 With density high at the middle and getting less as you go out. But there’s something fundamentally
2:08:12 different because the star only survives as long as it has fuel, right? I mean, then it turns into
2:08:17 a brown dwarf or white dwarf or whatever. But as a star, as a main sequence star, it is an out of
2:08:22 equilibrium system. But it’s more or less static, right? Like if I spill the coffee mug and it falls
2:08:29 in the process of falling, it’s out of equilibrium, but it’s also changing all the time. A specific
2:08:34 kind of system is where it looks sort of macroscopically stationary, like a star,
2:08:41 but underneath the hood, it’s burning fuel to beat the band in order to maintain that stability.
2:08:47 So as stars form that, that’s a different kind of complexity that comes to be.
2:08:51 Then there’s another kind of complexity that comes to be, roughly speaking at the origin of life.
2:08:57 Because that’s where you have information really being gathered and utilized by subsystems of the
2:09:04 universe. And then arguably, there’s any number of stages past that. I mean, one of the most obvious
2:09:10 ones to me is we talk about simulation theory, but you and I run simulations in our heads. They’re
2:09:16 just not that good, but we imagine different hypothetical futures, right? Bacteria don’t do
2:09:21 that. So that’s the kind of information processing that is a form of complexity. So I would like to
2:09:26 understand all these stages and how they fit together. The imagination. Yep. Mental time travel.
2:09:33 Yeah. The things going on in my head when I’m imagining worlds are super compressed representations
2:09:39 of those worlds, but they get to the essence of them. And maybe it’s possible with
2:09:43 non-human computing type devices to do those kinds of simulations in more and more compressed ways.
2:09:50 There’s an argument to be made that literally what separates human beings from other species
2:09:55 on earth is our ability to imagine counterfactual hypothetical futures.
2:10:00 Yeah. I mean, that’s one of the big features. I don’t know if it’s-
2:10:08 Everyone has their own favorite little feature, but that’s why I said there’s an argument to be
2:10:11 made. I did a podcast episode on it with Adam Bully. It developed slowly. I did different podcasts.
2:10:17 Sorry to keep mentioning podcast episodes I did, but Malcolm McIver, who is an engineer at Northwestern,
2:10:22 has a theory about one of the major stages in evolution is when fish first climbed on the
2:10:28 land. And of course, that is a major stage in evolution, but in particular, there’s a cognitive
2:10:33 shift because when you’re a fish swimming under the water, the attenuation length of light
2:10:39 in water is not that long. You can’t see kilometers away. You can see meters away,
2:10:45 and you’re moving at meters per second. So all of the evolutionary optimization is
2:10:51 make all of your decisions on a timescale of less than a second. When you see something new,
2:10:56 you have to make a rapid fire decision, what to do about it. As soon as you climb onto land,
2:11:01 you can essentially see forever, right? You can see stars in the sky. So now a whole new mode of
2:11:08 reasoning opens up where you see something far away. And rather than saying, look up table,
2:11:15 I see this, I react, you can say, okay, I see that thing. What if I did this? What if I did that?
2:11:21 What if I did something different? And that’s the birth of imagination eventually.
2:11:25 You’ve been critical on panpsychism.
2:11:27 Yes, you’ve noticed that, right.
2:11:31 Can you make the case for panpsychism and against it? So panpsychism is the
2:11:36 idea that consciousness permeates all matter. It’s maybe it’s the fundamental force or
2:11:43 physics of the way of the fabric of the universe.
2:11:48 Panpsychism, thought everywhere, consciousness everywhere, right?
2:11:52 To a point of entertainment, the idea of frustrations, which sort of as a fan is wonderful
2:12:00 to watch. You’ve had great episodes with panpsychists on your podcast where you go at it.
2:12:07 I had David Chalmers, who’s one of the world’s great philosophers, and he is panpsychism curious.
2:12:13 He doesn’t commit to anything, but he’s certainly willing to entertain it.
2:12:18 Philip Goff, who I’ve had and who’s a great guy, but he is devoted to panpsychism. In fact,
2:12:23 he is almost single-handedly responsible for the upsurge of interest in panpsychism in the
2:12:29 popular imagination. And the argument for it is supposed to be that there is something fundamentally
2:12:36 uncapturable about conscious awareness by physical behavior of atoms and molecules.
2:12:43 So the panpsychist will say, “Look, you can tell me maybe someday through advances of neuroscience
2:12:48 and what have you exactly what happens in your brain and how that translates into thought and
2:12:56 speech and action. What you can’t tell me is what it is like to be me. You can’t tell me what I am
2:13:04 experiencing when I see something that is red or that tastes something that is sweet. You can tell
2:13:11 me what neurons fire, but you can’t tell me what I’m experiencing.” That first person, inner,
2:13:16 subjective experience is simply not capturable by physics. And therefore, this is an old argument,
2:13:26 of course, but then the therefore is supposed to be I need something that is not contained within
2:13:31 physics to account for that. And I’m just going to call it mind. We don’t know what it is yet.
2:13:37 We’re going to call it mind. And it has to be separate from physics. And then there’s two
2:13:41 ways to go. If you buy that much, you can either say, “Okay, I’m going to be a dualist. I’m going
2:13:47 to believe that there’s matter and mind and they are separate from each other and they are interacting
2:13:51 somehow.” Or that’s a little bit complicated and sketchy as far as physics is going to go. So
2:13:57 I’m going to believe in mind, but I’m going to put it prior to matter. I’m going to believe that mind
2:14:02 comes first and that consciousness is the fundamental aspect of reality and everything else,
2:14:08 including matter and physics, comes from it. That would be at least as simple as physics comes
2:14:14 first, right? Now, the physicalist, such as myself, will say, “I don’t have any problem
2:14:22 explaining what is like to be you or what you experience when you see red. It’s a certain
2:14:28 way of talking about the atoms and the neurons, etc., that make up you. Just like the hardness
2:14:35 or the brownness of this table, these are words that we attach to certain underlying configurations
2:14:42 of ordinary physical matter. Likewise, sadness and redness or whatever are words that we attach
2:14:49 to you to describe what you’re doing.” And when it comes to consciousness in general,
2:14:55 I’m very quick to say I do not claim to have any special insight on how consciousness works
2:15:02 other than I see no reason to change the laws of physics to account for it.
2:15:07 If you don’t have to change the laws of physics, where do you think it emerges from?
2:15:09 Is consciousness an illusion? It’s almost like a shorthand that we humans use to describe a
2:15:16 certain kind of feeling we have when interacting with the world. Or is there some big leap that
2:15:22 happens at some stage? I almost never use the word illusion. Illusion means that there’s something
2:15:28 that you think you’re perceiving that is actually not there. Like an oasis in the desert is an
2:15:33 illusion. It has no causal efficacy. If you walk up to where the oasis is supposed to be, you’ll say,
2:15:39 “You are wrong about it being there.” That’s different than something being emergent or
2:15:44 non-fundamental, but also real. Like this table is real. Even though I know it’s made of atoms,
2:15:49 that doesn’t remove the realness from the table. I think the consciousness and free will and things
2:15:53 like that are just as real in tables and chairs. Oasis in the desert does have causal efficacy in
2:15:59 that you’re thirsty. It leads to draw incorrect conclusions about the world.
2:16:04 Sure, but imagining a thing can sometimes bring it to reality, as we’ve seen, and that has a kind of
2:16:13 causal efficacy. Sure, but your understanding of the world in a way that gives you power over it
2:16:21 and influence over it is decreased rather than increased by believing in that oasis.
2:16:25 That is not true about consciousness or this table. You don’t think you can
2:16:29 increase the chance of a thing existing by imagining it existing?
2:16:38 Unless you build it or make it. No, that’s what I mean. Imagining humans can fly if you’re the
2:16:44 right brothers. But that’s never to mention that humans are flying. In terms of counterfactuals,
2:16:50 in the future, absolutely imagination is crucially important, but that’s not an illusion.
2:16:54 The possibility of the future versus what reality is. I mean, the future is a concept,
2:17:04 so you can… Time is just a concept, so you can play with that. But yes, reality,
2:17:15 so to you, so for example, I love asking this, so Donald Hoffman
2:17:23 thinks that the entirety of the conversation we’ve been having about spacetime is an illusion.
2:17:32 Is it possible for you to steel man the case for that? Can you make the case
2:17:36 for and against reality, as I think he writes, that the laws of physics as we know them with
2:17:45 spacetime is a kind of interface to a much deeper thing that we don’t at all understand,
2:17:50 and that we’re fooling ourselves by constructing this world?
2:17:53 Well, I think there’s part of that idea that is perfectly respectable and part of it that
2:17:57 is perfectly nonsensical. I’m not even going to try to steel man the nonsensical part.
2:18:02 The real part to me is what is called structural realism.
2:18:07 We don’t know what the world is at a deep fundamental level. Let’s put ourselves in
2:18:15 the minds of people living 200 years ago. They didn’t know about quantum mechanics,
2:18:20 they didn’t know about relativity. That doesn’t mean they were wrong about the universe that
2:18:25 they understood. They had Newton’s laws. They could predict what time the sun was going to rise
2:18:30 perfectly well. In the progress of science, the words that would be used to give the most
2:18:38 fundamental description of how you were predicting the sun would rise changed because now you have
2:18:45 curved spacetime and things like that, right? And you didn’t have any of those words 200 years ago.
2:18:49 But the prediction is the same. Why? Because that prediction, independent of what we thought the
2:18:56 fundamental ontology was, the prediction pointed to something true about our understanding of
2:19:03 reality. To call it an illusion is just wrong, I think. We might not know what the best, most
2:19:10 comprehensive way of stating it is, but it’s still true. Is it true in the way, for example,
2:19:17 belief in God is true? Because for most of human history, people have believed in a God or multiple
2:19:25 gods. And that seemed very true to them as an explanation for the way the world is.
2:19:35 Some of the deeper questions about life itself and the human condition and why certain things
2:19:41 happen. That was a good explainer. So, that’s not an illusion. No, I think it was completely an
2:19:50 illusion. I think it was a very, very reasonable illusion to be under. There are illusions. There
2:19:54 are substantive claims about the world that go beyond predictions that we can make and verify,
2:20:01 which later turned out to be wrong. And the existence of God was one of them. If those people
2:20:08 at that time had abandoned their belief in God and replaced it with a mechanistic universe,
2:20:13 they would have done just as well at understanding things, right? Again, because there are so many
2:20:18 things they didn’t understand, it was very reasonable for them to have that belief. It
2:20:22 wasn’t that they were dummies or anything like that. But that is, as we understand the universe
2:20:28 better and better, some things stick with us, some things get replaced. So, like you said,
2:20:33 you are a believer of the mechanistic universe. You’re a naturalist. And as you’ve described,
2:20:41 a poetic naturalist. That’s right. What’s the word poetic? What is naturalism? And what is
2:20:47 poetic naturalism? Naturalism is just the idea that all that exists is the natural world.
2:20:52 There’s no supernatural world. You can have arguments about what that means, but I would
2:20:58 claim that the argument should be about what the word supernatural means, not the word natural.
2:21:03 The natural world is the world that we learn about by doing science. The poetic part means that you
2:21:08 shouldn’t be too, I want to say fundamentalist about what the natural world is. As we went from
2:21:17 Newtonian space time to Einsteinian space time, something is maintained there. There is a different
2:21:25 story that we can tell about the world. And that story in the Newtonian regime, if you want to fly
2:21:31 a rocket to the moon, you don’t use general relativity. Use Newtonian mechanics. That story
2:21:36 works perfectly well. The poetic aspect of the story is that there are many ways of talking about
2:21:41 the natural world. And as long as those ways latch on to something real and causally efficacious
2:21:48 about the functioning of the world, then we attribute some reality and truth to them.
2:21:53 So the poetic really looks at the, let’s say, the pothead questions at the edge of science.
2:21:59 It’s more open to them. It’s doing double duty a little bit. So that’s why it’s confusing. The
2:22:05 more obvious respectable duty is doing is that tables are real. Even though you know that it’s
2:22:11 really a quantum field theory wave function, tables are still real, they’re a different way
2:22:16 of talking about the underlying deeper reality of it. The other duty is doing is that we move
2:22:22 beyond purely descriptive vocabularies for discussing the universe onto normative and
2:22:28 prescriptive and judgmental ways of talking about the universe. This painting is beautiful,
2:22:34 that one is ugly. This action is morally right, that one is morally wrong. These are also ways
2:22:39 of talking about the universe. They are not fixed by the phenomena. They are not determined by our
2:22:46 observations. They cannot be ruled out by a crucial experiment. But they’re still valid. They might not
2:22:51 be universal. They might be subjective, but they’re not arbitrary. And they do have a role in describing
2:22:57 how the world works. So you don’t think it’s possible to construct experiments that explore
2:23:03 the realms of morality and even meaning. So those are subjective.
2:23:10 Yeah. They’re human. They’re personal.
2:23:13 But do you think that’s just because we don’t have a, the tools of science have not expanded
2:23:19 enough to incorporate the human experience? No, I don’t think that’s what it is. I think that
2:23:24 what we mean by aesthetics or morality are we’re attaching categories, properties to things that
2:23:31 happen in the physical world. And there is always going to be some subjectivity to our
2:23:35 attachment and how we do that. And that’s okay. And the faster we recognize that and deal with it,
2:23:39 the better awful be. But if we deeply and fully understand the functioning of the human mind,
2:23:46 won’t be able to incorporate that, no. That will absolutely be helpful in explaining why
2:23:51 certain people have certain moral beliefs. It won’t justify those beliefs. That’s right or wrong.
2:23:56 Do you think it’s possible to have a kind of general relativity, but that includes
2:24:00 the observer effect where the human mind is the observer? Sure.
2:24:05 So sort of like how we morph in the same way, gravity morphs space time. How does the human
2:24:13 mind morph reality and have a very thorough theory of how that morphing actually happens?
2:24:23 That’s a very pothead question, Lex. I’m sorry. We know you think it’s possible.
2:24:27 The answer is yes. I think that there’s no, I think that we’re part of the physical world
2:24:33 and the natural world. Physicalism would have been just as good a word to use as naturalism,
2:24:40 maybe even a more accurate word, but it’s a little bit more off-putting. So I did want to
2:24:44 snap your more attractive label than physicalism. Are there limits to science?
2:24:51 Sure. We just talked about one. Science can’t tell you right from wrong.
2:24:54 You need science to implement your ideas about right and wrong. If you are functioning on the
2:25:02 basis of an incorrect view of how the world works, you might very well think you’re doing right,
2:25:06 but actually be doing wrong. But all the science in the world won’t tell you which action is right
2:25:11 and which action is wrong. Dictators and people in power will sometimes use science
2:25:18 as an authority to convince you what’s right and wrong. Studying Nazi science is fascinating.
2:25:24 But there’s an instrumentalist view here. You have to first decide what your goals are,
2:25:29 and then science can help you achieve those goals. If your goals are horrible,
2:25:33 science has no problem helping you achieve them. Science is happy to help out.
2:25:38 Let me ask you about the method behind the madness on several aspects of your life.
2:25:42 So you mentioned your approach to writing for research and writing popular books.
2:25:48 How do you find the time of the day? What’s the day in the life of Sean Carroll?
2:25:52 So you don’t have a thing where in the morning you try to fight for two hours somewhere?
2:26:00 I don’t. I’m really terrible at that. My strategy for finding time is just to ignore
2:26:05 interruptions and emails. But it’s a different time every day. Some days it never happens,
2:26:11 some weeks it never happens. You’re extremely prolific. You’re able to have days where you
2:26:16 don’t write and still write the next day. Oh, wow. That’s a rare thing. A lot of
2:26:24 prolific writers will carve out two hours because otherwise it just disappears.
2:26:30 Right. No, I get that. Yeah, I do. Everyone has their foibles or whatever.
2:26:39 So I’m not able to do that. Therefore, I have to just figure it out on the fly.
2:26:46 And what’s the actual process look like when you’re writing popular stuff? You get behind a
2:26:50 computer? Yeah, get behind a computer. And my way of doing it, so my wife, Jennifer,
2:26:55 is a science writer. But it’s interesting because our techniques are entirely different.
2:27:00 She will think about something, but then she will free write. She’ll just sit at a computer and write.
2:27:05 Like, I think this, I think this. And then that will be vastly compressed, edited, rewritten or
2:27:11 whatever until the final thing happens. I will just sit there silently thinking for a very long
2:27:17 time and then I will write what is almost the final draft. So a lot of it happens. There might be
2:27:22 some scribbles for an outline or something like that. But a lot of it is in my brain before it’s
2:27:26 on the page. So that’s the case for the biggest ideas in the universe, the quanta book and the
2:27:30 space time motion book? Yeah, quanta and fields, which is actually mostly about quantum field theory
2:27:35 and particle physics. That’s coming out in May. And that is, I’m letting people in on things
2:27:44 that no other book lets them in on. So I hope it’s worth it. It’s a challenge because there’s a lot
2:27:48 of equations. I mean, you did the same thing with space time motion. You did something quite
2:27:52 interesting, which is like, you made the equation a centerpiece of a book. Right. There’s a lot of
2:27:58 equations. Book two is goes further in those directions than book one did. So it’s more
2:28:07 cool stuff. It’s also more mind bending. It’s more of a challenge. Book three that I’m writing right
2:28:13 now is called complexity and emergence. Oh, wow. That’ll be the final part of the trilogy. Oh,
2:28:20 that’s fascinating. So there’s a lot of probably ideas there. I mean, that’s a real cutting edge.
2:28:26 Well, but, you know, I’m not trying to be cutting edge. In other words, I’m not trying to speculate
2:28:32 in these books. Obviously, in other books, I’ve been very free about speculating. But
2:28:36 the point of these books is to say things that 500 years from now will still be true.
2:28:40 And so there are some things we know about complexity and emergence. And I want to focus
2:28:45 on those. And I will I will mention, I’m happy to say this is something that needs to be speculated
2:28:50 about, but I won’t pretend to be telling you what one is the right one. You somehow found the balance
2:28:54 between the rigor of mathematics and still accessible. Which is interesting.
2:28:58 I try. I mean, look, this these three books, the biggest ideas books are absolutely an experiment.
2:29:04 They’re going to appeal to a smaller audience than other books will. But that audience should
2:29:11 love them. Like my 16 year old self would have been so happy to get these books. I can’t tell you.
2:29:16 Yeah, in terms of looking back in history, those are books, the trilogy would be truly special
2:29:21 in that way. Work for Lord of the Rings. So I figured, why not me?
2:29:25 You wouldn’t talk. Yeah, different styles, different topics, same ultimate reality.
2:29:30 Like we mentioned, Mindscape Podcast, I love it. You interview a huge variety of experts
2:29:40 from all kinds of fields. So just several questions I want to ask. How do you prepare?
2:29:45 Like, how do you prepare to have a good conversation? How do you prepare in a way that
2:29:51 satisfies, makes your own curious mind happy? All that kind of stuff.
2:29:55 Yeah, no, these are great questions. And I’ve sort of struggled and changed my
2:29:58 techniques over the years. It’s over five year old podcast might be approaching six years old now.
2:30:03 I started out over preparing when I first started, you know, like I had a journey that I was going
2:30:11 to go down. Many of the people I talked to are academics or, you know, thinkers who write books.
2:30:16 So they have a story to tell. I could just say, okay, give me your lecture and then an hour later
2:30:22 stop, right? So the mistake is to sort of anticipate what the lecture would be and
2:30:28 to ask the leading questions that would pull it out of them. What I do now is much more,
2:30:33 here are the points, here are like the big questions that I’m interested in. And so I have
2:30:39 a much sketchier outline to start and then try to make it more of a real conversation.
2:30:46 I’m helped by the fact that it is not my day job. So I strictly limit myself to one day of
2:30:55 my life per podcast episode on average, some days take more. And that includes not just doing the
2:31:01 research, but inviting the guests, recording it, editing it, publishing it. So I need to be very,
2:31:07 very efficient at that. Yeah. You enforce constraints for yourself in which creativity
2:31:11 can emerge. That’s right. That’s right. And you know, look, sometimes if I’m interviewing a
2:31:17 theoretical physicist, I can just go in. And we’re interviewing an economist or historian,
2:31:23 I have to do a lot of work. Do you ever find yourself getting lost in rabbit holes that serve no
2:31:29 purpose except satisfying your own curiosity, and then potentially expanding the range of things
2:31:36 you know that can help your actual work and research and writing? Yes, on both counts. I do,
2:31:42 some people have so many things to talk about that you don’t know where to start or finish,
2:31:48 right? Others have a message. And one of the things I discovered over the course of these years
2:31:54 is the correlation with age. There are brilliant people, and I try very hard on the podcast to
2:32:00 sort of get all sorts of people, right? Different ages and things like that. And bless their hearts,
2:32:06 the most brilliant young people are not as practiced at wandering past their literal research,
2:32:14 right? They are less mastery over the field as a whole, much less how to talk about it. Whereas
2:32:20 certain older people just like have their patent answers, and that’s kind of boring, right? So
2:32:24 you want somewhere in between, the ideal person who has a broad enough of a scope that they can
2:32:31 wander outside their specific papers they’ve written, but they’re not overly practiced,
2:32:36 so they’re just giving you their canned answers. I feel like there’s a connection to the metaphor
2:32:40 of entropy and complexity, as you said there. Edge of chaos. You also do incredible AMAs,
2:32:46 and people should sign up to your Patreon because you can get to ask questions, Sean Carroll.
2:32:53 For several hours, you just answer in fascinating ways some really interesting questions. Is there
2:33:01 something you could say about the process of finding the answers to those? That’s a great one.
2:33:06 Again, it’s evolved over time. So the Ask Me Anything episodes were first, when I started
2:33:13 doing them, they were only for Patreon subscribers to both listen to and to ask the questions.
2:33:19 But then I actually asked my Patreon subscribers, “Would you like me to release them publicly?”
2:33:24 And they overwhelmingly voted yes. So I do that. So the Patreon supporters asked the questions.
2:33:29 Everyone can listen. And also at some point, I really used to try to answer every question,
2:33:35 but now there’s just too many. So I have to pick, and that’s fraught with peril. And my personal
2:33:41 standard for picking questions to answer is, “What are the ones I think I have interesting
2:33:45 answers to give for?” So that both means if it’s kind of the same old question about special
2:33:52 relativity that I’ve gotten 100 times before, I’m not going to answer it because you can just
2:33:56 Google that. It’s easier. There are some very clear attempts to ask an interesting question
2:34:05 that honestly just I don’t have an answer to. Like, “I read this science fiction novel.
2:34:10 What do you think about it?” I’m like, “Well, I haven’t read it, so I can’t help you there.”
2:34:15 “What’s your favorite color?” I can tell you what it is, but it’s not that interesting.
2:34:19 I try to make it a mix. It’s not all physics questions, not all philosophy questions. I will
2:34:27 talk about food or movies or politics or religion if that’s what people want to… I keep suggesting
2:34:32 that people ask me for relationship advice, but they never do. Yeah, I don’t think I’ve heard one.
2:34:38 I’m willing to do it, but I’m a little reluctant because I don’t actually like giving advice.
2:34:45 But I do, but I’m happy to talk about those topics. I want to give several hours of talking,
2:34:52 and I want to try to say things that I haven’t said before and keep it interesting,
2:34:57 keep it rolling. If you don’t like this question, wait for the next one.
2:34:59 What are some of the harder questions you’ve gotten? Do you remember? What kinds of questions
2:35:03 are difficult for you? Rarely, but occasionally, people will ask me a super insightful philosophy
2:35:10 question. I hadn’t thought of it in exactly that way, and I try to recognize that.
2:35:17 A lot of times, it’s the opposite where it’s like, “Okay, you’re clearly confused, and I’m going to
2:35:25 try to explain how the question you should have asked.” I love those. Yeah, why that’s the wrong
2:35:30 question or that kind of stuff. That’s great. But the hard questions, I don’t know. I don’t actually
2:35:37 answer personal questions very much. The most personal I will get are questions like,
2:35:41 “What do you think of Baltimore?” That much I can talk about, or “How are your cats doing?”
2:35:46 Happy to talk about the cats in infinite detail, but very personal questions I don’t get into.
2:35:51 But you even touch politics and stuff like this. Yeah, very happy to talk about politics.
2:35:56 I try to be clear on what is professional expertise, what is just me babbling,
2:36:02 what is my level of credence in different things, where you’re allowed to disagree,
2:36:06 whether if you disagree, you’re just wrong. People can disagree with that also, but I do think,
2:36:13 and I’m happy to go out on a limb a little bit. I’m happy to say, “Look, I don’t know,
2:36:18 but here’s my guess.” I just did a whole solo podcast, which was exactly that.
2:36:23 It’s interesting. Some people are like, “Oh, this was great,” and there’s a whole bunch of people
2:36:27 are like, “Why are you talking about this thing that you are not the world’s expert in?”
2:36:32 Well, I love the actual dance between humility and having a strong opinion on stuff,
2:36:37 which is a great, it’s a fascinating dance to pull off. I guess the way to do that is to just
2:36:43 expand into all kinds of topics and play with ideas and then change your mind and all that
2:36:48 kind of stuff. Yeah, it’s interesting because when people react against you by saying,
2:36:56 “You are being arrogant about this,” 99.999% of the time, all they mean is, “I disagree.”
2:37:03 That’s all they really mean, right? At a very basic level, people will accuse
2:37:12 atheists of being arrogant. I’m like, “You think God exists and loves you? You’re telling me that
2:37:18 I’m arrogant.” All of this is to say, just advice. When you disagree with somebody, try to specify
2:37:28 the substantive disagreement. Try not to psychologize them. Try to say, “Oh, you’re saying this because
2:37:33 of this.” Maybe it’s true. Maybe you’re right. But if you had an actual response to what they
2:37:39 were saying, that would be much more interesting. Yeah, I think I wonder why it’s difficult for
2:37:44 people to say or to imply, “I respect you. I like you, but I disagree on this, and here’s why I
2:37:52 disagree.” I wonder why they go to this place of, “Well, you’re an idiot,” or “You’re
2:38:00 egotistical,” or “You’re confused,” or “You’re naive,” or all the kinds of words. As opposed
2:38:10 to, “I respect you as a fellow who would be exploring the world of mysteries all around us,
2:38:16 and I disagree.” I will complicate the question even more, because there’s some people I don’t
2:38:21 respect or like. I once wrote a blog post. I think it was called “The Grid of Disputation,”
2:38:27 and I had a two-by-two grid, and it’s, “Are you someone I agree with or disagree with? Are you
2:38:34 someone who I respect or don’t?” All four quadrants are very populated. What that means is there are
2:38:44 people who I like and I disagree with, and there are people who agree with me, and I have no respect
2:38:50 for it all, the embarrassing allies quadrant. That was everyone’s favorite. I just think
2:38:55 being honest, trying to be honest about where people are, but if you actually want to move a
2:39:02 conversation forward, forget about whether you like or don’t like somebody. Explain the
2:39:06 disagreement, explain the agreement, but you’re absolutely right. I completely agree. As a society,
2:39:11 we are not very good at disagreeing. We instantly go to the insults.
2:39:15 Yeah. I mean, even on the deeper level, I think at some deep level, I respect and love
2:39:25 the humanity and the other person. Yep. You said that general relativity is the most beautiful
2:39:34 theory ever. So far. What do you find beautiful about it? Let’s put it this way. When I teach courses,
2:39:41 there’s no more satisfying subject to teach than general relativity, and the reason why is because
2:39:49 it starts from very clear, precisely articulated assumptions, and it goes so far. When I give
2:39:59 my talk, you can find it online. I’m probably not going to give it again. The book, one of the
2:40:02 biggest ideas, Talk, was building up from, you don’t know any math or physics. An hour later,
2:40:10 you know Einstein’s equation for general relativity. The punchline is the equation
2:40:17 is much smarter than Albert Einstein, because Albert Einstein did not know about the Big Bang.
2:40:22 He didn’t know about gravitational waves. He didn’t know about black holes, but his equation did.
2:40:27 That’s a miraculous aspect of science more generally, but general relativity is where it
2:40:36 manifests itself in the most absolutely obvious way. A human question. What do you think of the
2:40:44 fact that Einstein didn’t get the Nobel Prize for general relativity? Tragedy.
2:40:49 He should have gotten maybe four Nobel Prizes, honestly. The photoelectric effect was 100%
2:40:59 worth the Nobel Prize, and people don’t quite get this. Who cares about the photoelectric effect?
2:41:04 That’s like this very minor effect. The point is his explanation for the photoelectric effect
2:41:09 invented something called the photon. That’s worth the Nobel Prize. Max Planck gets credit
2:41:17 for this in 1900, explaining black body radiation by saying that when a little electron is jiggling
2:41:24 in an object at some temperature, gives off radiation in discrete chunks rather than
2:41:31 continuously. He didn’t quite say that’s because radiation is discrete chunks. It’s like having
2:41:39 a coffee maker that makes one cup of coffee at a time. It doesn’t mean that liquid comes in one cup
2:41:43 quanta, right? Just that you are dispensing it like that. It was Einstein in 1905 who said
2:41:50 light is quanta, and that was a radical thing. Clearly, that was not a mistake, but also special
2:41:56 relativity clearly deserved the Nobel Prize, and general relativity clearly deserved the Nobel Prize.
2:42:02 Not only were they brilliant, but they were experimentally verified, like everything you want.
2:42:06 So separately, you think? Yeah, absolutely.
2:42:08 Humans, whatever the explanation there. Edwin Hubble never won the Nobel Prize for
2:42:16 finding the universe was expanding. Yeah, but even the fact that we give prizes is almost
2:42:22 kind of silly. We limit the number of people that get the prize and all that. I think that
2:42:26 the Nobel Prize has enormous problems. I think it’s probably a net good for the world,
2:42:32 because it brings attention to good science. I think it’s probably a net negative for science,
2:42:38 because it makes people want to win the Nobel Prize.
2:42:41 Yeah, there’s a lot of fascinating human stories underneath it all. Science is its own thing,
2:42:48 but it’s also a collection of humans, and it’s a beautiful collection. There’s tension,
2:42:52 there’s competition, there’s jealousy, but there’s also great collaborations and all that kind of
2:42:58 stuff. Daniel Kahneman, who recently passed, is one of the great stories of collaboration in science.
2:43:09 So all of it, all of it, that’s what humans do. And Sean, thank you for being the person that makes us
2:43:17 celebrate science and fall in love with all of these beautiful ideas in science for writing
2:43:23 amazing books, for being legit and still pushing forward the research, science side of it, and
2:43:30 for allowing me and these podhead questions, and also for educating everybody through your own
2:43:39 podcast. Everybody should stop everything and subscribe and listen to every single episode
2:43:46 of Mindscape. So thank you. I’ve been a huge fan forever. I’m really honored that you would speak
2:43:51 with me. In the early days when I was still starting this podcast, it means the world.
2:43:55 I appreciate it. Thanks very much for having me on. Now that you’re a big deal, still having me on.
2:43:58 Thank you, Sean. Thanks for listening to this conversation with Sean Carroll. To support this
2:44:05 podcast, please check out our sponsors in the description. And now let me leave you with some
2:44:10 words from Richard Feynman. Study hard what interests you the most in the most undisciplined,
2:44:17 irreverent, and original manner possible. Thank you for listening and hope to see you next time.
2:44:33 [Music]
2:44:37 (gentle music)