Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas - 45 | Leonard Susskind on Quantum Information, Quantum Gravity, and Holography

Episode Date: May 6, 2019

For decades now physicists have been struggling to reconcile two great ideas from a century ago: general relativity and quantum mechanics. We don't yet know the final answer, but the journey has taken us to some amazing places. A leader in this quest has been Leonard Susskind, who has helped illuminate some of the most mind-blowing ideas in quantum gravity: the holographic principle, the string theory landscape, black-hole complementarity, and others. He has also become celebrated as a writer, speaker, and expositor of mind-blowing ideas. We talk about black holes, quantum mechanics, and the most exciting new directions in quantum gravity. Support Mindscape on Patreon or Paypal. Leonard Susskind received his Ph.D. in physics from Cornell University. He is currently the Felix Bloch Professor of Physics at Stanford University. He has made important contributions to numerous ideas in theoretical physics, including string theory, lattice gauge theory, dynamical symmetry breaking, the holographic principle, black hole complementarity, matrix theory, the cosmological multiverse, and quantum information. He is the author of several books, including a series of pedagogical physics texts called The Theoretical Minimum. Among his numerous awards are the J.J. Sakurai Prize and the Oskar Klein Medal. Web page Theoretical Minimum page Susskind Lectures on YouTube TED Talk about Richard Feynman Publications at Inspire Amazon author page Wikipedia

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Hey everyone, it's Kelpen. I'm inviting you to join the best. sounding book club you've ever heard with my podcast, Earsay, the Audible and I Heart Audio Book Club. Every episode, I nerd out with amazing guests and dive into the best new audiobooks available on Audible. It's the book club for your ears. Listen to Earsay, the Audible and IHart audiobook club on the IHart Radio app or wherever you get your podcasts. Hello, everyone, and welcome to the Mindscape podcast. I'm your host, Sean Carroll. And today's episode, I think it's safe to say, that has been anticipated for quite a while by a lot of people out there in Minescape Land.

Now, usually I'm an ornery guy, and if a bunch of people demand that I do this or that, I sort of resist doing this or that for exactly that reason. But in this case, the wisdom of the crowd was absolutely on the right track. So we're very happy to have Lenny Suskin on the podcast. To those of you who are physics fans, Lenny is very well known for his technical work, for his popular work, for his semi-technical books, the theoretical minimum, and within the physics community, extremely well known as a storyteller, a mentor, and a guiding visionary of the field. For those of you who don't know him, you're in for a treat,

both the personality that comes through very clearly and also the subjects that we're going to talk about. Lenny was one of the founding fathers of string theory. He has done an enormous amount of work in quantum field theory and the standard model, but today, at the age of 78, he is leading the charge to understand the black hole information loss paradox. This is the thing that Stephen Hawking bequeathed to us in the 1970s when he realized that black holes give off radiation, and if you throw information into a black hole, how does that information come back out in the radiation, or even does it come out? So Lenny, like I said, is one of the leaders in tackling this problem, which is probably the

single biggest clue that we have to ultimately constructing a theory of quantum gravity. So this has led people into things like the ADS-CFT correspondence, It from QBit, a whole bunch of very mind-stretchy ideas. So we're going to do our best in this podcast to bring those mind-stretching ideas down to earth, to make them understandable, and to convey some of the excitement that physicists, like Lenny and I, because we work in very, very similar fields,

really feel these days about the progress that's being made in the prospects for what's going to come very, very soon. So buckle up. This is going to be quite a ride, and I think you're going to like it. Let's go. This summer, A.G. Jeans looks to California Wine Country for inspiration, where style feels effortless, and time naturally slows down. The summer 2026 collection reflects that mindset with premium denim washes, relaxed silhouettes

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along with sun-fated sweatshirts, hats, and versatile essentials. Like a fine wine, AG denim is designed to evolve beautifully over time, growing richer with everywhere. AG's summer 2026 collection is available now at AGG, com. Use code summer 15 for 15% off, even if you've shopped with us before. Ask yourself, what are your best people spending their time on right now? Expense reports, receipt chasing, month in close that takes weeks. You become what you spend on, and that's not what you're building toward. Brex is the intelligent finance platform that eliminates that work before it starts,

AI agents that handle the manual stuff automatically. So your team can spend their time on what actually compounds. It's time to get a lot. at Brex AF. Learn more at brex.com slash AF. Lenny Saskin, welcome to the Mindscape podcast. Hi, how are you? So one question I'd like to ask of people who are identified as string theorists is, are you a string theorist? Do you call yourself that? I really don't. I really don't. I had a lot to deal with it in the beginning and then more recently,

but I like to just think of myself as a theoretical physicist. I think that's smarter because then you can do whatever you want, right? Yeah, you do what you're curious. about one time I was curious about that. The other times I was curious about other things, and I tend to follow where my nose leads. Right. Just so we can get it on the record, do you have a feeling for the present state and possible future of string theory as a field? Yeah, I think I do. I can't tell, and I don't know if it's going to be the, let's call it, the theory of everything. I hate that term, but I'll use it anyway. People get it. Whether it's

going to describe in any of its given forms, whether it's going to describe nature as we know it. But I can tell you what it has done. It's provided some very, very concrete, very, very precise, let me call them examples of theories, models, however you want to call them, are that contain gravity, that contain electrodynamics, that contain particles, that contain fermions and bosons, the various kinds of particles that we have in nature. and looks a lot like what we think nature looks like, at least when we look in the laboratory, we see particles, we see gravity, we see all these things,

and they're there. And so what this provides us with is a highly precise mathematical situation where we can try out ideas. And one of the ideas that was most important in all of this was the consistency of having quantum mechanics and gravity in the same mathematical theory. And I think by virtue of string theory and its spin-offs,

I think we know with certainty now that mathematically quantum mechanics and gravity can fit together sensibly and consistently. That is no small thing. And it might not be our world that we live in, but it's possible. That's right. It might not be, but we now know

the two can fit together that this idea that there was some breakdown where quantum mechanics and gravity simply could not fit together, I think we're quite sure that that is wrong. And one of the great sort of guide stars we have in this game is the entropy and evaporation of black holes, right? And you wrote a wonderful book that I'll plug the Black Hole War, written many wonderful books, but that was a great one.

No, not many. No, okay, a couple. A couple. I have to say they're wonderful. Oh, I don't. I wasn't arguing whether they're wonderful. I was arguing with as many. And so what do we learn?

I mean, let's imagine that the audience has heard the fact that black holes evaporate, but don't know any details. And we take it, many of us take it that there is some quantum mechanical lesson here. And in particular, there's information from which the black hole is made and we're not sure where it goes. So how do you think about this puzzle, this sort of legacy that Stephen Hawking left us? Right. Okay. So Stephen asked a wonderful question. It was a deep question.

It was a profound question. Let's not mix it up with a question of whether he answered it correctly. Sometimes questions can be more important than leave a bigger legacy than the attempted answers. And I think that's probably the case in Stephen's legacy, that his question is almost dominating the areas of physics that I've been interested in for the last 20, 30 years, completely dominating. Stephen simply asked a very simple question. He said, if something falls into a black hole and the black hole evaporates, well, almost by definition, something that falls into the black hole cannot get out of the black hole. And so what happened to the information that fell into the black hole?

Now, we can get into all kinds of questions of what we mean by information and so forth, but just basically the ability that in principle, looking at what comes out of the black hole, is it possible to reconstruct or fell in? In principle, given technologies, which is way, way, way beyond anything we're likely to ever have, but assume that we have it, and it's physically possible,

can one reconstruct or fell into the black hole? And there was a tension there. The tension had to do with the fact that in basic quantum mechanics, nothing is ever lost in the sense that if you know the quantum, state afterwards, you can reconstruct what the quantum state was before. That's a principle of quantum mechanics, and so in that sense, nothing can ever get lost.

On the other hand, black holes were places where things get lost. When anything falls past the horizon of a black hole, we were brought up to learn that they cannot get out. They're trapped. They're there forever. And so there was a tension there. Do they or don't they get out? Can you reconstruct, to put it in practice, what it meant was when a black hole evaporates,

it has evaporation products. Anything that evaporates leaves products coming out. There's a black hole, the products of evaporation of a black hole in principle, can you reassemble them and figure out what fell into the black hole? As simple as that. Stephen said no on the basis that black holes are places where you're not. you cannot escape from. And other people, myself included, said,

nope, you have to be able to, because basic quantum mechanics says you have to be able to. Who won? I would say quantum mechanics, won. So I want to bring up a, you know, there's a funny little paper by a philosopher, Tim Maudlin. I don't know if you read this paper. No, I don't.

But he makes the following point that our notion of time is a little bit subtle in general relativity. And in particular, we could imagine, calling the universe both the outside of the black hole and the inside of the black hole simultaneously. In other words, the universe is disconnected into the interior of the black hole and the exterior of the black hole. And then you could imagine that time just plugs on and information is not lost in the whole quantum wave function. It's just hidden forever in the black hole like it's a baby universe. Is that a possible resolution that you'd be sympathetic to?

No. Why not? Black hole evaporates. It's gone. It's gone in some slicing of the universe. in some of time's place. Oh, okay. In practice in the laboratory, the black hole is gone.

There's nothing left, but not even a little remnant that tells you there was a black hole there. All that's left is the outgoing hawking radiation. Principles of quantum mechanics said that that radiation has to carry the information. Now, can you wiggle and can you find ways to try to invade against that? against that, no doubt you can. But I think by now, your first question had to do about string theory. And I told you that string theory provides us an extremely precise tool in which we can investigate these questions in a mathematical context that we have a lot of confidence in. The answer that comes out of that highly mathematical but precise context is nothing is ever

lost from a black hole. So I think, so I think, This is a solid conclusion. As I said, it's not a small thing. And even Stephen, even Stephen conceded. He gave up the bat, right? Yeah. So one way of thinking about it. He did stick to his guns.

He did stick to his guns. He was a stubborn guy, but he would change his mind once it became. He did indeed. Absolutely clear. So one way that this is often put is if you throw a book into a black hole, the laws of physics, quantum mechanics, say that in principle you should be able to measure all of the radiation that comes out and reconstruct the book. Yeah, I mean, measuring is a loaded term in quantum mechanics. We have to be careful, but there is a sense in which... It's in there,

anyway, whether you can measure it or not, but the information is there. So let's just sort of go through some of these thought experiments. This is, after all, the thought experiment, just so the audience knows we haven't made any black holes and watched them evaporate, right? Oh, I don't know. No. No. How we have to do. We have not. So is it possible that somehow when the book falls into the black hole, its information is duplicated into the outgoing radiation and we get it that way? Well, that's an interesting question.

Can one model, one picture of what happens is as information falls into a black hole, it's exactly as you said, it gets duplicated, Xerox, let's call it. one copy falls into the black hole so people who fall into the black hole with the information see it smoothly pass through the horizon and the other copy is radiated back out

with the hawking radiation and then you have what's it we have our cake and eat it yeah we have our cake and eat it too this is an okay theory I think the problem with it is it's known in quantum mechanics that you can't not really duplicate information.

That quantum information, true quantum information cannot be duplicated. I thought I discovered this fact, and I call the no quantum Xerox principle. It had been discovered before by quantum theorists. It's called the no cloning theorem. It's one of the most central properties of quantum mechanics that you cannot faithfully reproduce the quantum state, and you can't duplicate it, as you said. So that left us with a funny situation. You can't even, at least within the usual rules of quantum mechanics,

save the situation by duplicating the information. On the other hand, what if you could duplicate the information, but nobody could ever in principle detect the fact that you duplicated in the information? Should that it feel better? Should we care what people can detect? Yeah, physics is an empirical science. And you start getting very, I know, I get very troubled when we put mathematical things into a theory that in principle can't be detected. I'm not saying you should never do that.

I'm not saying that under no circumstances should there be things in a theory which are, what's called, not empirically confirmable. But you should be a little bit nervous about it. If you discover that because of the laws of physics that you already trust, that even if you did, let's say, duplicate information, it would be the outcome would always be that you would get frustrated in trying to confirm

that you did something bad. I'll give you an example in a moment. Yeah. Okay. You might start to worry that you're thinking wrong. Ask yourself, what are your best people spending their time on right now? Expense reports, receipt chasing, month in close that takes weeks. You become what you spend on, and that's not what you're building toward.

Brex is the intelligent finance platform that eliminates that work before it starts. AI agents that handle the manual stuff automatically. So your team can spend their time on what actually. compounds. It's time to get Brex AF. Learn more at brex.com slash AF. Hey, everyone. It's Cal Penn. I'm the host of Earsay, the Audible and I-Heart audiobook club. This week on the podcast, I'm sitting down with Ray Porter, the narrator of Andy Weir's audiobook Project Hail Mary, massive sci-fi adventure about survival and science, and what happens when you wake up alone very far from Earth?

I really had to make a decision because I caught myself getting that frog in my throat and starting to get teary as I'm narrating some of these sections. And it's like, okay, yo, yeah, yo, is this indulgent? And I really thought about it. I was like, no, at this point, it would kind of be betraying the trust the author and the listener have in telling this story if I don't go through it. But there's places in this book that deeply emotionally affected me and I left it on the mic. That's great. Because it served the story. people will say like, oh my God, I cried at the end.

It's like, yeah, dude, me too. Listen to Earsay, the Audible and IHeart Audio Book Club on the IHeart Radio app or wherever you get your podcasts. So here's the example. Heisenberg taught us a long time ago that you can't know both the position and the momentum of the position and the velocity of a particle. Why not? Why can't you do both? A particle is just a little thing.

If it has a position, then it can have a position. then it can have a position at two different times. If it has a position at two different times, then it moved from one place to another. How can you not know the velocity at the same time? That's a good point. Yeah, it was a good point. And yet, it turned out that in quantum mechanics,

that no matter how hard you tried to be able to measure both the position and the velocity, doing one would always frustrate the other. And so it came to understand that really there is, a conflict, a tension, or whatever you want to call it, between knowing the position and the velocity of a particle, and that you simply can't do both. You simply can't do both.

So best then that your theory not allow it in principle, or better yet that the theory never allow an experiment, which can say quantum mechanics doesn't say a particle can't have a position and a velocity. It says nobody can ever detect both. of them. Yeah, okay. You know, as a never-ready-in, I would say that there's no such thing as the position or

the velocity. There's only the wave function. You can say it any way you like. I'll take the view that quantum mechanics doesn't say any such comprehensive things that a particle can't have a position of a velocity. What it says is that any experiment to design one will always frustrate the other. So you ask, were the thing falling into a black hole and so forth? Does it get duplicated?

It doesn't get duplicated. supposing you discover that using the rules of quantum mechanics and gravity, you will inevitably discover that any attempt to witness the duplication was always get frustrated. If you measure what's outside, you can't measure what's inside. If you measure what's inside, you can't measure what's outside, very much like position and velocity. Then maybe you'll say it's a principle similar to the uncertainty principle

that you simply can't do both. And that's my feeling about it now. So the thing that would have been imaginable is we're imagining some literal observer sitting outside the black hole, collecting some radiation from it, and then diving in really quickly and reading the book inside. Well, you try to do that. Apparently not. Apparently not. It always turns out, again, you always get frustrated. If you wait long enough on the outside to be able to collect this information in the hawking radiation,

you'll always find out that by the time you jump in, the first copy of it, the first clone of it, has fallen into the singularity and is gone. You can never get there. Right. And this is not a lightweight statement. This is based on calculation and geometry and so forth. You find out that an attempt to learn both,

what fell in and what came out at the same time gets frustrated. It takes too long to learn what came out so that when you jump in, what went in is gone into the singularity. And likewise, of course, if you just jumped in, you could read the book, but then you can never come back out again, right? So, yeah, so you get access to one copy of the book, but never to two. Yeah, that's what, yeah, right? And so what does this tell us about how we should think about black holes in quantum mechanics? Well, we should think about black holes in quantum mechanics in a way which is probably not too different than we think about quantum mechanics.

that you cannot know everything quantum mechanically, which classical physics tells you that you can know. Classical physics tells you you can know the position and momentum of a particle at the same time. It tells you that you can know the phase and the energy of an electromagnetic wave. Don't worry if you don't know what that means. It means something.

You can't know both, or at least classical physics says you can know both. quantum mechanics, you have to be consistent. You have to be very consistent. Use quantum mechanics as it was designed to be used and be very skeptical when your rules, when your principles seem to, how shall I say?

I'm running out of words. To lead you to some contradiction or something? Yeah, well, certainly when they lead you to a contradiction, then you should be appalled to go back to the drawing board. But when you're using concepts, which in principle by virtue of the rules of the game, cannot be confirmed. Yeah, oh, okay, okay, that point, yeah.

So, but this does have, this sounds maybe to the people out there, like this is an esoteric topic dealing with recovering information from black holes, which would be impractical anyway. But there seemed to be very deep consequence, for the nature of space and time itself. Like usually we think of information being located somewhere in space time. And this is saying that we shouldn't always ask that question. Yeah.

It is not a good question to ask where is the information. It may be a good question to ask in a particular way of gathering that information with a particular protocol for detecting that information. Where is it that it will be found? But more generally, different ways of gathering information may come to the conclusion. One way we would say it was here. Another way would say it was there without any contradiction because nobody can do both. Is there any worry that this way of talking gives so much emphasis to observers that we're being a little bit anthropocentric or something like that, that we're putting agency back into physics?

That's the expression. that wagon has already left the stable. Oh, that horse has already left the stable. By the phone, 1926. That was quantum mechanics, right? Okay, that's very fair. But are you, I mean, we're allowed to get on slight tensions here. When it comes to quantum mechanics,

are you a many worlds person, or are you agnostic about foundations of QM? Yeah, I think agnostic is the word. Agnosticism, okay. I just quote my old friend, Richard Feynman. this subject is so confusing, I can't even tell if there's a problem. He was asked exactly the same question. What do you make out of the foundations of quantum mechanics and so forth?

And his answer was, it is so confusing that I can't tell if there's a problem. And I feel that way. Quantum mechanics always works. I expect it will always work. I expect nobody will ever do an experiment that will violate it. On the other hand, there are some very, very deep and hard, I don't even know that they're hard questions to answer, maybe unanswerable,

about the relationship between, let's call it, reality and the mathematical symbols that we use on a piece of paper. Yeah. And I share both Feynman's skepticism of whether there's a problem there, and I share Hugh Everett's feelings that you should be able to describe quantum mechanics without collapsing the wave function, et cetera, et cetera. So one thing I do think is I don't think we will come to the end of the story until we do understand the relation between gravity and quantum mechanics.

Okay, that's interesting. Yeah, it's just, it's a sense that I have that too much in quantum mechanics and too much in gravity seem to influence each other and connect to each other in surprising ways that until we understand that connection, perhaps it's just, and also connecting with cosmology, horizons, all these things, that maybe until we understand those things better, it's a little bit premature

to have a final set of answers about quantum mechanics, but I don't know. My guess is it's not going to happen in my lifetime. I don't know. I'm optimistic. I'm working on it. You're optimistic. I'm 78.

Working hard. Okay, so just to get back to where we are. I didn't say your lifetime. No, I know, but I'm working hard for your lifetime. You seem very, very healthy. So we have the situation where we want quantum mechanics to be true, and we know some things about black holes. And so this leads us to this relatively profound suggestion that information doesn't have a unique location in space. And is this basically what you have dubbed black hole complementarity?

Yes, yes, very much so. Complementarity in the same sense, nobody could ever figure out what Bohr was talking about, incidentally. But to the extent that we do understand what Borr was talking about, complementarity pretty much in the same sense that information about position and information about momentum are complementary. You can't have both. You can have one, but not the other. And it depends on what experiment you do.

So I was trying to use the term in the same way Boer used it. And you were lucky enough that Boer was unclear about how he was using it. That's right. Well, it was so unclear that anything I said. You were safe. And that's closely related to another feature of black holes evaporating that you've also played a major role in, which is the holographic principle. Why are we, so holography somehow asks us to think of the entire black hole as either being or living on or being encoded on the surface, the area around the black hole. So why in the world would anybody think something that crazy?

Well, okay, the reason is the bounds on how much information can be stored in a region of space. And as I'm sure you know, there's a deep set of arguments that began with Beckenstein, Hawking, it hoofed, myself, and so forth, which concluded mathematically that the amount of information that can be stored in, in a region of space cannot be larger than the surface area of the region. That's unusual because normally in most physics, you would imagine the amount of information, number of bits that can be stored in a region of space

would be proportional to the volume of the region. Yeah. That's normal physics. I'm just putting a bit at every location in space. What's so hard about that? That's right. Put a bit of information at every point in space and then your number of bits will be proportional

to the volume. Indeed, what's so hard about that? Well, what's so hard about that is you wind up putting so much energy into the region of space that you create a black hole, and the black hole will be bigger than the region that you were trying to populate in the first place with information. So using ideas from black holes, from Hawking, from Beckenstein, and so forth, I think Atufth and I both came somewhat independently to the conclusion that you, under no circumstances could you ever put more information into a region of space than it's area.

Area measured in little plonky in pixels. And once you say that, you begin to think, well, maybe there should be some theory in which the interior volume of a region should be described by degrees of freedom that live on the boundary. Can you explain what a degree of freedom is? Yeah, it's a bit of information. of information. A bit of information, yeah. And I won't try to describe what a bit is. No, that's good. Zero and one. Zero and one. Yeah. Quantum mechanical version of that. The quantum mechanical version of zero and one.

Right. That there should be, if it's absolutely impossible that under any circumstances, you can ever contain more than an area's worth of information, again, perhaps that means there should be a description of it in terms of a theory which lives on the boundary. That was called the holographic principle. When the Toft first said it, nobody understood at all. There was a reason. No, no, there was a reason. He wrote a paper that I didn't even know about, and it was called dimensional reduction in gravity. What he meant was that the dimension goes from three dimensions, the volume to the surface area. The word dimensional reduction had an entirely different meaning to most physicists.

Entirely did. Don't worry about what it meant. It meant something entirely different. And since I was not in any way interested in dimensional reduction at that time, I never even looked at the paper. I didn't even know about it. Hey, everyone. It's Cal Penn.

I'm the host of Earsay, the Audible and I Heart audiobook Club. This week on the podcast, I am sitting down with Ray Porter, the narrator of Andy Weir's audio. book Project Hail Mary, massive sci-fi adventure about survival and science. And what happens when you wake up alone very far from Earth? I really had to make a decision because I caught myself getting that frog in my throat and starting to get teary as I'm narrating some of these sections. And it's like, okay, yo, yeah, yeah, yo, is this indulgent? And I really thought about it. I was like, no,

at this point, it would kind of be betraying the trust the author and the listener have in telling this story if I don't go through it. There's places in this book that deeply emotionally affected me and I left it on the mic. That's great. Because it served the story. People will say like oh my God, I cried at the end. It's like, yeah, dude, me too.

Listen to Earsay, the Audible and IHeart Audio Book Club on the IHeart Radio app or wherever you get your podcasts. Ask yourself, what are your best people spending their time on right now? Expense reports, receipt chasing, month in

close that takes weeks. You become what you spend on, and that's not what you're building toward. Brex is the intelligent finance platform that eliminates that work before it starts, AI agents that handle the manual stuff automatically, so your team can spend their time on what actually compounds. It's time to get Brex AF. Learn more at brex.com slash AF. Well, this is a lesson for young people out there. You can be one of the world's most famous physicist, but if the names, the titles of your papers are not that enticing, people are not going to read them. Yeah, I think that's true.

And I, he was thinking a little bit different than I was, but nevertheless, we came to the same conclusion that the maximum amount of information you could put in a volume was a surface area, and therefore there should be a theory of that type where everything is described by a surface set of degrees of freedom. We call it the hologram. I think he even used the word hologram some ways in his paper. I actually put it in the title.

Because a real hologram is a two-dimensional thing. Yes, yes, yes. A real hologram is a piece of film that's two-dimensional. If you look at it through a microscope, all you see is a bunch of random little dots and dots and, you know, just nothing you can make any sense out of.

But it encodes full three-dimensional information. So we both, independently called it that. I can tell you when the idea first came out, nobody noticed it with the Tuft at all. They did notice when I wrote it. Why? I think I was clearer.

I think I was clearer. Tuft was a very famous physicist. Nobel Prize winner, yeah. Yeah, yeah, yeah. One of my real heroes. Yeah, absolutely. Right. So nobody noticed it.

They noticed it when I wrote it. and I think a lot of people said, you know, those two guys used to be good physicists, but they lost their marbles. One of the few people who really did get excited about it was Ed Witten. He, from the beginning,

caught onto the idea. And really? Yeah, yeah. Ed Witten also, to those of you who don't know, like one of the most respected physicists around. Yeah, definitely. But the one who really made it precise and made it reputable, let's say, not only reputable,

turned it from a wild speculation to a almost clear-cut consequence of quantum mechanics and gravity and eventually to a tool. When a speculation goes from a speculation to a principle and then eventually becomes a tool of the subject, then, you know, that's progress. That's progress. And by the it, we're talking about this idea that all of this three-dimensional reality can be encoded on this two-dimensional.

Yeah, that was Juan Maldesana. Yeah. Juan Maldesana is now one of the very great physicists in the world. And he, again, he did not know about our work. Edward did. Witton did. Maldesana actually did not. He came to it his own strange way, actually from string theory, but the result was the same,

and he had a very, very precise mathematical version of it. It's the one that we call ADSCFT now. That's from string theory. And it was so mathematically precise and, you know, so convincing that it became, as I said, a principle and then a tool of physics. You know, I think this is sufficiently important and central that we should explain ADS CFD. There's six letters there. There's two things, ADS and CFT.

So ADS is a kind of space time. It's not the space time that we live in, a geometry, a geometry of space time. And why is it called ADS? That stands for anti-DICTER. Now, the CITR was a physicist, mathematician, physicist. from the early parts of the 20th century, who discovered a kind of space called De Sitter Space. I used to joke that his aunt discovered anti-Dissiter space.

De Sitter was classified actually as an astronomer. Yeah, yeah, he was an astronomer. Yeah, right. He was in astronomy. But he solved Einstein's equation for general order. And found the space called DeCitter space, which is incidentally thought to be the space, the kind of space we really live in.

Yeah, we're getting there. Right. but we understand that much less than we understand anti-decidus space. Now, anti-de-sidus space is in some sense the opposite of the city-space. It's not really the opposite. It has certain opposite properties. One of them is positive curvature. The other one has negative curvature.

It's not like a particle or an antiparticle. If you brought them together, they disappear. It's just, in some sense, anti-deciduspace has opposite properties to the sitter space. It turned out that was the one that was most amendable to mathematical analysis. I think, I'll just interject the one little extra piece of information. DeCitter's space is characterized by having a positive amount of energy in empty space. A positive amount of dark energy. Right.

Maybe you didn't want to use that term. I don't like to hear it. Dark energy, vacuum energy, whatever it is. But the space itself contains energy. An anti-Dissida space is the kind that has a negative, dark energy, vacuum energy. suffusing all of space. And that's the only thing in the universe. There's no particles.

There's no galaxies. There's no dark matter. Just vacuum energy everywhere. If it's a positive number, be decider. A negative number, which sounds crazy. And it's certainly not what we have in the world. But the space you get is anti-decider. Right.

And for reasons which are technical, string theory was very, very comfortable with anti-de-sidicid space and extremely to this day, uncomfortable with decider space. Yeah, it's an ongoing thing, like how to reconcile the observed fact that vacuum energy is possible with what string theory says should be the case. But Maldesanis simply took,

started with string theory, constructed out of the rules of string theory, this anti-discidus space, and analyzed it. And realized that it had this property that we call the holographic principle. He didn't even know the term, and his paper doesn't even refer to it.

Ed Witten, who was closely related to the string theory, centers of string theory, did, right of wrote a similar paper and called it holography. Holography.

An anti-desider space and holography. Right. So he was, but he was very familiar with my work and with Tuft's work. So the reason Juan did not mention

it was not because Juan has trouble mentioning other people's work. He just didn't know. No, that's right. So you had, based on black holes, this speculation that somehow

the information was encoded on the boundary of the black hole. Maldesena has an example of a specific spacetime, anti-desider space, where everything that happens in that space time is somehow encoded on a boundary with one lower dimension. Oh, no, in my work and in the tufts, the black hole is just a starting point. The eventual conclusion is that a region of space, no matter what's in it, black hole or no black hole would be described by such a boundary theory. but we had absolutely no idea how to build that kind of theory.

And, you know, it really was a speculation. I wouldn't say a wild speculation because I understood the reasons that we were pushed to it, but it did sound pretty out there. Maldesan's version of essentially the same thing was highly mathematically precise, and analyzable, you know, with theorem proof, proof, proof, proof. Equations, real equations. You don't get to speculate as much because there's equations that cover what you can do. I think the number of equations that appeared in the Tuft paper and Maya paper altogether,

the number of relevant equations. Both papers had a lot of equations. Most of them were irrelevant. The number of equations was very small. And Maldesan's paper, if I remember correctly, is now the most highly cited paper. So I'm taught. So this is important. Many thousands are.

So good. So Maldesan says, look, here's at least an example. We have one theory, but there's two different ways of talking about it. These two different ways seem utterly different. One is there's no gravity and it's a quantum field theory. And the other is there is gravity, but there's an extra dimension. There's one higher dimension of space. What have we learned from this weird construction?

Well, what do we ever learn? What do we learn? We learn we were... Okay, I'll tell you what I think we learned. Among other things, we learned many, many things. Gravity does not seem to be a thing that you quantize. I'll tell you what I mean by that. Ordinary theories like quantum electrode dynamics

or the theory of a harmonic oscillator or the hydrogen atom, the quantum mechanics of those theories was constructively. by a recipe. It was a recipe that was invented by Paul Dirac, and it starts with a classical theory, and then you apply some rules to the classical theory called quantization, and you produce a quantum theory of it. That never worked for gravity. It was the fact that it never worked, no matter how hard you tried, you always ran into infinities or information being lost or something really bad went wrong. It was that that made people think

that there was this tension between quantum mechanics and gravity and that there was a conflict between them. I think it's turning out the other way. I think the point is that they are so closely connected, so almost the same thing that the idea of quantizing one of them just, it separates them too much.

It separates them too much from the beginning to say this is this classic. theory and quantum mechanics, and we put them together, they're just too closely related. So you're saying in other words that other theories, you can start classically and then construct this quantum theory, but the right way to think about quantum gravity is just to be quantum from the start. Yes. And more than that, I think almost all quantum systems have features in them which are reminiscent of gravitational things, even though they may have very little to do with.

So we can find echoes of gravity and curved space time. That's right, of curved space time in ordinary simple quantum mechanical systems. Malda Sainz's construction was an example of this. Roughly speaking, he said if you could construct a sphere of material, a sphere of material, a shell, where the shell had certain properties that would allow it to be described by certain kinds of quantum mechanical field theories. And shells of matter are described by quantum field theories. That we can do.

Yeah. If we could construct just the right kind, then in every possible way, in the laboratory, in the laboratory, not in outer space, not the laboratory, if we could construct this shell and we could communicate with it by tapping on it, listening to what it does and so forth, that for all possible, purposes, we would discover that what was going on inside contained gravity, even though if we opened up the shell, we would find nothing in it. Very weird. I mean, how close is this to doable? Oh, I think it's getting there. We're not going to build such shells. That would not be the right

thing to do. We will build quantum, hopefully, we will build quantum computers that can quantum simulate those shells. Quantum simulating means that the actual quantum mechanics that's going on in the quantum computer is a direct reflection of the same quantum mechanics that went on in the shell or in Maldesanist constructions. We'll have these quantum computers. We will be able to interact with them by perturbing. You know, by how do you, we interact with computers.

Let me not explain how we interact with computers. We interact with them. We make measurements on them. We look at the screen, whatever. What will be going on inside those computers will be pretty much the same as what goes on in a black hole, in a quantum mechanical system that contains gravity. So I think I do believe that in the end we will be able to do a certain class of experiments

with quantum computers, whose results, if we took them literally, would say, you know, what's going on inside there? Gravity, black holes, all sorts of things. Even though if we opened up the quantum computer, all we would find would be circuits and... So what's the end point here? What the end point is, it may turn out that quantum gravity,

great theory of gravity, great theory of quantum mechanics, but it might eventually turn into a tool for quantum computation science. I think it's already going in that direction. It's already very much going in that direction. So by that we mean like building a better algorithm for a quantum computer to solve some problems? Building better algorithms. But the one thing that especially comes to mind is building a better error corrected. Error correction.

Error correction is a big deal in quantum computation. Let's just back up a bit because I think this is very valuable. What's so great? What is a quantum computer and what makes it so great? Why is it better than a classical computer? It's because quantum mechanics of a given number of qubits, what was a bit? A bit is a thing, is a switch, which is either on or off. A quantum bit, which is called a qubit, is a quantum mechanical version of that,

but it has more, it's a richer thing than a single bit. It can be in a superposition of states, not up, not down, but a up plus down and so forth. So a quantum bit is a more complicated thing. Now, quantum bits, a collection of quantum bits has an enormous capacity for what I call complexity, much more so than the corresponding classical. So the amount of information that you would necessarily have to provide to describe the quantum state of 100 cubits is vast. It's huge. But that also means that that 100 cubits can store an amount of information. Which is vastly bigger.

A qubit could be encoded in, you know, an electron that is either spinning clockwise or counterclockwise. So 100 electrons doesn't sound too hard to hang together and build into a machine. 400, 400 electrons, the amount of information that it would take to describe the state of 400 electrons is as big as the maximum amount of information that could ever be put into our entire observable universe. if it was packed as tight as possible. Classically. Right. So 400.

What's 400? 400 is, you know, there's nothing. 400 cubits in principle can store that much information. The question is, can you get out of the computer? And this is, you have to be very, very lucky to design an algorithm when not only can you store and answer questions involving that much information, but also be able to get it out of it. So people are doing it right now for a couple of qubits at a time, right?

In very simplified situations. But if we could make it dozens or hundreds, then a world opens up. A world opens up, and we're not exactly clear. We're really not exactly clear what that world is. We know that there are a handful of problems that we know that are much too hard for classical computers. The buzzword is that they're exponentially hard. problems which are exponentially hard, which cannot be solved by any reasonable classical computer,

that quantum computers can solve when the quantum computers are built. Hey, everyone, it's Cal Penn. I'm the host of Earsay, the Audible and I Heart audiobook club. This week on the podcast, I am sitting down with Ray Porter, the narrator of Andy Weir's audiobook Project Hail Mary, massive sci-fi adventure about survival and science. And what happens when you wake up alone very far from Earth? I really had to make a decision because I caught myself getting that frog in my throat and starting to get teary as I'm narrating some of these sections.

And it's like, okay, yo, yeah, yo, is this indulgent? And I really thought about it. I was like, no, at this point, it would kind of be betraying the trust the author and the listener have in telling this story if I don't go through it. But there's places in this book that deeply emotionally affected me, and I left it on the mic. That's great. Because it served the story. People will say like, oh, my God, I cried at the end. It's like, yeah, dude, me too.

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Learn more at brex.com slash AF. There's also another class of problems which are very interesting, which are simulating physical systems, quantum mechanical physical systems. We build into the quantum computer a set of rules, which are analogous to the rules which, let's say, govern a superconductor. And then we can examine in the computer

what happens when you do this, when you do that, you have a lot of control, things that would be too hard to actually do to the superconductor itself. So you get to explore changes, what happens if you change the computer, the superconductor a little bit this way, a little bit that way.

This is one of Richard Feynman's original motivation in pioneering quantum computing. Absolutely. So that's a world that may open. And there will probably be a wider class of problems that quantum computers can solve. But the problem with quantum computers is because they have so much capacity for complexity,

they're also very delicate. Little tiny errors that wouldn't have any significance at all for a classical bit. A classical bit is like a coin, which is either heads or tails. You put your coin on the table and you sneeze. What happens? Nothing. Nothing.

Pretty robust. Head stays heads. It's not susceptible to error because you sneezed. You sneeze on a quantum bit, no boy. You've entangled it now. It's all over. You've entangled it? You've made a mess out of it.

Okay. So that means that you have to be extra special in your protection. against errors. Errors are random flips of bits, random errors that classical bits are not terribly susceptible to, and quantum bits are very susceptible to. So that means you have to build in redundancy. You'll have to build in to your quantum computer redundancy and error correction, what's called error correction, so that if it happens, then an error is made, The computer by itself detects the error and corrects it. Okay, one of the really interesting things has happened

is that possibly one of the most interesting theoretical constructions about error correction that's happened in the last X years has come out of thinking about black holes. Remarkable fact. There's a dialogue going on, a very robust dialogue going on between this kind of gravitational holographic community

called the It from Cubit community. It from Cubit. Yeah, the It from Cubit community. Because John Wheeler suggested it from bit that reality emerged out of information. And you're saying, well, he was almost right. It's quantum information. Right. That's the gravity people.

That's the Maldesaners of the world. The witness, that's the thing I'm involved in and so forth. and at the same time, the serious hardcore quantum computer community, which is technological. They're building things, yeah, they're hardware. They're building things, right. Building things with technological purposes in mind. And the two communities are beginning, not only beginning,

they're for the last, I think, probably for about four or five years, five years now, I think there's been a very strong interaction between those two communities, with lessons learned from gravity, or quantum gravity, being transported into the computer science and lessons learned from the computer science transported. I'll give you an example.

We have these things in quantum gravity called wormholes that connect black holes, that can connect black holes if they're entangled the right way. I'm sure our listeners have seen science fiction movies We're holes in them. Yes, interstellar. Yeah, okay. Is it possible to send information into one black hole

and have it come out the other? We always thought the answer was no, that that would violate some basic principles, until a few of us started thinking about a phenomena that the quantum computer and the quantum communication community, that's called quantum teleportation. Quantum teleportation is a way of using entangled systems to send messages

which are entirely secure and which seem to go from one place to another without passing through the space in between. We now know that that's the same phenomenon as putting a thing into one black hole and having it appear through the wormhole coming out the other one. Okay, but just so that the people who are not experts

are grounded here, quantum teleportation we can see in the lab. You can do it in the lab. You can't see it happen. No, you don't see that. You don't see the information. But it's a real down-to-earth thing. Oh, yeah, no, no, no. Quantum teleportation is a real thing.

Because it sounds a little way out, but. It is an absolutely real thing. It has, it's hard to do. It's not easy to do, but it has been done. And it will be done more. And it's considered a technology for, you know, ultra-secure communication. But nothing actually disappears and then reappears, right? It's an information that is teleported, not like a dog.

Yeah. No, it's information that's teleported, but you put something in one end and it comes, and an equivalent thing comes out at the other end. Right. Right. That's called quantum teleportation. That's a real thing. And we now know that sending things through wormholes is this exact same mathematical phenomenon. You can't use it to send things faster than the speed of light. That doesn't happen.

Still true, yeah. It's still true. And you can't use it to make time machines. Oh, that's too bad. That's too bad. But you can use it for ultra-secure communication, which can't be detected by a use dropper. Okay. The principle can't be.

And so you're giving me the impression this is not just a bunch of enthusiastic theoretical physicists saying cool things that they think quantum computer people should care about. You're saying the quantum computer people actually do care about them. Yeah. And there's a lot of cross-talk. There's a lot of cross-talk. Look, I actually wrote a paper with a... Did we actually write the paper together? We did the work together.

Okay. With a computer scientist. Scott Aronson. Oh, yeah, with Scott Aronson. Scott Aronson. Future Mindscape podcast guest. He hasn't done it yet. That is something that if you would have told me 10 years ago,

I would write a paper with a computer scientist. I would have said, oh, you're crazy. I have no interest in that. But much more than that I wrote a paper with him, we talk a lot. not just Scott and myself, but Dorida Haranov, you know, a whole world who was devoted to the technology of quantum computation has now come together intellectually, into an intellectual community, which consists of these gravity, quantum mechanics people, the quantum information people, the quantum computation, quantum communication. And these are real technologists. So I have two questions.

I don't want to forget either one. The first one, let me put myself in a sort of skeptical position here and wonder about how abstract this all sounds. Like, here we are, we're in a room, there's chairs, there's tables, and you're saying that secretly it's all a two-dimensional screen of information. Is that like how secure, established, robust is that, or is this a bit of a cross our fingers, hope it's true kind of thing? Again, I'll say it again, the importance of string theory to this community and to this set of questions it has provided very, very exact, precise examples in which these things are true. Does that mean that they're true of the real world? Not necessarily.

Not necessarily. But it's plausible that it is, which is not... It's plausible that it is, but it sort of says, look, the mathematics of this is very, very secure. there's no question that this can happen. So it's definitely a useful tool. Right. The place where it happens, yes, it's definitely a useful tool for a number of things, but the place where it happens with precision is in these anti-de-sitter spaces.

What do we know about the world we really live in? It's de-sitter space. It's anti-de-sitters' nephew's space. And we don't know with the same confidence. We certainly don't know with the same confidence that, that the rules for the citter space are the same.

We don't. So is it possible that at the end of the day all this work ends up being really, really useful for people at Google who are designing algorithms for quantum computers

but doesn't help us understand the nature of space time? Do you want to tell your friends there that we're sitting in the Google office right now? For people out there in podcast land, we're actually doing its interview in an office at Google X. Why?

Because I consult for... For some reason, Google, Lex thinks that theoretical physicists are really important as senior consultants. So that's evidence for everything you've been saying, right? Right. But are computer scientists useful for theoretical physicists? Do you see the theory of everything emerging from the clouds here? Well, I don't know about the theory for everything. I've learned a new way to think.

In my old age, I'm not very good at it, the computer science way of thinking about things. but a lot of what I think about these days was ideas that were born out of computer science. The most important of which is called complexity theory, which turned out to have a direct application in the interiors of black holes. Complexity in what sense? How does a computer scientist think about complexity?

The complexity, you can think of a complexity of a thing or you can think of a complexity of a process. Of course, a thing is also a process. The thing is the process, the process of making it from something simple. Yep, fair enough. How do you assemble something? Yeah, the definition of computational complexity is the number of minimal simple steps that it takes to go from A to B. So if you're, let's talk about, I'll give you some examples of complexity.

My favorite was always, when I was young, I was interested in theorems. I don't prove theorems anymore. But it was interested in theorems. For someone else to do. I was very curious about the fact that some theorems are hard and some theorems are easy. What's the difference? I mean, they're either true or they're false.

What's this hard or easy? What does it mean? My teacher, my high school teacher, told me that the four-color map theorem is thought to be very, very hard. I said, what do you mean hard? It's either true or it's not true. How do you make sense

to have this idea of hard or easy? And I didn't know. Then I later learned out that there were theorems that were so hard that they were true, but they were so hard that they couldn't be proved, period. That was Mr. Gerdell's invention. And I got very curious, what does it mean to say that a theorem is harder than another theorem?

It turns out what it means is that it's more complex. What does more complex mean that the minimal number of steps to be able to prove it, not the number of steps that some mathematician might have used, but the absolute minimal number of steps, logical operations, starting with the postulates, and using logical operations and or and so forth,

that the complexity of a theorem is what the minimal number of steps to prove that theorem is. And it's a hard number. Given a theorem, you can say what the complexity of it is. Hard theorems are more complex than easy theorems. And this is a profound statement because a complex theorem in this sense might be very simple to state. Yes, indeed.

The four-color theorem is very simple to state, but very hard to prove, so that's a different notion of complexity. And the proofs of it involve a book's worth of equations one after another, plus a computer computation. Okay, yeah. So the number of minimal steps to prove the four-color map theorem is thought to be very large compared to proving X square plus Y squared equals Z square, what's that called, Pythagoras. Pythagoras is the right, yeah. And somehow this illuminates what's going on in the interior black holes.

Okay, so in quantum computation, You can say, supposing I want to do a certain calculation, what's the minimum number of basic unit quantum steps that it would take my quantum computer to carry out the calculation? Minimum, not the number that you designed. You may have used a lousy algorithm. God's number. Right. What's the absolute minimum? And that's called the complexity of the calculation.

A black hole evolving is carrying out a calculation. What does that mean? It just means that it's running. It's evolving. It's evolving. Quantum information is evolving. That's right. Quantum information is evolving.

And one of the very exciting things that for me was to discover that the computational complexity of the evolution of the black hole had a direct hole. meaning in terms of the volume of the interior of the black hole. That was very exciting to me. Why, I've always loved the ideas of complexity theory. I never thought they would come into physics. I've been fascinated by black holes. They've been especially interested in their interiors.

And to suddenly discover that the growth of the interior of a black hole is a case of a growing computational complexity was very exciting. is it having, it is again part of this dialogue between the computer scientists and the physicists doing these kind of gravitational black hole kind of things. And yeah, I expect that that will have some impact. Yeah, okay. This is all like cutting edge new stuff, right? It's very cutting edge.

It hasn't shaken out yet. And having these two communities, you know, some great thought may come out, some great consequence may come out of this interaction. And it may be impossible for anybody to ever really figure out exactly what the root to the great thought was. Was it what he said? Was it what she said? Was it what the computer scientist said? Was it getting communities like this together in a situation where they really do have common interest and enough common tools to be able to honestly interact?

I think has been just very, very exciting. And it was part of the thrill of theoretical physics, right? That we attack problems from different angles, and when they come together, we know something smells right. Yeah, that's right. And the other thing which makes you think you're on the right track is when something that you've been thinking about turns out the same mathematics or the same set of principles turn up in some other area and turn out to be useful in that other area. you know, really, really good stuff usually penetrates into several different directions, many directions, not just the directions that it was intended for.

And when that happens, you know that there's something good about what you're doing. So that is happening. It's also entering into what's called condensed matter physics. Condensed matter physics is the physics of materials. Some parts of condensed matter physics are now very heavily using the mathematics of black holes. Why, black holes are a kind of material. The horizons do things.

They have viscosity. They have electrical conductivity. The horizon of a black hole wobbles. It does stuff. And again, it's turned out that the mathematics of the surface of black holes turns out to be very similar to the mathematics of fluids, the mathematics of superconductors, mathematics of other things,

and that mathematics is being transformed back and forth between the two communities. So one of the biggest and hottest items in the gravitational subject is something called the SYK model. Okay. The SYK model was first invented by a condensed matter physicist by the name of Sashdev. S?

S. and Kiteye, who's the KEE, is also as a computer, quantum computer scientist. Like I'll take comedy. They invented this model, or at least Sashdev invented this model, to describe a certain class of materials which are called, what are they called, lousy conductors? Yeah, crummy conductors. I don't know. Yeah.

Something like that. Things that don't, not superconductors. Right. No, not superconductors. Right. The opposite. Right. And he invented.

to this model for that purpose. It turned out that Kataev looked at it, who was not a gravitational person at all, and realized from the little bit that he knew about black holes is that this model was describing something like a black hole. And now all the black hole physicists, this is the hottest item now, S-YK theory. I mean, it helps to be as brilliant as Alexei Kittaya

just to look at this and go, oh, that looks like a black hole, right? Right, right, right, but yes, that's right, but think about it. It was a material science guy, a condensed matter physicist, and a quantum computer guy who found the opening, and I think it's a serious opening, for new ideas about black holes. That is amazing. It's a wonderful synergy going on.

Interesting question. How long will it last? These kind of ideas and these kind of synergies that happen, they have a finite lifetime. They don't last forever. I think this one has probably at least a good 10 or 15 years left in it. Good. Good advice for the young people out there,

the middle-aged people out there. It will be part of the textbook subject. Of course, the subject will move on into other things, and this one will be the domain of older. Yep. We'll see. Let me bring it back to sort of wrap things up here. Let's bring it back to where we started in string theory,

because there's another obvious set of questions about gravity and space time, which are the cosmological questions, right? Universe and so forth. And you alluded, you alluded, you know, cagely to the idea that we should take seriously things that we can't even in principle observe if they're predicted by our theories. One example of that is the cosmological multiverse, which seems to be part of the string theory story. Do you still think that's true? I think so. I even wrote a book about it.

Yeah, the cosmic landscape. We'll plug that book to it. Yeah. You know, let me put it this way. I don't think anybody has a better idea for resolving some of the great. puzzles of cosmology, the great theoretical puzzles of cosmology, in particular the very, very strange, what are called fine tunings that we seem to see in nature, that parameters are extremely finely adjusted and we don't know why. There's, of course, a lot of almost anger at this

idea of a multiverse and so forth. We both felt it, yes? No, no, we both thought that. My answer is always, yeah, what do you have that's better? And the answer is never anybody has anything better. So it's the best idea we have right now for understanding why the parameters of nature are what they are. We don't have a better idea. And that's about all I would say about it with real conviction. Could it turn out to be wrong?

I suppose so, but I don't see how. But to say that I think it could turn out to be wrong is to say, say that I see some other possibility. And so just to be clear, the it in this case is the idea that there are regions of space very far away where the local laws of physics look different. That's the multiverse we're thinking about.

So it's a package. The idea that the universe is extreme, some parts of it we know with certainty. We know the universe is much bigger than the part we can see. How could we know that it's much bigger than the part we can see? The same way that we know

that the Earth is much bigger than the portion that I can personally see. We know it's much bigger not because we've been around the world, but because we see it's very flat. If we go out in a field, we see it's very flat. We know that it's probably going on

for a long distance because it's so flat. It probably doesn't terminate at the place where we see the horizon. That would be weird, yes. And the universe is the same way. The universe is the same way. It's very flat. It just seems to go on and on,

and there's no reason to think it stops at what the horizon is. So how much bigger? At least a thousand times bigger in volume. So that means there's a potential for lots of stuff out there that might be somewhat unrecognizable to us. More likely, it's millions and billions and billions and billions and billions of times bigger. So to say that the only things that are possible, the only kinds of environments, the only kind of behaviors that are possible are the kind that we see in our little tiny patch

is presumptuous. It's entirely possible that the universe is full of different kinds of patches with different kinds of behaviors. Just like the earth is full of different kinds of patches, Arctic, jungle, desert, that there could be a great deal going on out there. And so there may be very many different environments and only a small part of which are habitable.

Which are the parts that are inhabited? The parts that are inhabited are parts that are habitable. What does it take to be habitable? well, possibly some very special numbers. Numbers for the fundamental constants of nature parameters. Right. So it's not that everywhere in the whole universe is all the same and that we could live anywhere in it,

at least according to this idea. There are just a small, tiny fraction of the universe, which is of the type that is habitable. But where are we? We're in the place which is habitable. It's as dumb as that. It doesn't take a head. heavy philosopher to understand this concept.

And string theory helps us by sort of allowing for all these different laws of systems. String theory has many, many different solutions. Solution means a possible kind of world with different parameters, with different numbers. And so you take these things, the fact that we know the universe is much bigger than it is, than we can see, the fact that the parameters seem to be highly fine-tuned, and the fact that string theory gives rise to an enormous diversity of different possibilities, those are the three things that go into this idea. Is it a open and shut game? In other words, is it done? As the fat lady sung yet? No, I don't think so. But my usual answer to the critics of it is,

you've got anything better, buddy? And the answer is almost always no. It seems to me, maybe I think it back. The answer is always no. Always no. So far anyway, yes. Yeah. These wonderful ideas we've been discussing about qubits and quantum information in the emergence of space time, it seems to me that they haven't yet attempted to address mostly cosmological questions. What happens at the big thing? That's absolutely correct. Is this an open area? You think it's going to have a lot of interesting things?

Yes. I think it's an open area. And I won't try to predict what the next area will be that will be exciting because you're usually going to be wrong. Something will come up that you didn't expect. But, you know, just in the natural course of events, if science and the physics is going to answer these questions, it's got to address that question. How does, how do these ideas, bits, qubits, strings,

and all that kind of mathematics, holographic principle, ADS-CFT, or something different, how will they address the cosmological questions? And I think that's unknown. Right. It's unknown. Although there are plenty of people who have some ideas and the ideas may be good, I think they certainly haven't reached consensus.

Okay. And so for my really final question then, in addition to helping to reinvent the nature of space and time and to spending time here at Google X with the quantum computing folks, you've also written several books, right? Not only straightforwardly popular books, but the theoretical minimum series. And there's video lectures. I mean, what is the way that you think about your activities in that realm? Like, how important is that to you?

That's an interesting question. I did it for two reasons. One of them was a sort of emotional response to my father, of all things. My father was a very smart man. He was a plumber. He had a fifth grade education. He had a bunch of friends. They were all plumbers, rough characters, but they were all intellectuals.

This was, you know, a long time ago. Was part of New York? Was it? The Bronx. The Bronx. Okay. They'd sit around the table. I would say I was a little kid.

I would sit around with them. They'd talk about everything. They would talk about history. They would talk about science, all kinds of stuff. And it was a very strange mix of intellectualism and crackpotism. Why crackpotism? It wasn't that they were intrinsically crackpotty, in particular when it came to science.

It was that they had no access to be able to know what was real from what was a bit of crackpot science. And when I started to get really interested in science, I started to try to teach my father. It worked. I mean, you thought of things. But I always had this emotional sense that there were all these guys like my father out there. I would like to be able to teach them what real science is.

so they know the difference. The people I met when I got older that reminded me of a little bit, or they were actually scientifically pretty literate people, a lot of the older people that I met in the community in Palo Alto. These were people from everything from computer programmers to doctors to things like that, and they were very, very interested in science.

They were very interested in physics. and generally older. And at some point, I decided to try to teach in Stanford's Continuing Studies Program. The idea was to teach one quarter and to give them a sort of scientific American background into some of the modern physics. They loved it.

It was extremely popular, the one quarter, and so it went to the second quarter, and then a third quarter. But it was intended to be at the level of, let's call it, scientific American. they came to me at some point and they said enough with the scientific American stuff teach us physics, teach us the real things, show us the equations to do it right. And so I did.

I think I spent 10 or 12 or 13 years, I don't know how long, over and over teaching courses for these people. They weren't courses in the sense that anybody took an exam. They were just, you know, me lecturing. As a teacher, that's the fun part. Good. You figured it out. I did. I had all the fun part.

And at some point, I decided to take the lecture notes and write them up into books. So that was the origin of the theoretical minimum. Are we done? Is there another volume coming out? There will be sometime. Okay. Very good. Well, I completely agree.

I think you're right. I think there's an enormous audience if we do it well to explain science at a pretty good level. Yeah. Yeah. I think it's important to explain an honest level. An honest level allows you to use things like metaphor. and analogies, but you must explain what is a metaphor, what is an analogy, and you must also explain why it's deficient because they almost always are.

Yeah. So. All right. Well, you've done a fantastic job at it. Thank you. Thank you, Sean. What if you could have even more and more and more help to pursue your goals? At LPL Financial, we offer more ways for advisors and their clients to thrive.

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