The Equation That Changed How Physicists Think About Reality | Juan Maldacena - #554

Juan Maldacena is a theoretical physicist at the Institute for Advanced Study whose 1997 paper remains the most cited in the history of theoretical physics. We cover: -why wormholes and quantum entanglement may be the same thing -what actually happens to information when you throw something into a black hole -the reason Hawking radiation accidentally gave cosmologists the equation that explains why the universe has structure -whether science-fiction wormholes are ruled out by the laws of physics -the one unsolved problem Juan says matters more than black holes. The most important problem in quantum gravity is understanding the beginning of the Big Bang — not black holes. TIMESTAMPS 00:00 What If Einstein's Two Strangest Ideas Were One? 01:15 Juan Maldacena: The Most Cited Physicist Alive 03:25 What Would Einstein Most Want to Know Today? 07:45 The Holographic Principle Explained 09:20 What Happens When You Throw a Laptop Into a Black Hole? 11:00 Is Information Actually Lost Forever? 12:25 The Problem Juan Wants to Solve Before He Dies 13:50 Why Real Black Holes Don't Emit Hawking Radiation 15:25 How Black Hole Physics Accidentally Explained the Universe 17:00 Could Primordial Black Holes Be Dark Matter? 18:30 Real Observers Solving Imaginary Problems 21:15 Why Imaginary Numbers Keep Being Right 25:00 The Origin Story of AdS/CFT 27:05 Do We Actually Live in AdS Space? 29:00 Are Wormholes Real or Just Science Fiction? 32:10 Could AI Have Helped Einstein? 33:00 Can Science and Religion Coexist? ——— 📬 Get the transcript, fascinating bonus content, and my Monday M. A. G. I. C. Message: https://briankeating.com/yt 🌠 Have a .edu email and live in the USA 🇺🇸? You automatically win a meteorite: https://BrianKeating.com/edu 🔔 Subscribe: https://www.youtube.com/DrBrianKeating?sub_confirmation=1 🎯 Support Into the Impossible on Patreon — get my weekly M. A. G. I. C. Message, unfiltered bonus content, and live monthly Office Hours with me: https://www.patreon.com/drbriankeating ⭐ Join this channel for perks, monthly Office Hours, and your name in the Member Roster at the end of every episode: https://www.youtube.com/channel/UCmXH_moPhfkqCk6S3b9RWuw/join 📚 My books: Losing the Nobel Prize (memoir): http://amzn.to/2sa5UpA Think Like a Nobel Prize Winner: https://a.co/d/03ezQFu Focus Like a Nobel Prize Winner: https://a.co/d/hi50U9U Galileo's Dialogue (first-ever audiobook): https://a.co/d/iZPi9Un 🌐 More: 🏄‍♂️ Twitter: https://twitter.com/DrBrianKeating ✍️ Blog: https://briankeating.com/blog 🎙️ Audio-only: https://briankeating.com/podcast #intotheimpossible #briankeating #science #physics #astronomy #cosmology #podcast #universe Learn more about your ad choices. Visit megaphone.fm/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Speaker 00

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• 0:57Speaker 02

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• 1:22Speaker 03

  What if Einstein's two strangest ideas, wormholes and quantum entanglement, were the same idea? My guest today spent his career proving that they are.

• 1:31Speaker 04

  So-called Einstein Rosen paper on the fact that the full Rashid solution contains two black holes that are connected and the Einstein-Podolsky Rosen paper that talks about entanglement. And we now think that these two things are related.

• 1:43Speaker 03

  My guest is Juan Melde Senna, the physicist who in 1997 wrote the most cited paper in theoretical physics. The claim he just made that wormholes and entanglement are the same thing is called ER equals EPR. If he's right, the structure of spacetime is built out of quantum information itself.

• 2:00Speaker 04

  The information of the things you threw in is contained in this radiation. According to general relativity, we'll look like the information is lost. According to quantum mechanics, we would expect it to be preserved. There is a conflict between the two things. Quantum matter didn't obey this property, then you would be allowed to sense it as what in the speed of light. I think this is a beautiful consistency condition between the two theories.

• 2:19Speaker 03

  He also told me which problem in physics he'd most like to solve before he dies. The answer was not what I expected.

• 2:25Speaker 04

  The most important problem in quantum gravity is to understand the beginning of the big bang. That's really the problem that I would like most strongly to solve.

• 2:32Brian Keating?

  Juan Alasena, welcome to UC San Diego for your second appearance on the podcast.

• 2:35Speaker 04

  Yeah, thank you, Brian. It's a pleasure to be here.

• 2:37Brian Keating?

  You're here giving the Dashin lecture all the way from the Institute for Advanced Study, which I think is on Einstein Lane. Is that correct?

• 2:44Speaker 04

  Yeah.

• 2:45Brian Keating?

  Address is, I'm not doxing you, right? To say you're on one Einstein Lane. Here's Einstein over here. What do you think he'd be, you know, kind of most interested to learn, or if you could have 10 minutes alone with him, what would you tell him about?

• 2:55Speaker 04

  Well, I think black holes would be probably something he would be really interested in. I would particularly want to tell him, want to ask him whether he thought that his two papers from 1935 would be related, so-called Einstein Rosen paper and the fact that the full Rashid solution contains two black holes that are connected, and Einstein Podolsky Rosen paper that talks about entanglement. And we now think that these two things are related.

• 3:23Brian Keating?

  This E R equals E P R, right? That's one of the things you're known for, many, many things you're known for.

• 3:28Speaker 04

  One surprising thing would be that they, that they are a consequence of gravitational collapse and that are naturally produced in the universe. Now, in the last few years, really in the last few years, we had lots of experimental evidence for black holes, right? From collisions that produce gravity waves to imaging the matter near the black hole near this, of the black hole that is near the center of the Milky Way, to, you know, looking at stars that orbit this black hole. Yeah, so we have lots of evidence for these black holes now. Then the other surprise, I think, would be black hole thermodynamics. I think that would be something really interesting in the sense that there's a connection between the laws of thermodynamics and black holes, that black holes have an entropy, they have a temperature.

• 4:13Speaker 04

  I think that would be a lot of fun for him.

• 4:16Brian Keating?

  I mean, gravitational waves, another thing he predicted that he thought would never be observed, and I think he got a paper rejected, and then he said, I don't want to deal with a referee, and another thing that he did.

• 4:26Speaker 04

  Well, he first predicted gravity waves, then he thought maybe they don't exist, and then the referee said that, no, they do exist, you made a mistake here, and then...

• 4:35Brian Keating?

  That's what I say when people say peer review is bad.

• 4:38Speaker 04

  Yeah, well, I mean, this case was a good example of useful. Well, I guess you got a good reviewer.

• 4:46Brian Keating?

  That's right. Yeah, that led to multiple Nobel Prizes at Hals and Taylor and then LIGO and who knows what else it'll do. But yeah, I always tell my students aspire so that your blunders or things you don't think will ever work will lead to multiple Nobel Prizes.

• 4:59Speaker 04

  Yeah, yeah, and the cosmological constant, that was his biggest blunder. Now, yeah, now it's a central part of cosmology.

• 5:07Brian Keating?

  I want to talk today about the realities of black holes and of things like the holographic principle, which is among, again, many things you're known for in your amazing career. I was talking to a non-scientist, but a very smart layperson. And he was asking me, well, you know, if the holographic principle is correct, you know, some people say, well, we might be living inside of a black hole and things like that. But I always point out, you know, there's no such thing as an isolated hydrogen atom floating around the universe that truly can be solved by the Schrodinger equation. In other words, there's always perturbation. To my knowledge, there's no such thing as a Schwarzschild black hole either, right? That's perfect. There's a curve black holes. We know of the ergosphere surrounding them.

• 5:47Brian Keating?

  So in what sense is the holographic principle of the fact or proposition that we could be living inside of? Is that just pure theoretical because of the realities of real black holes?

• 5:57Speaker 04

  The holographic principle as applied to our universe, we don't know whether it's correct or not.

• 6:01Brian Keating?

  Could you explain the holographic principle first?

• 6:03Speaker 04

  The holographic principle is the idea that you can describe quantum gravity in some region of the universe by some theory of ordinary quantum mechanics that lives in the boundary of that region. It remains a big idea formulated this way. Now, in some special cases, some special universes, so universes which are infinitely big and so on, then we can go to a surface that is very, very far away and define there a very concrete theory that whose laws of physics we can define. And in that case, they are supposed to describe the interior of those universes. Those universes are not the universe we live in. They have slightly different, well, they have different laws of physics. They have a different value for the cosmological constant. But In those universes, there is a lot of evidence that this relationship is true.

• 6:48Speaker 04

  Now, there in those universes, you can consider black holes that are inside this universe. The black holes can have perturbation, matter around. And the idea is that those would be described by the theory that lives on the boundary. And there are some comparisons we can make. One, let's say, catch, or one thing that makes it hard is that the theory that lives on the boundary involves strongly interacting particles. And so it's not completely obvious how to solve this theory. So you have to apply some techniques. There are some things you can calculate, but not everything you would like to calculate. So that's in order to compare the two things. And we are learning more on how the dictionary gets built between this quantum description on the boundary and the gravity description in the interior.

• 7:33Brian Keating?

  When you say "lives on the boundary," what does that mean? Is that like a separate Hilbert space?

• 7:37Speaker 04

  "Lives in the boundary" means that these are particles that move on a space which is, has the geometry of the boundary. It doesn't have the extra dimension. And the idea is that you can think in two alternative ways. Either you have particles that live on that boundary, or you have the gravity description that lives in the interior. And the idea is that these particles are strongly interacting, and the gravity description is some kind of emergent property. It's not something that was there in the very beginning in the formulation of the theory, but it looks like it's an approximation to the underlying dynamics.

• 8:12Brian Keating?

  Does that gravitational theory, does that produce GR or something different?

• 8:18Speaker 04

  So the idea is that when these particles are strongly interacting, and in some special cases that we understand, then would produce general relativity. In fact, in the examples we understand, it produces general relativity plus string theory also at short distance. So there is some approximation where it's just general relativity with some particular matter content. And then also strings and stuff like that. Those are in the cases we we understand. We don't know whether string theory is necessary for this discussion, or whether this is valid more generally, or maybe string theory is the only way to quantize gravity. Those questions, we can remain agnostic.

• 8:54Brian Keating?

  Will it produce, you know, excitations and things like the fermions?

• 8:58Speaker 04

  Yeah, you can have fermions, you can have all that.

• 9:01Brian Keating?

  When you said strongly interacting, does that mean like the strong force? Or does this mean like the short range interaction?

• 9:06Speaker 04

  By strong interaction, I mean that the coupling between the particles is very strong. So that if you have two particles that collide, they will scatter very, very easily. The strong interactions are called strong because precisely the interactions are strong at the level of, let's say, inside a proton and so on. And in addition, the type of particles that we have also have interactions similar to the strong interactions. The so-called gauge theory, it's a type of interactions that involves the property, let's say, called color, which is a type of charge, but of which the sign is not just plus minus, but there are like three different types of charges. In nature, there are three different types. In these theories, we consider there is a large number of types.

• 9:51Speaker 04

  Yeah, so there are theories somewhat similar to the theories we have in nature, but not exactly the theories we have in nature. What we have are some examples of this involving this. Let's say, they are just theories and models. You could say it's a model of quantum gravity. And one of the advantages of this description and the reason that it was developed was that it could give a full quantum description of the gravitational spacetime. So we don't just get general relativity, but the quantum version of general relativity. And we hope that by having these models, we will understand the quantum gravity more. And then eventually, of course, the objective is in the end to understand quantum gravity in our own real world. So somehow to extract.

• 10:33Speaker 04

  Lessons from this to be able to apply them to our real world.

• 10:37Brian Keating?

  You know, just at a basic layperson level, you know, I'm not gonna do this, but you know, take your laptop, you're gonna be speaking later, throw it into a black hole. What happens and does it depend on what type of black hole it is?

• 10:48Speaker 04

  If you throw anything into a black hole, well, your laptop and so on, it will, it will fall and you will lose sight of it. So the time it takes light for going a distance of order the size of the black hole, all the information about that laptop is effectively lost to you. So in the sense that you will not see it anymore and any perturbation you had of the metric that was due to the fact that there was a laptop will be lost. So the influences decrease exponentially fast. Okay, this is fine. This is what happens with classical black holes. But as we were saying before, black holes have some entropy. And entropy in physics, we interpret it as arising from statistics. And it's a measure of how many states the black hole can have,

• 11:32Speaker 04

  how many, if you wish, bytes can be stored in this, or qubits can be stored in this black hole.

• 11:38Brian Keating?

  On the surface or on the volume?

• 11:40Speaker 04

  Well, the formula for the entropy is just the surface. So then you might be tempted to say it's in the surface. But in the classical solution, the matter falls in and goes into the black hole. So you, you could be free to say it's in the interior. What that somehow suggests, this picture that the black holes have a finite amount of entropy, is that that information is not completely lost somehow. In fact, when you throw in the computer into the black hole, the area, the mass of the black hole grows a little bit and the area grows a little bit. And the entropy becomes larger. It becomes larger by an amount which is bigger than the, than the entropy that was, than the amount of information that was in the, in your laptop. And you can use the laws of physics to, to show that this is always the case.

• 12:23Speaker 04

  Whenever you send something into the black hole, the, the entropy always increases. The question is, is this lost forever or not? And in principle, you could say it's lost forever. You may think because the, you know, it goes into the black hole and then, well, it will never come out, according to classical physics. But the new aspect is that this thermal effects, in particular Hawking radiation, implies that the black hole will emit something, emit some radiation that in the first approximation is thermal and carries no information. But it's saying that the black hole will start losing mass, right? So it will get smaller. And eventually, the black hole might perhaps disappear completely and we can get some radiation. And you could wonder where the information of the things you've thrown in

• 13:08Speaker 04

  is contained in this radiation. If it is contained, it will be contained in a very subtle way. But the question is whether, in principle, it's contained. The reason we're asking this question is not because we are desperate to find this information, but we are a little bit desperate. But the reason we are desperate is just that because it's a problem that will force us to understand quantum mechanics and gravity together and how things work. Because according to general relativity, we'll look like the information is lost. And according to quantum mechanics, we would expect it to be preserved. And so there is a conflict between the two things. And we hope that by solving this conflict, we will learn better quantum gravity. The most important problem of quantum gravity is not the black hole information problem.

• 13:47Brian Keating?

  No.

• 13:48Speaker 04

  The most important problem in quantum gravity is to understand the beginning of the Big Bang. So understand.

• 13:53Speaker 00

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• 14:23Speaker 04

  What happened in the very beginning? That's really the problem that I would like most strongly to solve, right? But the black hole information problem has the advantage of being a more concrete problem and that we have some tools to address it. So that's why there is effort and progress in this problem.

• 14:41Brian Keating?

  And getting back to my question about real black holes that aren't static, that have charge, that spin. Is that true? Is it also true that, you know, you get the exact same Hawking radiation? Or if not, in a curve, maximal curve black hole? So we should say what that is. But in a black hole with an ergosphere, like interstellar, you know, think about gargantua, real black holes, do they?

• 15:02Speaker 04

  Have the same phenomena? The question is whether real black holes are emitting Hawking radiation. The problem is that the temperature for real black holes that we've known, we know they exist, they have masses of order solar mass or higher. Those black holes have a temperature which is very small, many orders of magnitude smaller than the temperature of the cosmic microwave background. So even if the black hole didn't have any matter swirling around, which they do, so, and that matter is at even higher temperatures, even then, even just the cosmic microwave background would be swamped in the Hawking radiation in the sense that the cosmic microwave background would fall into the black hole and the Hawking radiation would be a tiny effect. So the answer is no. For the, for the big black holes, Hawking radiation is an irrelevant phenomenon

• 15:47Speaker 04

  and it, of course, hasn't been observed and there is little, well, it's probably not going to be observed anytime in the foreseeable future for astrophysical black holes. This would make you think, why, why people think about Hawking radiation if it is such an irrelevant thing? But I would like to point out, to point something out, which is that this phenomenon of Hawking radiation is inspired the theoretical development of, or discovery of some other phenomenon, which is the generation of fluctuations in an expanding cosmology. So in a black hole, there is a horizon, or there is a vision you can't observe and can access, and that's somehow ultimately responsible for this thermal effects.

• 16:30Speaker 04

  If you live in a universe that is expanding fairly rapidly, like as we think it was during the early epochs of inflation, then you expect a similar thermal effect and those, that, that temperature and the associated phenomenon will change the properties of the inflaton and will produce fluctuations in the shape of the inflaton. And we think that that's the leading theory for the generation of the primordial fluctuations, so the fluctuations that make the, the universe not perfectly uniform. So the universe is somewhat uniform at large scales, but not perfectly uniform. Well, as you know very well, you've been studying this. Inhomogeneities during your whole career and made wonderful discoveries. But it's ultimately a similar.

• 17:14Speaker 04

  We think they also arose from quantum fluctuations and it's the same phenomenon as Hawking radiation. So in this case, learning something for black holes. So Hawking's paper was earlier than the papers that discussed this phenomenon in inflation. Helped us understand something about cosmology that now forms part of more or less standard cosmology, I would say. And we similarly hope that understanding these other aspects of black holes will help understanding, you know, earlier epochs of cosmology. Right. So in some sense, the idea that phenomena discovered from black holes could be helpful for cosmology has already happened. And we hope to repeat this. That's our hope.

• 17:54Speaker 03

  Hold on to that because what Wang just said about black holes accidentally gave cosmologists the equation and explains what the universe has structure at all. That's on a small footnote and that's where I come in.

• 18:06Brian Keating?

  We've only discovered black holes with much more large masses than the sun. And yet the ones that are most likely to produce observable Hawking radiation are the small ones. And that kind of always meant to me, you know, for people that conjecture that say primordial black holes could be dark matter or could have truly existed since the dawn of time, basically. That's sort of is hard to reconcile. So what do you make of attempts to solve the, you know, missing matter problem? And even recently solved some dark energy phenomena using black holes, basically, which may or may not be primordial.

• 18:42Speaker 04

  From the particle physics point of view and from the model building point of view, they are not the most. I would say they are not the most natural thing or not the simplest thing you could think about for dark matter. So there are maybe other particle physics ideas that might seem more likely, but well, we'll see. I mean, maybe, maybe they are. And of course, if dark matter is black holes in the range where they are allowed, then Hawking radiation would be relevant. So, I mean, would be. President and would be, you know, bigger than the temperature would be higher than the CMB temperature.

• 19:15Brian Keating?

  You are known and kind of remarkable to me because you study things, you know, at the forefront of theoretical physics, but you also aren't afraid to take on philosophical, you know, kind of discussions. And one of the, you know, papers I think I read from 2024 is called Real Observers Solving Imaginary Problems paper. What is that? What was the purpose of that paper? And I want to talk later about your Beauty and the Beast paper, you know, such great titles.

• 19:41Speaker 04

  That paper had to do with computations in the Citter space, more precisely. It is sometimes useful to consider the Euclidean version of some space times. Euclidean version is basically you take the, you know, the usual universe and you make the time, you change the sign in the metric in the time direction. That makes a space which is purely spatial. And in the case of an expanding the Citter universe, that is a sphere. So you can consider Einstein gravity on a sphere. We would expect that that type of universe to be computing the thermodynamics of the Citter space. The reason is the following, that evolution in imaginary time

• 20:23Speaker 04

  or this procedure I've just mentioned is useful because if you solve that evolution, you're basically calculating the thermal partition function or you're calculating some thermodynamic properties of the system. This is something that is true for any physical system. And if you do that for the Citter space, you would expect that it should be telling you about the thermodynamics of the Citter. Now, this is not a new idea. This idea well goes back to Gibbons and Hawking.

• 20:51Brian Keating?

  Hawking, yeah.

• 20:52Speaker 04

  Yeah. And if you do that, then you get that the Citter space has some entropy, which is the area of the horizon, so formula very similar to the black hole entropy formula. In fact, that paper was the same time as they discussed also the same thing for black holes. Now, so all of this is perfectly nice and so on. But if you calculate the first quantum correction, so calculate not just the Einstein action for the sphere, but also the Quantum fluctuations, including the quantum fluctuations. The quantum fluctuations, they would give a negative value for the partition function. So the number of states would be negative. And depending on the dimensions, in some cases, it's imaginary. It's some i to the power of the number of dimensions of spacetime.

• 21:36Speaker 04

  So this was something confusing that was found by. But I think Hawking already noticed that there were some issues with some sign. Polchinsky calculated more precisely what the sign is. More recently, with trying to understand better the physics of the Citter space, it was understood that in order to construct the Hilbert space, it was useful to include an observer. So that you include an observer and the degrees of freedom of the observer were important. Some of the degrees of freedom to define the Hilbert space. And so what that paper did was notice that if you don't consider just the sphere, but the sphere with the trajectory of a particle, then there are some other minus signs from the trajectory of these particles or some other i's that cancel the.

• 22:21Speaker 04

  And then you get something nice and positive. Well, actually, in the paper, I originally got something positive. Then Victor, a student of mine, pointed out a mistake. Then I got something negative. And then eventually, a group from Stanford with Douglas Stanford and collaborators, they found another mistake and so now it's positive.

• 22:42Brian Keating?

  Triple negative.

• 22:44Speaker 04

  Yeah, triple negative. Well, that's how many things work in science.

• 22:48Brian Keating?

  I remember reading A Brief History of Time. I started reading it in high school. I couldn't finish it until I, in fact, I didn't finish it until about five years ago. But it was a good thing I didn't because I don't think I could have understood kind of what he was doing in that book until much, much later. But one of the things when he brings up this, you know, kind of what's called a Wick rotation, right? He brings it up and he says, well, imagine we're just going to build this as a trick. You know, we're just going to do a trick. We're going to introduce imaginary time, you know, the number square root of negative one in front of the time component. And when we do that, it's called a Wick rotation. And then we can solve all these things as if it's taking place in Euclidean space. So it's, but don't worry, dear reader, it's just a simple. And then the rest of the book is just basically assuming that's true. And then he goes on to say, you know, and then we'll have the mind of God. What do you make of this?

• 23:33Brian Keating?

  I mean, what is the reality of it? I guess I'm asking Wigner's question, you know, why is math so? Useful, like, one of the things that always blows my mind, and I try to impress it on my students is, you know, in classical mechanics, we have Lagrangians, we have Poisson brackets, you can do all sorts of things. If you take a Poisson bracket and commutation bracket, you get, you know, the product of these things, and they cancel out. The Poisson bracket for classical observers is zero. But if you, if you say it's quantum mechanical, you do the commutation relation, you get the square root of negative one, and all of a sudden, all of quantum mechanics can emerge from it. It's sort of bizarre, right? What level are these things tricks? I mean, when you see the imaginary number and you talk about in this paper,

• 24:12Brian Keating?

  is it real? Maxwell's fields have imaginary solutions too. They're not real, but we can observe only real things. So where, where does a person go with this?

• 24:20Speaker 04

  I like a story that the, apparently Lorentz, so that's the same person of the Lorentz transformations. He was tasked with calculating how water gets into the various canals and how to design some dams and so on. So he, with some people, they wrote a report and how this should be calculated. And in the beginning of this report, he says, well, we're going to use complex numbers. But it's just a trick and at the very end, all the heights of the water and so on are going to be real. Don't worry about it. And I guess at the time, it was thought it would be necessary to explain this point. Now, any engineering student uses complex numbers to solve this, this type of problems with oscillations and so on.

• 25:05Speaker 04

  And yeah, well, it's a trick, but it's a trick that simplifies. In that case, it's a trick that simplifies the, the, the calculation. And in this case, maybe similar. So everything we measure, we always measure real numbers. And so the imaginary numbers, that's how they were invented for discussing the roots of polynomials and so on. But they, they are useful tricks and I, yeah. But it's true that it's a trick that is used so often and so much that it seems that there is something deep about it.

• 25:34Brian Keating?

  When we think about all the other mathematical structures, so you start off with the square root of negative one, you get quantum mechanics, you get all sorts of interesting phenomena. Then you have spin one half particles can be described by these SU2 matrices, two by two matrices that are complex. And then later you could have SU3, you could have quaternions. And then I think there are octonions, but then nothing like people obviously could keep going, right? All the powers of two. But does anything correspond to whatever hexa dec- has a hexadecimal team?

• 26:05Speaker 04

  Well, the problem is the complex numbers have many of the properties of ordinary numbers. And once you start going to these other ones, they don't have all the properties of ordinary numbers and you start losing some of the properties. So they become, I would say they become less useful. I mean, quaternions were invented and they could be useful for describing rotations in space, but they are not used that much. I mean, it's not something, I'm not sure whether engineers use it, for example, for this purpose.

• 26:32Brian Keating?

  I think they're using like AI and some AI application, I guess for rotations.

• 26:36Speaker 04

  Maybe they're used for some things.

• 26:38Brian Keating?

  I want to talk about one of the things you're most known for when I was getting my PhD, you know, in late 90s at Brown.

• 26:45Speaker 00

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• 27:14Brian Keating?

  I remember some conference and everyone's so excited and at the end they did the Macarena, but they called it the Maltese. Take us back to those times, you know, this ADS-CFT. What is it? How did you come upon it? Give us the origin story.

• 27:29Speaker 04

  Well, ADS-CFT is this connection between, you know, universes which are large and with negative cosmological constant. So that's an ADS anti-de Sitter spacetime. So the Sitter is the one with positive cosmological constant. This is with negative cosmological constant. And CFT is a type of field theories or field theories. There's theories that we use to describe relativistic particles. And conformal means it has some scaling symmetry. And the idea is that these two are connected. It's this instantiation of the holographic, this holographic idea. So it's a concrete example. Yeah, so that conference took place after this paper and after people had worked on it.

• 28:12Speaker 04

  And there are many other interesting properties. And so Jeff Harvey wrote this song. I mean, the Macarena was the song that was popular at the time.

• 28:23Brian Keating?

  What do you say to people that often, you know, have said the mathematics? Like with string theory is beautiful, but we live in a, we certainly don't live, seem to live in ADS space. So is it just pure, again, like a Wick rotation? Is it something that we should use as a useful tool? Or could it describe reality and we just haven't found evidence for it?

• 28:40Speaker 04

  Well, we made a sign error.

• 28:45Brian Keating?

  Okay, typo, we gotta retract it. Paper is zero citations.

• 28:50Speaker 04

  Yes, yes. So the Sitter space is much closer to our universe. And I would very much like to have something, I mean, we, everyone would very much like to have something like this in the Sitter space. And hopefully, understanding the anti-de Sitter case will be useful for understanding the Sitter case. I hope that the understanding of the Sitter case would have happened already. And I hope it will happen soon. And but maybe we'll need maybe a new conceptual idea. So people who say that this is not the physical universe are correct. But, you know, we hope it's close enough that we can extract some lessons.

• 29:25Brian Keating?

  The other thing we talked about briefly in our last conversation, four years ago, I can't believe it, was wormholes and even humanly traversable wormholes. What is a human traversable wormhole? What good is it other than for solving a lot of issues in Hollywood where you're off to tomorrow?

• 29:40Speaker 04

  Yeah. Before I discuss what the wormhole is. So in Einstein theory, the structure of spacetime is dynamical and curved. So the spacetime can be deformed, right? Okay, so it can be deformed a little bit and. You know, when Einstein developed his theory, he thought, okay, these deformations will be small. Then there were some even larger deformations, like black holes, and okay, that's more drastic. But then you can have some other types of deformations where you drill a hole in spacetime and you connect to another region of space. So you can have, for example, a spacetime like this. Imagine a membrane. You dig a hole in these two portions of the membrane and you somehow connect them. But you connect them through a tube that is not embedded in this spacetime. It's just a very short tube.

• 30:22Brian Keating?

  Right. My Klein bottle over there.

• 30:25Speaker 04

  Yeah, something exotic like this. And the question is, are these configurations allowed? Are they possible in general relativity? Science fiction authors love it because you could go in one end and come out in the other and you could travel faster than the speed of light, for example. This is something that they could allow if they were possible. But it would be a little funny because the structure of special relativity and general relativity is based on the idea of a maximum speed for propagation of signals. In general relativity, you are not allowed to put any spacetime. So you're not allowed to say, oh, I have this spacetime. You have to obey certain equations, right? And the equations roughly say that the curvature of your spacetime should be equal

• 31:10Speaker 04

  to the density of matter. Then you can say, okay, fine, if I want to build some spacetime, I just put appropriate matter and then I will be able to have any spacetime I want. But then there is a catch because matter has to obey certain properties. You cannot have matter, let's say, with negative energy or things like this. At least in classical physics, you can't have that. And once you put in that constraint on the types of matter you are allowed to have, then you forbid this type of wormhole that would allow you to propagate faster than the speed of light. That is also forbidden in the full quantum theory. In the quantum theory, we think that in the quantum mechanics, you are allowed to have a little bit of negative energy, but not enough to have a wormhole that would allow you to travel faster than the speed of light.

• 31:53Speaker 04

  So those types of science fiction wormholes are not allowed according to the laws of physics as we know them. And this is not something that depends on the detailed structure of the standard model, but it's something that depends on relativistic quantum field theory. So the principles of relativity, which are the principles on which this whole picture of spacetime is based, and the principles of quantum mechanics, they do not allow such a thing. I think this is a beautiful consistency condition between the two theories because the... This issue with this wormholes, which is some property of general relativity, they depend on some quantum property of matter. If quantum matter didn't obey this property, then you would be allowed to violate the, you would be able to send signals faster than the speed of light.

• 32:33Brian Keating?

  Right.

• 32:34Speaker 04

  Creating these wormholes. So those are not allowed, and we, this is a nice theoretical result, important theoretical result. But this does not forbid wormholes that where it would take longer for you to go, right? So you could imagine a non-trivial topology where, where there are two holes and they're connected by a long tube, and it takes you longer to go through the tube, at least I've seen from someone outside, than the time it takes to go between the two mouths. And recently, it's became possible to construct some solutions that are of this kind. So they require certain types of matter, in particular, charged fermions, which are massless and so on. So they could exist as solutions at very microscopic scales, where you can approximate

• 33:17Speaker 04

  the fermions of nature as being massless, though those would be very tiny. Or you could say, well, I have some very special type of dark matter that is this, is a dark matter specially designed to make wormholes. And then you could have a very, very big wormhole that could be humanly traversable, that the person can traverse.

• 33:36Brian Keating?

  Meter scale, right?

• 33:38Speaker 04

  Yeah, well, for, to make them this way, you need them to be actually much bigger than meter scale. And the reason is, is kind of interesting. It's because, so these are structures where there is some spacetime curvature. And we, we are quite sensitive to tidal forces. So you need them to be roughly the size of the Earth to, for, for it not, not to kill you when you're traveling.

• 34:02Brian Keating?

  Well, that's beneficial. We could transport whole planets, you know? Why stop at astronauts when you could have all people?

• 34:07Speaker 04

  That size is just so that the curvature is small enough that they would, would not kill you.

• 34:11Brian Keating?

  Ah, right. I see. If you and Einstein were together in 1913, you know, or 1911, say, after his happiest thought about, you know, falling on an elevator and, and experiencing no gravitational field, and you gave him an L M and a G P T and a G P U, and you had the most powerful system, do you think he could have come to, or you guys together could do stuff that you couldn't do without an A I? In other words, someone, you know, operating at the highest levels of theoretical physics, what level of, I mean, I use L M's all the time, but I don't see them creating new physics anytime soon.

• 34:47Speaker 04

  Well, we'll see. We'll never say never.

• 34:51Speaker 04

  The field is advancing quickly and we'll see. We'll see what happens. Yeah.

• 34:54Brian Keating?

  I was an altar boy in the Catholic Church in Westchester County, actually in Chappaqua, New York, where the Clintons now live, as it turns out. And I loved it. I thought it was awesome. It was 1984, 1985. And then the Pope John Paul II, who was, in my opinion, the greatest Pope in history, maybe. I loved him. They came out with a decision that Galileo was right, but they never really forgave him. And I understand that you remember the Catholic Scientist Society. How do you reconcile? Like, do you feel like there's a tension? I always thought they should just say, he was right, he was pardoned. How do you reconcile the so-called, you know, kind of tension between science and religion?

• 35:34Speaker 04

  I think, yeah, the Galileo thing was a very unfortunate case. But there are many other cases of, you know, scientists that reconcile their faith with their... And we're talking about cosmology, for example, Le Maitre, who was one of the people who created the Big Bang Theory. He was a priest and he reconciled. So I think that there isn't a fundamental issue, but, you know, as science progresses, we have to change how we understand religion. And also, religion can illuminate some scientific, well, not some scientific questions, but some issues that arise because of science.

• 36:17Brian Keating?

  Right, yeah.

• 36:17Speaker 04

  I know we have now very powerful weapons and, you know, that have some responsibilities that are very, very important, very moral responsibilities that are...

• 36:28Brian Keating?

  Yeah, and how to adapt. You know, people are so obsessed with artificial intelligence, but I kind of feel like we need artificial wisdom. Like, intelligence is plentiful, but somehow it's more important to get wisdom. And I don't see science providing wisdom. It provides knowledge. I mean, that's what science means in Latin, right? But it doesn't mean wisdom, so. Yeah, from my perspective, they can be partners, you know, science and religion. I don't see them as foes or in opposition. But yeah, people that try to derive one from the other, like prove that the big bang happened using the Torah, you know, using the Bible. I think that's not, not great.

• 37:02Speaker 04

  When the cosmic microwave background was detected. So the Pope wanted to say, said actually that now we saw the, you know, the beginning of the universe, the hand of God and so on. And, and Lemaître told him, don't, don't wade into this, just don't, don't say anything. Because.

• 37:22Brian Keating?

  Yeah, that's right. It could, it could change. And back then they thought the earth was, you know, younger, older than the universe. Right. That was quite embarrassing. Well, let's see, we got to get you to your talk, but before we do, I have a gift for you, not a Nobel Prize, but it's called the Keating Prize. It's not too arrogant of me. So it has Arthur C. Clarke on the front, because the podcast comes from him. And it says, the Keating Prize for impossibly good imagination. And, and then a meteorite, which is a fragment of the early solar system that somehow magnetically attaches to the monolith on the back and has your name on the side. Juan Maldese, and thank you so much for coming to San Diego.

• 37:59Speaker 04

  Thank you very much.

• 38:02Brian Keating?

  And you'll add it when you win the Nobel Prize, you could add them together.

• 38:04Speaker 04

  Right, right.

• 38:05Brian Keating?

  Great, thank you so much for being on and stay tuned, watch the lecture on black hole entropy and thermodynamics coming up next.

• 38:11Speaker 03

  Juan told us today that he thinks the structure of spacetime is built out of quantum entanglement and that the deepest problem in physics isn't black holes, it's the big bang. Now, if that changes how you think about reality, hit subscribe and turn on notifications. Drop a comment, let me know what problem you think Einstein would most like to see solved if he came back. And you'll want to go deeper and check out Juan's two-part lecture on my second channel, Keating Experiments. I'll link that here. And if you want to go deeper, you're going to want to watch my conversation with Leonard Susskind, talking about the black hole wars, using the language that he and Juan invented. The link is right here. Don't forget to like, comment and subscribe and I'll see you next time.

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