Juan Maldacena The Emergence Of Spacetime

TITLE: Juan Maldacena: The Emergence of Spacetime CHANNEL: Curt Jaimungal DATE: 2026-05-04 ---TRANSCRIPT--- Wormholes are a bit like leaky pipes. Not everything is fitting together. The singularity is not a place inside the black hole. It’s a place in the future. I think it’s common maybe not to feel good and to feel that maybe you’re not good enough, but well, eventually you’ll make your contributions. Professor Juan Maldisena has spent 30 years probing deeper and deeper into what the heck spacetime is. In this episode, we explore how it comes down to the claim that geometry in some cases is what entanglement looks like from the inside. Today, we discuss what er equals EPR actually means. And we discuss black hole interiors, where singularity is just the name for for things we don’t understand.

Most of us intuitively assume there’s a sort of view from nowhere that you can just do quantum mechanics from the outside, look in, and write down what’s there. Professor Juan Maldisena discusses why we can’t. On this channel, I interview researchers regarding their theories of reality with rigor and technical depth. My name is Kirchaungal. I’ve included further resources on my Substack at kjimong.com if you’re interested. Stick toward the end for advice for students, for you the listener, as we see a strikingly honest, vulnerable, and inspiring perspective from the man who wrote the most cited paper in theoretical physics during his graduate studies. Even Juan Maldisena didn’t feel good enough. If you’re struggling with something similar, hang in there. Keep studying, keep pushing forward, and don’t give up. Professor, it’s an honor to meet you. Well, it’s great to be with you. What is spacetime made out of? Well, spacetime in the theory of general relativity, it’s not made out of anything. It’s a primary concept. It’s the main dynamical object of the theory. Um the the question about what it is made of is uh only relevant for a more fundamental theory, some other theory. Um and we think uh by thinking about the quantum mechanics of spacetime that it can be at least convenient sometimes to think of it as made of something else. And um the something something else could be uh cubits or some other fundamental quantum degrees of freedom that live uh in the boundaries of the spacetime that live far away. Yeah, that’s a relationship that we’ve been studying quite a bit in the last maybe 20 years or more. Yeah. So it’s a picture where spacetime is immersion from some other degrees of freedom that live on the boundary space time. So in in that picture there is this boundary this region far away that uh serves as a framework as some overall space where or spacetime where the degrees of freedom that describe the interior of the spacetime live. Um so in that description we are listing that boundary. You said it could be quantum degrees of freedom. Yes. What else could it be? Well, it could be other things. I mean, in in in physics, uh, when we say something is made out of something else, right? Uh, we we think about some more fundamental things. Um, and in different examples, they are made of different things. In general, in all of physics, in most of physics, it’s made out of particles. We say, I mean, that’s what let’s say we we learn in high school and so on that matter is made out of particles. Uh in um modern physics we describe things not as made out of particles but made out of fields like uh the electromagnetic field, the electron field, the Hicks field, uh various fields uh and that we think is the basic fabric of uh reality of nature at short distances. That’s that’s the theory we really have experimental confirmation uh from. So then uh all those fields live in some space time. The spacetime is given is some kind of fixed arena where everything happens. Um but in general relativity spacetime itself moves and changes and we we think that in some sense it’s described by some other field and um whereas the fields of of matter we describe them quantum mechanically. The field that makes the metric or the space-time geometry uh we describe only classicalally. We we only know how to describe it classicalally any we try to we can describe it quantum mechanically in an approximate way and that approximate theory uh can give us some answers but it cannot give us all answers somehow it uh we know it fails in some cases the most important place where it fails it’s in the beginning of the universe and this is the main reason we want to find the better theory to find out what happened in the beginning of the universe another place where it failed is in the interior of black holes and that’s another reason for trying to understand the better theory. So we know of places in the universe where the current our current understanding of physics breaks and um the idea is to develop at least some theory that can describe such uh such things and then try to figure out of course in some way that that theory is the correct theory. We don’t know whether the theories we have right now of quantum spacetime more complete theories where they are the correct theory or not. And what would you say is the primary reason for the difficulty of combining GR with quantum theory? So the popsai is that oh one is discrete and one is continuous or one is linear, one is nonlinear. How do you see the incompatibility between the two? Um well I I I think the main issue is that uh in in quantum mechanics there is usually some time some order between operators and in uh order between in the measurements we make in general relativity um and in gravity uh spacetime can have uh different geometries different topologies we don’t know what the order is. Also another conceptual difficulty is that in quantum mechanics we have some observer who’s outside the system and in uh gravity everything is somehow inside the system. So we cannot have an observer that has no mass that has you know no energy that measures things from outside. Um whatever observer exists inside the universe has its own energy and yes these are some of the features that make it uh difficult. I mean there there are some other technical things people often discuss that are special to let’s say four spac-time dimensions which is that there are certain infinities that appear in the calculations but that that I would say it’s a more technical issue rather than a more conceptual issues like the ones we mentioned earlier. Now what’s the difference between a conceptual issue and then a mathematical one? Well, I would say mathematical one is is one where the issue is not there let’s say in two dimensions but it’s there maybe in four dimensions. So from this point of view it looks like an accident that we live in four dimensions. We would still have a bunch of conceptual issues related to quantum gravity but we would not uh have perhaps that technical issue we just mentioned or that technical issue will be a little easier. But the other questions are still there in two dimensions. We still have black holes that are confusing in uh finite universes. We have the issue of uh having to include the observer and exactly what the how we should do that. Yeah. So those are issues that still remain. Earlier you mentioned observers. Now Whitten along with Pennington and some other collaborators helped fix the what happens inside a black hole interior with type three to type two algebbras and some technicalities. Is that observer that’s talked about in those papers the same sort of observer that’s talked about in say the foundations of quantum mechanics? Yeah, it’s related. I mean their description is uh perturbative. So starting with a particular background and considering small fluctuations around that background. It’s in the context of uh what would sometimes call semiclassical gravity. So I I mentioned previously that uh gravity quantum gravity makes sense as in a certain approximation and that’s the approximation in which they they did their discussion and so it’s it’s an extension of what uh of those methods to deal with situations where maybe we have a close tun so specially compact universe or a region of the universe where the observer can access only to a portion of the universe such as the the center space. um and and let’s say it’s an improvement relative to what we had before. Um so that’s uh that’s what it does. Um it does not answer the questions of uh you know what happened in the beginning of the big bang or how the observer actually emerged in the very beginning and so on. So that that does not do but but but well this is normally in physics right. we we do some things we understand uh some aspects and make uh incre make progress and there are still important questions. Was it a surprising result to you? Yeah, I think it was a beautiful way of um improving the our understanding of uh the so-called generalized entropy. So let me mention let me say a little bit more what that is. Um so if you have a black hole the black hole uh has an entropy leading in the leading approximation which is the area of the horizon in plank units. Um this is a very large number for a microscopic black hole. Uh but this entropy was then um supposed to have a quantum correction that comes from Hawking radiation. So Hawin radiation is some radiation that is you know pretty well it’s some thermal radiation that comes out of the black hole. And if you’re looking at this radiation and you’re looking it um from the point of view of an observer who states outside the black hole and you approach the the boundary of the horizon so the boundary of the region that is accessible uh then it looks like it’s hotter and hotter and hotter and naively computed the the contribution to the entropy coming from that would be infinite. Um however we we think that that infinite contribution somehow combines with this area contribution which is very large and gives really something that should be viewed as finite. And uh what these papers did is they they understood how to combine uh how to derive an expression for the entropy that would be finite or would describe uh changes in this entropy um in a consistent way without ever having to talk about any infinity or cancellations of infinities. So it’s it’s a better way to think about black hole entropy in the semiclassical theory. It answers some questions of black hole entropy. Again, it doesn’t answer all the questions, but it answers some some important questions. It answers the questions that are related to, let’s say, the interaction of a black hole with with some amount of matter. An amount of matter which um is not big enough to change the mass of the black hole in an appreciable way. But it describes that matter in a completely quantum mechanical way. And and and the entropy of that matter in a completely quantum mechanical way without infinities or anything. Uh right now these infinities are some technical issue which uh occurred this paper solved and uh made the theory more more reasonable. You said it solves some problems of black holes, some important problems. That means that there are some other important problems that may be left unsolved. So what are the greatest unsolved problems about black holes? Uh I would say the the greatest problem is understanding uh better the black hole interior. So in the black hole interior the space-time curvature becomes infinite. Uh that that infinite means that something happens that we don’t know how to describe. So if if you don’t know what this means we don’t know either we don’t know what happens at the so-called singularity. So singularity is just the name for for things we don’t understand. But so the the Einstein equations themselves uh predict that as you evolve them towards the interior you hit this uh singularity. The singularity is not a place uh sort of inside the black hole. It’s a place in the future. You go to the interior of the black hole and you find this singularity in your future. So you can’t avoid it. It’s a bit like a big branch singularity somewhat similar. So a singularity where the whole that whole region of the universe collapses. Um I mean one way to think about it is that um you know the universe is generally expanding right and so we’re all happy when the universe expands but in some regions where a lot of matter gets concentrated the the universe starts starts collapsing and uh in these regions you produce a small big crunch. Uh so a region where the space time curvature becomes infinite the opposite of a big band and that is not visible from the outside is behind the so-called black hole horizon. So we don’t directly get any signal from this region but if you were someone who’s falling into the black hole you you would get into this region and you would collapse together with the rest of the matter making the black hole. Now, so the fact that the the space-time curvature becomes very large suggests that in these regions the quantum effects will become important and so if a full theory should say exactly what happens there and should give us a more complete description and um we we don’t yet have uh such a thing. Um what what what we do have are some theories that can describe aspects of black holes as seen from the outside. So if you remain outside the black hole and ask uh what what we think are very precise questions about the black hole then we have uh some we think we know what would happen at least conceptually. Part of what we were I mean what we were discussing previously about uh the entropy and the algebbras and the work of Pennington uh is related to describing the black hole from the outside. I have a question about Elissio and Tiachi, if I’m pronouncing that correctly. Either way, I’ll place a link on screen and in the description. If I recall correctly, they showed that semiclassical near extreal black holes that their thermodynamics breaks down at the Schwarzene scale. Now, does that break holography or is that just specific to JT gravity? Mhm. Yeah. Let let me try to describe this uh general area. Um so there there are black holes and there are charged black holes. So black holes can carry charge and if you uh have a large charged black holes as it evaporates the temperature becomes smaller and it emits energy but it keeps its charge and so there is some state that that it um approaches where um it has the minimal mass consistent with that charge and that’s a non-zero mass. So it’s and its hawking temperature goes to zero. That’s soal an extrema black hole. And the black hole develops a geometry that has a a near horizon geometry with a very long uh near horizon region if you wish. Um and um and it develops a kind of scaling symmetry if you wish. It becomes self similar in some way. So you you can get closer and closer and closer to to the horizon and the black hole looks the same. Um okay. So you can have one of these very big black holes and if if if it the black hole is very big then um you would expect that the quantum corrections quantum gravity corrections are very small. You might think but if you go to very very close to zero temperature or very very close to extremality then uh there is a very particular quantum corrections that becomes important um and and that’s uh very interesting because um it’s an example of a controllable situation in quantum gravity where only one aspect of the geometry becomes quantum mechanical but the the rest the remaining aspects of the geometry are still classical and we don’t have to worry about them. So it’s a case where you can really quantize gravity. You can quantize if you wish a particular degree of freedom of gravity. In some sense it is as if the black hole becomes very long and this length can have fluctuations quantum fluctuations and you can treat them uh precisely and um and this exact quantum treatment of the of the fluctuations is something that was really only understood this aspect uh of extreal black holes was only understood in the last uh maybe decade or so or so and um it went through a sequence of developments uh starting with some toy models based on condensed matter analogies and so on. So it was a pretty interesting story that I won’t uh give you in detail and um this particular paper that you quoted by uh Ilasu and Trichi they analyze this for you know fourdimensional black holes in general relativity that carry very big electric charge and um they they showed that uh the entropy as uh you approach extremality uh goes to becomes small I I I should say that the important aspect is that these quantum corrections change some qualitative aspects of these black holes. Um in particular these black holes seem to uh violate the third law of thermodynamics. So the third law of thermodynamics says that as you decrease the temperature of the system all the way to zero, its entropy should also go all the way to zero. Um now these black holes if you just treat them purely classicalally ignoring the quantum corrections uh they at extremality they have zero temperature but non zero entropy. Um and once you include the quantum corrections the the entropy uh sort of decreases essentially all the way to zero. Um so this correction has the important feature of making this black hole also now consistent with the third law of thermodynamics more similar to ordinary quantum systems that that’s one uh that’s one aspect and well there are other other aspects in this highly quantum regime that are also very interesting. I subscribe to the economist their science and their AI coverage is among the best I found anywhere and I say that as someone who reads plenty of it. 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That’s economist.com/toe for 35% off. Now, what about the island formula? Yeah. So, this is related to the black hole information paradox. So there is a problem that well was pointed out originally by Hawking that uh after he discovered Hawkin radiation he thought well a black hole uh can collapse in many different ways uh but the radiation it emits seems to be a featureless thermal radiation that’s largely independent of what fell in and that does not seem to be consistent with the unitarity of quantum mechanics. I mean the black hole might evaporate completely and you you get this featureless thermal radiation. Um and of course something similar happens when you burn a piece of paper and so on. But in that that particular case if you work hard enough and so on uh you would be able to recover the information at least in principle the laws of uh are understood to to the extent that in principle should allow you to do it. I mean it might be super expensive not worth the trouble and so on. But in principle, it’s like in detective shows when they try to take the shredded documents and put it together. It’s a more complicated version of that. Exactly. It’s the same thing. I mean, when you shred a document in principled information is there. If you’re patient enough, you can put it back together. But yeah, if you throw the document into a black hole, can you do that? And the laws of physics. And so we think that the laws of physics are such that they would allow you to do that. Um and and that is true in some according to some conjectures that use string theory and so on. Um now the the question was how to see that more directly from the gravity point of view. So what Hawkins said is well I can calculate something called the the amount of quantum information that you get in the radiation. This is sometimes called the fun entropies. It’s a measure of quantum information and if you calculate it, it looks like it it is increasing. So you you you send something in and then you get something bigger and it was a mystery of how you would see this from the gravity point of view. So the island formula well it came uh came after developments by uh and Harry Angelard Maran Maxfield and a separate paper by Pennington. Um so this formula is a way to compute the entropy of the outgoing radiation. So there is a a way to compute the entropy of the outgoing radiation which is similar to the entropy of the black hole that we saw before the the area formula. So it’s a new type of area formula that uh allows us to compute the entropy of the radiation and it gives us an entropy which is consistent with the idea that you are preserving information. So as the black hole uh evaporates and when it it’s close to evaporating completely then the entropy goes it becomes smaller. So uh in the sense that all the and that is interpreted as saying that all the information is coming out. So it it makes oh it’s something that uh closes a bit the the development of black hole entropy in the from the 70s. So in the 70s there there were people noticed that um black holes as seen from the outside they behave very much like thermodynamic systems. They have a they have a temperature they have a hawking radiation. All of these are theoretical discover discoveries. They are not uh things you can uh that have been checked in experiments. We can discuss experiments later. Um um and and the the the area of a black hole also classically always uh increases and and Aaron Wall also showed that if you even even if you include quantum corrections the the area of black holes increases uh the area of black holes sorry together with the the radiation that is outside the total entropy increases. Um so that’s uh the so-called second law. So the second law of thermodynamics applied to black holes and what happened in uh started around uh starting around 2006 or so was that uh a new formula was discovered for black hole entropy. It’s was discovered by Ryu and Takayanag um two Japanese physicists um one of them working here in the US and um they found a new formula that calculates a slightly different type of entropy. So in thermodynamics uh there are two types of entropy. One is the one that appears in the second law. It’s got sometimes called the Bolsman entropy. Um but then there’s a second kind of entropy which measures the amount of uh fine grain information. So quantum information it uh it measures the amount of information that you have about the system if um you are able to measure things precisely as much as you can. So if you had infinite resources so so let’s go back to your example of shredding a piece of paper. So if you share a piece of paper, you nly lose the information that is in the piece of paper. But it’s someone you know with a lot of resources and the resources of the FBI etc. Uh it can in principle recover the information, right? So for someone with uh enough resources, information is not lost, right? Um and um but there are ways of scrambling something enough so that the information is lost. If you if you really take that information away and uh store it somewhere else, uh you take out all the little letters and you just leave the shredded paper and you took out all all the letters. Yes. You you uh make sure the information is not there. And um so so the the the the other new form of entropy or this new this form of entropy well is a standard form of entropy that people use to describe quantum system systems measures that second kind of information the information available to someone who uh has infinite resources and that’s what this new formula gives us. It gives us another and gives us the formula for the entropy in terms of an area. It’s not the area of the horizon. is the area of some other surface that um that depends on the geometry of the interior. So you are supposed to um look at the interior of the black hole and find the the area the yeah the the surface with some extreme area. So the area which let’s say it’s a minimum just informally speaking. So some region where the area is a minimum and that minimum value of the area is that entropy. So it’s a formula that is surprisingly simple, very similar to the formulas of black hole entropy and it represents an extension of those ideas of black hole thermodynamics to to and and it represents a connection between uh space-time geometry and uh quantum information because this this notion of entropy I should say this notion of entropy is intimately connected to with the Shannon entropy the the notion of uh classical information. Mhm. Actually as a side comment Shannon entropy was invented after forman entropy and uh um after the this quantum version of the entropy that we’re discussing right now. Anyway, so so this these concepts of entropy these are this is a concept of entropy that is very important for class both classical and quantum information uh theory. Um and um and now we have a connection between black well through black holes with uh between quantum information and space-time geometry and uh this development of that you mentioned of the alien formal and solar all are all in this uh general area. Yes, it’s a beautiful formula. I’ll place it on screen and put links in the description. Now I haven’t kept up with the literature but my understanding is that it’s in a uklitian path integral or lives in a uklidian path integral but I don’t know if there’s a lorencian derivation and if not is there’s some reason for the impediment. Why is it so tricky? Well, the the derivation uh in both involves path integral techniques. Um and the the reason there is some ukidian evolution is because it’s usually necessary to prepare a state. Um so it’s usually convenient to prepare a state by doing so-called ukidian evolution. That’s very common for the thermal state where uh a thermal state can be viewed by evolving over a ukidian time on a circle. um um but we don’t think that um this is uh fundamental to to the derivation and some some researchers have suggested more natively let’s say Lorenzian derivations of the the formula uh for example Dom Marshong and other collaborators from UC Santa Barbara uh have emphasized this purely it’s more purely Lorenzian derivation of this formula Um there’s always a little bit of uklidian evolution because um you need you get you need to get an answer for the entropy which is um you know a real answer for the entropy and ukidian path integrals give you faces give you lorencian path integrals you mean sorry lensian path integrals give you phases you know interference of waves and so on so but but so recently there there’s been an understanding ing of uh of this formula from a mostly Lorenzian point of view which is quite useful and make well it’s very useful and makes it makes the derivation more general more general than what was done previously. Yeah. So this formula is is being better and better understood I should say and and applied to new new situations and this island formula is one of the breakthroughs. There are a couple breakthroughs of the past 15 years or so. One is the page curve and this island formula has a relationship to the page curve and that helps derive it. So can you please outline that? Well, the formula is is interesting because um it it connects black holes with quantum information. It gives us the amount of true m fine grain information that the black hole has. That that’s what makes it interesting. Now a great breakthrough of the last few years was that the realization that you could apply this very same formula to compute the for computing the entropy of hawking radiation and this this involved also some conceptual uh breakthrough how how to apply this formula but but fundamentally it it’s the same formula the page curve is is a it’s a curve that person called don page uh proposed as uh how the information of hawking radiation as it comes out of the black hole. You have the Hawking radiation coming out of the black hole. Then you can calculate uh the information that comes out as a function of time and and or the entropy containing that radiation as a function of time. And what you find is that this uh if the black hole evolves in a unitary way according to the rules of quantum mechanics uh then that information should grow and then decrease again and decrease back back to something small when the black hole evaporates completely. uh that that’s what’s called the page curve and uh it was always a challenge to calculate this page page curve and uh using this new formula we can calculate the page curve and it gives that type of qualitative behavior. Now speaking of constructing something your dad helped fix elevators and when you were 12 if my understanding is correct that you built a working model with rusty blocks. Mhm. Is that correct? Yeah, that that’s correct. Yeah, I I my my my dad was a very hands-on person and he liked to, you know, fix the car, fix the washing machine, everything. And then I always love to watch it. It was a family activity, I guess. Yes. Now, that led you to going into engineering, but was there anything there that you now still carry over with you into physics? Well, I I I guess I view physics as a bunch of things that need to be fixed and formulas that maybe we’re not fixing wires or, you know, mechanical parts, but we’re trying to build formulas and make all the formulas work together and fix a kind of conceptual architecture that tries to, you know, extend the conceptual architectures that we have uh right now. Right? That’s what theoretical physics uh I think it’s about. to expanding the conceptual tools that we have to describe nature. Which of your results over the years do you think your dad would understand on site if Well, they’re quite abstract obviously, but if you were to explain it to him, which one do you think he would like the most? Well, maybe yeah, maybe something more concrete like uh how you know how energy flows after in particle collisions. So with Diego Hoffman we did some calculations of that and you know now thanks to the efforts of researchers like Iron Malt and others uh we that they are being measured in great detail in colliders. So that’s I think something that can be understood. Now my understanding is Leonard Suskin’s dad was a hands-on person as well I believe a plumber. Mhm. You and Lenny have a a great relationship and you sent an email to him many years ago. ER equals EPR. Just a single word or equation. And he understood it. What did he understand? Well, he understood it because we had been discussing that for quite a while. So the the connection between entanglement and space-time geometry is part of the the developments spurred by the this new formula of black hole entropy derived by Ruayanagi that uh as I said suggested the connection between quantum information and the spa the geometry of spacetime. Um this is the type of information that we sometimes uh want to measure when we want to quantify entanglement. So this this this information measure is also useful for for that purpose. Uh so entanglement is a subtle form of correlation we can have in quantum systems and um and and of course entanglement was discovered originally by originally by Einstein Posski and Rosan and is usually known by the the acronym of EPR. So now in the same year surprisingly in the same year Einstein and Rosson wrote another paper uh which was the the first paper noticing that the original shride solution doesn’t describe one black holes but essentially two black holes that there are two uh regions two outside regions um and they didn’t understand everything but they understood this this part about the the the black hole and it was later understood more completely was later understood completely by Kuskal you know about 30 years later but yeah and that is sometimes called the Einstein Rossen bridge so the idea that there is uh there are two as two regions that look like flat space and they are connected through two sort of black hole horizon and the the two asmtotic regions share a single interior and this this if you look at that uh region at the moment of time then it looks like a bridge between the joints is two separate uh two separate spatial geometries. So this also sometimes called the wormhole and so er stands for ein and rosson. So some people read that as saying that the wormhole is entanglement. Would you be so bold or would you say that more like entanglement gives rise to some sort of geometric connection? How do you see it? Well, so the the I think um so the idea is that uh if if so we were talking about before about black holes and we’re saying that if we are looking at the black hole from the outside we can view it as a quantum system. Now when we look at the Russia solution we really have two black holes. So what’s the connection between the two black holes? The idea is that these um two black holes are two separate quantum systems that are entangled with each other in a very particular entangle state. Um it’s called the thermofield double if you wanted to know the name of this particular entangle state. Um so but if they’re entangled in this particular entangle state then the idea is that they give rise to a geometry connected geometry uh which is that of the full sh flat solution or the Einstein Rossen bridge. Um so in this particular example I think the arguments that the entanglement creates uh this space-time geometry or this space-time connection is fairly clear and quite uh convincing I would say. Um now then the question is whether this is true in any case that we have entanglement. That’s a wilder claim and and I I I would say that we cannot make this claim in a meaningful way in the follow in the sense that um we don’t know a description of geometry that is um I mean if it is true uh it should be true in in some generalized sense of geometry. So it’s definitely not true if our notion of geometry is the usual geometry that appears in Einstein’s equations because if we have just a spin a half particle that two spin and a half particles which are entangled with each other there’s no connected geometry of any kind uh of the conventional kind but perhaps so so er equal to PR is a bit like an aspiration so it’s like a slogan or a principle that perhaps uh a more complete theory of gravity should have so there might be a new notion of geometry that um would generalize Einstein’s notion of geometry and in which this uh property would be true even for a spin half particle. Earlier you said that you like to view physics problems as something that needs fixing. What are you fixing right now? Um I’m trying to understand the wormholes. Um so wormholes are a bit like leaky pipes in the sense that the lo that surrounds them is um not everything is fitting together. So we know we think we know a bunch of things uh about the theory of quantum gravity in the presence of wormholes. We like the effects of wormholes in some cases that they produce uh uh very interesting effects. For example, they are important for justifying this island formula that you just mentioned. Um the the the derivation of that formula involves a certain kind of wormhole. On the other uh they seem to give uh rise to the idea that the the constants of nature are not quite fixed but they maybe are arbitrary and we should average over them. And in some of our uh models of uh string theory and so on, the the the constants of nature are really fixed. And so we we don’t know how to make everything compatible. I I perhaps didn’t explain clearly what the problem is. The problem is a bit subtle. Um perhaps a short way to say it is that uh there are many ideas that surround wormholes and they’re not all compatible with each other. The question is um we probably need to modify some ideas or we need to maybe they have some subtleties that we haven’t understood and and this is um I mean not only what I’m working on but many other people are are also uh working on this area. It’s one of the hot topics in in our field at the time. This video is sponsored by Short Form. If you want a free trial and an exclusive $50 off their annual plan, then go to the link in my description, shortform.com/e. If you’re like me, you’ve encountered books that are so dense, finishing them is actually just the beginning. Short form helps with that. Their book guides go far beyond pastiche summaries. They critique, they add context, they include interactive exercises and connect ideas across authors. Take Girdlebach or The Master and His Emissary. two of the most demanding reads in consciousness studies on the popular market. My method is I read the guide first, then the book, then I read the guide again. So, it’s a tptic of engagement that cements understanding, better understanding for me. The GEB guide maps recursive structures in a way that exhibits intellectual plyotropy where one insight branches into consciousness, computation, and self-reference simultaneously. Shore form covers philosophy, science, and psychology. Ipso facto, the intellectual core of this channel. They publish new guides weekly and subscribers vote on what books get covered next. Their browser extension, Short Form AI, summarizes articles and YouTube videos with a single click. Go to shortform.com/e for a free trial and an exclusive $50 off your annual subscription. That’s shortform.com/e. What about traversible wormholes? Ah, okay. So, that’s uh those are fun. Uh um do you also view those like toy models or do you view do you think that in our universe they may already be realized or they may be realized in the future? I don’t think they exist in our universe. Um well I think it’s highly unlikely that they exist in our universe. Maybe that would be a more polite way to say or more to say that. Um so um well it’s connected to what we discussed before that if you entangle systems in certain black holes in a certain way you could uh get a connection and that connection uh gives you a wormhole but it’s not a traversible wormhole. So uh and you cannot send signals and you cannot travel through this wormhole and that’s consistent with the idea that uh it can be interpreted as entangled states because using entangle states you cannot send signals. However, if you bring this uh black holes closer together and you let them interact um then you let them exchange a bit of information and so on then they could develop uh this Einstein Rossen bridge can change a little bit. the whole geometry of the black hole could change and it could form a wormhole. So wormhole would be a traversible wormhole. It could form a traversible wormhole and traversible wormhole is not quite a black hole. So you will not have a black hole horizon and it would be some structure where you enter through one of the mouths of the wormhole and you exit through the other m mouth of the wormhole. Um in the wormholes that are constructed this way, they they don’t allow you to travel faster than the speed of light in the space where the wormholes are sitting. So they are not like the science fiction wormholes that you might have heard in science fiction stories. And um everyone gets excited about wormholes because the you might you think oh they will allow us to travel you know to the next galaxy and back in short time that those type of wormholes we don’t think are compatible with the laws of physics. So the ones that allow you to violate causality um or causality in the ambient space. So, but these are these are not these are a bit more like long long detours, but it’s a long detour that you take uh using uh a wormhole and um let me let me make an analogy for this um for this this kind of traversible wormholes that are the ones that uh we think exist please. Yeah. Well, before I make the analogy, I should say that they are classically forbidden. So classicalally they cannot exist according to the classical equations but they can exist thanks to quantum corrections because in quantum physics energies can be negative and then then they they can exist but well now to the analogy. So you know in the um there’s this fine way to travel between two points uh on the surface of the earth which is to dig a tunnel and then uh go down and then go up to the other point and then if you had a tunnel with no friction then just and you go with a little cart and with no friction then you would slide down this tunnel and come out back out in the other side and in maybe half an hour 40 minutes you could be in any place on the surface of the world. Now there are some small reasons why this uh would not work in practice or might be hard to realize in practice. You know the s the interior of the earth is very hot. The there are no frictionless uh things and etc. Just tiny reasons. Yeah. Yeah. There there are a few there are a few technical inconveniences. Okay. But in principle it is possible. So these traversible wormholes are a bit like this. Uh imagine the surface of the earth as a structure of spacetime and the traversal wormhole has this tunnel that you build through the earth and well the earth is is nothing there is no nothing that exists there. So that that’s roughly the something um somewhat similar and they will allow you to go between these two points and uh they will allow you to go from your point of view very fast. So because there’s a large gravitational field, you would uh not experience a very long time in going through these wormholes. So someone from the outside would see that it takes you, I don’t know, 10,000 years to go through the wormhole. But if you go and you travel yourself, maybe would take you maybe one second. Again, this these are not things that will exist in nature. They would require laws of physics, which are, you know, new particles that we haven’t seen and stuff like this. So I I’m this is these are some solutions that are compatible with the general principles of nature but not the particular laws of nature that we have in in our in our universe. Speaking of wormholes, there was a hub about how there was a publicity campaign about quantum computers creating literally creating wormholes in the lab. Those were some of the pop sai headlines. So what’s the actual truth behind the headlines? Yeah. So that that is in this context of uh entangling quantum. So there there is the idea that if you uh have a quantum system that uh is complex enough um it could create space time it could create an immersion spaceime right um now this is this is an interesting point and what is an a subject that we’ve been uh investigating and I guess in with the promise of quantum computers is that they can make all kinds of new materials and new quantum states of matter. And this would be a very fun and interesting quantum state of matter that might allow us to test some of these ideas that we have about quantum gravity. Now uh this also in particular tells you that if you create two cl two matter two pieces of matter in this way that have an immersion geometry and that uh then they’re entangled with each other then they would create a wormhole that connects them. Um and um now what this uh these authors did they they created a quantum simulation that had what they said was the simplest version of this wormhole. Um, so they took the simplest model that displays something simil some phenomenon similar to this wormhole and they paired down to they pair down the model to this bare bare rare essentials and um and they found some features that were a little bit like uh the features of this wormhole. So it’s it’s a correlated quantum system. They had a small number of cubits uh maybe I think maybe seven cubits on each on each side. Um and so people have uh some people say well no you don’t have enough cubits this is not a system that is complex enough uh but I think it’s I would say it’s a first step in in this direction and I think as as you know quantum computers will become more and more powerful they will um make uh better and better versions of this type of thing and at some point uh people will say well this really is looking like a wormhole Right. I mean, it’s a bit like this question of when a sand pile becomes a sand pile, right? So, um the Yeah, if you have seven grains of sand, okay, maybe it’s not a sand pile, but once you have uh enough grains, you it will become more universally recognized as a sand p sand pile. So, they simulated a wormhole but didn’t actually create a wormhole or what? Well, I yeah, it’s debatable of whether in this type of setups you either simulate or create. So this is a bit of a language question. So um so if you if you have a condensed if you create a quantum simul quantum state of matter that has a certain property let’s say super fluid or whatever are you creating it or simulating it? It’s you know once you once the the quantum computer allows you a level of control that you can do many things you can create a quantum state that uh has u various properties and the question is whether you are simulating them or creating them I think it’s a bit philosophical and why are you working on these leaky pipe wormholes right now what is it about them that excites you well is that there is a conceptual problem so there there’s a concept There’s a there there is they are interesting because they give interesting effects um and they also raise questions and we are trying to and this is there is something not completely understood around them and that’s uh that’s what is interesting that by understanding them better we’ll understand quantum gravity in a in a deeper way I think uh so these wormholes were a mystery since let’s say the 80s when people started thinking about this. Um I mean soon after this Hawin information that is related to the Hoken information problem and uh researchers uh discussed them at the time. Uh Colan and Gstrometer discussed them. Um and and the situation was confusing at the time. It wasn’t clear what the rules were for for quantum gravity and um they didn’t seem to be present in string theory in any way we understood. But now now there are many effects that these wormholes are have been well are pro have proven useful for for deriving for example these formulas for deriving properties about the black hole or the energy spectrum of black holes. Uh there is a series of beautiful works by papers by Sad Shanker and Stanford where they show that um certain aspects of quantum chaos are reflected in a wormhole closely associated to the shell solution. Um and yeah so so warps are doing wonderful things for us and we we should understand how to fit those wonderful things with the confusing aspects. Speaking of Strawer, he treats the BMS group as a physical symmetry. Mhm. And that’s what undergurs celestial holography. I haven’t heard you comment on celestial holography on that program. What do you make of it? Yeah. So, so this this this program’s been uh recently understanding uh subtle properties of gravity in flat space. So um instead of trying to understand you know gravity at very short distances they look at uh gravity at long distances but they try to understand deeply the symmetries of flat space and and um what happens when you emit uh some waves of radiation at very long distances and and so on. And they’ve uncovered lots of uh interesting symmetries. I mean the the BMS group is something that was uh recognized in the 60s. Uh but they found a interesting connection between this uh BMS group and some other features of scattering amplitudes and and other gravitational phenomena such as the so-called gravitational memory effect and um things like that that involve scattering of gravity waves. of them now. Yeah. So this is a very interesting aspect and so in in the past in this uh symmetries were important for deriving let’s say dualities between uh gravity systems and quantum field theories. So there there was a beautiful paper of Brown and Heno uh who found that the three-dimensional anti-itter space or particular three-dimensional negatively curved space uh has a two-dimensional boundary and on this boundary it has a certain symmetry which is the same type of symmetry you have in um quantum critical systems in in you know oneplus 1 dimensions. Um so um and and then it was then used to propose the duality between uh these two systems. Um and well strummature himself uh u use this to argue for this duality in general ways. So yeah, so so this is uh is something that has proven to be important in the past and now um we don’t know whether flat space gravitational physics in flat space has some alternative description in terms of uh some quantum system. Um, of course, if we had that alternative description, we could describe black holes and so on in flat space in a in a more complete way. Uh, and that’s one reason for for for trying to find it and is somehow also viewed as a stepping stone towards trying to understand the more realistic cosmology. So the the cases where we understand a quantum fully quantum mechanical description more precisely are cases with negative curvature. cases like hyperbolic space and then flat space is intermediate and then let’s say a sphere or an expanding universe are the the more interesting ones. Now I know that you’re also working on DSCFT correspondences. Why is that so tricky? Um well the adsf is this relationship between negatively curved spaces and field theories or quantum systems at the boundary. DS is means the sitter. So the sitter was uh you know Dutch astronomers who proposed this expanding universe and that is a good description of our universe at late times or also at very early times. Um and the the structure of these two spacetimes is very fairly similar and uh they both have a kind of boundary in in the in the case of an expanding universe that boundary is what happens very far in the to the future. So very far into the future there is a kind of surface at uh that looks spatially flat. It has the structure of threedimensional threedimensional space with no time. Um and it’s natural to think that perhaps uh there is some you know statistical theory that describes uh such universes. So it would be a universe where it would be a description where the time would be emergent and and the the main difficulty is that we don’t have nice guesses. So we don’t um we in the case of uh the anti-de case we had the help that in those cases we can have additional symmetries that such as uh symmetrical super symmetry which uh is very technically useful and useful for uh generating examples um of this relationship. Um but in the sitter case it’s more complicated to generate examples um that that now maybe we don’t generate examples because this relationship is not true. Maybe the relationship is would be intrinsically approximate. So we don’t know exactly why we we haven’t managed to find an example. It might be that we we just don’t have the right techniques to to find it. And supposing that Desi’s results are correct and that there may be time varying dark energy. Does that put a monkey wrench in DSCFT or does that help DSCFT or it would just be some small mere technicality? Well, I mean of course it would be a very interesting aspe of the universe that the dark energy is changing. Um I I I it’s not logically connected with the SCFT in any uh obvious way. So we we could have varying dark energy or not dark not varying dark energy. Of course if we vary have a varying dark energy the structure of the universe in the far future will not be the sitter. Um I mean it might still be the sitter because the dark energy might eventually stop varing. Right now something that that is more worrisome is that some of the desi fit fits have suggested that the value of the dark energy could be of the equation of state could be go below minus one that would be a more severe blows of under our understanding of physics. So I suspect that that um that probably will not be true. But uh why though? Um well because there are deep principles uh the the principles of um you know no negative null energy. So these principles that enforce uh causality that enforce you know traversability traversability or l traversability of wormholes and so on. So um I I I suspect that these principles are are more important than well are very important and I find them very sacred. Of course, if if if it was true that they’re violated, it would be super interesting and it would be the biggest news in the last, you know, 100 years. Um, but I very much doubt that uh that that’s what will happen. I I I suspect that one once they fit the data in a different way or and so I I don’t think that there is uh enough evidence to claim that uh this equation of state parameter which is called W is less than minus one. Another pillar is unitarity. And I’ve heard many physicists say that they’d rather sacrifice locality for unitarity. That unitarity is sacred. So can you please give a taste as to why why is unitarity so important? Um well um unitarity is related to the conservation of probability. So um we if if it wasn’t true then we would have trouble with uh probabilities. So probabilities might be you know not positive or bigger than one and we would have some some bigger problem. So we wouldn’t know how to interpret a theory where that’s not true. Um yeah so it might be that uh we might need to give up some other principle of quantum mechanics and or some other aspect of the structure of quantum mechanics that that might be possible like uh this discussions of including the observer or some of the problems we face in quantum gravity with the merchants of time maybe will require us eventually to give up some of some of the structure of quantum mechanics but I I I I I wouldn’t necessarily say that we give up locality. I would say that we give up some manifest locality in some of our current descriptions. U it’s not completely obvious we are we are giving up locality. I think we just not manifest in the the ways that we describe it. What’s a manifest locality versus an actual locality? Well, the problem is that locality is hard to define when uh you don’t have a spaceime, right? Or when you don’t have a a background space, a fixed spaceime, right? So, uh when we are thinking about quantum gravity or first of all, when we’re thinking about quantum mechanics, um let’s say just for a single particle, uh the particle doesn’t have a well- definfined position and momentum everywhere. It can have various possibilities for its momentum and its velocity. Right? Um similarly when we do quantum field theory, a field does not have to have a definite uh uh form in everywhere in spaceime but it could be it could have probabilities of having various uh different possible forms. Um now when we talk about gravity then uh what we are saying is that we have a metric or a space-time geometry and it’s not fixed. we we can have uh various probabilities of having different spac-time geometries. So uh you know two points that are on one surface may be far on that surface but on some other surface they are smaller and both are have some probability right so that they’re and so when you say that you cannot say a priori where two points are points are far away or not there there is always some the probability of having some surface where they are close by. Um and that’s why locality is uh sometimes a little harder to define. uh but may maybe it is local in some in the sense that maybe for each each of the particular surfaces that appears the you know the theory uh is local in that sense and there is some some notion of causality that you cannot if you have a space that is let’s say asmtotically flat or which is flat far away or this negatively curved space is far away then far away you have a structure a coal structure that you define by coal structure what I mean is that you know two points might be uh reachable by an observer who travels at less than the speed of light and that’s uh usually well that we think will be preserved by the the bulk spacetime even though it’s fluctuating and so on even though there can be fluctuations where the points are closer and so on in the end of the day if you want to send the signal you cannot send it faster than light so causality continues to be preserved In that sense for the entanglement wedge reconstruction there’s a objection from Harlo. So please explain what’s being presupposed. Explain this whole situation. Uh well I I think uh we can discuss this a little bit. So yeah um Almary uh Dong and Halo uh wrote a very interesting paper where they propose uh some analogies between quantum error correction and the way that holograph is supposed to work. So basically quantum error correction and the map between the bulk uh degrees of freedom and the boundary or some analogies between the way that the bulk is embedded in the boundary theory and the way that the quantum information is embedded in in a quantum error correct correcting code. Um yeah and this uh this well was was very nice and then uh was a wonderful paper and then there were a series of developments um uh involving other techniques uh that people use to describe quantum systems uh um that involve something called tensor networks. So tensor networks are roughly like uh neural networks but to describe quantum systems. They’re they’re something analogous to neur neural networks but this mathematical or or physical procedure if you wish to to encode a series of cubits in a complex uh quantum system. Um yeah and this shed some light on you know how perhaps the black hole could be embedded into the boundary theory and other aspects of the holographic uh dictionary. Now in this ads CFT correspondence or in holography generally when there’s a duality between the boundary and the bulk is there reason to privilege one ontologically than the other. So for instance some will say that the bulk emerges from the boundary. Yeah. Yeah. That’s a popular thing to say now because we uh we think we understand better the boundary theory. Um but uh I I think it might be that at some point we’ll understand the bulk theory also well enough that uh we will view it as a true duality between the two things and that both will be ontologically similar. When I was speaking to Eric Verinde he said no he said the boundary is real and then I said why why if they’re dual? He said well they’re not exactly dual. Mhm. Yeah. I mean there is a sense in which the the geometrical concepts of general relativity are have some limitation right I mean the the general relativity itself is what we call an effective field theory so it’s some theory that it’s well defined at long distances but at shorter distances has some problems uh it’s not quite well defined so so we already know it should be replaced by something else however we think that we can replace it by string theory for example Um and then if you take that point of view then okay the the bulk theory in principle is also well defined. Um except that string theory is a work in progress. It’s not a theory that has been completely understood. It’s many aspects can be understood but some remain mysterious and it is possible that in the future we’ll understand this bul theory well enough so that uh will be equivalent to the understanding we have of the boundary theory. But for now that’s an that’s a speculation. Are many people working on M theory currently? Um well M theory I think is viewed as a kind of phase or a region of the of some consistent theory which also sometimes is called strength theory and and it’s a phase where it’s 11 dimensional. So and uh yeah some some people are are working on some aspects of this and I mean I I personally with Aiden Herder she wrote a paper a few years ago a couple of years ago uh talking about a very interesting proposal from M for for M theory that was u that is based on a certain matrix model. So the idea is that you can discuss scattering amplitudes in that theory using a simple quantum mechanical matrix model. Yeah. So people continue to to work on this subject. It’s part of the whole uh area of quantum gravity and some questions might be interesting to ask in that 11dimensional context. Some might be interested in in a different context. Your office at the IAS is infamously fairly empty on the walls. Why? Well, maybe I never bother too much to put something on the walls. And it does. It has it has a few pictures of uh but not too much. Yeah. Is that a reflection of your mind? You like to keep it clean or who cares? Uh yeah. Yeah. Yeah. I like to keep it clean. Keep uh you know people focused on the blackboard and keep keep us al focused on the blackboard when we come and discuss our my office. What is it like to live in your mind? I don’t know anything else. What does your wife say? Um, well, she I guess she finds sometimes a little strange our obsessions with certain things and uh that that’s same with my wife. Maybe I’m a little obsessive, I guess she says. Yeah. Yeah. Same. Yeah. Are you able to turn off your mind or do you just keep turning on problems? Uh, no. I’m I’m not able to turn it off too easily. Yeah. Yeah, that’s right. That’s one of the things she complains about. Yeah, same. Yeah. And I constantly have ideas. I constantly have to pause what we’re doing and then I have to jot it down and interrupt. I don’t know. Are you the same? Yeah. Yeah. No, I I I I disconnect, but it takes me a while. So it it so yeah. What’s your average day like? Do you have a routine where you set work from say 8:00 a.m. to 5:00 p.m. or what have you and you say I’m not going to work outside of that? Yeah, I have something like that. So yeah, usually I’m in my office from 9:00 a.m. to 7:00 p.m. Um and um yeah, I like to do exercise in the morning and you know go to the office and and the office we you know we discuss with uh various people with other researchers with post dogs visitors um and that that’s a big part of the day. I mean that’s a big part I think of scientist life is to uh talk to other people and uh you know interchange ideas and make progress together. Yeah. What exercises I usually just go jogging. Oh okay. Sorry I’m super curious about your work schedule. So, do you fix that you’re going to be alone closed door from 12 to 3:00 p.m. and you can take meetings before that or or is it just varied? No, I usually allowed myself to be interrupted. So, I like to be available and well, usually I arrange meetings with students or post dogs. It is sort of a bit more chaotic. So, and um and well, usually there are fixed times for seminars and things of that kind. What’s the difference between a good PhD student and a great PhD student? Well, a I guess a great PhD student has new ideas and you know not comes up with great problems and comes up with you know things that blow you blow your mind away. So that things that you are unexpected, things that you think are wrong when you first hear them and then eventually you realize that, oh no, that’s right. That’s a great idea. Yeah. Many times I’ve told my students that what they said were wrong and they ended up being great ideas. Why don’t you give us a flavor of that? Um well for for example uh person called Leoix came with an interesting generalization of uh this formula for the of Ruenta Kayanagi that we were talking about and I um and and I and I said well I I I thought I thought it was probably wrong and uh and and not justified and and but in the end it was correct and um yeah unfortunately he didn’t publish it the other people published it. So that that was uh one of my failures as an adviser. Uh many people may not know this but your breakthrough paper from 97. It was assigned essentially or inspired by a boring project that was given to you by Kalen your adviser. Well, why don’t you tell that story and then I want to know if you’re working on a a boring project right now that you think may lead to something. Well, I think uh of course Kurt Kalan is one of the greatest found I mean founding fathers of the standard model of particle physics and and and all that and um he had a great intuition for great problems and one of the problems he gave me was to work on uh some statistical models in or field theories in hyperbolic space. uh and yeah so that I thought that was a boring boring problem but I did it because he was my adviser he told me but it it it uh gave me some tools that then were useful later um have you assigned a boring problem to one of your PhD students that that turned out to be well no one can match your your brilliance and breakthroughs but you understand well I I I I don’t know whether my problems were as good as Kurt Cullen’s problems I yeah may maybe well I I I guess I think they are they are somewhat interesting but yeah maybe my students thought they were boring you said that you are a perpetual student. So who are you learning from right now? Well I I guess we I learned from other people. I learned from my students. I learned from uh you know our post dogs. I learned from other researchers and we were con constantly learning new things. So okay let’s be concrete. What’s something in recent memory that they taught you? Well, for example, we were talking about this uh new formulas for black hole entropy that written in uh Pennington derived. I mean, I had no idea of those techniques, those ma mathematical techniques. I had to learn them. Uh they they taught me these techniques to me. So, a new way to think about it. Uh now Cyborg, another colleague of yours mentioned that you go after problems that most people stay away from. So is that just his characterization of you? It’s it’s a mischaracterization or or would you say that’s correct? Like what is it about you? I don’t think uh well I think many people work on the same problems that I work on. I I feel um I mean people are interested in black holes. People are interested in quantum mechanics and black holes and I I feel I’ve always been in a in a community of people who have been interested in similar things. What’s a result then that you hope is true but let’s say you have see ads has plenty of evidence and there’s no proof. So I was about to say what’s something you hope is true but you have no proof of but proof is quite strong. So I I’ll be even weaker and say what’s something you hope is true that you have even little evidence of. Okay, I’m going to mention something that I hope is true. Uh some people think it’s not true. Uh and so is that uh when you have the theory of inflation, the field range is uh has finite and the field can you know in other words that the the field cannot move much during the in inflationary time. Um uh now the reason for thinking that this might be true is well various things about wormholes. So it’s not very clear. Um and and some some people most notably Eva Silverstein think that this is not true and we’ve always debated this. Um but if it were true it would be interesting because it would be a falsifiable prediction. So from ideas of quantum gravity. Um uh unfortunately I cannot argue whether it’s true or not. It it if it were true it would it would uh predict that uh the experiments in the near future would not see gravity waves. Um I mean the the experiments that look for gravity waves in the CMBB would not see gravity waves. Um one one part of me hopes that that this is true so that we could have a a prediction. Uh the the other part of me hopes that we will see gravity waves from inflation. Um I think that would be more exciting. Um uh because it would uh tell us you know quantum gravity is real and well or inflation probably happened and yeah that would be real that that that’s one of the most uh I think exciting prospects to see this this gravity waves. That was a long and somewhat technical answer to your question. You were probably hoping for a more flashy answer. Well, just so you know, this podcast, it skews quite technical. Now, the obviously it can’t just be with this many subscribers and watchers just technical, but there are many artists and truck drivers and so forth, but we skew it toward academics and researchers in physics, philosophy, math. So, speaking of flashy, you have a paper from 2 years ago or so. See, on YouTube, you can be flashy with your titles, with your thumbnails, and so forth, but there’s not many paper titles that are grabbing. And so, when I find one, I always find it fun. You have one called, oh gosh, let me let me recall this correctly. Real observers solve imaginary problems. Yeah. So, well, this this was a bit of a player on word. So the the there are some uh calculations that you can do in the sitter space which is this cos cosmology. There is a ukidian version of the sitter space and um as I as we mentioned before sometimes the ukidian uh versions of certain space times can be useful for calculating thermal properties of those spaces. Um and in this case uh it would this the uklidian version of the sitter space is a sphere and um it is supposed to calculate the the space as seen from an observer who’s in the center of that space and the thermodynamic observations of that observer. This is something that was discussed originally by Gibos and Hawin in the 70s. this part it’s closely connected to their work on black hole thermodynamics. Now when you compute so that that’s a leading classical effect and then if you compute the quantum corrections you find the funny feature that the quantity that is supposed to be the number of states or giving counting the total number of states of this this thermodynamic system um is is not uh positive. So it can have uh an i. So the i the the imaginary unit uh to some power that depends on the dimensions. So in different dimensions you get different different answers. Um sometimes it’s positive, sometimes there is an i, sometimes it’s negative and so on. Um and so the the idea is that um that if you consider a slightly more complicated uh discussion where you include an observer that is moving in this space so you don’t just do the empty space but you do the space with the observer as uh you mention mentioned this paper you know gender and longo pennington and witten they found from the lorencian point of view so from the real time point of view that it was useful to put an observer but also from this ukidian uh point of view if you put an observer then now the number uh becomes positive. Um so in that sense uh putting an observer a real observer observer that is there in the system solves this problem with the imaginary numbers that were appearing. So that’s the that’s what that title is about. And as I was going through it, does it mean that there is no view from nowhere in quantum gravity that you require an observer? Uh yeah. Yeah, that’s uh that’s what uh happens here. Even in perturbation theory around a fixed background, well in perturbation theory, it depends on the background. So it has a bit to do with the choice of time. So when you um in internal relativity there is no a priority notion of what time is. So you can redefine your times and so on. Um and but if there is something going on in your space time like uh you know for example in inflation or in cosmology where the universe is expanding then um in our universe we can think of time as the mean density of the universe. So as the universe dilutes then time progresses right and we can use that to measure the passage of time or or we can you know we can have a clock that I mean if we have an actual clock then the clock also measures the passage of time but well it’s a physical system that has some parts and it’s moving and so on and that uh measures so of of course well the somehow the physics definition of what time is the more practical definition is what clock measures The question is, can you measure time without the clock? And the idea is that you you can’t. So interesting. You would need a clock of some kind to be able to talk about time. And that’s what these observers are doing. And um they’re providing some type of clock and um and also some some location in the universe to say where you are. So we we we are where you know where the earth is and that’s our location. So you say we can’t measure time without a clock or you can’t measure duration without a clock or are those two the same to you? Um I would say the same deal view. Yeah. So then would you also say that we can’t measure length without a ruler? Yeah. Or is that somehow different in this? No no it’s the same. Yeah. when people talk about general relativity even in Einstein thinking of general relativity when he was imagining a system of rulers of of observers that were moving in uh space and they were exchanging information synchronizing their clocks and you know comparing the rulers and so on. Um and in the classical theory you can consider those observers as having no mass and no energy and not doing anything. But once you consider the quantum theory, you cannot do that. And you have to take take them into account as physical systems. And so it’s it’s part of what makes the quantum theory a little more complicated. What’s the justification for having a single minus sign? So in other words, a a single time direction. Why not have two time directions or three or what have you? Um I don’t have a good answer. So with a single minus um we understand what the theory is like and we are used to that and um I don’t know if it is possible to have more minus signs. I mean it gets very confusing and so on but just just that it is confusing doesn’t mean it’s impossible but so I I don’t know if you can uh it’s not what we seem to have in nature but uh yeah I mean I know there are two times theories like bars I believe um well it’s true that sometimes uh you can introduce more than one time um and this is an idea that goes back to drag for in his treatment of uh scaling varian theories like electromagnetism and so on. Um but usually if you do that then you also have uh the second second time together with some special coordinates form what’s sometimes called projective coordinates which uh where where you eventually end up removing again the time that you introduced. So you somehow introduce one extra time and then you end up removing it again. But it’s useful to introduce it to make manifest some of the symmetries of this the theory and and and it’s true. I mean that I don’t view this as a fundamentally new introduction of time. I mean that that is something we understand and you know I’ve used it myself in some of the papers but um I I guess the question is whether really we have more than one physical time that I I I don’t know. Hi everyone. Hope you’re enjoying today’s episode. If you’re hungry for deeper dives into physics, AI, consciousness, philosophy, along with my personal reflections, you’ll find it all on my Substack. Subscribers get first access to new episodes, new posts, as well, behind the scenes insights, and the chance to be a part of a thriving community of like-minded pilgrimmers. By joining, you’ll directly be supporting my work, and helping keep these conversations at the cutting edge. So, click the link on screen here, hit subscribe, and let’s keep pushing the boundaries of knowledge together. Thank you, and enjoy the show. Just so you know, if you’re listening, it’s cur ji mu n g- a l.org. kurtjongle.org. Spacetime is doomed. Um well I I I think uh that that tries to echo uh the Mikoski’s dictim that space is doom is I mean we we won’t have space again we won’t have time we have now spacetime and now they say well spacetime is doomed um what that means is that um the the new theory of you know quantum spacetime will have to use a different concept that there will a new concept and we don’t really know what the new concept is. So um that that’s that’s what this quotation tries to emphasize and what do you make of it? Uh it’s a challenge for us to find the new concept that there should be a new fundamental concept and I I would say that we don’t know what the new fundamental concept of quantum gravity is. So uh the the basic concept in the same way that spacetime is the basic concept for general relativity um we don’t know what the fundamental concept for quantum gravity is two approaches come to mind so one is Verendai’s entropic gravity yeah and then another is nema’s positive geometries so have you looked deeply into them enough to talk about what interests you about them and potentially what disinterests you about Well, the I think this idea of entropic gravity seems to be um very useful and relevant when we think about what happens near a black hole horizon and it’s uh related to this uh interesting uh connections between gravitational and thermodynamics in the context of near black hole horizons. It’s not clear how this um extends beyond uh you know black hole horizons. Um and also more recently there as we were saying there is a deeper connection between quantum information and and gravity. So it’s not just uh thermodynamics or just entropy is just this more fine grained uh concept that that’s probably what um Eric Verinda thinks has in mind but uh yeah so maybe some version of that might might be true. I mean all these are ideas for some some ways to get at the correct structure and Nema’s Nema’s discussion is connected a bit to to the well it’s the idea that you study scattering amplitudes and by from far away and try to reconstruct the spaceime uh where that scattering is occurring and determine the scattering amplitude through some other alternative theory or other other alternative principles where and then the um the usual space time will come out uh yeah out of something else but but we we haven’t yet found it so there there it’s certainly something people are I mean it’s it’s a challenge as as we said and people are trying to meet this challenge in different ways yeah and those those are well you know interesting ideas and in particular this idea of uh looking at the deep mathematical structures and scattering amplitudes is very promising because you know you might or might not find spacetime but you’ll definitely understand and and people have definitely understood the scattering amplitudes in ways that uh you know are very useful and powerful for colliders. So I mean these methods that they’ve developed are are now used in uh particle accelerators um and and are useful for computing other things like cosmological observations and and many other applications. What is it about Nema that you admire? like what’s different about it? Well, he has unbounded energy. He has an amount of energy that is is pretty amazing. and uh and he can go from some mathematical you know discussions about the structure of grasmanions or whatever they are and uh to uh aspects of phenomenology in particle physics and he’s follow he is able to follow many different subjects uh he’s he’s he’s very amazing actually my brother who is at the University of Toronto is a professor in math finance was also at the University of Toronto as an undergrad with Nema and said that I’ve always remembered Nema’s energy. Yeah. Yeah. So, what about Whitten? What is it about Whitten that you admire? What is it like to collaborate with Whitten? Um, well, it’s it’s wonderful. I mean, he he’s very fast uh and yeah, very deep as I I was saying. Yes, it’s really great. He has a depth of understanding that is uh you know light years ahead from definitely from mine. Yeah. Uh again he he has a both both a knowledge of math and physics that is amazingly broad and amazingly deep. You know many people you can be deep but not very broad but you but you can be very broad and not very deep. But he’s both things in an amazing way. you know he he knows things better than the specialists in those fields. So and uh than this in many many different fields. So Strowmaner had a paper recently February of this year I believe where he used GPT one of the models of GPT to to find glue on amplitudes. Now do you use AI? Do you hope to use AI? Uh yeah I hope to use AI. I I I use it in a simple way. No, I I never I haven’t written a paper with it. I I use it to help me, you know, with some calculations, formulas as a as probably you and many of the of our listeners are are using AI. Um uh yeah I think I found a really interesting way to use it and uh and they found an interesting formula and I’m I’m actually discussing this formula with one of our with a collaborator of Andes who’s a postto at our institute uh Alfredo Gavara. So we are um yeah we’re analyzing some other we’re doing some follow-up work on on this paper and studying some of the consequences of this formula. When I was speaking to Edward Frankle, he said that who also collaborated with Whitten. He said Whitten speaks in fully formed sentences that are like an essay where each word matters. Yeah, it’s true. I mean, he he understands it completely and then then then you sometimes say, “Oh, no, but it’s better to think about it this other way.” And then you realize, “No, no, really, the way that Edward said it in the beginning, that that was the best way to say.” Yeah. So yeah. So going back to AI, so for LLMs, like what do you personally use them for? Do you use them for idea generation or do you just you use them to learn about some topic or just something mundane like, hey, make this make this recipe for me for different amounts of people? Well, maybe learning about topics I don’t know that are well known so that you know, you know, as as a better Google search as um um I do some formulas with it. I I asked it to check some formulas and um I mean for doing integrals it can be better than mathematica sometimes uh but then then you check it with Mathematica. So Mathematica is this uh computer program of symbolic manipulation that we we use it. So so it’s good for you know discovering new formulas and proposing new formulas. Now it’s not always right and it’s can can make mistake as as as the warning sign tells you it’s true. Yes. Uh but it’s useful. It’s it’s very useful. I I I I have no doubt that it will be more useful in the future. Yes. Yeah. The progress in the past few years is remarkable. Yes. So as you know the podcast is watched by many researchers, many PhD students, many blah blah blah. So many of them will like to know how the heck do those in the top of their fields use AI. So you just mentioned that you use it to teach yourself and potentially to generate new formulas and so forth. Can you be more specific because there are some ways of using it that are poor and there are some ways I’m sure that are better than others. I would advise people not not to look not to imitate what I do in AI. I I think uh I think maybe I feel already like a dinosaur that I’m not learning it fast enough. Um and uh you should try it and you should uh explore yourselves and uh find new ways uh yourselves to to to do it. Um I I don’t have a good answer myself. I I haven’t found myself a a really good way to use it. I’m sure that there are wonderful ways to use it. uh it’s a very it’s a powerful technology and I really encourage you to experiment. I tell I tell this to my students, you know, they come and they say, “Well, I tried this calculation. Did you did you try it with AI? Did you Yeah. Sir, what’s a piece of advice that you’ve received? Could be from your advisor, could be from anyone that you keep coming back to or that you pass on to your students?” Um well it sounds tright but just be open-minded and uh you know try to learn question things uh understand things deeply don’t don’t go too fast just uh you know don’t repeat what everyone says just understand things uh your own way I think those are those are very important uh things to to Can you tell me about some way of understanding that was someone else’s way didn’t work for you and then you reformulated it to your own way? It’s it’s hard to give an example but it it often happens that people um sometimes sometimes there is something called lore right that uh you know people in the field maybe repeat but they haven’t really thought about through very much and and and then if you you know work through something and do the calculations from the beginning go to basics and you you you you really understand it and It’s just that people were repeating things without having done the calculation without really having understanding it without having understood it. Now examples uh well I remember when I started working on cosmological perturbation theory I found lots of confusing things and I was very confused about uh how these calculations were done in a curve space in an expanding universe and so on. I had to go back to basics, you know, had to do it with a harmonic oscillator, a time dependent harmonic oscillator and so on. And that that was that was useful to me. And um I I feel sometimes in some pieces in the literature, people were, you know, inventing some grand general schemes that were not not respecting the were not working for the simplest case of uh for the harmonic oscillator. Um yeah, and that was useful for me to go through this just to understand and develop things yourself. And then once you you understand things by yourself, you understand what other people are saying, right? You say, “Oh, this is what this paper was saying.” And you you you can then read the other paper other other papers and understand what they are saying. There’s a lecture that you gave at Harvard called the Chiloquium if I’m not mistaken. And in it, you said that you didn’t feel good enough as a graduate student. Mhm. And many graduate students feel like that. So what changed for you? Um well I I yeah I think I think it’s common uh maybe not to feel good and to feel that um maybe you’re not good enough you know there are the the people from the past that did some great things but this is not you know your time now things are more difficult and um and yeah many people go perhaps to graduate school thinking that they will go in and do all kinds of uh you know amazing in discoveries from day one and the you know the reality is that you uh you need to learn many things you need to know what’s known you need to and well it takes some time but well eventually you’ll you’ll make your contributions feel first there will be you know small contributions then maybe a slightly bigger contributions you’ll become uh specialist in your field and then maybe you’ll have a great idea at some point you’ll uh make an important contribution and And I think if you if you are in your career in science, you’ll you’ll make some of these contributions along the way. And sometimes you’ll make them in the beginning of the career. Sometimes you’ll you know wait long you make but eventually you’ll make uh big contributions and and sometimes maybe you make contributions which you didn’t realize they were big and then they become bigger later and uh Yes. Yeah. And uh and yeah maybe some things that were thought to be big then they are not big. So we’ll see. uh but it’s all part of uh contributing to science and I think um yeah that’s uh that’s what’s important of course there is the issue of careers and finding a job and so on and uh that’s uh always challenging. Uh did you once tell Lisa Randall that you’d like to write a science fiction paper? Uh it is possible. Uh oh yes yes yes I did. Yeah. So um yeah yeah you I yeah I wrote a kind of science fiction paper uh on this traversible wormholes. So the the the stuff we were talking about before uh is relate related to traversible wormholes was using one of her ideas. So uh something called the random model. Yes. Yeah. And to be clear to people who didn’t read it, it’s not science fiction in the sense that it has a a hero and then an an antagonist, an evil villain. No, there’s no villains. It’s just it’s just the the discussion of this traversible wormholes assuming you know this um okay so the random model is a possibility for physics beyond the standard model and there are various versions of it and there’s one particular version which uh allows for the construction of worm traversible wormhole solutions and uh I mean part of uh the work that Brandon and Sundum did was that to notice that uh those particular uh models for physics beyond the standard model were compatible with all current measurements. Um no and well of course they make some new predictions for the future but they at least compatible with all the current things we know. So uh they’re not ruled out and so one of those versions which are not ruled out could allow for traversible wormholes. of course that they are not ruled out doesn’t mean that they are likely to exist that and and also it doesn’t mean that constructing these wormholes I mean we constructed mathematically the worst as solutions uh we didn’t give a physical procedure for for making them so so it’s a mathematical curiosity if you wish but it it um it it kind of shows that um it gives an interesting scenario where traversible warm could exist and would be relatively big. Professor, I just wanted to thank you for spending so much time with me and I appreciate it. Sure. It’s been a pleasure and uh good luck. Good luck to everyone. Hi there, Kurt here. If you’d like more content from Theories of Everything and the very best listening experience, then be sure to check out my Substack at curtjongle.org. Some of the top perks are that every week you get brand new episodes ahead of time. 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