 Okay, it's a great pleasure for me and honor to be here with Juan Maldesena in conversations about physics and how to do physics and about the ICTP. So Juan Maldesena is giving the Salam distinguished lectures here at the International Center for Theoretical Physics. We are in fact at the moment sitting in the Salam's office, Palma office and he has been telling us about connections between quantum mechanics and space time and very exciting developments at the very frontiers of physics. He is a professor at the Institute for Advanced Study in Princeton and is widely regarded as one of the leading physicists of his generation and he has made pioneering and very original contributions to several aspects of quantum gravity and cosmology and in string theory and perhaps his most influential work is what is known as the ADS-CFT Holographic Correspondence which has been a very active field of research for the last 20 years or so. So maybe let me start my questions starting with the ADS-CFT correspondence. So at some level it's a, I think for all the practitioners in this field, it's clearly something very deep that we have uncovered about an equivalent, quantum equivalence between a theory of gravity and the theory without gravity but sometimes I get a feeling that even our colleagues in the very nearby fields don't fully appreciate how remarkable is this equivalence and even more so for a common person on the street. So maybe can you try to bring out what you think are the most, a bit about the ADS-CFT correspondence? Yeah, so this is a relationship between a theory of interacting particles and a theory of gravity and it has a few interesting aspects. So one of them is that the theory of gravity seems to live in more dimensions than the theory of interacting particles. So somehow it looks as if one of the dimensions of space-time arises in an emergent way, so emergent is like in some approximate way or some way that was not manifest in the original description. And this also has enabled us to understand the aspects of black holes because you can have a black hole in the interior and it's described by a thermal system or some system in the boundary theory. And then this leads to a description which is fully consistent with the loss of quantum physics because it's embedded in the black hole in the ordinary theory of quantum physics. But also it seems to give an interesting picture for what space-time is. So it looks like the degrees of freedom that describe space-time do not live, there are no atoms for space-time that live locally in space but they live at the boundary far away. And while this is an aspect of quantum gravity, I think we are yet to discover the true lessons of this for cosmology for example and for the big bang singularity and so on that they probably will have some lessons or maybe it's even weirder than this idea CFD where so in this correspondence the direction that is emergent is the radial direction so it's a space-like direction. In cosmology we expect that perhaps the emergent direction is the time directions there might be some description perhaps without time and then yeah well now I'm speculating about the applications to cosmology. It's a very interesting question. But even without speculation so we've come to this realization by thinking about objects in strength theory, these d-brains so-called d-brains which are some type of soliton or some solutions of strength theory that have a very precise description found by Polchinski and using them and also the relationship between them and solutions of gravity, black brains and black holes, one was led to this correspondence but we still don't fully understand how it works so we think it's true and we can check some of the consequences of the relationship, some detailed mathematical formulas so if the relationship is true then you derive that certain mathematical equalities should hold and they do hold but in general the relationship relates strongly interacting systems of quantum particles to some gravitational systems some let's say Einstein gravity systems but one of the issues is that it's very hard to study this interacting systems of quantum particles so it's hard to find an explicit correspondence or the explicit map between the two variables. So in physics sometimes we have different descriptions of the same phenomenon so I mean the very simple case is that if we have a signal that varies in time we can give the signal as how it changes in time or we can give all its frequency components for example and those two descriptions of the signal are equivalent and this is an equivalence we understand in great detail and we understand completely explicitly and but in this other case we don't have such an explicit understanding that would be one of the goals would be to have an explicit understanding or mapping between the two types of variables that the variables we use to describe space time like metrics and the geometry of space time and the variables we used to describe the quantum systems the quantum mechanical system so yes I think this your work has influenced the work of many of us in the field last year in Princeton there was this 20 years of ADSRT and it was still and all the talks there were still it doesn't look like this idea has run out of steam so if you look at this last 20 years what development has sort of surprised you the most? Well there were many interesting surprises I there were surprises for me in terms of applications perhaps the application to condense matter is for me was the most surprising I thought that the systems are very different but nevertheless they were interesting lessons for condense matter and vice versa the condense matter physicists have given us also interesting perspectives in terms of the same more formal developments I think this connection between entanglement and geometry perhaps the best example is there's a formula that Ruh and Takayanagi derive for computing the entanglement entropy of a region that is a kind of generalization of the famous Hockenbeckenstein formula which computes the entropy of a black hole and so yeah I think those are yeah that that's that was an interesting development that has led to again surprising more surprising connections so the idea that entanglement is fundamental for determining the structure of space time then you can have connected space times even though the Hamiltonian interactions are not even that though the two systems are not connected so somehow space time seems to capture the correlations present in the quantum state so it's a way as a way to describe the the correlations present in the way function yes actually that brings me to a question that also I have asked myself sometimes that so at some level ADS safety correspondence is to teach us something about quantum gravity about black holes and so on and but there is another point of view that maybe ADS safety is just a kind of a framework for studying strongly cup you know it's not like the gravity is just some kind of effective description right right and how do you view what is your take on this do you view well ADS safety is something deep about gravity or do you regard this as a kind of a framework for gravity emerges as just effective description well yeah I think if I think I think the question is whether gravity is a bit like hydrodynamics so hydrodynamics is a theory that we used to describe fluids but hydrodynamics is not the most basic description of the system so we have some interacting particles that interact the loss of quantum mechanics or classical physics or whatever and then long distances or for insufficient in certain circumstances we can also have a hydrodynamic description the hydrodynamic description is in some sense approximate it can fail sometimes you can phone shock waves you can perhaps have some singularity so and and we are not surprised by this and it's also description where we lose information where entropy can increase so on now one perspective is that well maybe gravity something similar so gravity is an effective description of the quantum system and it will be approximate and well it breaks down sometimes and well it might be then replaced by something else and there's something else that replaces is the quantum system so you have to go back to the quantum system so that's a perspective that is actually quite popular and some people think about it this way but even if you accept this perspective it's a kind of effective description that keeps more information about the entanglement of the system and even microscopic aspects of entanglement so for example we just discussed this Rittak Kyanagi formula this formula is not computing the entropy of the system the usual entropy of the system is computing the microscopic entropy of the system defined so called fine-grained entropy and so that to me tells me that the gravity knows more is keeping some very detailed information now you can say well okay I'm skipping this data information but only the total value it doesn't know about the particular microstate so you could also say that but well it's a little more than hydrodynamics at least in that sense and I think the original idea was that it was a true duality that there were two possible descriptions one was the gravitational description and certainly strength theory gives us full perturbative description that makes sense and we think that even non-perturbatively we can define this theory and for some observers they some very supersymmetric observables you can actually do the full computation and some this perturbation series so that that was always viewed as the evidence that there is a theory of gravity that is independently defined it could be defined by strength theory for example and yeah I'm inclined to view this the second option I think gravity is too beautiful and too interesting to be an effective theory and I suspect that maybe once well this is now speculation well the previous statement was also speculation that once we understand gravity well enough actually the quantum system will be some will be immersion will appear when we cut the gravity description into two pieces so we we introduce some boundaries and we separate we separate the full gravity solution into a piece that we want to describe and the rest that we view as measuring or some classical aspect this classical let's say measurement apparatus that is measurement measuring the I see that's an interesting point of view but entanglement yes somehow you break the entanglement a bit yes yes okay so we're coming to I mean we'll come back to science but before that I also wanted to ask you a bit about so ICTP has this other important aspect to its mission that on one hand we have excellence in science right and on the other hand we have inclusion right of people who are normally left out of the scientific right and that was certainly the guiding principle for right and you yourself come from Argentina you have did your early work in Argentina then you went to Princeton so and I know that you are also a frequent visitor I think I remember also giving lectures with you a long time ago you have been here maybe I don't know 20 times 10 times yeah and you are the member of our scientific council so you have sort of seen ICTP is functioning so yeah so do you well I think ICTP plays a real singular role in connecting science to the countries to let's say well the old days they were called third world let's say least developed countries developing countries and then and indeed it had an important role to play well in Argentina it was pretty important for scientists to connect to the developments in the latest developments in science so for example that there was my my former advisor in Argentina he well did some his PhD in Argentina on some aspects of statistical mechanics and then he came here to the ICTP to the postdoc and at the time everyone here in the ICTP was very excited about string theory was one of the main well it was after 1984 with all these developments and so he came here and everyone was doing string theory so well he joined if he wanted to talk to everyone anyone here and well he he he learned string theory here he did some work on those string theory subjects here then he went back to Argentina and started a group doing string theory and well I joined them and I during my let's say master thesis early bachelor thesis and so on so yeah and that was very useful for me and also for other people in Argentina another example is Horacio Cassini so he also had a similar story yeah yeah so he did his work on particle physics in Argentina then he came here to the ICTP he started here working on entanglement in quantum field theory and well then he went back to Argentina and continued working for this field for this field for a long time and not many people were paying attention to this field and and he he and also Marina Huerta well they both came here and went back to Argentina they they obtained really beautiful results and using these methods that revealed as deep truths about quantum field theory so basically what they did was to show that there are certain certain quantities related to entanglement well and basically entanglement tropists are monotonic when you sort of decrease when you go in the RG flow direction so when you coarse grain theory and people did this was kind of a holy grail in quantum field theory was to find quantities that decrease when you follow the sea renormalization from yeah that's right so in two dimensions it was known that it was true and they found the proof of this in three dimensions they also prove the old results into dimensions using these techniques and also in four dimensions which also had previously been shown using other things so but they had a unified description of all dimensions yes using using all these techniques and well they found also many other things interesting things related to entanglement and he well they also created a group in yes and lectured last spring school yeah okay yeah and you see that they well it comes back to here to the ICP and yeah so so these are actually are two really very important sort of success stories I mean in a even in the indirect waves just guiding somebody in the right direction or sending people in the yeah exciting directions of science has this big effect on right right I mean the one of the ideas of ICP was to be a place where scientists from from developing countries can meet and one of the difficulties of doing science in the developing countries is the lack of connections and the lack of well the lack of communication so of course now with the internet you can you can see people's papers and so on but there isn't a face-to-face contact what is exciting yeah and what is exciting and the news and and all that and one of the ideas of ICP was to serve as a place where this could happen and I think it's working yeah it's working well if it has led to ADS-CFT success yeah so and okay I mean and what yeah so this is certainly a one very important aspect of ICTP's mission but okay now as a leader in this field and also being for example an advisor to the ICTP what areas of I mean where do you think ICTP is doing a very good job and or where we could improve what do you have any thoughts on that about well I think the the schools are an important part the associates programs it's also an important part yeah I think what is most those are yeah I think those are very very effective programs and I think they just to bring people here and yeah yeah yeah yeah bring people here making what the talk to each other interact with the the faculty here interact with other visitors that come for schools so in a meeting place as a meeting place I think that was actually his original yeah and I think yeah because it's a it's a question to be asked because you know in 50 years things have changed as you said countries which were regarded as third world are now the different bracket in India and China or big countries are in a different bracket so one needs to rethink about how to reorient ICTP's mission a little bit but right you said that some of these aspects still remain true today and right right and maybe we can do things different maybe maybe we can get money try to persuade for example one in this initiative in Brazil I think that's one way to spread our mission or yeah so that's another thing that the CTP was doing is to create sort of daughter centers yeah so that use the expertise that has been built here building an international institution to create other more local regional international centers for example I've been involved with one in Brazil there is also a similar one there that was started later in Argentina of creating a place where again would be a place for people to meet within the country and perhaps with neighboring countries perhaps bringing scientists from abroad so a center that would work well with international standards and the ICTP can guide them to to do this can oversee it yeah so and it's it's working developing and I think the CTP has done this in various places of the world I'm only involved in these two places yes also we have in yeah now we have an important Beijing yeah okay so maybe coming back to science when you see briefly mentioned I mean clearly one of the very exciting scientific developments has to do with cosmology and anti-disseter space that where the ADS-CFT correspondence comes and the deseter space in which we seem to be living in they're kind of closely related but at the same time they're very far from each other yeah yeah one has yeah one has somehow positive what we call positive curvature and the other one has negative curvature so we got the sign wrong you say why do you study this one if we live in a positive curve space with positive curvature why do we study the one with negative curvature and the reason is that for the one with negative curvature we understand more and in physics sometimes it's easier to well it's useful to study a case where at least you can say something then get some intuition perhaps we can say something also for the case with positive curvature in many ways they are not they're not too different I mean in fact if you do calculations purely in gravity you can translate between the calculations done in both cases just the level of solving the classical equations and so on if you solve the classical equations with negative curvature you can immediately get the solution to sign yeah you flip the sign you don't have to resolve the equation same same solutions can be come apply for the other case and indeed this led to some interesting ways of thinking about cosmological correlators so it's cosmological observables right that are computing in inflation so indeed the period of inflation is the period where this universe was well approximated by the cedar space and yeah so but we don't have clear examples of a full duality where you have you know the full quantum system for a system that is due to Einstein gravity there are some examples for systems that are related to higher spin gravity so some other kind of theory of gravity sometimes called so also was illic gravity where where you don't only have the gravity on but you have any number of particles with higher spins so yeah yeah certainly I guess this exploration of non-gaussian it is in cosmology was kind of inspired by you could say why yeah yeah yeah that's right that's okay but one has not had this something as concrete as the idea safety cosponsor yeah that's right that's right yeah the non-gaussian it is heard the well so you have the primordial fluctuations that have been studied using what they did they printed in the cosmic microwave background and are very close to Gaussian but non-gaussian it is a small statistical deviations from let's say the bell curve have have the potential to teach us something about the interactions that were present during inflation and yeah this is an this was an active field I mean people are looking for this non-gaussian it is they've been looking for this non-gaussian it is for a long time and yeah and hopefully with the new newer experiments will have the statistical power to to see this tiny non-gaussian it is so what we know is now is that well the non-gaussian it's not big they're very small but hopefully they will see them at some point and teaches something about the very early universe I mean this is the best probe that we have of the very early universe and we can call we can call this we can think of this as a kind of cosmological accelerator accelerator which or microscope that is allowing us to look at the universe when it was very small yeah the highest possible the highest energies that could be accessible by observations and and so the roughly speaking the non-gaussian it is tell us about the particle collisions that occurred in the very early universe so there were there were particles or waves in the very early universe and their collisions produce some non-linearities and that's this non-gaussian that they get translated in our present observations as this deviation that is statistical deviations from Gaussian anything I have another question about sort of maybe it's a bit about sociology of science so in the past there was kind of a much more direct link between theory and observation meaning whatever you imagine you could more or less imagine doing an experiment to your time or oftentimes the data was ahead of the theory and prompted we are somehow in a kind of a peculiar historical situation where our theories are extremely powerful and their structure is so rigid that we have great confidence for example the Higgs boson is an example where most more or less all physicists were convinced that okay spontaneous symmetric broken gage theories is the right framework okay how the details will work out they were the you know an electroveic unification salamence was responsible for it but it took 50 years and billions of dollars and thousands of physicists working on it so there is clearly some additional criteria of logical consistency and which is as important as verification I mean even before it want to think about verifying it with an experiment right so that seems to guide the research and it's even more true in string theory so do you want to sort of comment on this that how does one in a world where we don't have immediate contact with experiment how does a physicist choose good problems or right right right so as you said the the electrobic theory was an example where people could have some idea of how physics could be at the distances much shorter than the ones they could measure at the time and indeed it took many years to get to those shorter distances string theory or quantum gravity involves a shamping distances which is much bigger much bigger so it's like roughly the shamping distances between our scale and the scales that the LHC can probe today right so if we if we shrunk to those scales then quantum gravity would we have to build another LHC yeah that's right but so you might think well you I mean many things could happen and indeed the many things would happen but we think that the the the framework we could use through all those scales is the framework of quantum field theory and that framework would have to be changed when we get to gravity and and that that I think we we know and and and string theory tries to use a theory that for how this framework could be changed and I think that it's very difficult to put together quantum quantum mechanics and gravity and the fact that string theory managed to do it says that well this is only an interesting example at least to study and that it's a problem that looks so constrained that they might have a simple unique solution or much like in case of quantum gauge theory yeah yeah or at least we might have a unique framework or we maybe the framework is more general than string theory maybe it's not the current understanding of string theory but it tells us that string theory is only an interesting example or toy model to study or at the very least it's that it might be the full thing but I think our current understanding of string theory is probably to elementary but yeah and it's a worthwhile problem that can show and and perhaps one of the surprises is that by thinking about this very abstract problem you learn some things about ordinary problems yes that you could teach you an example yesterday yeah that's right that it could teach you something about quantum field theory about what strong interacting quantum systems and so I didn't have to be this way yes maybe it was something so there is theory but it turns out this so there is very closely connected to ordinary physical theories it's perhaps not surprising because actually string theory arose not from some esoteric thinking but by trying to fit you know string the strings that were observed in experiments the strings of quantum chromodynamics so and so it's closely connected connected to physics yeah in fact the EDS CFT is an example because the CFT is a very close cousin in many of this EDS 5 example is a very close cousin of the quantum chromodynamics which is sort of a glue that holds the nucleus together yeah it's closely related to it's not exactly quantum chromodynamics but with some minor modification that is closely related to theory of gravity so I think there is that connection but okay but what are the criteria sort of I mean if you were to if a young graduate student is asking yeah question well I think what guides your well I think the criterion is that the community of the community in general yeah it's so I like to joke the strings sounds it's an acronym that means solid theoretical research into natural geometric structures and that's roughly the thinking of the community in the sense that well you you have you can theorize how much you want but it has to be at least mathematical it has to be mathematical consistent there has to be some solid formula and the formula might be have very narrow application or it might only hold for some very special thing but at least it has to be correct within at least self-consistent if we use theorize so much that the formula even is not self-consistent yeah then there is no formula then then it's a little more suspect and I mean if it is officially interesting we can totally no formula but and and also also some natural geometric structures what does that mean I mean it's there surprising thing about nature is that there are some theories that are very powerful like quantum field theory that describes describe systems of interacting particles describes the standard model it's incredible successes and this is a structure that has lots of subtlety and and you can study it on its own and and much of the the work in our field is actually understanding aspects of quantum field theory even before trying to understand quantum gravity and understanding those aspects will probably teach us something about quantum gravity because any theory of quantum gravity has to recover quantum field theory and general properties of quantum field theory and so and and I called it geometric because you know by special relativity we know that what quantum filter is the the theory that puts together space time so the space time of general special relativity and quantum mechanics together and that does this successfully and well quantum gravity should do that but with general relativity quantum mechanics and general relativity together so it's only a first step and well of course general relativity is a theory of geometry and geometry is very important their interest in geometries like black holes and cosmological geometries that they are still very confused they are still very confusing they were very confusing classically so it took 50 years to understand the Schwarzschild solution so the simplest solution of Einstein's equations it took 50 years to understand what the classical geometry was not to detect the gravitational waves yeah yeah this is one but I'm now talking about the theory so just theoretically understanding what the geometry that the Schwarzschild solution describes took 50 years now of course it took many more years to understand how to see black holes in nature and whether black holes are are or not present in nature and well now we have great evidence that they are present but perhaps the most beautiful evidence is from the gravity wave detection that we had very recently and yeah and probably we'll understand those those things better and better but maybe there are other exotic objects that we haven't thought about waiting to be discovered yeah I mean I guess one comment I want to say that somehow when a common person says theory you know I think the physicists when they mean theory they really have some quite elaborate and very closely fitting structure in mind like quantum field theory but as for a common person theory means okay I have a theory that it will rain today or I have a theory that so sometimes there is a yeah so that the fact that the the theories we make of physics have to be consistent with all previous experiments and our previous observations puts a huge constraint how huge that constraint is sometimes yeah yeah it's a very very big constraint so the constraints also well just the constraints of special relativity and quantum mechanics are very very strong and they lead to quantum field theory and two particles yeah yeah they lead to antiparticles and predictions of this kind so yeah antiparticles is a generic general prediction of quantum field theory there they are theory independent in the sense that I mean they are present for electrodynamics for chromodynamics for yeah so it just follows from the theoretical constraints of the theory yeah that's right very general so I mean another example is Hawking radiation yes that's a that's another example that just quantum field theory in the presence of horizons and so on predicts this thermal radiation that's also generic prediction of quantum field theory I mean there's a very close casting of Hawking radiation where you don't have to talk about the black hole which is to even be in flat space and you are an accelerating observer in flat space sometimes called radiation so that the accelerating observer would see a temperature that fact is also a very generic prediction of quantum field theory they also closely related is this primordial perturbations and then the quantum fluctuations that right right right right yeah that's right yeah and and this yeah maybe I maybe one could say emphasize the fact that now people sometimes say okay well Hawking radiation okay hasn't been seen if you talk a lot about Hawking radio why do you care about Hawking radiation you can calculate it for astrophysical black holes and it's very very tiny you'll never see it at least in the black holes that have masses for the solar masses which are the ones that naturally form so why do you study such a silly thing so but the point is that this this effect is also present during cosmology very closely and it's it's the same thing it's the same the same thing there is a cosmological horizon and it's basically a same effect and and that's the fact is it's crucial for describing aspects of our universe so the the fact that the universe is not perfectly homogeneous and and it describes the long-distance structure of the universe very well so it's not that this effect plays a central role in explaining how the universe is like like it is all the galaxies why they are so yeah not uniform structures in the observe it somehow came from this very subtle quantum yeah situation which you think is why is it important but it's actually all important yeah yeah it's okay so and so maybe I should ask you maybe the final question I should ask you so you've been talking I guess your lectures alarm lectures are on sort of some of your recent work on quantum entanglement and space time it's a very beautiful connection between entanglement and quantum mechanics and geometry of space time yeah so maybe can you tell us a bit about what are the exciting future directions for your research of all these developments that you have been recently occupied with can you see something about that well I think well in this relationship between entanglement and geometry is basically understanding how how the basic quantum description of space time builds the space time and part you could say understanding how how holography works part is to understand better the interior of black holes because the interior to describe the interior of the black hole this disconnection to entanglement seems crucial and to solve to solve black whole paradoxes and so on seems to be an important element and we don't understand it well enough to solve all the paradoxes we but we do understand that it is important for thinking about the paradoxes so for example going back to the Schwarzschild solution one aspect that was understood about the Schwarzschild solution is that it describes two black holes and the idea is that these two black holes are due to separate quantum systems but they are in an entangled state so understanding more this connection is seems to be interesting that's now where where is it leading so I think it's hopefully leading to a new picture or some picture of what space time is and how space the space time and the quantum are related so because it's interesting that you can have a connected space time that comes from this connected quantum systems because it's saying that the local physical loss of that you see in this universe which is some some some evolution law and so on are somehow present in the patterns of entanglement of the quantum system they are internal to that pattern and how to think about this is well what we are trying to understand yeah so they are sort of connected in this extra dimension that even though the systems are far away from each other there is this extra dimension that we talked about right right right and they're connected in that right extra space time dimension and yeah and this this connection is has some interesting implications for quantum teleportation which is a general quantum mechanical property of the systems of entangled systems and yeah and hopefully one will learn to use some of these ideas for cosmology in some way that's always that's always in the back of your mind yeah that's in the back of everybody's mind how to apply some of these things to cosmology there hasn't been a killer application but yeah given the history of string theory you know you know it was applied I mean the I mean we have already seen an example which was Hawking radiation right this car this car for black holes then apply to cosmology and yes okay it was really exciting and to hear I mean it was very thank you for a very nice pedagogical introduction to your ideas and work and it was a great fun to talk to you yeah thank you I look forward to be here yeah