Andrew Strominger: Black Holes, Quantum Gravity, and Theoretical Physics | Lex Fridman Podcast #359
a black hole is a mirror and the way it's a mirror is if light a photon bounces off your face towards the black hole and it goes straight to the black hole just Falls in you never see it again but if it just misses the black hole it'll swing around the back and come back to you and you see yourself from the photon that went around the back of the black hole but not only can that happen the black hole the photon can swing around twice and come back so you actually see an infinite number of copies of yourself the following is a conversation with Andrew strominger theoretical physicist at Harvard whose research seeks to shed light on the unification of fundamental laws of nature the origin of the universe and the quantum structure of black holes and event Horizons this is the Lux Friedman podcast the supported please check out our sponsors in the description and now dear friends here's Andrew strominger you are part of the Harvard black hole initiative which has theoretical physicists experimentalists and even philosophers so let me ask the big question what is a black hole from a theoretical from an experimental uh maybe even from a philosophical perspective so a black hole is defined theoretically as a region of space-time from which light can never Escape therefore it's black now that's just the starting point many weird things uh follow from that basic definition but that is that is the basic definition what is light they can't escape from a black hole well light is uh you know the stuff that comes out of the Sun that stuff that goes into your eyes light is one of the the stuff that disappears when the lights go off this is stuff that appears when the lights come on um of course that could give you a Beth a medical definition but or physical mathematical definition but I think it's something that we uh will understand very intuitively what is light black holes on the other hand we don't understand intuitively they're very weird and one of the questions is about black holes which I think you were alluding to is you know why doesn't light get out or how is it that there can be a region of space-time from which light can't escape it definitely happens we've seen those regions we have spectacular pictures especially in the last several years of those regions um they're there in fact they're up in the sky thousands or millions of them we don't yet know how many um but the proper explanation of why light doesn't escape from uh a black hole is still a matter of some debate um and one explanation which perhaps Einstein might have given is that light carries energy you know it carries energy because you know we have uh photo cells and we can take the light from the sun and collect it turn it into electricity so there's energy in light and anything that carries energy is subject to a gravitational pull gravity will pull at anything with energy now it turns out that the gravital gravitational pull exerted by an object uh is proportional to its mass and so if you get enough Mass in a small enough region um you you can prevent light from escaping and let me flesh that out a little more um if you're on the Earth and you're on a rocket ship leaving this the surface of the Earth and if we ignore the friction from the air um if your rocket accelerates up to 11 kilometers per second that's escape velocity and it can if there were no friction it could just continue forever to the next galaxy on the moon which has less Mass it's only seven kilometers per second so but going in the other direction if you have enough mass in one place the escape Velocity can become the speed of light if you shine light straight up away from the earth it doesn't have too much trouble it's going way above the escape Velocity and um but if you have enough Mass there even light can't escape the escape Velocity and according to Einstein's theory of relativity there is an absolute speed limit in the universe the speed of light and nothing makes any sense nothing could be self-consistent if there are objects that could exceed light speed and so uh in these very very massive regions of space-time even light cannot escape and the interesting thing is Einstein himself didn't think that uh these uh objects would call the black holes could exist but let me actually Linger on this yeah that's incredibly interesting there's a lot of interesting things here first of the speed limit how Wild is it to you if you put yourself in the mind in the time of Einstein before him to come up with a speed limit of that there is a speed limit that and that speed limit is the speed of light how difficult of an idea is that is it you know you said from a mathematic mathematical physics perspective everything just kind of falls into place but he wasn't perhaps maybe initially had the luxury to think mathematically he had to come up with it intuitively yes so like what how common intuitive is this notion to you well is it still crazy no no so it's a very funny thing in physics the best discoveries seem completely obvious in retrospect yeah even my own discoveries which of course are far lesser than Einsteins but many of my papers many of my collaborators get a little confused we'll try to understand something we said we've got to solve this problem we'll get all confused finally we'll solve it we'll get it all together and um then we'll all of a sudden everything will fall into place we'll explain it and then we'll look back at our discussions for the precedings of months and literally be unable to reconstruct how confused we were yeah and how we could ever have thought of it any other way so not only can I not fathom how confused Einstein was before he when you know when he started thinking about the issues I can't even reconstruct my own confusion from from two weeks ago uh you know so the really beautiful ideas that physics have this very hard to get yourself back into the mindset of course Einstein was confused about many many things doesn't matter if you're a physicist it's not how many things you got wrong it's not the ratio of how many you got wrong coming you got right it's the number that you got right so Einstein didn't believe black holes existed even though he predicted them and I went and I read that paper which he wrote you know Einstein wrote down his field equations and 1915 and short Shield solved them and discovered the black hole solution three or four months later in very early 1916 and um 25 years later Einstein wrote a paper so with 25 years to think about what this solution means yeah wrote a paper in which he said that black holes didn't exist and I I'm like whoa you know if one of my students in my general relativity course wrote this you know I wouldn't pass them you get I get to see mine oh you wouldn't pass them okay all right good to see minus okay same thing with gravity waves you didn't believe oh he didn't believe in gravitational waves either he went back and forth but he wrote a paper and I think 34 saying that gravity waves didn't exist because it people were very confused about what a coordinate transformation is and in fact this confusion about what a coordinate transformation is has persisted and we actually think we were on the edge of solving it a hundred years later well what a hundred years later what is coordinate transformation as it was a hundred years ago to today let's imagine I want to draw a map with pictures of all the states and the mountains and then I want to draw the weather forecast what the temperatures are going to be all over the country and I do that using one set of weather stations and I number the weather stations and you have some other set of weather stations and you you do the same thing so the coordinates are the locations of the weather stations yeah they're how we describe where the things are at the end of the day we should draw the same map that is coordinate invariance and if we're telling somebody uh we're going to tell somebody at a real physical operation we want you to stay as dry as possible on your drive from here to California we should give them exactly the same route no matter which weather stations we use or how we you know it's a very trivial it's it's the labeling of points is an artifact and not in the real physics sure so it turns out that that's almost true but but not quite there's some subtleties to it the statement that you should always have the same give it this the same kind of trajectory the same kind of uh instructions no matter the weather station yeah yeah there's some very delicate subtleties to that which begin to be noticed in the in the 50s it's mostly true but when you have a space-time with edges it gets very tricky how you label the edges and space-time in terms of spacer in terms of time in terms of everything just based on either one okay space or time that gets very tricky and Einstein uh didn't didn't didn't have it right and in fact he had an earlier version of general relativity in 1914 which he was very excited about um which was wrong um gave it wasn't fully coordinate and variant it was only partially coordinated variant it was wrong it gave the wrong answer for bending light to the Sun by a factor of two there was an expedition sent out to measure it during World War One they were captured before they could measure it and that came that came Einstein four more years to clean it to clean his act act up by which time he had gotten it right so it's a very it's a very tricky business but once it's all laid out it saves uh it's it's it's clear then what do you think Einstein didn't uh believe his own equations and didn't think that black holes are real well why was that such a difficult idea for him well something very interesting happens in short Shield solution of the Einstein equation I think his reasoning was ultimately wrong but let me explain to you uh what it was um at the center of the black hole behind the horizon in a region that nobody can see and live to tell about it as a center of the black hole there's a singularity and if you pass the Horizon you go into the singularity you get crushed and that's the end of everything now the word singularity means that um it just means that Einstein's equations break down they become infinite you write them down you put them on the computer when the computer hits that Singularity it crashes everything becomes infinite there's two so the questions are just no good there now that's actually not a bad thing it's a really good thing and let me explain why um so it's an odd thing that Maxwell's Theory and Newton's Theory never exhibit this phenomena you write them down you can solve them exactly they're really Newton's theory of gravity they're really very simple theories you can solve them well you can't solve the three body problem but um you can certainly solve a lot of things about them nevertheless there was never any reason even though Maxwell and Newton perhaps fell for this trap there were never any reason to think that these equations were exact um and every there's no equation well there's some equations that we've written down that we still think are exact some people still think are exact my view is that there's no exact equation everything is an approximation everything is an approximation are you trying to get as close as possible yeah so you think are you saying objective truth doesn't exist in this world we could discuss that but that's a different that's a different thing um we wouldn't say Newton's theory was wrong it had very very small Corrections Inc incomplete small Corrections it's actually a puzzle why they're so small so if you watch the procession of Mercury's perihelia this was the first indication of something going wrong according to Newton's Theory mercury has an elliptical orbit the long part of it moves around as other planets come by and perturb it and so on and so this was measured by leveria in 1859 and he compared Theory and experiment and he found out that the perihelion process moves around the Sun once every 233 centuries instead of every 231 centuries okay now this is the wonderful thing about science why was this guy are we didn't get any idea how much work this is you know but of course he made one of the greatest discoveries of the history of science without you know even knowing what it what good it was going to be so that's how small that that was the first sign that there was something wrong with Newton yeah nah so the corrections to Newton's law are very very small but they're definitely there the corrections to electromagnetism they're mostly the ones that we see are mostly coming from Quantum effects and so so the corrections there for uh Maxwell's equations is when you get super tiny and then the corrections for the um for Newton's uh laws gravity is when you get super big that though that's when you acquire Corrections that's true but I would phrase it as saying when it's super accurate you know if you look at the Bohr atom Maxwell electromagnetism is not a very good approximation to the force between the proton and the electron the quantum mechanics if you if you if you didn't have quantum mechanics the electron would would spiral into the proton and the atom would collapse it's Quantum you know so that's a huge correction there sure so every Theory gets corrected as we learn more dude just be no reason to suppose that it should be otherwise well how is this really to the singularity why the singular so when you hit the singularity you know that you need some Improvement to Einstein's theory of gravity and that Improvement we understand what kind of things that Improvement should involve it should involve quantum mechanics Quantum effects become important there it's a small thing and um we don't understand exactly what the theory is but we know there's no reason to think you know Einstein's theory was invented to describe weekly curves things the solar system and so on it it's incredibly robust that we now see that it works very well near the horizons of around black holes and so on so so it's a good thing that the theory drives itself that it predicts its own demise Newton's gravity had its demise there were regimes in which it wasn't valid Maxwell's electromagnetism had its demise there was uh regimes in which Quantum effects greatly modified the equations but general relativity all on its own found drove a system which originally was fine would perversely wander off into a configuration in which Einstein's equations no longer applied so to you the edges of the theory are wonderful the failures the edges are wonderful because that keeps that keeps us in business so that one of the things you said I think in your Ted Talk that uh the the the fact that quantum mechanics and uh and relativity don't describe everything and then they Clash is wonderful all right I forget the adjective you used but it was something like this so why is that uh why is that interesting to you in that same way that there's contradictions that create Discovery there's no question in my mind of course many people would disagree with me that now is the most wonderful time to be a physicist so so people people look back at at um it's a classical thing to say among physicists uh I wish it were 1920. right quantum mechanics had been just understood uh there was the periodic table there was but in fact that was such a rich thing um that uh well so that what a lot of exciting stuff happened around 1920. it took it took the whole it took a whole century to sort out the new insights that we got especially adding some experimental stuff into the into the bunch actually making observations and adding all the experimental things all the computers also help with visualizations and all that kind of stuff yeah yeah yeah it was a whole sort of Wonderful Century I mean the seed of general relativity was the incompatibility of Maxwell's Theory of the electromagnetic field with Newton's laws of gravity they were incompatible because if you look at Maxwell's Theory there's a contradiction if anything goes faster than the speed of light but Newton's theory of gravity the uh gravitational field the gravitational force is instantaneously transmitted across the entire universe so you could you know if you had a a a a friend on you know in another galaxy with a very sensitive measuring device that could measure the gravitational field that could just take this cup of coffee and move it up and down and Morse code and they could get the message instantaneously over another galaxy that leads to all kinds of contradictions it's not it's not self-consistent it was exactly in resolving those contradictions that Einstein came up with the general theory of of relativity and it's fascinating how this contradiction which seems like maybe it's kind of technical thing led to a whole new vision of the of the universe now let's not get fooled because lots of contradictions are technical things we haven't set up the we run into other kinds of contradictions that are Technical and they they don't seem to you know they would just we understood something wrong we made a mistake we set up our equations in the wrong way we didn't translate the formalisms as opposed to revealing some deep mystery that's yet to be uncovered yeah yeah and so we never we're never very sure which are which are the really important ones but to you the difference between quantum mechanics and general relativity seem the the tension the contradiction there seems to hint at some deeper deeper thing that's going to be discovered in the century yes because that one has been understood since the 50s poly was the uh first person to notice it and Hawking in the early 70s gave it a really much more visceral form um and people have been hurling themselves at it trying to reduce it to some technicality but nobody has succeeded and the efforts to understand it have led to uh all kinds of interesting relations between Quantum systems and and applications to other fields and and so on well let's actually jump around so we'll return to black holes I have a million questions there but let's let's go into this unification uh the battle against the contradictions and the tensions between the theories of physics what is quantum gravity maybe what is the standard model of physics what is quantum mechanics what is general relativity what's quantum gravity uh what are all the different unification efforts okay so again five questions yeah it's a theory that describes everything with astonishing accuracy it's the most accurate theory in the history of human thought Theory and experiment have been successfully compared to 16 decimal place we have that stenciled on the door where where I work you know it's a it's an amazing it's an amazing feat of the human mind it describes um the electromagnetic interaction unifies the electromagnetic interaction with the so-called weak interaction which you need some good tools to even view the weak interaction and then there's the strong interaction which binds the quarks into protons and the forces between them are mediated by something called Yang Mills Theory which is a a beautiful mathematical generalization of electromagnetism in which the analogs of the photons and themselves carry charge and um so this uh the final piece of this of the standard model everything in the standard model has been observed and its properties have been measured the final particle to be observed was the Higgs particle served like a over a decade ago the Higgs is already a decade ago I I think it is yeah wow time flies but you better check me on that yeah it's it's true but so much fun has been happening it's so much fun it's been happening and so that's all um that's that's all pretty well understood there's some things that miter might not around the edges of that you know Dark Matter neutrino masses some sort of fine points or things we haven't quite measured perfectly and so on but it's largely a very complete uh complete Theory and we don't expect anything very new conceptually in the completion of that anything contradictory but I'm new because can't you think contradictory yeah I'll have some wild questions uh for you on that front but yeah anything that yeah because there's no gaps it's so accurate so precise it's predictions it's hard to imagine something yeah yeah yeah and it was all based on something called let me not explain what it is let me just throw out the buzzword renormalizable quantum field Theory they all fall in the category of renormalizable quantum field Theory I'm going to throw that at a bar later to impress impress the girls good luck thank you all right so uh they all they all fall under that rubric gravity will not will not will not put that suit on so the force of gravity Cannot Be Tamed by the same renormalizable Quantum field Theory to which the all the other forces so eagerly submitted what is the effort of quantum gravity what are the different efforts to um to have these two dance together effectively to try to unify the standard model and um and general relativity any kind of model of gravity sort of the one fully uh consistent model that we have that reconciles that it it would that sort of tames gravity and reconciles it with quantum mechanics uh is string theory and its cousins and we don't know what or if in any sense String Theory describes the world the physical world but we do know that it um is a consistent reconciliation of quantum mechanics and general relativity and moreover one which um which is able to incorporate particles and forces like the ones we see around us so it hasn't been ruled out as an actual sort of unified theory of nature but there also isn't a in my view some people would disagree with me but there isn't a reasonable uh possibility that we would be able to do an experiment in the foreseeable future which would be sort of a yes or no to to string theory okay so you've been there from the early days of string theory you've seen his developments what are some interesting developments uh what do you see as the also the future of string theory and what is string theory well the basic idea which emerged in the early 70s was that if you uh you take uh the notion of a particle and you literally replace it by a little Loop of string that strings are sort of softer than than particles what do you mean by softer well you know if you hit a particle if there were particle on this table a big one and you hit it you might bruise yourself sure but if there was a string on the table you would probably just push it around and and this the source of the infinities in Quantum field theories that would particles hit each other it's a little bit of a it's a little bit of a a jarring effect and and uh I've never described it this way before but it's actually scientifically accurate but if you throw strings at each other it's a little more friendly one thing I can't explain is how wonderfully precise will the mathematics is that goes into describing String Theory we don't just wave our hands and throw strings around and you know there's some very um compelling mathematical equations that describe it now what was realized in the early 70s is that if you replace particles by strings these Infinities go away and you get a uh consistent theory of gravity without the infinities and um that may sound a little trivial but at that point it already been 15 years that people had been searching around for any kind of theory that could do this and it was actually found kind of uh by accident and there are a lot of accidental discoveries uh in this subject now at the same time it was believed then that string theory was an interesting sort of toy model for putting quantum mechanics and general relativity together on paper but um but that it couldn't describe some of the very idiosyncratic phenomena that pertain to our own Universe in particular the form of so-called parity violation our another term for the bar later tonight yeah yeah parity violation so so if you go to the bar and I already got the renormalizable quantity and you look in the mirror across the bar yes the universe that you see in the mirror is not identical you would be able to tell if you show your your your your your the lady in the bar the photograph that shows both the mirror and you there's a difference if she's smart enough she'll be able to to tell which one is the real world and which one is you now she would have to do some very precise measurements and if the photograph was too grainy it might not be possible but as Principle as possible why is this interesting why is it does this mean that there is some um not perfect determinism or uh what does that mean there's some uncertainty no it's a very interesting feature of the real world that it isn't parody of invariant and string theory it was thought could not tolerate that and um then it was learned in the mid 80s that not only could it tolerate that but if you did things in the right way you could construct a world uh involving strings that reconciled quantum mechanics and general relativity which looked more or less like the world that we live in and now that isn't to say that strings Theory predicted our world it just meant that it was consistent that the the hypothesis that string theory describes our world can't be ruled out from the get-go and it is also the only proposal for a complete theory that would describe our world still nobody will believe it until there's some kind of direct experiment and I don't even believe it myself sure which is a good place to be mentally as a physicist right always I mean Einstein didn't believe his own uh equations right with the black hole okay well that money was wrong about that but but you might be wrong too right uh so do you think string theory is dead if you were to bet all your money on um no the future of strength I think it's a a logical error to think that string theory is either right or wrong or dead or alive what it is is a stepping stone and uh an analogy I like to draw is yangmil's theory which I mentioned a few minutes ago in the context of standard model yangmail's theory was discovered by Yangon Mills in the 50s and they thought that the symmetry of Yang and Mills Theory described the relationship between the proton and the neutron that's why they invented it that turned out to be completely wrong it does however describe everything else in the standard model and it had a kind of inevitability you know they had some of the right pieces but not the other ones sure they didn't have it quite in the right context and it had an inevitability to it and it eventually sort of found its place and it's also true of Einstein's theory of general relativity you know he had the wrong version of it in 1914 and he was missing some pieces and you wouldn't say that that his early version was right or wrong he'd understood the equivalence principle it understood space-time curvature he just didn't have everything I mean technically you would have to say it was wrong and technically you would have to say Yang and Mills were wrong and I guess in that sense I would believe just odds are we always keep finding new wrinkles odds are we're going to find new wrinkles and strings Theory and technically what we call String Theory Now isn't quite right but we're always going to be wrong but hopefully a little bit less wrong every time except exactly and I I would you know bet the farm as they say do you have a farm you know I say that much more seriously because not only do I have a farm but we just renovated it simply before I read it so before I renovated better get the far my wife I spent five years renovating it before I you were much much looser with that statement but now I really means something no no it really means something and and I would bet the farm on the um on the uh guess that 100 years from now String Theory will be viewed as a stepping stone towards a greater understanding of of nature and and it would I mean another thing that I didn't mention about strings theory is of course we knew that it solved the Infinities problem and then we later learned that it also solved Hawking's puzzle about what's inside of a black hole and you put in one assumption you get five things out you somehow you're doing something right you know probably not everything but you're you're you know there's some good signposts and there have been a lot of good signposts like that it is also mathematical toolkit and you you've used it you've used it with comrade Waffa uh maybe we can sneak our way back from String Theory into black holes yeah um what was the idea that you and Cameron valpha developed with the holographic principle and string theory were we able to discover through through this through string theory about black holes or um that connects us back to the reality of black holes yeah so that is a very interesting story I was interested in black holes before I was interested in String Theory I was sort of a reluctance strength there is in the beginning I thought I had to learn it because people were talking about it but you know once I studied it I I grew to love it first I did it in a sort of dutiful way these people say they've claimed quantum gravity I ought to read their papers at least and then the more I read them the more interested I got and I began to see you know they they phrased it in a very clumsy way the description of string theory was was very clumsy and mathematically clumsy or just mathematically yeah it was all correct but but mathematically clumsy but it often happens that in all kinds of branches of physics that um people start working on it really hard and they sort of dream about it and live it and breathe it and they begin to see inner relationships and they see a beauty that is really there they're not they're not deceived they're really seeing something that exists but if you just kind of look at it you know you can't you can't grasp it all in the beginning and and um so our understanding of string theory in uh uh in 1985 was almost all about uh you know weekly coupled waves of strings colliding and so on we didn't know how to describe a big thing like a black hole and so you know in string theory of course we could show that strings in theory in some limit reproduced Einstein's theory of general relativity and corrected it but we couldn't do any better with black holes than um before my work with command we couldn't do any better than Einstein and tortill had done now um one of the puzzles um you know if you look at the Hawking's headstone and also Boltzmann's headstone and you put them together you get a formula for their really Central equations in 20th century physics I don't think there are many equations that made it to headstones and and they're really Central equations and you put them together and you get a formula for the number of gigabytes in a black hole now a short shelves description the black hole is literally a hole in space and there's no place to store the gigabytes and it's not too hard to and this really was wheeler and beckenstein and wheeler beckenstein and Hawking to come to the conclusion that if there isn't a sense in which a black hole can store some large number of gigabytes that quantum mechanics and gravity can't be consistent we got we got to go there a little bit so uh so how is it possible I won't say gigabyte say there's some information so black holes can store information how is this thing that sucks up all light and it's supposed to basically be you know be super homogeneous and boring how is that actually able to store information where does the store information on the inside on the surface uh where where's yeah and what's information I'm liking this ask five questions to see which one you actually answer oh okay I'm going to ask you about that I should try to memorize them and answer each one in order just to answer them I don't know I don't know what I'm doing I'm desperately desperately uh trying to figure it out as I go along here so um Einstein's Black Culture short sort of black hole they can't store information this stuff stuff goes in there and it just keeps flying and it goes to the singularity and it's gone however Einstein's theory is not exact it has Corrections and string theory tells you what those Corrections are and so you should be able to find some way of some alternate way of describing the black hole that enables you to understand where the gigabytes are stored so what Hawking and beckenstein really did was they showed that physics is inconsistent unless a black hole can store an a number of gigabytes proportional to its area divided by four times Newton's constant times Planck's Constant and that's another wild idea you said area not volume exactly and that's the holographic principle the universe is so weird and that's the holographic principle that's called the holographic principle that is it's the area we're just jumping around what is the holographic principle what does that mean well is this some kind of weird projection going on what what the heck uh well I was just before I came here writing an introduction to a paper and the first sentence was the as yet imprecisely defined holographic principle blah blah blah so nobody knows exactly what it is but roughly speaking it says just what we were alluding to that um really all the information that is in some volume of space-time can be stored on the boundary of that region so this is not just about black holes it's about any any areas based on any area space however we've made sense of the holographic principle for black holes we've made sense of the holographic principle for something which could be called anti-decider space which could be thought of as a giant as the black hole turned into a whole universe and um we don't really understand how to talk about the holographic principle for either flat space which we appear to live in or asymptotically the sitter space which astronomers tell us we actually live in as the universe continues to expand so it's one of the one of the huge problems in uh physics is to you know apply or even formulate the holographic principle for more realistic well black holes are realistic we see them but um yeah in in more General context so from a general statement of the holographic principle what's the difference in flat space and uh asymptotic decider space so flat space is just an approximation of like the world we live in so like uh uh the sitter space at some time I wonder what that even means meaning like uh asymptotic over what okay so for thousands of years you know until the last half of the 20th well sorry until the 20th century um we thought space time was flat can you elaborate on flat or what do we mean by flat well like the surface of this table is is flat let me just give an intuitive explanation surface of the table is flat but the surface of a basketball is curved so the universe itself could be flat like the surface of a table or it could be curved like a basketball which actually has a positive curvature and then there's another kind of curvature called the negative curvature and curvature can be even weirder because that kind of curvature I've just described is the curvature of space but Einstein taught us that we really live in a space-time continuum so we can have curvature in a way that mixes up space and time and that's kind of hard to visualize because you have to step what a couple of Dimensions up so it's hard to you have to step a couple but even a if you have flat space and it's expanding in time you know we could imagine we're sitting here this room good approximation it's flat but imagine we suddenly start getting further and further apart then space is flat but it's expanding which means it's space time is curved automates about space time okay so what's the what's the sitter in anti-disitter space the three simplest space times are flat space time which we call minkowski's base time and negatively curved space-time anti-decider space and positively curved space-time the sitter space and so astronomers um think that on large scales even though for thousands of years we hadn't noticed it beginning with Hubble we started to notice that space time was curved space is expanding in time means that space time is curved and the nature of this curvature is affected by the matter in it because matter itself causes the curvature of space-time but as it expands the matter gets more and more diluted and one might ask when it's all diluted away is space-time still curved and astronomers believe they've done precise enough measurements to determine this and they believe that the answer is yes the universe is now expanding eventually all the unit matter in it will be expanded away but it will continue to expand because uh well they would call it the dark energy Einstein would call it a cosmological constant in any case that the in the far future matter will be expanded away and we'll be left with empty decider space okay so there's this cosmological Einstein's cosmological constant that now hides this thing that we don't understand called Dark Energy what's dark energy what's your best guess at what this thing is why do we think it's there it's because of this it comes from the astronomers dark energy is synonymous with positive cosmological constant and um uh we think it's there because the astronomers have told us it's there and um they they know what they're doing and we don't know really really hard measurement but they know they really know what they're doing and we have no freaking idea why it's there another big mystery another another reason it's fun to be a physicist and if it is there why should it be so small why should there be so little why should it have hit itself from us why shouldn't there enough be enough of it to substantially cons curve the space between us and the moon why did there have to be such a small amount that only the crazy best astronomers in the world could find it well again the same thing be said about all all the constants all of the content be said about gravity can't they be said about the speed of light like why is the speed of light so slow so fast so slow relative to the size of the universe can't it be faster or no well the speed of Lights is a funny one because you could always choose units in which the speed of light is one you know we measure it in kilometers per second and it's 100 86 000 or miles per second is 186 000 miles per second and but if we had used different units yeah then we could make it one but you can make dimensionless ratios so um you know you could say why is the time scale set by the expansion of the universe so large compared to the time scale of a human life or so large compared to the time scale for a neutron to Decay you know yeah I mean ultimately you know the reference the temporal reference frame here is a human life maybe isn't that the important thing for us uh descendants of Apes isn't that a really important aspect of physics like uh because we kind of experienced the world we Intuit the World Through The Eyes of the these biological organisms I guess mathematics helps you escape that for a time but ultimately isn't that how you wonder about the world absolutely that like a human life yeah time is only 100 years because if you think of everything um if you're able to think in I don't know in billions of years uh then maybe everything looks way different maybe universes are born and die and maybe all these physical phenomena become much more intuitive that we see at the Grand scale of general relativity well that is one of the a little off the track here but that certainly is one of the nice things about being a physicist it's you spend a lot of time thinking about you know insides of black holes and billions of years in the future and and it's sort of uh gets you away from the day-to-day uh into into another fantastic realm um but I was answering your question about how there could be information in a black hole yes so Einstein only gave us an approximate description and we now have a theory that corrects it string theory and now sort of was the Moment of Truth well when we first discovered String Theory we knew we knew from the get-go that string theory would correct what Einstein said just like Einstein corrected what Newton said um but we didn't understand it well enough to actually compute the correction to compute how many gigabytes there were and sometime in the early 90s we began to understand the mathematics of string theory better and better and it came to the point where it was clear that this was something we might be able to compute and it was a kind of Moment of Truth for string theory because if it hadn't given the answer that beckenstein and Hawking said it had to give for consistency String Theory itself would have been inconsistent and we wouldn't be doing this interview well that's a very dramatic statement but yes uh that's not the most that's not the most dramatic thing I mean but like okay that's very life and death you mean like that that uh because string theory was Central to your work at that time is that what you mean well String Theory would have been inconsistent yeah okay so that would be a string theory would have been inconsistent but those inconsistencies can give birth to other theories like you said the inconsistency right something else could have happened yes yeah it would have been a major a major uh change in the way we think about string theory if it and it was a good thing that you know one supposition that the world is made of strings solves two problems not not one solves the infinity problem and it solved the Hawking's problem and also the way that it did it was very uh was very beautiful um it it gave an alternate description so alternate description thing of things are are uh are very common I mean we could to take a simple example this bottle of water here is 90 percent full I could say it's 90 full I could also say it's ten percent empty those are obviously the same statement and they're it's trivial to see that they're the same but there are many statements that can be made in mathematics and mathematical physics that are equivalent but might take years to understand that they're equivalent and might take the invention or discovery of whole new fields of mathematics to prove their equivalent and this was one of those we found an alternate description of the certain black holes and string theory which we could prove was equivalent and it was a description of the black hole as a hologram that can be thought of a holographic plate that could be thought of as sitting on the surface of the black hole and the interior of the black hole itself sort of arises as a projection uh or the near Horizon region of the black hole arises as a projection of that holographic plate so the two descriptions were the hologram the three-dimensional image and the holographic plate and the whole gram is what Einstein discovered in the holographic plate is what we discovered and this idea that you could describe things very very concretely in string theory in these two different languages of course took off and was applied to many uh many different many different contexts within string string theory so you mentioned the infinity problem in the Hawking problem uh witch-hawking problem the the that the black hole destroys information or that the what which Hawking problem are we talking about well there's really two Hawking problems they're very closely related one is how does the black hole store the information and um that is the one that we solved in some cases so it's sort of like you know your your smartphone how does it store at 64 gigabytes well you rip the cover off and you count the chips and there's 64 of them each with a gigabyte and you know they're 64 gigabytes but that does not solve the problem of how you get information in and out of your smartphone you have to understand a lot more about the Wi-Fi and the internet and the cellular and and that's where Hawking radiation this prediction it starts that's where Hawking radiation comes in and that problem of how the information gets in and out you can't you couldn't have explained how information gets in and out of an iPhone without first explaining how it's stored in the first place so just to clarify the storage is on the plate on the flight on the holographic plate and then it projects somehow inside the the bulk the the space time is the Hologram the Hologram but man I mean did you have any intuitive when you sit late at night and you stare at the stars do you have any intuitive understanding what a holographic plate is um like that there's two Dimension no projections that store information how a black hole could store information on a holographic plate I think we do understand in in great mathematical detail and also intuitively and it's very much like an ordinary hologram or you hold up a holographic plate and you sh it contains all the information you shine a light through it and you get an image which looks three-dimensional yeah but why should there be a holographic plate why should there be yeah why that is the Great thing about being a theoretical physicist is anybody can very quickly stump you if they going to the next level of wise yeah so if I can just keep asking yeah you could just keep asking and it won't take you very long to so the trick in being a theory a theoretical physics is finding the questions that you can answer sure so so the questions that we think we might be able to answer now and we've partially answered is that um there is a holographic explanation for certain kinds of things and string theory Sure we've answered that now we'd like to take what we've learned and that's what I've mostly been doing for the last 15 20 years I haven't really been working so much on string theory proper I've been sort of taking the lessons that you we learned in string theory and trying to apply them to the real world using only assuming only what we know for sure about the real world so on this uh topic you you co-wrote co-author the paper with Stephen Hawking called soft hair on black holes yes that makes the argument against Hawking's original prediction that black holes destroy information can you explain this paper yeah and the title yeah okay so um first of all um the hair on black holes is a word that was coined by the greatest phrase master in the history of physics John Wheeler invented the word black hole and he also said that uh he made the statement that black holes have no hair that is every black hole in the universe is described just by its mass and spin they wrote they can also rotate as was later shown by Kerr and um and this is very much unlike a star right every Star of the same mass is different in a multitude of different ways different chemical compositions different motions of the individual molecules every star in the universe even of the same mass is different in many many different ways black holes are all the same and that means when you throw some in Einstein's description of them which we think must be corrected and um if you throw something into a black hole it gets sucked in and if you uh throw in a red book or a Blue Book the black hole gets a little bigger but there's no way within Einstein's theory of telling how they're different and that was one of the assumptions that Hawking made in his 1974-75 papers in which he concluded that black holes destroy information you can throw encyclopedias thesis defenses the Library of Congress it doesn't matter it's going to behave exactly the same uniform way yeah so what what Hawking and I showed and also Malcolm Perry um is that one has to be very careful about what happens at the boundary of the black hole and this gets back to something I mentioned earlier about when two things which are related by a coordinate transformation are and are not equivalent and um and what we showed is that they're very subtle imprints when you throw something into a black hole they're very subtle imprints left on the horizon of the black hole which you can read off at least partially what went in and um so this invalidates uh Steven's original uh argument that the information is destroyed and that's a soft hair that's the soft hair right so and soft is the word that is used in physics for things which have very low energy and these things actually carry no energy there are things in the universe which carry no energy you said I think to Sean Carroll um by the way everyone should go check out Sean Carroll's mindscape podcast it's incredible and Sean Carroll is an incredible person I think he said there maybe in a paper I have a quote you said that a soft particle is a particle that has zero energy just like you said now and when the energy goes to zero because the energy is proportional to the wave of length it's also spread over an infinitely large distance if you like it's spread over the whole universe it somehow runs off to the boundary what we learned from that is that if you add a zero energy particle to the vacuum you get a new state and so there are infinitely many vacuole plural for vacuum which can be thought of as being different from one another by the addition of soft photons or soft gravitons right can you uh elaborate on this wild idea if you like it spreads over the whole universe when the energy goes to zero because the energy is proportional to the wavelength it also spreads over an infinitely large distance if you like it's spread over the whole unit it's spread over the whole universe what um can you explain these soft gravitons and photons yeah so the soft gravitons and photons um have been uh known about since the 60s but exactly what we're supposed to do with them or how we're supposed to think about them um I think what has been well understood only recently and in quantum mechanics the energy of a particle is proportional to Planck's constant times its wavelength so when the energy goes to zero the wavelength gets goes to Infinity now if something has uh zero energy and it's spread all over the universe in what sense is it actually there that's yeah that's been the confusing thing to make a precise statement about when something is and isn't there now the simplest way of seeing so people might have taken the point of view that if it has zero energy and is spread all over the universe it's not there we can ignore it um but if you do this you'll get into trouble and one of the ways that you'll get into trouble is that even though it has zero energy it doesn't have zero angular momentum if it's a photon it always has angular momentum one if it's a graviton it's uh angular momentum two so you can't say that the state of the system with the Zero Energy Photon should be identified with the one without the Zero Energy Photon that we can just ignore them because then you will conclude that angular momentum is not conserved and the angular momentum is not conserved things won't be consistent and um and of course you can have a lot of these things and typically you do get a lot of them and when you you can actually do a calculation that shows that every time you scatter two particles you create an infinite number of them infinite number of the soft photons the Zero Energy ones yeah and so these are and they're somehow everywhere but they're everywhere but they're also contained information or they're able to store information and they're able to store information they're able to store an arbitrary large amount of information so what we pointed out is so what these things really do one way of thinking of them is they rush off to the edges of the universe spreading out all over the space is like saying they rush off to the energy edge of the universe right and that includes if the interior of the black hole is not considered part of the universe that includes the edge of the black hole so we need to set up our description of physics so that all the things that are conserved are still conserved in the way that we're describing them and that will not be true if we ignore these things we have to keep careful track of these things and people had been sloppy about that that and we learned how to be very precise and careful about it and this and once you're being precise you can actually uh that makes you can actually answer this kind of very problematic thing that Hawking suggested that black holes destroy information well what we showed is that there's an error in the argument that all black holes are the same because they hadn't kept track of these uh these very subtle things and um whether or not this is the key error in the argument remains to be seen or whether this is a technical Point yes but it is an error it is an error and uh Hawking obviously agreed with it Hawking agreed with it and he was sure that this was the he was sure that this was this was a critical error that this was the critical error in that understanding this would would would get us the whole story and and and and and that could well be what was it like working with Stephen Hawking on this particular uh problem because it's kind of a whole journey right well you know I I I I love the guy I he's so passionate about physics uh he just yeah his his Oneness with the problem and uh I mean it's so his mind is all occupied by the world that's uh yeah and let me tell you there's a lot of other things with his illness and with his celebrity and yeah a lot of other things a lot of distractions pulling it is uh at his mind he's still there that's right I remember him turning down tea with Lady Gaga so we could spend another hour on our paper that my friend's dedication what did you learn about physics what did you learn about life uh from uh having worked with Stephen Hawking well he was one of my great teachers of course he's he's older than me and I was I was reading his um his textbooks in in um in graduate school and um uh you know I learned a lot about uh relativity from him I learned about passion for a problem I I learned about um not caring what other people think you know I mean physics isn't interesting culture even if you make a great discovery like Walking Dead people don't believe everything you say in fact people love to disagree it's it's a it's a a culture that uh cherishes disagreement and and so you know he kept ahead with what he believed in and sometimes he was right and sometimes he was wrong do you feel pressure from the community so for example with strength Theory I was very popular for a time there's a bit of criticism or is less popular now do you feel the forces of the community as it moves in and out of different fields or do you try to stay like how difficult is it to stay uh intellectually and mathematically independent from the community personally uh I'm lucky I'm well equipped for that I when I started out in graduate school the problem of quantum gravity was not considered interesting he still did it anyway I still did it anyway I I'm a little bit of a contrarian I guess and I think that has served me well um and uh people are always sort of disagreeing with me and they're usually right but I'm right enough and like you said the contradiction ultimately paves the path of Discovery yeah uh let me ask you just on this tension we've been dancing between physics and Mathematics um what to you is an interesting line you can draw between the two uh you have done some very complicated mathematics in your life to explore the laws of nature what's the difference between physics and Mathematics to you well um I love math I think my first love is is physics and the math that I've done I've I've done to because it was needed and serve as a physics in service of physics but then of course in the In the Heat of it it has its own appeal and uh In the Heat of it I like it sure it has its its own appeal and I certainly enjoyed it and ultimately I would like to think I wouldn't say I believe but I would like like to think that there's no difference between physics and Mathematics that or mathematics is realized in the physical world and all physics has a for a mathematical basis that they're really the same thing I mean why would there be math that had no physical manifestation it seems a little odd right you have two kinds of math some that are relevant to the real world where they don't have to be contradictory but you can have a can't you not have mathematical objects that are not at all connected to the physical world so I mean this is to the question of is math discovered or invented I said to you math is just discovered and and and there's a deep linkage between the two yeah yeah yeah do you find it all compelling these ideas uh like something like Max tag Mark where our universe is actually fundamentally a mathematical object that math is our universe is mathematical fundamentally mathematical in nature my expertise is a a physicist doesn't add anything to that it's not really you know physics is you know I was once very interested in philosophy and you know physics physics I like questions that can be answered that it's not obvious what the answer is and that you can find a an answer to the question and everybody will agree what the answer is and that there's a an algorithm for for getting there um not that these other questions aren't interesting um and they don't somehow have a way of preventing presenting themselves but to me the interesting thing is to is is motion in what we know is learning more and understanding things that we didn't understanding before things that seemed totally confusing having them seem obvious that's wonderful so I think that's those questions are there I mean I would even go further you know the whole Multiverse I don't I don't think there's too much we concrete we're ever going to be able to say about it this this is fascinating because you spend so much time in string theory which is devoid from a connection to the physical world for a long time like it did not devoid but it it travels in a mathematical world that seems to be beautiful and consistent and seems to indicate uh that it could be a a good model of the laws of nature but there's it's still traveling independently because it's very difficult to experimentally verify but there's a promise with Laden in it in the same way Multiverse or uh you can have a lot of kind of very far out there questions or your gut and Instinct and intuition says that maybe in 1500 200 years you'll be able to actually have strong experimental validation right I think that with string theory um I don't think it's likely that we could measure it but we could get lucky in other words just to take an example about 10 or 20 years ago it was thought that they had seen a string in the sky in it that it was seen by um you know uh doubled stars that were gravitationally lensed around the gravitational field produced by some long string there was a line of double instant now the signal went away okay but people were hoping that they'd seen a string and it could be a fundamental string that it somehow gotten stretched and that would be some evidence for string theory there was also bicep two which it was a the experiment was wrong but it could have happened it could have happened that we got lucky and this experiment was able to make direct measurements certainly would have been measurements of quantum gravity if not string theory so it's a logical it's a very logical possibility that we could get experimental evidence from string that is a very different thing than saying do this experiment here's a billion dollars and after you do it we'll know whether or not strings are real but I think it's a crucial difference it's measurable in principle and we don't see how to get from here to there if we see how to get from here to there in my eyes it's boring right so when I was a graduate student they knew how to measure the Higgs boson took 40 years but they did it I not just say that stuff is boring I don't want to say that stuff yeah but I I you know you know would Magellan set out he didn't know we could get around the world there was no map you know so I don't know how we're gonna um connect in a concrete way um all these ideas of string theory to the real world and you know when I started out in graduate school I said what is the what is the most interesting problem that there might be the deepest most interesting problem that there might be progress on in 60 years and I think it could be you know uh that you know in another 30 years that maybe we'll learn that uh we have understood how black holes store information you know that doesn't seem wild that that we're able to abstract what we learned from string theory and show that it's operative and and and and and you know I mean the Bose Einstein condensate they did you know they if you win Bose and Einstein uh predicted it when was that the 30s maybe early 30s it took they were there were 20 orders of magnitude that were needed in order to Improvement in order to measure it um and they did 50 years later so and then you couldn't have guessed how that had happened how they could have gotten that and it could happen that we I don't think we're gonna light see the heterotic string Spectrum in an accelerator but but it could be that things come around and and uh in an interesting way and somehow it comes together and the fact that we can't see to the end isn't a reason not to do it you know we just you know what did they do when they were trying to find the Pacific right they just they took every route they just tried everything and that's what we're doing and we're taking and I'm taking the one that my nose tells me is the best you know and other people are taking other ones and that's good because we need every person taking every route and you know if somebody on another route uh uh find something that looks really promising you know I'm gonna make a Portage over the mountain and get on their street you know so the fact that you don't see the experiment Now isn't to me a reason to give up on what I view as the most fundamental Paradox in 20th century 20th in present physics 20 21st century physics absolutely you can see that it's possible you just don't know the way but that's what I mean why some of the philosophical questions could be formulated in a way that's explorable scientifically so uh some of the stuff we've talked about but you know for example this topic that's become more okay to talk about which is the topic of consciousness uh you know to me as an artificial intelligence person that's a very practically interesting topic but there's also philosophers uh Sean Carroll loves to argue against them but there's the philosophers that are pen psychist I'm not against philosophers it's just not as fun I don't it's not a fun right but they they uh they start a little flame of a fire going that some of those Flames I think eventually become physics So eventually it becomes something that we can really like having them around is really important because you'll discover something by modeling and exploring black holes it's really weird and having these ideas around like the ideas of pan psychists that Consciousness could be a fundamental force of nature just even having that crazy idea swimming around in the background could really spark something where that you were missing something completely and it's just that's where the philosophy done right I think is very useful that's where even the you know these thought experiments which is very fun and sort of the the tech sci-fi world that we live in a simulation uh that you know taking a perspective of the universe as a as a computer as a as a computational system that process information which is a pretty intuitive notion but you can just even reframing it that way for yourself could really open up some different way of thinking and then you have I don't know if you're familiar with Stephen wolfram's work of like cellular atom and complexity yeah I did a podcast with Stephen Hey Stephen that's awesome I I to me forget physics to get all that uh cellular automata make no sense they're so beautiful they're so like they're from simple rules you can create complexity I I just don't think you know he wrote a book A New Kind of Science um basically hinting at which a lot of people offended as like we don't have a good way to talk about these objects we don't we can't figure out what is happening here these simple these trivial rules can create incredible complexity all right he's totally right about that yeah and I and physicists I guess don't have don't know what to do with that don't know do with cellular automata because you can describe the simple rules that will govern the system but how complexity can emerge like incredible complexity yeah of course wolfram's version of that is that physicists will never be able to describe it right yeah exactly he tries to prove that it's impossible what do you make of that what do you what do you uh what do you make about the tension of being a physicist and potentially not being able to it's like a Freud or somebody that maybe uh Sigmund Freud maybe you'll never be able to actually describe the human psyche uh is that a possibility for you that you will never be able to get to the core fundamental description of the laws of nature yeah so I had this conversation with Weinberg yeah how'd it go so Weinberg has this book called dreams of a final Theory yeah and I had this conversation with him I said why do you think um there's ever going to be a final Theory why should there ever be a final Theory I mean what does that mean do physics departments shut down we've solved everything um and you know what is it doesn't it seem that every time we answer some old questions we'll we'll just find new ones and that will just keep going on forever and ever he said well that's what they used to say about the Nile they were never going to find the end and one day they found it yeah so I don't I don't know um String Theory doesn't String Theory doesn't look like a candidate to me for a final Theory it as it stands now it doesn't get to the bottom of the world yeah it seems to me that even if we kind of solved it and we've did experiments there still would be more questions like why are there four dimensions instead of six it doesn't seem to have any anything that in it that would explain that you could you can always hope you know that there's something that we don't know about string theory that we'll explain it but but it but it still doesn't look like it's going to answer every question and um why is there one time not two you know why is this but you know it doesn't seem like it's I don't even know what it would mean to answer every question but to answer every question obviously so when you refer to The Theory of Everything you'll be able to have a if it exists it would be a theory that allows you to predict precisely the the behavior of objects in the universe and their their movement right what what about them their movement yeah like like precisely no matter the option right that's true so so that would be a really interesting State of Affairs if we could predict everything but not necessarily understand everything so for example let's just forget about gravity I mean we're not too far from that situation if we forget about gravity the standard model in principle given a big enough computer predicts almost everything but if you look at the standard model it's kind of a laundry list with neutrino masses and all that stuff they're they're hundreds of free parameters where do they come from is there an organizing principle is there some further unification sure so so being able to predict uh everything is not the only goal that physicists have so on the way to trying to predict you're trying to understand that's actually probably the goal is to understand yeah but but but right we're more interested in understanding than actually than actually doing the predictions but the predictions are more focusing on how to make predictions is a good way to improve your understanding because you know you've understood it if you could do the predictions yeah one of the interesting things that might come to a head with is our artificial intelligence there's an increasing use of AI in in physics we might live in a world where AI would be able to predict perfectly what's happening and so that will as physicists you'll have to come to to the fact that you're actually not that interested in prediction I mean it's very useful but you're interested in really understanding the Deep laws of nature versus a perfect predictor yeah like uh you want to play Just within AI yeah AI people are trying to understand what it is that the AI Bots have learned in order to produce whatever they produce for sure but you still don't understand deeply especially because they're getting you know uh especially language models if you're paying attention uh the systems that are able to generate tax they're able to have conversations chat gpts the recent manifestation of that there they just seem to know everything they're trained on the internet they seem to be very very good at uh something that looks like reasoning they're able to generate uh you can ask them questions they can answer questions it just feels like this thing is intelligent right uh and I could just see that being possible with physics you ask any kind of physical question and it'll be able to very precise about a particular star system or a particular black hole you'll say well these are the numbers it's it'll perfectly predict and then sure you can understand uh how the neural network is the architecture is structured actually for most of them now they're very simple you can understand what data is trained on huge amount of data you're giving a huge amount of data from a very nice telescope or something and then but it seems to predict everything perfectly you know how a banana Falls when you throw it like everything is perfectly predicted you still don't have a deep understanding of what governs the whole thing um and maybe you can ask it a question it'll be some kind of Hitchhiker's Guide to the Galaxy type answer uh that you know it's a funny world uh we live in of course it's also possible that there's no such deep simple governing laws of nature behind the whole thing I mean you there's something in US humans it's possible that wants it there to be yeah it doesn't have to be right right I do what's where do you again you're betting the you already bet the farm but if you were to have a second Farm do you think there is a theory of everything that we might get at so um simple laws in the whole thing I don't I don't I I honestly I I don't know but I'm pretty confident that if there is we won't get to it in my lifetime I don't think we're near it but doesn't it feel like they're like the fact that we have the laws we do they're relatively simple already that's kind of incredible it's just there seems to be there seems to be simple laws that govern things right by Theory of Everything you mean Theory a theory of of everything an algorithm to predict everything but a simple algorithm a relatively simple algorithm to predict everything so for me it would be a sad day if we arrived at that without answering some deeper questions sure of course it definitely is but the question yes but one of the questions before we arrive there we're going to ask does such a destination even exist so because the asking the question and the possible answers and the process of trying to answer that question is in itself super interesting is that is is it is even possible to get there well there's an equals mc squared type of there's a function okay you can have many parameters but a finite number parameter function that can predict a lot of things about our universe well okay but just to sort of throw one thing in in order to answer every question we would need a theory of the origin of the universe right and that is a huge uh task right so and the fact that the Universe seems to have a beginning defies everything we know and love right because we we you know one of the one of the basic principles the physics is determinism that the past follows from the the present follows from the past the future follows from the present so on but if you have the origin of the universe if you have a big bang that means before that there was nothing and you can't have a theory in which something Falls from nothing so somehow sounds like you don't like singularities well I thought for somebody that works with black holes you'll get used to them by now no no I I like this because it's it's so hard to understand I like it because it's hard to understand but but it's really challenging us it's not a I don't think we're close to solving that problem so even uh and strength string theory has basically had nothing there's been almost nothing interesting said about that in the last many decades so String Theory hasn't really looked at the big bang it hasn't really tried to get to see the origin not not successfully not not in there there aren't compelling papers that lots of people have read that that people have taken it up and tried to go at it but but there aren't there aren't there aren't compelling String Theory doesn't seem to have a trick that that helps us with that puzzle do you think we'll be able to sneak up to the the origin of the universe like reverse engineer it from experimental from theoretical perspective like okay if if we can well it's what would be you've already gotten yourself in trouble you see because you use the word reverse engineer so if you're going to reverse engineer that means you know you you forward engineering means that you take the present and determine the future reverse engineering means that you take the present and determine the past but estimate the best but yes sure but but but if the past was nothing how are you ever gonna reverse engineer to nothing well that's hard to do run up against the nothing right until they have mathematical models that break down nicely to where you can actually start to infer things let's work on it no but do you think that that it could maybe but it is people try to do things like that yeah but have not succeeded it's not it's not something that we we you know we're getting a pluses in sure let's pretend we live in a world where in a hundred years we have an answer to that yeah what would that answer look like who what department is that from What fields left led us there what uh uh not what feels what set of ideas and theoretical physics um is it experimental is it theoretical like what can you imagine possibly could have possibly lead us there is it through gravitational waves and some kind of observations there is it investigation of black holes is a simulation of universes is it uh maybe you start creating black holes somehow I don't I don't know uh maybe some kind of high energy physics type of experiments well I have some late night ideas about that that aren't really ready for Prime drugs okay sure but you have some ideas yeah yeah but but um and many people do it could be that some of the advances in Quantum information Theory are important in in that they kind of go beyond taking Quantum systems and just replicating themselves but combining them with others do you think since uh you highlighted the issue with time and the origin of the universe do you think time is fundamental or or emergent I think ultimately it has to be emergent yeah what does it mean for time to be emergent well because let's review what it means for space to be emergent yes what it means for space to be emergent is that um you you have a holographic plate and you shine some light that's moving in space and it produces an image which contains an extra spatial dimension and time just goes along for the ride so what we'd like to do and indeed there is some rather concrete work in this direction though again I would say even within our stringy Community we're not getting a pluses on these efforts but what we'd like to do is to see um examples in which the extra space-time dimension is time in other words usually what what we understand very well mathematically is how to take systems uh in some number of space-time dimensions and rewrite them as a plate in fewer space dimensions what we'd like to do is to take systems with one time and some number of space dimensions and to rewrite them is a system that had only space dimensions in it had no time evolution and there's some fairly concrete ideas about how to do that but they're not you know you universally accepted even within the stringy community but isn't it wild to you yes for to be emergent how do we Intuit these kinds of ideas as human beings for whom space and time seems as fundamental as apples and oranges both Illusions okay they're both illusions even time you co-authored a paper titled Photon rings around warped black holes first of all whoever writes your paper titles you like uh the soft hair and the and and the term black hole in the Big Bang you're very good at coming up with titles yourself anyway you co-authored a paper title Photon rings around warped black holes in it you write quote recent work has identified a number of emergent symmetries related to the intricate self-similar structure of the photon ring so what are Photon Rings what are some interesting characteristics of a photon ring um so that was a paper with Dan coppets and Alex lipska that just came out and this is uh paper is kind of a wonderful example of what happens when you start to talk to people who are you know way out of your comfort zone of those different stuff and look at the world a different way and and some two or three years ago um I'm I'm part of this uh the black hole initiative I'm also part of this event horizon telescope collaboration that took the famous uh though I had nothing to do with the experiment but uh that that took the famous picture of the of the Donut of of m87 and um through conversations with them which started out in an effort to understand the image that they'd seen so it's a great thing for somebody like me a theoretical physicist lost seemingly lost in string land to be presented with an actual picture of a black hole and ask and to be yes to be asked what can we learn from this so you know with some help uh uh from uh you know Michael Johnson um and a bunch of other people venturized in collaboration we came up with a fantastic beautiful answer using Einstein's theory that is both shaping the future of now it is shaping the future of improved photo black hole photographs what do you want to concentrate on in the photograph you know just point at the sky and click no you don't do that you you optimize for various features and um it's it's both shaping that and in the process of talking to them and thinking about how light behaves around a black hole it they black holes just have so many magic tricks and they do so many weird things and the photon ring is among the weirdest of them we understood the this Photon ring and in the process of this we said hey this Photon ring has got to be telling us something about the puzzle of where the holographic plate is um outside of a ordinary astrophysical black hole and we nailed it for the stringy black holes but they have a somewhat different character with a stringy black hole the the black holes that describe us the black holes that are contained in string theory and they have different structure in them sure well basically can we step back so what was the light in the image taken in 2019 not taken to 2018 um uh presented in 2000 so here's the puzzle um what they really saw so the black holes tend to gather stuff that swirls around it yeah and they don't know what that stuff is made of they don't know what its temperature is they don't know what kind of magnetic fields there are around there so the form of the image has a lot of unknowns in it that it's dependent on many other things other than the geometry of the black hole so most of what you're learning is about the stuff now the stuff the swirling stuff the hot swirling stuff is interesting as hell but it's not as interesting as the black hole which are the which are the most in my view the most interesting things of the universe so you don't want to just learn about the stuff you want to learn about the black hole that that it's swirling around so one of the at the very first step of the very primitive level this is just a big leap for human civilization to be able to see a black hole and the way you can see it is because there's stuff around it but you don't get to learn much about the black hole you get to learn more about the stuff just from the image yeah okay but you're not going to learn about the details before you've even seen it because there's too many parameters there's too many variables that govern the stuff yeah so then we found a very wonderful way to learn about the black hole and here's how it works a black hole is a mirror and the way it's a mirror is if light a photon bounces off your face towards the black hole it goes straight to the black hole just Falls in you never see it again but if it just misses the black hole it'll swing around the back and come back to you and you see yourself from the photon that went around the back of the black hole but not only can that happen the black hole the photon can swing around twice and come back so you actually see an infinite number of copies of yourself like with a little bit of a delay with a little bit of a delay right this is awesome yeah and in fact I mean we're not used to an object that bends light like that right yeah so you're gonna get some trippier effects and in fact one of my my students has made a really awesome computer animation of this which I'm going to show at a public lecture in a couple weeks where the audience will see infinitely many copies of themselves so so if you so it's a black hole is like a Hall of Mirrors you know like a department store where you go and there's there's the three mirrors and you see infinitely many copies of yourself yeah think of the black hole as the mirror you know and you know you go in there with your clothes if you want to know about your clothes you just look at the direct image you're not learning anything about the configuration of mirrors but the relation of of um the image you see in front of you to the one you see at the side and the next one and then so on depends only on the mirrors it doesn't matter what clothes you're wearing so you can go there a thousand times wearing different clothes but each time there will be the same relation between the subsequent images and that is how we're going to learn about the black holes we're going to take the stuff that is swirling around and we're going to tease out the subsequent images and look at their relation and there's some very beautiful really beautiful mathematics which we were surprised to realize with the volumes and volumes of papers on black holes in their properties this particular because it was a physical question that had never been asked in exactly this way so basically you're looking at the the relationship with the subsequent images the relationship but those are ultimately formed by photons that are swirling around the photons that are orbiting so the photon ring are the photons that orbit around and Beyond so like orbit and lose orbit like they are they yeah like so uh wow and that starts to give you what can you possibly figure out mathematically about the black hole can you the geometry of it does this geometry the spin um and you can verify things behaving you know we have never seen a region of space-time with such high curvature I mean yeah the region around a black hole is crazy it's not like in this room right the curvature is everything you know you spend probably enough time with the math and the photons can you put yourself in that space so we're like having a conversation pretty peaceful comfortable flat space are you able to put yourself in in a place around a black hole yeah I'm able to imagine that kind of thing yeah so for example and actually there's a wonderful movie um Interstellar yeah and um in that movie you know Kip Thorne of course is a great theoretical physicist experimental who later won the Nobel Prize um for ligo and that movie is very accurate scientifically and there's some funny statements in there that of the you know 100 million people who saw that movie there can't be more than 10 or 20 understood about why Matthew McConaughey is ejecting the trash in a certain direction in order to but you know for example if I wear a spinning black hole right here if I was spinning fast enough you wouldn't be able to stay still there you'd be have to be orbiting around like that you know you'd have to have your microphone on them but I wonder what the experiences with the actual experience because I mean space itself is curved well space gets very curved you get crushed you know my body gets ripped apart because the forces are different on different parts sure okay so that would be but if but if it can be less curved so that the curvature is very noticeable but you're not ripped apart the fact that this was just nonchalantly stated is is just beautiful like two biological systems discussing uh which level of curvature is required to rip apart said biological system very well um so you propose in the paper that a photon ring of a warped black hole is indeed part of the black hole hologram a photon ring of a warped black hole is indeed part of the black hole hologram uh so what can you Intuit about the hologram and the the holographic plate from looking at the photon rings well this paper is exploring a new idea it's not making it's not it's it's not making a new Discovery so to speak it's exploring an idea and and and um the ins and outs of it and and what might work and and what might not and this Photon ring somehow everybody always thought that the holographic plate sat at the Horizon of the black hole right and that the quantum system that describes the black hole is inside the horizon and it in fact um we think it's plausible and we give some evidence in some soluble examples in this case in an example in one lower Dimension where we can handle the equations better that the quantum system that describes the black hole should correspond to a region of space-time which is includes the photon ring so it's bigger hmm so that that would be the holographic plays so that would be the holographic play all of that I mean we didn't prove this we we we uh you know we put it out there with hadn't really been considered previously we put it out there and um it does seem more plausible than the idea that it sits literally at the Horizon and it is a big outstanding problem of of how you have a holographic reconstruction of black holes like m87 do you think uh there could be experimental further experimental data that helps explore some of these ideas that you have for Photon rings and holographic plates through Imaging and through like higher and higher resolution images and also just more and more data I wish so but I don't think so and but what I think already has happened and will continue happen is that the you know there there are many different ways that uh theorists and observers can interact the gold standard is the theorist makes a prediction The Observer measures it and confirms it or the Observer uh makes a discovery and the theorist explains it um but there's a lot less than that which is really kind of the bread and butter of those are dramatic moments when that happens right those are once in a lifetime moments when that happens but the bread and butter is more when and it already has already happened they came to us and said what what is the interesting theoretical things we can understand in this swirl around the black hole and we give an answer and then that in turn jogged us to think about the holographic principle in the context of m87 a little bit differently and so it's a useful and in the same vein it's useful to talk to the philosophers and it's useful to talk to the mathematicians and you know a lot of you gotta we just gotta you know we don't know where we're going we just gotta like do everything let me ask you another sort of philosophical type question but not really actually um it seems that thought experiments are used so it's not just mathematics that makes progress in theoretical physics but thought experiments too they did for Einstein as well they did for a lot of a lot of great physicists throughout history over the years how is your ability to generate thought experiments it's just your intuition about some of these weird things like quantum mechanics or string theory or quantum gravity or yeah even general relativity how's your intuition improved over the years have you been able to make progress the hard part in physics is most problems are uh either doable most problems the theoretical calculation that are theoretical physicists would do there there's no end of problems whose answer is uninteresting can be solved but the answer is uninteresting there's also no end of problems that are very interesting some of which you've asked me but we don't have a clue how to solve them and when first presented with the problem almost every problem is one or the other it's it's the jackpot when you find one that isn't one or the other and it seems like there's a gray area between the two right that's where you should be looking well I wouldn't describe it as a gray area I would describe it as a knife edge this is a very small area there isn't like a huge area with a side hear life problems that are doable and people want to know the answer and that's in some deep sense that's where timing is everything with physics with with science with Discovery timing I mean I think earlier in my career I I aired more on the side of problems that were not solvable the ambition of Youth yeah what what what made you fall in love with physics at first if we can go back to the early days you said black holes were there in the beginning but what made you do you remember what really made you fall in love you know I wanted to I wanted to reach nirvana and I sort of realized that wasn't going to happen and then after that I wanted to know the meaning of life and I realized I wasn't probably wasn't going to figure that out and then I wanted to like understand you know Justice and socialism and World things and couldn't figure those out either and the simplest smaller and smaller problems I mean most of this stuff talking about adolescence you know but but uh it was the biggest problem that I thought that there was uh a prospect of but not a hundred percent you know and I was definitely ready to spend my my life in the wilderness knocking my head against the wall but I haven't had to I haven't solved them but I've said enough interesting things that you're you're you're you're you're interviewing me so so I'm not in the wilderness but yeah so do you remember the the early days of if you feel nostalgic when you think back to the ideas the circumstances that let down that led you down this the path towards black holes those theoretical physics doors the tools of physics stores this really fascinating world of theoretical physics well I wouldn't I wouldn't add Nostalgia to it because it's not like a you know a summer in Italy or something it's it's like there there's there's results that are there that that people are and that's what's that's what's so gratifying I mean of course one's name disappears from these these things um unless you're Einstein or Newton or something you're people are not gonna remember my name in 50 years almost no basically every nail will be forgotten in hundreds of years yeah yeah are you able to by the way um love the idea the exploration of ideas themselves without the names yeah yeah that's what I'm saying so I have not I hope someday but I have not um you know there are some experiments now to verify some of my predictions about you know properties of gravity and so on but I I have not like you know most of what I've done is in the you know it could happen still it's still a logical possibility that everything having to do with string theory and the uh I mean as I as we mentioned I'm betting the farm that it's not but but it is a lot indeed a logical possibility that that people always say can you believe Lex Fred men interviewed Elon Musk and Kanye West and and he interviewed strominger who was on this this working on this theory that just completely went into the yeah it completely went into the toilet you know I'm gonna make I'm gonna get uh with a wife I don't have I'm gonna make a public statement she'll be on stage I'll say I'm really sorry I made this giant mistake of platforming this wild-eyed physicist that believed for decades in the power of theoretical physics yes no like you said so that could happen it could happen it could happen it's it's in the and of course if that couldn't happen yeah it wouldn't be real exploration right absolutely and um so but I I you know I do take a lot of satisfaction that some of the things I discovered are at the minimum mathematical truths and they're still so you don't have that sort of nostalgic feeling of it being something that was gone and and uh and I'm still making discoveries now that I'm as excited about we'll see if they hold the test of time that that stand the test of time that these other ones did that but that I'm as excited about as I was about those when I when I when I made them I am easily excitable as my friends will tell you well one interesting thing about you is and I have been very excited about things which turned out to be completely wrong you know well that's the the excitement is a precondition for uh for breakthroughs um but you're also somebody like just like you said you don't have a cynical view of the modern state of physics so so there's a lot of people that glorify like the early days of string theory and that you know all these yeah yeah yeah but you're saying like this this to you uh might be one of if not the most exciting times to be a theoretical physicist like uh when uh the alien civilizations 500 years from now that visit Earth will look back they'll think the 21st century some of the biggest discoveries ever were made in the 21st and they have a when they have a measurement of string theory the fund's over then we have to go on to something new you know no there's deep there's oh there's going to be the fun is over oh man um but there is an end to the Nile right I mean there that's there's is there who told you some Weber guy uh uh let me let me ask you another trippy out there question uh so uh again perhaps unanswerable from a physics perspective but do you wonder about um alien civilizations do you wonder about other intelligent beings out there making up their own math and physics trying to figure out the world do you think they're they're out there it is hard to understand why there were given that there's so many planets and of course there's Drake's formula and we don't exactly know what the but I mean I think fermi's Paradox that you know is a real Paradox and I I think they're there probably are and I think it's very exciting that uh you know we might you know find some it's it's a logical possibility that we could learn about it I mean to me it's super interesting to think about aliens from a perspective of physics because so any intelligent civilization is going to be contending with the ideas but you're trying to understand the world around it so I think that the alien I think that the universe is filled with alien civilizations so they're all have their physicists right they all have their they're all trying to understand the world around them and it's just interesting to me to imagine all these different perspectives all these different Einsteins yeah like trying to make sense of like though they might be more different than we think they might be different in a way that we haven't even thought of like smarter or different just just different something that we don't even we're not even able to describe now we just haven't thought about it you know man yeah this is a really frustrating thing when we think from uh me as an AI person you start to think about what is intelligence which is consciousness you start to sometimes again uh night evening thoughts is um how little we understand how narrow our thinking is about these Concepts yeah yeah that that it could be intelligence could be something could be intelligent and be very different intelligent in a very different way they won't be able to detect because we're not keeping an open mind open enough mind and that's kind of sad because to me there's also just a strong possibility that aliens or something like alien intelligence or some fascinating beautiful physical phenomena are all around us and we're too dumb to see it for now or to close-minded to see it there's something we're just deeply missing whether it's uh like fundamental limitations of our cognitive abilities or just because our tools are too primitive right now or like the way we it's like you said like uh the idea seemed trivial once you figure it all out looking back yeah but that kind of makes me sad because there could be so much Beauty in the world we're not seeing because we're too dumb there surely is and that's I guess the process of Science and physics is to keep exploring to keep exploring to find the thing that will uh in a century seem obvious well it's something we know for sure I mean the brain we don't really understand and that's got to base some fabulously beautiful story that uh I'm hoping some of that story will be written through the process of trying to build a brain so the process of engineering intelligence not just the neuroscience perspective of just looking at the brain but trying to create it uh but yeah that story hasn't been written almost at all which is the early days of figuring that one out but see like you said that math is discovered so um aliens should at least have the same math as us right I think so maybe different symbols oh well they might have discovered different they might have discovered it differently and they might have had a different idea of what a proof is sure yeah we're very uh we're very like black and white with the proof thing yeah maybe they're looser right well so you can you can know something is true to first of all you never know something is true with 100 uncertainty I mean you might have had a blackout it's just to be it's never a hundred percent right you you might have had a momentary lapse of Consciousness it's a key step in the proof and nobody read it and whatever okay so you never know for sure but you can be have a preponderance of evidence which makes it and preponderance of evidence is not accepted very much in mathematics and and that was sort of how you know the famous ramanujan work he worked he had formulas which he guessed at and then he gathered a preponderance of evidence that that you were sure they were true and so there might be or something completely different you know they might function in a different in a very different way let me ask you um kind of a heavy question for a physicist but one on nuclear weapons just in general what do you think about nuclear weapons where like philosophical level where brilliant physicists and Brilliant engineering leads to thing that can destroy human civilization sort of like some of the ideas that you're working on have power when it when engineered into machines into systems is there some aspects I don't know what the brilliant had to do with it because of course you know Oppenheimer and all that okay they did it really fast but if you didn't have Oppenheimer yeah you know I mean would all have happened anyway it it's the it had a reality of its own the possibility of making a nuclear didn't it didn't depend on the fact that the physicists who built it were brilliant maybe that bed it up by a year two years or but by now we'd have nuclear weapons it's it's something that so the ideas have momentum and that they're um yeah Unstoppable right the possibility of making nuclear weapons was discovered right it was there before we didn't we didn't it's not like somebody made it right um without Picasso you know there would never have been a Guernica but without Oppenheimer that would surely have still been an atom bomb but timing matters right timing is very important there's a guy of course of course of course but the timing matter the timing mattered there um but I I yeah uh okay I mean you could try to make a case for stopping uh no no no it's the case of carrying the burden of the responsibility of the power of ideas when manifested through into systems so there's not it's not a game it's not just a game a fun mathematics the same with artificial intelligence you have this you know a lot of people in AI yeah you know in a lot of people in the AI Community it's a fascinating fun puzzle how to make systems more and more intelligent how to you have a bunch of benchmarks you try to make them perform better and better and better and all of a sudden you have a system that's able to all smart people it's now able to be used in geopolitics it's able to create a super intelligent Bots that are able to at scale control the belief of a population of people and now you can have world wars you can have a lot of really risky instabilities they really are incredible and so to think like there's some responsibility this is not sort of um it's a beauty and a power and a terror to these ideas you know yeah at that moment it was certainly a question for Oppenheimer and everybody who participated in that what it what is the responsible way to serve Society when you're sort of accidentally in this position of being at the Forefront of a development that has a huge impact on society I I don't see my work the likelihood of having a huge impact on the development of society itself but if I were you oh I love this working on AI yeah I think that there is a possibility there and that it is as a responsible scientist that it's really not a good thing to say I'm just the scientist here and I'm figuring out what's possible because you're in a role where you you're you know you have more of a a Podium to influence things than other people and it's your responsibility is it citizen of the planet or or let me phrase it a little less shouldy it's you know you have an opportunity yeah as a citizen of the planet to make the world a better place which which it would be sad too to bypass yeah it's a nice world without going it'd be nice to keep it going for a little bit longer Andrew um I'm really honored that you sit down with me this is uh thank you for your work thank you it was it was a really great conversation I really enjoyed it you really uh you really covered a lot I can't believe you're able to discuss at this level on so many different uh topics so it's a pleasure it was it was super fun thank you thanks for listening to this conversation with Andrew stromiger to support this podcast please check out our sponsors in the description and now let me leave you some words from Warner Heisenberg not only is the universe Stranger than we think it is stranger than we can think you for listening and hope to see you next time