Brian Greene and Leonard Susskind: World Science U Q+A Session

[Music] do [Music] [Music] [Music] hello everyone here we are again for another one of our live conversations and these are live conversations i always am amused when i see in the chat people say is this live is this live yeah it's live if you're watching it live right if you're not watching it live it's not live and i don't know maybe just to uh establish that fact it's always good to uh respond in real time there is one question here from someone in fiji i don't know where the the question runt question went i should say had to do with black holes and string theory uh if you want to ask that question again our participant from fiji i'll try to look at it but the main event today as you all know is a conversation with lenny suskind who's really one of the great theoretical physicists of our time so it's an exciting guest to have on the program as many of you know lenny has had profound impact in string theory black hole physics quantum mechanics elementary particle physics i mean it's um quite a lot quite a lot of accomplishments and we'll obviously only be talking about part of those insights but i think we'll likely skew heavily on black holes you know as you reach the end of a year there's a tendency to think back on the good things the bad things summarize you know all these best of lists worst of lists things of that sort if i was going to focus upon the best of physics or let's say theoretical physics in 2020 even perhaps even earlier black holes would be near the top perhaps at the top you know we've had um a great run and it is continuing onward and we're going to discuss today some of the puzzles that remain but some of the deep puzzles of the past that today just are not as puzzling any longer it's not that everything has been resolved it's rare that deep puzzles just get fully thoroughly resolved but there are many things that were very unclear even as early as uh 15 20 25 years ago that now have really been unraveled and lenny and a bunch of other folks that will no doubt make reference to critical to making that happen so lenny will join us at about 1 30 or so i'll do a little bit of q a maybe just a couple questions if there's something to get us going uh and then i'll maybe give a i don't know a little background to the conversation that we're going to have with lenny so here's a question from captain vyom how are dark energy and dark matter related captain vyam asks and i like questions like that because the answer is short basically i don't know and nobody does know how these dark things are related they may be related they may not be related remember dark matter is this idea of matter that is out there we believe in space and we come to that conclusion because when we take account of the gravity that can be exerted by the matter that's not dark which means matter that we can see gives off light reflects light matter of that sort the amount of gravity that such matter can exert just is not enough to account for the motions that we see through astrophysical measurements astrophysical data right i mean the analogy that i'd like to use i think it's a pretty good one if you have a a bicycle wheel that's wet as it spins you know that the water droplets fly off as the wheel turns similarly in galaxies that are spinning if they're spinning at a sufficiently fast rate stars should be flung outward and we see galaxies for which the stars should be flung outwards but they're not which must mean there's something else out there that's holding those stars inside of those galaxies the belief is that there's additional matter beyond the matter that we can see with our telescopes and that dark matter is responsible for the gravitational pull that's keeping those stars from flying outwards like the water droplets good that's dark matter i should say there's a lot of dark matter we think when you do these calculations there's on the order of four or five times as much dark matter as there is ordinary matter you know the stuff that we're made of dark energy is a different beast dark energy the most convincing evidence for it are these observations that we've discussed in this series from time to time of the accelerated expansion of space space is not only getting larger over time that was a shock right that was the shock that initially was really confirmed through the observations of edwin hubble but not only is universe getting bigger it's getting bigger at an accelerated clip so the expansion is speeding up how can expansion speed up right galaxies they pull on each other with the force of gravity force of gravity we usually think of as pulling things inward but yet something's pushing outward and the remarkable thing is that in albert einstein's general theory of relativity gravity can actually be repulsive it can push outward not if the source of that gravity is a clump like a star a clump like a galaxy a clump like a planet rather if there's a diffuse energy that is spread uniformly throughout a region of space then under modest assumptions it will give rise to a repulsive push an outward push that can drive the expansion of space to accelerate and because this energy does not itself give off light we call it dark energy so there are the two dark things dark matter dark energy and again i gave you the amount of dark matter dark energies in terms of the energy mass budget of the universe is even more substantial on the order of 70 percent of the mass energy of the universe is this dark energy so that are these two components now are they related many people have written down theories which suggest that they are none of these have really gained the consensus of the scientific community as yet but who knows you know they both have dark in their name is that the end of the connection or is it deeper i don't know all right anyway so that's uh a good question to get us going here mark kennedy also has how much do you miss mama joys i i don't know what that means but it does ring a bell somewhere deep in my childhood or something i know what you're talking about mark i think or maybe not maybe it's something that i don't want to remember i don't know but anyway i can't answer the question i don't remember exactly what it means a quick one more before we head on to a little bit of background uh physics forever asks what are strings made of we'll talk a little bit about string theory here today no doubt lenny suskind founding father pioneer of string theory i'm sure it will come up in our conversation and look when you think about any proposal for what stuff is made of and you think back on the history of ideas any proposal for what stuff is made of seem to ultimately have finer stuff inside of it right molecules made of atoms atoms yeah nucleus with electrons in these fuzzy quantum orbits the nucleus has smaller things protons and neutrons inside protons and neutrons smaller things called quarks if string theory is right big f really we'll talk about that then inside of these particles are these little vibrating filaments these string-like filaments of energy so if that's the correct picture natural question is is that the end of the line have we reached the smallest of the matroyska dolls if i'm pronouncing that correctly no doubt somebody will correct me you know the russian dolls each has a smaller doll inside of it is it that strings have some smaller entity inside of them too i don't know could be string to the end of the line could be that there are finer ingredients in some sense strength even suggests that possibility with things called d0 particles at some level maybe we'll talk about that a little bit too but it's a natural question and one that uh we don't fully uh know the answer to that's two questions that we don't know for the answer to maybe there's a question here that i can actually answer that would be that would be nice give me a little uh boost to keep on going um um yeah another question about string theory let me just find uh uh tishia patel can we describe the fabric of spacetime is made of particles i seem to be drawn to the questions that i can't answer here this uh this afternoon and in in a sense yes so in the quantum mechanical framework the forces that influence how things move the electromagnetic force and nuclear forces they're all part of the standard model of particle physics which envisions that those forces are indeed communicated by a particle right for the electromagnetic force photons nuclear forces w and z bosons and so forth now gravity is a force of nature as well einstein tells us the gravity is associated with space-time the geometrical structure of space-time determines the gravitational influence that a body moving through that region of space time will experience now if you believe as most of us do that quantum mechanics and general relativity have got to come together then the paradigm of a force being communicated by a particle will spill over to the gravitational influence as well and that gives rise to this notion of gravitons no one has ever seen a graviton we're not surprised by that it's the smallest packet of the weakest of nature's forces so it's not as though we have expected to see it lighting up our detectors and it hasn't so not a mystery but if this idea is true then you can actually think of the fabric of space which is the medium of gravity as being a whole huge collection of gravitons that are arranged in a coherent pattern that manifests on large scales as the geometry of space-time but on small scales would have this distinct particulate quantum mechanical description so i can't say the answer is absolutely yes but certainly there is a natural intellectual pathway that would take us to that picture all right good let's um i'm happy to again answer questions as they come through and as in the conversation with lenny i'll try to look and sometimes it's hard some of you have asked why for instance in the conversation with roger penrose that we had last week why i didn't turn to some of the questions that you ask and honestly i just get so carried away in the conversation sometimes with the person i'm speaking with that breaking away to try to scan through the list of questions can be difficult we do have somebody looking at those questions right now so if you ask something that's really on point highly relevant to wherever we are in the conversation with lenny i'm more than happy to try to bring your questions into the conversation so um i'm absolutely for the open to that without a doubt all right so a little bit of background lenny will join us about 10 12 minutes so look we're all now familiar that black holes are an observationally confirmed quality of reality right the event horizon telescope we have a little black hole image from them of course we can bring it up on the screen where you know in the most direct way you see a black hole right i mean a black hole is nothing but a region in space where light can't get out and the event horizon telescope imagery is one that makes that you know the most straightforward confirmation of that idea so i see it on my screen here hope you guys see it on yours which would be good i think you do now black holes really came out of the mathematics of carl schwarzschild way back in 1916 1917 or so and it's an idea that you know is interesting exciting but it kind of became a bit of a backwater of physics for a long period of time it's really john wheeler who i think we have a little image of john wheeler nice to sort of see it was john wheeler who brought the study of einstein's general relativity and the puzzles of black hole physics he really brought that into mainstream physics research so he's really responsible for that and and john wheeler had a very particular puzzle about black holes that we're going to be talking about with lenny suskind wheeler worried he was a very genteel man he worried that if he was drinking a hot cup of tea okay there's a lot of entropy in a hot cup of tea the disorder of the water molecules bouncing to and fro in that cup of tea he worried that if there was a nearby black hole he could take his cup of tea with the hot water with all that entropy and just throw it into the black hole and then he worried that if the cup of tea and the entropy went to black hole since black holes don't let anything to come out the entropy in some sense would be gone and he worried about that because the second law of thermodynamics tells us that entropy should always go up and wheeler said i seem to be able to use a black hole to thwart the second law of thermodynamics i just keep dumping entropy into a black hole it's gone and in that way entropy in the rest of the universe the observable universe perhaps is the only part that we observers ever have access to would go down so he mentioned this puzzle to his student student named jacob beckenstein a brilliant student and and beckenstein came up with an idea to resolve this puzzle and the idea is that black holes do have entropy black holes do have entropy they're not the simple pristine objects as indicated by einstein's general theory of relativity where when you look at the mathematics from carl schwarzschild back in 1916 you find in some sense that black holes seem to be the simplest of all things in the universe you specify their mass or also their charge and their angular momentum just give a couple numbers and you completely nail down what that black hole is whereas for us to nail down most things in the world around us you got to get a lot more information than just three numbers to nail it down right if you wanted to describe the exact physical state of my body good luck so many particles and so many interactions that to write down the quantum mechanical wave function you know the exact quantum description of my body would be pretty pretty tough in the exact classical description would be pretty tough for black hole the exact classical description was quite simple and that led people to wonder how in the world a black hole could have entropy entropy you need something to carry the disorder something that is able to be affected by say john wheeler's cup of hot tea falling inside but beckensen had a beautiful argument and the argument a little too long to go through it completely right now but the upshot of the argument was that if you know the area of the event horizon of a black hole then you know the entropy of the black hole he basically says take the area of the event horizon of black hole break it up into little plank size squares what's the plank length remember that's this little tiny length at which gravity quantum mechanics really come together 10 to the minus 33 centimeters is the plank length so the plank length squared a little square whose size or the plank length would be about 10 to the minus 66 centimeters squared a really tiny area but take the entire surface area of the event riser black will break it up into these little plank-sized squares and that number of those plaquettes plank plaquettes is the entropy of a black hole now stephen hawking heard about this and thought it was nonsense didn't think that black holes could have entropy and set out to disprove beckenstein's ideas that was his goal and remarkably stephen hawking wound up confirming beckenstein's ideas and going even further he not only showed that black holes do have entropy surprise number one but then he said look in a very mathematical way that i'll just say in words you said look any object that has entropy it also has heat remember heat entropy really wetted together and any object that has heat has a temperature and therefore black holes in some sense should glow with a particular temperature in fact you give an exact formula for what the temperature of a black hole would be don't need the details but it goes like one over the mass of the black hole so now black holes are not even these simple pristine objects as einstein's math seemed to initially suggest but now they carried entropy their surface area they carried temperature and because they carry temperature they glow which means that things in some sense are coming out of a block on a very precise way they're not coming out from deep inside the black hole they're really coming right from the edge the event horizon of the black hole but this now raised the puzzle and the puzzle is okay look if the black hole is glowing if it's radiating so-called hawking radiation in fact we have a little hawking radiation image maybe we can jump to that just to show up what it would look like to have a black hole hawking and you see these uh particles of radiation streaming out from the event horizon of a black hole so here's the puzzle as this black hole continues to radiate and as it gets smaller its mass goes down i said the temperature goes like one over the mass so the intensity of the radiation goes up and up and up over time here's the deep question does that radiation have any memory any imprint of the detailed objects that fell in to the black hole to construct the black hole now if you throw in your your iphone your computer whatever into the black hole a lot of information in there does the information that goes in come out with this radiation that is the big puzzle and stephen hawking famously said that the radiation does not carry an imprint of the information of what fell in and lenny suskind and gerard tuft and others but they were really the pioneers of these ideas they would not accept stephen hawking's declaration and we're going to talk about what ultimately lenny suskind describes as the black hole war a war that i think he would agree that he won he and his colleagues have won and we'll go through that as well as the next stage in the battles that are now being waged today and i see that lenny has joined us so let me just give a quick introduction many of you know lenny of course through his wonderful popular books the black hole war the cosmic landscape he is the felix block professor of theoretical physics at stanford university and since 2009 he's been director of the stanford institute for theoretical physics he has made tremendous contributions to physics across the board string theory black holes quantum mechanics elementary particle physics so it's a it's a pleasure to welcome lenny to the discussion lenny how are you doing good how are you brian i'm doing pretty well where are you right now in uh at stanford at stanford no i haven't been at stanford in stanford university since early march yeah yeah but i'm a mile and a half away in my house and you've been teaching me like like all of us you've been doing the zoom teaching yeah yeah uh this quarter i wasn't teaching next quarter i'll be teaching and um yeah but i've been doing a lot of lecturing a lot of communicating with the public and so forth and my physics colleagues zoom is a wonderful thing yeah you try to imagine what it would be like if there weren't these tools i mean if this would have happened ten years ago uh yeah it would have been uh quite an impact well i probably would have um sat down at my desk with a piece of paper in front of me and written some good papers yeah as it is i had so much contact and so much fun uh with my uh with my colleagues uh physics has done a marvelous job of keeping us all from actually working and uh so yeah who knows now remember at the beginning of this there was a lot of chatter among uh physicists that were now in the position of isaac newton black death and if newton would have had zoom maybe we would come up with the universal law of gravity yeah yeah i think i think we'd still be in uh thinking that gravity was due what did they think the gravity was due to then friction i don't know i don't know uh action at a distance or something no that was newton yeah [Music] yeah oh you mean prior to newton yeah yeah yeah yeah uh yeah i don't know if the things one things want to fall right oh that's right aristotle i guess very uh anthropomorphic view of right so so if zoom if zoom exit existed and when was it 1687 or something around then yeah we'd still be thinking that things want to fall yeah okay hey i wanted to ask you uh i heard just because of where i am at the moment um i am about 15 minutes from a place called the beaver kill is it true that you you are wow what are you doing up there so we have a uh just a house up here in the middle of nowhere so since march we kind of disappeared up here and somebody mentioned to me that you spent time as a kid in in oh yes when i was like just not just as a kid as a young man too my father was a fly fisherman he taught me how to fly fish the beaver kill is one of the great eastern um trout fly fishing uh rivers and yes i spent a lot of time on it yeah ever come uh i mean in recent times do you uh ever come back these ways no i i would love to but just too far away so where is your where is your place livingston manor no we are about 45 minutes from livingston manner we go the other direction so if you head out um toward andy's we're near andy's new york i don't know if you know that little town i don't know where it is yeah most people who stop at beaver cold don't know what happens further down the road it's like north of 96th street in manhattan nobody knows about it exactly right so we're on the other side of the tracks in some sense but uh so i was a very urban kid extremely urban but still uh you know several times of summer we go up to the beaver kill get in the car drive up there take our flies uh fly rods and fish in the river yeah and your dad was a was a plumber he was a plumber yeah and he expected you to go in the family business you really did yeah and and you did did you do so you did some plumbing right yeah for 10 years maybe from about the age of 12 or 13 until i was 22 and went away to graduate school and and so what did your dad think about your i can't say that i liked it that was not a very good plumber and but did your dad i mean did you support your move from the family yeah well that's a story i've told many many times but yes you did in the end in the end he was uh really troubled when i was already married i had children or one child and uh i had decided i wanted to be a physicist i went to see my father and told him and he was pretty upset in fact he didn't know what a physicist was he he had a fifth grade education he did not know what a physicist was and he made the mistake of thinking that i said pharmacist no no you're not going to work in a drug store but i said the one word which was guaranteed to i not intentionally i said like einstein and you know at that time especially among jews in new york einstein was a magic word yeah he said are you any good at this i said well i hope so i think so right and uh yeah that was it that was it he said okay you're gonna be einstein and that was it and i failed him i failed everything yeah you i think you've done quite well for yourself young man um so i want to get into some of the um you know the scientific ideas contributions that you that's what i'm here for but can we spend a moment on on other things for a moment that many in the audience will know you from namely the books the wonderful books written you know because i remember i don't remember exactly what it was but i think it was in amsterdam you told me that you were writing i must have been the cosmic landscape i think it was your first book and and you mentioned to me what a blast you were having you know writing this stuff so so look it's a great service to the public but presumably this is something that you deeply enjoy doing and maybe come through in the right i did i got so into it i got so into it that um you know i would i took a leave of absence to write the book i think for pretty much a whole year and i had some sabbatical leave leave for absence and i would get up you know eight o'clock in the morning sit down at the computer and by five o'clock in the afternoon my wife would have to drag me away it was really fun uh i really enjoyed writing about physics but it turned out that i also really enjoy writing about people well of course i enjoy writing about myself we all do no i don't know if we all do but right well you but it's interesting that you say that about people because the one i don't know if you remember this event i think i was speaking like to the queen or something like that at holland but we only spoke for a woman and and and and i wanted to like there was one piece of advice i wanted to give you one right you didn't need it obviously but the one thing that i would have said was when i wrote my first draft of my first book elegant universe i left out all the people i was afraid that like somebody would get mad that i didn't give them the right credit and so i said look let me just leave everybody out they'll like make it simple and the book was i went back and put all the people back in and and then when i put the people in it came to life it came to life yeah you know yeah well i enjoyed writing about the people from the beginning i've always enjoyed the history of physics i enjoyed uh learning how the people how the people thought i've always been very very interested in how particularly physicists but more generally how people think about things how their minds work and uh and um so i i found it very very interesting to go back over my oh not not my whole life but uh the uh the episodes that took place and trying to figure out you know what was going on in people's minds and uh then when i came to write about it i just it was just natural for me to to take it from that point of view yeah i mean you write very much like you speak i try to do that too i mean are you basically letting it just flow as if it's a conversation when you're writing is that the process yeah it's interesting my first time around when i write whatever i write i write it in the language that we all learned to write in when we became physicists the dull boring really technical uh way of writing and then i have to go back over it and eventually it starts to flow eventually but each time i start to write each new subject the book had what 12 chapters i don't remember and each chapter started the same way it sounded like the sort of things we write when we write physics papers and then looking at it you know i'm not sure if i write well but i do read well and i do have some sense of when things work right i look at boy that doesn't work that really doesn't work and uh so i start thinking about how to explain things better and after a while that technical jargon stuff sort of forms a backbone for writing but then it starts to flow and it becomes a sort of natural language so the first time around not natural language second time around yes it becomes i i also oh the other thing i do is i read what i wrote out loud yeah that's very helpful yeah i read it out loud and see what did that sound like yeah no it's very similar to what i would i do as well and it's interesting you mentioned the way we're all trained to write as physicists because i was reading not that long ago the original davis and germa and in that paper they described in a different way the accident in the laboratory when they were doing you know electron scattering off of nickel crystal and they describe what happened they turned up the intensity too high they ruptured the vacuum chamber they had to clean off the nickel crystal by doing that they changed the scattering centers and in that way led to data that confirmed what we now call the double slit experiment but today if people had that experience they wouldn't write about the accident they would just write about the result right and and so we have taken some of the human side out of things very much so yeah yeah i find that sad um yeah i don't know what to do about it so so what do you think that we do as physicists i mean do you think that we are revealing truths realities or just making models of of of the world oh gee you know that's yeah that's a tough question it's as tough as any question i can think about about the question of what is reality does reality exist are we finding it or are we just making models of it and um i think my tendency is to say put that aside and let's just uh let's just go with our curiosity i'm curious about how this works let me try to figure it out um it's a pretense but i like to pretend that i'm more of a car mechanic than a philosopher trying to not a car mechanic but somebody trying to figure out how the car works uh that's of course a little bit pretentious i'm not a car mechanic but um but i i do like to think of myself as somebody trying to figure out how a piece of machinery works a piece of quantum machinery so it's not like a car and just uh try to understand how its parts work how they fit together what's going on how does quantum mechanics fit together with gravity and not worry too much about these very heavyweight questions that i know that i'm not going to be able to answer yeah but i mean it's totally remarkable that we can even even do that right i mean einstein famously said the most incomprehensible thing about the universe is that it's comprehensible now there's two ways of interpreting that one is the fact that the universe has these patterns but two that we can figure them out yes i just i i just gave a lecture i give the lecture simon's institute i think and uh i began by pointing out exactly that that somehow evolution created a species just one step above the monkeys that uh can not only ask questions about how nature works but follow the thoughts and figure out how nature works and that is a very very remarkable fact yeah that i i don't know anybody understand that does anybody understand how evolution gave us the ability to figure out quantum mechanics i don't know but doesn't it it's very hard to imagine you know natural selection would tend to select those things that help for survival and those of our forebears who sat down and thought about quantum mechanics on the african savanna are the ones that probably got eaten because they didn't have a lion that was going up you know tripped over their feet that's right yeah totally um you know there's this there's also this moment if you remember but in in the the the television adaptation of the elegant universe that you were in um there was a moment in the beginning where i'm lecturing to a dog you don't know that at first i'm just doing you know einstein's equations on the blackboard and there's some student who's not getting it in the camera pans and it's a dog and the point there was there are many intelligent creatures on this planet that don't have this ability as far as we know you know to figure out how to figure out these things so what is it that allowed us us to do that and so yes it is did you ever did you ever did you ever read kafka's investigations of a dog i have not i don't think i have tell me well kafka was making phonophysicists basically and uh he invented a dog that became curious and tried to figure out how the world worked and among other things he tried to figure out where food comes from he didn't understand why uh how food appeared in his dish he didn't understand how it circled from above and came down and he made all kinds of theories about the [Music] so it's very funny it's very funny i think it was i think it was making fun of physicists yeah no it sounds uh it sounds kafka-esque and it's very very much that's right so before we move on to actual science there's only one other thing i wanted to mention too uh in in of the black hole war there is an unusual and wonderful chapter where you describe a very personal stretch of time when you were utterly obsessed with stephen hawking utterly obsessed with wanting him to see the light and almost like in your words you know ahab going after the the white whale you were like tracking him down no when you were right i mean it's an exciting chapter to read but when you were writing that and i sometimes go through this too was there a question of am i revealing too much of myself is this you know you know i no i i don't i was having fun yeah i was having fun writing sort of enjoying uh enjoying reminiscing and thinking about my experiences i i wasn't i wasn't that kind i wasn't worried about that no yeah it certainly shows how physics is not a nine to five undertaking right i mean it's uh it's uh it's all it's all encompassing when you're working with these kinds of questions all right so yes it's it's a combination of 24 hours a day and maybe three days out of the year three days out of the year something happens the other uh the other 24 hours a day uh sort of frustration banging your head on the wall well it's good for students to hear that i i mean as you know i mean the number of graduate students that i've encountered who just wonder when the big breakthrough is going to happen you know when and and and it's rare you know it's just unfortunately rare you know um so i wanted to talk a bit about some of the breakthroughs of uh in some sense the last 25-30 years but we'll hopefully get toward toward the end i mean the things that involved here general relativity quantum mechanics thermodynamics statistical mechanics black holes entropy entanglement you know worm i mean it's just an ending list of of and now and now computer science is coming from your time totally so can we just do a brief run through of the general relativity side of things the puzzles that um were apparent to you in gerard a tough not apparent to many others and then where things stand today on all that so you know i made mention before you came on to john wheeler and his teacup worrying about what would happen if he threw his teacup into a black hole where the where would the entropy go when did you start worrying about questions like that well okay so i didn't learn really much about it until oh roughly 1980 1981 which was seven or eight years after beckenstein had made his great breakthrough and discovered that black holes have entropy before that i was obsessed like most of my generation of theoretical physicists with problems of elementary particle physics i it was 1980 or 81 i can't remember exactly it was at a small conference that took place in the mansion of a rich man in uh in san francisco werner earhart as a matter of fact some of you may know his name um and uh yes guy or something right right right he was also a physics groupie and um anyway stephen was there gerard the tuft was there and a bunch of other people that i can't remember right now but uh i think i don't know whether gerard knew very much about what stephen was talking about beforehand but stephen got up and said black holes lose information that what goes into a black hole cannot come out uh of course in practice that's sort of true but in some very deep fundamental sense he said there was a contradiction with uh a basic rule of physics and the basic rule of physics is nothing really ever gets lost information never gets lost it's called unitarity in quantum mechanics but it says that uh that what goes in comes out even if it may come out in a horribly scrambled form that's extremely un uh extremely difficult to disentangle anything that goes into a black hole has to come out anything that goes into anything into a fire into a furnace whatever has to come out even if it comes out in the form which is very difficult to decipher okay steven said black holes are not like that black holes are different what goes in in order for it to come out it would have to exceed the speed of light and we can't exceed the speed of light and so black holes are fundamentally different than anything else in nature and in fact they must break the rules of basic quantum mechanics um that just felt wrong i realized the depth of the question the question was clearly very very deep and very very important i can't even explain why it sounded wrong so much of what i knew about physics was ultimately based on a rule of physics you can call it what com what goes in has to come out you can call it unitarity you can call it reversibility any number of different terms for it um and everything i knew about physics rested on that linchpin that uh of unitary let's call it it had to be wrong stephen was giving up quantum mechanics prematurely not only prematurely wrongly he had asked an extremely deep question and in my mind and in the tufts come to too easy and answer just the quantum mechanics was wrong i just wanted do you think that's because stephen was coming at the world from a general relativity perspective yeah yeah i think i think so but i think that was part of it um but he certainly had a very deep understanding of quantum mechanics he could not have done what he did had he not have that that deep understanding of quantum mechanics i think maybe there was also an element that he got so excited by his own uh revelation that quantum mechanics had to be wrong that he found it psychologically hard to let go of it i think but i i knew stephen it was hard to know stephen i know for certain that he admired me i certainly admired him um to get into his head was so difficult for the just because of the physical barrier the physical barrier of him not being able to uh to communicate uh so i can't really say what was in his head but he insisted uh as rod and i insisted that he was wrong and uh from that time forward i think i felt that that was the biggest problem that i could address the rest of my uh whatever abilities i had to try to uncover what was going on there but strangely you've also noted that not many other people elevated issue the way you did yeah the that's right the general relativity community for the most part just thought that stephen was right he said okay information is lost in uh in black holes that sort of stunned me but i guess maybe uh well the the particular component of the general relativity people were people who did know quantum mechanics and did appreciate quantum mechanics uh so it wasn't that for a lack of appreciation of quantum mechanics but somehow they just um uh took it as given that what goes into a black hole can't come out and that was the end of the story for them uh the particle physics community which was much more centered on the rules of quantum mechanics the rule that i just called unitarity was an essential component of of particle physics they tended not to be too interested in gravity the reason was i think feynman said it he said that gravity is so remote so remote in the sense of being the gravity quantum mechanics of gravity here is so remote that it'll be 500 years before we'll be able to learn anything about it and so the particle physics community had other fish to fry they had the standard model they had quantum chromodynamics uh and so forth and so they just you know they said don't bother us with this gravity crap right it's uh there was very very few people who uh really got terribly excited by this very deep observation of stevens and so i sort of felt in the wilderness for i'm not sure how long maybe a period of 10 years were these ideas pretty much percolating the back of your mind because you were also engaging with all these other areas too that were more in the forefront at that time yeah yeah no it was not in the back of my mind it was in the front of my mind but yes that was uh very troubling and and and so uh were you in gerard sort of comrades in arms on this kind of thing or were you both going your own ways and communicating now and then gerard and i are friends we've always we've been friends for a long time uh you know he's a bit of a contrarian and he's a dutchman which makes him a little bit bristly my description of my relationship with him was so very often i would consider i would say to sarah zoron i completely agree with you and his answer would be yes but i completely disagree with you so but we do yeah we uh we certainly communicated quite a bit and um for me for my part i felt uh comforted by the fact that gerard had the same views that i did i considered him a very great physicist and i i saw him as being extremely deep and the fact that he had pretty much the same views that i had was it was uh comforting and and so the picture that you guys began to develop which now has been elevated to a principle holographic principle can you just tell us a little bit about what led to this perspective that that the information somehow exists on the surface of a black hole when an entity falls in yeah um right i was good it is literally true i've been thinking a lot about the information falling into a black hole and to me the big puzzle was it seemed like information had to be in two places at the same time if you were to if you were to take the view that gerard and i did that information that falls into a black hole is ultimately radiated back out in the hawking radiation and at the same time that information can fall through the horizon of the black hole then it seemed like there was something going on that said that information somehow can be in two places at the same time or if not in two places at the same time very much like quantum complementarity that either you look at it one way or the other way but you don't try to you don't try to look at it both ways uh so somebody falling into a black hole sees the information going into the black hole somebody outside sees it outside something funny going on about the localization of information it was literally true um i've told the story a number of times i think i even wrote about it and people tend to disbelieve me they think that i'm sort of making up a little story it was not a story i was walking uh in stanford also stanford at that time there was a small science small physics display uh we called it a museum and and the display there was a hologram there was a hologram of a very very pretty girl and the there was the the picture of the of the girl was surrounded by a film and i knew what a hologram was but it just hit me there's an example of information that's being stored in the film strictly speaking it's being stored in the two-dimensional film and if you looked at the film you would just see a bun a little carefully through a microscope all you would see would be random markings with no particular structure that you would recognize and if you did the right thing with that film mainly shine light on it but it doesn't do the right translation dictionary you would reconstruct a full three-dimensional object in the interior of this space which was surrounded by the film it was literally true i looked at that and i said could it be that in some quantum mechanical way the surface of a black hole the horizon of the black hole is functioning as a kind of hologram where information can be thought of as being stored on the boundary on the horizon itself being like the film itself and at the same time a translation dictionary would allow you to think of that as information that fell into the black hole that that was sometime around 1993 gerard was thinking i guess very very something very very similar and uh he published he published the idea i uh i didn't even i didn't even know that he had published it i found out after writing my own paper on it somebody uh somebody told me that he had written a paper i couldn't find the paper but i spoke to gerard and he told me what he was thinking it was very similar and so i just attributed it to him the basic idea but holographic way of thinking about it was your image of how to explain what the black hole was doing then presumably yeah yeah but there was a generalization of it which i think we both thought about but gerard really really expressed it very clearly that it wasn't just a black hole if you take a region of space no matter what is in it whether it's a black hole or anything else you should be simply because a black hole carries more information than anything else that a region of space cannot have more information in it than that that which can be described as being on the surface of the region of space rather than in the volume and so it became more than just the theory of black holes it became a theory that if you like of a piece of the universe if you take a piece of the universe it should be possible to describe it in terms of degrees of freedom mathematical structures that in some sense live on the surface of the region of space so right i mean that's just shocking it looks very shocking okay i i called my paper the world as a hologram um i don't remember what azeroth called his yes it was not only shocking it was also thought to be a big crackpot yeah um not too bad uh yes when i would talk about it i got the feeling sort of mildly amused that uh people had thought that both gerard and i had lost our marbles we were once good physicists but here we were um a little bit crazy but it might be good to explain to people how nuts i mean we're used to physics being you know things take place in the volume of space you can tinker with it you can tinker with there but you're in some sense saying that's illusory because the actual fundamental degrees of freedom don't fill out that much they're not as numerous as that and they're not as numerous as that and if you right and if you try to excite more than a certain amount of structure in that regional space you will inevitably create a black hole that's bigger than that region and so there really is a limitation on how much how many bits of information you can stuff into a region of space and it's not the volume that's the area so that's a that's a that's a well accepted thing the first person who really got excited by this apart from gerard and myself and um who called me up wanted to know more class asked me please come to his institution and talk about it was in fact it it wouldn't edwin yeah he got very excited by him right yeah it's not his style it's not his style of thing but you know he's extraordinarily smart and he's a great physicist and he saw he saw that yeah but maybe that was right yeah i mean in some sense this is perhaps among the most radical breaks with human intuition that have been developed over the course of we're talking evolution before certainly evolution imprinted in us a view of the world yeah look at the world it's funny it's so it's so widely accepted now that nobody really seems to understand why or why it was considered so radical well that's always what happened young young people here yeah you know um so things go from a crazy conjecture to well crazy speculation to a conjecture to a tool of the trade in almost no time at all and of course you you of course know why it suddenly became a tool of the trade it was the work of one one of the senior yeah when we get to that so so so you are saying that even in this room that i'm sitting or any room that anybody else is sitting in some sense there is an illusion going on the illusion that we have the freedom to change things within this volume at will is wrong the actual capacity of change is far less and can be fully described by data that lives on a surface that surrounds that region yes that is what it says yeah so so it's understandable that look i mean obviously you always command a great respect but i can imagine people hearing that and saying this is going one step too far and it just doesn't seem like it could be right yes and then as you say i'm sorry go ahead well i've always admired the the great detective sherlock um one of his uh i cannot i can never quite remember the quote but uh when you've what was it when you tried everything uh and everything doesn't work that which is um may seem most improbable but if it's all that's left it has to be the truth do you remember the quote it's a sherlock holmes quote uh yeah i mean the justification we've tried everything and nothing works uh nothing sensible works whatever's left over it must be the truth no matter how crazy yeah have little doubt that somebody in the chat will uh we'll give us that quote in just just a moment uh but so then for the black hole story then you know if i pull something out and i throw it into a black hole how would this description assist us in dealing with the the paradox the problem of the information well the information was ah okay so the way i thought about it at the time which may have not been i'm not sure it's exactly the right way to think about the way that i thought about it at the time was the problem was that information that falls into the black hole cannot escape because it can't exceed the speed of light on the other hand the hawking radiation must be carrying off the information and how did it escape from the black hole well the intuition was that really that it really was like a hologram that the stuff on the inside was to some extent illusory as you say i don't know if illusory is quite the right term but uh some sense a consequence of the mathematics of the hologram and that the reality is really stored on the surface just above the horizon of the black hole and that eventually the evaporation of the black hole was tantamount to the evaporation of this film of the uh of the holographic film itself which would which contained all of the information about what was inside the hologram but when it's evaporated there's no interior there's nothing left because there's no image left in the interior because the hologram which was in some sense the reality has evaporated but the information was not destroyed it was carried off by the evaporation products so that was a little bit naive but that's the way i thought about it at the time and some of the things that you had to contend with to make it less naive right there was this issue that i think you you and others raised that you know you throw an encyclopedia in if it if it goes in and yet also leaves its information on the surface you seem to have these two copies of the nation and there's this famous no cloning theorem in quantum mechanics that tells you you can't actually create those kinds of copies yeah so that's right so i was uh that that was right the other person who thought about that a lot was um uh john pressgill and john preston taught me classical mechanics when i was an undergraduate you know yeah right so yeah there was this problem of the replication of information that in quantum mechanics the replication of quantum information is not permitted and so when something falls into a black hole according to the usual rules uh it uh it can't be replicated in two places at the same time my answer to that at the time was by thinking about various duncan experiments i convinced myself i think correctly uh that it was absolutely impossible for any actual observation or experiment to see both copies at the same time so it was much like um much like ordinary quantum mechanics where you can either describe a particle as having a velocity or momentum or you can describe it as having a position but you can't describe it as having both at once i called it quant black hole complementarity that it was okay to have this duplication of information as long as nature itself prevented any observation from uh from seeing both replicas of the information and uh so through a series of arguments i was able to convince myself and other people that it was impossible to have uh knowledge of both what falls in and what comes out if you're outside the black hole you see what comes out if you fall in with the material filling you see it fall in you can't be both outside and inside so it's really all okay i think it more or less worked out that way and and so taking that complementarity idea one step forward it raises the following question so many people who study black holes know that if you're looking at an object falling in from the outside something curious happens you see the object getting closer and closer to the horizon but you don't actually see it falling over the edge you see it redshifted you see it time dilated and so forth but you never actually see it crossover so in some sense it gets plastered on the outside yeah in retrospect would that have led one to think in the holographic idea i mean was that part of your thinking that that that got you there absolutely absolutely that uh that was very much part of the thinking um i knew actually i think from reading uh kip thorne's book on the membrane part kip thorne and other people's book on the membrane paradigm for black holes yeah membrane paradigm this was a technical book this was not a highly technical book more technical than i wanted it to be i had known about this picture that everything that falls onto a black hole forms a sort of sedimentary structure that gradually sinks towards the horizon but never passes it at least that's the picture that somebody watching it from the outside says and so from that picture i again there was a sort of sherlock holmes moment it has to be that way even though in a frame of reference which is falling with the material falling in it appears to pass through the horizon it still must be consistent to think about it as uh from the outside as having been plastered within let's say a plunk distance from the horizon so yes that was absolutely an important part of it and yet the thing that you that you mentioned in there which is among the things that makes it hard for at least a general person to fully understand this to the freely falling individual they pass through the horizon as if there is nothing there there's no impact at all but from the outside perspective there's a lot going on at the horizon because all the stuff as you say is getting plastered there and ultimately being emitted through the hawking radiation how about how do how to help someone put these two perspectives together or do you simply admonish them using black hole complementarity and say look take one perspective take the other you're going to drive yourself crazy if you try to think about them both at the same time right same kind of craziness you know a particle what is a particle a particle is a thing with a position if it's anything at all it's a thing with a position if it has a position and every time it has to have a velocity so how do you reconcile in your head that a particle has either a position or a velocity or not both and not both it's the same kind of craziness eventually you give up and you say those are the rules of quantum mechanics you uh you follow heisenberg with his microscope trying to locate a particle and both see its velocity in his position his gadon his thought microscope and what did you discover you discovered that whether or not you think a particle has both a position and a velocity you simply cannot measure both so from an operational point of view there can't be a contradiction because you cannot have access to both kinds of information that was my that was my opinion about the black hole problem that it was a problem of complementarity and quantum mechanics in which the two kinds of information were incompatible no experiment could could reveal both what's going on as the material fell through the horizon and at the same time watch from the outside i can't help but ask though so when we talk in quantum mechanics about particles not having a momentum and a position simultaneously when i one always needs to ask myself are we talking in terms of the way reality is or the way reality is measured because for instance there is the de bruy boom approach to quantum mechanics i don't know if this is an approach that has excited you at all at times it has me this is an approach in which particles do have positions and momentum simultaneously in terms of the the properties that they actually have the ontology the stuff that's real in the world even though as you say you can't measure both but yet they do have both whereas in conventional quantum mechanics they really don't have both simultaneously look at the wave function it's either this wave function or that wave function it can't be eigen states if we use slightly technical language of both the position operating the momentum operator would a de bruy boom like interpretation have any role in black hole complementarity is there some sense that these things are there you just can't measure them you're not talking about the particle now you're talking about the information in the black hole yeah first of all i'm not a partisan of the uh the the broadly uh yeah most aren't yeah no no and if you ask me why i would say well it's some kind of cooked up theory of a single particle but we live in a world of quantum fields we live in a world of multi-particle systems and it gets so baroque and so complicated when you try to yeah think about quantum field theory in that language that i just don't think it goes anywheres yeah i mean obviously there are people in that community who have a different perspective but that's another guy i know and uh you know who knows yeah look i am i'm a person who knows how to use i think i know how to use quantum mechanics i know how to examine experiments from the point of view of quantum mechanics am i puzzled about the whole quantum mechanics business of reality and non-reality and uh and what the whole um interpretation of quantum mechanics is yes i am but i think it was feynman who said it best uh when he was asked about exactly this he says i'm so puzzled about quantum mechanics that i can't even tell if there's a problem or not yeah yeah no it's uh well i mean the measurement problem seems like a real issue that we have to resolve at some point right we have an algorithm without a mechanism well i know i think there is a mechanism i think the mechanism of entanglement between the apparatus and the system measured is is a consistent description of what happens during a measurement the and do you take a mini world's view of things or i mean is it just the way i wouldn't go that far i wouldn't go that far i i find it just very puzzling okay you want me to tell you when i stop thinking about this where i tend to go yeah i wouldn't mind all right yeah um so imagine this is where i start imagining i want to make a model of the world in which these kind of questions might come up might even have answers so take a giant tin can but tint can i just mean a volume of space let's forget the holographic principle let's even forget gravity or quantum gravity and so forth we have a volume of space which is isolated closed volume of space and uh somebody puts a bunch of particles into it how many particles oh i don't know 10 to the 50th or maybe 10 to the 80th what is the number of protons in the universe tend to be into the 88 or puts those particles into it uh and allows it to evolve okay it evolves quantum mechanically the world's kind of rules are quantum mechanics and after having evolved for a while we want to ask the question um if not if but is there an experiment going on at coordinates x y z in which an experiment is being done on a single elementary particle and what are the probabilities of the result of that experiment well if you start thinking if you start thinking about that how you would analyze that you begin with the fact that the quantum wave function the quantum state of this collection of particles is vastly big it has enormous amounts of irrelevant information in it these are the branches of the wave function most of the wave function describes things or does not describe a world in which there is even a planet at your coordinates x y z let alone an observer let alone an observer doing a certain experiment some tiny tiny portion of this wave function of this state vector or whatever we call it uh some tiny tiny tiny portion of it buried in an enormous gigantic branching tree of possibilities may describe the question that you're trying to answer the uh the whole quantum description is excessive it has much too much information in it and how do you extract out anything interesting from it it would be absolutely impossible to be compared with a classical description in the classical description there either is or is not an observer doing a certain experiment someplace sure because either is that particles are in definite locations and you can simply go and see whether there are particles forming an observer doing an experiment the quantum wave function is vastly vastly bigger in mathematical complexity and because of that it contains information so big that almost all of it would be relevant to any experiment okay then you could say well all right look nevertheless i'm going to follow that wave function and what do we do with the wave function we do experiments we do experiments and use the wave function to get probabilities so what's an experiment the people inside this tin can if there are any people and mostly there won't be people uh they're not capable of doing an experiment on themselves we can imagine opening up the tin can some observer who built the system can then open the tin can look inside and do an experiment okay what you're going to find with overwhelming probability he will just find some totally random mess of particles it can measure the locations of the particles or it can measure their velocities you can't do both and they'll just get a bunch of numbers which has nothing to do whatever with the questions you might have wanted to ask about what's going on in there if you wanted to get better information about probabilities you could do it but you could only do it by doing many many many replicas of the same experiment but my point is here this is just getting excessive it's getting out of control we don't know what the hell we're doing we would not know how to use even if somebody provided us with the exact wave function we wouldn't know what to do with it so my feeling is at that level we really don't understand quantum mechanics but that's it that's a that's a really important remark though right because i think so if if for you if we view quantum mechanics as the fundamental paradigm uh and we want you know it to apply to everything the stuff in the tin can the person from the outside looking into the tin can right um uh the the statement that we don't know what to do with that structure is fundamentally pointing to a deep ignorance in in in the very basic laws of the universe right yeah yeah absolutely you yeah um so my friend my friend gerard he believes he's ideas about this yeah he's been he has ideas about it i don't agree with the ideas i do again again he's asking very very deep questions whether his answers are the right answers i i of course don't know i don't think i agree with them but uh you know my feeling is anybody who takes the time to sit down and think about what quantum mechanics is and how it relates to the world and so forth can't help but uh have this sense of confusion the sense of right puzzlement yeah i mean there's often there's often the sense that einstein's resistance to quantum mechanics was a remnant of a dinosaur-like attitude and it's not no there's a real issue that einstein many issues that is focused upon and it's still with us um so so with that i i you know you mentioned juan maldasena and the work that ultimately brought the holographic principle this idea that the information would be stored on the surface to its most refined form that journey of course was the end point of a journey that begins with other work that you initiated which is string theory um so if we could just pivot for a moment to string theory don't forget don't forget our friend joe polzinski yeah absolutely but going further back so in the early days of string theory we're now in the early 1970s um you know this this this idea came along um largely at first through the study of interaction strong interaction physics with formulae that were written down that seemed to do a good job of describing certain kind of data sets you looked at this mathematics and within that you saw the glimmers of string theory so just just take us back to what that was like it's a long long time ago yeah more than 50 years uh it was it was late 60s 1969 or 68 or 69 yeah um the questions and the physics were not about gravity they were not about gravitons not even about photons they were about hadrons hadrons means subnuclear particles protons neutrons mesons the things which we now know are made out of quarks uh there was a lot of experimental data about them they were strongly interacting which means when they collide they made lots of junk and all kinds of stuff but they also seemed to have a capacity that an electron for example would not have electron has spin okay but you can't spin an electron like you can spin a basketball you can take a basketball you can still can think of it as not spinning or you can spin it up and you can spin it up even faster eventually of course if you spin it too fast it'll break apart but the basketball has all sorts of rotational states uh an electron has only one or one or two states of existence it's its rotational properties hadrons seem to be more in a certain sense like basketballs you could rotate them up and uh the proton could be spun up so that at a higher angular momentum whole sequences of states of higher and higher spin seemed to grow out of spinning up a proton so uh that was that had nothing to do with me that was experimental physics from the 60s a physicist a very good physicist by the name of gabriele veneziano who i didn't know at the time had constructed a formula for the scattering of hatrons and it had embedded in it all this information about the possibility of spinning up the particles which collided and it was called the veneziano formula i'm sure that you know what it is here but right uh and it was a very simple formula it it what did it describe it to describe the amplitude or the probabilities for the scattering of two particles to result in different outcomes it was just a function of two variables a variable called s in a variable called t it doesn't matter the momentum of the particles it was very simple it had a product of two functions two well-known common garden varieties oh let's tell people there were gamma functions we can they were gamma functions right a gamma function two gamma functions in the numerator and one gamma function of the denominator and that's all that it was okay so somebody came and showed me that that formula and the thing that struck me was not all the physics that went into it but the simplicity of it and i noticed that it did have this structure that described let's call them the excited states of the particles that were colliding or the excited states of the particles that could occur in the intermediate states of the collision and the other thing that i noticed is that those excited states in their energy were equally spaced they looked like harmonic oscillators they have the spectrum of energies of a harmonic oscillator or equally spaced energy levels true enough the equal spacing was in the mass squared not in the mass but never mind that it was a detail and so i looked at this thing and i said my goodness this must be some kind of harmonic oscillator and so i started experimenting around i i knew a lot about harmonic oscillators one of the few things i knew about and so i made a model the model was a harmonic oscillator which meant two particles connected by a spring and a third particle a photon would scatter off one of the particles in the spring you could think of the two particles as being charged particles and the charged particles would scatter a photon the photon would be absorbed and re-emitted by the charged particle and i worked out the corresponding amplitude for that and it looked an awful much like the veneziano amplitude not quite not quite it looked enough like it that that i wrote a little paper called the harmonic oscillator analogy for the viniciano amplitude and it was during that period when i was writing the paper that i realized i i knew that it wasn't quite right i knew it was just an analogy that i realized that if you added more oscillators and turned the spring into a string i called it a rubber band at that time that you would get exactly the minutiano amplitude right that was exciting i mean it was uh something uh i don't know how to describe the feeling that you get when you when you encounter a thing like that uh so it worked i knew at that point exactly that that what this thing was and i was pretty certain that hadrons really worked that way it was kind of interesting the psychology of it i was very excited because i thought i was the only one in the world who knew that yeah when you think you're the only one in the world who knows something and you're about to tell everybody it and we think okay that's that's uh something exciting yeah what i didn't count on is that there was somebody else who knew exactly the same thing exactly bamboo [Music] bamboo of course was a great physicist at that time he was uh one of my heroes so on the one hand i was terribly disappointed to find out that somebody else knew it on the other hand if it had to be somebody else better that it was nambu i felt a certain exhilaration in having discovered something that the great mambo had discovered and when you wrote that those paper they weren't exactly met with standing ovation right no they were they were rejected by the physical review letters yeah and so i mean was that it was that a sense of terrible objection or yeah or did you just say that those are silly people and they don't just get it you know oh i said both i've been rejected by silly people who just don't get it right right now somebody wrote somebody wrote a referee's report saying uh there there's no new information no new experimental data in this paper and there are other derivations of the veneziano model so i don't recommend publication of it and yes indeed i was very hurt and i was not a famous physicist at that time but you resubmitted someplace else presumably and uh it was yeah i did resubmit the physical review it was published yeah uh but it took a long time and during that long time i was um right not happy by 1984 so i was a graduate student i started graduate school in 1984 and um you know by that time string theory had just bubbled up to the surface where there was a basic feeling that if you weren't doing string theory you were missing the boat and it was the most i mean for me was it you know if i look back on my years the most exciting period there because all of these greats in physics were buzzing around saying here it is you know this is the thing that we have been looking for at that time did you think that the pendulum had swung too far the other way you know from you know initial rejection that was wrong but was it you know i guess uh greenspan's you know uh you know uh whatever the word was you know euphoria that wasn't you know justified by where we were no i i i didn't feel that um i did feel that it wasn't addressing this one question which i thought was so central the question of the black holes and then until it did that uh that it had to be considered um well what we would call perturbation theory i didn't think it got to the true depths of the uh very very hard problems about quantum gravity but i i think i along with a lot of other people thought that there was a reasonable hope that some version of it not the precise versions that people were working with i knew that they couldn't work they were too super symmetric they were too special but i did think that there was a good hope that some particular version of it would um nail the particle physics uh spectrum that would explain the three generations of uh you know the three families of quarks the electron and neutrino all that sort of stuff or at least be able to accommodate those particles some particular version some particular what we call a compactification which you know very well some collabo manifold or something which i still know very little about that that would nail apart you want to know i'm happy to have a yeah right you said you certainly can um but you know after a period of trying and uh and particularly a number of things dissuaded me from that the things which dissuaded me from it were cosmological the cosmological constant well lack of supersymmetry in the world um the cosmological constant the uh the successive inflation and all those things together with an observation that i think was joe polczynski and raphael busso that who estimated the number of possible solutions of string theory that by that i mean the number of possible string theories that you could have and that was gigantic they said 10 to the 500 the number may be vastly bigger than that and so it came down then to saying we have a theory which has so many possibilities that looking for the right one would be like a needle in the haystack and or a word much much worse than a needle in a haystack and uh so i began to feel that the that the effort to find a precise version of string theory which would describe particle physics was probably misguided maybe buried in this huge heap of possibilities there was something but how do we make a theory instead of finding the right precise one how do we make a theory or something useful about this observation that there are 10 to the 500 possibilities yeah and that for a period i think that led me um to be a bit of a pariah in the string theory community they didn't want to hear that yeah now i remember you and i talking about this i think was in sweden or something at some gathering and i asked you remember i asked you do you really believe this are you just trying to stir up the other stranger you know i'm not a star yeah you know it's definitely definitely genuine so we'll come to you know the development that ultimately yields uh the holographic refinement if you will but um let's go back to string theory for a minute yeah please yeah yeah i i think string theory has had a gigantic success but the success is not was not in reproducing the particle spectrum and that sort of thing it was providing a an example an existence example uh for a theory which allowed quantum mechanics and gravity to be combined consistently yeah and that's no small thing that it i'm not talking about the early versions of string theory i'm talking about the later things things like matrix theory and ads cft and all these things that juan invented and so forth which produced a very a series of very very precise and exact let me call them universes uh in which quantum mechanics black holes and gravity coexisted consistently so after that happened if somebody came and told you look there's a contradiction between quantum mechanics and gravity was a contradiction between black holes and standard predictions of gravity you could say no there is no such contradiction we know that because we have an extremely precise set of examples and i consider that to be you know a great triumph of string theory yeah no i fully agree and i'm glad you raised that point because i think as as you know i know and as many people watching this know there's a lot of chatter out there a lot of nonsensical chatter out there which tries to make the case that string theory is useless it's collapsed it's gone away something's wrong with the theory and and and the point that you're making of course is the right one which is sure there are many unresolved issues about connecting string theory to the real world but to show that gravity and quantum mechanics can coexist in a coherent consistent manner a mathematically calculable matter a precise mathematically calculated manner that justifies the existence of string theory and all the work that's being done to develop it forward and it's just really sad when i see people get the wrong impression from the blogs and the books and things that that are out there it used to be in the old days the physicists would do their work and nobody paid any attention and it was great now everybody pays attention and everybody has an opinion and everybody gets extremely contentious and political and all that kind of crap and uh yeah it's not nice yeah no exactly so uh a breath of fresh air for the the actual situation to be more accurately described um but um let's now turn to the work that was spearheaded initially by joe polczynski and ultimately results in the achievement of juan maldasena which comes through the technology of string theory and ultimately connects with the earlier part of our discussion which is this holographic idea it's a tough subject to discuss but can you give a brief summary of what juan found yeah um let me first say something about both these two people and more than these two people are currently evolving generation we tend to think of the generation of einstein and heisenberg and schroedinger something extraordinary exceptional that never happened again in physics i don't think that's true i think people like juan and joe ochinski and alda cena and the and also the great string theorists were every bit i don't know about einstein einstein was a very special case but they're every bit this uh the uh the equals of the people in the early part of the 20th century who invented quantum mechanics and so forth i'm just saying that because i want people to know that that the heroic age of physics has not passed uh completely okay now you asked me about what juan did and what joe did yep so it when i say it it will sound like it's not as much as it really was it's hard to it's hard to explain how something could be so important and at the same time be very simple but let me just say it was extremely important joel invented or discovered that in string theory there were contained uh a set of objects they're called d brains they were not the strings of string theory but they were sets of objects planes or lines or even points which were more massive than strings but strings could simply end on them so they were like let's see if we can make an analogy um imagine a wire kind of thing yeah yeah yeah right beads on a wire that kind of thing except the strings could be attached to those beads okay there were new structures and string theory that were that nobody knew about nobody realized they were there a whole new set of objects they were heavier and they were called d brains joe worked out the theory of those and sort of startled i i would say certainly startled me yeah the one person who i think everybody was startled i think edwin was sort of close may have been close to some of these ideas uh and um so there's no object existed there's no objects existed and there's no objects which existed let's say there were exa examples of three brain three brains three brains would have three dimensions uh two brains would be like membranes the structures these structures could have objects on them living on them like you said beads the theory of those objects that lived on them was actually quantum field theory itself quantum fields living on these d brains these quantum fields were all right it doesn't matter what they were but what what juan realized is that these structures these brains these surfaces had all of the ingredients that were necessary for them to be the holograms of the holographic principle [Music] and he basically worked that out a very very precise and exact version of the holographic principle in which the hologram itself that means the film was replaced by a d brains collection of these d brains and that uh the image of this hologram was like the actual reality described by string theory so to put it short it was a very precise instantiation of the holographic principle and uh it was revolutionary it was shocking and it came directly out of joe's uh joe pochinski's construction of the d brains but it has truly revolutionized this area of physics yeah and with this new holographic construction which did not rely on perturbation theory it didn't rely on recoupling and so forth it provided worlds that we could study in which quantum mechanics and gravity we knew very very precisely that uh that they were consistent that they contained black holes and so that uh that that very much revolutionized the subject and shortly thereafter so ed edwin writes a paper where he shows if you have a black hole in the bulk the volume if you will the holographic description is just hot quantum field theory which has no issue with the unitarity that you and gerard have been arguing for that's correct yeah and and that basically at least in an abstract level nails the case that the information has got got to come out how did the community also wrote a paper together it's the only paper i ever wrote with that in which we connected the ideas of the deep brains with the holographic principle right and so at that point was there still any resistance from the hawking general relativity or was that basically it um you know it was well i was going to say something i would probably regret saying we can always cut it out of the one we post if you want to try it all right that was i was going to say it was like trump uh trump refusing to admit that he lost the election hey don't regret but of course of course there was no similarity whatever between stephen walking and the dominant oh yeah the stephen hawking were lit yeah i see that analogy would have some issues yeah yeah that has some trouble with it but i don't mind if you say it but let me just make perfectly clear and plain stephen was a hero and a hero and a hero and that's it right but uh yeah there was it it did have that sense that people were fighting a dying battle that uh there was no point in even fighting it anymore it was finished right um so within some period of time that issue was was laid to rest um but it's still but but it's not as though that story is over right i mean so once you know that the information comes out of a black hole by virtue of translating the question into this holographic question where the answer is manifest obvious that there's no loss of information there's still the issue of the details of how the information gets out and so for instance joe polczynski who you mentioned before together with three collaborators writes a paper that raises some tension some issues in trying to think about how the information comes out suggesting possibility that there's a firewall at the edge of a black hole and so forth was that it was not a surprising paper was that one of those papers very and for a while i was i i wasn't sure if they were right or not but um it was very disconcerting what uh yeah well joe don maroff and two students at the time one uh i've met mary and the other james sully uh wrote a paper in which saying it was logically impossible that after a certain period of an evaporating black hole back oil is evaporating after it has evaporated half of its not its energy but half of its entropy that it was simply logically impossible for the black hole to have a smooth horizon a horizon that you could fall through in other words all the features that we thought were true of horizons of black holes it was and they even said that it was logically impossible for this black hole complementarity idea to be correct right uh the argument was very convincing it made use of quantum entanglement uh it was a simple argument it said on the one hand it said that the consistency of the horizon required that the interior of the black hole and the exterior of the black hole be entangled quantum mechanically entangled on the other hand it said that the radiation which is carried off by the hawking radiation will necessarily be entangled with a black hole and the third element is a thing cannot be entangled simultaneously with two other things maximum can't have right you can't have the outside of the black hole being tangled with the radiation and at the same time with the interior of the black hole and so something has to give and their conclusion was that what gave was that the horizon of the black hole has to become an impenetrable barrier so that in effect the black hole doesn't have an interior um this did you lose sleep on that one i mean like yeah i totally guess i lost sleep on that one um yes very much so and to my mind at first i was convinced they might be right uh that old black holes would have this problem i i didn't think so after a while i just thought no this is much like the hawking thing this is a great question it's a deep question very very fundamental i suspect they've taken the easy answer and that the hard answer is the one which will allow these two things to coexist at the same time the smoothness of the horizon and the entanglement structure so uh and the answer to that i think is the answer that what's called now e r equals epr juan and i started to discuss this over email it's the only it's the only time i've ever collaborated successfully over email was with maldasena um i just sent him a message at some point and said one this is crazy there's something wrong with us can we try to figure out what's wrong with it and we batted ideas back and forth oh for at least a month maybe for more than a month daily emails i could never get him to answer me on sunday but uh he was with his family on sunday but for six days a week we batted back and forth ideas um and we have both been perturbed by one particular thing that there was a particular construction it was one of juan maldasena's early constructions it was called the eternal black hole which was really two black holes two entangled black holes and it seemed to violate what joe and company had said the black hole on one side was completely entangled with a black hole on the other side and yet it didn't seem to have this trouble that they claimed would be there if the black hole was entangled with anything else and so both of us had been troubled by that and at some point juan sent me a very cryptic message it was about the things we were talking about but i think it had one it was a very short message and it said er equals epr and when i saw that i said holy smoke that is what's going on and what i'm sure you want to get to that so i'll let you ask the questions you want to ask well you know it's it's a it's a beautiful and and and deep result uh maybe just for the audience we have the uh a little picture of the epr paper we can bring that up just to show people so it's 1935 einstein padolsky and rosen write this paper about quantum entanglement that distant particles can be somehow connected by some threads of of quantum entanglement and then the er you're referring to just for the audience uh is another paper uh that einstein writes with rosen podolski got kicked off or something i don't know where podolski was and that one that had to do with uh wormholes now now i believe it's the case that nobody thought there was any einstein didn't think there's any connection between these two 1935 papers they were just different yeah i i don't believe he had any inkling whatever if he did he was even smarter than einstein yeah yeah because once in the realm of quantum mechanics the others in the realm of general relativity um and and then in essence what the email that you're referring to that juan sent you was saying hang on these two papers there is a deep connection between well they're the same thing same thing yeah um talking about i mean it wasn't that we weren't with that we hadn't been yeah talking about very closely related things uh but you said this in such a succinct way when i saw that i said that has to be true yeah that has to be true and so i very quickly put the story together and i wrote it up one rewrote it i rewrote it and um and uh there's this but this was another very crazy idea that um another with another sherlock holmesian story to it that has to be true because nothing else can possibly be true so the idea you know entanglement is a connection between potentially distant systems which um has a certain quantum mechanical connectivity to these systems wormholes is a connection between possibly distant systems they can have wormholes connecting them distant black holes we went through all the properties of entanglement in particular for example the idea that you can't send a message from one place to another using entanglement even though it's you know people always think you can send messages yeah faster than lighter you can't you can't exceed the speed of light and if you look at it from the entanglement point of view there's a there's a reason why and wormholes something similar that uh wormholes connecting two distant places you would think you could send a signal through in both cases you get frustrated by different things in one case it's by the properties of quantum mechanics and the other it's by the properties of general relativity and a whole set of things about entanglement seem to be parallel to the things about wormholes juan's message was just a uh capping the story off and the with the clever er equals cpr uh phrase because he had been thinking about these things for a long time this was not totally new to him um it hit me very suddenly i mean very quickly i knew that this was the answer to the amps of the um the firewall paradox and um so you can playing to us how it answers it i mean yeah yeah yeah yeah um what's behind how do you describe the things which are behind the horizon of a black hole just behind the horizon of a black hole well whatever they are they are things which are quantum mechanically entangled with the degrees of freedom just in front of the horizon uh you know the story the story is you understand the evaporation of black holes is by pairs being produced pairs of particles one foot one of negative energy falls into the black hole one of positive energy goes flying off and forms a hawking radiation and those two particles are entangled so it is important that the interior and the exterior of the black hole be entangled on the other hand after the radiation has been thoroughly uh half the radiation has gone out the black hole is entangled with its own radiation and presumably it can't be entangled with both okay so let's see where was i good one resolution of the paradox is that the things behind the horizon of the black hole are actually the same as the things in the outgoing radiation that was just behind the horizon of the black hole is nothing but or the information content is the same as the information content of the outgoing radiation well how can that be the outgoing radiation is very far away the interior of the black hole is close by how can they be the same thing and the answer is they can only be the same thing if there's a secret passageway between the distant talking radiation and the black hole and that's what we call the einstein-rosen bridge if the outgoing radiation could be thought of as another system connected by entanglement to the black hole then there could be this kind of tunnel or wormhole between them so that the outgoing radiation would basically be in some sense also behind the horizon of the black hole right and that that it's it's it's i can't do it i can't do a decent job on this without a piece of paper and a pencil and some equations sure but that's the way the thing went and um it was it was fairly convincing to people er equals epr was kind of was accepted pretty quickly by this community but in the very recent past um a whole gang of very young people well al mary himself who was ahmed was one of the people on the firewall paper jeff pennington other people very very young not much more than students in fact jeff was a student really sort of nailed this idea that the that the outgoing distant hawking radiation is really the same as the interior of the black hole they have done for that idea what maldasena himself did for the um for the uh holographic idea nailed it in place with a very very sharp new version of it which you know it's too hard to explain yeah no it's beautiful work it is it is complex but just for an overarching idea you know when we think about the other forces not gravity nuclear forces electromagnetic force we of course have a well-defined quantization procedure whereby we take the classical articulation of these forces and we bring them into the quantum world right gravity seems to i mean through er equals epr in some sense the statement is gravity knows classical gravity knows about quantum mechanics at some level it's it's somehow different in in that way so what what's the lesson there is is is gravity smarter than we thought it was and that it already and or quantum mechanics or quantum mechanics are smaller than the other way yeah that's right um yeah no i make this point repeatedly that like i sometimes call it gr equals qm i got i got memory of this kind of uh these little equals yeah yeah uh what yeah okay so you asked me about quantization of gravity let me just let me just uh explain what that means as you said there's a set of rules for taking classical systems harmonic oscillator an atom or a orbiting system and converting it to quantum mechanics the rules were invented primarily by dirac in 1990 around 1930 and um we've learned that that's the way you take a a theory and convert it to a quantum theory they work for electrodynamics it works for yang mills theory it works for the standard model and so forth and so on it even works for string theory uh not not in quite the same way it never worked for gravity anytime anybody tried it it always led to disaster various kinds of disasters black holes were one of them infinities or another it never worked and i think what we're learning now is that gravity and quantum mechanics are too closely connected to pull them apart and then put them back together again by quantity by quantization by the rules of quantization um that they're just it's not that they it's not that they're inconsistent with each other they're too close they are too much the same thing to pull them apart and then quantize what you've pulled apart and go back so er equals epr as an example there's a whole bunch of examples now of basic quantum phenomena which are very generic quantum phenomena which reappear in another guys through the holographic principle which appear reappear in other guys as gravitational phenomena i won't go through them but they are like er equals epr two a basic quantum mechanical thing on one side a basic gravitational thing on the other side and finding out that they are in some sense equivalent or very parallel to each other so i don't think we've put the whole thing together yet i don't think we have a complete story about how these things fit together it's getting more complete but but we don't have a complete story but i think the evidence is kind of leaning in the direction that gravity is not something to quantize it's already quantized it just doesn't makes it it is quantum mechanics the two go together hand and fist uh and are too close to to separate that's my view that's my view um what do you think it means for the future of of space time say i mean space time is always something that we sort of put in from the get-go in our theories will it be the case as many of us have suspected that space-time is just some emergent thing that arises in certain environments but is not a fundamental ingredient to build your theory out of i think that could be that could be what it's telling us [Music] a bunch of quantum mechanical degrees of freedom where the right kind of entanglement structure can emerge as a space time and uh just a whole bunch of qubits you know quantum mechanical bits linked together by the right kind of entanglement quantum entanglement structure can have the properties of space-time that's one direction that seems um a possible direction but you know i've learned not to try to predict too far in the future uh surprises happen it's the rule that surprises happen the only thing about surprise is that it's not surprising that surprises happen it's just that whenever they happen they're very surprising now it'd be very surprising if there were no surprises yeah generally when you have a good set of ideas the surprises don't reverse the direction of uh that you're going in but they can make it bend off at an angle yeah and so i sort of learned not to try to get ahead of myself and to try to guess what uh where things are going because i'll usually be 50 right and 50 wrong and i can't guess which part is right which is wrong you know let me ask you one final final thing you've been very generous with your time lenny i appreciate that um i'm often asked uh where based on what you anticipated say in the 1980s are we further ahead than where you thought we'd be are we not as far ahead uh can you sort of grade how well we as a community have done if you look back 30 40 years what's your view on that well these these uh these uh things which are just one step ahead of the monkeys have followed the dot dots in a way that i could not have imagined it's been a collective effort there's been a few people who've really stood out uh i think i won't mention names right now a few people have really stood out but it has been a collective effort and um it i i'm just blown away and amazed by how far we have gotten in understanding these questions i didn't no i did not i i also thought like feynman that would be 500 years before we'd really have answers to them yeah that 500 years has turned out to be what 25 30 so something like that yeah it's spectacular absolutely i i find it very spectacular and as i said to my mind it means that the heroic age of physics uh has not gone away yeah yeah i totally agree so again thanks so much for joining us and by the way if you ever like in the beaver kill or want to come to the beaver first i'll remember that revisit your fly fishing days uh i don't i don't have my fly rod anymore yeah i'm sure we can i'm sure we can arrange something anything have you ever tried it i've never tried fly fishing you know i'm vegan so somehow there's part of me that doesn't want to do that can you catch and release yeah but you can just think you can just take the hook off the fly yeah and just cast the flies that's very relaxing hey that's get your waders and go out into the paper kill and uh just take the hook off the fly you have to you know it's a little cold right now as you can see outside we got about three feet of snow behind me oh beautiful beautiful but when uh when it warms up yeah you gotta wait for what you can't get a license until uh you know fishing license until about april i forget oh i didn't know that okay again obviously i'm an amateur yeah you get arrested is that true wow all right well i'll keep all that in mind with the advice as well uh so thanks and look i look forward to seeing you at some point when this endemic thing uh is over but thanks for joining us here everybody thank you very much thank you bye-bye bye all right everybody that was a real fun conversation with uh lenny susskind we covered a lot of ground it's uh it's three o'clock i'm pretty tired guys um let me just see if there any questions in the chat that i might address before we call it a day here um yeah many people are uh thanking lenny for joining us and i thank you guys for thanking him um let us see uh any final questions before we uh we wrap it up um someone asked can we cite the work that was mentioned um there's not one particular paper that really sums up everything that we spoke about certainly if you want to know the story of the of the black hole war up to the early years of 2000s i know maybe 2005 2006 you should check out lenny's book uh the black hole war you know you should buy copies for all your friends it's christmas time it's a great gift that's a a wonderful read that goes through the journey in much more detail than we did here today of the information paradox and hawking's realizations and beckenstein's insights and ultimately all the way through the holographic ideas of suskind and a tuft as well as juan maul the cena's you know incredible breakthrough in the mid-1990s where string theory gave as lenny described this concrete realization this concrete instantiation of the holographic idea so that's a great that's a great resource to take a look at and beyond that it's mostly the research literature there's not a whole lot else really to look at at the moment um somebody's asking uh simon anthony is asking would roger penrose agree with what we discussed here today and some of it he would certainly agree with some of it i think he'd be quite skeptical about as you heard you know i think penrose has a visceral reaction against string theoretic ideas and just think they're the wrong direction so i presume he would consider any insights that emerge from tools that have been developed by string theory he would be suspicious of those conclusions but you know the point that lenny is making it's a point that i've made a lot too but it's it's just great to hear it from different people even if string theory is not the right theory of the world the fact that we were able to write down a mathematical system that puts together gravity and quantum mechanics in a systematic coherent sensible logical paradox-free manner that's powerful because if for instance if hawking was right his initial view i mean hawking changed his mind toward the end of his life and actually did come on board and did take on this perspective that the information is not lost so the the analogy and already lenny disavowed it so this shouldn't be taken any personally but the analogy with like trump resisting the election uh hawking absolutely came around to this perspective but his earlier view that the information would be lost were that to be true that argument was so general the one that he made that it should apply to any theory that successfully puts gravity and quantum mechanics together therefore it should apply to the string theoretic description of black holes and the fact that it doesn't even if string through is not the right theory of the world that shows there's a deep issue a problem with hawking's argument and indeed there is a problem with hawking's argument you need to include all these other features of gravity and quantum mechanics that we discussed here today that have been developed by many people the late joe polczynski brilliant physicist who tragically sadly died not too long ago a young man uh and and the great work of juan maldisan and many others including the young physicist that lenny was mentioning so would would roger disagree sure but i don't think he would disagree with the logical links as being described whether or not string theory is the right theory the world the logical links are the ones that allow you to conclude that hawking's initial proposal that the information was lost could not be correct okay anything else before we uh wrap it up um no i think we're good okay so again as you see we listened to the suggestions you make a number of people had suggested that we speak with uh lenny susscon i wanted to do that anyway so it's always nice when there's consonants between the direction that we are headed in this series that we've been having and the people you'd like us to bring on feel free to make more suggestions we'd love to hear where you would like these conversations to go with topics people and we will likely do another one of these in probably two weeks we've been sort of getting into that rhythm once every two weeks but maybe we'll do one before then my teaching at columbia has just wrapped up which gives me a little bit more time to think about things of this sort i should also mention that the epr and the er papers that we made reference to it turns out that we're since those are 1935 we're in the 85th anniversary of both of those papers and in celebration of that and in celebration of this unexpected link between these ideas we are going to have a world science festival program that is going to explore the connection between entanglement and wormholes i don't know exactly when that's going to be available we're in the process of putting that program together but just so you know for those of you who want to hear more about this and hear other voices weigh in on the subject this will be one of our world science festival programs in the not too distant future so if you want to know about that you should follow me on twitter i'm going to be announcing as these programs become available that's at begreen at twitter you should follow world science festival i don't know what your screen looks like right now but somewhere on your screen i think there's a button where you can follow world science festival both in terms of youtube or the social channels twitter or facebook as well okay that then wraps us up here for today thanks for joining us and i look forward to carrying on these uh conversations in the not too distant future okay brian green signing off bye [Music] you

Channel: World Science Festival

Views: 231,711

Rating: 4.8472157 out of 5

Keywords: Leonard Susskind, Brian Greene, Nobel Prize, gravitational waves, black holes, quantum mechanics, singularity, event horizon, radio telescopes, astronomy, general relativity, gravity, New York City, NYC, world science festival, World, Science, Festival, Big Ideas Series, string theory, world science u, Stanford University, quantum field theory, quantum statistical mechanics, quantum cosmology, Theoretical Physics

Id: xk48z8N-sl0

Length: 128min 2sec (7682 seconds)

Published: Thu Dec 17 2020

Reddit Comments

[u/Dualblade20]

This is awesome. I watched Susskind's lectures on classical mechanics almost a decade ago online and I loved how his teaching felt at the time. I haven't really heard or seen much about him since then.

[u/Kuyosaki]

fucking love world science festival

[u/BlazeOrangeDeer]

It's live now, and Susskind joins 24 minutes in.

[u/WolfmanJacko]

This is amazing thank you for getting this out there!