Leonard Susskind on Richard Feynman, the Holographic Principle, and Unanswered Questions in Physics

what I wanted to start with is you've often been characterized as someone with like non-traditional you know kind of out there ideas some of which have become you know part of the physics Canon some of which who knows what happened who they all became part of the physics Canon every single one of them I never made a mistake of course alright well thanks for coming on the podcast you're the first person who's never made a mistake I was curious who is your who do you think is your most outlandish friend wait come back to your previous question for a moment a battement I am very mainstream I am not at all a alternative thinker this this is some misconception which I don't know how it happened but my physics has been extremely mainstream it may have been that at the beginning of each of some of the ideas people were not quite ready but they very quickly caught on but it's just not true that I was some kind of alternative um-hmm what should we call it I don't know what the right word is yeah some kind of radical thinking not at all not at all no I spent a lot of time thinking about what you would call in conflicts of principle okay situations where things were not fitting together properly and thought a lot about them and eventually came to the conclusion that you had to change things or that you had to break break the mold a little bit and I think that's probably where this reputation came from but these were things that really there were no alternatives to what is what was it that Sherlock Holmes said do you remember the quote when you've tried it when you've tried all possibilities and I forget the exact but roughly speaking when you've tried everything and it doesn't work whatever remains must be the truth no matter how I've landed it should or something likes a little bit like oh yeah a little bit like Occam's razor but in particular when you've tried everything and it doesn't work there may still be something left that you haven't that you haven't tried yet because you thought it was too outlandish well you gotta try it mm-hmm and I think that probably is the source of some of this mythology about me as a as a radical but I am the most conservative physicist imaginable hmm okay so what's an example done of a friend who is more outlandish more radical than you Oh Freeman Dyson he's I can't exactly call him a friend but I know I'm a little bit he is what you might call a contrarian he enjoys running against the grain and he sometimes has some brilliant and smart things like all contrarians he's got a very large probability of being wrong and he's willing to my friend Hirata tuft I don't know if you know his name is a very famous physicist he's also a bit of a contrarian he's far more out there than I've ever been was dick Fineman a contrarian no he was about as mainstream as you can be hmm but also had his own he had his own very special scientific personality and I suspect that's also true of me that that my way of thinking my way of doing things is probably different than most people and so um yeah it did lead to this contrary in this view of me as a contrarian as a radical but it was absolutely wrong and do you see physics like kind of birthing more contrarians in the the modern paradigm works experiments are so expensive too you know kind of executes at this point or do they have to be kind of more mainstream to get things done well unfortunately I think you have to be really mainstream sometimes I think too much okay sometimes I think too much by mainstream now I mean people are often trained within a framework which is fairly tight and rigid and I sometimes think maybe a little more free thinking out there might be useful free thinking but that doesn't mean being a contrarian a contrarian is somebody who was contrary just for the sake of being right inquiry yeah well because I mean I you know read your PDF page don't some interviews with you and I heard about you being a plumber working with your father yeah and now here we are at Stanford and you're you know kind of like of the industry right you're here yeah I took me a long time to feel part of the team I feel an outsider yeah my background was a little bit strange for so it took me a long time to not feel like an outsider and then all of a sudden I found that I was the ultimate insider and how do you deal with that it's hard but just ignore it okay I don't think yeah fair enough I'm I'm interested in the physics problem I'm not gonna let it go I spend most of my time thinking about it and not the not agonizing all the things then where do you go to think of new ideas because that that's something that you mean here I go to the bathroom with or take a shower no I'm kind of curious where your ideas have come from over the course of your career though almost always came from some sense that things were not fitting together properly what I call a conflict of principle or a paradox one of the early things that I worked on was called what's called quark confinement why don't quirks come out of particles and the pure in the laboratory okay okay they seem to exist they seem to be part of the proton and neutron and so forth and they seem to be stuck inside and never come out all right and that was a that appeared to be a paradox because from all that we knew about the subject is quantum field theory the the subject that governs particle physics from all that we thought we knew any kind of particle that exists should be possible to get to kick it out and observe it directly in the laboratory so there was a there was a paradox there they seemed to exist and yet they seemed not to exist something was wrong and that's the kind of thing that captures me and gets me going and I I don't I don't want to let it go until I feel I understand it so someone from from Twitter asks a question related to this their name is claudio and they think are they asked do you think the graviton can be experimentally found so similar well of course there's a sense in which it's already been but this is I think I know what they mean but gravitons in great or photons or some large class of particles when they're in sufficient abundance just behave like wave fields so the electromagnetic field is a collection of photons but you can't sir but that doesn't mean you can detect them as individual photons easily radio waves for example would be very very difficult to detect this individual photons alright we've seen gravitational waves that means we've seen large numbers of gravitons yeah I mean I don't know how many it zillions and zillions and zillions of them I think what the what the person was asking about is the possibility of seeing them individually that seems very very hard I don't see any easy route to that and in fact I guess I don't see any route to it at all but it's but ultimately I think it's a technological problem okay ability you know if you could build an accelerator as big as the galaxy and so forth and so on and harness a hundred thousand stars to take all of the energy that they produce and run the accelerator with it you could make gravitons so it's a technological problem okay so it's a little bit like the space Lego the space like oh wait the space like goes in is a trivial technology it'll be really what's the space like Oh what does it look like Oh in space yeah that's true real by comparison like my friends yeah no no no it's trivial in the sense that in principle we can do it we will do it and it probably doesn't involve any technological hurdles which are which are insurmountable mm-hmm building a machine that could produce gravitons at least for the next million years is gonna be insurmountable oh it's not I I think it's not gonna be done right on the other hand maybe I'm wrong so let's go to some of some of your other ideas so you know you're credited as one of the creators of string theory which is extremely mainstream which is super me yeah but it wasn't it wasn't when we started it correct so that's right so that's where the idea of me as a radical came from but now it's mainstream where did the idea come from oh well the idea came from them from asking about the structure of particles which are known as hydrants these are protons neutrons massan's they're common things that make up the nucleus and there was a lot of work experimental as well as theoretical which showed that these particles were not elementary particles that they were composites of some sort you could spin them you can't take a point and spin a point the point is too small to have you wasn't mean to rotate a point okay okay whatever protons in neutrons were you could spin them up you could increase their angular momentum they they seemed to be capable of being vibrated and excited in all sorts of ways there was there was some mathematical work it was very mathematical and didn't have to do with strings yeah but which caught some of the properties of these hydrants and I got interested in it and just looked at it looked at some of the formulas and said oh those formulas are interesting I wonder what they mean or got it a little more and I said oh there's something vibrating there's some some kind of concept of vibration going on and it was just a matter of thinking about about it for a few weeks and saying oh the strings they're elastic strings hmm and with each of these like were you deeply knowledgeable in the field before this okay no I was deeply knowledgeable about quantum mechanics ever at least or was I deeply knowledgeable even that I think I was but yeah I I had a very very good education it was self education about quantum mechanics about classical mechanics I did not have much of an education about particle physics but it was unnecessary somebody showed me formula and there was a mathematical formula I knew what a proton was I knew what a neutron was I knew that if you collided them stuff come out of them and I also knew that that they had these properties of being capable of being excited and spun up and so forth so I didn't know that but that was easy I mean you know I just told you when you now know it too they showed me a formula and the mathematical formula had some pieces in it that I recognized that seen it before I've seen it in the context of basically elementary quantum mechanics I'd seen it before and I looked at it and at first I thought oh this thing is just a pair of particles on the ends of a spring meaning to say the mathematics of it was the mathematics or what's called a harmonic oscillator okay okay but I looked at a little more and a little more and a little more and eventually I realized that the formula was representing the interaction of particles which themselves will string like string like meaning elastic threads let's call them and so I worked it out and published it and that was the story and then in your your the Cornell lectures from 2014 or something like the messenger messenger lectures yeah you you kind of like offhandedly said that despite being one of the creators of the string theory you weren't the biggest believer in the world right now oh okay um I probably did say that and what I had in mind was something like this I do believe in string theory in the following sense it's a mathematical theory it's a consistent theory and it contains both quantum mechanics and gravity that makes it a very very valuable laboratory for trying out ideas it in itself doesn't mean it is the theory of the real world okay my guess is the theory of the real world may have things to do with string theory but it's not string theory in its formal rigorous mathematical sense we know that we know that we know that the formal the formal I mean mathematically rigorous structure that string theory became it became mathematical structure of great rigor and inconsistency that it in itself as it is cannot describe the real world of particles it has to be modified it has to be generalized that has to be put in a slightly bigger context so the exact thing which is good I call string theory which is this mathematical structure is not going to be able by itself to describe particles will will it will what does correctly describe particles be a small modification of it or a big modification and that's what I don't know okay but I do know the value of it in as a laboratory for investigating quantum mechanics and gravity and that's that's remarkable okay because that the question that I've been wondering and then it's sort of straightforward but why does there have to be a grand unified theory well there has to be that why does it have to be or do people want it we don't I don't know what people think I know what I think it's not tolerable to have inconsistencies in the theory of nature where one piece of the theory says one thing another piece of the theory says another thing and they're saying inconsistent things they have to be made consistent at the present time we're in the business of trying to put together a consistent framework for the combination of gravity and quantum mechanics elementary particles there are inconsistencies in what we know about elementary particles we're trying to put those together when we put them together and make a consistent story out of all of this we'll call that a grand unified theory that's it and it's inconsistent I mean sorry it's intolerable not to have a consistent story you get different answers by doing different versions of it that can't stand so that's my answer today okay I'll do it okay so when when you look at physics as it stands right now where do you see the cracks that you wanna be focus that like the most important thing you could possibly be working on right now which the yeah well a grand unified theory and in other why don't think of it that way I don't think of it that way at the moment people like myself John fresco one male the sane a wonderful and great physicist have gotten focused on the connection between quantum mechanics and gravity for many years it was thought that quantum mechanics and gravity simply don't fit together for a variety of reasons including things that Stephen Hawking had said which were brilliant I don't think correct but brilliant anyway it really looked like there was an inconsistency between quantum and gravity quantum mechanics governs all other parts of nature but of course gravity also covers a large part of nature and to have inconsistent theories is as I said intolerable so the puzzle of putting together quantum mechanics and gravity is the one which is front and center for me and I think front and center for Theoretical Physics right now there are also well let's conflicts there are conflicts in our understanding of elementary particles we don't understand how they can behave certain ways that they do behave one of the problems of the it's just a name but it's called the gauge hierarchy problem it's an apparent almost inconsistency in the what's called the standard model of particle physics there are other questions about how it does fit together with gravity we went we make great progress in understanding elementary particles for a long time and it was always progress till in hand-in-hand with experimental developments big accelerators and so forth we seem to have run out of new experimental data even though there was a big experimental project the LHC at CERN what if that is a great big machine that produces particles and collides them mm-hmm and I would say I don't want to use the word disappointingly well I will anyway disappointingly it simply didn't give any new information so particle physics has run into what I suspect is a temporary brick wall it's been basically since their early 1980s that it hasn't changed and [Music] so I don't see at the present time for me much problem much profit than pursuing it hmm gravity and quantum mechanics or what fascinate me well what are the other large unanswered questions that people are pursuing at this point like because clearly it's not just you working on this right no I think other other things well in the context of there are huge problems in cosmology in all of this all of this cosmology is about quantum mechanics and gravity hmm early cosmology so-called inflationary theory is about how quantum fluctuations imprinted themselves on the universe and led to the things galaxies planets and so forth so quantum mechanics and gravity are the foundations of cosmology but we don't understand how they fit together at all not that not that particularly in the cosmological context we really just don't understand how they fit together the dark energy the thing that's called dark energy is a puzzle it's not the puzzle of why is there dark energy it's the puzzle of why isn't there a lot more of it hmm the dark energy is a tiny tiny minuscule fraction of what it could be why is it so small 10 to the minus 120 of what the natural expectation for it would be so for many years people thought there was no dark energy mm-hmm we call it the cosmological constant but it's the same thing as what people call dark energy okay we have no idea and so for originally we thought it wasn't there at all hmm it's also yeah Einstein invented the cosmological constant and then said it was his worst mistake because it doesn't seem to be there well it was there but it was there at a level which was so minut that it took until the 1990s to discover any evidence for how is it measured it's measured astronomically and bye-bye modern observational cosmology counting galaxies counts and all kinds of the quasar counts all sorts of stuff but the the main point is in the end it turned out that it was there the stark energy but it was there it's such a small incredibly small value that it took all that time they get any evidence for and we don't know why it isn't bigger more of it hmm that's the puzzle not why is there but why is it not there in larger abundance do you have a hypothesis well the usual hypothesis is that you know the usual hypothesis the only one that I think makes any sense which is outlandish there's no question Cavendish it's not mine I'm jealous no it's not mine but I think it's the only thing that does at the moment seem to make any sense is to say the universe is extremely big much bigger than we can see and varied varied means that has properties which are different from place to place that's a good theoretical idea it makes it that it does fit together with the equations and so forth that the universe is vastly bigger than the part we can see and that as you scan over the whole thing you'll find places where the constants of nature are one thing are the places where the constants of nature are another thing some places where this cosmological constant is more or less normal which means much much bigger than then it is here in our neighborhood some places where it might even be smaller but then the question becomes in what kinds of environments can we exist and even ask the questions my friend Steve Weinberg in around 1987 made an argument that if the cosmological constant were any bigger than a certain magnitude that galaxies could not have formed and if galaxies couldn't for stars can't form planets can form when we can't be here so he said the answer is the universe is very big and varied and we are where we can be that's all we just did B that's called the anthropic principle and it's a widely hated idea physicists definitely among scientists it is yeah it's a widely hated idea but it just might be right so I was listening to a radio interview with you and you said similar to this that there there was a discovery that there are relatively few ways of organizing matter than we thought there would be and the hello was talking about that's a good question but my question is like could you explain because you said they're relatively few ways that don't turn into black holes oh um I don't remember exactly what I was talking about okay but here's what I can tell you almost all the matter or almost only information in the universe is in the form of black holes if you take some matter and and just generically populate the the world with matter you will find in a very quick amount of time that it's mostly all black holes hmm our world is mostly all black holes it really is in the sense that the information stored in matter is at least let me think I think about 10 to the 10th a factor of 10 to the 10th more information stored in black holes and than anything else even though black holes seem very rare in the universe they contain almost everything can you define information just for people yeah that's what's what's in a computer but this one bit penis bits the bit we calm qubits because the quantum bits but yeah bits and the bits which determine here's what we might say we take the universe as it is we can run it forward in time and that'll tell us what it will be hmm we can also try to run it backward in time to all find out what it was like in the beginning in order to do that you have to have every single bit accounted for you try to run things backwards you'll make mistakes very quickly unless you have accounted for everything so the question is how many bits of information do you need in order to run backward and find out what the world was like in the beginning and that number of bits is about 10 to the tenth times bigger than all the known bits in ordinary material in the universe protons neutrons electrons and so forth where is it hiding we now know that it's hiding in black holes gotcha okay so I I briefly encountered this through the holographic principle that you worked on yeah and one one question that I couldn't fully wrap my there's another example of something which is considered a little bit radical at first a little bit nuts but of course it's now extremely mainstream yeah very mainstream but the I mean I would push back a little bit you know okay go ahead well like anything that's fringe that becomes popular you can say is mainstream but it was French in the beginning no its mainstream in the sense well it wasn't fringe in the beginning people just didn't recognize how essential it was to the logic it took a little while mm-hmm it took a little while for people to realize yes this was the only way it could be it wasn't just that it became popular this is not a popularity contest physics is not a popularity contest a contest for brief periods of time sometimes things become popular hmm but they don't last if they're just popular they last if they have value explanatory value predictive power value and the value of leading to a consistent framework in that sense the holographic principle is now completely mainstream and as why is it mainstreamed it's mainstream for the reasons that I thought had to be correct it just had to be correct it couldn't not be correct I worked at it can you give a brief explanation because I this is this was a hard one yeah well it has to do with black holes it had to do with black holes and the information law worship which it had to do with this discussion about information being lost in black holes which right there Stephen Hawking's very very brilliant insight even though I think he get the final answer wrong was very brilliant insight to ask what happens to the information that goes into black holes is it lost is it lost the universe if it's lost that would be a major change in physics in which an ordinary physics information is never lost now Stephen also said that black holes evaporate well a natural answer might be that that the information comes out in the evaporation but it can't come out in the evaporation if fell into the black hole because nothing can get out of a black hole okay so there was a there was my favorite kind of situation a clash of principles the answer turned out to be in this holographic idea that as let me let me say it in a way which is not exactly correct but as close as I can get without writing a bunch of equations on the blackboard the information that falls into a black hole has can be thought of as both falling into the black hole and also getting stuck on its horizons two versions of it almost as though the information was Xerox that the horizon of the black hole and one half of it sent in and the other half stored on the horizon now the real real ed the real curse statement was more like saying the stuff on the horizon is a kind of hologram of the stuff that falls in mm-hmm so it's really only one thing but represented in two different ways and then once you said that the stuff that falls into the black hole can be thought of as a hologram that never does fall through the horizon then you can imagine that when the black hole evaporates this hologram evaporates with it and carries off the information now that's that's yeah so this is the check perfect sorry another very challenging right and I'm not sure that they can I I think if you really really wanted to know and you were willing to spend three or four days talking about it with me I could probably reduce it to something which was both correct and incomprehensible but not in 15 minutes yes I'm in 15 minutes it's just this the way it is and so okay but the point was that black hole horizons are behaving like Holograms of anything that falls into the black hole mm-hmm but then when thinking about it further we realized that the whole world could be in a black hole you can't tell that's not in the black hole okay in particular the entire universe has a horizon out at very large distances which is very much like a black hole horizon and what kind of inside it so that leads to the conclusion that that we here in the interior must have another representation as a hologram out at the boundary of the universe now this this was a strange idea that certainly was a strange idea I felt driven to it because I could see no way other than that to incidentally wasn't just me it was also her ah they're tough to put this idea forward and it was a little bit out there it certainly was out there it wasn't it didn't come in from the cold shall we say until the work of one maldacena mm-hmm who made a really rigorous beautiful version of it which now everybody believes the mathematics of it was came it was a string theoretic construction hmm where one showed how at least in certain setups the universe would have to be regarded as a hologram a hologram saying it's a hologram is a bit of an analogy yeah but that that would be represented as information stored on the surface on the outer surface of the world rather than in three dimensions as we normally think about it inside yeah yeah that you know one really nailed that with such mathematical precision that it just became part of our standard it became a tool okay that's a good thing when things go from being they say softn start out as very speculative then they become something a little bit better than speculative conjectural conjectural is better than speculative and the end processes they just become a tool of physics hmm things that everybody uses all the time because it has a predictive value a mathematical value the holographic principle is a tool now so yeah it's stuck so why why does it have to be holographic so in other words say it it's mapped around I'm gonna have to bring this into a 3d world right so there's a 3d sphere call it a black hole right why is it ha graphic versus a 2d image for example it isn't happy rent it's a 2d you mean why can't it just be like a picture on the wall yeah well the picture on the wall is 2-dimensional it may deceive you it you know a clever painter can can paint the painting which when you look at it you think you see three-dimensional things but you never do you don't then in particular if you move your head around from side to side you can't see what's behind the flower mm-hmm there's nothing behind the flower and you can you were just deceived into thinking there was something three-dimensional layer but how would you check it was three-dimensional you would check I was three-dimensional by going around to the other side and see if something's there well if you move your head around with that the the picture of my that the plant on my wall there you will not see anything behind the plant there's just nothing there it's strictly two-dimensional on the other hand it is possible to map a three-dimensional world onto two dimensions but never in a way in which the two-dimensional stuff looks anything like the thing you're mapping it will look random it will look it will look like a simply confused jumble or little tiny scratches you can see that if you if you can get a hole of a real hologram which a hologram does map three-dimensional space onto a two-dimensional film and somehow look at the film through a microscope or something you'll see that there's nothing on that pit on that film which resembles anything like the thing that it's representing it's just a bunch of little tiny scratches and and random noise almost so you can't map the three dimensions to two dimensions without really um making it totally discontinuous the word is mathematically discontinuous but yet it does contain the same information that's the same thing about this holographic principle the horizon really did store all of the stuff that fell into the black hole but in a way which you could not easily reconstruct it's more like a hologram than it would be like a photograph mm-hmm and how does the reconstruction happen so say say we are in a black hole for a real hologram all you have to do is shine the right kind of light on it right reconstruct the image not here here it will be a mathematical reconstruction if somebody gave you the quantum state of the horizon of a black hole and you were smart enough if you meant that nobody smart enough but with sufficient that kind of technology of quantum computation and so forth and if we knew the precise rules by which black holes evolved we could reconstruct from the quantum state of the horizon we could reconstruct what fell in what's inside and and so forth mhm we could reconstruct that world that fell into the black hole this is not something which is easy it is far from mathematically tractable with present computers and so forth but in principle it is possible okay if somebody showed you the hologram incidentally of just the you know a patch of flowers or something and just gave you the film and didn't allow you to shine light on it just said reconstruct from that you think in a little bit yeah yeah yep eventually you probably could but but it would be very hard mm-hmm and multiply that out to the universe well and and they had quantum mechanics which escalates the story hugely gotcha slight tangent have you followed any of these ideas around we live in a simulation these simulation hypotheses yeah it doesn't seem to me to add anything mmm what does that mean there's the idea that we live in a simulation mean that there was a simulator that somebody simulated us I believe so yeah I think we live in a computer program based on our ideas but I would say of course we live in a computer program the program is called the laws of nature and that computer is the world so it's a yeah but then somebody would say oh that's not what I meant I said what did you mean by saying we live in a computer I think they meant that there was a computer programmer who programmed it for some purpose mm-hmm is do we live in a computer program that somebody programmed for compare for a purpose I have no idea in North East I would love to know but you know then I would ask I'm a curious person I would ask them okay if there is that guy out there let's not give him a name did the program the program of read program the the the simulation who programmed him right what the what are the laws by which he functions does he does he satisfy the laws of quantum mechanics he or she probably neither it's probably a sex-free environment who knows who knows yes lever program them that's right yeah and then who programmed the program who program the program and so forth there isn't satisfy it just doesn't lead to any satisfying visitors yeah this this reminded me I was listening to your Caltech your Fineman lecturer and Tex and you said something really nice which was a Fineman didn't much like philosophers philosophizing about science and in the context of machine learning which your son works on yeah do you find yourself in the same camp you're just like back to basics about the technical aspects or do you philosophize or and let yourself philosophize first let me say something gonna find them okay okay find me this claim that is this like philosophy he did dislike philosophy but I'll tell you what that means in a minute and yet he was the most philosophical of all physicists he really was he was a deep philosopher when I say he didn't like philosophy I meant he didn't like a certain style of thinking that was full of jargon full of the full of I'll use his word baloney where people who didn't know what they were talking about pontificated and used fancy words like ontological which I never knew what that meant I know some words and when you use them but I don't know what they mean oh yeah as a substitute for simple thinking yeah okay that is what he didn't like and yet I think in some ways in some deep way he was an extraordinarily philosophical person if you read his works you know I don't mean his physics works if you read things he wrote about the world the ordinary world they're very very philosophical but they're also incredibly simple and they cut through all the crap and there was a crap that he didn't like okay yeah yeah I would say the same about mathematics he didn't like the overly fancy mathematics but he was very math a very good mathematician hmm and and what we were talking about before we started recording like he was also quite moral right in in his philosophy of the world you know he was affected by that I mean one analyst alamos as well yeah he had a very you know hated the fact that he had participated he hated the fact that he had participated in the invention of nuclear weapons and he doubly hated the fact that he had so much fun doing it it's fair did you interact with any of any other people that worked on the the bomb Hans bethe okay Hans bethe was one of my thesis advisors yes so I did but I didn't talk with hunter hunts was not he was a friend but he wasn't a friend in the same way that Fineman was he wasn't a soul mate okay in that that depth you talk about with Fineman did you find that with your adviser did he yeah did he have the same sort of grief around the way it created oh well I can't all right I know the answer to that okay but not from him directly I know that from the answer from that just because its historical yes he he was very upset about the bomb and he as much as anybody worked hard very very hard for disarmament and nuclear disarmament Fineman did not finding just said okay I'm gonna do physics and that's my that's what I'm gonna do yeah and he didn't work Han State Homs was very very active in nuclear disarmament so I do know that he regretted it yeah but I don't know it directly from him hmm I'm wondering what the parallels might be today because I think there are so many engineers working on incredibly technical things that who knows what the implications might be or I mean already are you could see say with Facebook other things you know yeah on the other hand the enormous amount of good that has come from technology of all kinds so I think you can't not work on it how do you yeah at what point do you stop and say this is dangerous well I think it's probably built into a some people curiosity the the need to explore and they're just gonna do it mm-hmm it's it's not I don't believe it's the physicists job to decide what what should and shouldn't be discovered from a physicists point of view everything should be discovered if possible it is the job of politicians and are the people of that ilk to to make sure that things are not misused the misuse of nuclear weapons was not really the scientists who built them they were worried about the Nazis getting them if there was misuse this is also to debate about whether nuclear weapons were misused or were they used well to end the war and all that sort of stuff if they were misused it wasn't the scientists the scientists didn't want to see the bombs used so they were given a problem toward a double problem part number one of the problem was the Nazis are gonna build it if we don't and the second part the second problem was have you built it mm-hmm they had no choice I don't believe they had any choice except to go and do it both the scientists and as human beings the fact that it got misused I don't believe was the scientists themselves and if anything those people tended to be very traumatized by the fact that they had built weapons you said he didn't work on disarmament but do you think Heath any of his focuses later in life were related to I don't know making improving the world I think he would have said you improve the world by discovering what the world is I think he would have said that that's my job as a physicist when I say my I actually mean mine too but but I meant his that is his job to find out as much about the world as can be found out and he was very good at it he advanced our knowledge of the world how Iike how it gets used as something as not he did not see is his responsibility does that align with your personal philosophy your reason I think so I think so look if I were to suddenly discover something that I knew was gonna be exceedingly dangerous I would and I was absolutely certain that it was destructive and so forth first of all I don't think you could hide it you can't hide it it's gonna come out if I don't come out okay yeah so all you can do is warn all you can do is warn people that that this is there it will be discovered you've got to worry about it well beta did that I think Fineman didn't his reaction to it was my job on earth is to learn about the world and I'm gonna focus on that and I I am NOT responsible for all the evil in the world and I will and I am I can be responsible for uncovering what nature is like mm-hmm because I'm just curious what how you've stayed motivated and been so prolific yeah with your career well I think I'm also a curious person cured I mean weird other people can decide that I mean that I have a sense of curiosity about the world right and it just doesn't go away I mean I don't die I didn't say to myself I'm going to continue to do physics until I'm 78 years old and there's now you know I didn't plan that I just get curious about things and [Music] that's it I don't have a choice what are you most curious about right now gravity and quantum mechanics how they fit together what in particular whether the laws of gravity are really just the laws of quantum mechanics a little bit hidden my guess is that almost everything we know about gravity is coming straight from quantum mechanics and that there are equivalent rules of quantum mechanics which reflect the gravitational things this is gonna get us into technical discussion the search you want to do it yes do it no yeah no I mean if I get dropped if some of the listeners have to drop that's okay but certain people will like it a lot yeah right it's good so one of the things that was discovered by myself and one Melda saying there's probably more but now the saying it in myself we wrote a paper together it's called the ER equals EPR hypothesis oh this is a great story incidentally let's let's back up for a minute let me tell you the story about Einstein and ER and EPR okay ER stand for two names Einstein and Rosen EPR stands for three names Einstein Podolsky and Rosen in one year 1935 after it was generally deemed to the Einstein had you know was basically finished as a physicist or at least something like ten years Einstein wrote two papers which nobody paid too much attention to for many years one of them was the ER paper and it was about wormholes it was about solutions of the Einstein field equations which had this wormhole character where they were wormholes connecting distant regions of space they were called einstein-rosen bridges if you look up einstein-rosen bridges you will find that there are bridges which connect with different regions of space a black hole in one place and a black hole in another place has a has a connection between him and that was solutions of Einstein equations the other paper that he wrote the same year was about something called entanglement and entanglement is something that can happen to quantum systems when they get correlated and and it's a very non-local kind of thing it's purely quantum mechanical it does not obviously have to do with gravity and these were two separate things I do not believe that Einstein had all had any idea that they were connected the einstein-rosen bridges and the idea of entanglement and one of the really odd things was that the own very recent years we found out that entanglement and einstein-rosen bridges are the same thing that in particular an example would be if you have two black holes black holes have all kinds of internal structure to them that quantum mechanical objects okay if the two black holes are entangled they will have an einstein-rosen bridge connecting them if the two black holes have an einstein-rosen bridge they will be entangled we found out that they are the same thing quantum entanglement and the kind of connectivity between systems that were called einstein-rosen bridges so this was this was a weird quirk of history that in the same year Einstein discovered both of things almost certainly didn't have any inkling that they were the same course maybe he did but what did the two papers say if they ultimately became the same thing one paper said there are solutions of my equations in which distant black holes are connected by wormholes okay the short run shortcut between them isn't element of look that's about black holes that was about that was not about quantum mechanics okay that was about Einstein's general theory of relativity which is a completely classical non quantum mechanical thing the other thing is he was thinking about quantum mechanics and discovered this odd non-local connection that systems can have that we call entanglement as far as I know as I said he didn't draw any conclusions about any relationship between these two things that happened in 2013 long long after Einstein had been dead for many many years as a consequence of the mathematical study of black holes it was largely one mile the same as discovery I happened to be on this paper with him because we were working on something together mm-hmm and that drawing out the ultimate conclusions of that finding out what it really means how it brings quantum mechanics together with with gravity has been the essential focus of my own thinking for at least five years now hmm yeah and trying to make a theory out if it trying to build a comprehensive theory and what was the technical the technical part you wanted to get to the technical part had to do with something called quantum complexity theory these wormholes that connect you know you might think if you have a wormhole connecting to distant places you could jump in one and come the other yeah okay no the problem is the wormhole grows and it grows so fast that you can't get through it it's as if you had a tunnel new jersey and new jersey new york city the Holland Tunnel or the Lincoln Tunnel and you go in one end of the tunnel and of course you can come out the other end but what if the tunnel was growing while you went in and it was growing so fast yeah that they grew faster than you then you're speeding car well then you can't get out the other end right yep that's the way these are these einstein-rosen bridges behave okay okay so the question is what is what is the the quantum mechanical meaning of the growth of these wormholes the answer appears to be that they are connected with something called complexity theory complexity theory is a computer science concept it tells you how hard it is to reverse something and the complexity of the growing Lincoln Tunnel would be a measure of how hard it would be to shorten the tunnel again so that you could get through okay so this question of quantum complexity theory has been sort of focused on what I've been thinking about other people think about different things this is a main scent the main focus of a lot of work on what's going on both here Princeton all over the world and where we'll go I don't know it's just fun to think about it's going to pay and they pay us to do it yeah it's not a bad gig so there there was a related question from Twitter for you so Noah asked could quantum teleportation be used in the future as a means of intergalactic communication no no in order to do quantum teleportation you cannot do quantum teleportation without at the same time sending classical information from one place to another classical information means you know the dots and dashes most dots and dashes you can have two entangled systems and you can send information through the entanglement but not without sending a code to decode the mm-hmm without without sending a code classically from one place to another and that will take the amount that that will take time so you don't speed up communication if it would take you a hundred thousand years to communicate from one into the galaxy to the other end of the galaxy in any kind of normal sense it will take you that same hundred thousand years to do quantum teleportation so yeah you could use quantum teleportation to teleport stuff over vast distances but it won't be any faster it'll be more secure right more secure means more secret you won't be able to crack it mm-hmm but but that's what quantum teleportation does for you it gives you absolute 100% security that no classical none quantum mechanical protocol could ever give you but it can't be done faster okay good to know related Ryoka ryoga digital asked Ryoka it's it's a like a brand it's just someone with an avatar how do you think quantum theory will shape technology in the future that's a very good question of course it's already shaped technology completely in the presence on going yeah yeah I mean all the electronics in the world is all based on quantum mechanics but it's a but it's particularly simple quantum mechanics quantum mechanics of a small number of electrons and things like that the quantum mechanics that we're exploring now is the quantum mechanics of massive entanglement large number of qubits those are quantum bits which are massively entangled with each other and how that can be used to do things that no classical computer can do I can't tell for sure how it's going quantum computers will probably be built they will be built to try to exploit this massive idea of entanglement what problems will it solve is unclear this conceivably could be the people who build quantum computers and not figure out what to be able to do with them now I don't think that will happen is one thing that you can do with the quantum computer and that's to simulate quantum systems in a way that classical computers couldn't classical computers can never be built big enough to explore more than four hundred more than I'm actually more than probably a hundred qubits one hundred qubits doesn't seem like very much no classical computer can do the calculation of following what one hundred qubits do so if you're interested in some quantum mechanical system and you want to study it the most efficient way to study it is not to program it for a classical computer that will never go very far but the program is on a quantum computer and then you have a good chance to be able to explore it so that's a scientific purpose for it you want to understand how certain chemistry chemical molecules behave the big chemical molecules which are too big to do on a classical computer you run it on a quantum computer you want to understand new materials quantum mechanics materials that depend for their properties on quantum mechanics classical computer for the most part can't do it you'll be able to simulate it on quantum computers will they be able to solve problems that are the usual kinds of problems that computers you hope computers can solve that that remains to be seen the last thing I was wondering is now so you're both an accomplished physicist but you're also a physics educator ah for better or worse right all of your all of your videos your books you clearly have a knack for communicating these ideas that's nice to hear at least it works for me yeah if you can impart any particular ideas across the population or about physics and understanding what would they be you know I don't really know let me ask a totally different answer a totally different question why did I start teaching for the public sure I think the simple answer is that it was fun I like teaching I get two things out of teaching I like to perform in that sense I have a bit of Fineman in me mm-hmm and I yeah I get a kick out of performing that's one thing there's another element to it I find that the process of figuring out how to explain things is very very helpful in formulating new ideas to me teaching is absolutely essential for doing physics I much of my physics began with trying to figure out how to explain something it doesn't almost doesn't matter whether it's explaining to another physicist or explaining to a layperson in particular I found that trying to explain things to a layperson I explain them honestly explain them not through fake analogies but to try to give an honest and clear explanation of something often really focused my ideas on how everything works and so it had it has value to me that was above and beyond just the fun of teaching I did find that teaching the for the public the public Stanford's continuing studies was especially valuable this way the students students they were all they were any year anyways between 50 years old and 95 that was actually true there wasn't 95 year old lady who and she was she followed the she knew what she was doing yeah so I found that the curiosity there they had some degree of technical background they tended to know a bit of mathematics just a bit through calculus and they they were very curious about physics I found teaching them to be especially gratifying and I really would spend a lot of time figuring out how to explain hard things to them in the process I often found that I understood them so much better hmm so that was why I got into teaching and the public called the public sector or whatever mm-hmm I don't know that there's any particular thing that I would want to convey to them you know there's some obvious answers you want to convey to them that science makes sense you want to convey to them that scientists are an phonies that they really do sometimes know the answers to things that there are facts and so forth there of course all these things are true was i motivated by that not really I was just motivated by having fun and and enjoying teaching I think there was one more thing my my father had a bunch of friends they were plumbers and they were funny characters they were sort of intellectuals but none of them in passed a fifth grade they were very curious about all sorts of things some science some history and stuff and they were mildly crackpot II mm-hmm why were they crack Bobby they were crack Pauline up because they were intrinsically crackpots they were crack party because they had no venue in which they could find out what was real science from fake science they were plumbers they couldn't go askin physicists is this real or is that not real and I always felt some sense that I would have liked to be able to go back in time to to my father and his friends and tell them what was real and what was fakie stuff yeah I was and that I don't know emotionally I think that sort of did come into hmm the reason why I liked teaching these people it reminded me of yeah yeah that's great well thank you so much for your time okay you

Channel: Y Combinator

Views: 255,471

Rating: 4.9002266 out of 5

Keywords: YC, Y Combinator, Leonard Susskind, Richard Feynman, Physics, Stanford, Caltech, Holographic Principle, String Theory, Podcast, Interview, Video, Craig Cannon

Id: CQAcLW6qdQY

Length: 66min 19sec (3979 seconds)

Published: Thu Dec 06 2018

Reddit Comments

[u/TotallyNotAstronomer]

Wow, he is 80 years old, I remember him looking so much younger in his old recorded lectures on youtube.

Then again, I did watch them nearly a decade ago... Maybe I'm not that young anymore either.

[u/orbituary]

Susskind is my favorite.

[u/ChuckChuckelson]

What podcast is this?

[u/Henhouse808]

I could literally listen to this man speak about things I barely comprehend for hours upon hours.

[u/no8airbag]

great stuff

[u/TheEagle93]

Going to grab popcorns for this one!

[u/RustArtist]

What's the friend's name Susskind mentions at 3:05

https://youtu.be/CQAcLW6qdQY?t=186

[u/qows]

Bruhhhh I just did for the first gta?