Leonard Susskind: Quantum Mechanics, String Theory and Black Holes | Lex Fridman Podcast #41
the following is a conversation with Leonard Susskind he's a professor of theoretical physics at Stanford University and founding director of Stanford Institute of theoretical physics he's widely regarded as one of the fathers of string theory and in general is one of the greatest physicists of our time both as a researcher and an educator this is the artificial intelligence podcast perhaps you noticed that the people have been speaking with are not just computer scientists but philosophers mathematicians writers psychologists physicists and soon other disciplines to me AI is much bigger than deep learning bigger than computing it is our civilizations journey into understanding the human mind and creating echoes of it in the machine if you enjoy the podcast subscribe on YouTube give it five stars and iTunes supported on patreon or simply connect with me on Twitter at lex friedman spelled fri d ma a.m. and now here's my conversation with leonard susskind you work two more friends with richard fineman house he influenced you changed you as a physicist and thinker what I saw I think what I saw was somebody who could do physics in this deeply intuitive way his style was almost a closed his eyes and visualize the phenomena that he was thinking about and through visualization I'll flank the mathematical highly mathematical and very very sophisticated technical arguments that people would use I think that was also natural to me but I saw somebody who was actually successful at it who could do physics in a way that that I regarded as simpler more direct more intuitive and while I don't think he changed my way of thinking I do think he validated it he made me look at it and say yeah that's something you can do and get away with practically even get away with it so do you find yourself whether you're thinking about quantum mechanics or black holes or string theory using intuition as a first step or step throughout using visualization yeah very much so very much so I tend not to think about the equations I tend not to think about the symbols I tend to try to visualize the phenomena themselves and then when I get an insight that I think is valid I might try to convert it to mathematics but I'm not a math and then a natural mathematician or I'm good enough at it I'm good enough at it but I'm not a great mathematician so for me the way of thinking about physics is first intuitive first visualization scribble a few equations maybe but then try to convert it to mathematics experiences that other people are better at converting into mathematics and I am and yet you've worked very counterintuitive ideas so how that's true that's naive is something Connor into every their rewiring your brain in new ways yeah quantum mechanics is not intuitive very little of modern physics is intuitive intuitive or what does intuitive mean it means the ability to think about it with basic classical physics the physics that that we evolved with throwing stones splashing water or whatever it happens to be quantum physics general relativity quantum field theory are deeply unintuitive in that way but you know after time and getting familiar with these things you develop new intuitions I always said you rewire and it's to the point where me and many of my friends find many my friends can think more easily quantum-mechanically than we can classically we've gotten so used to it I mean yes our neural wiring in our brain is such that we understand rocks and stones and water and so I'm sort of evolved evolved for you do you think it's possible to create a wiring of neuron like state devices that more naturally understand quantum mechanics understand wave function understand these weird things well I'm not sure I think many of us have evolved the ability to think quantum mechanically to some extent but that doesn't mean you can think like an electron that doesn't mean an another example forget for a minute quantum mechanics just visualizing four dimensional space or five dimensional space or six dimensional space I think we're fundamentally wired to visualize three dimensions I can't even visualize two dimensions or one dimension without thinking about it as embedded in three dimension in space if I want to visualize a line I think of the line as being aligned in three dimensions right well I think of the line as being aligned on a piece of paper with a piece of paper being in three dimensions I never seem to be able to in some abstract and pure way visualize in my head the one dimension the two dimension the four dimensions the five dimensions and I don't think that's ever gonna happen the reason is I think our neural wiring is just set up for that on the other hand we do learn ways to think about five six seven dimensions and we learn ways we learn mathematical ways and we learn ways to visualize them but they're different and so yeah I think I think we do rewire ourselves whether we can ever completely rewire ourselves to be completely comfortable with these concepts I doubt so that it's completely natural there was a tour it's completely natural so I'm sure there's some what you could argue creatures that live in it two-dimensional space yeah and there are and um well it's romanticizing the notion of course we're all living as far as we know in three-dimensional space but how do you how do those creatures imagine 3d space well probably the way we imagined 4d by using some mathematics and some equations and some some tricks okay so jumping back to a fireman just for a second he had a little bit of an ego yes what do you think ego is powerful or dangerous in science I think both both both I think you have to have both arrogance and humility you have to have the arrogance to say I can do this nature is difficult nature is very very hard I'm smart enough I can do it I can win the battle with nature on the other hand I think you also have to have the humility to know that you're very likely to be wrong on any given occasion everything you're thinking could suddenly change young people can come along and say things you won't understand and you'll be lost and flabbergasted so I think it's a combination of both you better recognize that you're very limited and you better be able to say to yourself I'm not so limited that I can't win this battle with nature it takes a special kind of person who can manage both of those I would say and I would say there's echoes of that in your own work a little bit of ego a little bit of outside of the box humble thinking I hope so so it was their time where you complete you felt you looked at yourself and asked am i completely wrong about this oh yeah but the whole thing about specific things the whole thing that way which cold thing me and me and my ability to do this thing oh those kinds of doubts those first of all did you have those kinds of doubts no I had different kind of doubts I came from a very working-class background and I was uncomfortable in academia for Oh for a long time but they weren't doubts about my ability of my they were just the discomfort and being in an environment that my family hadn't participated in I know nothing about as a young person I didn't learn that there was such a thing called physics until I was almost 20 years old so I did have certain kind of doubts but not about my ability I don't think I was too worried about whether I would succeed or not I never I never felt this insecurity am I ever gonna get a job that veteran never occurred to me that I wouldn't maybe you could speak a little bit to this sense of what is academia for because I do feel a bit uncomfortable in it mm-hm there's something I can't put quite into words what you have that's not doesn't if we call it music you play a different kind of music than a lot of academia what how have you joined this Orchestra how do you think about it I don't know that I thought about it as much as I just felt it yeah you know thinking is one thing feeling is another thing I felt like an outsider until a certain age when I suddenly found myself the ultimate insider in academic physics and that was a sharp transition in the world I wasn't the young man I was probably 50 years old you were never quite it was a phase transition you were never quite free milk in the middle yeah that's right I wasn't I always felt a little bit of an outsider the beginning a lot and outside Earth my way of thinking was different my approach to mathematics was different but also this my social background that I came from was different now these days half the young people I meet their parents were professors my that was not my case so yeah but then all of a sudden at some point I found myself at they're very much the center of maybe not the only one at the center but certainly one of the people in the center of a certain kind of physics and all that put away I mean I went away in a flash so maybe maybe a little bit with Fineman but in general how do you develop ideas do you work their ideas alone do you brainstorm with others oh both both very definitely both the earth the younger time I spent more time with myself now because I'm at Stanford because I'm because I have a lot of ex students and I you know people who who are interested in the same thing I am I spend a good deal of time almost on a daily basis interacting brainstorming as you said it's a it's a very important part I spend less time probably completely self focused and the paper and just sitting there staring at it what are your hopes for quantum computers so machines that are based on that have some elements of leverage quantum mechanical ideas yeah it's not just leveraging quantum mechanical ideas you can simulate quantum systems on a classical computer simulate them means solve the Schrodinger equation for them or solve the equations of quantum mechanics on a computer on a classical computer but the classical computer is not doing is not a quantum mechanical system itself of course it is that everything is made of quantum mechanics but it's not some functioning it's not a functioning as a quantum system it's just solving equations the quantum computer is truly a quantum system which is actually doing the things that you're programming it to do you want to program a quantum field theory if you do it in classical physics that program is not actually functioning in the computer as a quantum field theory it's just solving some equations physically it's not doing the things that that the quantum system would do the quantum computer is really a quantum mechanical system which is actually carrying out the quantum operations you can measure it at the end it intrinsically satisfies the uncertainty principle it is limited in the same way that quantum systems are limited by uncertainty and so forth and it really is a quantum system that means that what you what you're doing when you program something for quantum system is they're actually building a real version of the system the limits of a classical computer classical computers are enormous ly limited when it comes to the quantum systems enormously limited because you probably heard this before but in order to store the amount of information that's in a quantum state of 400 spins that's not very many 400 I can put in my path with 400 pennies in my pocket so we'll be able to simulate the quantum state of 400 elementary quantum systems qubits we call him to do that would take more information than can possibly be stored in the entire universe if it were packed so tightly that you couldn't pack anymore in right 400 cubits on the other hand if your quantum computer is composed of four hundred qubits it can do everything four hundred qubits can do what kind of space if you just intuitively think about the space of algorithms that that unlocks for us so there's a whole complexity theory around classical computers measuring the running time of things and PE so on what kind of algorithm is just intuitively do you think it's you know mocks for us okay so we know that there are a handful of algorithms that can seriously be quantum of classical computers and which can have exponentially more power and this is a mathematical statement nobody's exhibited this in the laboratory it's a mathematical statement we know that's true but it also seems more and more that the number of such things is very limited only very very special problems exhibit that much advantage for a quantum computer others of standard problems to my mind as far as I can tell the great power of quantum computers will actually be to simulate quantum systems if you're interested in a certain quantum system and it's too hard to simulate classically you simply build a version of the same system you build a version of it you build a model of it that's actually functioning as the system you run it and then you do the same thing you would do the quantum system you make measurements on it quantum measurements on it the advantages you can run it much slower you could say why bother why not just use the real system and why not just do experiments on the real system well real systems are kind of limited you can't change them you can't like them you can't slow them down so that you can poke into them you can't modify them an arbitrary kinds of ways to see what would happen if I if I change the system a little bit so I think that quantum computers will be extremely valuable in in understanding quantum systems at the lowest of the fundamental laws they're actually satisfying the same laws as the systems that they're simulating that's right okay so in the one hand you have things like factoring in factoring is the great thing of quantum computers factoring large numbers that doesn't seem that much to do with quantum mechanics right it seems to be almost a fluke that a quantum computer can solve the factoring problem in a short time so though and those problems seem to be extremely special rare and it's not clear to me that there's gonna be a lot of them on the other hand there are a lot of quantum systems chemistry there's solid-state physics there's material science there's quantum gravity there's all kinds of quantum of quantum field theory and some of these are actually turning out to be Applied Sciences as well as very fundamental Sciences so we probably will run out of the ability to solve equations for these things you know solve equations by the standard methods of pencil and paper and solve the equations by the method of classical computers and so what we'll do is we'll build versions of these systems run them and run them under controlled circumstances or we can change them manipulate them make measurements on them and find out all the things we want to know so in finding out the things we want to know about very small systems right now the is there something we can also find out about the macro level about something about it the function and forgive me of our brain biological systems the the stuff that's about one meter in size versus much much smaller well what the only excitement is about among the people that I interact with is understanding black holes that falls black holes are big things there are many many degrees of freedom there is another kind of quantum system that is big it's a large quantum computer and one of the things we learned is that the physics of large quantum computers is in some ways similar to the physics of large quantum black holes and we're using that relationship now you asked you didn't ask about quantum computers or systems you didn't ask about black holes you asked about brains yeah I bought stuff that's in the middle of the - it's different so but black holes are there's something fundamental about black holes it feels to be very different in the brain yes and they also function in a very quantum mechanical way right okay it is first of all unclear to me but of course it's unclear to me I another I'm not a a neuroscientist I have I don't even have very many friends who are neuroscientists I would like to have more friends who are neuroscientists I just don't run into them very often among the few neuroscientists I've ever talked about about this they are pretty convinced that the brain functions classically there is not intrinsically a quantum mechanical system or doesn't make use of the of the special features entanglement coherent superposition are they right I don't know I sort of hope that wrong with just because I like the romantic idea that the brain is a quantum system and but I think that I think probably not the other thing big systems can be composed of lots of little systems materials the materials are that we work with and so forth are three large systems and a large piece of material but they're Bagan they're made out of quantum systems now one of the things that's been happening over the last a good number of years is with discovering materials and quantum systems which function much more quantum mechanically then than we imagine topological insulators this kind of thing that kind of thing those are macroscopic systems but they just superconductors superconductors I have a lot of quantum mechanics in them you can have a large chunk of superconductor so it's a big decent material on the other hand it's functioning and its properties depend very very strongly on quantum mechanics and to analyze them you need the tools of quantum mechanics if we can go on to black holes mm-hmm and looking at the universe as a information processing system as a computer as a giant computer what's the power of thinking of the universe as an information processing system but what is perhaps its use besides the mathematical use of discussing black holes and your famous debates and ideas around that to human beings or life in general as information processing systems well all systems are information processing systems you poke them they change a little bit they evolve all systems or information processes there's no extra magic to us humans it certainly feels consciousness intelligence feels like magic sure though where does it emerge from if we look at information processing what are the emergent phenomena that come from viewing the world is an information processing system here is what I think my thoughts are not worth much of this if you ask me about physics my thoughts may be worth something yes if you ask me about this I'm not sure my thoughts are worth anything but as I said earlier I think when we do introspection when we imagine doing introspection and try to figure out what it is when we do and we're thinking I think we I think we get it wrong I'm pretty sure we get it wrong everything I've heard about the way the brain functions is so counterintuitive for example you have neurons which detect the vertical lines you have different neurons which detect lines at 45 degrees you have different neurons I never imagined that there were whole circuits which were devoted to vertical lines in the brain yeah doesn't seem to we where when my brain works my brain seems to work put my finger up vertically or if I put it horizontally or if I put it this way or that way it seems to me it's the same the same circuits that are it's not the way it works the way the brain is compartmentalized seems to be very very different than what I would have imagined if I were just doing psychological introspection about how things work my conclusion is that we won't get it right that way but how will we get it right I think maybe computer scientists will get it right eventually I don't think that any ways near it I don't even think they're thinking about it but by computer eventually we will build machines perhaps which are complicated enough and partly engineered partly evolved maybe evolved by machine learning and so forth this machine learning is very interesting by machine learning will evolve systems and we may start to discover mechanisms that that have implications for how we think and for what what does consciousness thing is all about and we'll be able to do experiments on them and perhaps answer questions that we can't possibly answer by by introspection so that's a really interesting point you've in many cases if you look even at string theory when you first think about a system it seems really complicated like the human brain and through some basic reasoning then trying to discover a fundamental low-level behavior of the system you find out that it's actually much simpler you one have you you know is that generally the process and to do you have that also hope for biological systems as well for all the kinds of stuff we're studying at the human level of course physics always begins by trying to find the simplest version of something an analyzer yeah I mean there are lots of examples where physics has taken very complicated systems analyze them and found simplicity in them for sure I said superconductors before it's an obvious one a superconductor seems like monstrously complicated thing with all sorts of crazy electrical properties magnetic properties and so forth and when it finally is boiled down through its simplest elements it's a very simple quantum mechanical phenomenon called spontaneous symmetry breaking and which we in other context we learned about and we're very familiar with so yeah I mean yes we do take complicated things make them simple but what we don't want to do is take things which are intrinsically complicated and fool ourselves into thinking that we can make them simple we don't want to make I don't know who said this but we don't want to make them simpler than they really are right okay is the brain a thing which ultimately functions by some simple rules or is it just complicated in terms of artificial intelligence nobody really knows what are the limits of our current approaches you mentioned machine learning how do we create human level intelligence it seems that there's a lot of very smart physicists who perhaps oversimplify the nature of intelligence and think of it as information processing and therefore that it doesn't seem to be any theoretical reason why we can't artificially create a human level or super human level intelligence in fact the reasoning goes if you create human level intelligence the same approach you just used to create human level intelligence should allow you to create superhuman level intelligence very easily exponentially so what do you think that way of thinking that comes from physicists is all about I wish I knew but there's a particular reason why I wish I knew I have a second job I consult for Google ah not for Google for Google X I am the senior academic advisor third to a group of machine learning physicists at now that sounds crazy because I know nothing about the subject I know very little about the subject on the other hand I'm good at giving advice so I give them advice on things anyway I see these young physicists who are approaching the machine learning problem there is a myth there is a real machine learning problem mainly why does it work as well as it does it nobody really seems to understand why it is capable of doing the kind of generalizations that it does and so forth and there are three groups of people who have thought about this there are the engineers the engineers are incredibly smart but they tend not to think as hard about why the thing is working as much as they do how to use it obviously they provided a lot of data and it is they who demonstrated that machine learning can work much better than you have any right to expect the machine learning systems are systems that the system is not too different than the kind of systems if this is a study there's not all that much difference between quantum construction of mathematics physically yes but in the structure the mathematics between a tension network designed to describe a quantum system on the one hand and the kind of networks that are used in machine learning so they're more and more I think young physicists are being drawn to this field of machine learning some very very good ones I work with a number of very good ones not on machine learning but having lunch on having lunch yeah and I can tell you they are super smart they don't seem to be so arrogant about their physics backgrounds that they think they can do things that nobody else can do but those physics way of thinking I think will add will I had great value to UM will bring value to the machine learning I believe it will and I think it already has and what time scale do you think predicting the future becomes useless in your long experience and being surprised at new discoveries sometimes a day sometimes 20 years there are things which I thought we were very far from understanding which practically in a snap of the fingers or a blink of the eye suddenly became understood completely surprising for me there are other things which I looked at and I said we're not gonna understand these things for 500 years in particular quantum gravity the scale for that was 20 years 25 years and we understand a lot and we don't understand it completely now by any means but we're I thought it was 500 years to make any progress it turned out to be very very far from that it turned out to be more like 20 or 25 years from the time when I thought it was 500 years so for me can we jump around quantum gravity some basic ideas in physics what is the dream of string theory mathematically what is the hope where does it come from what problems are trying to solve I don't think the dream of string theory is any different than the dream of fundamental theoretical physics altogether understanding a unified theory of everything I I don't like thinking of string theory as a subject unto itself with people called string theorists who are the practitioners of this thing called string theory I much prefer to think of them as theoretical physicists trying to answer deep fundamental questions about nature in particular gravity in particular gravity and it's connection with quantum mechanics and who at the present time find string theory a useful tool rather than saying there's a subject called string theorists I don't like being referred to as a string theorists yes but as a tool is it useful to think about our nature in multiple dimensions the strings vibrating I believe it is useful I'll tell you what the main use of it has been up till now well has had a number of main uses originally string theory was invented then I know there I was there I was right at the spot where it was being invented literally and it was being invented to understand hey groans hey drones are sub-nuclear particles protons neutrons mesons and at that time the late 60s early seventies it was clear from experiment that these particles call hydrants had could vibrate could rotate could do all the things that a little closed string can do and it was and is a valid and correct theory of these hydrants it's been experimentally tested and that is a done deal it had a second life as a theory of gravity the same basic mathematics except on a very very much smaller distance scale the objects of gravitation are nineteen orders of magnitude smaller than a proton but the same mathematics turned up the same mathematics turned up what has been its value its value is that it's mathematically rigorous in many ways and enabled us to to find to find mathematical structures which have both quantum mechanics and gravity with rigor we can test out ideas we can test out ideas we can't test them in the laboratory that nineteen orders of magnitude too small or things that were interested in but we can test them out mathematically and analyze their internal consistency by now forty years ago thirty five years ago so forth people very very much questioned the consistency between gravity and quantum mechanics Stephen Hawking was very famous for it rightly so now nobody questions that consistency anymore they don't because we have mathematically precise string theories which contain both gravity and quantum mechanics in a consistent way so it's provided that um that certainty that quantum mechanics and gravity can coexist that's not a small thing that's a very huge thing it's a huge thing Einstein be proud Einstein he might be appalled I don't know I'm like a very much yeah he would certainly be struck by it yeah I think that maybe at this time its biggest contribution to physics in illustrating almost definitively that quantum mechanics and gravity are very closely related and not inconsistent with each other is there a possibility of something deeper more profound that still is consistent with string theory but is deeper that is to be found well you could ask the same theme of quantum mechanics is there something exactly yeah yeah I think string theory is just an example of a quantum mechanical system that contains both gravitation and in quantum mechanics so is there something underlying quantum mechanics perhaps something deterministic so have something deterministic my friend far out it wolf whose name you may know he's a very famous physicist Dutch not as famous as he should be but the heart dispels names it's hard to say his name you know it's easy to spelling ' he's only person I know his name begins with an apostrophe and he's one of my heroes in physics and it's a little younger than me but it's nonetheless one of my heroes the Tufte believes that there was some sub structure to the world which is classical in character the deterministic in character which somehow by some mechanism that he has a hard time spelling out emerges as quantum mechanics I don't the wavefunction is somehow emergent the wavefunction and not just the wavefunction but the whole making the whole thing that goes with quantum mechanics uncertainty and pango meant all these things are emergent do you think quantum mechanics is the bottom of the well as is the right here I think is here I think is where you have to be humble here's where humility comes I don't think anybody should say anything is the bottom of the well at this time yes I think we I think we can reasonably say I can reasonably say when I look into the well I can't see past quantum mechanics I don't see any reason for it to be anything beyond quantum mechanics I think a tuft is a Sperry interesting and deep questions I don't like his answers well again let me ask if we look at the deepest nature of reality with whether it's deterministic or unobserved is probabilistic what does that mean for our human level of ideas of free will is there any connection whatsoever from this perception perhaps illusion of free will that we have and the fundamental nature of reality the only thing I can say is I am I am puzzled by that as much as you are the illusion of it the illusion of consciousness the illusion of free will the illusion of self does that connect to how can a physical system do that and and I am as puzzled as anybody there's echoes of it in the observer effect yeah so do you understand what it means to be an observer I understand it at a technical level an observer is a system with enough degrees of freedom that it can record information and which can become entangled with the thing that's measuring entanglement is the key when a system which we call an apparatus or an observer same thing interacts with the system that it's observing it doesn't just look at it it becomes physically entangled with it and it's that entanglement which we call an observation or measure now does that satisfy me personally as an observer hmm yes and no I find it very satisfying that we have a mathematical representation of what it means to observe a system you are observing stuff right now yeah the conscious level right is you think there's echoes of that kind of entanglement in our macro scale yes absolutely for sure we're entangled with quantum mechanically entangled with everything in this room if we weren't of and it was just well we wouldn't be observing it but on the other hand you can ask though I really am I really comfortable with it and I'm uncomfortable with it in the same way that I can never get comfortable with five dimensions my my brain isn't wired for it are you comfortable with four dimensions a little bit more because I can always imagine the fourth dimension this time so the arrow of time are you comfortable with that arrow do you think time is an emergent phenomena or is it's fundamental to nature that is a big question in physics right now all the physics that we do or at least that the people that I am comfortable with talking to my my friends yeah my friends no we all ask the same question that you just asked in space we have a pretty good idea is emergent and it emerges out of tan tangle mint and other other things time always seems to be built into our equations as just what Newton pretty much were for Newton modified a little bit by Einstein would have called time and and mostly in our equations it is not emergent time in physics is completely symmetric forward and bathymetric so you don't really need to think about the area of time for most physical phenomena the most microscopic phenomena no it's only when the phenomena involves systems which are big enough for thermal Amyx to become important the entropy to become important for small subsets a small system entropy is not a good concept an entropy is something which which emerges out of large numbers it's a probabilistic idea it's a statistical idea and it's a thermodynamic idea thermodynamics requires lots and lots and lots of little sub structures okay so it's not until you emerge at the thermodynamic level that there's an arrow of time do we understand it yeah I think I think we understand better than most people think that most people say they think we understand it yeah I think we understand it it's just a statistical idea the you mean like second law thermodynamics entropy and so on yeah the pack of cards and you're flinging it in the air and you look what happens to it yeah but what's random we understand it doesn't go from random to simple it goes from simple to random but do you think it ever breaks down what I think you can do is in a laboratory setting you can take a system which is somewhere intermediate between being small and being large and make it go backward a thing which looks like it only wants to go forward because of statistical mechanical reasons because of the second law you can very very carefully manipulate it to make it run backward I don't think you can take an egg Humpty Dumpty who fell on the floor yeah and reverse that but you can in a very controlled situation you can take systems which appear to be evolving statistically toward randomness stop them reverse them and make them go back what's the intuition behind that how do how do we do that how do we reverse it a clue you're saying closed system yeah pretty much closed system yes did you just say that time travel is possible no I didn't say time travel is possible I said you can make a system go backward in time and you don't like it go back you can make it reverse it steps you can make it reverse its trajectory yeah how do we do what's the intuition there does it have is it just a fluke thing that we can do at a small scale in the lab that doesn't have what I'm saying is you can do it on a little bit better than a small scale you can certainly do it with a simple small system small systems don't have any sense of the arrow of time atoms atoms uh no sense of the arrow of time they're completely reversible it's only when you have you know the second law of thermodynamics is the law of large numbers say you can break the law because it's not you can break German isn't the break it but it's hard it requires great care the bigger the system is the more to care the more the harder it is you have to overcome what's called chaos and that's hard and it requires more and more precision for 10 particles you might be able to do it with a with some effort 400 particles it's really hard for a thousand or a million particles forget it but not for any fundamental reason just because it's technologically too hard to make the system go backward so so note no time travel for engineering reasons oh no no no what is time travel time travel time travel to the future that's easy yes you just close your eyes go to sleep and you wake up in the future yeah yeah good nap gets you there yeah good map gets you there right but in reversing the second law of thermal is a very difficult engineering effort I wouldn't call that time travel because it gets to me too mixed up with what the science fiction calls time-travel right this is just the ability to reverse a system you take the system and you reverse the direction of motion of every molecule in it that input you can do it with one molecule if you find a particle moving in a certain direction let's not say a mama particle a baseball you stop it dead and then you simply reverse its motion in principle that's not too hard and it'll go back along its trajectory in the backward direction just running the program backwards running the program backward yeah okay if you have two baseball's colliding well you can do it but you have to be very very careful to get it just right now ten baseball's really really tore better yet tend ten billiard balls on an idealized frictionless billiard table mm-hmm okay so you start the balls all in a triangle right and you're whack them yep depending on the game you're playing you the wacom where you're really careful but the you're welcome and they go flying off in all possible directions okay try to reverse that try to reverse that imagine trying to take every billiard ball stopping it dated sometime at some point and reversing its motion so it was going in the opposite direction if you did that with tremendous care it would reassemble itself back into the triangle okay that is a fact and you can probably do it with two billiard balls maybe with three billion balls if you're really lucky but what happens is as the system gets more and more complicated you have to be more and more precise not to make the tiniest error because the tiniest errors will get magnified and you'll simply not be able to do the reversal so yeah you could that but I wouldn't call that time travel yeah that's something else but if you think think of it it just made me think if we think the unrolling of state that's happening as a program if we look at the world so the idea of looking at the world as a simulation as a computer but it's not a computer it's just a single program a question arises that might be useful how how hard is it to have a computer that runs the universe okay so there are mathematical universes that we know about one of them is called anti de sitter space where we and it's quantum mechanics well I think we could simulate it in a computer and a quantum computer classical computer all you can do is solve its equations you can't make it work like the real system if we could build a quantum computer or big enough one robust enough one we could probably simulate a universe a small version of an anti-de sitter universe and that the sitter is a kind of cosmology so I think we know how to do that the trouble is the universe that we live in is not the anti-de sitter geometry it's the decent or geometry and we don't really understand the quantum mechanics at all so at the present time I would say we wouldn't have the vaguest idea how to simulate a universe similar to our own you know we could ask oh we could we build in the laboratory a small version a quantum mechanical version the collection of quantum computers entangled and the couple together which would reproduce the the phenomena that go on in the universe even on a small scale yes if you were anti de sitter space know if it's the sitter space can you a slightly describe the sitter space and anti-de sitter space yeah what are the geometric properties of big different they differ by a this is the sine of a single constant called the cosmological constant one of them is negatively curved the other is positively curved the anti-de sitter space which is the negatively curved one you can think of as an isolated system in a box with reflecting walls you could think of it as a system of quantum mechanical system isolated in an isolated environment the sitter space is the one we really live in and that's the one that's exponentially expanding exponential expansion dark energy whatever you want to call it and we don't understand that mathematically do we understand not everybody would agree with me but I don't understand they would agree with me they definitely would agree with me that I don't understand it what about their an understanding of the birth the origin no the bing bang so knows what normally theories there are theories my favorite is the one called eternal inflation the infinity can be on both sides on one of the sides and none of the sides so what my real opinion okay infinity on both sides oh boy yeah yeah that's why is that your favorite because it's the the most just mind-blowing no because we want a beginning no why do we want a beginning I practiced it was the beginning of course and practice it was a beginning but could it have been a random fluctuation in an otherwise infinite time maybe in any case the the eternal inflation theory I think if correctly understood it would be infinite in both directions how do you think about infinity Oh God so okay of course you can think about mathematically I just finished this I just finished this discussion with my friend Sergey Brin yes how do you think about infinity I say well Sergey Brin is infinitely rich how do you test that hypothesis okay essential good lines all right yeah so there's no there's really no way to visualize some of these things like ya know this is a very good question those physics have any is does infinity have any place in physics right right and well I can say is very good question so what do you think of the recent first image of a black hole visualized from the event horizon telescope it it's an incredible triumph of science in itself the fact that there are black holes which collide is not a surprise and they seem to work exactly the way they're supposed to work will we learn a great deal from it I don't know I can I I we might but the kind of things we learn won't really be about black holes why there are black holes in nature of that particular mass scale and why they're so common may tell us something about the structure evolution of structure in the universe but I don't think it's going to tell us anything new about black holes but it's a triumph in the sense that you go back a hundred years and it was a continuous development general relativity the discovery of black holes LIGO the incredible technology that went into LIGO it is something that I never would have believed was gonna happen you know 30 40 years ago and I think it's a magnificent the structure magnificent thing this evolution of general relativity LIGO high precision ability to measure things on a scale of 10 to the minus 21 so so you're just astonishing though we zoom all this just happy for us to this right picture is it different you know you've thought a lot about black holes is it how did you visualize them in your mind and is the picture different than you know lies that no it simply confirmed you know it's a magnificent triumph to have confirmed confirmed a direct observation yeah that Einstein's theory of gravity at the level of black hole collisions actually works is awesome and it's really awesome you know I know some of the people who were involved in that they just thought married people yeah and the idea that they could carry this out I just don't I'm shocked yeah just these little Homo sapiens yeah just these little monkeys got together right and took a picture of slightly advanced lemurs I think what kind of questions can science not currently answer but you hope might be able to soon well you you've already addressed them what is consciousness for example do you think that's within the reach of science I think it's somewhat within the reach of science but I think that now I think it's in the hands of the computer scientists and the neuroscientists I'm not a physicist perhaps with the helper haps at some point but I think physicists will try to simplify it down to something that they can use their methods and maybe they're not appropriate maybe we maybe we simply need to do more machine learning on bigger scales evolve machines machines not only that learn but volve their own architecture as a process of learning evolve in architecture not under our control only partially under our control but under the control of a machine learning I'll tell you another thing that I find awesome you know this Google thing that they taught the computers how to play chess yeah yeah okay they taught the computers how to play chess not by teaching them how to play chess but just having them play against each other against each other itself against each other this is a form of evolution these machines evolved they evolved and intelligence they evolved in intelligence without anybody telling them how to do it and we're not engineered they just played against each other and got better and better and better that makes me think that machines can evolve intelligence what exact kind of intelligence I don't know but in understanding that better and better maybe we'll get better clues as to what there goes on your life and intelligence is last question what kind of questions can science not currently answer and may never be able to answer yeah is there an intelligence out there that's underlies the whole thing you can call them with the G word if you want I can say are we a computer simulation with a purpose is there an agent an intelligent agent that underlies or is responsible for the whole thing does that intelligent agent satisfy the laws of physics does it satisfy the laws of quantum mechanics is it made of atoms and molecules yeah there's a lot of questions and I don't see this it seems to me a real question it's an answerable question well it's answerable the questions have to be answerable to be real some philosophers would say that a question is not a question unless it's answerable this question doesn't seem to me answerable by any known method but it seems to me real there's no better place to end monitor thank you so much for talking about okay you