The Quantum Origins of Gravity by Leonard Susskind

so we gather here today to celebrate physics in memory of Oskar Klein the idea here is that I will say a few words about him and his role in physics I'm both on board by the way professor here at physical now after that Lara stole asius will take over and introduce our speaker today and there will be small ceremony after that we we shifted that to before the talk so we can have an uninterrupted Anantara interrupted you know celebration with a talk and questions so let's start let's start yes so Oscar Klein he was a Stockholm physicist and my grandmother's generation more importantly he was a young physicist in Copenhagen in the twenties when modern physics was born and he was not just a spectator he contributed to many important developments important still today Colusa Klein theory the klein-gordon equation you're inclined second quantization the Klein Sheena formula and decline paradox for today's lecture the Klein paradox is possibly the most relevant so what a paradox is for they aren't possible to measure in the lab are they hmm paradoxes disclose the tension between accepted principles so physics thrives and on this tension it challenges our theories and directs our attention for the future on Christmas Eve 90 years ago Oscar Klein's manuscript on his paradox was received by the journal citing fear physique it dealt with the relativistic particles in an external potential intense debate followed today this paradox as what's called is resolved but it's still relevant the most relevant aspect of it is the connection to Hawking radiation from black holes which I turn to now this is supposed to depict this potential step right in a sufficiently strong potential pairs of particles and antiparticles are created this happens outside the event horizon of a black hole and as I said this creation of particles and antiparticles is part of the resolution to the client paradox the particles in the pair they are correlated with one another and if one of them is absorbed by the black hole Hawking argued that information would be lost this is intention with basic quantum mechanics and constitutes the information paradox so our speaker today has played a decisive role in this resolution in the resolution of the information paradox and it's very suitable thus to have him in our list of eminent Cline lecturers now I leave the word to Lauriston osseous who is going to chair the session thank you so on behalf of the Oscar Cline Memorial Lecture Committee it is gives me great pleasure to introduce the speaker for 2018 oskar klein lecture professor Leonard Susskind from Stanford University who will also receive the 2018 Oscar Cline medal here today professor Susskind is a world-renowned theorist who has made key contributions to many branches of physics he has made contributions to elementary particle physics to string theory and to quantum gravity he received his PhD from Cornell University in 1965 this was followed by a postdoctoral fellowship at Berkeley and then since 1967 when 1967 he joined the faculty of Yeshiva University in New York and then since 1978 he has been at Stanford University where he is the Felix Bloch professor of physics now professor Sorkin's list of physics achievements is quite long and I will just list or mention a few highlights he did groundbreaking work on the theory of core confinement he as also also on lattice gauge theory an early theory of baryogenesis in the early universe and he's also one of the originators of the so-called Technicolor theories of electroweak symmetry breaking he was one of the founders of string theory in the late 1960s and string theory in quantum gravity have been major themes in his research throughout among his many seminal contributions to gravitational theory and string theory I think stands out is the holographic principle which among other things underlies Malthus anus celebrated anti-de sitter conformal field theory ideas CFT correspondence and his recent work has focused on the application of ideas from quantum information theory quantum computing such as quantum complexity to the physics and thermodynamics of black holes and other holographic systems and I believe we'll hear more about this in his talk this afternoon in addition to his research accomplishments Susskind is highly regarded as an educator and he's an ambassador of physics to the general public he has written several text books books on popular science and you will find many of his lectures online he's a member of the US National Academy of Sciences the American Academy of Arts and Sciences he was awarded the Sakurai prize in 1998 the Homer Antioch Prize in 2008 and now in the 2008 Oscar Kline medal and so before we start the talk I would like to yield the floor to my colleague Edward Madsen who will present the client medal and then we will continue with the proceedings okay thank you very much so I will be very brief I will basically just hand over the medal but first I will take this opportunity also to invite you to a reception after their talk so this will be on the third floor outside of the restaurant I don't know if I can can i yeah I will have solar closers I'll do it manually okay is that fine good okay so I have the middle here so please Leonard it's a pleasure to hand you the Oscar Klein madam so there we go Oh so in case you didn't catch it this was actually the thing you saw booth first slide there okay Wow so okay now I give that to you thank you oh right about 25 hours ago I woke up in Palo Alto I have not been asleep since this is I can handle that being up for 25 hours not as well as I could when I was 20 years younger well it's a dangerous thing to do before you give a public lecture I don't expect to fall asleep but I think it's quite possible that I won't recognize some of my own transparent my own slides up there so if I start to ramble and get a little bit incoherent well I'll just ask you to forgive me because I'm pretty tired I wonder if I have some friends here how low the vecchio has he here hello Paulo oh you long time okay well let's uh let's begin I had some things I was going to say about oskar klein in particular the connection between the klein paradox and hawking radiation I've been scooped too late nevertheless I will say in many different circumstances my career has crossed paths with Oscar Klein's in particular that's the one I wanted to most emphasize but I'll leave it to Lou to have said that anyway let's let's begin let's see how does this work what I want to talk about with you today is quantum mechanics and gravity as was mentioned that's been a subject which I've been interested in for a long time in fact I would say it's the only subject I've been interested in for a long time and physics subject let me begin with a question look this is too small to see here it's gravity a thing to be quantized like the harmonic oscillator a fing to be quantized like the harmonic oscillator the hydrogen atom electrodynamics or young oh there it is I got it ok good thank you it's gravity going over this over here or evil so I can stay in front of it this is this gonna work no maybe we can move the whole table over good yeah ok good ok good yes is gravity a thing to be quantized quantized is a particular recipe it's a recipe for taking a classical system a classical theory of some kind and to convert it into a quantum mechanical theory it's a recipe that goes back to Dirac and it consists of a number of steps the first step is you take the degrees of freedom in this case the gravitational field the metric of space-time and you would place it by operators operators quantum mechanical operators and then you invent a thing called a Hilbert space Hilbert space replaces the phase space of classical mechanics you replace commutator sorry you replace Poisson brackets by commutator z' this is just a recipe and finally Hamilton's equations Hamilton's equations of motion become the Heisenberg equations of motion the important thing here is that there is a recipe for taking a classical theory and converting it to a quantum theory you can do in other ways there's the path integral approach of my friend Richard Fineman what's the usual goal the usual goal is to construct the set of Fineman diagrams diagrams that will allow you to calculate processes like for example the scattering of particles due to gravitational forces the scattering of light by the Sun all kinds of processes that you can calculate quantum mechanically and to some extent it works but not so well once you start actually trying to use it and trying to push it to its logical conclusions all hell breaks loose everything comes out infinite black holes lose information all kinds of bad things happen that aren't supposed to happen and so I would say to some extent perhaps the more than some extent it's been a failure the quantization of gravity as if it were just a classical theory to apply the recipe to okay well this Sun has led some people to say more than some people almost everybody the thing that there's some really serious tension or even a contradiction between gravity and quantum mechanics I want to read you what I had to say about this I'm reading now what's in quotes there I've taken from my own writings I take exception to this view Oh incidentally to take exception to something in English means I just don't agree with it I think exception to this view I think exactly the opposite is true it may be too strong to say that gravity and quantum mechanics are exactly the same but to those of us who are paying attention we may already sense that the two are inseparable that neither makes sense without the other and two things make me think so I'm going to describe those two things today and go into some detail not technical detail but conceptual detail the first of them is called er equals EPR I'll tell you but right now I hadn't intended to tell you now I will tell you anyway er stands for Einstein and Rosen Einstein and Rosen well you've heard of Rosen but Einstein was sort of his assistant I am tired says for Einstein and Rosen who wrote a famous paper about gravity in 1935 EPR stands for Einstein Podolsky and Rosen Podolski was another one of Einstein's collaborators who wrote a famous paper about quantum mechanics that same year in 1935 having little idea I suspect I don't believe they had any idea that the two were intimately related einstein-rosen and einstein-podolsky-rosen now what are they we're gonna spend a little bit of time explaining what they are and how odd and bizarre they are let's start with the idea of a formation of a black hole very simple a cloud of particles indicated by a bunch of little points collapses under its own weight to due to its own gravitation the particles get pulled together by gravity and what happens to them eventually they form a black hole that's the basic process that we'll be interested in and of course there's no reason why this should only be one black hole in the world two clouds of gas somewhere as in distant parts of the universe could independently collapse and form two black holes we could call one of them Alice's black hole I suppose that's the one on the left Bob's black hole he's a little fellow can you see Bob I can't yeah there's Bob Bob that's Bob's black hole there's Alice's black hole over there and just two black holes what do they have to do with each other not much they were formed independently they're uncorrelated they're not interacting with each other because they're too far away from each other and so we would say that they're statistically and dynamically and an always independently Cho of each other no interactions no correlations and if perchance Alice happened to fall into her black hole that would be the end of her if Bob fell into his black hole that would be the end of him and the story okay now what I want to imagine now is that we formed the black or linoleum that we come to sell I am tired let me come to another issue now that was black holes we'll have more to say about black holes clearly but before we do I want to talk about entanglement and tangle mint is the thing that Einstein Podolsky and Rosen discovered okay what is entanglement and tangle meant between systems it could be the entanglement between two electrons it could be entanglement between two photons it could be entanglement between well any two things entanglement is a quantum mechanical it's the quintessential quantum mechanical property that systems can have pairs of systems that never makes any sense classically if I were to tell you that I know everything that there is to know about a pair of systems these two things over here if I know everything that can be known about the come the composite system made out of these two things one of these are things anyway ah if I were to tell you that I know everything or that everything that can be known about system is known you will say well that must mean that everything about the parts of the system are known how can it possibly be that you know everything that can be known about a system and not also know everything about the two parts of the system well in quantum mechanics that can happen you can know everything about a pair of electrons this is supposed to be a pair of electron spins the spin can be up or it can be down two electrons and for those who know a little bit about entanglement you'll recognize this as an entangled state in which you know nothing about the first particle what do I mean by you know nothing about it it could equally well be likely to be up or down so if it's equally likely to be up or down you know nothing the second particle can easily can equally well be down or up again you know nothing and yet this is as complete a description of this state of an electron of a pair of electrons as you can possibly have what do you know what you do know is correlation you know that if the first electron is measured to be up the second one is definitely down if the first one is measured to be down the second one is necessarily up it's either this or this that's the character of entanglement something that never happens classically as I said if you classically if you know everything there is to know about a system you know everything about its parts not so quantum mechanically two particles which are entangled in this way are usually called Bell pairs Bell doesn't stand for the thing that you make a sound with John Bell the physicist John Bell who spoke about a great deal of this in a did a great deal to it but this is essentially the einstein-podolsky-rosen idea of entanglement and we usually use a little graphical notation to indicate that two particles are entangled namely just draw a line between them and that indicates that they're entangled in this funny way okay now let's imagine again that we're going to create two black holes but instead of just creating them independently what we're going to imagine is that we create first a bunch of Bell pairs a large number of particles which are entangled with another large number of particles this particle is entangled with this one this one's entangled with this one we create them in the laboratory and then we separate them separating them does nothing to destroy the entanglement and so what we wind up with is two entangled clouds of particles and then on each side the particles collapse to form black holes what does that make well it makes two entangled black holes that means that if you measure something about one black hole you learn something about the other one if you measure something about this one you learn something about this one two entangled black holes einstein-podolsky-rosen entanglement in this case between black holes and this is something that it's not something he's gonna do in a laboratory we're not gonna make black the laboratory but in principle this is something that could be made okay what's new about it how are they entangled black holes different than the unentangled black holes and here is where something strange happens I'm not going to prove it I'm going to tell you this is the outcome of many years of research the last 20 years of research 20 25 years of research we'll talk a little bit about it but I'm gonna tell you right now what has been learned now take a look at this this is supposed to be space Y had let bent it over like that bending over like that there's nothing real to it if I have a piece of paper and that piece of paper is flat if I bend it like that that does not change geometry of the sheet of paper intrinsic called the intrinsic geometry the intrinsic geometry is the same whether this way or this way or this way or this way but just so I can make a point I've taken space and folded it over so that this region over here is close to that region on the diagram these two places are very far apart if Alice wants to get the Bob she has to walk all the ways around here Alice and Bob have created a pair of entangled black holes the new thing because they're entangled is that an einstein-rosen bridge fall forms between them an einstein-rosen bridge is popularly called a wormhole it's a route through space which short circuits the long distance between these this is up to my mind one of the craziest things we've discovered in the last 20 years that the existence of entanglement creates spatial connectivity between two regions which would otherwise be very very far apart now does that mean let's just does that mean that Alice can send a signal to Bob short-circuiting the raw the long distance between them no it doesn't entanglement does not provide a way of sending messages it's correlation but it's not message sending Alice cannot when we're gonna come to the reason why soon enough why Alice can't use this wormhole this einstein-rosen bridge to send a message directly to Bob she cannot violate the rule that to send a signal from one place to another place that it goes faster than the speed of light it would be going faster than the speed of light at least the speed of light around this direction if Alice could send a message to Bob there are two ways to think about this from the point of view of entanglement there's a theorem the theorem says that entanglement can never be used to send the signal from one place to another it's correlation but it's not signal sending from the point of view of general relativity from the point of view of these wormholes it's the non traversable character of wormholes that you cannot cross from one to the other from one side to the other we're gonna we're gonna explain why that's true but not yet okay but nevertheless despite the fact that Alice and Bob cannot send messages through the wormhole there is something else that's extremely odd and this is a property of general relativity it's a property of these einstein-rosen bridges namely that Alice and Bob can decide to jump into their respective black holes and meet at the center so let's let's just review we take these entangled particles we separate them out to the ends of the universe we collapse them into black holes and then Alice jumps into her black hole Bob jumps into her black his black hole and they can meet in the center after a very short time that is a remarkable conclusion if it's true but the only thing whoops let's go back the only thing is that they're meeting at the center can never be um shall I say can never be reported to the outside they are stuck behind the horizons of these black holes they've jumped in there behind the horizons of the black holes and they cannot report in the outside world that they've discovered some each other on the inside so somebody on the outside will not experience any indication that something very strange has happened because of this entanglement okay for the experts for the experts here's the proof the proof is a Penrose diagram if you've seen Penrose diagrams before you'll know what this is if you haven't it's just what I said on the previous slide this is a picture of diagram a mathematical diagram of the two black holes that were created very very far apart but in which you can pass from one to another through a wormhole what this picture is saying what the previous picture is saying is that Bob and Alice can jump into their respective black holes and meet at the center all right so that's a mathematical proof did you catch the proof no equations but a mathematical proof that two entangled black holes and they are entangled two entangled black holes can be used not to send a message through notice you cannot send a message from this side over here to the side over here I would exceed the speed of light but the two participants can jump in and meet at the center that's a crazy thing but it is crazy given that it's crazy you might ask and you might sensibly ask how can I have the boldness to be so sure that such things are right but I really know what I'm talking about I'm making up things maybe I'm making up things but do make up things sometimes no I'm not making up things all of this comes from about 25 years of experience combined experience of 25 years of many physicists it began with something called the holographic principle which I won't go into I was involved in it heavily but it's it's part of the story we won't go there right now the holographic oops excuse me the holographic principle in another language is called oops I'm wonderin tired it's called gauge gravity to our B I'll talk about that a little more one version of it is called matrix theory it goes back to 1996 but most of all the most important thing that goes into these arguments is something called a DSC ft I'm going to tell you what that is it was discovered by Juan maldacena in 9th in 1998 and it has been the main driver of research in quantum gravity for at least the last I would say the last 20 years I think 20 years yeah and it has been extraordinarily fruitful so let me tell you what it is the holographic principle is the idea that a regional space can be described by degrees of freedom and physics on the boundary of the region of space that idea became extremely precise with the work of maldacena and here's what it says it says in a kind of space called ad SIDS stands for anti-de sitter there's a kind of space called de sitter space and it was invented by de sitter this was the space that was invented by the sitter's ant sorry and thank this is the opposite of the sitter space whatever whatever the sitter space is this is the opposite but this is the way to think about it it's a space time time runs up word space runs across and it's a space that you can think of you can think of it as being in a finite cavity in a finite spherical box with reflecting walls so that anything that hits the wall of the box bounces off everything takes place within this box time runs up space runs across and it's a space-time that has gravity in it it can have black holes it can have people the people can fall toward the black holes and it's called the bulk space it's three-dimensional I wrote D dimensional here but it's three-dimensional space in here the holographic principle or a DSC ft says exactly this state the saw system is equally described mathematically by a theory of degrees of freedom that live on the boundary of the space purely on the boundary and those degrees of freedom constitute a quantum field theory a quantum field theory whose degrees of freedom are entirely restricted to the boundary of space that quantum field theory has Fineman diagrams and particles and so forth but they don't move into the bulk they live only on the boundary and while the same is discovery is that these these two theories are exactly the same and it described the same thing that is also crazy but here the mathematics is extremely precise and so it will so much true the things I'm going to tell you are consequences of this idea you don't have to believe any of them but if you don't believe them I think you have to not believe in this kind of gauge gravity or ABS CFT duality which as I said is mathematically very precise ok but now what are we gonna go next next what I want to tell you is the kind of quantum field theory that describes the boundary of space or that sorry it doesn't describe the boundary of the scribes everything inside the space but it lives on the boundary it's a field theory on the boundary that kind of quantum field theory is very familiar kind of quantum field theory but it occurs usually you think of quantum field theory is having to do with elementary particles but the place where quantum field theory also really has had a tremendous impact in physics is in condensed matter physics just a lump of material if it's quantum mechanical and of course all of the material are quantum mechanical but typically described by quantum field theories those quantum field theories for example can govern the phonons propagating in a crystal lattice the fluctuations of a superfluid superconductors magnetic systems quantum Hall system topological insulators and those quantum field theories are very similar to the kind of quantum field theories that go into this gauge gravity duality the kind of quantum field theories that live on the boundary of space in fact there's no known objection or obstruction there engineering a strongly coupled large n gauge Theory on a shell of silicon you'd have to dope the silicon you have to put in some extra stuff but this when I say silicon that doesn't have to be silicon that's just a metaphor for a piece of material are constructing a set of degrees of freedom that live on a shell that have the property of behaving exactly like the kind of quantum field theory that gauge gravity duality is all about so I want you to imagine that we created such a shell in a laboratory there's a shell of matter it's a shell it's hollow there's nothing inside it and it has been engineered in just such a way that it is described all of its properties of the scribes by a quantum field theory that lives on that shell that has just the properties of the kind that maldacena thought about when he talked about the relation between anti-de sitter space and the boundary quantum field theory so there it is and now Alice who's the experimentalist in this laboratory wants the check is there in some sense an equivalence between the things that go on on the shell and some kind of theory that lives in a bulk in a hypothetical bulk I'm not going to tell you where the bulk is but I'll will ask so she's gonna do some experiments on the shell the first thing she does for the shell is she taps on it tapping on it will create excitations on it now from the point of view of the shell looking at the shell you might expect that the perturbations create waves which go around the shell and focus at the opposite end it's true that's what would happen on the other hand if you were a believer in the existence of a bulk then you might say that tapping on the shell over here shoots a particle into the bulk which propagates across to the other side in both cases you'll find that something goes across from this end to this end in a certain amount of time and in fact if you calculate in the anti-de sitter space geometry how long it takes for a signal to go through the bulk it's exactly the same as the time that it takes to go around the boundary so that's a check Alice does this she taps on it over here she goes and looks over here and she says oh my goodness Oh something came through the bulk or she can say something went through the no difference she can do other kinds of experiments in fact she can get together with Bob Alice will tap on this end Bob will tap on that end that will create signals which according to the bulk theory propagate into the interior from both sides scatter as if they were particles scattering and then come back out to the boundary and if Alice waits over here she will see a signal over here and Bob will see a signal over here or will they conclude either there was some rather magical conspiracy in stuff propagating around the boundary or they will say yes there is this bulk boundary correspondence and one way to think about it is by tapping on the shell we send something into this hypothetical bulk which scattered off the other thing and they wound up near the boundary one can calculate these things and in fact that is what they would see here's another example for some reason I imagined two taps one from over here one from over here now in this case we imagine that there was something already in the bulk what does it mean that there's something in the bulk from the point of view of the boundary don't worry somehow some energy was sent into the system so that some kind of object is believed to be by Alice Alice believes that some kind of object at the center of the bulk of the geometry here and so she taps on the system that creates a signal that goes into the bulk if you believe in the bulk hits the object over here and if the object is of the right kind that might start it ringing if it was a bell it would start it ringing if it rang it would send out waves the waves were propagate out to the boundary and by standing out near the boundary of the shell here the shell the boundary out near the boundary she would discover that kind of waves propagated out she would feel it she would be able to detect these waves hitting the boundary and she would conclude that yes there was something at the center it had some properties and they could be probed by from the boundary from her outside laboratory by tapping on the shell so the lesson to be learned here is that Alice can either think that the boundary theory created waves which went around the boundary in very very complicated ways or she can think equally well that yes there really is a bulk on the inside of the shell and that stuff can propagate in hit things scatter and be detected at the boundary now one of the things that we can create in the bulk is a black hole how do we create a black hole I just have to tell you this the way you create a black hole starting with just this show the shell of silicon you heat it up that's all you heat it up just heating it up creates energy on the shell and the dual description of that the description of that from the point of view of this a DSC ft holographic description is that a black hole is obvious but that our one of the things that goes with this idea how can you heat the shell one way you can heat the shell is just illuminate it with laser until it's heated it creates a black hole inside the bulk of course you can't see behind the black hole horizon for the usual reason nothing can escape from the black hole but you could do other experiments having created the black hole you could hit it with something and watch the black hole ring black holes do ring they have quasi normal modes which are like wringing modes so Alice could in fact attempt to discover whether there's really a black hole at the center of this shell and she would discover that there is we create two black holes same way we create two shells let's start with two shells we create two shells and again illuminate each one separately create two black holes and that's all we just have two black holes two independent black holes in two different shells but we can do something else or at least Alice can do something else she can use a phenomenon called spontaneous parametric down-conversion spontaneous down conversion is a process in which a photon an electromagnetic photon or high frequency one or relatively high frequency that's why it's labeled by blue it's a crystal and splits into two photons it's a nonlinear process one photon becomes two photons but moreover the two photons are entangled entangled the same way that Einstein thought about then go and be sorbed by the two separate silicon shells and both will get heated but now by virtue of the fact that the photons which heated them were entangled the two shells or the two black holes will also be entangled well we argued a few minutes ago that entangled black holes have einstein-rosen bridges between them they're connected by wormholes Alice and Bob can attempt to send things into the silicon shell there two silicon shells remember one of the over there one is way over here they can send surrogates into the shells by tapping on them by doing things those surrogates can fall into their respective black holes and the prediction is crazy again that they will find each other and meet each other inside the two black holes now these are real black holes at least they don't seem like the real black hole they're just descriptions of the behavior of the silicon shells descriptions of the behavior through this idea of gauge gravity duality but yet if you believe the mathematics of gauge gravity duality you'll forced to conclude that Alice and Bob can send in signals they may be they may even be able to worm this to their own ways into these shells and meet at the center of black holes which aren't even really there or at least nobody can find them in the laboratory is two laboratories this the tooth to the two shells nobody in a laboratory can find this wormhole which connects them but yet they will find each other that's the prediction that's the crazy prediction can it be tested can it be tested given that the surrogates or Alice and Bob themselves only meet behind the horizon and the answer is yes it can be here I'll show you what they have to do I've abstracted the whole picture of the two black holes and their wormholes by just drawing a wormhole between two two circles over here these are two horizons the two horizons of Alice's and Bob's black hole inside their respective silicon and here's what Alice wants to do she has some kind of system in her possession it's a quantum system it could be a spin pointing in some direction whatever it is she wants to send that the Bob as a message or as a present she wants to send it through the wormhole to Bob now can she do it I will tell you the answers yes she can do it by a protocol a protocol a procedure which goes as follows the first thing she does let me go back the first thing she does is she drops the quantum system that she wants to send in into her black hole okay that sent it into the black call into the wormhole between them then she makes a measurement of some kind she measures something on her horizon this is the horizon of her black hole she measures a couple of things it almost doesn't matter what she measured measures she makes some message some measurements and she records them in her notebook maybe um just a few numbers that she measures about her black hole then she sends her mat her notebook to Bob now first of all what's in Alice's notebook is pure gibberish it's random she made these measurements quantum measurements on the system she just got random gibberish why just because that's the way these quantum measurements would work she gets random rubbish and in particular that rubbish has no correlation or no memory to the system that she threw into the black hole but when Bob gets it he has gotten something which you can think of as a code for what to do next he gets this little piece of information which itself has no information about what Alice sent in but it gives Bob an instruction what to do next Bob follows the instruction whatever it is he applies his instruction to his black hole maybe he rotates a magnet maybe he bounces a ping pong ball off the office black hole whatever the instruction is that Alice sent and guess what happens next the quantum system that Alice sent in pops out on Bob side the message went through the wormhole doing this with the two silicon shells Alice can Alice and Bob can convince themselves that indeed there was a wormhole and that the system the present that Alice sent the Bob passed through the wormhole okay moreover in passing through the wormhole whatever that system was it may detect things inside the wormhole there may be photons in there there may be other particles when whatever it was that Alison through comes out the other side it may have been affected by the things that were discovered inside the black hole and so when he goes through the wormhole and comes out the other end whatever that system is it record a message that can be delivered to Bob saying I found this or that inside the wormhole so not only can you send the message but you can explore the wormhole now is this fantasy it's just a concoction that I made up no it's actually called quantum teleportation quantum teleportation is a procedure that was invented not by gravitational physicists not by a DSC ft mile designers and so forth it was invented by computer scientists quantum computer scientists who were interested in exploring quantum mechanical ways to send very secure messages messages that could not be eavesdropped upon this is the sort of iconic figure of quantum teleportation Alice and Bob create a pair of entangled particles as an extra particle over here which Alice on her side wants to send through to Bob when I say a particle to send through the Bob I mean the quantum state of the particle she has a particle in a quantum state and she wants to she wants to send that quantum state the Bob this is the iconic figure again Alice combines the system that she's trying to send with her half of the entangled system makes a measurement sends a couple of pieces of classical information and Bob then knows what to do so that his qubit over here comes out in the state of the thing that she was trying to send through this is called quantum teleportation thing that has been done in the laboratory by now it's a hard thing to do but it has been done in a laboratory and you can ask how did the information get from this side to this side it did not go through this classical stuff that was sent across here that was Dredd gibberish one can say either you can say well it's just quantum teleportation or you can say it passed through a sort of proto wormhole that correspond it to the fact that you have entanglement between these two things you can do the same thing except in a much more elaborate way which much much more reflects what happens in these blackhole prototypes that were made in the silicon shells not by using silicon shells but by using quantum computers if we ever develop quantum computers this time goes up in this picture this is supposed to be a quantum computer of processing information I admit they're acting with each other here's another computer over here what goes on down here is simply the statement that the two quantum computers were made out of entangled matter you've entangled two quantum computers this is something that can be done I mean in principle it can be done probably in practice can be done through quantum computers can be entangled in this way you can ample degrees of freedom a couple qubits on Alice's side and let them interact with the rest of the quantum computer and if you do the protocol correctly out will pop whatever you put in over here on Bob's side you'll have your you have your choice you can either say this is just quantum teleportation nothing new or you can say that the entanglement created a wormhole between these two and this system over here went through the wormhole and popped out on the side over here the two descriptions are quite equivalent you can do something else you can modify the initial state of the quantum computer I've just done one extra little perturbation over here somebody gives a little knock to one of these qubits over here what does that do that well it just affects the quantum computer in a certain way but from the point of view of the black hole picture other or the wormhole picture this perturbation over here is roughly equivalent to making a new photon or a new particle inside the wormhole you perturb the wormhole it's a little bit different perhaps it has an extra photon the effect of this perturbation over here will influence what comes out on the right-hand side instead of the system going through and coming out unmolested let's say it will be affected by this another way to say it is when you send something through the wormhole when it comes out it has a record of what it encountered in the interior which it can report to the outside in other words it allows you to explore the interior of the wormhole and this this is something that in my opinion will actually take place in laboratories when we learn to you've to build quantum computers there's no reason why this can't be done in a laboratory if we have more than a few qubits it will be impossible to study or the standard quantum mechanical methods how will we study it by thinking of it as a gravitational wormhole now how I've been going on okay we have a few more minutes I wish I had my glasses yeah okay yeah I originally I began well began by telling you there were there were two things that made me think that quantum mechanics and gravity are so tightly connected together that they really can't be separated one of them was this er equals EPR einstein-rosen bridges that's a gravitational thing EPR that's a quantum mechanical thing and they seem to be the same the other thing which is very interesting I think is the idea well yeah of quantum complexity and the growth or the expansion of space wormholes grow I'll show you a little cartoon that I made to show you that wormholes grow if I have a wormhole there's a wormhole connecting to otherwise distant places what happens in time to that wormhole that wormhole starts to grow applause thank you the wormhole grows this is this is a consequence of Einstein's equations of motion for the gravitational field wormholes grow okay I'm gonna I'm gonna do it again but I'm gonna put a little extra something in I'm going to imagine that when we started there was a particle on the left side Alice's side and Alice wants to send the particle through the Bob while the wormhole is growing watch what happens the wormhole never gets through sorry the particle never gets through because the wormhole was growing the particle can't keep up with the growth of the wormhole and the result is you can not this is the non traverse ability of wormholes why can't you traverse them because they grow at such a rate that you get frustrated the particle can't get through by the time the wormhole is separated this is the gravitational origin or when I say gravitational I mean the general relativity origin of the fact that wormholes are not reversible that you can't send signals through them the quantum mechanical explanation is a statement that entanglement does not allow communication or doesn't yeah oops one more and do once more I like I like doing this no applause that time okay all right what I just described to you I'm gonna show you in a technical language the language of Penrose diagrams Penrose diagrams are a technical way of describing the geometry of space-time here for those who know again this is for those who know this is the Penrose diagram of two black holes connected by entanglement on the left side here this is Alice out here this is Bob out here and this is only for those who know what I'm talking about if you don't know it's just what I previously said and what what the picture is supposed to do is convince you that I know what I'm talking about okay all right this is Alice aside here's the outside of Alice's black hole here's the outside of Bob's black hole but and the entanglement between them somehow creates a new region of space that you can think of as the wormhole here's the wormhole over here okay let's take a look here notice that the wormhole grows this is time time goes up the wormhole grows with time it's small over here it gets bigger it gets bigger it gets bigger it gets bigger the question is what is it from the point of view of quantum mechanics which is growing when a black hole when they--when the wormhole inside the black hole grows this is a question that should have been asked 20 30 40 years ago and for some reason or another nobody asked it let me go back a step and talk about another thing which grows another thing which we do know what it is that grows here's another Penrose diagram this is a traditional Penrose diagram for an ordinary black hole that's been created by a shell of incoming matter again I apologize for those who are not familiar with Penrose diagrams but I'll try to explain what's going on here's the interior of the black hole here's the exterior of the black hole here's the horizon the horizon starts small and it grows this distance or nothing sorry not this distance but the area of the horizon grows the black hole starts small and starts to grow and it grows until the shell of matter passes the horizon and then it stops growing do we know what's growing in this case I mean we know what the area of the horizon is growing but do we know from a quantum mechanical perspective what's growing yes we do it's entropy it's the entropy of the black hole and in fact the fact that the area of black holes always grows is nothing but the second law of thermodynamics entropy grows with time and entropy is area okay now let's come back to this issue of what is it that's growing on the inside of a black hole in the einstein-rosen bridge why does it what is it that's growing when the einstein-rosen bridge grows okay is it entropy no it's definitely not entropy for two reasons first of all these two black holes were in thermal equilibrium to begin with you'll have to accept that that these two black holes weren't perfect thermal equilibrium and their entropy will not grow it's as met large as it can be to begin with and furthermore it grows forever this is T equals infinity out here or at least classically it grows for a very very long time whereas entropy very quickly stops growing when a system comes to thermal equilibrium a black hole comes to thermal equilibrium extremely rapidly and so there's no chance that it's entropy that grows what is it well what it seems to be is something called quantum complexity so I need to explain to you we have I think we still have ten minutes to explain what quantum complexity is and what it has to do with black holes this is these are surprising ideas that that are no more than five years old the idea that complexity is what governs the growth of the space behind the horizon let's start with the difference then how different classical States think of a computer the state of a computer is described by the state of a bunch of bits zeros and ones to describe the state of a register inside a computer or a memory it's just described by a series of zeros and ones a set of binary digits if there are n bits then there are n binary digits in the description of the state of the computer of a state of the memory of the computer so it takes n binary digits to describe the state of n bits what about a quantum register a register made out of quantum mechanical qubits how complicated is it well each one of these states of the classical computer is itself a possible state of a quantum computer but the most general state of a quantum computer is a superposition of these is a superposition a quantum mechanical superposition and it requires two - the end it requires 2 to the n complex coefficients here to describe that quantum mechanical state so whereas this took n binary digits to describe the corresponding quantum mechanical state requires 2 to the enth complex coefficients here the number of distinct quanta classical States was 2 to the N here the number of corresponding quantum states is e to the 2 to the N so the this quantum states are vastly richer in their potential for complexity than the corresponding classical things how do we describe its complexity just a you know what all its complexity complexity can mean all sorts of things to all kinds of people my my relationship with my mother-in-law is complex all sorts of things that can mean but by complexity we mean something very very definite it's called computational complexity computational quantum complexity again we imagine a quantum computer it's running time goes horizontally now you put an input into the quantum computer and then the qubits interact with each other these little red circles here are called gates logical gates and the computer processes the initial state the initial state could be a bunch of zeros that's a simple state it processes them and spits out a quantum state of some more complex character the definition of complexity is the minimum number of gates that it takes that the complexity of sy starting with something simple and easy to describe how many gates does it take the minimum number to get you to the state of interest that minimum number is called the quantum computational complexity of sorry it's a property of the state of the system how complex is it how hard is it to make okay so it's the minimum number of gates now the maximum entropy of a system of n bits or in even n qubits is in the maximal complexity is 2 to the N so complexity is a kind of thing which can grow to vastly bigger size than entropy and that's why it continues to grow for very very long periods of time after a system has come to thermal equilibrium this is something computer scientists and in particular quantum computer scientists didn't know mostly physicists didn't know it all right so the hypothesis and this hypothesis fits it fits extremely well that the growing wormhole that the size of the wormhole in here the volume of it is described quantum mechanically is described quantum mechanically in terms of the complexity of the state of the system the relationship is that the volume of the wormhole divided by Newton's constant times a certain length scale which has to do with anti-de sitter spaces and so forth is equal to the complexity and this is a banal fairly well tested hypothesis and it's surprising again it's surprising because it's another link between quantum information on the one hand something purely quantum mechanical and something purely relativistic or general relativity the space behind the horizon ok so we have these two aspects on the one hand the growth of the volume of the wormhole on the other hand the growth of complexity in a quantum mechanical system as it evolves again a tight-knit close connection between quantum mechanics and gravity which five years ago or six years ago was totally unexpected can and bomb measure the fact that the complexity is increasing of their wormhole yeah they can here's what they have to do in a quantum mechanical theory the vacuum fluctuates there are field fluctuations electromagnetic field fluctuations whatever typically they fluctuate so that if in one place the field gets a little bigger in some other place it goes in the opposite direction so that's constant them and and they're correlated if I take field current field fluctuations in one region of space to get the wormhole for a minute these are just two very distant places if we get the wormhole the field fluctuates over here Alice measures those fluctuations the field fluctuates over here whatever field it is it fluctuates over here Bob Mayo that's how Alice now that's Alice that's Bob sorry Alice Alice and Bob measure the fields in their neighborhood they fluctuate up and down they're too far apart to compare the fluctuations but they can measure them and they can record them they can keep track of the time at which the fluctuation or the field of the electric field went up the time note which went down and keep a record of it now the first thing that they find because of these entangled black holes oh whoa what's that gusto though because of the large separation between Alice and Bob these fluctuations the fluctuations are there but the correlation between them would be very weak they're so far apart that there's hardly any correlation distance between things is a measure of how strong the correlation is on the other hand in this circumstance with this entangled pair of black holes Alice and Bob would discover essentially as the status of a theorem that the correlations are not as small as they might have discovered because of the long distance between these in fact the correlation starts out large and with time the correlation fall why does the correlation large to begin with because there's another route between Alice and Bob where correlations can propagate between the other route namely the wormhole because the wormhole is there a correlation between Alice's Bob's measurements is large on the other hand because the wormhole grows with time and they separate that correlation will get smaller and smaller l here is the distance through the wormhole between Alice and Bob and that's the way we expect the correlation between Alice and Bob to decrease on the other hand we can study complexity in a purely quantum mechanical context and we would discover that the complexity the sorry the correlation between two quantum computers would also do the same kind of thing as the complexity increased as a complexity of the of the system increased we will also find that correlation decreases they have exactly the same form one of these is a general relativity prediction having to do with the growth of space the other is a quantum circuit or quantum mechanical or a quantum computer expert would calculate and he will also calculate the same kind of thing and basically get the same answer now Alice and Bob what are they gonna do with their information Alice has recorded the fields the fluctuations in her notebook she's timed them going up over here went down at this time went up at this time Bob has done the same thing but there's nothing they can do with that unless they can talk to each other and typically they can't talk to each other through the wormhole but what they can do is after they make the record the long table record of the fluctuations that they've witnessed after the experiment is over now I love doing this all right Obama necklace Bob can take the long route bring his information over to Alice and they can check that the correlation functions decreased in a certain manner and an in fact that reflected the growing complexity you can do the same thing with quantum circuits or you don't need silicon shells to do it you don't need black holes to do it you can do the same kind of thing with entangled quantum circuits or quantum quantum computers and you'll find the same behavior what does this tell us well one interpretation is just quantum mechanics made the correlation decrease because that's that's a calculation you can do without having anything to do with black holes or anything at all or you can discover and notice that the way the correlation decreases is exactly as if there was a wormhole between these and the wormhole grew with time my main message here I would say is well okay well in five more minutes two more minutes then I won't try to do the next thing I'll just tell you what I think the upshot is the the thing which is to me most exciting now about this and which I think should provoke some response from experimental physicists that appears another in this way this way of thinking you actually can do experiments which have the status of experiments on quantum gravity you do not have to have very heavy objects you do not have to have Planck scale accelerators you need silicon shells or better yet quantum computers that can simulate those silicon shells well I Silicon as I said them silicon is a metaphor and doing the right kind of experiments testing this ad SC ft connection is in a sense doing experiments that are testing ideas which are ideas of the connections between quantum mechanics and gravity the claim is that any experiment of this kind will be consistently interpreted either as a quantum mechanical experiment or as an experiment involving space time and gravity wormholes whatever you have there are many examples of this I've only touched the surface I won't go into them here let's not let's come back and ask me about that if it but um yeah okay come back and ask me about these pictures if you want to ask questions but um I think the main thing that I wanted to emphasize is this possibility in the future and then in the relatively near future I think of actually doing experiments which bear directly on the quantum theory of gravity that is something that I find exciting ok I have described the point of view it's my point of view it's a point of view which I am responsible for responsible not in the sense that I necessarily invented it but if it's wrong I will take responsibility for it you know like um like my president said I'll take responsibility I'm not sure you know what I'm talking about but but I will say these ideas were extremely heavily influenced by other people and in particular very very influenced like two of my colleagues one of whom is Joe polchinski who terribly unfortunately is no longer with us this is a great blow to physics to me personally and to everybody who knew Joe I think I would like to dedicate this lecture at the job polchinski who died prematurely and to one maldacena was alive and kicking and and doing wonderful physics I think these two people more than anybody have contributed to my own views about about these things I don't think it will be too much for me to claim that they they they shared the same views I would be putting words in their mouth but yes they did share the same word of use okay so good we can stop here [Applause]

Channel: TheOskarKleinCentre

Views: 189,522

Rating: 4.8044376 out of 5

Keywords: Leonard Susskind, gravity, general relativity, quantum mechanics, cosmology

Id: ruJgtjpSoPk

Length: 77min 52sec (4672 seconds)

Published: Mon Mar 18 2019