The greatest lecture ever. Leonard Susskind on Quantum Gravity Black Holes and Paradoxes

I don't mean that the laws of physics exist or that they are precise and mathematical I mean the fact that an animal whose closest relative is the chimpanzee was able to ask about these laws there he is up there but was also able to navigate through a sea of wrong ideas and eventually hit on relativity quantum mechanics the standard model of Elementary particles and more that is a absolutely remarkable fact that is not a physics fact it's probably a bi biology fact but it just blows me away what are the tools that our ancestors used intellectual ancestors I don't I don't mean the monkeys I mean Newton Einstein and so forth what were the tools that they use to be able to answer these questions well there were theoretical tools thought experiments apparent conflicts of principle paradoxes and of course mathematics but some people would say the most important tool was of course experiment today we're going to talk about a subject quantum gravity which is so remote the scales of distance so infinitesimal the energy so enormous that direct experiment is entirely out of the question at least for now the phenomena they're the phenomena which are at the intersection of quantum mechanics and gravity quantum gravity to give it a name so we theorists are on our own can we make progress well it seems that we have made serious progress over the last two decades maybe even revolutionary progress okay as I said the phenomena are so note that the possibility of experiments is out of the question and it means we're really on our own we have made progress and I'm going to tell you some of the little pieces I'm not going to give you the whole story I couldn't possibly but I'm going to try to give you some feel for what some of the pieces have been that I think are adding up to a revolution okay so where are we the whole subject of quantum gravity probably goes back to the first day that anybody thought about quantum mechanics and thought about gravity but the modern era of it I think started in 1958 or sometime around then when theoretical physicists asked the question should we quantize gravity now what does quantise mean quanti was a an expression which meant a procedure a procedure that you do on a classical system the classical system could be a harmonic oscillator it could be an atom it could be electrodynamics ordinary electrodynamics a procedure which if you know about that procedure you recognize the equations if not it doesn't matter there are equations and the founders of the subject called fining dit Weinberg wheeler Hawking their attempts were organized around trying to describe scattering processes processes where particles come in and particles go out they interact under the influence of gravity and here and there they may may emit some gravitational waves gravitons they invented finan diagrams for uh for gravitation well it was a disaster it was a disaster everything they tried to compute came out infinite or came out minous came out nonsensical and that led to what I would call an era of angst and confusion a time when it just did not look as if quantum mechanics and gravity were compatible now how could they not be compatible they have to be compatible the world has both and we can't live in a world of inconsistency and that was then how do you fit them together can you fit them together and it looked impossible today the situation is different today as as far as we can tell not only can gravity and quantum mechanics fit together but it's almost as if they were the same thing or two sides of a coin a single coin I'm going to tell you a little bit about some of the things that went into that all right I always start with digressions i I'm a great digress I digress all the time I'm going to digress about something you've all seen Holograms what is a hologram you've probably all seen them the first Holograms were something like this you had a region inside a cavity of some sort and the cavity was not a cavity just a round region and the cavity the surface of the cavity was a film a photographic film and if you looked carefully at that film even through a microscope all you would see was little scratchy um meaningless rubbish noise it seemed to encode nothing nothing recognizable on the other hand if you shined light on it of the right kind what is called coherent light all of a sudden voila an image would form but a threedimensional image right in the middle of this cavity a three-dimensional image of whatever it was that have been photographed what is a hologram from an abstract point of view a a hologram is a two-dimensional mathematical representation of a three-dimensional portion of the world now how do you manage to take a three-dimensional thing and map it into two Dimensions well it's possible but it is always at the cost of the two-dimensional image being looking like a random hash how you put it back together again that maybe just shining light on it or or it may be some much more complicated mathematical procedure but that's what a hologram is and what you can say about a hologram is that things are not where you think they are in the Hologram the information the um it's called the information of what is in the Hologram is in the film the image is in the bulk what we today call the bulk things or the information encoding things are not where you think they are I want you to keep that in mind now let's come back to quantum gravity in the 1990s this was the period when Steven Hawking and I were having our fun debating and uh Raphael was a student thought experiments principally initiated by Steven himself about black holes led to something called the holographic principle what was the holographic principle and what is the holographic principle principle today it's the idea that a region of space with everything in it it could be astronomical space it could just be this room or even the whole universe that the information encoding everything taking place in this three-dimensional world is encoded on the boundary of that region as a kind of quantum hologram it's of course impossible for me to describe the mathematics of the quantum hologram here and we'll try but that's the message and again things are not where you think they are or at least things are not where the information which is encoding them would lead you to believe they are okay let's come back now to this idea that gravity and quantum mechanics may be so closely related that they really are just two sides of the same coin the evidence for that is a whole whole bunch of parallels between gravitational phenomena we'll talk about what that means in a moment and Quantum phenomena things that we had no idea were connecting things from two radically different fields of physics are turning out to be parallel to each other and perhaps even not just related to each other but the same thing the connection is through this holographic principle the gravitational phenomena are the phenomena which are like the image the three-dimensional image the encoding of that three-dimensional world is in the form of the quantum mechanical hologram correspondences are correspondences between the two of these so I will give you some examples let's go to the most primitive or basic of gravitational phenomena you all know what that that is if you fallen out of bed I've fallen out of bed a number of times regularly you know what gravitation is it's falling falling in a gravitational field and if I wanted to if I wanted to express it abstractly I would say that the gravitational force just like any other force is a tendency for things to accelerate in this picture here the apple is accelerating it's accelerating downward caused by the gravitational field of the Earth you can rewrite the you know the equation f equals ma of course you can rewrite that as f equals the rate of change of momentum the momentum of the apple is increasing as it falls so you can say that falling is the tendency for momentum to increase in the presence of a gravitational field that's one side of the coin the falling side the other side of the coin the quantum side is something so radically different that it's hard to imagine that it has anything to do with it you take some Quantum system now this Quantum system I'm imagining is the Quantum system encoding the Hologram this bunch of squiggles and uh and random looking bits of information out at the boundary of the region of interest and you come along and you tap the system you perturb the system you might you might uh hit it with an extra electron or you might do whatever it is that you do to it at some spot you perturb the system that perturbation this is a quantum mechanical fact will start to spread throughout the system its influence will spread throughout the system like an epidemic you touch one cubit if you know what a cubit is that Cubit will touch a few more cubits few more cubits will touch a few more cubits and the effect of of perturbing the system will spread there's a notion of size it's like the size of an epidemic the size of an epidemic simply means the number of sick people and the number of sick people has a tendency to grow here's an example for the experts on Quantum computation there's a Quantum circuit and it proceeds from left to right if you perturb it with a with a green Cubit that perturbation will spread throughout the quantum computer and that's the phenomena of scrambling of information scrambling now what on Earth does this have to do with falling I'll give you an example it comes from a setup called ads CFT that may not mean anything to you it's fine doesn't matter what is this this is the boundary far away the boundary encoding the Hologram the bulk of space the interior is in here imagine coming and perturbing the Hologram perturbing means just hitting it or something that information that you've done so starts to spread and starts to spread throughout the Hologram and there is this notion of the size of the perturbation you can calculate these things and the calculation as I said is purely quantum mechanical maybe a bit of quantum field Theory but no gravity and what do you find you find that the rate of change of size in this setup is exactly equal to the mass of a an object which was created at this point times the gravitational acceleration these quantities here you get from elsewhere but they're well defined in the context and what does it say well what would gravity say gravity would say that the time derivative of the momentum of the particle is falling is equal to mg and so we see these two different fields of physics entirely different coming together and giving rise to an equation I don't know if Galileo would have recognized it in quite this form but it was Galileo's equation and in order to make sense of it you have to believe that momentum of the object which was created is simply the size of the perturbation now you may not understand that you may say he's talking gobbly G and the main thing that I want you to get from this is again this correspondence between gravitational things this unexpected correspondence between gravitational things and Quantum things here's another example instead of a flat plane being the Hologram we can imagine the Hologram as a sphere surrounding some place at the center of the diagram the center of the picture there might be a black hole or planet or other Mass if you do the same calculation in this context of calculating how the size of a perturbation grows again purely Quantum mechanically what do you find you find The Marvelous formula that the rate of change of the size mass times the acceleration of the inflowing object is just equal to the product of the two masses Newton's constant and the distance between them squared in other words Newton's law of gravity quantum mechanics the growth of size Gra gravitation gravitational attraction to me that is very stunning correspondence okay let's come out to black holes and paradoxes it was a famous Paradox of Step Hawkings uh I'm going to oversimplify it not just Stephen Hawkings Paradox but a later Paradox called the um the firewall Paradox please if you're a theoretical physicist don't shoot me for the way I explain this because it's going to be over simplify black hole Paradox we have a black hole The Horizon of the black hole is simply this circle here and we throw something into the black hole that has some information and encyclopedia and I'm going to call that encyclopedia a there's no escape from a black hole or at least as far as we knew um in 1990s there is no way that anything can escape from a black hole but the black hole can evaporate that was something that Steven Hawking discovered that black holes can evaporate and they can shrink at some point they shrink enough that something strange happens namely there is not enough information enough area enough whatever it is in that remaining black hole to encode the encyclopedia that you threw in it's called a page point and all of a sudden the encyclopedia simply cannot be there anymore where is it or where are its bits of information they're in the Hawking radiation so the encyclopedia gets transferred or the information of the encyclopedia gets transferred to the um to the radiation that's a quantum mechanical principle that uh that's we've known about for a long time and if you take that radiation imagine somebody takes that radi grabs all the photons puts them in a box and squeezes that box down into some small box somewh then what this is telling us the quantum mechanics is that the encyclopedia again I emphasize by that I mean the bits that comprise the information in the encyclopedia is transferred from the black hole to the radiation or to the black hole that the radiation might have been compressed into in other words as the black hole shrinks it cannot hold any information and if it can't hold any information nothing can fall into it anymore and one says that there is a firewall at the Horizon firewall doesn't mean in the sense of burning up it means in the sense of an information firewall that no information can fall into the black hole anymore if every time you try to do so it pops out and it appears far away in the radiation that's the idea now this idea of a firewall was very very badly at odds with what we knew about general relativity general relativity always said that you can always put things into the black hole and they will simply stay there so this led to this Paradox the so-called firewall Paradox could say it this way either there is a firewall or well some sometimes call Raphael were you the inventor of a equals RB I think you might have been yes I think he was a matter of fact the idea is a generalization of the holographic idea that things are not where you think they are and that in fact the encyclopedia which is in fact behind the Horizon of the black hole but its bits its bits of information are found far away in the Hawking radiation don't worry about what a and RB stand for what it says is that the information comprising the black the the thing inside the black hole is far away far away on Alpha centuri in some other system in other words it's an extreme version of this holographic idea that things are just not where you think they are well that seemed too crazy I think even Raphael thought it was too crazy right it did seem too crazy but one thing it seemed to suggest that if somebody far away manipulated the radiation in this box it would immediately have an effect on the interior of the black hole and an effect which if somebody jumped into the black hole would detect what was done far away and that seemed totally inconsistent with the idea that close things can affect close things but they can't affect bar things well it needed a new idea resolving this puzzle required a new idea and the new idea is called E equals epr Einstein ER stands well let's first do Eep how many people here know the who what epr stands for good good fraction of you good it stands of course for Einstein podowski and Rosen but it also is the phenomena of entanglement quantum entanglement now I'm not going to tell you exactly what quantum entanglement is it's what Einstein called spooky action at a distance I'm just going to tell you a very very simple version of it two things they could just be two electrons or they could be two nuclei or they could be two macroscopic objects are entangled if if by measuring one of them you find out certain kinds of quantum information about the other one no matter how far away that other one is let's just call that entanglement now all my physicist friends know that I'm being oversimplified but there is this phenomena of sharing information between two different distant systems that's called epr entanglement and it's a very mysterious phenomenon and I'm not going to explain it now we'll just say it exists that was the year 1935 when Einstein Podolski and Rosen discovered or at least uh let's let's make it simple discovered entanglement it was incidentally a very good year for Einstein Einstein I think had three really good years 1905 when he discovered speci when he discovered all of modern physics except for Gravity except for the rules of gravity 1915 or so when he completed the general theory of relativity and understood gravity and 1935 which is much less famous in which he discovered this phenomena of entanglement but the same exact year he discovered something else called Einstein Rosen Bridges Einstein or sometimes called wormholes wormholes are a solution of of Einstein's equations in which you have two black holes far away from each other with a kind of tunnel of space between them you can't see that tunnel of space it's in the some interior space that can't be seen but the two very distant objects are connected by a I call it a tunnel I've called it a bridge I've called it a wormhole all the same idea you've seen these things in science fiction people jump into wormholes and so forth I always thought it was nonsense but not completely what's the idea if you could go as fast if what it sounds like is you can jump into one black hole over here and pop out over here well we'll see that you can't do that but nevertheless that's what a wormhole resembles the science fiction idea of a bridge between very distant places now what does ER that's the bridge Einstein Rosen Bridge have to do with epr other than they have two letters in common nothing before 2013 nobody and I'm absolutely convinced that Einstein was among that nobody had any idea that entanglement and wormholes or Einstein Rosen Brites had anything to do with each other and after 201 2013 they had everything to do with each other in fact the idea goes with the acronym E equals e you would call it P equals 1 but nobody does ER the idea of a bridge between distant regions of space and the idea of entanglement are the same idea so I'm going to show you a little bit about how that works the rectangle here is supposed to be space a big region of space over here on Earth we have a bunch of particles far away on Alpha centuri we have another bunch of particles two clouds of particles those particles have never been in contact with each other they don't know about each other they're completely separate with no prior interaction between them I'm going to take this sheet of space and fold it over over not because it's folded over but just want I want to draw it that way to make a point but it is still true that this Cloud over here is far from this Cloud because you have to go around this long way to get there what happens if you let those those clouds of particles collapse shrink they form black holes that's what a black hole is it's the shrinkage and collapse of a star for example these could form a star eventually and after a star they could form black holes and those black holes will be completely separate from each other with no connection between them on the other hand let's do something else now let's take a bunch of entangled electrons or a bunch of entangled particles half of them are over here and half of them are over here now how do you create well the uh the green line here just indicates that this particle is entangled with this one this particle is entangled with this one no not that one this one how do you create such a situation to create it you have to create the entangled particles near each other you can't create entangled particles far from each other you have to bring the particles together you've got to let them interact with each other and they will become entangled but once they're entangled you can take the half of them that's down here separate it well let's say we take this half of them and bring them all around here so that we wind up with two clouds of of entangled particles now we let gravity do its work and when gravity does its work again it creates two black holes but the black holes are now connected by an Einstein Rosen bridge in other words entanglement and wormholes are in some sense the same name two black holes which are entangled will necessarily have a bridge between them to black holes which are unentangled will not this is what's called E equals epr and it was a major Discovery uh it seemed ludicrous at first but it very quickly caught on is now part of the standard uh lore what can you do with it okay so let's imagine now that we do have such a wormhole connecting two very distant black holes one of them Franklin has control over control means he can do things to it he can jump into it if he wants the other one lus has control over and they're very far away one is on Alpha centu the other is on Earth but they have this Einstein Rosen Bridge connecting them well with enough care and enough fine tuning they can arrange these black holes so that they can eat jump into their own black hole and in a very short period of time can meet at the center and Shake Hands what they cannot do at least without some further considerations which I'll come through if I have time I may not have time lonus cannot jump into one pass through and come out the other one that can't happen now the fact that it can't happen is both known from the quantum mechanical point of view it's called a no signaling theorem for entanglement and for wormholes it's called the non-traversable of wormholes the impossibility of traversing through them and it turns out those are the same phenomena one quantum mechanical the other gravitational now let's come back to a equal RB the encyclopedia a in black hole number one or leftand black hole here is in coded in the radiation in Region 2 over here our problem before was that sounded crazy because somebody manipulating the radiation over here could perturb what's inside the black hole and just such a way that somebody who jumped into this black hole over here would detect that a very very distant Observer had done something to this group of photons over here that sounded out landish but now we know that if these photons over here are entangled with the black hole which they will be that an Einstein Rosen bridge and from outside you can't see that Einstein Rosen bridge but the Einstein Rosen bridge will open up and so anything anybody does over here will affect what's behind the Horizon of the original black hole in other words a equals RB makes perfect sense again it has to do with this basic idea that information is not where you think it is this is a radical example of it what about the Wormhole side of it how can it be why should it be that somebody who goes into one black hole an alpha centuri Can't Get Through the Wormhole and come out well one worm one end of the Wormhole is in New York the other end is in California think of it as a tunnel between the two places why can't you D drive through that tunnel and the reason is that Einstein Rosen Bridges and this is a gravitational phenomena tend to stretch and expand with time that's the solution of Einstein's equations gravitational equations wormholes grow so if who was it Franklin I can't remember Franklin or lonus if lonus tries to drive into the New York side he will encounter the fact that the Wormhole is growing and in fact it will grow so fast that he cannot outrun the growth of the Wormhole and will simply get never get through to the other side and come out the other side that's the non- traversability of wormholes you can't even send a light signal through because even a light signal will not go fast enough to outrun the growth of the Wormhole what that's the that's the gravitational side of it is there a Quantum side of it yes there is and I don't have time to tell you what it is I will tell you what it is I don't have time to explain it but I'll tell you that it's a computer science concept it's called the growth of complexity the black hole Quantum state of the system is becoming more and more complex it's very much like this information scrambling that we talked about having to do with falling the quantum state of the wormhole evolves it becomes more and more complex and that complexity translates into the growth of the Wormhole so these are all these very very remarkable correspondences which tend to make us think that they're not just that there are deep connections between quantum mechanics and gravity but it's some level as I said I'll say it again they're two sides of the same coin okay so we have the idea of a Quantum hologram encoding information which could be far from where the object that it's encoding is and what is the other side of the coin the other side of the coin is gravity I like this picture it's my favorite one of all there are criticisms oh incidentally one might point out that most of these ideas grew not just out of a combination of quantum mechanics and gravity but String Theory how String Theory got into it I haven't really said very much about but let me tell you that all of the precise examples all the mathematically precise examples of this correspondence tend to come from systems which were invented or discovered in string theory of string theory quantum gravity has been the victim of an enormous amount of criticism the criticism which first of all things is unjustified but what does it have to do with the criticism I would say stems from the fact and I think it is a fact that good science almost always spreads its influence far and wide into many fields of of not just physics but even outside of physics and in particular into engineering into technology and that's a pattern that we've seen over and over and over again special relativity led to nuclear energy general relativity we use it for navigation by satellite Believe It or Not quantum mechanics the list of technological advantage advances and quantum mechanics was not invented by people trying to do um technology was invented by people who were curious about the atom uh quantum mechanics among other things it led to the MRI machine but so many things I had that the list would would go on and on Quant Quantum electrodynamics trying to understand the quantum mechanics of electrons and photons in particular photons led to the laser or at least as closely connected with the laser and so forth and so on what about quantum gravity general relativity and its connection to Quantum Mechanics it seems so infinitely remote with no connections or applications to the rest of science it could be that that's true it could be we're just stuck with that but that has not been what is happening first of all this connections between quantum mechanics and gravity have led to new insights into strictly uh phenomena which seem to have nothing to do with gravitation for example the surface of a black hole The Horizon of a black hole behaves as if it were made of a fluid that's something that General relativist General relativists discovered a long time but not just a fluid but a Quantum fluid whatever that means one can use the fact by knowing enough about black hole physics and knowing enough about general relativity you can compute properties of fluids that were too hard to compute otherwise here's one example of something that was inspired by the connection between fluid mechanics black hole r s in quantum mechanics it's abound on the viscosity of fluids now that doesn't seem to have anything to do with either of those subjects well it's a little bit quantum mechanical it is quantum mechanical Ada is theu is a viscosity of a fluid the stickiness of it s on the right hand side is the entropy per unit volume of the fluid the heat per unit volume what was discovered in the context not discovered by people doing fluid dynamics people comparing properties of black quantum mechanical black hole horizons with fluids is that the viscosity is always greater than equal to some number that includes H bar that includes the quantum constant times the entropy density will that have impact into fluid dynamics and into uh probably there are things called strange Metals strange metals are a form of matter that was discovered by condensed metaphysicist are they important in technology I don't really know but they were discovered about 30 years ago and they were met metallic systems which behaved just differently than ordinary Metals it's turning out that those strange metals are mathematically identical to certain special kinds of black holes called extremal black holes or near extremal black holes both sides are quantum mechanical one side is also gravitational extremal black holes the other side is the pure quantum mechanics of certain materials information scrambling the thing which I told you accounts for the falling of the apple as it as it accelerates in the gravitational field information scrambling is an important thing in quantum computer science the information scrambling from black holes led to a bound again another bound that a certain constant called the opo exponent in information scrambling is always less than some other constant that now is considered a reliable fundamental bound on how fast information can spread through a quantum mechanical system I told you that Linus cannot get Through the Wormhole well I was a little bit too pessimistic with a little bit of help from something called classical in the exchange of classical information these little purple dots being sent from one side to the other that's just ordinary Morse code for example but has no information about it about Linus or about anything else inside the Wormhole with a little bit of help from a little bit of classical information you can slow down that growth of the Wormhole you can slow it down enough so that indeed Linus can get through it that phenomenon which was discovered in the context of gravity and quantum gravity has led to a new protocol and new experiment for quantum teleportation quantum teleportation is a real thing you can teleport information what it means you can't exceed the speed of light but you can send information in a way that is completely hidden 100% hidden cannot be decoded by an eavesdropper and so it's led to new protocols for quantum teleportation experiments are now being done to confirm that this can happen not in black holes but in quantum computers I'm not allowed to tell you that the experiments are successful because I promise not to mention that they're being done and that they're working out success y so I won't Quantum complexity Theory the growth of worm holes the whole idea of gravitation being controlled by the growth of complexity has led to many new insights into how complexity of quantum systems evolve we've seen advances coming from gravity in error correction error correction is the big hangup in trying to build a quantum computer it's too easy to make errors in a quantum computer you have to error correct for them and a whole new insight into error correction has come from thinking about gravitational systems again and finally in the hands of one of my favorite physicists Juan Mala uh much of what we're talking about has had a very interesting influence in cosmology in inflationary cosmology so far from being a totally isolated thing outside the framework of any other science this quantum gravity is beginning to have an effect which let's just put it this way condensed metap physicists and quantum computer physicists and theoretical cosmologists are being forced to learn what ads CFT means and they are learning learning it they're learning it and using it so this is exciting this is a very very exciting period in the development of physics it is also one which is very very difficult to explain to a general audience you know when you give a lecture like this or what are link and say you can please half of the people half of them blah blah the time well you can you can please half of the audience half of the time and so forth and so on I think the real truth is in a lecture like this you can you you you're lucky if you can uh if you can satisfy any piece of the audience even a little bit because the ideas are complicated the difficult and so forth you do your best uh you do your best to try to explain and I've tried to explain as well as I can I hope you have gotten something out of it I hope there's at least one person who has gotten something out of this lecture uh and I thank you for [Applause] listening [Applause] what about the Wormhole side of it how can it be why should it be that somebody who goes into one black hole an alpha centuri Can't Get Through the Wormhole and come out well one worm one end of the Wormhole is in New York the other end is in California think of it as a tunnel between the two plac places why can't you D drive through that tunnel and the reason is that Einstein Rosen Bridges and this is a gravitational phenomena tend to stretch and expand with time that's the solution of Einstein's equations gravitational equations wormholes grow so if who was it Franklin I can't remember Franklin or lonus if lonus tries to drive into the New York side he will encounter the fact that the Wormhole is growing and in fact it will grow so fast that he cannot outrun the growth of the Wormhole and will simply get never get through to the other side and come out the other side that's the non-traversable of wormholes you can't even send a light signal through because even a light signal will not go fast enough to outrun the growth of the Wormhole what that's the that's the gravitational side of it is there a Quantum side of it yes there is and I don't have time to tell you what it is I will tell you what it is I don't have time to explain it but I'll tell you that it's a computer science concept it's called the growth of complexity the black hole Quantum state of the system is becoming more and more complex it's very much like this information scrambling that we talked about having to do with falling the quantum state of the Wormhole Evol it becomes more and more complex and that complexity translates into the growth of the Wormhole so these are all these very very remarkable correspondences which tend to make us think that they're not just that there are deep connections between quantum mechanics and gravity but it's some level as I said I'll say it again they're two sides of the same coin okay so we have the idea of a Quantum hologram encoding information which could be far from where the object that it's encoding is and what is the other side of the coin the other side of the coin is gravity I like this picture it's my favorite one of all there are criticisms oh incidentally on might point out that most of these ideas grew not just out of a combination of quantum mechanics and gravity but String Theory how String Theory got into it I haven't really said very much about but let me tell you that all of the precise examples all the mathematically precise examples of this correspondence tend to come from systems which were invented or discovered in string theory of string theory quantum gravity has been the victim of an enormous amount of criticism the criticism which are first of all things unjustified but what does it have to do with the criticism I would say stems from the fact and I think it is a fact that good science almost always spreads its influence far and wide into many fields of of not just physics but even outside of physics and in particular into engineering into technology and that's a pattern that we've seen over and over and over again special relativity led to nuclear energy general relativity we use it for navigation by satellite Believe It or Not quantum mechanics the list of technological advantage advances and quantum mechanics was not invented by people trying to do um technology it was invented by people who were curious about the atom uh quantum mechanics among other things it led to the MRI machine but so many things I have that the list would would go on and on Quant electrodynamics trying to understand the quantum mechanics of electrons and photons in particular photons led to the laser or at least as closely connected with the laser and so forth and so on what about quantum gravity general relativity and its connection to Quantum Mechanics it seems so infinitely remote with no connections or applications to the rest of science it could be that that's true it could be we're just stuck with that but that has not been what is happening first of all this connections between quantum mechanics and gravity have led to new insights into strictly uh phenomena which seem to have nothing to do with gravitation for example the surface of a black hole The Horizon of a black hole behaves as if it were made of a fluid that's something that General relativist General relativists discovered a long time but not just a fluid but a Quantum fluid whatever that means one can use the fact by knowing enough about black hole physics and knowing enough about general relativity you can compute properties of fluids that were too hard to compute otherwise here's one example of something that was inspired by the connection between fluid mechanics black hole in quantum mechanics it's abound on the viscosity of fluids now that doesn't seem to have anything to do with either of those subjects well it's a little bit quantum mechanical it is quantum mechanical Ada is theu is a viscosity of a fluid the stickiness of it s on the right hand side is the entropy per unit volume of the fluid the heat per unit volume what was discovered in the context not the discovered by people doing fluid dynamics people comparing properties of black quantum mechanical black hole horizons with fluids is that the viscosity is always greater than equal to some number that includes HR that includes the quantum constant times the entropy density will that have impact into fluid dynamics and into uh probably there are things called strange Metals strange metals are a form of matter that was discovered by condensed metaphysicist are they important in technology I don't really know but they were discovered about 30 years ago and they were met metallic systems which behaved just differently than ordinary Metals it's turning out that those strange metals are mathematically identical to certain special kinds of black holes called extremal black holes or near extremal black holes both sides are quantum mechanical one side is also gravitational extremal black holes the other side is the pure quantum mechanics of certain materials information scrambling the thing which I told you accounts for the falling of the apple as it as it accelerates in the gravitational field information scrambling is an important thing in quantum computer science the information scrambling from black holes led to a bound again another bound that a certain constant called the aeno exponent in information scrambling is always less than some other constant that now is considered a reliable fundamental bound on how fast information can spread through a quantum mechanical system I told you that Linus cannot get Through the Wormhole well I was a little bit too pessimistic with a little bit of help from something called classical in the exchange of classical information these little purple dots being sent from one side to the other that's just ordinary Morse code for example but has no information about it about Linus or about anything else inside the Wormhole with a little bit of help from a little bit of classical information you can slow down that growth of the Wormhole you can slow it down enough so that indeed Linus can get through it that phenomenon which was discovered in the context of gravity and quantum gravity has led to a new protocol and new experiment for quantum teleportation quantum teleportation is a real thing you can teleport information what it means you can't exceed the speed of light but you you can send information in a way that is completely hidden 100% hidden cannot be decoded by an eavesdropper and so it's led to new protocols for quantum teleportation experiments are now being done to confirm that this can happen not in black holes but in quantum computers I'm not allowed to tell you that the experiments are successful because I promise not to mention that they're being done and that they're working out success y so I won't Quantum complexity Theory the growth of wormholes the whole idea of gravitation being controlled by the growth of complexity has led to many new insights into how complexity of quantum systems evolve we've seen advances coming from gravity in error correction error correction is the big hangup in trying to build a quantum computer it's too easy to make errors in a quantum computer you have to error correct for them and a whole new insight into error correction has come from thinking about gravitational systems again and finally in the hands of one of my favorite physicists Juan Mala uh much of what we're talking about has had a very interesting influence in cosmology in inflationary cosmology so far from being a totally isolated thing outside the framework of any other science this quantum gravity is beginning to have an effect which let's just put it this way condensed metap physicists and quantum computer physicists and theoretical cosmologists are being forced to learn what ads CFT means and they are learning in it they're learning it and using it so this is exciting this is a very very exciting period in the development of physics it is also one which is very very difficult to explain to a general audience you know when you give a lecture like this or what are the link and say you can please half of the people half of them blah blah the time well you can you can please half of the audience half of the time and so forth and so on I think the real truth is in a lecture like this you can you you you're lucky if you can uh if you can satisfy any piece of the audience even a little bit because the ideas are complicated the're difficult and so forth you do your best uh you do your best to try to explain and I've tried to explain as well as I can I hope you have gotten something out of it I hope there's at least one person who has gotten something out of this lecture uh and I thank you for [Applause] listening

Channel: Emergence

Views: 84,238

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Id: OqyMVHh10uE

Length: 55min 1sec (3301 seconds)

Published: Sat Mar 16 2024