THE 2022 OPPENHEIMER LECTURE: THE QUANTUM ORIGINS OF GRAVITY

good evening everybody is this one is this on hi oh it is working hi good evening i'll wait maybe a few more seconds while people settle in but as you do uh let me introduce myself i'm james and elitis i am chair of the physics department and it is my pleasure to introduce uh the 23rd annual annual j robert oppenheimer lecture um [Applause] so what i'm going to do right now is i'll give you an introduction to what the openheimer lecture is about before i hand over to my colleague professor rafael busso to introduce tonight's speaker the oppenheimer annual series was established in 1998 and is made possible by the generosity of jane and robert wilson as well as steve and eileen krieger the series was brought has brought a who's who of of physicists theoretical physicists from around the world past oppenheimer lecturers have included cn yang freeman dyson helen quinn charlie kane andre linde murray galman stephen hawking kept thorne and marvin cohen who's joining us via zoom i believe many prominent theorists in the fields of particle physics condensed metaphysics astrophysics cosmology and amo have stood where i am standing today robert oppenheimer was born in 1904 growing up in an upper-class family in manhattan he graduated from harvard majoring in chemistry entered cambridge the other cambridge across the atlantic uh in 1924 as a graduate student hoping to work with ernest rutherford who many of you may know as the second most famous new zealand physicist um he was then leaving in 1926 and so oppenheimer finished his phd with max born in gottingen he published more than a dozen papers while with born mostly focused on the new theory at the time of quantum mechanics this included his most famous born oppenheimer approximation that simplifies molecular physics by separating slow nuclear motion from the faster electron motion more than 90 years ago in 1929 after two years of postdoctoral study mostly in europe oppenheimer returned to the u.s he accepted an associate professorship right here in berkeley where he remained for nearly 15 years during this period he published his famous paper with volkov establishing the tolman oppenheimer volkov limit on the maximum mass of a neutron star the mass above which a star must collapse into a black hole he also developed the theory of that collapse and black hole formation both topics are of keen interest today as many of you may know with the recent observation of gravitational waves from black hole and neutron star mergers at berkeley oppenheimer's group typically eight to ten graduate students and half a dozen post docs met with oppenheimer every day with open while open height with oppenheimer probing them about their progress hans baker noted that probably the most important ingredient he brought to his teaching was his exquisite taste he always knew where the important problems are the scientific leadership of oppenheimer demonstrated at berkeley complicated his later life and his role in science as many of you know in 1942 he was selected to lead world war ii's manhattan project's engineering lab cited at los alamos near a ranch oppenheimer owned his leadership of his effort culminated in the successful trinity test and later the political decision to use atomic weapons against japan a decision that troubled oppenheimer for the rest of his life after world war ii oppenheimer became the public face of science and technology featured on the covers of time and life magazines this period of his life came to a close with a controversial loss of his security clearance in 1954. that's his badge shown to the right there a time when the new cold war and mccarthyism were stoking fears there was some resolution about a decade later when president kennedy presented oppenheimer with the nation's fermi award my notes say kennedy but the but the picture clearly shows lyndon johnson i think but anyway one of them definitely gave him the award oppenheimer's legacy at berkeley is a simpler one summarized by a plaque on the fourth floor of physics south hall with another quote from hans beta in these corners in these corner offices 1929 to 1942 j robert oppenheimer created the greatest school of theoretical physics the world has ever known berkeley physics strives to continue this legacy today so with that introduction it is now my great pleasure to introduce my colleague from uc berkeley physics professor rafael busso to introduce tonight's oppenheimer lecturer dr lenny suskind thank you james well it's a great pleasure and honor to introduce dr leonard suskind lenny is a legend he's also a friend and a mentor to me personally um he has made many great discoveries and don't worry i will not tell you about all of them he's won great prizes like the sakurai prize what i want to talk about is how that's really just half of the picture lenny more than almost any other great theoretical physicist that i've had the privilege to meet has taught us how to think how to pick important topics how to tackle them and the best way that i can try to convey that is by by telling you the story of how i first met lenny which he probably doesn't actually know um this was in in 1994 and i was an undergrad i was trying to decide between grad school at stanford or in cambridge england i didn't get into berkeley is anybody here who no anyway so so i went to stanford to sort of check the place out and my hope was to meet with andre linde but he had broken his leg so he couldn't meet with me after after all but but this guy comes running in wearing running shorts sweaty shirt um and uh you know i tell him i'm thinking about grad school i'm interested in quantum cosmology and black holes and so on and and he sits me down and he gives me this this very clear lecture about how stephen hawking is completely wrong about black holes and this whole idea of the wave function of the whole universe there's nobody you can see the whole universe doesn't make any sense um and this completely shocked me i'd already spent a little bit of time in cambridge and you know it's a place that definitely sees itself as the center of the universe and shall not be criticized um so this was this was uh remarkable um and i i went to cambridge anyway but but what but what lenny had explained to me he explained in such a way that it stayed and it planted a seed and even though i became steven's student and that was great it was a wonderful time i i came to realize that lenny was probably right about black holes and stephen hawking was wrong that information really does come out of black holes and is not lost and you know before you knew it stephen hawking decided that lenny was right about black holes now this is just an example of i think how lenny operates he's good at making people think the right way about something he's good at guiding the field and for generations of physicists coming and going he has he has guided us in the right directions he's championed sometimes unpopular ideas and topics that people wouldn't touch and he's got a stellar record on those picks now how did he do that i don't really know how to explain it it's some sort of irresistible clarity in the way he thinks about things and conveys those thoughts it's it's a radical inevitability it's some contradiction in you know you you hear it and at once it's like my goodness this is completely crazy and yet you realize there's no other option and fortunately i don't have to explain it to you because we're about to hear lenny explain some things to you so i welcome professor leonard tuskett it's very nice to be here the last two and a half years i haven't been more than 30 miles from my house i'm now about 35 miles from my house wonderful okay good okay so thank you raphael i've known the name oppenheimer since 1945 when i was five years old sometime after after august 6th i think it was i asked my grandfather grandpa what's an atom bomb he said that he didn't know how it worked but he did know the one most important thing he said it was a giant bomb that had been built by a jewish man named oppenheimer that's all i know about oppenheimer for about 15 or maybe a little more years when i started to read his papers his physics papers in fact i did read a number of them and one of them had a great influence on me a paper with a young i i'm sure he was a student or just a young physicist by the name of heartland schneider a brilliant young physicist that gave rise to the modern theory of black holes even if it's not explicit everything in my lecture today rep rests on the foundation that oppenheimer and snyder laid in 1939 which by the way was a year before i was born okay let's begin i maintain that the biggest puzzle about physics is that it exists at all 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 bio biology fact but it just blows me away what are the tools that our ancestors used intellectual ancestors i don't mean the monkeys i mean newton einstein and so forth what were the tools that they used 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 remote 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 quantize mean quantize 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 iraq finding dewitt 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 the here and there they may emit some gravitational waves gravitons they invented feynman 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 meaningless 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 can be compatible they have to be compatible the world has both and we can't live in a world where of inconsistency and that was then how do you fit them together can you fit them together it looked impossible today the situation is different today 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'm a great digressor 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 are 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 was called coherent light all of a sudden voila an image would form but a three-dimensional image right in the middle of this cavity a three-dimensional image of whatever it was that had been photographed what is a hologram from an abstract point of view 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 may be just shining light on it 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 that'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 stephen hawking and i were having our fun debating and rafael was a student thought experiments principally initiated by stephen himself about black holes led to something called the holographic principle what was the holographic principle and what is the holographic 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 i will 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 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 connected 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 the correspondences are correspondences between the two of these so i'll 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've 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 m a 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 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 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 qubit if you know what a qubit is that qubit will touch a few more qubits a few more qubits will touch a few more qubits and the effect 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 if there are anybody know about quantum computers nobody good okay then this picture doesn't mean much to you this is a quantum circuit and uh it proceeds from left to right if you perturb it with a with a green qubit 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 adscft that may not mean anything to you it's fine it doesn't matter uh let's let me just oh i just realized i have a um a laser pointer built into here it's not really a laser pointer yeah good all right 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 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 this 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 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 the 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 gravitation gravitational attraction to me that is very stunning uh correspondence okay let's come out to black holes in paradoxes it was a famous paradox of stephen hawkings uh i'm going to oversimplify it not just stephen hawking's 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 oversimplified the 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 an 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 uh in the 1990s there is no way that anything can escape from a black hole but the black hole can evaporate that was something that stephen 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 the 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 the radiation that's a quantum mechanical principle that that we've known about for a long time and if you take that radiation imagine somebody takes that radiation grabs all the photons puts them in a box and squeezes that box down into some small box somewheres then what this is telling us the quantum mechanics is that the encyclopedia again i emphasize that 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 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 a this paradox the so-called firewall paradox you could say it this way either there is a firewall or or sometimes called raphael were you the inventor of a equals rb i think you might have been yes i think he was as 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 r b 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 centauri 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 rafael thought it was too crazy 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 r equals epr einstein er stands well let's first do epr how many people here know the who what epr stands for a good to a good fraction of you good it stands of course for einstein podolski and rosen but it also is the phenomena of entanglement quantum entanglement now i'm not going to tell you exactly what the 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 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 podelski and rosen discovered or at least 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 especially discovered all of modern physics uh 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 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 some interior space that can't be seen but the two very distant objects are connected by a i call it a tunnel i call it a bridge i call it a wormhole all the same idea you've seen these things in science fiction people jumping 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 or 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 bridges had anything to do with each other and after 2013 they had everything to do with each other in fact the idea goes with the acronym er equals epr you could call it p equals one but nobody does er the idea of a bridge between distant regions of space and the idea of entanglement of 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 centauri 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 not because it's folded over but just why 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 colors 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 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 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 thing two black holes which are entangled will necessarily have a bridge between them two black holes which are unentangled will not this is what's called er equals epr and it was a major discovery it seemed ludicrous at first but it very quickly caught on is now part of the standard 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 linus has control over and they're very far away one is on alpha centauri 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 each 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 or if i have time i may not have time linus 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 the no signaling theorem for entanglement and for wormholes it's called the non-traversability 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 equals rb the encyclopedia a in black hole number one or left-hand black hole here is encoded 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 in 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 outlandish 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 or 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 in alpha centauri 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 ja drive through that tunnel and the reason is that einstein rows and 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 a franklin i can't remember franklin or linus if linus 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 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 the 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 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 in some level as i said i'll say it again there are 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 okay whoops what happened here oh my is a totally different subject let's see if we can get it back how can i let you good here we are all right now let me address let's see how much time do i have raphael am i running out of time 15 seconds oh 15 minutes i don't need 15 minutes lots of time for questions okay 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 a string theory quantum gravity has been the victim of an enormous amount of criticism the criticism which first of all thing 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 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 are curious about the atom uh quantum mechanics among other things it led to the mri machine but so many things that i had that the list would go on and on quantum electrodynamics trying to understand the quantum mechanics of electrons and photons in particular photons led to the laser or at least it's 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 relativistic 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 holes and quantum mechanics it's a bound 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 the fl 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 nanophysicists 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 extreme or 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 does 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 japanese 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 experiments 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 are successful because i promise not to mention that they're being done and they're working out successfully 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 hang up 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 alvasena 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 metaphysicists and quantum computer physicists and theoretical cosmologists are being forced to learn what ads cft means and they are 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 did lincoln say you completes half of the people half of them all the time well you can please half of the audience uh 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're lucky if you can uh if you can satisfy any piece of the audience even a little bit because the ideas are complicated they'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 listening [Applause] okay we have some time for a few questions um from the audience i also have some from zoom would you like to take a ques ask a question well i got a couple on zoom so oh [Music] okay how are you there's an experiment going on on using drones to detect using drones to detect quantum waves drones drones there's stuff flying around in ukraine drones drones drones yeah stones yes and they're used they're you're planning on uh there's experiments going on and using these to detect quantum waves how are they uh okay i guess they it's an interesting thing i have no idea i'm interested in the drone so okay that's that's i'm sorry tell me well i never heard of maybe okay okay we have a question hi hello just out of curiosity did you draw the charlie brown drawings yourself or did you find this having a little difficulty hearing can somebody uh closer did you draw the cartoons yourself can i draw the cartoons did you draw those cartoons the peanuts cartoons did you draw them yourself oh yeah some of them some of them i just copied but they were all hand drawn by me yes yes [Music] my first um my first uh ambition had been to be a painter not a house painter a a picture painter the problem was i had no talent whatever or at least my talent was maybe being able to draw charles schultz cartoons but i really wanted to be picasso didn't work out i have a couple of questions for online um how can a wormhole grow faster than the speed of light well locally each piece of it is growing the rule is not that one thing can't exceed the speed of light it's one thing cannot pass another close by faster than the speed of light the universe for example in some sense grows faster than the speed of light the accelerated expansion of the universe tells you that if you're far enough away just the just the hubble law that things will be moving away from you faster than the speed of light but it does tell you that you can't get information from uh from behind from that far away and that's what creates a horizon it creates a cosmic horizon the same is true here the wormhole can grow but if it's growing that fast you simply you can't get through it for one thing and you can't receive signals uh from far away along the wormhole unless this idea of classical information being sent back and forth can come and slow the uh the growth of the wormhole and that is something that um that uh there's mathematics for it and in in some way it's being tested in the laboratory not on real wormholes but in entangled quantum computers thanks for the talk so if the information in everything is really all scrambled up and delocalized why does it act in any way that makes sense to us like why do things like why is physics local to us that is a really good question now if you just take some random quantum system it will have some features that look somewhat gravitational in character but it will not have the degree of locality that we expect in the real world it's only very very special quantum systems very very special candidates for the holographic screen let's say which show the kind of locality which you're asking about those special quantum systems are generally gauge theories whatever if you don't know what that means that's okay and they're also very very strongly coupled very difficult systems to analyze so you're pointing at one of the most important questions to understand what the nature of the very very special systems that behave with this kind of ultra locality that the real world seems to have and that's that's as far as i can go with it because i think the answers are yet to be given first of all fantastic lecture it was great but i'm not going to admit i'm not going to state that i understood no no but i mean one of the the a person who happens to have worked on strange metals and thought a little bit about quantum computing uh how much of the connections between uh the quantum gravity and everything you've been saying and and these other fields that some of us work on is because of the mathematics discovered in the process of of working out theories string theory and working with theories and how much is some deeper connection between the phenomena okay i think the connections are deeper certainly certainly that's part of it just the mathematics seems entirely similar you probably know about the such dev d a category theory that's an example yeah they're the but a lot of the physical phenomena uh experienced in that system are identical to the physical phenomena that you would expect in a near extreme or black hole so i'm hesitant to say that it's just the mathematics so don't think of it i think there is a real physical similarity if that's what you're asking i think there is yeah we'd have to sit down and talk about that obviously but uh i have another question from online if i'm if i may yeah so uh can you say a few more words about the growth of quantum complexity and how it corresponds to the non-traversability words you wouldn't believe it and i have but um i'm not sure this is the venue for it quantum complexity well i'll tell you what quantum complexity is first of all let me tell you what complexity is complexity means lots of different things to different people you might think for example a beautifully designed building or something or a beautifully designed car is complex no it's less complex than almost anything you can think about uh in that sense complexity is used in other words i have a very complex relation with my mother-in-law unfortunately i had well fortunately i had a very good relation with my mother-in-law but you could use the term that way in fact a very special thing is meant complexity if you have a given problem complexity is a measure of the shortest number of steps to solve that problem let me give you an example theorem proving you start with a bunch of axioms and now you have some theorem that you think might be a theorem and so you go to try to prove it and you prove it okay it took you a certain number of steps that number of steps might not be the cheapest and least number of steps that it takes you to prove the theorem what am i being misstep steps are the use of the uh of the logical um axioms together with the rules of logic each one being used once is a step your proof might involve 550 steps the complexity of a theorem is a measure of the least possible number of steps that it would have taken to prove the theorem another example from quantum mechanics or quantum computation is you have a quantum computer and you're going to put into it some simple state and you want to run that computer and get to another state another state which may have some interesting information in it okay quantum state of a bunch of qubits or something like that you can think of different ways of getting there there are different routes different routes different series of gates different series of processes that could bring you to that um to that target state the complexity of the state is a measure of the fewest number of simple operations that can get you there that's the notion of complexity now that's exactly the notion that's used in this wormhole situation the wormhole grows which is simply another way of well which means that the quantum state of the uh of the two black holes evolves with time it's a general feature that complexity increases with time in other words you go to states which are harder and harder to get to which would take few more and more steps to get to them and curiously interestingly there appears to be a connection between how complex the quantum state of the black holes is and how big the wormhole is that seems to be something that's fairly well confirmed by now and so it is believed that the growth of the interior of the wormhole is equivalent that's the gravitational side that's the general relativity side equal to the tendency for quantum complexity to increase with time for a complex uh for a chaotic quantum system so i'll have to leave it at that because uh as i said i could say a lot more but it would take more time than we can expand that was great hello uh so i was wondering since you said that the center of a black hole or at least the holographic surface of the center of a black hole acts like a fluid uh would that would solving the horizon which is a kind of holographic surface yes okay so if it acts like a fluid with solving uh the navier-stokes equation be a step forward to figuring out what happens at the center of life i rather think frankly i think it will happen the other way that people will be able to solve the gravitational equations for this fluid and uh make steps that direct attack on the navy stokes equations would be too hard for so my guess is it will happen the other way that not solving the navy stokes equation won't teach you that much about black holes because i think it's just too hard to do but solving black hole equations which are a lot easier because they're just einstein's equations might very well teach us a lot about um uh dynamics of fluids that would be my best guess as to which way the information will flow there but there are other experts on that subject here and um i think you're an expert right yeah yeah he has a nori well i don't know who's somebody's in there oh i kind of question uh here i don't know if he knows oh anyways i was wondering if there is a relationship between the surface and the entropy between the what oh it's here hello i was i was wondering if uh so you mentioned there is a uh bound on the viscosity uh of this of the black hole uh and the entropy uh so since the viscosity somehow seems to be on the surface of the black hole is there a relationship between the surface of the black hole and the entropy and if so why black holes and horizons have entropy they have a uniform entropy distributed over the horizon they also have viscosity so it was realized by i think it was a condensed matter physicist the sun son that that no matter what he tried to do to the black hole he could never get the viscosity of the surface fluid to be less than a certain amount which was proportional to the entropy uh and uh that was a pure black hole study and it turned out that when people looked at experiments on the most on the least viscous fluids no matter how they manipulated the fluids experimentally nobody found the fluid who has that ex that ex exceeded them the uh the bound whose sun and staronets uh who discovered this bound uh i just took that as an example there's nothing particularly special there are lots and lots of examples like this this one was easy to explain there's a nice simple equation that goes with it and uh and um it's so down to earth that it's very easy to understand not understand where it comes from why it's true but what it says um sir okay i have two questions um the first one is um i'm not very familiar with this quantum gravity before this lecture and there's one thing that seems very well to me is that um where is the quantum holograph and does it physically exist and this is a you're asking where the client yeah i think the answer is that you take any region any region whatever this region here of space and you want to link the room and you want to ask how the information is encoded and the answer is that the amount of information you need to describe the interior of this room is never more than the surface area of the boundary of the room and so you can always say you pick your region and then the answer will be on the boundary of that region so it's a mathematical statement uh what about the universe well the universe does have a horizon uh the natural place for the hologram describing the entire universe would be at the horizon of the uh at the cosmic horizon of the universe but that's something that is still being studied so the natural the natural thing to say is we take the whole entire observable universe it's bounded by something called the cosmic horizon and that would be the natural place where you might want to say the hologram was now i'm not sure that all my colleagues agree with that and that's still still something that i think is um work in progress okay and the second one is rather and is related to a former question how do you define on two points are nearby as we discussed that you cannot travel um greater than at a speed greater than the speed of light between two nearby points and to what extent does these two points can be uh can be considered as nearby as you stated send messages back and forth in small amount of time you'll say they're nearby i'm not sure what else to say i don't know raphael wendy said two points are nearby yeah okay hi thank you for the lecture um for sure oh yeah here uh so the famous quote-unquote historical dilemma is that the copenhagen interpretation of quantum mechanics is not irreversible is not reversible while the schrodinger equation is very much time symmetric do you believe that these deeper insights into the workings of quantum mechanics and as they relate to general relativity as you laid out in the lecture we'll deepen our understanding of our interpretation of the wave function or even perhaps change or offer a new interpretation of the wave function i'm only going to say that that's a wonderful question i think that is a really really good question is all of i've always felt that the puzzles of quantum mechanics the many worlds interpretation all the very very confusing things about quantum mechanics would only eventually get solved when we understand the connection with gravity i still think that but i don't if i knew how to do it how to make those connections i would have published them the no no i mean my feeling is that the answer to your question is understanding these aspects of quantum gravity will tell you more about the inner workings of quantum mechanics but nobody has really um actually that's not been a primary concern of the people doing this kind of work they tend to be very focused uh pragmatic people if you can call a theoretical physicist pragmatic who will tend to solve problems that can be solved this is part of the art of being a good theoretical physicist is to identify those problems which are hard enough that the results of them of a solution will matter and be important but we're not so hard that we'll just sit around for a hundred years scratching our butts over them and so the general feeling of my friends and so forth and i sort of share it is that these problems of the foundations of quantum mechanics are real problems but boy they've been around a long time people have struggled with them people not been able to make any real sense out of it it's not even clear they're real you know what feynman said about it he said the problems are so confusing that he can't even tell if there's a real problem and that is the way it feels you know it's like thinking about consciousness or uh so the pragmatic side of physicists tends to make them steer away from problems like that will they come back to it perhaps but at the present time i think all of this has not impacted the um the most fundamental understanding of quantum mechanics which is disappointing in a way okay we have time for two more questions i think one at the front here yes i also have a question about we talked before about collapsing a set of entangled particles into a wormhole at one ear and one at alta centauri and i wanted to ask if that in turn also means that if we have a wormhole and it decays that we also get back a set of entangled particles i am still having a little trouble hearing can anybody repeat it some voices it's nothing special some voices i simply have difficulty there's a certain range of frequencies yeah absolutely yeah for example you have these two entangled black holes which form the wormhole um what happens if those two black holes evaporate they each just get transformed into a radiation particles that go out but those particles will be entangled so that's exactly right okay one last question just yet yeah last question because i'm beginning to fade try now okay i'll say that again i'm i'll ask a much simpler question than i was i was planning to um you mentioned that there's uh a definition of the leoponoff exponent that has to do with quantum gravity i'm i was just wondering if you could say more about what system that exponent is referring to and where that definition comes from uh well it's a wide variety of systems but any system the the most well understood case i would say is this syk model which is also a theory of strange metals the yampanov exponent has to do with exponential growth when anything grows exponentially it it varies like e to some constant times time that constant in this context is called the yapon of exponent um what is it that's growing well the simplest way to say it is it's the growth of the region of influence of the perturbation in other words this growth of what i call size earlier um what did you ask me what system is it referring to any kind pretty much any kind of quantum chaotic system but as i said the best understood one at this point is this um syk theory of strange metals which is also a theory of extreme or near-extremal black holes s-y-k that's the initials sashtev yeah and kitayev so i feel that's where most of it has been worked out in the greatest detail well lenny i think the only way that people would be left unsatisfied is only insofar as they're hungry for more so thanks again professors [Music] [Applause] [Music]

Channel: UC Berkeley Events

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Length: 78min 22sec (4702 seconds)

Published: Wed May 11 2022

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The sound level is all over the place