Einstein and the Quantum: Entanglement and Emergence

[Music] tonight's program is a tale of two papers each written by albert einstein each written in 1935 and each according to einstein on completely different totally unrelated subjects the tale of these two papers went its way right up to the present as some scientists now propose that these two papers are actually intimately connected a proposal with profound implications for our understanding of the small quantum mechanics and our understanding of the large einstein's general theory of relativity some background prior to the 1900s we seemingly had a perfectly good understanding of the laws governing the physical universe handed down to us by the great isaac newton these are the laws we still teach high school kids around the world the laws of so-called classical physics and for those of you who like math here are some of the essential equations but those are details at the core of newton's perspective is the idea that if you specify how the world is right now the positions and velocities of all ingredients and you specify the forces acting on those ingredients the laws predict how the world will be at any subsequent moment the framework it's it's sleek it's elegant it's powerful and it's rigid in its determination of the future and it worked the predictions about the future where a pendulum would be as it swung where a ball would land when thrown where the moon would be in its orbit around the earth all of these predictions and every other that was made all of them were spot on accurate that is until the early years of the 20th century when scientists gained the capacity to explore the micro world the world of of molecules of atoms and subatomic particles because in this realm the predictions from newton's math were they were wrong the data revealed that new rules were needed to understand the microscopic realm and remarkably within just a few decades a generation of scientists including the likes of albert einstein max planck niels bohr werner heisenberg irwin schrodinger max born and many others this group of brilliant thinkers ushered in a new understanding called quantum mechanics now quantum mechanics it comes with its own powerful mathematical formulation but again one can grasp the central new idea of quantum mechanics even without the math whereas newton said tell me how the world is now and i will tell you how the world will be tomorrow quantum mechanics says tell me how the world is now and i'll tell you the probability that the world will be one way or the other tomorrow and according to quantum mechanics these probabilities provide the deepest and most precise description of the physical unfolding god completely gone according to quantum mechanics was the rigid certainty of the classical world now much has been made of einstein's resistance to the probabilistic nature of quantum mechanics the famous and frequently quoted god does not play dice but einstein did not deny that quantum probabilities provided a spectacularly accurate description of the microscopic realm instead he firmly believed that quantum mechanics was only a provisional theory one that would ultimately be replaced by a deeper understanding that would not rely on probabilities and toward this end einstein he worked tirelessly to expose qualities of quantum mechanics that he hoped would be so obviously unacceptable so counter to any reasonable person's expectation of how the world works that everyone would have to agree with his view that quantum mechanics was not the final story and in 1935 with two colleagues boris podolski and nathan rosen einstein believed he had finally found the ultimate quantum mechanical achilles heel with a property inherent to quantum mechanics called quantum entanglement briefly put einstein his colleagues found that according to the math of quantum mechanics if two objects interact and then widely separate a subsequent measurement on one of those objects revealing one or another quality would have an instantaneous influence on the other object regardless of the distance between them einstein called this strange quantum connection he called it spooky spooky action at a distance and it troubled him deeply his his straightforward intuitive belief was that widely separated objects are independent of one another but quantum entanglement seemingly denied that by providing an invisible quantum connection capable of linking the distant objects together or as the name says entangling them einstein could not accept this quantum view of reality and so he concluded that something had to give quantum mechanics could not be the full and final story that's the upshot of the 1935 so-called epr paper now i want you to note the date on this paper may 1935 in july of 1935 einstein and rosen published another paper on a seemingly completely different subject years earlier in 1915 einstein had ripped another rug out from under the newtonian worldview showing that newton's approach to gravity needed to be replaced by his own general theory of relativity in which the force of gravity which newton quantified but never explained is finally laid bare to move under the influence of gravity according to einstein is to surf the warps and curves in space-time now shortly after einstein announced this general theory of relativity to the world a german scientist named carl schwarzschild found the first exact solution to einstein's new equation showing how a spherical body like a star warps its environment and within that solution is a startling realization if a if a spherical body is crushed to a small enough size the space-time curvature is so extreme that a kind of point of no return forms if anything crosses this so-called event horizon it's impossible to escape the gravitational pull even even a beam of light can't escape so the region appears dark we have what modern parlance is called a black hole in his 1935 paper with rosen einstein wasn't actually thinking about black holes but it turns out the black holes provide the right language for the modern interpretation of the result which is this einstein and rosen found that two black holes can be joined together in space-time connecting one region of the universe to another through a kind of bridge what we now call an einstein rosen bridge or perhaps a more popular terminology a wormhole now wormholes have been a staple of science fiction but they're also an arena of scientific study although they are still surely fully hypothetical theoretical physicists have learned a great deal about wormholes in the intervening decades but here is the thing were you to have asked any of those researchers if there was any connection between wormholes and einstein's other 1935 paper the one on quantum entanglement they would have said no at least until very recently as a surprising new connection has been proposed a proposal with potentially far-reaching consequences not just for black holes but even for the very structure of space and time themselves tonight we are honored to have three guests who will lead us through the wonderful worlds of general relativity quantum mechanics black holes entanglement and some of the cutting-edge developments that may link these ideas together our first guest is mark von ramsdonk who is a professor of physics at the university of british columbia he was a canada research chair a sloan foundation fellow is currently a simon's investigator and is a recipient of the canadian capcrm medal in theoretical and mathematical physics he joins us now from vancouver welcome mark next we have anna alonso serrano a postdoctoral researcher at the max planck institute for gravitational physics her work lies at the interface between gravitational theory and quantum theory including aspects of the black hole information paradox thermodynamics of space-time as well as notions of entropy quantum cosmology quantum field theory and wormhole she joins us from berlin welcome anna and finally we have leonard suskin the felix block professor of theoretical physics at stanford university he's a member of the national academy of sciences the american academy of arts and sciences an associate member of the faculty of canada's perimeter institute for theoretical physics and a distinguished professor at the korea institute for advanced study suskine is one of the founders of string theory and is widely regarded as one of the most influential physicists of our time i could go on but suffice it to say that without suskin's profound contributions many of the key ideas that we will be discussing here tonight simply put they would not be known he joins us from his home in stanford california welcome lenny and welcome to you all let's jump right in and discuss einstein's 1935 paper on quantum entanglement and so mark what do you think what motivated einstein to write this paper was he trying to advance our understanding of quantum mechanics or perhaps was he trying to kind of undercut it entirely yeah i mean i was looking over that paper again last night and uh it's it's really clear that he's he's uncomfortable with some aspects of quantum mechanics uh he's he's trying to show in a sense that uh the current understanding of quantum mechanics can't be complete he he uh with his colleagues feels that maybe there's something wrong with it and it's it's like he's they've found the the thing that will will maybe prove that there's got to be something more there that maybe there's some extra information in addition to the wave function yeah so so lenny you know there's this interesting historical fact that schroedinger long before the 1935 paper he said something like i don't have the exact words but he said something like entanglement is not one but the feature of quantum mechanics that distinguishes it from a classical worldview do you agree with that assessment of entanglement yes i i think that's fair i think that's fair um to say that entanglement it's certainly the most peculiar and um different thing about quantum mechanics i think it's fair to say it's the central core of the um of the difference between classical physics and quantum mechanics so ana i'm just curious because i think of all the the folks participating in this conversation i think you and i have perhaps the greatest age differential when i was an undergraduate entanglement never came up it wasn't in the lecture and it was a it was a good class i went to like a good place it wasn't in the textbook how about how about for you did you learn entanglement in your first course in quantum mechanics yeah that's a great question because it was something uh it surprised me a lot in fact not really at the beginning it was something that appeared pretty late for what it is as uh was said before that being the most peculiar and characteristic point uh was not so much mentioned and i was also surprised like uh neither any of the history of how quantum mechanics was developed and the this point that was erased by einstein and the different ideas about interpretation of quantum mechanics and it's not something that you are teaching right now neither in a bachelor or a degree so yeah so mark we've said a few words about and damn we haven't actually said what it is so can you just give us uh your best description of what quantum entanglement is actually all about yeah absolutely so i think the underlying idea here is the quantum superposition idea i would say that in some sense is the core of quantum mechanics that if if you have a physical system and it can exist in two different configurations then quantum mechanics says that it could also exist in what we call quantum superpositions of those configurations so let's say i had a classical object like a coin it could you flip it it could be heads it could be tails and in quantum mechanics what we say is that well it could also be in all of these other configurations that are part heads part tails they don't have a definite result until you go and and you you interact with that system and measure it to see if it's heads if it's tails so so that's sort of the basis and then with quantum entanglement it's just applying that to systems that have multiple parts so maybe you look at two of these coins now and if you flip both of those then in quantum mechanics you could have weird combinations like partly heads tails partly tails heads so there's all these different combinations and the idea with quantum entanglement if you go in and measure that if you go and look what are the coins saying then if you find heads for the first one then you're definitely going to find heads for the second one if you find tails for the first one you'll definitely find tails for the second one so it's this correlation between the two states even though either one of those things is uncertain right right and and because these correlations they hold regardless of how far apart the objects may be einstein famously described this as spooky as spooky action at a distance right you do a measurement on one of these coins or one of these particles and it has a kind of instantaneous impact on the partner you you measure one coin you find heads the other one is definitely heads or definitely tails whatever the configuration of the objects may be but but einstein himself seemed to focus on another aspect a related aspect of quantum entanglement which is this in composite systems you can know everything that there is to know about that composite system and yet not know anything specific about the individual pieces now lenny is is that a part of entanglement that you think is really more at the essence of quantum entanglement than spooky action at a distance yes uh that's exactly the way i think about entanglement and what's weird about it that you can know you have two systems my two hands uh you know god gave us thumbs what a lucky thing one up one down or one down one up and so forth okay an entangled state of my thumbs is a very peculiar thing it could be both of them can be up both of them down they can be pointing toward you they can be pointing away from you and so forth an entangled state is exactly what you said it's a situation where you can know everything that can be known everything that can be known that nature that the rules of quantum mechanics say that you can know you can know everything that can be known about the composite system and know nothing at all about either of its parts now how can that be i'm absolutely certain that that's what freaked einstein out how can it be that you know everything that can be known about the composite system and know nothing about the parts what do i mean about not knowing anything about the parts well you simply if you make a measurement on one on your on this thumb here it's randomly up or down if you make a measurement on the other thumb it's randomly up or down you know nothing about it on the other hand you know something else you know correlation you know that if one thumb is up then the other one is down if one thumb is down then the other one is up or something like that so einstein felt deeply that this could not be the end of the story he understood quantum mechanics there's no doubt that he completely understood this uh and wasn't in the dark about what all this meant he simply couldn't he couldn't come to terms with the idea that you could know everything that can be known about a system and know nothing at all about its parts um that was deep that was extremely deep and um we're still seeing the repercussions of it today very much so in fact and and so anna if it freaked out einstein right does it freak out you this this idea that quantum mechanics describes composite systems in this strange way yeah of course i think i think nobody that have learned quantum mechanics can say that was not freaking out first time that was uh learning these things and i think getting the from our classical world where we are used to think and to have intuition it's not easy to change our minds to uh learn how to take intuition of that i think it's a change of the tip i'm thinking about that system that they say that is a non-separable system is uh that's the one of the key words is the non-separability there of the of the whole system so then you cannot know individually anything completely so mark historically speaking what did einstein do with this freaked out reaction to quantum mechanics did he just leave it at that did he have a potential solution where was he hoping he could push the field by emphasizing this weird quality of quantum systems yeah so einstein thought that this idea that you could have an entangled state widely separated where the two parts were so far apart that they weren't interacting at all so then the idea that you could measure one part and then immediately know what's going on with the other part specifically in the paper they they just tried to show that they tried to show a contradiction that the standard uh story in quantum mechanics with a pair of particles uh so so there's the heisen per uncertainty principle statement that you can't know the momentum and the position at the same time so they wanted to set up kind of a contradiction to show that if you have one of these entangled pairs of particles then by looking at one of the two particles in different ways you could either say look at the momentum and then you would know the momentum of the other one because of the entanglement or you could look at the position and then you would know the position of the other one because of the entanglement and then they go on to say that well because i could have done either one of those things and because well there's no way that that could have had an have an effect on the other side because it's not interacting it's too far separated then they suggest that uh well that's that sort of shows that you could know both the momentum and the position of the other one and that's sort of a contradiction with the heisenberg uncertainty principles so things just don't seem self-consistent right i guess with the idea being that if somehow a measurement in new york is telling you something about a particle in los angeles then it must be according to einstein and colleagues that the particle in los angeles always had that quality even if quantum mechanics wasn't able to tell you what that quality was it can't be that what you do in new york is literally bringing that quality to a definite circumstance that just seems kind of crazy and it's an interesting historical fact again that's worth noting einstein wasn't very happy with the epr paper this is this wonderful quote which i happen to have here because schroedinger wrote einstein a letter after the epr paper kind of uh congratulating him on it that was a really good observation einstein then writes back to schroedinger here's what he says he goes i was very pleased with your detailed letter which speaks about the little essay the essay being the 1935 epr paper then einstein goes on for reasons of language this was written by podolski after many discussions but still it has not come out as well as i really wanted on the contrary the main point was so to speak buried by the erudition so he he wasn't thrilled with how this this paper turned out and lenny i think it's because einstein wasn't pleased as mark recounted that the epr paper focuses on heisenberg uncertainty principle it focuses upon a counterfactual right it imagines that you could measure this or you could measure that you know a contrarian can say well you can't measure them both on the first particle so can you really draw any inference about the qualities of the second so he saw this as much more general i think than what the epr paper even even discussed is that your take on it as well yeah very much so um you know the the letters epr we all know what they stand for right when i first learned about it i was told that epr stood for elements of physical reality which is a term that appears in their paper when i learned about epr it was elements of physical reality and it was very clear that that was what was bothering einstein what is real what is real what uh what is at the bottom of everything what is really unambiguous what is um objective as opposed to what's uh in our minds and what we decide to measure and don't decide to measure and so forth what is real einstein was a realist i mean he was a realist and that he believed always deeply that there's something real and something underlying that is objective almost classical yet he knew that quantum mechanics didn't work that way i don't know that quant that einstein was trying to overthrow quantum mechanics or that he was trying to find a contradiction i more or less read the paper as just saying isn't this very weird this can't be the end of the story that's more or less the way i read it and with no particular prescription about where it was going to go from here so i mean this is probably the same with all of you but if i was to count up the uh the number of times that i've been asked by members of the general public okay do you really understand quantum entanglement i think it's the question i've been asked more than than any other question and i don't mean to get all meta on this but i think the answer in part depends on what one means by the term understands right um we can make predictions on and so forth but i mean ono what's your view if someone interrogates you and says do you understand quantum entanglement what do you say yeah in fact this is something that i'm i've been asked a lot of times by family friends and in several talks and as you pointed out it's very interesting the point that how do you do we understand um the understanding of the quantum mechanics because sometimes the most powerful part of this theories of quantum mechanics and the developments further in the future like quantum filtering and everything are the extremely accurate predictions that they have uh given us it's it's incredible how well they work how they allow us to to make things work even in our normal life the devices that we are using for this conversation and everything so i think it's that's a real understanding how we can use this to predict and to create new things but the difficult part of this understanding is how you transmit to the general audience because when you want to transmit things to the general audience you tend to use metaphors or you know similarities with real life and as i said before this is not in our daily intuition so then it's when it's problematic like oh why what what is my understanding of this when i compare it with my daily life and yeah that's a tricky part now there is another tricky part of quantum mechanics a kind of gap in our understanding of quantum mechanics sets really just behind much of the territory we've been discussing which is that quantum mechanics describes reality as a kind of fuzzy haze of possibilities that only resolves into the kind of definite reality we all experience when the world is observed or or measured or interacted with in the right way and yet our quantum mechanical understanding has at least so far not articulated how that transition from fuzzy to definite how it actually happens a transition that is of course essential to entanglement since we're talking about an observation here having a spooky connection to an observation over there now lenny does this gap this so-called quantum measurement problem does it trouble you or do you see it as more of a detail that we will one day resolve oh i don't think it's a detail by any means i think it's at the very center of all of the confusion about quantum mechanics which i share i mean i don't feel i understand quantum mechanics at that level and there was feynman who said i'm so confused by the whole thing that i can't even tell if there's a problem and i think a lot of us feel that way on the one hand we have to use quantum mechanics we know that quantum mechanics works if we want to make a theory of the world it's got to be quantum mechanical on the other hand we're so confused about what the um the basic conceptual framework of dividing the world into what's observed and what's not observed that uh that i personally think there's there is going to be another quantum revolution that other quantum revolution is going to come to grips with this in a way that we can't conceive of yet but i also have the probably a lot of people think that okay my guess and i wouldn't be surprised if um if mark shares it is that we won't get that deeper understanding until we understand the relationship between quantum mechanics and gravity quantum mechanics and the geometry of the world and we don't fully understand that yet we're making a lot of progress but um i think it's not until then will we have a chance at understanding these deeper issues that's a a great segue so why don't we turn to gravity general theory of relativity where we're gonna ultimately take up a second of einstein's 1935 papers so mark einstein gives the world this beautiful theory of gravity the general theory of relativity where now gravity is translated into geometry warps and curves and space and time and so forth a sort of beautiful encapsulation of how the force of gravity actually works 1919 it's confirmed or sort of confirmed depending how you look at the history of it through the bending of starlight by the sun but then interest in general relativity seems to taper off i mean why was that why wasn't that beginning why isn't that beginning of a sort of new era of gravitational study why did it sort of begin to die away yeah that's a a good question uh i i'm not sure i i know the answer i mean i i'm guessing that it was probably seen as uh still maybe a little bit esoteric and mathematical and difficult to maybe compare with experiments that we could readily do in laboratories so it may have been that uh it was still seen as just not quite practical physics yet but maybe maybe lenny wants to comment i live through that era i'm old enough to have been part of that era when grown-up physicists didn't think grown up in quotes physicists did not think about general relativity part of the reason mark said part of it well that that it was very very hard to get experimental data about it but the other side of the coin is that it was very easy to get lots and lots of experimental data about other things there were other fish to fry there was condensed matter physics there was elementary particle physics and so i think a large amount of it was just physicists had other fish to fry lots and lots of data and so it was kind of natural that those who wanted to get ahead who wanted to do things who wanted to you know get their hands on and maybe win nobel prizes would be going in the direction that um that seemed well that would seem like it was going to yield to new theory whereas gravity was beautiful it was pristine it was um it was special but there wasn't too much you could do with it in the way of experimental data it was just laid out there by einstein and all its beauty and all its glory but what do you do with it so that was my understanding of it at the time yeah and so what about the 1935 paper he was sort of uh okay so then the other that's the other 19 you're talking about the other 90 the other 1935 other 1935 paper yeah i don't know what was on einstein's mind frankly um he uh he didn't like black holes i don't even know if he knew in that paper that he was talking about a pair of black holes i'm not even sure that it was actually einstein who cooked up the uh the solution that we call einstein rosen um he just as far as i can tell he and rosen simply discovered a solution of the einstein field equations which uh contained this wormhole connecting different regions of space yeah yeah it connects two distant points of space with a tunnel uh with a with a short tunnel it's as if you could um had a tunnel between new york and san francisco and new york and palo alto but if you go in the tunnel it's only a mile and a half inside the tunnel from one end to the other uh so exactly what was on his mind i don't know and certainly i am certainly convinced that he had no inkling at all about the possible connection with the other 1935 paper yeah i mean we're gonna we're gonna come to that in just a little bit but let's spend a little more time on black holes which are intrinsic to the modern day interpretation of the einstein rosen bridges you know having two black holes that are connected by a bridge by this kind of throat and for our conversation here perhaps the most formative insight about black holes comes to us from stephen hawking who way back in the 1970s showed that black holes are not as black as their name suggests i mean hawking found that when you include quantum effects black holes can actually radiate away particles from their edge from their horizon so anna tell us what is the idea here so hawking starting thinking about uh what's happening there in the in the horizon so was using this um theory this quantum field theory to uh calculate what's happening with the fields there so then is um this uh simplified version of thinking about it that is that in this um quantum field theory we allow for the creation of particles some uh when we consider this this is when we consider quantum mechanics apply to fields and that you have um more complex uh theory and in this case you allow for this creation of particles so it can be the point that this creation of particles takes place uh at the horizon of the black hole so then one of the particles falls into the black hole and the other goes outside the black hole being these particles entangle as we have been discussing before and in this case what you have is like uh in the end what do you find is that there is some radiation emitted by the black hole some quantum emission right so so just to summarize those important points on according to quantum mechanics pairs of particles they can materialize in empty space and although in ordinary empty space those particles would then quickly collide annihilating one another near the horizon of a black hole once its particle can fall in leaving its partner free to fly off to radiate away into space and as you also rightly pointed out on this radiation brings entanglement into the black hole story because those pairs of erupting particles are entangled that's a point we're going to come back to in just a moment but first mark once we learn that black holes radiate these particles that ana is making reference to that there's actually stuff that in some sense is coming out from a black hole that stuff it takes away energy and so the black hole gets smaller in the process and will ultimately disappear and this raises a deep concern because the fundamental principle in physics is that information is never lost now when something falls into a black hole right if it's a book an iphone whatever you might have imagined that the information contained in the object is not lost it's just inaccessible since it's now inside the black hole itself but when we learn that black holes radiate and ultimately disappear that vision no longer works the black hole can't hide the information forever so the radiation better carry that information back out with it but at first blush it doesn't seem that the radiation does that giving rise to the so-called black hole information problem or black hole information paradox so mark can you tell us about that and then we can discuss how lenny's going to solve it for us yeah so with this black hole radiation that anna described so you've got your black hole what it looks like if you were to measure what's coming out of the black hole it looks just like say an object with some temperature emitting the thermal radiation corresponding to that temperature and one of the interesting things about the detailed calculation that hawking does is that what comes out really doesn't care about what formed the black hole what what maybe collapsed to produce that black hole in the first place it just is the same kind of purely random thermal radiation coming out and so it it seems that you lose information uh about what formed the black hole and that's kind of in conflict with a basic assumption or a basic property of quantum mechanics in quantum mechanics you could always sort of run the clock backward you could recover what it was that was the initial state given this if you had complete knowledge of the state later on and so there was a tension there that if you completely accepted um quantum hawking's calculation then there seems to be a tension with quantum mechanics and so you could decide okay well you need to modify quantum mechanics if if gravity is going to be a part of it or you have to come up with some some you know more creative and brilliant idea so lenny this loss of information regarding what falls into a black hole was a potential blow against all the hard one understanding because according to quantum mechanics no information is ever lost according to quantum mechanics a detailed form of radiation coming out of a black hole should depend on what fell in so you and your comrade in arms gerard the hardest of all names in physics to pronounce you both were having none of it and over the course of many years you both developed your own versions of similar arguments claiming that information is not lost making use of incredibly strange but wonderfully powerful idea known as the holographic principle so tell us how that goes and i gather that string theory was an important inspiration in your thinking here okay i had had some experience in the field of string theory and there was one thing that had always puzzled me about string theory the more carefully you look at a quantum string the more precision that you bring to bear on the measurement of the kind of of the quantum string the more you will find that it's spread out over all of space if you look at it casually with not too great a resolving power you'll find the string localized a little string a little particle you'll find it localized in some localized place if you look at it more carefully with more precision you will find out that it's spread over a bigger distance if you look at it really carefully you'll find out that it's spread out over a larger distance it occurred to me that when a particle falls into a black hole somebody watching it from the outside because everything slows down as a thing falls towards a black hole its internal motions slow down somebody watching it from the outside gets to see it with more and more precision because it slows down it's like looking at an airplane propeller if it's going too fast you can't see what it is if it's slowed down you can get a look at it and see what it is and see it in more detail same thing with this vibrating string as it falls into a black hole somebody watching it would get to see it with better and better resolution and therefore we get to see it spread out more and more in just such a way that instead of falling into the black hole it would get spread out not only over the horizon but spread out way out to infinity and so i just prayed on my mind i got to thinking about it and had the idea that somehow the fundamental degrees of freedom and string theory are stored very far away and that the relationship between the actual things that you see and the actual storage of information was very much like a hologram uh at about the same time maybe even a little bit earlier my good friend that tuft had a very similar idea again that the information in a system any system is stored far away at the boundaries of the world a very weird idea an extremely weird idea and um we got together we talked about it we both published articles on it um i was actually unaware of the fact that he had used the term hologram in describing this earlier and i called my paper the world as a hologram i think tufts paper got lost people didn't notice it for some reason the uh the bold uh title the world is a hologram i think caught a few people's attention at the time so the idea was that information never really falls into the black hole it's always stored far away but it's only the kind of um image of this information that falls into the black hole and can we think of the information as being on the horizon echoing i think that's one way of thinking about it but you can even go further than that you can say if you get enough resolution power you will find that as you get better and better resolution power it sort of hovers above the black hole to larger and larger distances away from the black hole uh where exactly is the horizon that depends on how much detail you look at it where is the information it again depends on what detail you look at it with your resolving power of your camera the speed of your camera if you like it's not uh in the black hole it's not on the black hole ultimately it's out at the boundaries of space wherever the boundaries of space are now the space have a boundary who knows and so we come to a pretty wild state of affairs which seems to solve the problem right information regarding whatever object falls into a black hole is not lost because that information gets stored holographically outside the black hole itself which in principle means that the information is recoverable good but then comes a paper in 2012 by four physicists amiri marl palchinsky and sully a paper referred to in the field as amps which shall we say shocked the community and it has to do with the entangled nature of the hawking radiation that comes out of a black hole which anna you alluded to earlier namely the erupting particles near the black holes horizon are entangled but for the outgoing radiation to embody information about whatever fell in that radiation can't be fully random instead there also has to be entanglement between the early radiation which is now traveled far from the black hole and later radiation which is still near the black hole itself because only with such entanglement can there be information storage now for the problem that amps pointed out quantum mechanics doesn't allow such three-way entanglement cheekily referred to in the field as the monogamy of entanglement so lenny what to do so mark's work was very very instrumental in this now what uh before yes again i have to bring up another person uh on this paper that i wrote that uh that had to do with the two 1935 papers for once in my life i was very definitely the second author on a paper the first author was juan maldasena who had been thinking about very related things for a long period of time and uh i i should mention that before we go any further okay so what uh what was i thinking i had i and also i think rafael busso had the following idea um the information right behind the black hole right behind the horizon of the black hole let's see how to say this right the exterior of the black hole cannot both be entangled with what's just behind the horizon and the hawking radiation unless the information right behind the black hole and in the hawking radiation are the same thing now that sounds again crazy how can what's right behind the black hole is my black hole over here okay then the hawking radiation is miles and miles and miles no many many many light years away how can the information of what's right behind the horizon of the black hole be the same thing as the information in the hawking radiation very very far away nevertheless i said it must be true it must be true that the entanglement is not broken it's just that the information right behind the black hole is the same as in the hawking radiation and that was my opinion that somehow in some mysterious way the interior of the black hole just behind the horizon was really the same thing as the hawking radiation very far away another nutty idea that sounds somewhat similar to the holographic idea information that you thought was nearby really very far away uh juan maldasena and i got to talking over the internet we started collaborating and trying to figure out what was going on with the amps paradox at some point juan sent me a very cryptic message and in the cryptic message he simply wrote down e r equals epr i knew instantly what he was talking about that's that just was an explosion in my brain when he said it i knew exactly what he was talking about but he said it in such a clear way now what did it mean e.r stood feinstein rosen e.p.r stephen einstein podolski rosen remember what the two of them are einstein rosen wormholes epr entanglement what he was saying was that if there's entanglement then there must be wormholes connecting the uh the regions that are entangled if you took the whole thing together what it seemed to say was that there must be wormholes or a wormhole connecting the distant radiation to the interior of the black hole they're not as measured through the wormhole they're really not very far away so that what we see as radiation very very far from the black hole is connected through through a hidden wormhole to things just behind the horizon of the black hole that's what juan meant when he wrote down er equals epr that um that if there's entanglement there must be wormholes there must be spatial connectivity that in fact did turn out to be the salute as we believe that is the solution of the amps paradox that the entanglement between the exterior and the interior is not broken it just reappears as the entanglement between the exterior of the black hole and the hawking radiation now einstein wrote those two papers like two months apart that's incredible do you think that he had any inkling that there is any connection between them no i don't think so there's no evidence whatever uh and no i i do not think so so it's another one of those crazy examples where einstein is just somehow able to put his finger on things that that that last didn't matter um i think i i really think this is just accidental but who can say einstein was was extraordinarily unusual but he wasn't supernatural and i don't think he could have um he didn't have all the things which were necessary to um to formulate a reason for connecting those two ideas yeah yeah absolutely so mark lenny made reference to your work that played an important role in the remarkable results that he was just recounting i i want to finish up with our our final scientific subject to to explore that insight of yours a little bit and maybe we can do it in two pieces because part of it makes use of an insight of juan maldasena the physicist that lenny made reference to this uh generalized version if you will of holography you know describing some region of space by degrees of freedom and physics that lives on a boundary if you can give us a uh just an intuitive sense of of that first and then we can go on to how that insight is then leveraged into this result about how space is held together by quantum entanglement yeah so i think that the the way i would describe holography is the idea that what you see in a gravitational universe um is really not it's somehow encoded in a system that's more fundamental that's not really present in that universe at all so it does happen to have the same geometry as the boundary of space and some examples but i think maybe the more general idea is that you've got some kind of more or less ordinary quantum system that doesn't have gravity in some of the examples it's basically you can imagine a spherical shell of some exotic material and that thing is able to encode the physics of a gravitational universe and then there's a basic question well how does it do that how does how do you store the information about geometry of space and the presence of black holes and radiation all this how is that stored in this much simpler kind of quantum system and we've eventually come to an understanding that actually thinking about this story of entanglement is really important for understanding how is that information about gravity encoded in this in this more ordinary system so cena gave a really interesting example where he wanted to describe not just a a single ordinary black hole but a kind of more exotic that is basically the one that einstein and rosen thought about so one way to describe that is you have a black hole in one universe or one part of a universe and another black hole and maldocino was thinking about the case where that was in an entirely disconnected separate universe and so there's this configuration where geometrically those two things are connected such that if one person went into the horizon of one black hole and another person from the other universe went into the horizon of another black hole they could they could meet in the middle so there's a spatial connection between these things and so while the cena said um how do we describe this thing using this holographic approach and he says because there's two universes that you know otherwise apart from this wormhole they're just separate universes well this should involve two copies two sets of uh two copies of this quantum system that we were talking about before so you can imagine having two of these spherical shells and he says that if you want to build one of the wormholes well you put a lot of energy in one to get a black hole you put a lot of energy in the other to get a black hole in the other space-time but then you entangle them in this very special way and that's the only thing that connects them and he says well this is the natural description in quantum mechanics of that space time where you have the two parts of the two universes connected by a wormhole and uh it just struck me that really because this entanglement is the only thing that makes these two systems have anything to do with one another then maybe that's the thing that is just directly responsible for creating this wormhole as i said you did a study theoretical study of course where you were kind of snipping the threads of quantum entanglement and recognizing its impact on the connectivity of space and that to me really brings it home so you can describe how that goes yeah yeah so i was wondering you know is this something that is specific to this black hole space time that's kind of an exotic thing having two universes connected by a wormhole so is the is it that entanglement makes these exotic wormholes or is it just the more basic thing that entanglement is the thing that is responsible for any space being connected with any other space and so what i thought to do was just look at a simpler example where you go to the case where you just have one of these quantum mechanical systems describing a completely boring empty space time so just like we were talking about with the near the black hole horizon you have an ordinary kind of empty patch of space there's lots of entanglement between the fields here and the fields here so that's the same in these holographic quantum mechanical systems they have lots of entanglement between the left side of this ball and the right side of the ball and so i wondered what would happen if you envisioned altering the state altering this state you'd have to add energy but to you add energy in such a way that it removes entanglement between two sides you can kind of follow it through and ask what happens to the space time and it seemed that as you removed entanglement between one side and the other then what happens to the space-time is it kind of pulls itself apart like like pulling a piece of gum apart it gets you know thinner the connection and eventually would snap and be basically just two disconnected pieces once you've removed the entanglement from the underlying quantum system and then you can just keep doing that and now you'd have two pieces now you could divide up this ball into more pieces remove the entanglement between those further pieces and if you just kept going and going it seems that on the gravity side you really have nothing left at all and so when you say there's no when you say there's nothing there at all i gather you mean that in some sense spacers keeps cutting up into smaller and smaller kind of pixelates to the point where no connectivity is left and it's just gone yeah that's that's right so you just have you know we think of maybe space as being the background for everything else but with einstein space became a dynamical entity and then you could start to ask the question maybe where does it come from is it always there and this picture this holographic picture suggests that even space in order to exist you need quantum entanglement between these underlying physical entities so it's it's kind of bizarre like we went into studying string theory and holography to really to understand quantum gravity and then when you get to this kind of observation it suggests that even if you're trying to understand classical gravity then there's quantum mechanics behind that yeah that takes us to our to the final question i wanted to ask that's a perfect segue in so so on a final question that i'd like everyone to weigh in on in traditional approaches to quantum mechanics the way we often teach it to our students is we have a classical system and then we quantize it we employ certain rules of quantum mechanics to take whatever system that we might be studying using newton's equations and translate it into a system using schrodinger's equation you know the quantum degrees of freedom now when we've tried to do that straightforwardly with gravity we've always run into headaches which is what has motivated approaches that many people follow from loop quantum gravity to string theory and so forth now we're learning that in some sense quantum mechanics knows about space space at quarter einstein is the arena it's warps and curves of space that give rise to gravity so in some sense quantum mechanics seems to know about gravity so is that quality of quantum mechanics that was overlooked for a long time do you think that might have been the reason why we had such trouble using the standard quantization rules and trying to apply them to gravity itself what happened at least to me in the last years and inspiring all the proposals that they have mentioned is that i'm not sure if it's coming from quantum mechanics as a basic setup that has a control over what happens there but what is nowadays clear is that quantum mechanics is giving us the necessary tools to understand this connection between gravity and quantum realm in the sense of this precisely this entanglement so lenny big picture people have been trying to put gravity and quantum mechanics together for a long time you've of course spearheaded the most um insightful approach to that which is string theory does these recent developments do they they change your thinking on the relationship between quantum mechanics and gravity i think what's going on and i very strongly believe what's going on is that quantum mechanics and gravity are simply so deeply connected so intimately connected uh so inseparable that trying to separate them into the classical theory of gravity and quantum mechanics will inevitably lead and then put them back together again separate them into these two things and then put them back together by the procedure of quantization will just be the wrong thing except that quantum mechanics and gravity are almost the same thing or that they're at least so closely linked that you can't pull them apart and you will never be led to the idea of trying to quantize gravity uh so yes my opinion is that that's the direction things are going in and it's why i said in the beginning that perhaps the ultimate mysteries of quantum mechanics will only be revealed when we much better understand the connection with uh with gravity but we'll see yeah absolutely so it's been a fascinating conversation covered a lot of ground motivated by einstein's 1935 papers and it's an exciting time in physics and who knows where it's all going to lead but it's so remarkable to me that we're actually talking about things like what is space made of the kind of question that like as a high school kid many of us worried about and thought about but never thought there'd be any insight so congratulations all the work that you have all spearheaded to push the frontiers forward and thank you so much for spending some time today to talk about it [Music] so [Music] you

Channel: World Science Festival

Views: 264,888

Keywords: Brian Greene, black holes, worm hole, quantum mechanics, string theory, General Theory of Relativity, Albert Einstein, physics, MaxPlanck, Niels Bohr, Erwin Schrödinger, Werner Heisenberg, Max Born, wormhole, Leonard Susskind, Mark Van Raamsdonk, Ana Alonso-Serrano, space-time, spacetime, Do wormholes exist?, What is a wormhole?, What is a black hole?, best science talks, EPR paper, ER paper, singularity, Best science Events, New York City, NYC, Quantum entanglement

Id: ntxC5KMC4y0

Length: 65min 36sec (3936 seconds)

Published: Thu Mar 31 2022

Reddit Comments

[u/birkir]

WSF just posted their latest 'panel discussion', where Brian Greene introduces a topic and moderates a discussion.

These programs tend to be aimed at interested and motivated but unfamiliar audience with no prerequisites. They are always a joy though, even for professional physicists, since the guests tend to be renowned and the visuals stunning.

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To discuss, ultimately, the topic of ER=EPR, he's joined here by:

• Leonard Susskind, well known comedian to users of this community already,
  
• Ana Alonso-Serrano, of 'The time traveler's guide to the quantization of zero modes' fame, to name one of her many recent hits,
  
• Mark Van Raamsdonk, author of the greatest title for a PhD thesis in string theory: 'Making the most of zero branes and a weak background'

[u/birkir]

Susskind:

At some point Juan sent me a very cryptic message, and in the cryptic message he simply wrote down:

ER = EPR

I knew instantly what he was talking about. That... that just was an explosion in my brain, when he said it.

Now what did it mean?

ER stood for Einstein, Rosen.
EPR stood for Einstein, Podolski & Rosen.

Remember what the two of them are?

ER: Wormholes
EPR: Entanglement

What Juan was saying was that if there's entanglement, then there must be wormholes, connecting the regions what are entangled.

If you took the whole thing together, what it seemed to say was that there must be wormholes, or a wormhole, connecting the distant radiation to the interior of the black hole. They're not – as measured through the wormhole – they're really not very far away. So that what we see as radiation very, very far from the black hole is connected through through a hidden wormhole, to things just behind the horizon of the black hole.

That's what Juan meant when he wrote down ER = EPR: That if there's entanglement there must be wormholes. There must be spatial connectivity.

That in fact did turn out to be the soluti– as we believe, that is the solution of the AMPS paradox. That the entanglement between the exterior and the interior is not broken, it just reappears as the entanglement between the exterior of the black hole and the Hawking radiation.

Brian Greene:

— Now, Einstein wrote those two papers, like, two months apart. Do you think that he had any inkling that there is any connection between them?

Susskind:

— No. I don't think so. There's no evidence whatsoever, and... no, I– I do not think so.