Brian Greene and Leonard Susskind: World Science U Q+A Session

Hello everyone, here we are again for another of our live conversations and these are live conversations. It always amuses me when I see in the chat people say: "this live is this live, yes, it is live if you are watching it live, right? I'm not watching it live, it's not live and I don't I know, maybe just to establish that fact. It's always good to answer in real time. There's a question here from someone in Fiji. I don't know where the question was, I should say it has to do with black holes and the. string theory.

If you want to ask that question again, our participant from Fiji I will try to see it, but the main event today, as everyone knows, is a conversation with Lenny Suskind who is really one of the great theoretical physicists of our time, so he's an exciting guest on the show, as many of you know, lenny has had a profound impact on string theory, black hole physics, quantum mechanics, elementary particle physics, I mean, it's quite a lot of achievements and obviously we will only talk about some of that knowledge, but I think we will probably lean a lot towards black holes.

You know, when you get to the end of a year, there's a tendency to think about the good things that the bad things sum up, you know all these best of lists, worst of all lists, things like that if I were to focus on the best of physics or let's say theoretical physics in 2020, even maybe even before, black holes would be near the top, maybe at the top. You know, we've had a great run and it continues forward and today we're going to discuss some of the riddles that remain, but some of the deep riddles of the past that today are no longer so baffling, that's not it. everything has been resolved, it's rare that deep puzzles are completely resolved, but there are a lot of things that weren't very clear even 15 20 25 years ago that have now really been unraveled and lenny and a group of other people who no longer will. doubt, reference the critical for that to happen, so lenny will join us around 130 or so.

I'll ask a little bit of questions, maybe just a couple of questions if there's anything to get us going, and then maybe I'll give an i. I don't know a little bit of background on the conversation we're going to have with lenny, so here's a question from captain vyom about how dark energy and dark matter are related. Captain vyam asks and I like questions like that because the answer is short basically. I don't know and no one knows how these dark things are related, they may be related, they may not be related, remember dark matter is this idea of ​​matter that is out there, we believe in space and we come to that conclusion because when We take into account the gravity that non-dark matter can exert, which means that the matter we can see emits light reflects light.

Matter of that type. The amount of gravity that such matter can exert is simply not enough to explain the movements we see. through astrophysical measurements, astrophysical data, right, I mean the analogy that I would like to use. I think it's pretty good. If you have a bicycle wheel that is wet while spinning, you know that water droplets fly away when the wheel spins in a similar way in galaxies. that are spinning, if they spin at a fast enough speed, the stars should be thrown outwards and we see galaxies in which the stars should be thrown outwards, but they are not, which must mean that there is something else out there that keeps those stars inside those galaxies, the belief is that there is additional matter beyond the matter we can see with our telescopes and that dark matter is responsible for the gravitational attraction that prevents those stars from flying outward like water droplets. .

Well, that's dark matter. I must say that there is a lot of material. dark matter we think that when you do these calculations there is on the order of four or five times more dark matter than ordinary matter, you know, the stuff that we are made of dark energy is a different beast dark energy the most compelling evidence for Are these observations that we have discussed in this series from time to time about the accelerated expansion of space, space is not only getting bigger over time, which was a shock, right, that was the shock that was initially actually confirmed through observations by Edwin Hubble. but not only is the universe getting bigger, it's getting bigger at an accelerating rate, so the expansion is accelerating, how can the expansion accelerate the right galaxies?

They attract each other with the force of gravity, the force of gravity that we generally think of as pushing things inward, but still something is pushing outward and the remarkable thing is that in Albert Einstein's general theory of relativity the gravity can actually be repulsive it can push outward not if the source of that gravity is a group like a star a group like a galaxy a group like a planet rather if there is a diffuse energy that is distributed uniformly throughout a region from space then under modest assumptions it will give rise to a repulsive push, an outward push that can drive the expansion of space to accelerate and because this energy does not emit light we call it dark energy so there are two dark things dark matter dark energy and again I gave you the amount of dark matter dark energies in terms of the energy mass budget of the universe is even more substantial on the order of 70 percent of the mass energy of the universe is this dark energy, so now These two components are related.

Many people have written theories suggesting that neither of these have really gained consensus from the scientific community so far, but who knows, they both have dark energy. in his name is the end of the connection or is it deeper, I don't know either way, so that's a good question for us to pursue here. Mark Kennedy also has How Much Do You Miss Mom Joys? I do not know what. That means, but she sounds somewhere deep in my childhood or something. I know what you're talking about, Mark, I think or maybe I don't, maybe it's something I don't want to remember.

I don't know, but I can anyway. I don't answer the question, I don't remember exactly what one more question means before moving on to some context. Physics always asks what strings are made of. We'll talk a little bit about string theory here today, no doubt. Lenny Suskind, founding father, pioneer of string theory, I'm sure that will come up in our conversation and we'll see it when you think about any proposition about what things are made of and you think about the history of ideas, any proposition about what things are done. it seems that ultimately it has finer things inside it, right molecules made of atoms, atoms, yes, nucleus with electrons in these fuzzy quantum orbits, the nucleus has finer things, protons and neutrons inside protons and neutrons, smaller things called quarks, if string theory is correct, big for us.

We'll talk about that, so within these particles there are these little vibrating filaments, these string-like filaments of energy, so if that's the right picture, the natural question is: at the end of the line have we reached the smallest of matroyska dolls? pronouncing that correctly, no doubt someone will correct me, you know, Russian dolls each have a smaller doll inside them, do ropes also have a smaller entity inside them? I don't know, it could be a rope to the end of the line. that there are finer ingredients in some sense, the force even suggests that possibility with things called d0 particles at some level, maybe we'll talk about that a little bit too, but it's a natural question and we don't fully know the answer.

They're two questions that we don't know the answer to, maybe there's a question here that I can actually answer, that would be nice, give me a little push to move forward, um, yeah, another question about string theory. I just found uh uh tishia patel can we describe that the fabric of spacetime is made of particles? I seem to be drawn to questions that I can't answer here this afternoon and in a sense, yes, within the framework of quantum mechanics, forces that influence how things move, the electromagnetic force and the nuclear forces, they are all part of the standard model of particle physics that predicts that those forces are actually communicated by a particle, just for the electromagnetic force, photons, nuclear forces, w and z bosons, etc. now gravity is a force of nature and Einstein tells us that gravity is associated with space-time, the geometric structure of space-time determines the gravitational influence that a body moving through that region of space-time will experience now if you believe like most of If we make that quantum mechanics and general relativity have to come together, then the paradigm of a force communicated by a particle will also be extended to gravitational influence and that gives rise to this notion of gravitons.

No one has ever seen a graviton. It doesn't surprise me that it's the smallest package of nature's weakest forces, so it's not like we expected to see it light up our detectors and it's not a mystery, but if this idea is true, then you can really think about tissue. of space, which is the medium of gravity, as if it were a huge collection of gravitons arranged in a coherent pattern that manifests on a large scale like the geometry of space-time, but on small scales it would have this distinctive quantum mechanical particle. description, so I can't say the answer is absolutely yes, but there is certainly a natural intellectual path that would lead us to that image, okay, well, I'll be happy to answer questions again as they arise and as in the conversation with Lenny I will try to look and sometimes it is difficult some of you have asked me why, for example, in the conversation with Roger Penrose that we had last week why I did not resort to some of the questions that you asked and, honestly, I understand Sometimes getting carried away Because of the conversation with the person I'm talking to, breaking away to try to go through the list of questions can be difficult.

We have someone looking at those questions right now, so if you ask something that's actually relevant, it's very relevant. Wherever we are in the conversation with Lenny, I'm more than happy to try to include your questions in the conversation, so I'm absolutely open to that, without a doubt, okay, so a little background, Lenny will join we. about 10 12 minutes, so look, now we are all familiar that black holes are a quality of reality confirmed by observation. Just with the event horizon telescope we have a small image of the black holes of them, of course, we can show it on the screen where you know. the most direct way to see a black hole, I mean, a black hole is nothing more than a region in space where light cannot leave and the telescope images of the event horizon are the ones that make you know the most direct confirmation of that idea.

I see it on my screen here, I hope you guys see it on yours, which would be nice. I think they know it now. Black holes really came out of Carl Schwarzschild's mathematics back in 1916, 1917 or so and it's an idea that you know is interesting. It's exciting, but it became a physics backwater for a long period of time. It's actually John Wheeler who I think we have a little image of John Wheeler. It's good to see that it was John Wheeler who brought the study of Einstein's general theory. relativity and the puzzles of black hole physics, he really brought it into mainstream physics research, so he's really responsible for that, and John Wheeler had a very particular puzzle about black holes that we're going to talk about. with Lenny Suskind Wheeler, concerned because a very gentle man was concerned that if you were drinking a cup of hot tea, well, there's a lot of entropy in a cup of hot tea, the disorder of the water molecules bouncing back and forth in that cup of tea.

He was worried that if there was a black hole nearby he would take his cup of tea with the hot water with all that entropy and just throw it into the black hole and then he was worried that if the cup of tea and the entropy went into the black hole, since black holes don't let anything out, the entropy in some sense would disappear and that worried him because the second law of thermodynamics tells us that entropy should always increase and Wheeler said it looks like I can use a black hole to thwart the second. law of thermodynamics I keep throwing entropy into a black hole, it no longer exists and that way the entropy in the rest of the universe (the observable universe may be the only part that we observers have access to) would decrease, so he mentioned this puzzle to your student. he named jacob beckenstein a brilliant student and beckenstein came up with an idea to solve this puzzle and the idea is that black holes have entropy black holes have entropy, they are not theHe had the same views as me.

He was a great physicist and I saw him as someone extremely profound and the fact that he had more or less the same opinions as me was comforting and hence the image that you began to develop and that has now been raised to a higher level. principle holographic principle can you tell us a little bit about what led to this perspective that information somehow exists on the surface of a black hole when an entity falls? Yeah, sure, I was fine, it's literally true, I've been thinking. A lot about information falling into a black hole and for me the big puzzle was that it seemed like the information had to be in two places at the same time if you were to take the view that Gerard and I made that information. falls into a black hole, it eventually radiates back in Hawking radiation and at the same time the information can fall through the horizon of the black hole, then it seemed that something was happening that said that the information may somehow be in two places at the same time. at the same time or else in two places at the same time, very similar to quantum complementarity, that you either look at it one way or another, but you don't try, you don't try to see it in both ways. someone falling into a black hole sees the information entering the black hole someone outside sees it outside something funny is happening with the location of the information was literally true.

I've told the story several times, I think I even wrote about it. and people tend not to believe me, they think I'm making up a little story, it wasn't a story, I was walking at Stanford, also at Stanford, at that time there was a little

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exhibit, little physics, we called it a museum and and on the screen There was a hologram There was a hologram of a very, very pretty girl and there was the image of the girl It was surrounded by a film and I knew what a hologram was but I realized that there is an example of The information that is stored in the film Strictly speaking, it is stored on the two-dimensional film and if you were to watch the film you would just look at a bun with a little attention through a microscope, all you would see would be random marks with no particular structure that you would recognize and if you did the right thing with that film mainly lighting it but it doesn't make the translation dictionary right, you would reconstruct a complete three-dimensional object inside this space that was surrounded by the film, it was literally True, I looked at that and said, could it be that somehow quantum mechanics the surface of a black hole, the horizon of the black hole, is functioning as a kind of hologram where you can think that the information is stored at the edge of the The horizon itself is like the movie itself and at the same time a translation dictionary would allow you to think of that as information that fell into the black hole that that was sometime around 1993.

Gerard was thinking, I guess it's something very, very similar and uh. published, he published the idea, I didn't even know he had published it, I discovered it after writing my own article about it, someone, someone told me that he had written an article and I couldn't find the paper, but I talked to Gerard and He told me what he was thinking. It was very similar, so I just attributed it to him. The basic idea, but the holographic way of thinking about it was your picture of how to explain what the black hole was doing at that moment. presumably yes, yes, but there was a generalization that I think we both thought of, but gerard actually put it very clearly that it wasn't just a black hole if you take a region of space no matter what's in it, whether it's a hole black hole or anything else you should be simply because a black hole carries more information than anything else, that a region of space cannot have more information than what can be described as being on the surface of the region of space instead of in volume and so it became more than just the theory of black holes, it became a theory that if you like a piece of the universe, if you take a piece of the universe, it should be possible to describe it in terms of degrees of mathematical freedom. structures that in a sense live on the surface of the region of space so true, I mean, that's shocking, it seems very shocking, okay, I called my article the

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as a hologram, I don't remember what Azeroth called his, yes, not only was it. shocking, he was also thought to be a big nut, yes, not so bad, yes, when I talked about it, I had the feeling that I was a little amused that people had thought that both Gerard and I had lost our minds, some Once we were good physicists. but here we were a little crazy, but it would be nice to explain to people how crazy it is, I mean, we are used to physics, you know, things happen in the volume of space, you can play with it, you can play there, but in a sense you're saying that's illusory because the real fundamental degrees of freedom aren't as crowded, they're not as numerous and they're not as numerous and if you're right and if you try to excite more than a certain amount of structure in that regional space, you will inevitably create a black hole that is larger than that region and therefore there is really a limitation as to how many bits of information you can put into a region of space and it is not the volume that is the area so that It is something well accepted, the first person who really got excited about this apart from Gerard and me and whoever called me wanted to know more.

Class asked me to please come to your institution and talk about it. In fact, it wouldn't be like that, Edwin, yes, it would be He was very enthusiastic about him, right, yes, it's not his style, it's not his style of doing things, but you know he's extraordinarily intelligent and he's a great physicist and he saw, he saw that, yes, but maybe that's how it was. right, yeah, I mean, in a sense, this is perhaps one of the most radical breaks with human intuition that have developed over the course of we're talking about evolution before certainly evolution imprinted on us a

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view, yeah, look at the world, it's funny. so it's so widely accepted now that no one seems to really understand why or why it was considered so radical, well that's always what happened with the young people here, yeah, you know, so things go from wild guess to speculation crazy, to a guess to a tool. of the trade in almost no time and of course you of course know why it suddenly became a tool of the trade, it was the work of one of the higher ups, yeah, when we get to it, then, then, it's saying that even in this room in which I am sitting or in any room in which someone else is sitting, there is somehow an illusion: the illusion that we have the freedom to change things within this volume at will is wrong, the real capacity for change is much less. and it can be completely described by data that lives on a surface surrounding that region, yes that's what it says, yes, so it's understandable, look, I mean obviously you always inspire great respect, but I can imagine people listening to that and saying this is going to happen. one step too far and it just doesn't seem like it could be right, yeah, and then when you say, sorry, go ahead, well, I've always admired the great detective Sherlock, one of his own, uh, I can't, I never quite can .

Remember the quote, but when you know what it was like when you tried everything, and everything doesn't work, which is, it may seem very unlikely, but if it's all that's left, it has to be the truth, remember the quote, it's a sherlock holmes quote uh yes, i mean justification, we've tried everything and nothing works uh, nothing sensible works, whatever remains must be the truth, no matter how crazy it is, yes, i have little doubt that anyone In the chat he will give it to us. I quote in just a moment, uh, but then, for the black hole story, then you know, if I took something out and threw it into a black hole, how would this description help us deal with the paradox, the information problem? the information was fine, so the way I thought about it at the time, which may not have been the case, I'm not sure that's exactly the right way to think about it the way I thought about it at the time, the problem was that the information falling into the black hole cannot escape because it cannot exceed the speed of light, on the other hand, Hawking radiation must be carrying the information and how it escaped from the black hole, well the intuition was that it really was like a hologram that the things inside were to some extent illusory as you say, I don't know if illusory is the correct term but to some extent it is a consequence of the mathematics of the hologram and that reality is actually stored on the surface simply over the horizon of the black hole and that eventually the evaporation of the black hole was equivalent to the evaporation of this film of the holographic film itself that would contain all the information about what was inside the hologram, but when it evaporated there is no interior there is nothing left because there is no image left inside because the hologram that in a sense was reality has evaporated but the information was not destroyed, it was carried away by the products of evaporation, so it was a bit naive, but that's the way in what I thought about it at the time and some of the things you had to deal with to make it less naive, there was an issue that I think you and others raised that you know you should include an encyclopedia if it works. and yet it also leaves its information on the surface, it seems that you have these two copies of the nation and there is this famous non-cloning theorem in quantum mechanics that tells you that you can't actually create those kinds of copies, yes, that's right, That's how it was. uh, that was true, the other person who thought a lot about that was um uh, john pressgill and john preston taught me classical mechanics when I was a student, you know, yeah, sure, so yeah, there was this problem of the replication of information that in quantum mechanics the replication of quantum information is not allowed and therefore when something falls into a black hole according to the usual rules it cannot be replicated in two places at the same time, my response at that moment was to think In several of Duncan's experiments, I became convinced, I think correctly, that it was absolutely impossible for any real observation or experiment to see both copies at the same time, so it was very much like ordinary quantum mechanics, where you can describe a particle as if had a speed. or impulse or you can describe it as having one position, but you can't describe it as having both at the same time.

I called it black hole quantum complementarity. It was fine to have this duplication of information as long as nature itself prevented any observations. By seeing both replicas of the information and, through a series of arguments, I was able to convince myself and other people that it was impossible to have knowledge of both what falls into and what comes out if you are outside the black hole. look what happens if you fall with the filling material, you see it fall, you can't be inside and outside at the same time, so everything is fine, I think it more or less worked that way, so we carry that idea of ​​complementarity one more step.

Go ahead, the next question arises, so many people who study black holes know that if you are looking at an object falling from the outside, a curious thing happens: you see the object getting closer and closer to the horizon, but in reality it doesn't. You see it. when you fall over the edge, you see it redshifted, you see it time dilated, etc., but you never actually see it crossed, so in a sense it is stuck on the outside, yes, in retrospect, that would have taken one to think about the holographic idea, I mean, it was that part of your thinking that got you there absolutely absolutely that uh that was very much part of the thinking um I knew it actually I think reading Kip Thorne's book on the part of the membrane kip thorne and other people's book on the membrane paradigm for black holes yes membrane paradigm this was a technical book this was not a highly technical book more technical than I wanted it to be I knew about this image that everything that falls on a black hole forms a kind of sedimentary structure that gradually sinks towards the horizon but never passes it at least that is the image that someone looking from the outside says and then from that image again there was a kind of moment from sherlock holmes it has to be like this although in a falling frame of reference with the material falling into it appearing to pass through the horizon, it must still be coherent to think of it from the outside as if it had been plastered on the inside, say to a considerable distance from the horizon, so yes, that was an absolutely important part and yet what you mentioned there, which is one of the things that makes it difficult for at least one general person to fully understand this for the Freely falling individual passes by the horizon as if there is nothing there, there is no impact, but From the outside perspective, there is a lot happening on the horizon because everything you say gets stuck there and ultimately, It is emitted through Hawking radiation.

ThatWe can remove it from the post if you want to try it, okay, that's what I was going to say, it was like Trump, uh, Trump refused to admit that he lost the election. Hey, no regrets, but of course, there was no similarity between Stephen Walking and the dominants, oh yeah, Stephen Hawking were on, yeah, I see the analogy would have some problems, yeah, yeah, that has some problems, but no I don't care if you say it, but let me make it perfectly clear and simple, Stephen was a hero and a hero and a hero and that's all, but uh yeah, there was, I had that feeling that people were fighting an agonizing battle that uh , there was no point in fighting it anymore, it ended well, um, so within a period of time that The topic was put aside, um, but still, but it's not that that story is over, I mean, once that you know that the information comes out of a black hole by virtue of translating the question into this holographic question where the answer is.

Clearly there is no loss of information, there is still the question of the details of how the information comes out and, for example, Joe Polczynski, who you mentioned before, along with three collaborators, writes an article that generates some tension and some questions at the same time. trying to think about how the information coming out suggests the possibility of there being a firewall at the edge of a black hole and so on, it wasn't a surprising article, it was that one of those articles was very and for a while I wasn't sure if They were right. or not, but it was very disconcerting what, yeah, well, joe don maroff and two students at the time, one, i met mary and the other, james sully, wrote an article saying that it was logically impossible that after a certain period of an evaporating black hole, since the oil is evaporating after having evaporated half of its energy, but half of its entropy, that it was simply logically impossible for the black hole to have a smooth horizon, a horizon that you could fall through, in other words, all the characteristics that we thought were true for the horizons of black holes, they were true and they even said that it was logically impossible for this idea of ​​complementarity of black holes to be correct, right , the argument was very convincing, it made use of quantum entanglement, it was a simple argument.

He said on the one hand that the consistency of the horizon required that the interior of the black hole and the exterior of the black hole be quantum mechanically entangled. On the other hand, he said that the radiation that is carried by hawking radiation will necessarily become entangled with a black hole and the third element is a thing that cannot be entangled simultaneously with at most two other things, you cannot be right, you cannot have the outside of the black hole entangled with the radiation and at the same time with the inside of the black hole and then something has to give and his conclusion was that what gave was that the horizon of the black hole has to become an impenetrable barrier so that, in effect, the black hole does not have an interior um this Did you lose sleep over that?

I mean, yeah, I totally think I lost sleep over that. um, yeah, very much so, and in my opinion at first I was convinced that they might be right, uh, that the old black holes would have this problem if I did. I don't believe it, after a while I just thought no, this is a lot like street vending, this is a great question, it's a deep question, very, very fundamental, I suspect that they have taken the easy answer and that the answer difficult is the one that will allow these two things to coexist at the same time the softness of the horizon and the intertwined structure so uh and the answer to that I think is the answer that what is now called e r is equal to epr juan and I started to discuss This email is the only one.

The only time I have successfully collaborated via email was with Maldasena. I just messaged him at some point and said, This is crazy. There is something wrong with us. Can we try to figure out what's wrong with it? and we bounced ideas back and forth, oh, for at least a month, maybe for over a month, daily emails, I could never get him to respond on Sundays, but he was with his family on Sundays, but for six days a week, we exchange ideas and forward ideas um and we have both felt disturbed by one thing in particular that there was a particular construction it was one of Juan Maldasena's first constructions it was called the eternal black hole which in reality were two black holes two entangled black holes and seemed to violate what Joe and company had said that the black hole on one side was completely entangled with a black hole on the other side and yet it didn't seem to have the problem that they claimed would be there if the hole black was intertwined with anything else, and so we had both been worried about that and at some point Juan sent me a very cryptic message it was about the things we were talking about but I think he had one it was a very short message and it said er es equal to epr and when I saw that I said holy smoke, that's what's going on and I'm sure you want to get to that so I'll let you ask whatever questions you want to ask, you know, it's a beautiful and profound result uh. maybe just for the audience we have a little image from the epr article that we can show just to show people that it is 1935 einstein padolsky and rosen write this article about quantum entanglement that distant particles can be connected in some way by some threads of quantum entanglement and then what you're referring to just for the audience is another article that einstein writes with rosen podolski was expelled or something like that, I don't know where podolski was and that one that had to do with wormholes now Now I think it's true no one thought Einstein existed, they didn't think there was any connection between these two 1935 articles, they were just different, yeah, I don't think he had a clue, if he did he was even smarter than Einstein, yeah.

Yes, because once in the realm of quantum mechanics the others in the realm of general relativity um and then, in essence, what the email that you're referring to that Juan sent you said, wait in these two articles, there's a deep connection between good. they're the same thing, the same thing, yeah, talking about that, I mean, it wasn't like we weren't with it, we hadn't been, yeah, talking about very related things, huh, but you said this so succinctly when I saw it. that what I said, that has to be true, yeah, that has to be true, so I quickly put together the story and I wrote it, I rewrote it, I rewrote it and um and uh there's this, but this was another very crazy idea that um another with another Sherlock Holmes story that has to be true because nothing else can be true, then the idea that you know entanglement is a connection between potentially distant systems that, um, has some quantum mechanical connectivity with these systems.

Wormholes are a connection between potentially distant systems. systems, they can have wormholes connecting them, distant black holes, we looked at all the properties of entanglement, in particular, for example, the idea that you can't send a message from one place to another using entanglement, even though you know that people always think you can send it. messages yes, faster than the lighter, you can't, you can't exceed the speed of light and if you look at it from the point of view of entanglement, there is a reason why and wormholes are something similar to wormholes. worm connecting two distant places, you would think. you could send a signal in both cases you will be frustrated by different things, in one case it is the properties of quantum mechanics and in the other the properties of general relativity and a whole set of things about entanglement seem to parallel the things about holes worm John's message was just to close the story and the phrase with the intelligent er equals cpr uh because he had been thinking about these things for a long time this was not totally new for him, I realized very suddenly, I want say, very quickly I knew that this was the answer to the firewall paradox amplifiers and um, so you can play us how it responds.

I mean, yeah, yeah, yeah, yeah, what's behind it? How do you describe the things that are behind the horizon? a black hole just behind the horizon of a black hole, well, whatever they are, they're things that are quantum mechanically entangled with the degrees of freedom just in front of the horizon, uh, you know the story, the story is, you understand that The evaporation of black holes is when pairs of particles are produced, one foot, one of negative energy falls into the black hole, one of positive energy flies out and forms falconry radiation and those two particles become entangled, so it is important that the interior and the outside of the black hole being entangled on the other hand after the radiation has been completely removed, half of the radiation has come out, the black hole is entangled with its own radiation and presumably it can't be entangled with both, okay, like this Let's see where the good resolution of the The paradox is that the things behind the horizon of the black hole are actually the same as the things in the outgoing radiation that was just behind the horizon of the black hole is nothing more than or the information content is the same as the information content of the outgoing black hole. radiation well how can that be?

The outgoing radiation is very far away. The interior of the black hole is close. How can they be the same? And the answer is that they can only be the same if there is a secret passageway between the distant talking radiation. and the black hole and that is what we call the Einstein-Rosen bridge, if the outgoing radiation could be considered as another system connected by entanglement to the black hole then there could be this type of tunnel or wormhole between them so that the outgoing radiation basically, in a sense, also behind the horizon of the black hole, and that's, I can't do it, I can't do a decent job on this without a piece of paper, a pencil and some equations, sure, but that's the way which was the thing and um it was quite convincing for people er equals epr it was accepted quite quickly by this community, but in the very recent past um a whole gang of very young people, well, mary himself, who was ahmed, was a of the people in the role of the firewall, jeff pennington, other people very, very young, not much more than students, in fact, jeff was a student, he really hit the nail on the head with the idea that distant outgoing Hawking radiation is actually the same as the inside of the black. hole they've made for that idea what Maldasena himself did for the holographic idea he nailed it into place with a very, very sharp new version that you know is too hard to explain, yes, no, it's beautiful work, it's complex . but just for a general idea, you know, when we think about the other forces, not gravity, not nuclear forces, not electromagnetic force, of course, we have a well-defined quantification procedure by which we take the classical articulation of these forces and we take them to the quantum world, the right gravity.

It seems like I mean that via er equals epr in some sense the claim is that gravity knows classical gravity knows about quantum mechanics at some level it's somehow different in that sense so what's the lesson What is there is that gravity is smarter than we thought it was and that already and or quantum mechanics or quantum mechanics are smaller than the other way around, yes, that's true, um, yes, no, I make this point repeatedly that, as I sometimes call it, gr equals qm, I have memory of this kind of uh, these little ones are the same, yeah.

Yeah, what, yeah, okay, so you asked me about gravity quantification. Let me explain to you what that means. As you said, there is a set of rules for taking classical systems, a harmonic oscillator, an atom or an orbiting system, and converting it to quantum mechanics. the rules were mainly invented by dirac in 1990 around 1930 and we have learned that that is the way you take a theory and turn it into a quantum theory they work for electrodynamics they work for Yang Mills theory they work for the standard model and so on and so forth, it even works for string theory, eh, no, not in the same way, it never worked for gravity, every time someone tried it, it always led to disaster, various types of disasters, black holes were one of them infinite or another, it never worked and I think what we are learning now is that gravity and quantum mechanics are too closely connected to separate them and then put them back together by quantity, by quantization, by the rules of quantization, um, that's just not that it's not like that.

They are inconsistent with each other They are too close They are too equal to separate them and then quantify what you have separated and go back so that er equals epr as an example, now there are a lot of examples of basic quantum phenomena that are very generic quantum phenomena that reappear in other types through the holographic principle that appear reappear in other types as gravitational phenomena. I will not analyze them,but they're like er equals epr two, a basic quantum mechanics thing on the one hand. a basic gravitational thing on the other side and finding out that in some sense they are equivalent or very parallel to each other, so I don't think we've put it all together yet, I don't think we have a complete story about The way these things fit together is becoming more complete, but we don't have a complete story, but I think the evidence leans in the direction that gravity is not something to quantify, it's already quantified, it just doesn't. quantum mechanics the two go together hand and fist uh and they're too close to separate them that's my opinion that's my opinion um what do you think it means for the future of spacetime?

I mean, space-time is always something that we sort of put from the beginning of our theories, is this the case, since many of us have suspected that space-time is simply something emergent that arises in certain environments, but isn't it a fundamental ingredient from which to build your theory? I think that might be that might be what's telling us a bunch of quantum mechanical degrees of freedom where the right kind of entanglement structure can emerge as a space-time and, uh, just a bunch of qubits, you know. , bits of quantum mechanics linked together by the right kind of entanglement.

The quantum entanglement structure may have the properties of space-time. That's a direction that seems like a possible direction, but you know, I've learned not to try to predict too much into the future. Surprises happen, it is the rule that surprises happen. The surprise is that it's not surprising that surprises happen, it's just that whenever they happen they are very surprising. Now it would be very surprising if there were no surprises. Yes, generally, when you have a good set of ideas, surprises don't reverse direction. that you're going in, but they can make it bend at an angle, yeah, so I learned not to try to get ahead of myself and try to guess where things are going because usually I'll be 50 right and 50 wrong and I can't guess which part is right and which is incorrect.

You know, let me ask you one last thing. You've been very generous with your time, Lenny, I appreciate it. They often ask me where I am. about what you anticipated in the 1980s, are we further along than you thought we would be? Aren't we that far ahead? Can you rate how well we've done as a community if you look back 30 or 40 years? What is your opinion? Well, these things that are just one step ahead of the monkeys have followed the dots in a way I couldn't have imagined. It has been a collective effort. There have been some people who have really stood out.

I think I won't mention names at this time. Some people have really stood out, but it's been a collective effort and I'm impressed and surprised by how far we've come in understanding these questions. I did not do it. I didn't, I also thought like Feynman that it would be 500 years before we really had answers. Yeah, 500 years turned out to be 25 30, so something like that. Yes, it is spectacular. I find it very spectacular. I said it in my mind, it means that the heroic age of physics, uh, hasn't disappeared, yes, yes, I totally agree, so again thank you very much for joining us and by the way, if you ever like kill the beaver or do you want to come to the beaver first.

I'll remember, revisit your fly fishing days, uh, no, I don't have my fly rod anymore, yeah, I'm sure we can, I'm sure we can fix something, anything, have you ever tried it? I have never tried. fly fishing you know I'm vegan so somehow there's a part of me that doesn't want to do that can you catch it and release it yeah but you might think you can just take the hook off the fly yeah and just cast the flies, that's very relaxing, hey, that's it, grab your boots and go out and kill paper and uh, just remove the fly hook, you have to know that it's a little cold right now, as you can see outside , we have about a meter of snow behind me, oh, beautiful, beautiful. but when it's hot yeah you have to wait until you can't get a license until you know, fishing license until about April I forget oh I didn't know okay again obviously I'm an amateur yeah , they arrest you.

It's true? Well, it's ok. I will take all that into account with the advice as well. So thank you and I hope to see you at some point. This endemic thing is over, but thanks for joining us here, everyone, thank you very much, bye, bye, okay, everyone, it was a very fun conversation with Lenny Susskind, we covered a lot of ground , it's three o'clock. I'm pretty tired guys, let me see if there are any questions in the chat that I can address before we end the day here, yeah, a lot of people are thanking Lenny for joining us and I thank you for thanking him, let us.

Look, any final questions before we finish. Someone asked: can we cite the work that was mentioned? There isn't one particular article that really sums up everything we talked about. Certainly if you want to know the history of the Black Hole War up until the early 2000s. I know maybe 2005 2006 you should check out Lenny's book. The black hole war. You know you should buy copies for all your friends. It's Christmas. It is a great gift. It is a wonderful read. the journey in much more detail than we took here today of the information paradox and the realizations of hawking and the ideas of beckenstein and, ultimately, through the holographic ideas of suskind and a tuft, as well as john maul , the incredible dinner breakthrough in the mid-1990s, where string theory gave as lenny described this concrete realization, this concrete instantiation of the holographic idea, so it's a great resource to take a look at and beyond that, It's mainly research literature, there's not much else to look at. the moment someone asks, uh, simon anthony asks, would Roger Penrose agree with what we discussed here today?

And part of it, I would certainly agree with some of that, I think I would be quite skeptical, as you heard, you know, I think Penrose has a visceral reaction against the ideas of string theory and just thinks that they're going in the wrong direction, so which I suppose would consider any knowledge that emerges from the tools that have been developed by string theory. I would be suspicious of those conclusions, but you know the point that Lenny is. It's a point that I've made a lot too, but it's great to hear it from different people, even if string theory is not the correct theory of the world, the fact that We were able to write a mathematical system that brings together gravity and quantum mechanics in a systematic, coherent, logical, sensible and paradox-free way, that is powerful because if, for example, Hawking was right in his initial view, i.e.

Hawking changed his mind towards the end of his life and actually came on board and took this on. perspective that information is not lost, so the analogy and Lenny already rejected it, so this should not be taken personally, but the analogy with Trump resisting the election, uh, Hawking, absolutely came to this perspective, but his previous view that the information would be lost if it were true that the argument he presented was so general that it should apply to any theory that successfully unites gravity and quantum mechanics, therefore it should apply to the string theoretical description of black holes and to the fact that even if String Through is not the correct theory of the world that shows that there is a deep problem, a problem with Hawking's argument and, in fact, there is a problem with Hawking's argument, it is necessary to include all these other features of gravity and quantum mechanics that we discussed. here today that have been developed by many people, the late Joe Polczynski, brilliant physicist who tragically died not long ago, a young, uh, and the great work of Juan Maldisan and many others, including the young physicist that Lenny was mentioning, as well that roger I don't agree, sure, but I don't think you disagree with the logical links as described, whether or not string theory is the correct theory, the world, the logical links are what allow you to conclude that Hawking's initial proposal that the information was lost might not be correct, okay, one more thing before we finish, um no, I think we're good, okay, so again, as you see, we listened to the suggestions that you made, several people had suggested that we talk to uh, lenny susscon, i wanted to do it. that anyway, it's always nice when there are consonants between the direction we're going in this series we've been having and the people you'd like us to bring in, feel free to make more suggestions, we'd love to hear where you are.

I'd like these conversations to address topics, people, and we'll probably do another one of these in about two weeks. We've been getting into that groove once every two weeks, but maybe we'll do one before my teaching at Columbia is over. I just finished, which gives me a little more time to think about things like this. I should also mention that from the epr and er documents that we referenced it turns out that, since it is 1935, we are on the 85th anniversary of both articles and to celebrate that and to celebrate this unexpected link between these ideas, we are going to have a program of world

science

festival that will explore the connection between entanglement and wormholes, I don't know exactly when. that will be available, we are in the process of putting that program together, but just so you know, those of you who want to hear more about this and hear other voices weigh in on the topic, this will be one of our global science festivals. shows in the not-too-distant future, so if you want to know about that, you should follow me on Twitter.

I will announce it as these programs become available. That's on begreen on Twitter. You must follow the world science festival, I don't know what. Your screen looks like it does now, but somewhere on your screen I think there is a button where you can follow the World Science Festival both in terms of YouTube and the social channels of Twitter or Facebook. Well, that wraps it up here for today, thanks for joining us. And I hope to continue these conversations in the not-too-distant future. Okay, Brian Green, signing off. Bye bye.