Leonard Susskind is a professor of theoretical physics at Stanford University and he’s regarded as one of the fathers of string theory.

He’s written several books including: The Black Hole War, The Cosmic Landscape, and the Theoretical Minimum series.

He also has over 100 lectures on YouTube.

---

Topics

0:00 – Being perceived as an outsider physicist

4:00 – The perils of becoming too mainstream

5:45 – Where his ideas come from

7:00 – Claudio asks – Do you think the graviton can be experimentally found?

9:45 – The origins of String Theory

15:15 – Why should there be a grand unified theory?

16:30 – Quantum mechanics and gravity

19:50 – Large unanswered questions in physics

27:30 – Holographic principle

38:00 – Simulation hypothesis

40:15 – Richard Feynman on philosophy

42:00 – Feynman and the bomb

46:00 – Improving the world by discovering what the world is

49:00 – ER and EPR – Black holes and entanglement

56:00 – Noah Hammer asks – Could quantum teleportation be used in the future as a means of intergalactic communication?

58:00 – rokkodigi asks – How do you think quantum theory will shape technology in the future?

1:01:30 – Why teach physics for the public?

---

---

Subscribe

iTunes
Breaker
Google Play
Spotify
Stitcher
SoundCloud
RSS

---

Transcript

Craig Cannon [00:00:00] – Hey, how’s it going? This is Craig Cannon, and you’re listening to Y Combinator’s podcast. Today’s episode is with Leonard Susskind. Leonard’s a professor of theoretical physics at Stanford University. He’s regarded as one of the father’s of string theory. He’s written several books, and if you’re just getting into physics, I’d recommend checking out his Theoretical Minimum series. He also has over 100 lectures on YouTube and I’ll link those up in the show notes. All right, here we go. What I wanted to start with is you’ve often been characterized as someone with non-traditional, kind of out-there, ideas. Some of which have become part of the physics cannon, some of which, who knows what happened?

Leonard Susskind [00:00:39] – They all became part of the physics cannon. Every single one of them. I’ve never made a mistake.

Craig Cannon [00:00:45] – Of course, all right, well thanks for coming on the podcast. You’re the first person who’s never made a mistake. I was curious, who is your, who do you think is your most outlandish friend?

Leonard Susskind [00:00:56] – Wait, come back to your previous question for a moment. I am very mainstream. I am not at all a alternative thinker. This is some misconception, which I don’t know how it happened, but my physics has been extremely mainstream. It may have been that at the beginning of some of the ideas, people were not quite ready, but they very quickly caught on. It’s just not true that I was some kind of alternative, what should we call it? I don’t know what the right word is.

Craig Cannon [00:01:35] – Some kind of radical thinker.

Leonard Susskind [00:01:36] – Not at all, not at all, no. I spent a lot of time thinking about, what shall we call them? Conflicts of principle. Situations where things were not fitting together properly, and thought a lot about them, and eventually came to the conclusion that you had to change things, or that you had to break the mold a little bit. That’s probably where this reputation came from. But these were things that, really there were no alternatives to. What was it that Sherlock Holmes said? Do you remember the quote? “When you’ve tried all possibilities,” and I forget the exact quote, roughly speaking, “When you’ve tried everything and it doesn’t work, whatever remains must be the truth, no matter how outlandish,” or something like that.

Craig Cannon [00:02:31] – A little bit like Occam’s razor.

Leonard Susskind [00:02:33] – Well, a little bit like Occam’s razor, but in particular, when you’ve tried everything and it doesn’t work, there may still be something left that you haven’t tried yet because you thought it was too outlandish. Well, you got to try it. That probably is the source of some of this mythology about me as a radical. But I am the most conservative physicist imaginable.

Craig Cannon [00:03:00] – Okay, so what’s an example then of a friend who is more outlandish, more radical than you?

Leonard Susskind [00:03:05] – Freeman Dyson. I can’t exactly call him a friend, but I know him a little bit. He is, what you might call, a contrarian. He enjoys running against the grain. He sometimes says some brilliant and smart things. Like all contrarians, he’s got a very large probability of being wrong.

Craig Cannon [00:03:32] – He’s willing to.

Leonard Susskind [00:03:33] – My friend, Gerard ‘t Hooft, I don’t know if you know his name. He’s a very famous physicist. He’s also a bit of a contrarian. He’s far more out there than I’ve ever been. Was Dick Feynman a contrarian? No. He was about as mainstream as you can be. But, also, had his own, he had his own very special, scientific personality, and I suspect that’s also true of me. That my way of thinking, my way of doing things, is probably different than most people. It did lead to this contrarian, this view of me as a contrarian, as a radical, but it was absolutely wrong.

Craig Cannon [00:04:19] – Do you see physics kind of birthing more contrarians in the modern paradigm where experiments are so expensive to execute at this point? Or do they have to be more mainstream to get things done?

Leonard Susskind [00:04:35] – Unfortunately, you have to be really mainstream, sometimes I think too much. By mainstream now I mean people are often trained within a framework, which is fairly tight and rigid, and I sometimes think maybe a little more free thinking out there might be more useful. Free thinking, but that doesn’t mean being a contrarian. A contrarian is somebody who is contrary just for the sake of being contrary.

Craig Cannon [00:05:11] – I read your Wikipedia page, saw some interviews with you, and heard about you being a plumber.

Leonard Susskind [00:05:18] – That was true.

Craig Cannon [00:05:18] – Working with your father which was true. Now here we are at Stanford, and you’re kind of like “of the industry.” You’re here.

Leonard Susskind [00:05:27] – It took me a long time to feel part of the, to not feel an outsider. My background was a little bit strange for a, so it took me a long time to not feel like an outsider. Then, all of a sudden, I found out I was the ultimate insider.

Craig Cannon [00:05:43] – And how do you deal with that? It’s hard.

Leonard Susskind [00:05:45] – I just ignore it. I do my thing.

Craig Cannon [00:05:48] – Fair enough.

Leonard Susskind [00:05:50] – I’m interested in the physics problem, I’m not going to let it go, I spend most of my time thinking about it, and I’m not agonizing about other things.

Craig Cannon [00:06:00] – Then where do you go to think of new ideas? Because that’s something that–

Leonard Susskind [00:06:03] – You mean do I go to the bathroom? Or do I take a shower, or what?

Craig Cannon [00:06:06] – Yeah, you take a shower. No, I’m curious where your ideas have come from over the course of your career.

Leonard Susskind [00:06:10] – They almost always came from some sense that things were not fitting together properly. What I call a conflict of principle or a paradox. One of the early things that I worked on was called quark confinement. Why don’t quarks come out of particles and appear in the laboratory? They seem to exist, they seem to be part of the proton, the neutron, and so forth, and they seem to be stuck inside and never come out. That appeared to be a paradox, because from all that we knew about the subject that’s quantum field theory, the subject that governs particle physics, from all that we thought we knew, any kind of particle that exists, should be possible to kick it out and observe it directly in the laboratory. There was a paradox there, they seem to exist, and yet they seem not to exist. Something was wrong. That’s the kind of thing that captures me and gets me going. I don’t want to let it go until I feel I understand it.

Craig Cannon [00:07:21] – Someone from Twitter asked a question related to this. Their name’s Claudio, and they asked, do you think the graviton can be experimentally found? Similar–

Leonard Susskind [00:07:32] – Well, there’s a sense in which it’s already been but this was, I think I know what they mean, but. Gravitons in great, or photons, or some large class of particles, when they’re in sufficient abundance, just behave like wave fields. The electromagnetic field is a collection of photons, but that doesn’t mean that you can detect them as individual photons, easily. Radio waves, for example, would be very, very difficult to detect as individual photons. All right, we’ve seen gravitational waves, that means we’ve seen large numbers of gravitons. I don’t know how many, zillions and zillions and zillions of them. I think what the person was asking about is the possibility of seeing them individually. That seems very, very hard. I don’t see any easy route to that, and, in fact, I guess I don’t see any route to it at all, but, ultimately, I think it’s a technological problem. If you could build an accelerator as big as the galaxy, and so forth, and so on, and harness 100,000 stars to take all of the energy that they produce, and run the accelerator with it, you could make gravitons. It’s a technological problem.

Craig Cannon [00:08:55] – Okay, so it’s a little bit like the space LIGO?

Leonard Susskind [00:08:59] – The space LIGO? Wait, the space LIGO is a trivial technology, what is the space LIGO?

Craig Cannon [00:09:07] – LIGO in space? That’s trivial?

Leonard Susskind [00:09:10] – By comparison!

Craig Cannon [00:09:10] – By comparison, okay.

Leonard Susskind [00:09:11] – No, no, no, no, I don’t want to insult my friends. No, no, no, no. It’s trivial in the sense that in principle, we can do it, we will do it, and it probably doesn’t involve any technological hurdles which are insurmountable. Building a machine that could produce gravitons, at least for the next million years, is going to be insurmountable.

Craig Cannon [00:09:42] – Oh, wow, okay.

Leonard Susskind [00:09:43] – It’s not, I think it’s not going to be done. On the other hand, maybe I’m wrong.

Craig Cannon [00:09:49] – Let’s go to some of your other ideas. You’re credited as one of the creators of string theory.

Leonard Susskind [00:09:55] – Which is extremely mainstream in all of this.

Craig Cannon [00:09:57] – Which is super mainstream.

Leonard Susskind [00:09:58] – But it wasn’t when we started it.

Craig Cannon [00:10:00] – Correct, well that’s where the idea–

Leonard Susskind [00:10:02] – Right, so that’s where the idea of me as a radical came from, but now it’s mainstream.

Craig Cannon [00:10:07] – Where did the idea come from?

Leonard Susskind [00:10:09] – Oh, well, the idea came from asking about the structure of particles which are known as hadrons. These are protons, neutrons, mesons, they’re common things that make up the nucleus. There was a lot of work, experimental as well as theoretical, which showed that these particles were not elementary particles, that they were composites of some sort. You could spin them, you can’t pick a point and spin a point, the point is too small to, what does it mean to rotate a point? Whatever protons and neutrons were, you could spin them up, you could increase their angular momentum, they seemed to be capable of being vibrated and excited in all sorts of ways. There was some mathematical work, it was very mathematical, and didn’t have to do with strings, but which caught some of the properties of these hadrons, and I got interested in it, and just looked at it, looked at some of the formulas, and said, “Oh, those formulas are interesting, I wonder what they mean.” Looked at it a little more and I said, “Oh, there’s something vibrating, there’s some kind of concept of vibration going on.” It was just a matter of thinking about it for a few weeks and saying, “Oh, they’re strings, they’re elastic strings.”

Craig Cannon [00:11:39] – With each of these, were you deeply knowledgeable on the field before or–

Leonard Susskind [00:11:44] – No, no, I was deeply knowledgeable about quantum mechanics. At least, well, was I deeply knowledgeable, even that, I think I was but, yeah, I had a very, very good education, it was self-education about quantum mechanics, about classical mechanics, I did not have much of an education about particle physics. But it was unnecessary, somebody showed me a formula and it was a mathematical formula. I knew what a proton was, I knew what a neutron was, I knew that if collided them stuff came out of them, and I also knew that they had these properties of being capable of being excited and spun up, and so forth. I did know that, but that was easy. I just told you and you now know it too, okay? They showed me a formula, and the mathematical formula had some pieces in it that I recognized, I’d seen it before. I’d seen it in the context of, basically, elementary quantum mechanics. I’d seen it before and I looked at it, and at first I thought, “Oh this thing is just a pair of particles on the ends of a spring.” Meaning to say the mathematics of it was the mathematics of what’s called a harmonic oscillator. I looked at it a little more, and a little more, and a little more, and eventually I realized that the formula was representing the interaction of particles, which themselves were string like. String like meaning elastic threads, lets call them. I worked it out and published it, and that was the story.

Craig Cannon [00:13:28] – In the Cornell lectures form 2014, or something like that.

Leonard Susskind [00:13:33] – Oh the messenger lectures.

Craig Cannon [00:13:34] – The messenger lectures. You kind of offhandedly said that, despite being on of the creators of the string theory, you weren’t the biggest believer in the world right now.

Leonard Susskind [00:13:44] – I probably did say that, and what I had in mind was something like this. I do believe in string theory in the following sense, it’s a mathematical theory, it’s a consistent theory, and it contains both quantum mechanics and gravity. That makes it a very, very valuable laboratory for trying out ideas. It in itself doesn’t mean it is the theory of the real world. My guess is, the theory of the real world may have things to do with string theory but it’s not string theory in it’s formal, rigorous, mathematical sense. We know that, we know that. We know that the formal, by formal I mean mathematically, rigorous structure that string theory became. It became a mathematical structure of great rigor and consistency that it, in itself, as it is, cannot describe the real world of particles. It has to be modified, it has to be generalized, it has to be put in a slightly bigger context. The exact thing, which I call string theory, which is this mathematical structure, is not going to be able to, by itself, describe particles. Will what does correctly describe particles be a small modification of it or a big modification? That’s what I don’t know, but I do know the value of it as a laboratory for investigating quantum mechanics and gravity, and that’s remarkable.

Craig Cannon [00:15:30] – Okay, because the question that I’ve been wondering, it’s sort of straight forward, but why does there have to be a grand unified theory?

Leonard Susskind [00:15:40] – Well, there has to be, why does there have to be?

Craig Cannon [00:15:43] – Why do people want it?

Leonard Susskind [00:15:44] – We don’t, I don’t know what people think, I know what I think. It’s not tolerable to have inconsistencies in the theory of nature, where one piece of the theory says one thing, another piece of the theory says another thing, and they’re saying inconsistent things. They have to be made consistent. At the present time, we’re in the business of trying to put together a consistent framework for the combination of gravity and quantum mechanics. Elementary particles, there are inconsistencies in what we know about elementary particles. We’re trying to put those together. When we put them together and make a consistent story out of all of this, we’ll call that a grand unified theory. That’s it. It’s intolerable not to have a consistent story. You get different answers by doing different versions of it. That can’t stand. That’s my answer to it.

Craig Cannon [00:16:49] – When you look at physics, as it stands right now, where do you see the cracks that you want to be focused, is that the most important thing you could be working on right now?

Leonard Susskind [00:17:01] – Which?

Craig Cannon [00:17:03] – Yeah, well, a grand unified theory. In other words–

Leonard Susskind [00:17:06] – Well, I don’t think of it that way, I don’t think of it that way. At the moment, people like myself, John Preskill, Juan Maldacena, wonderful and great physicists, have gotten focused on the connection between quantum mechanics and gravity. For many years it was thought that quantum mechanics and gravity simply don’t fit together. For a variety of reasons, including things that Stephen Hawking had said, which were brilliant. I don’t think were correct, but brilliant anyway. It really looked like there was an inconsistency between quantum mechanics and gravity. Quantum mechanics governs all other parts of nature, but, of course, gravity also covers a large part of nature, and to have inconsistent theorists is, as I said, intolerable. The puzzle of putting together quantum mechanics and gravity is the one which is front and center for me. Front and center for theoretical physics right now. There are also, well, conflicts, there are conflicts in our understanding of elementary particles. We don’t understand how they can behave certain ways that they do behave. One of the problems, it’s just a name, but it’s called a gage hierarchy problem, it’s an apparent, almost inconsistency in the, what’s called the standard model of particle physics. There are other questions about how it does fit together with gravity. We made great progress in understanding elementary particles for a long time, and it was always progressed, though, in hand-in-hand with experimental developments, big accelerators and so forth. We seem to have run out of new experimental data,

Leonard Susskind [00:19:05] – even though there was a big experimental project, the LHC at CERN, whatever that is? A great big machine that produces particles and collides them. I don’t want to use the word disappointingly, well, I will anyway, disappointingly, it simply didn’t give any new information. Particle physics has run into, what I suspect is a temporary brick wall, it’s been, basically since the early 1980s, that it hasn’t changed. I don’t see at the present time, for me, much profit in pursuing it. Gravity and quantum mechanics are what fascinate me.

Craig Cannon [00:20:06] – What are the other large, unanswered questions that people are pursuing at this point? Clearly it’s not just you working on this.

Leonard Susskind [00:20:13] – No, other things. Well, in the context of, there are huge problems in cosmology. In all of this, cosmology is about quantum mechanics and gravity. Early cosmology, so called inflationary theory, is about how quantum fluctuations imprinted themselves on the universe and lead to the things, galaxies, planets and so forth. So, quantum mechanics and gravity are the foundations of cosmology, but we don’t understand how they fit together at all. Particularly in the cosmological context. We really just don’t understand how they fit together. The dark energy, the thing that’s called dark energy, is a puzzle. It’s not the puzzle of why is there dark energy? It’s the puzzle of why isn’t there a lot more of it? The dark energy is a tiny, tiny, minuscule fraction of what it could be. Why is it so small? 10 to the minus 120 of what the natural expectation for it would be. For many years, people thought there was no dark energy. We call it the cosmological constant, but it’s the same thing as what people call dark energy. We have no idea, so for originally, we thought it wasn’t there at all. Einstein invented the cosmological constant and then said it was his worst mistake because it doesn’t seem to be there. Well, it was there, but it was there at a level which was so minute that it took until the 1990s to discover any evidence for it.

Craig Cannon [00:22:03] – How is it measured?

Leonard Susskind [00:22:04] – It’s measured astronomically and by modern observational cosmology, counting galaxy counts and all kinds, the quasar counts, all sorts of stuff. But the main point is, in the end, it turned out that it was there, this dark energy, but it was there at such a small, incredibly small value, that it took all that time to get any evidence for. And we don’t know why it isn’t bigger, more of it. That’s the puzzle, not why is it there? But why is it not there in larger abundance?

Craig Cannon [00:22:45] – Do you have a hypothesis?

Leonard Susskind [00:22:47] – Well, the usual hypothesis is that, the usual hypothesis, the only one that I think makes any sense, which is outlandish, there’s no question it’s outlandish, it’s not mine.

Craig Cannon [00:23:02] – Are you jealous?

Leonard Susskind [00:23:03] – No, it’s not mine but it’s the only thing that does, at the moment, seem to make any sense, is to say the universe is extremely big, much bigger than we can see, and varied, varied means it has properties which are different from place to place. That’s a good theoretical idea, it makes it, it does fit together with the equations and so forth, that the universe is vastly bigger than the part we can see. As you scan over the whole thing you’ll find places where the constants of nature are one thing, other places where the constants of nature are another thing. Some places where there’s cosmological constant, there’s more or less normal, which means much, much bigger than it is here in our neighborhood. Some places where it might even be smaller. But then the question becomes, in what kinds of environments can we exist, and even ask the questions? My friend Steve Weinberg, in around 1987, made an argument that if the cosmological constant were any bigger than a certain magnitude, that galaxies could not have formed, and if galaxies couldn’t form, stars can’t form, planets can’t form, and we can’t be here. He said the answer is the universe is very big and varied and are where we can be, that’s all.

Craig Cannon [00:24:29] – It just is.

Leonard Susskind [00:24:29] – We’re just where we can be. That’s called the anthropic principle and it’s a widely hated idea among physicists.

Craig Cannon [00:24:35] – Definitely amongst scientists, anything that just is.

Leonard Susskind [00:24:37] – Yeah, it’s a widely hated idea but it just might be right.

Craig Cannon [00:24:41] – I was listening to a radio interview with you and you said, similar to this, that there was a discovery that there are relatively few ways of organizing matter than we thought there would be.

Leonard Susskind [00:24:54] – I wonder what the hell I was talking about.

Craig Cannon [00:24:56] – That’s a good question. But my question is, could you explain? Because you said there are relatively few ways that don’t turn into black holes.

Leonard Susskind [00:25:06] – I don’t remember exactly what I was talking about. But here is what I can tell you. Almost all the matter, or almost all the information in the universe is in the form of black holes. If you take some matter and just generically populate the world with matter, you will find, in a very quick amount of time, that it’s mostly all black holes. Our world is mostly all black holes, it really is. In the sense that the information stored in matter is at least, let me think, I think about 10 to the 10th, a factor of 10 to the 10th more information stored in black holes than in anything else. Even though black holes seem very rare in the universe, they contain almost everything.

Craig Cannon [00:26:09] – Can you define information, just for people.

Leonard Susskind [00:26:14] – It’s what’s in a computer.

Craig Cannon [00:26:17] – Bits?

Leonard Susskind [00:26:19] – Bits, the bits. We call them qubits because they’re quantum bits but, yeah, bits. And the bits which determine, here’s what we might say, we take the universe as it is. We can run it forward in time, and that’ll tell us what it will be. We can also try and run it backward in time, or find out what it was like in the beginning. In order to do that you have to have every single bit accounted for. You try to run things backward, you’ll make mistakes very quickly, unless you’ve accounted for everything. The question is, how many bits of information do you need in order to run backward and find out what the world was like in the beginning? That number of bits is about 10 to the 10th times bigger than all the known bits in ordinary material in the universe, protons, neutrons, electrons, and so forth. Where is it hiding? We now know that it’s hiding in black holes.

Craig Cannon [00:27:25] – Gotcha, okay. I briefly encountered this through the holographic principle that you worked on. One question that I couldn’t fully wrap my head around–

Leonard Susskind [00:27:37] – See, there’s another example of something, which was considered a little bit radical at first, a little bit nuts, but, of course, it’s now extremely mainstream, very mainstream.

Craig Cannon [00:27:49] – I would push back a little bit.

Leonard Susskind [00:27:50] – Okay, go ahead.

Craig Cannon [00:27:51] – Well, anything that’s fringe that becomes popular, you can say is mainstream, but it was fringe in the beginning.

Leonard Susskind [00:27:58] – No, it’s mainstream in the sense, well, it wasn’t fringe in the beginning. People just didn’t recognize how essential it was to the logic. It took a little while. It took a little while for people to realize, yes this was the only way it could be. It wasn’t just that it became popular, this is not a popularity contest, physics is not a popularity contest. For brief periods of time, sometimes things become popular, but they don’t last if they’re just popular. They last if they have value, explanatory value, predictive value, and the value of leading to a consistent framework. In that sense, the holographic principle is now completely mainstream. And why is it mainstream? It’s mainstream for the reasons that I thought had to be correct. It just had to be correct, it couldn’t not be correct.

Craig Cannon [00:28:55] – It worked out. Can you give a brief explanation? Because this was a hard one.

Leonard Susskind [00:29:00] – Well it has to do with black holes, it had to do with black holes, and the information law, which it had to do with this discussion about information being lost in black holes, which was, Stephen Hawking’s very, very brilliant insight, even though I think he got that final answer wrong, it was very brilliant insight to ask what happens to the information that goes into black holes? Is it lost, is it lost to the universe? If it’s lost, that would be a major change in physics, in which, in ordinary physics, information is never lost. Now, Stephen also said that black holes evaporate. Well, a natural answer might be that the information comes out in the evaporation, but it can’t come out in the evaporation if it fell into the black hole because nothing can get out of a black hole. Okay, so there was, my favorite kind of situation, a clash of principles. The answer turned out to be in this holographic idea that as, let me say it in a way which is, not exactly correct, but as close as I can get without writing a bunch of equations on a blackboard. The information that falls into a black hole can be thought of as both falling into the black hole, and also getting stuck on it’s horizons. Two versions of it. Almost as though the information was Xeroxed at the horizon of the black hole, and one half of it sent in, and the other half stored on the horizon. Now, the real correct statement was more like saying that the stuff on the horizon is a kind of hologram of the stuff that falls in. It’s really only one thing,

Leonard Susskind [00:30:57] – but represented in two different ways. And then once you said that the stuff that falls into the black hole can be thought of as a hologram that never does fall through the horizon, then you can imagine that when the black hole evaporates, this hologram evaporates with it and carries off the information. Now that’s–

Craig Cannon [00:31:19] – This was the challenging part of it.

Leonard Susskind [00:31:21] – It’s very challenging. Right, and I’m not sure that they can… I think, if you really, really wanted to know, and you were willing to spend three or four days talking about it with me, I could probably reduce it to something which was both correct and comprehensible, but not in 15 minutes.

Craig Cannon [00:31:45] – Yeah, fair.

Leonard Susskind [00:31:46] – Not in 15 minutes, that’s just the way it is. Okay, the point was that black hole horizons are behaving like holograms of anything that falls into the black hole. But then, when we’re thinking about it further, we realized that the whole world could be in a black hole. You can’t tell that it’s not in a black hole. In particular, the entire universe has a horizon, out at very large distances, which is very much like a black hole horizon, and we’re kind of inside it. That leads to the conclusion that we, here in the interior, must have another representation as a hologram out at the boundary of the universe. Now, this was a strange idea, this certainly was a strange idea. I felt driven to it because I could see no way other than that, incidentally, it wasn’t just me, it was also Gerard t’ Hooft who put this idea forward. It was a little bit out there, it certainly was out there. It didn’t come in from the cold, shall we say? Until the work of Juan Maldacena, who made a really rigorous, beautiful version of it, which, now, everybody believes. The mathematics of it was, and it was a string theoretic construction, where Juan showed how, at least in certain setups, the universe would have to be regarded as a hologram. A hologram, saying it’s a hologram is a bit of an analogy, but that it would be represented as information stored on the surface,

Leonard Susskind [00:33:35] – on the out of surface of the world, rather than in three dimensions, as we normally think about it.

Craig Cannon [00:33:41] – Inside.

Leonard Susskind [00:33:42] – Juan really nailed that with such mathematical precision, that it just became part of our standard, it became a tool. That’s a good thing, when things go from being, they often start out as very speculative, then they become something a little bit better than speculative, conjectural, conjectural is better than speculative, and the end process is they just become a tool of physics, things that everybody uses all the time because it has a predictor value or mathematical value. The holographic principle is a tool now. So, yeah, it’s stuck.

Craig Cannon [00:34:24] – Why does it have to be holographic? In other words, say it’s mapped around, I’m going to have to bring this into a 3D world, right? There’s a 3D sphere, call that a black hole. Why is it holographic versus a 2D image, for example?

Leonard Susskind [00:34:42] – It is a 2D image. It’s a 2D, you mean why can’t it just be like a picture on the wall? Well, a picture on the wall is two dimensional. It may deceive you. A clever painter can paint a painting, which, when you look at it, you think you see three dimensional things.

Craig Cannon [00:35:04] – But you never do.

Leonard Susskind [00:35:05] – You don’t, and in particular, if you move your head around from side to side, you can’t see what’s behind the flower. There is nothing behind the flower, and you were just deceived into thinking there was something three dimensional there. But how would you check it was three dimensional? You would check it was three dimensional by going around to the other side and see if something’s there. Well, if you move your head around the picture of that plant on my wall there, you will not see anything behind the plant, there’s just nothing there, it’s strictly two dimensional. On the other hand, it is possible to map a three dimensional world onto two dimensions, but never in a way in which the two dimensional stuff looks anything like the thing you’re mapping. It will look random, it will look like a simply confused jumble of little, tiny scratches. You can see that if you can get a hold of a real hologram, which a hologram does map three dimensional space onto a two dimensional film, and somehow look at the film through a microscope or something, you’ll see that there’s nothing on that film which resembles anything like the thing that it’s representing. It’s just a bunch of little, tiny scratches and random noise, almost. You can’t map the three dimensions to two dimensions without really making it totally discontinuous, the words mathematically discontinuous, but, yet, it does contain the same information. That’s the same thing about this holographic principle. The horizon really did store all of the stuff that fell into the black hole,

Leonard Susskind [00:36:54] – but in a way which you could not easily reconstruct. It’s more like a hologram than it would be like a photograph.

Craig Cannon [00:37:01] – How does the reconstruction happen? Say we are in a black hole.

Leonard Susskind [00:37:05] – Okay, for a real hologram, all you have to do is shine the right kind of light on it to reconstruct the image. Not here. Here it would be a mathematical reconstruction. If somebody gave you the quantum state of the horizon of a black hole, and you were smart enough–

Craig Cannon [00:37:23] – You could re-map it.

Leonard Susskind [00:37:23] – I assure you that nobody’s smart enough, but with sufficient technology of quantum computation, and so forth, and if we knew the precise rules by which black holes evolve, we could reconstruct, from the quantum state of the horizon, we could reconstruct what fell in, what’s inside, and so forth. We could reconstruct that world that fell into the black hole. This is not something which is easy. It is far from mathematically tractable with present computers and so forth, but, in principle, it is possible. If somebody showed you the hologram, incidentally, of just a patch of flowers, or something, and just gave you the film, and didn’t allow you to shine light on it, just said, reconstruct from that–

Craig Cannon [00:38:22] – You couldn’t.

Leonard Susskind [00:38:22] – You’d have a hell of a time. Yeah, eventually you probably could, but it would be very hard.

Craig Cannon [00:38:30] – And multiply that out to the universe.

Leonard Susskind [00:38:32] – And quantum mechanics, which escalates the story hugely.

Craig Cannon [00:38:38] – Gotcha. Slight tangent, have you followed any of these ideas around we live in a simulation, the simulation hypothesis?

Leonard Susskind [00:38:48] – It doesn’t seem to me to add anything. What does that mean? Does the idea that we live in a simulation mean that there was a simulator, that somebody simulated us?

Craig Cannon [00:39:03] – I believe so, yeah. That we live in a computer program.

Leonard Susskind [00:39:07] – Yeah, yeah, yeah, but I mean–

Craig Cannon [00:39:08] – Based on our ideas of what it might be, yeah.

Leonard Susskind [00:39:09] – No, no, but I would say, of course we live in a computer program, the program is called the laws of nature and that computer is the world. So, it’s, yeah. But then somebody would say, oh, that’s not what I meant. I’d say, what did you mean by saying we live in a computer? I think they meant that there was a computer programmer who programmed it for some purpose. Do we live in a computer program that somebody programmed for a purpose? I have no idea.

Craig Cannon [00:39:36] – Nor do you spend much time–

Leonard Susskind [00:39:37] – I would love to know, but then I would ask, I’m a curious person, I would ask then, okay, if there is that guy out there, lets not give him a name, the programmer, the programmer who programmed the simulation, who programmed him?

Craig Cannon [00:39:53] – Right.

Leonard Susskind [00:39:54] – What are the laws by which he functions? Does he satisfy the laws of quantum mechanics, he or she? Probably neither, it’s probably a sex-free environment.

Craig Cannon [00:40:04] – Who knows?

Leonard Susskind [00:40:04] – Who knows, exactly?

Craig Cannon [00:40:06] – Whoever programmed them decides.

Leonard Susskind [00:40:07] – Right, right, and then who programmed the programmer who programmed the programmer? And so forth. It doesn’t satisfy, it just doesn’t lead to any satisfying answers.

Craig Cannon [00:40:15] – This reminded me, I was listening to your Cal Tech, your Feynman lecture at TEDX, and you said something really nice, which was Feynman didn’t much like philosophers philosophizing about science. In the context of machine learning, which your son works on. Do you find yourself in the same camp? You’re just like back to basics about the technical aspects, or do you philosophize, or let yourself philosophize?

Leonard Susskind [00:40:43] – First let me say something about Feynman. Feynman claimed to dislike philosophy. He did dislike philosophy, but I’ll tell you what that means in a minute, and yet he was the most philosophical of all physicists. He really was. He was a deep philosopher. When I say he didn’t like philosophy, I meant he didn’t like a certain style of thinking that was full of jargon, full of the, full of, I’ll use his word, baloney, where people who didn’t know what they were talking about pontificated and used fancy words, like ontological, which I never knew what that meant.

Craig Cannon [00:41:26] – I know a lot of words and when you use them, but I don’t know what they mean, all of the time.

Leonard Susskind [00:41:29] – As a substitute for simple thinking. That is what he didn’t like. Yet, I think in some ways, in some deep way, he was an extraordinarily philosophical person. If you read his works, I don’t mean his physics work, if you read the things he wrote about the world, the ordinary world, they’re very, very philosophical, but they’re also incredibly simple and they cut through all the crap. It was the crap that he didn’t like. I would say the same about mathematics, he didn’t like the overly fancy mathematics, but he was a very good mathematician.

Craig Cannon [00:42:12] – And what we were talking about before we started recording, he was also quite moral, right? In his philosophy of the world, he was affected by that, I mean, as well.

Leonard Susskind [00:42:22] – Hated the fact that he had participated, he hated the fact that he had participated in the invention of nuclear weapons, and he doubly hated the fact that he had so much fun doing it.

Craig Cannon [00:42:39] – That’s fair.

Leonard Susskind [00:42:40] – Right.

Craig Cannon [00:42:41] – Did you interact with any other people that worked on the bomb?

Leonard Susskind [00:42:47] – Hans Bethe. Hans Bethe was one of my thesis advisors, yes, so. I did, but I didn’t talk with Hans, Hans was not, he was a friend, but he wasn’t a friend in the same way that Feynman was. He wasn’t a soulmate.

Craig Cannon [00:43:09] – Okay. And that depth you talk about with Feynman, did you find that with your advisor?

Leonard Susskind [00:43:18] – Who, Hans?

Craig Cannon [00:43:19] – Did he have the same sense of grief around what he had created?

Leonard Susskind [00:43:23] – Oh, well, I can’t, all right, I know the answer to that but not from him directly. From that, just because it’s historical. Yes he was very upset about the bomb, and he, as much as anybody, worked hard, very, very hard for disarmament, for nuclear disarmament. Feynman did not. Feynman just said, “Okay, I’m going to do physics, and that’s what I’m going to do.” He didn’t work, Hans did, Hans was very, very active in nuclear disarmament. I do know that he regretted it, yeah, but I don’t it directly from him.

Craig Cannon [00:44:08] – I’m wondering what the parallels might be today because I think there are so many engineers working on incredibly technical things that, who knows what the implications might be? Already are, you could say, with Facebook, other things.

Leonard Susskind [00:44:20] – On the other hand, the enormous amount of good that has come from technology, of all kinds. You can’t not work on it. At what point do you stop and say, this is dangerous? Well, I think it’s probably built into some people, curiosity, the need to explore, and they’re just going to do it. I don’t believe it’s the physicists job to decide what should and shouldn’t be discovered. From a physicists point of view, everything should be discovered if possible. It is the job of politicians, and other people of that ilk, to make sure that things are not misused. The misuse of nuclear weapons was not really the scientists who built them, they were worried about the Nazis getting them. If there was misuse, there’s all sorts of debate about whether nuclear weapons were misused, or were they used well to end the war? And all that sort of stuff. If they were misused, it wasn’t the scientists. The scientists didn’t want to see the bombs used. They were given a problem, a double problem. Part number one of the problem was the Nazis are going to build it if we don’t, and the second problem was how do you build it? They had no choice, I don’t believe they had any choice except to go and do it. Both as scientists and as human beings. The fact that it got misused, I don’t believe was the scientists themselves. If anything, those people tended to be very traumatized by the fact that they had built weapons.

Craig Cannon [00:46:31] – You said he didn’t work on disarmament, but do you think any of his focuses later in life were related to improving the world?

Leonard Susskind [00:46:41] – He would have said you improve the world by discovering what the world is. He would have said that it’s my job as a physicist, when I say my, I actually mean mine to, but I meant his, that it’s his job to find out as much about the world as can be found out. And he was very good at it, he advanced our knowledge of the world. How it gets used, he did not see as his responsibility.

Craig Cannon [00:47:16] – Does that align with your personal philosophy, your reason to work on this?

Leonard Susskind [00:47:19] – I think so. Look, if I were to suddenly discover something that I knew was going to be exceedingly dangerous, I would, and I was absolutely certain that it was destructive, and so forth, first of all, I don’t think you can hide it. You can’t hide it, it’s going to come out.

Craig Cannon [00:47:40] – Eventually.

Leonard Susskind [00:47:40] – It’s going to come out.

Craig Cannon [00:47:41] – Okay.

Leonard Susskind [00:47:42] – All you can do is warn, all you can do is warn people that this is there, and it will be discovered, you’ve got to worry about it. Bethe did that, I think Feynman didn’t. His reaction to it was, my job on earth is to learn about the world and I’m going to focus on that. I am not responsible for all the evil in the world, and I can be responsible for uncovering what nature is like.

Craig Cannon [00:48:24] – I’m just curious how you’ve stayed motivated and been so prolific with your career.

Leonard Susskind [00:48:31] – I think I’m also a curious person. I don’t mean weird, other people can decide that. I mean that I have a sense of curiosity about the world. And it just doesn’t go away. I didn’t say to myself I’m going to continue to do physics until I’m 78 years old.

Craig Cannon [00:48:48] – And then I’m out.

Leonard Susskind [00:48:49] – No, I didn’t plan that, I just get curious about things. That’s it. I don’t have a choice.

Craig Cannon [00:49:01] – What are you most curious about right now?

Leonard Susskind [00:49:02] – Gravity and quantum mechanics. How they fit together.

Craig Cannon [00:49:05] – What in particular?

Leonard Susskind [00:49:10] – Whether the laws of gravity are really just the laws of quantum mechanics, a little bit hidden. My guess is that almost everything we know about gravity is coming straight from quantum mechanics, and that there are equivalent rules of quantum mechanics which reflect the gravitational things. This is going to get us into technical discussions, which–

Craig Cannon [00:49:32] – Lets do it.

Leonard Susskind [00:49:33] – You want to do it?

Craig Cannon [00:49:34] – Yeah, let’s do it.

Leonard Susskind [00:49:35] – No. No.

Craig Cannon [00:49:37] – If I get dropped, if some of the listeners have to drop, that’s okay, but certain people will like it a lot.

Leonard Susskind [00:49:43] – Right. Good. One of the things that was discovered by myself and Juan Maldacena, probably more by Maldacena than myself, we wrote a paper together, is called the ER=EPR hypothesis. Oh, this is a great story, incidentally, lets back off for a minute, let me tell you the story about Einstein and ER and EPR. ER stand for two names, Einstein and Rosen. EPR stand for three names, Einstein, Podolsky and Rosen. One year, 1935, after it was generally deemed that Einstein was basically finished as a physicist for at least something like 10 years, Einstein wrote two papers, which nobody paid too much attention to for many years. One of them was the ER paper, and it was about wormholes. It was about solutions of the Einstein field equations, which had this wormhole character where there were wormholes connecting distant regions of space. They were called Einstein-Rosen bridges. If you look up Einstein-Rosen bridges you will find that they’re bridges which connect with different regions of space, a black hole in one place, and a black hole in another place has a connection between them. That was solutions of Einstein equations. The other paper that he wrote the same year was about something called entanglement. Entanglement is something that can happen to quantum systems when they get correlated and it’s a very non-local kind of thing.

Leonard Susskind [00:51:31] – It’s purely quantum mechanical, it does not obviously have to do with gravity. These were two separate things, I do not believe that Einstein, at all, had any idea that they were connected. The Einstein-Rosen bridges and the idea of entanglement. One of the really odd things was that, in very recent years, we’ve found out that entanglement and Einstein-Rosen bridges are the same thing. That, in particular, an example would be if you have two black holes, black holes have all kinds of internal structure to them, they’re quantum mechanical objects. If the two black holes are entangled, they will have an Einstein-Rosen bridge connecting them. If the two black holes have an Einstein-Rosen bridge, they will be entangled. We’ve found out that they are the same thing. Quantum entanglement and a kind of connectivity between systems that were called Einstein-Rosen bridges. This was a weird quirk of history, that in the same year, Einstein discovered both of these things, almost certainly didn’t have any inkling that they were the same, maybe he did but I don’t think so.

Craig Cannon [00:52:39] – What did the two papers say? If they ultimately became the same thing.

Leonard Susskind [00:52:43] – One paper said there are solutions of my equations in which distant black holes are connected by wormholes. A shortcut between them–

Craig Cannon [00:52:54] – Is entanglement.

Leonard Susskind [00:52:57] – That’s about black holes, that was not about quantum mechanics. That was about Einstein’s general theory of relativity, which is a completely classical, non-quantum mechanical thing. The other thing is he was thinking about quantum mechanics and discovered this odd, non-local connection that systems can have, that we call entanglement. As far as I know, as I said, he didn’t draw any conclusions about any relationship between these two things. That happened in 2013, long, long after Einstein had been dead for many, many years, as a consequence of the mathematical study of black holes. It was largely Juan Maldacena’s discovery, I happened to be on the paper with him because we were working on something together. Drawing out the ultimate conclusions of that, finding out what it really means, how it brings quantum mechanics together with gravity has been the essential focus of my own thinking for at least five years. Trying to make a theory out of it, trying to build a comprehensive theory.

Craig Cannon [00:54:24] – And what was the technical part you wanted to get to?

Leonard Susskind [00:54:31] – The technical part had to do with something called quantum complexity theory. These wormholes that connect… You might think if you have a wormhole connecting two distant places, you can jump in one and come out the other.

Craig Cannon [00:54:48] – That’d be great.

Leonard Susskind [00:54:48] – No, the problem is the wormhole grows, and it grows so fast that you can’t get through it. It’s as if you had a tunnel, New Jersey and New York City, the Holland Tunnel or the Lincoln Tunnel, and you go in one end of the tunnel, and, of course, you can come out the other end, but what if the tunnel was growing while you went in, and it was growing so fast that it grew faster than your speeding car? Well then you can’t get out the other end.

Craig Cannon [00:55:17] – Right, yep.

Leonard Susskind [00:55:18] – That’s the way these Einstein-Rosen bridges behave. The question is what is the quantum mechanical meaning of the growth of these wormholes? The answer appears to be that they are connected with something called complexity theory. Complexity theory’s a computer science concept. It tells you how hard it is to reverse something. The complexity of the growing Lincoln Tunnel would be a measure of how hard it would be to shorten the tunnel again so that you could get through? This question of quantum complexity theory has been, sort of focused what I’ve been thinking about. Other people think about different things. This is a main focus of a lot of work on what’s going on, both here, Princeton, all over the world. And where it will go, I don’t know. Its just fun to think about, and they pay us to do it.

Craig Cannon [00:56:26] – It’s not a bad gig. There was a related question from Twitter for you. Noah asked, “Could quantum teleportation be used in the future as a means of intergalactic communication?”

Leonard Susskind [00:56:40] – No. No, in order to do quantum teleportation, you cannot do quantum teleportation without, at the same time, sending classical information from one place to another. Classical information means dots and dashes, Morse code dots and dashes. You can have two entangled systems, and you can send information through the entanglement, but not without sending a code, to decode, but not without sending a code, classically, from one place to another, and that will take time. You don’t speed up communication. If it would take you 100,000 years to communicate from one end of the galaxy to the other end of the galaxy in any kind of normal sense, it will take you that same 100,000 years to do quantum teleportation. You could use quantum teleportation to teleport stuff over vast distances, but it won’t be any faster. It will be more secure, more secure means more secret, you won’t be able to crack it. But, that’s what quantum teleportation does for you. It gives you absolute, 100% security that no classical, non-quantum mechanical protocol could ever give you, but it can’t be done faster.

Craig Cannon [00:58:14] – Okay, good to know. Related, Riokko Digital asked–

Leonard Susskind [00:58:21] – Who, who, who?

Craig Cannon [00:58:22] – Riokko, it’s just like a brand, it’s just someone with an avatar. “How do you think quantum theory will shape technology in the future?”

Leonard Susskind [00:58:33] – That’s a very good question. Of course, it’s already shaped technology, completely, in the present.

Craig Cannon [00:58:38] – It’s on-going, yeah.

Leonard Susskind [00:58:39] – All the electronics in the world is all based on quantum mechanics, but it’s particularly simple quantum mechanics. Quantum mechanics of a small number of electrons, and things like that. The quantum mechanics that we’re exploring now is the quantum mechanics of massive entanglement. Large numbers of qubits, those are quantum bits, which are massively entangled with each other, and how that can be used to do things that no classical computer can do. I can’t tell for sure how it’s going, quantum computers will probably be built. They will be built to try to exploit this massive idea of entanglement. What problems will it solve is unclear. It conceivably could be that people will build quantum computers and not figure out what they’ll be able to do with them. Now, I don’t think that will happen. There’s one thing that you can do with a quantum computer and that’s to simulate quantum systems in a way that classical computers couldn’t. Classical computers can never be built big enough to explore more than 400, actually more than, probably 100 qubits, 100 qubits doesn’t seem like very much. No classical computer can do the calculation of following what 100 qubits do. If you’re interested in some quantum mechanical system and you want to study it, the most efficient way to study it is not to program it for a classical computer, that will never go very far, but to program it on a quantum computer, and then you have a good chance to be able to explore it. That’s the scientific purpose for it.

Leonard Susskind [01:00:36] – You want to understand how certain chemical molecules behave, the big chemical molecules, which are too big to do on a classical computer. You run it on a quantum computer. You want to understand new materials, materials that depend, or their properties, on quantum mechanics. Classical computer for the most part can’t do it, you’ll be able to simulate it on quantum computers. Will they be able to solve problems that are the usual kinds of problems that you hope computers can solve? That remains to be seen.

Craig Cannon [01:01:16] – The last thing I was wondering is now, so you’re both an accomplished physicist, but you’re also a physics educator.

Leonard Susskind [01:01:24] – Okay, I hope so.

Craig Cannon [01:01:25] – For better or worse, right? All of your videos, your books. You clearly have a knack for communicating these ideas.

Leonard Susskind [01:01:33] – Oh, that’s nice to hear.

Craig Cannon [01:01:34] – At least it works for me. If you could impart any particular ideas, across the population, about physics and understanding, what would they be?

Leonard Susskind [01:01:46] – I don’t really know. Let me answer a totally different question, why did I start teaching for the public?

Craig Cannon [01:01:55] – Sure.

Leonard Susskind [01:01:57] – The simple answer is, that is was fun. I like teaching, I get two things out of teaching, I like to perform. In that sense, I have a bit of Feynman in me. I get a kick out of performing. That’s one thing. There’s another element to it, I find that the process of figuring out how to explain things is very, very helpful in formulating new ideas. To me, teaching is absolutely essential for doing physics. Much of my physics began with trying to figure out how to explain something. It almost doesn’t matter whether it’s explaining to another physicist or explaining to a lay person. In particular, I found that trying to explain things to a lay person, I explain them honestly, explain them not through fake analogies, but to try to give an honest and clear explanation of something, often really focused my ideas on how the thing works. It has value to me that was above and beyond just the fun of teaching. I did find that teaching for the public, for the public, Stanford’s continuing studies, was especially valuable this way. The students, students, they were any where’s between 50 years old and 95. That was actually true, there was a 95 year old lady who, and she followed the, she knew what she was doing. I found that their curiosity, they had some degree of technical background, they tended to know a bit of mathematics, just a bit, through calculus. They were very curious about physics. I found teaching them to be especially gratifying, and I really would spend a lot of time

Leonard Susskind [01:04:15] – figuring out how to explain hard things to them. In the process, I often found out I understood them so much better. That was why I got into teaching in the public sector, or whatever. I don’t know that there’s any particular thing that I would want to convey to them. There’s some obvious answers, you want to convey to them that science makes sense. You want to convey to them that scientists aren’t phonies, that they really do, sometimes, know the answers to things, that there are facts, and so forth. Of course, all these things are true. Was I motivated by that? Not really, I was just motivated by having fun and enjoying teaching. I think there was one more thing. My father had a bunch of friends, they were plumbers, and they were funny characters. They were sort of intellectuals, but none of them even passed the fifth grade. They were very curious about all sorts of things, some science, some history, and stuff. They were mildly crack-potty. Why were they crack-potty? They were crack-potty not because they were intrinsically crackpots, they were crack-potty because they had no venue in which they could find out what was real science from fake science. They were plumbers, they couldn’t asking physicists is this real, or is that not real? I always felt some sense that I would of liked to be able to go back in time to my father and his friends, and tell them what was real and what was fakey stuff. Emotionally, I think that sort of did come into the reason why I liked teaching these people.

Leonard Susskind [01:06:17] – They reminded me of my–

Craig Cannon [01:06:19] – Yeah, that’s great. Well, thank you so much for your time.

Leonard Susskind [01:06:23] – Okay, it was good.

Craig Cannon [01:06:26] – All right, thanks for listening. As always, you can find the transcript and video at blog.ycombinator.com. If you have a second, it would be awesome to give us a rating and review, wherever you find your podcasts. See you next time.