Leonard Susskind on Richard Feynman, the Holographic Principle, and Unanswered Questions in Physics

What I wanted to start with is you've often been characterized as someone with like non-traditional, you know, kind of out there ideas. Some of which have become, you know, part of the physics canon; some of which, who knows what happened. Who they all became part of the physics canon? Every single one of them? I never made a mistake, of course.

Alright, well, thanks for coming on the podcast. You're the first person who's never made a mistake. I was curious, who do you think is your most outlandish friend?

Wait, come back to your previous question for a moment! A battement! I am very mainstream. I am not at all an 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. But it's just not true that I was some kind of alternative, um, what should we call it? I don't know what the right word is. Yeah, some kind of radical thinking? Not at all! Not at all. No, I spent a lot of time thinking about what you would call conflicts of principle—situations where things were not fitting together properly. I 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. I think 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 it, when you've tried all possibilities—and I forget the exact, but roughly speaking when you've tried everything and it doesn't work, whatever remains must be the truth. No matter how?

I've landed it, should or something like that? Oh yeah! 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 gotta try it!

I think that probably is the source of some of this mythology about me as a radical, but I am the most conservative physicist imaginable.

Okay, so what's an example of a friend who is more outlandish, more radical than you?

Oh, Freeman Dyson! He's—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 and he sometimes has some brilliant and smart things. Like all contrarians, he's got a very large probability of being wrong, and he's willing to—my friend Hirata Tuft, I don't know if you know his name—is 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! Hmm, but also had his own—he had his own very special scientific personality. I suspect that's also true of me—that my way of thinking, my way of doing things is probably different than most people. And so, yeah, it did lead to this contrary, this view of me as a contrarian, as a radical, but it was absolutely wrong.

And do you see physics kind of birthing more contrarians in the modern paradigm? Works? Experiments are so expensive too, you know, kind of executes at this point or do they have to be kind of more mainstream to get things done?

Well, unfortunately, I think you have to be really mainstream sometimes. I think too much. 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 useful. Free thinking! But that doesn't mean being a contrarian. A contrarian is somebody who is contrary just for the sake of being right—an inquiry.

Yeah! Well, because I mean, I, you know, read your PDF page, done some interviews with you and I heard about you being a plumber working with your father. Yeah, and now here we are at Stanford and you're, you know, kind of like of the industry right? You're here!

Yeah, it took me a long time to feel part of the team. I feel an outsider! Yeah, my background was a little bit strange for so it took me a long time to not feel like an outsider—and then all of a sudden, I found that I was the ultimate insider.

And how do you deal with that?

It's hard, but just ignore it. Okay? I don't think—yeah, fair enough. I'm interested in the physics problem. I'm not gonna let it go. I spend most of my time thinking about it and not the—not agonizing.

All the things—then, where do you go to think of new ideas? Because that's something that—you mean here?

I go to the bathroom or take a shower. No, I'm kind of curious where your ideas have come from over the course of your career though. 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 what's called quark confinement. Why don't quarks come out of particles in the pure, in the laboratory?

Okay, okay.

They seem to exist; they seem to be part of the proton and neutron and so forth, and they seem to be stuck inside and never come out. Alright? And that was—a that appeared to be a paradox because from all that we knew about the subject, it'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 get to kick it out and observe it directly in the laboratory. So, there was a paradox there.

They seemed to exist and yet they seemed not to exist. Something was wrong, and that's the kind of thing that captures me and gets me going. And I—I don't—I don't want to let it go until I feel I understand it!

So someone from Twitter asks a question related to this. Their name is Claudio and they think—or they asked, do you think the graviton can be experimentally found?

So, similar—well, of course, there's a sense in which it's already been. But this is—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. So the electromagnetic field is a collection of photons, but you can't—sir—but that doesn't mean you can detect them as individual photons easily. Radio waves, for example, would be very, very difficult to detect these individual photons.

Alright, we've seen gravitational waves. That means we've seen large numbers of gravitons. Yeah. I mean, 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 it's—but ultimately I think it's a technological problem. Okay? You know, if you could build an accelerator as big as the galaxy and so forth, and so on, and harness a hundred thousand stars to take all of the energy that they produce and run the accelerator with it, you could make gravitons! So it's a technological problem.

Okay, so it's a little bit like the space Lego—the space Lego, oh wait! The space Legos in a, it's a trivial technology. It'll be really—what's the space Lego? Oh, what does it look like?

Oh, in space? Yeah, that's true. Real by comparison! Like my friends.

Yeah, 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, which are insurmountable.

Mm-hmm. Building a machine that could produce gravitons at least for the next million years is gonna be insurmountable!

Oh, it's not—I think it's not gonna be done. Right. On the other hand, maybe I'm wrong.

So let's go to some of your other ideas. So, you know, you're credited as one of the creators of string theory, which is extremely mainstream.

Which is super mainstream! Yeah! But it wasn't—it wasn't when we started it.

Correct! So that's right! So that's where the idea of me as a radical came from! But now it's mainstream. Where did the idea come from?

Oh, well the idea came from them—from asking about the structure of particles which are known as hadrons. These are protons, neutrons, and mesons—they're common things that make up the nucleus. And 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 take a point and spin a point! The point is too small to have—you wasn't mean to rotate a point!

Okay, okay.

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—yeah, 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."

Or I got in a little more and I said, "Oh, there's something vibrating! There's some kind of concept of vibration going on!" And it was just a matter of thinking about it for a few weeks and saying "Oh! The strings! They're elastic strings!"

Hmm. And with each of these, were you deeply knowledgeable in the field before this?

Okay—no! I was deeply knowledgeable about quantum mechanics! Ever, at least—or was I deeply knowledgeable? Even that I think I was! But, yeah, I—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 there was a mathematical formula. I knew what a proton was; I knew what a neutron was! I knew that if you collided them, stuff comes out of them!

And I also knew that they had these properties of being capable of being excited and spun up and so forth. So I didn't know that—but that was easy! I mean, you know? I just told you! When you now know it too! They showed me a formula and the mathematical formula had some pieces in it that I recognized! That seeing 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!

Okay, okay.

But I looked at it 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! Let's call them! And so I worked it out and published it, and that was the story!

And then in your Cornell lectures from 2014 or something—like the Messenger lectures?

Yeah, you—you kind of like offhandedly said that despite being one of the creators of the string theory, you weren't the biggest believer in the world right now.

Oh, okay! Um, 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 its formal, rigorous mathematical sense. We know that—we know that—we know that the formal, I mean mathematically rigorous structure that string theory became.

It became a mathematical structure of great rigor and inconsistency that in itself, as it is, cannot describe the real world of particles! It has to be modified, it has to be generalized! That has to be put in a slightly bigger context. So the exact thing which is good, I call string theory, which is this mathematical structure, is not going to be able by itself to describe particles! Will it—will what does correctly describe particles be a small modification of it or a big modification? That's what I don't know!

Okay, but I do know the value of it as a laboratory for investigating quantum mechanics and gravity, and that's remarkable!

Okay, because the question that I've been wondering—and then it's sort of straightforward—but why does there have to be a grand unified theory?

Well, there has to be that! Why does it have to be? Or do people want it? 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! And it's inconsistent—I mean, sorry, it's intolerable not to have a consistent story! You get different answers by doing different versions of it, that can't stand! So that's my answer to that!

Okay! I'll do it!

Okay, so when you look at physics as it stands right now, where do you see the cracks that you want to be focused—that like the most important thing you could possibly be working on right now?

Which is, yeah, well—grand unified theory! And in other words, why don't you think of it that way? I don't think of it that way at the moment! People like myself, John Fresco, Juan Maldacena—a wonderful and great physicist—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 correct, but brilliant! Anyway, it really looked like there was an inconsistency between quantum 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 theories is, as I said, intolerable!

So the puzzle of putting together quantum mechanics and gravity is the one which is front and center for me! I think front and center for theoretical physics right now! There are also—well, let's—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 of the—it's just a name—but it's called the gauge 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 went—we make great progress in understanding elementary particles for a long time, and it was always progress till in hand-in-hand with experimental developments, big accelerators and so forth! We seem to have run out of new experimental data!

Even though there was a big experimental project, the LHC at CERN—what if that is a great big machine that produces particles and collides them?

Mm-hmm.

And I would say—I don't want to use the word disappointingly—well, I will anyway—disappointingly it simply didn't give any new information! So particle physics has run into what I suspect is a temporary brick wall! It's been basically since their early 1980s that it hasn't changed!

So I don't see at the present time for me much profit than pursuing it! Hmm, gravity and quantum mechanics are what fascinate me!

Well, what are the other large unanswered questions that people are pursuing at this point? Because clearly it's not just you working on this, right?

No! I think other other things—well, in the context of there are huge problems in cosmology! In all of this—all of this cosmology is about quantum mechanics and gravity! Hmm, early cosmology—so-called inflationary theory—is about how quantum fluctuations imprinted themselves on the universe and led to things like 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—not that—not that 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!

Hmm.

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? So for many years people thought there was no dark energy. Hmm. We call it the cosmological constant, but it's the same thing as what people call dark energy! Okay? We have no idea, and so for originally we thought it wasn't there at all!

Hmm.

It's also, yeah! 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 minut that it took until the 1990s to discover any evidence for!

How is it measured?

It's measured astronomically and b—by modern observational cosmology—counting galaxies, counts, and all kinds of the quasar counts—all sorts of stuff! But the main point is, in the end, it turned out that it was there—the 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! Hmm, that's the puzzle—not why is there, but why is it not there in larger abundance?

Do you have a hypothesis?

Well, the usual hypothesis is that, you know, the usual hypothesis—the only one that I think makes any sense—which is outlandish, there's no question—Cavendish! It's not mine, I'm jealous! No, it's not mine! But I think 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—that it does fit together with the equations and so forth. That the universe is vastly bigger than the part we can see and that as you scan over the whole thing, you'll find places where the constants of nature are one thing, and other places where the constants of nature are another thing. Some places where this cosmological constant is more or less normal—which means much, much bigger than then 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! So he said the answer is the universe is very big and varied and we are where we can be! That's all!

We just did B! That's called the anthropic principle—it's a widely hated idea! Physicists definitely among scientists!

It is, yeah! It's a widely hated idea, but it just might be right!

So I was listening to a radio interview with you and you said similar to this that there—there was a discovery that there are relatively few ways of organizing matter than we thought there would be!

And the—hello! I was talking about—that's a good question! But my question is like could you explain? Because you said there are relatively few ways that don't turn into black holes!

Oh! Um, I don't remember exactly what I was talking about! Okay, but here's what I can tell you—almost all the matter, or almost only 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!

Hmm.

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 anything else. Even though black holes seem very rare in the universe, they contain almost everything!

Can you define information just for people?

Yeah! That's what's—in a computer! But this one bit—pieces, bits! The bit—we call them qubits because the 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!

Hmm.

We can also try to run it backward in time to all 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 have accounted for everything! So 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? And that number of bits is about 10 to the tenth 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!

Gotcha! Okay, so I briefly encountered this through the holographic principle that you worked on!

Yeah! And one—one question that I couldn't fully wrap my—there's another example of something which is considered a little bit radical at first! A little bit nuts! But of course it's now extremely mainstream!

Yeah, very mainstream!

But the—I mean, I would push back a little bit, you know?

Okay, go ahead!

Well, like anything that's fringe that becomes popular, you can say is mainstream, but it was fringe in the beginning!

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!

Mm-hmm.

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! A contest for brief periods of time—sometimes things become popular!

Mm-hmm.

But they don't last if they're just popular! They last if they have value—explanatory value, predictive power value! And the value—of leading to a consistent framework! In that sense, the holographic principle is now completely mainstream! And as—why is it mainstreamed? 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!

I worked at it!

Can you give a brief explanation? Because I—this was a hard one!

Yeah! Well—it has to do with black holes! It had to do with black holes and the information loss problem which it had to do with this discussion about information being lost in black holes! Which right there, Stephen Hawking's very, very brilliant insight—even though I think he got the final answer wrong, 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 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 a—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 the 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 its horizon! 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—the real curse statement was more like saying the stuff on the horizon is a kind of hologram of the stuff that falls in! Mm-hmm! So it's really only one thing 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—that's—yeah, so this is a check!

Perfect! Sorry!

Another 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 incomprehensible! But not in 15 minutes!

Yes, I—I—exactly! In 15 minutes, it's just the way it is!

And so, okay! But the point was that black hole horizons are behaving like holograms of anything that falls into the black hole!

Mm-hmm.

But then, when thinking about it further, we realized that the whole world could be in a black hole! You can't tell that's not in the black hole!

Okay, in particular, the entire universe has a horizon out at very large distances which is very much like a black hole horizon! And what kind of inside it? So 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—this was a strange idea that 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 Hirata Tuft—we put this idea forward! And 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!

Mm-hmm.

Who made a really rigorous, beautiful version of it, which now everybody believes! The mathematics of it was came—it was a string theoretic construction!

Mm-hmm.

Where one 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!

Yeah!

But that—that would be represented as information stored on the surface—on the outer surface of the world rather than in three dimensions as we normally think about it! Inside—yeah, yeah!

That—you know, one really nailed that with such mathematical precision that it just became part of our standard! It became a tool! Okay, that's a good thing when things go from being, they say, soft and start out as very speculative! Then they become something a little bit better than speculative!

Hmm.

Conjectural is better than speculative! And in the end processes they just become a tool of physics!

Hmm.

Things that everybody uses all the time because it has a predictive value—a mathematical value! The holographic principle is a tool now!

So, yeah, it's stuck!

So why—why does it have to be holographic? So in other words, say—say it's mapped around—I’m gonna have to bring this into a 3D world, right? So there's a 3D sphere—call it a black hole, right? Why is it holographic versus a 2D image, for example?

It isn't holographic! You mean why can't it just be like a picture on the wall?

Yeah!

Well, the picture on the wall is 2-dimensional! It may deceive you! You know, a clever painter can paint the painting which when you look at it, you think you see three-dimensional things! But you never do! You don't!

Then in particular, if you move your head around from side to side, you can't see what's behind the flower!

Mm-hmm.

There's nothing behind the flower! And you can—you were just deceived into thinking there was something three-dimensional layer! 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 with that, the—the picture of my—that the 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—it will look like a simply confused jumble or 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 pit—that 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! So you can't map the three dimensions to two dimensions without really making it totally discontinuous! The word is 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! But in a way which you could not easily reconstruct! It's more like a hologram than it would be like a photograph!

Mm-hmm.

And how does the reconstruction happen? So say—we are in a black hole! For a real hologram, all you have to do is shine the right kind of light on it, right?

Reconstruct the image!

Not here! Here! It will be a mathematical reconstruction! If somebody gave you the quantum state of the horizon of a black hole, and you were smart enough (if you meant that nobody's smart enough), but with sufficient—that kind of technology of quantum computation and so forth! And if we knew the precise rules by which black holes evolved, we could reconstruct from the quantum state of the horizon, we could reconstruct what fell in!

What’s inside, and so forth!

Mm-hmm.

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!

Okay! If somebody showed you the hologram incidentally of just—you know—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, you'd think in a little bit! Yeah!

Eventually, you probably could! But—but it would be very hard!

Mm-hmm.

And multiply that out to the universe?

Well, and—and they had quantum mechanics, which escalates the story hugely!

Gotcha! Slight tangent—have you followed any of these ideas around we live in a simulation—these simulation hypotheses?

Yeah! It doesn't seem to me to add anything!

Hmm, what does that mean—the idea that we live in a simulation mean that there was a simulator that somebody simulated us?

I believe so! Yeah! I think we live in a computer program based on our ideas! 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 a—yeah! But then somebody would say, "Oh, that's not what I meant!" I said, "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!

Mm-hmm.

Is—do we live in a computer program that somebody programmed for a purpose? I have no idea! I'd love to know! But, you know, then I would ask—I'm a curious person! I would ask them, "Okay, if there is that guy out there, let's not give him a name—did the program—the program of the simulation—who programmed him?"

Right.

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! Who knows? Who knows? Yes! Lever—a program them! That's right!

Yeah! And then who programmed the program? Who programmed the program? And so forth! There isn't—satisfy it just doesn't lead to any satisfying visitors!

Yeah! This reminded me—I was listening to your Caltech—your Feynman lecture and text, and you said something really nice which was—Feynman didn't much like philosophers philosophizing about science! And 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—and let yourself philosophize?

First let me say something about Feynman!

Okay, okay!

Feynman did this claim that is—this—like 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—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! I know some words and when you use them, but I don't know what they mean!

Oh, yeah!

As a substitute for simple thinking!

Yeah! Okay! That is what he didn't like! And yet I think in some ways, in some deep way, he was an extraordinarily philosophical person! If you read his works—you know? I don't mean his physics works; if you read 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!

And there was the crap that he didn't like!

Okay, yeah!

Yeah! I would say the same about mathematics! He didn't like the overly fancy mathematics, but he was a very math—a very good mathematician!

Hmm.

And—and what we were talking about before we started recording—like, he was also quite moral, right? In—in his philosophy of the world, you know? He was affected by that!

I mean, one analyst, Alamos, as well!

Yeah! He had a very—you know—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!

It's fair! Did you interact with any of the people that worked on the bomb? Hans Bethe?

Okay, Hans Bethe was one of my thesis advisors! Yes, so I did! But I didn't talk with him! Hans was not—he was a friend! But he wasn't a friend in the same way that Feynman was! He wasn't a soul mate!

Okay! In that depth—you talk about with Feynman, did you find that with your advisor? Did he—yeah—did he have the same sort of grief around the way it created?

Oh, well, I can't! Alright? I know the answer to that! Okay? But not from him directly! I know that from the answer from that just because it’s historical!

Yes, he—he was very upset about the bomb! And he—as much as anybody—worked hard—very, very hard—for disarmament! And nuclear disarmament! Feynman did not! Feynman just said, "Okay, I'm gonna do physics, and that's my—that's what I'm gonna do!"

Hmm.

Yeah, and he didn't work! Hans was very, very active in nuclear disarmament, so I do know that he regretted it! Yeah, but I don't know it directly from him!

Hmm. 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 or—I mean, already are! You could see, say, with Facebook or other things, you know?

Yeah! On the other hand the enormous amount of good that has come from technology of all kinds! So I think you can't not work on it!

How do you—yeah! At what point do you stop and say this is dangerous?

Well, I think it's probably built into some people's curiosity—the—the need to explore! And they're just gonna do it!

Mm-hmm.

It's not—I don't believe it's the physicists' job to decide what should and shouldn't be discovered! From a physicist's point of view, everything should be discovered if possible! It is the job of politicians and the people of that ilk to—to make sure that things are not misused!

The misuse of nuclear weapons was not really the scientist who built them! They were worried about the Nazis getting them! If there was misuse—that's also to 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!

So they were given a problem toward—a double problem! Part number one of the problem was the Nazis are gonna build it if we don't! And the second part, the second problem was, have you built it?

Mm-hmm.

They had no choice! I don't believe they had any choice except to go and do it—both the scientists and as human beings! The fact that it got misused, I don't believe was the scientists themselves, and if anything, those people tended to be very traumatized by the fact that they had built weapons!

You said he didn't work on disarmament, but do you think any of his focuses later in life were related to—I don't know—making—improving the world?

I think he would have said you improve the world by discovering what the world is! I think he would have said that! That's my job as a physicist! When I say my—I actually mean mine too, but—but I meant his—that is 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 is something that is—not—he did not see as his responsibility!

Does that align with your personal philosophy? Your reason?

I think so! I think so! Look, if I were to suddenly discover something that I knew was gonna 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 could hide it! You can’t hide it! It’s gonna come out if I don’t come out!

Okay, yeah!

So all you can do is warn! All you can do is warn people that—that this is there, it will be discovered, you gotta worry about it! Well, Beta 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 gonna focus on that! And I am NOT responsible for all the evil in the world, and I will—and I can be responsible for uncovering what nature is like!"

Mm-hmm.

Because I'm just curious what—how you've stayed motivated and been so prolific!

Yeah! With my career? Well, I think I’m also a curious person, curious—I mean, weird! Other people can decide that! I mean that I have a sense of curiosity about the world, right? And it just doesn't go away! I mean, I don’t die! I didn’t say to myself, "I'm going to continue to do physics until I'm 78 years old!" And there’s now—you know, I didn't plan that! I just get curious about things and [Music] that's it! I don’t have a choice!

What are you most curious about right now?

Gravity and quantum mechanics! How they fit together! What in particular? 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 gonna get us into technical discussion—the search—you want to do it?

Yes! Do it!

No! Yeah! No, I mean if I get dropped if some of the listeners have to drop, that’s okay! But certain people will like it a lot!

Yeah, right! It’s good! So one of the things that was discovered by myself and Juan Maldacena—there’s probably more, but Maldacena and myself, we wrote a paper together! It’s called the ER equals EPR hypothesis!

Oh, this is a great story! Incidentally—let's back up for a minute! Let me tell you the story about Einstein and ER and EPR!

Okay!

Okay? ER stands for two names: Einstein and Rosen! EPR stands for three names: Einstein, Podolsky, and Rosen! In one year, 1935, after it was generally deemed that Einstein had—you know—was basically finished as a physicist, or at least something like ten 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 they were wormholes connecting distant regions of space! They were called Einstein-Rosen bridges! If you look up Einstein-Rosen bridges, you will find that there are bridges which connect different regions of space—a black hole in one place and a black hole in another place has a connection between them!

And that was solutions of Einstein's equations! The other paper that he wrote the same year was about something called entanglement! And entanglement is something that can happen to quantum systems when they get correlated! And it's a very non-local kind of thing! It's purely quantum mechanical! It does not obviously have to do with gravity!

And these were two separate things! I do not believe that Einstein had any idea that they were connected—the Einstein-Rosen bridges and the idea of entanglement! And one of the really odd things was that the—very recent years, we found out that entanglement and Einstein-Rosen bridges are the same thing!

Hmm.

That in particular, an example would be if you have two black holes—black holes have all kinds of internal structure to them that quantum mechanical objects! Okay? 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 found out that they are the same thing—quantum entanglement and the kind of connectivity between systems that were called Einstein-Rosen bridges! So this was a weird quirk of history that in the same year Einstein discovered both of things—almost certainly didn't have any inkling that they were the same!

Of course, maybe he did! But what did the two papers say if they ultimately became the same thing? One paper said there are solutions of my equations in which distant black holes are connected by wormholes! Okay? The short run—shortcut between them! Isn't it an element of look?

That's about black holes! 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 this paper with him because we were working on something together!

Mm-hmm.

And that 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 now! Hmm, yeah! And trying to make a theory out of it! Trying to build a comprehensive theory!

And what was the technical—the technical part you wanted to get to?

The technical part had to do with something called quantum complexity theory! These wormholes that connect—you know, you might think if you have a wormhole connecting to distant places, you could jump in one and come the other!

Yeah!

Okay? 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 Jersey—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 they grew faster than you—in your speeding car? Well then you can't get out the other end!

Right! Yep!

That's the way these Einstein-Rosen bridges behave! Okay? So 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 is a computer science concept! It tells you how hard it is to reverse something!

And 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! Okay? So this question of quantum complexity theory has been sort of focused on what I've been thinking about—other people think about different things!

This is the main scent—the main focus of a lot of work—on what's going on, both here, Princeton, and all over the world! And where we'll go, I don't know! It's just fun to think about! It's going to pay—and they pay us to do it!

Yeah! It's not a bad gig!

So there was a related question from Twitter for you—so Noah asked, could quantum teleportation be used in the future as a means of intergalactic communication?

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, you know, the dots and dashes—most 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 the—mm-hmm—without sending a code classically from one place to another! And that will take the amount that—that will take time!

So you don't speed up communication! If it would take you a hundred thousand 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 hundred thousand years to do quantum teleportation!

So, yeah! You could use quantum teleportation to teleport stuff over vast distances, but it won't be any faster! It will be more secure, right?

More secure means more secret! You won't be able to crack it!

Mm-hmm!

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!

Okay, good to know! Related, Ryoga digital asked—Ryoga is—it’s a like a brand, it’s just someone with an avatar—how do you think quantum theory will shape technology in the future?

That's a very good question! Of course, it's already shaped technology completely, in the presence, on-going!

Yeah!

I mean, all the electronics in the world is all based on quantum mechanics, but it's a—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 number 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! This conceivably could be the people who build quantum computers and not figure out what to be able to do with them! Now, I don't think that will happen!

There's one thing that you can do with the 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 four hundred—more than—I’m actually more than probably a hundred qubits! One hundred qubits doesn’t seem like very much! No classical computer can do the calculation of following what one hundred qubits do!

So 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!

So that's a scientific purpose for it! You wanna understand how certain chemistry—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—quantum mechanics materials that depend for their properties on quantum mechanics! Classical computers 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 computers—you hope computers can solve? That remains to be seen!

The last thing I was wondering is—now, so you're both an accomplished physicist, but you're also a physics educator!

Ah, for better or worse! Right? All of your—all of your videos, your books—you clearly have a knack for communicating these ideas!

That's nice to hear! At least it works for me!

Yeah! If you can impart any particular ideas across the population—or about physics and understanding—what would they be?

You know, I don't really know! Let me ask a totally different answer! A totally different question! Why did I start teaching for the public?

Sure!

I think the simple answer is that it 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!

Mm-hmm.

And I—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 doesn't almost matter whether it's explaining to another physicist or explaining to a layperson! In particular, I found that trying to explain things to a layperson, I explain them honestly—not through fake analogies—but to try to give an honest and clear explanation of something often really focused my ideas on how everything works!

So it had value to me that was above and beyond just the fun of teaching! I did find that teaching for the public—the public Stanford's continuing studies was especially valuable this way! The students—they were all—they were anywhere from 50 years old to 95! That was actually true! There was a 95-year-old lady who—and she was—she followed the—she knew what she was doing!

Yeah!

So I found that the curiosity there—they had some degree of technical background! They tended to know a bit of mathematics—just a bit—through calculus! And they—were very curious about physics! I found teaching them to be especially gratifying!

And I really would spend a lot of time figuring out how to explain hard things to them—in the process, I often found that I understood them so much better!

Hmm.

So that was why I got into teaching and the public—called the public sector or whatever!

Mm-hmm.

I don't know that there's any particular thing that I would want to convey to them! You know, there's some obvious answers! You want to convey to them that science makes sense! You want to convey to them that scientists are phonies! That they really do sometimes know the answers to things! That there are facts and so forth! There, of course, all these things are true!

Was I motivated by that? Not really! I was just motivated by having fun and—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—passed a fifth grade! They were very curious about all sorts of things: some science, some history, and stuff, and they were mildly crackpot-y!

Hmm!

Why were they crackpots? They were crackpots because they had no venue in which they could find out what was real science from fake science! They were plumbers! They couldn't go asking physicists, "Is this real, or is that not real?"

And I always felt some sense that I would have liked to be able to go back in time to—to my father and his friends and tell them what was real and what was fakey stuff!

Yeah!

I was—and that—I don't know—emotionally! I think that sort of did come into mm-hmm, the reason why I liked teaching these people! It reminded me of!

Yeah!

Yeah, that's great! Well, thank you so much for your time!

Okay! You!