Leonard Susskind

One of the most interesting events in modern theoretical physics took place in January 1981 in the attic of a private mansion in San Francisco. The host, self-appointed saver of the world and self-help seminar guru, John Paul Rosenberg, who since 1960 had gone by the name of Werner Erhard—Werner in honour of physicist Werner Heisenberg, Erhard in honour of German politician and economist Ludwig Erhard—organised one of his beloved private EST conferences, gathering the leading lights of theoretical physics. Towards the end of the event the ‘wild’ Stephen Hawking offered the mathematical proof that information fallen into black holes disappears, leaving no trace when the black holes themselves evaporate. According to the story, only two of those present disagreed, convinced that the proof couldn’t be right since it violated the grounding principles of quantum mechanics, including the fundamental law of reversibility or ‘information conservation’ thereby making quantum mechanics impossible. The two were a genius Dutch physicist, Gerard ’t Hooft, and Leonard Susskind (1940). The episode of disagreement started a more than 25-year-long ‘black hole war’ (described with gusto and in detail by Susskind in his 2008 book The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics). The war formally ended on 23 April, 2007, when Hawking conceded that information could survive the evaporation of the black hole, and consequently admitted that his former proof was wrong. This clash of minds is just one of the cases in which Leonard Susskind, whom many regard as the world’s number one theoretical physicist, entertained a view regarded as impossible by the majority, only later for it to become the leading view or even common sense among physicists.

It is well known that uniting relativity and quantum mechanics is the Holy Grail for contemporary physicists. Susskind’s work revolves around those two extremes—massive objects of cosmological scale that bend the fabric of spacetime, and the tiniest excitations that change whenever we try to measure them. He is one of the founding fathers of string theory—a theory that invokes one-dimensional vibrating strings and symmetry considerations in order to discover deep truths about nature; some of its later variations assume that very dense, eleven-dimensional space is a necessary precondition for the existence of strings. Susskind’s thinking echoes his interests—on the one hand, it’s highly abstract and first-principle-centric; on the other, it’s intuitive and visual. Perhaps this combination allowed him to develop and make openly accessible his ‘Theoretical Minimum’ lecture series (on Stanford’s YouTube channel) that covers the entire subject of modern physics. He has been applying string theory to develop ’t Hooft’s idea of the world as a hologram—the idea that the universe can be described by the information on its surface. Most recently, Susskind proposed connecting gravity and quantum mechanics using wormholes. By his own admission, ten years ago he didn’t have the slightest idea that he’d be working seriously with a concept as science-fictional as this. The conjecture is called ‘ER = EPR’: ‘ER’ refers to wormholes, or Einstein–Rosen bridges, which are black holes that connect two otherwise unconnected regions of spacetime; ‘EPR’ stands for the Einstein–Podolsky–Rosen paradox or quantum entanglement—the state of a particle system in which particles, although far away from each other, still act as though they are linked or entangled. The conjecture suggests that two entangled particles are connected by a wormhole and may form the basis for a theory of everything.

Susskind is the director of the Stanford Institute for Theoretical Physics (SITP), but some say that the ‘S’ in that acronym stands for ‘Susskind’s’. At the end of our conversation, enchanted by the liveliness of his mind, I asked, ‘Under what conditions would you visit Latvia?’ Leonard Susskind replied, ‘I would come if you promised to introduce me to the three most interesting human beings in Latvia.’

Arnis Rītups and Stanislav Filatov

…you are saying that you will compensate me with a chocolate bar if I am not useful, but you will not compensate me if I am useful?

Yes.

But that is crazy!?

The world is crazy.

I am still not sure if I want to do this. If I now say that I have nothing useful to say to you, I will get compensated?

You will get a bar of chocolate.

And if I say to you something interesting or useful, then I won’t get compensated?

Yes.

I have nothing to tell you.

How can I convince you to speak?

What is you background, first? You are not a scientist?

No. I have studied philosophy for nine years.

But not technical, mathematical philosophy or logics?

No, history of philosophy.

You are Polish, yes?

No, I am Latvian.

O, so am I.

No way! It’s impossible!

My grandfather was Latvian, Latvian Jew. His name was Herman Meltz. How would that mean anything to you?

It might. But it does not.

He came to the United States in 1900.

We could say that there was no Latvia at that moment. It was part of Russia.

There was no Latvia? May be it was 1905. Where did the name Latvia come from? Because he always said that he came from Latvia.

May be it was a retrospective projection of later historical events?

May be. Was not the area known as Latvia?

Among some anarchists perhaps.

O, that would fit. The whole family was a bunch of anarchists.

Was your grandfather a plumber like your father?

That was on the other side of the family. This is on my mother’s side.

Then you are a genuine Jew?

O, yah. I am not religious, I have no religion whatever in that sense…

Most of the interesting people are Jews and I cannot understand why?

Most of the interesting people are Jews? Well, you are interesting and you are not a Jew. Do not ask me how I know it—I don’t.

I’d like to begin with an issue that’s been puzzling me ever since I started talking with scientists: the status of scientific theory. You’ve introduced several theories. What is the status of theory itself? 

The status of theory itself—that could be many, many things. You could ask me: What’s the status of quantum mechanics? Or: What’s the status of general relativity? I would take that to mean: How certain are we that it’s true? But you could ask me a broader question as in: What do we mean by theory? Or: Is theory a good way to do science? So which question are you asking?

I’m asking you to relate a few of the theories you’ve proposed to some kind of strict knowledge.

The theories I’ve mainly been concerned with in the last 10–15 years are about very remote phenomena, phenomena which are either so terribly small the best accelerators can’t begin to see them, or so big—bigger than the observable universe—that we can’t observe them. Many questions in physics are beginning to revolve around things so remote from an experimental point of view that it’s really not 100% clear they will ever materialise as consensus science or textbook science. You know how things work… In the beginning science is no different than art or anything else where people just have opinions. Nobody knows if their opinions make any sense. And slowly, slowly but surely—partly by making the theories more and more consistent, by connecting them with experiments, by connecting them with observation—after some period a little kernel of truth, objective truth, is distilled out of the whole thing, and everybody agrees on it. That’s called consensus science: science that has established itself as textbook science. There’s no way of knowing, when you’re dealing with things which are very remote, whether five hundred years from now anybody will even have heard of them, whether they will be part of consensus science.

As I understand it, they’re remote not only from experimental science, but also from what we can observe.

Experiment and observation are the same thing. 

Then where did you get the theories? 

They came out of the need for consistency between things we already knew.

And why do you need consistency?

Oh! You can’t tolerate having inconsistent theories which contradict each other. 

Because you assume that reality is consistent? 

I assume that reality is consistent. Could be wrong!

Exactly. Why do you assume that reality is consistent? 

Well…I don’t even know what it would mean for it to be inconsistent. You might ask: How do you know? I think that’s not the right question. ‘Inconsistent’ simply means that you can get two different answers. 

Or five. 

No, let’s say two answers which contradict each other. Reality can’t contradict itself. I don’t know what that means! Maybe it means something to a philosopher but it means nothing to a physicist. You could imagine a different thing; you could imagine that two different theories are non-overlapping, that they talk about totally different things for which there’s no intersection. In that case you might say not that they’re inconsistent, but that they don’t have to be consistent. It is not known whether the theory of gravity—the general theory of relativity—is consistent with quantum mechanics. But we certainly know there have to be overlaps between them. We certainly know there are things in the world which involve both quantum mechanics and gravity. If two of them are inconsistent and give inconsistent answers about certain questions then that’s intolerable to a physicist! Physics is all about a consistent description of nature. 

But to a large extent, don’t both these theories whose consistency you and others are searching for depend on what questions you ask at the very beginning? 

Yes, of course, they’re dependent on the questions we ask, and we try very hard to formulate questions which make sense. Some questions are not really questions. 

Give me a vivid example. 

‘Is there a God?’

That’s not a question? 

It sounds like a question but you can’t imagine how it could possibly be answered definitively—at least scientifically. 

So you think a question by definition must be a scientific question? 

No, I don’t think all questions are scientific questions. What I’m really talking about is whether a question is scientific. Now, what can I mean by: Is there a God? I might mean: Was there an intelligence that created the universe? It sounds like a real question! On the other hand, you can’t imagine it could be answered—at least I can’t. 

And the fact that you can’t imagine the answer…

It makes me suspect that it’s not a real question, that it’s not a scientific question. I think generally scientists take a view—maybe it’s a hidden view, but they take the view—that real questions are questions which, in principle, not only have answers but can be answered. You could say: Well, there’s no reason why that’s what you have to mean by ‘a question’. That’s what a scientific question is: one which not only has an answer, but for which there’s a procedure for finding the answer.

There seems to be a relationship between scientific questions and imagination. You said you can’t imagine how the question could be answered. Freeman Dyson describes string theory as a product of imagination….

First of all, I must say about Freeman…he’s a great man and all, but he’s a contrarian. He will almost always take the opposite direction from everybody else—he loves to do that. I think it has prevented him from being the great physicist he should’ve been. I shouldn’t be talking about him….

Let’s forget about Dyson and concentrate on the relation between imagination and string theory, which you’ve helped to develop. 

As I said, science, when it’s first being formulated, is as human as everything else. It involves imagination to the same degree that art involves imagination. Much of my work has been stimulated by pictures in my own head and all kinds of things; other people function differently—they use their imaginations in different ways. But the final product at the end—the little core of knowledge, the little core of scientific fact—is not a thing of imagination. It’s a thing that can be reproduced in a laboratory; it’s a thing that can be…

None of the things you’re talking about, the unobservable phenomena, can be reproduced in a laboratory. 

That’s what I was saying, that there’s a fear among the people who think about these questions: cosmological questions, questions about things which are at present too small for us to see. There’s a fear of it never becoming that kind of little kernel of truth. 

Do you share this fear? 

Oh, yes! Absolutely. 

If it never happened, wouldn’t that be a wasted lifetime then? 

No, no, I don’t worry about wasting my lifetime…. No. I’m a curious person, I indulge my curiosity. I don’t really care what happens after I’m dead. I don’t care whether anybody remembers me because I will not be here to get any benefit from it. I’m pursuing what I’m interested in, that’s it. 

So you agree that those things are pure speculations in a way. 

Certainly not pure speculations. 

So string theory is not pure speculation? 

No! Pure speculation is also a random speculation. ‘Oh, I think the moon is made of cheese’ is a pure speculation—no better than the pure speculation that the moon is made of…err…peanut butter! How is one better than the other? On the other hand, if you say, ‘The moon is made of rock!’ ‘How do you know the moon is made of rock?’ ‘Well, I don’t know if the moon is made of rock, but the Earth is made of rock, and the moon seems to be the same kind of thing as the Earth. Most likely it was created in the same event or series of events that created the Earth so I think it’s a good bet that the moon is made of rock.’ That’s an educated guess, if you like. It’s not a random speculation, it’s not a pure speculation—it’s trying to pull together facts you already know. I would say the sort of things my colleagues and I think about are like that—we know this, we know that, we know the other. They don’t seem to fit well together; that’s why we’re trying to create a single theory that sits in the middle and is mathematically consistent. ‘Mathematically consistent’ just means it doesn’t lead to contradictions and you can be quantitative about it—you can calculate things and test them. 

Test them? 

Test them mathematically—test their internal consistency, make sure you get the same answers no matter how you approach them, whichever way you approach them. If the theory leads to inconsistency, you throw it out! I wouldn’t call that pure speculation; I would call that trying to pull together things we already know—things we already know about the universe, about elementary particle physics, about gravitation—which don’t fit comfortably together in present physical theories, but saying to ourselves there’s one reality that contains quantum mechanics, gravity and all these different things. That one reality has to have a consistent mathematical framework. You could call it faith, you could call it whatever you want to call it. You could call it experience, you could call it that line of thinking’s historical success, but we expect our theories to be consistent. And if they’re not, out they go.

Was there a theory you contemplated for a while and then found to be wrong and inconsistent?

Oh yes. Yes. Yes, indeed.

Could you give me one understandable example?

Well, I don’t know how understandable it is…. One theory I was sort of famous for which turned out to be wrong was called the theory of dynamical symmetry breaking, sometimes called ‘technicolour theory’. It was a theory which was an early replacement for the Higgs boson. You know what the Higgs boson is? You’ve heard of it anyway. 

Sure. And I’ve heard it explained several ways. 

Right. This was a theory which attempted to pull together things we already knew about—the weak interactions, the standard model of particle physics and other things—without introducing reasons. One didn’t want to introduce the Higgs boson because of something called gauge hierarchy, which was, and still is, a very unstable element in the standard model of particle physics. But! The important point is that after basically a year, a year and a half, of thinking about it, inconsistencies—both internal inconsistencies and inconsistencies with empirical facts—began to surface. You know, you don’t immediately say: Junk it! You hope you can make it right, you hope you can find some tricks, some missing ingredient you hadn’t put into the theory. But after a while you just encounter one contradiction after another and see that it was wrong, just wrong! Stephen Hawking attempted to prove that quantum mechanics was wrong. In a brilliant insight—really, really brilliant insight—he came to the conclusion that quantum mechanics would necessarily be broken, that it would be destroyed by black holes. Now I think even he believes he was wrong about that. 

But it took some 25 years. 

It took 25 years! How did we come to the conclusion he was wrong if nobody was ever able to make an experiment? Well, his argument was fundamentally a mathematical argument. It was fundamentally an argument about the mathematical inconsistency of general relativity with quantum mechanics in the presence of black holes. By analysing the mathematics we just eventually discovered that Stephen was wrong! That at the mathematical level he was wrong. 

You discovered, not we. 

Yeah, yeah, yeah…very few people were involved, but it wasn’t only me. But that’s correct. Once the mathematical idea or mathematical construction was in place lots of other people caught on, realised how it worked and made it extremely precise—extremely mathematically precise—to the point that now it’s become consensus science. It’s interesting, it became consensus science without any experiment. But forget what I did! What Stephen himself did—the idea that black holes evaporate—where did that come from? That came from putting together general relativity with quantum field theory. These were two ideas, two completely different mathematical frameworks. Stephen said, ‘They have to fit together.’ And in an attempt to fit them together he discovered that black holes evaporate. It was no experiment—nobody will ever do an experiment to test whether black holes evaporate—but it’s absolutely consensus science. 

One thing apparently worried you when Stephen Hawking formulated his mathematical theory of things evaporating into black holes—you’ve said it on many occasions: some bits of information cannot disappear, whereas the suggestion was that they do disappear. Why shouldn’t they be able to disappear?

That’s the difference between somebody who knows some technical physics and somebody who doesn’t. All of physics as we know it was fundamentally based on the idea that… Let’s say what information disappearing means. Information disappearing means the distinction between two things disappears over a period of time. It means two different starting points can lead to the same endpoint. 

And you consider that an inconsistent idea? 

It’s inconsistent with everything we knew about physics before that. All of physics—classical physics, quantum physics—was based fundamentally on distinctions being preserved. Can you imagine a physical world in which distinctions are not preserved? Yes! Yes, you can write equations but… Everything we know through thermodynamics, statistical mechanics, classical mechanics and quantum mechanics was all absolutely based on this idea that distinctions are frozen into a system and cannot disappear. 

Just to clarify… For instance—if we could imagine such a scenario—annihilation of the universe is a clear example of all distinctions being gone!

Yes. 

So all assumptions are based on another assumption—that the universe cannot be annihilated. 

I think there was a difference between… You’re right and certainly we don’t know that the universe cannot disappear. We certainly don’t know that. 

All distinctions would disappear. 

Yeah. No, we were talking about… That’s right. Once it comes to the universe as a whole, we don’t know what we’re doing. But when we were talking about experiments that could take place in a laboratory—controllable experiments, the kind we’re normally familiar with, or phenomena that don’t involve the universe as a whole—everything we knew depended fundamentally on information being preserved. Hawking’s idea just felt wrong. My sensor for wrong things and right things is good. But can you know exactly what said that can’t be right? We certainly had some arguments, but in the end it wasn’t science; it was intuition: it didn’t feel right, it didn’t smell right. It came from some inner sense of… I think it’s fair to say that some people in science have it and some people don’t.

An idea which seems to have become consensus is that the laws which govern everything are rigid, so to speak, while simultaneously, all the constants are being fine-tuned for certain things to emerge. Does the rigidity of laws and fine-tuning of constants tell us anything about the origins of the whole? 

Personally I think it has a great deal to do with it. The fine-tuning of constants…again this is a feeling more than something I can prove…the fine-tuning of constants needs an explanation. When a constant of nature could be anything but it’s fine-tuned to an extraordinary accuracy, and all we can correlate it to is that if it weren’t fine-tuned in that way life wouldn’t be possible, we wouldn’t be possible… We wouldn’t be here to ask the question! It requires some explanation. Either we have an independent explanation for why the number is what it is—and I think we’re talking about a number called the cosmological constant, but there are various constants in nature… It just appears extremely unlikely that certain numbers in physics are exactly what they are, and our only explanation is that if they were slightly different we couldn’t be here—the world would not, the laws of physics would not be such that life could exist. So we come to the question: Is it just an accident that a certain number is exactly what it is, or might there be some connection with the fact that we exist? All right, so there’s a very simple example—I’ve used this example over and over and over again; other people have also but I may have been the first; I don’t know. The Earth’s temperature happens to lie in a fairly narrow range. If it were much larger, water would boil and the Earth would have no water on it; if it were much lower, everything would be frozen solid ice. So you could ask: Is it an accident? What kind of lucky accident is it that the planet we live on happens to be fine-tuned in just the right way for us to be here? The answer, of course, is not that the Earth’s temperature is fine-tuned. The answer is that there are many, many, many planets out there—a huge number of planets!—of all varying kinds of temperatures: some hotter, some colder and some tiny fraction in this narrow fine-tuned range. Where do we exist? We exist where life is possible! It is tautology. It doesn’t require a deep thinker to say: Life will exist on planets where life is possible. The idea that’s been around for a long time, due partly to my colleague, Andrei Linde, and other people, is that the universe is simply very, very big. And very diverse. In the same sense that planets can differ from each other, regions of this very, very big universe can differ from one another. You use the word ‘rigidity’ to describe the laws of physics. I suppose that would mean the laws of physics are only one particular thing and they don’t vary from place to place.

Or from time to time. 

Or from time to time. But every indication from fundamental physics—from string theory, quantum field theory, many, many things—suggests the laws of physics can change from place to place. 

And from time to time? 

And from time to time. Of course, the laws of physics—if they are really laws—must not change. If the law is a law, which means it cannot change, then it cannot change. But that’s not what we mean by the laws of physics. The laws of life do not determine exactly what life is like. What are the laws of life? Let’s take the laws of DNA molecules. The laws of DNA molecules do not say that a human being must have five fingers on each hand, five toes on each foot and two ears. If we’re looking for an explanation of why human beings have two ears, five fingers and five toes, we’d better not look in the laws of DNA. Because we won’t find it there. But nevertheless we could say it’s a law of human beings that they have five fingers. That’s a law of human life; it’s a law of human beings. Well, don’t be too surprised if on the next planet we discover intelligent creatures with six fingers, or four fingers, or ten fingers! Because it may be a law of human beings but it’s not a law of DNA. 

So that’s a contingent fact. 

That’s a contingent fact. Right, so the question is now whether the facts that we see about physics in our neighbourhood of the universe are laws…

…or contingent facts.

Or contingent facts. And we don’t know! We don’t know with any certainty, but several different things coming from different directions are suggesting that the laws we see here are contingent facts. Among other things, they are not very beautiful or simple. 

Really?! I have heard that beauty is one of the criteria for judging the truthfulness of equations. 

Equations, yes, but actual facts about what the laws of physics are… How many particles are there? How many different kinds of particles? Oh, there are quarks, there are leptons, there are gauge bosons, there’s the Higgs boson, there’s this and that…. They’re all over the place. 

Like a zoo. 

Yes. What are their masses? Oh, they range from this number to that number to that number to that number—all over the map. What are nature’s constants? Just random numbers that apparently have no connection to each other. The fact is although there’s a lot of pattern there’s also a lot of randomness. Randomness—I don’t know if that’s quite the right word…inelegance. There’s nothing particularly special, nothing that captures the eye like ‘This is the beauty of the pure kind’ that a physicist would like to believe the laws of nature are. No, it’s not like that. It’s a hundred different kinds of particles, a hundred and fifty different constants of nature. Where did they come from? They just seem random. This suggests the laws of physics we see here are no more fundamental than the fact that a person has five fingers or an elephant has a crazy nose or an octopus has eight arms. They’re just contingent facts. That is what I think a growing number of physicists… They’re not happy about this! Nobody was ever happy about this. You know, they just had this idea they’d find one unique, special explanation of everything. It would be beautiful; it would satisfy all the criteria of elegance and aesthetics. They didn’t want to believe that the laws of physics we’ve been studying—particularly the laws of particle physics—they didn’t want to believe these were just contingent facts, random facts. Disappointment! Big disappointment. But I think more and more physicists are accepting that this may be the case. We don’t know for sure how it will play out in the end. We don’t know for sure that somebody won’t come up with an explanation for why all the constants in the particles are exactly as they are. We have only studied, only know about, one universe. We only know about one set of laws of physics, and it may or may not be that trying to explain every detail of these laws is like trying to explain why creatures have five fingers. It may work and we may actually discover that we do understand why the laws of physics are what they are, but in my opinion we’re more likely to discover it’s a contingent fact, and if we go far enough away we’ll discover worlds or parts of the universe where the facts are just different. But we’ll find out. Eventually we will find out. 

When I think about this view, I have a hard time understanding what even qualifies as a law if all currently known constants can also be perceived as contingent. What criteria must be fulfilled for something to qualify as a law? 

Yeah… At what point you actually achieve final laws I don’t know. DNA is a good law for biology but of course it’s not the final law. There are laws of chemistry, laws of physics underlying it and so forth. 

So it only explains a certain realm or a certain scale of events. 

Yes. What you think of as a final law may not be the final law. It’s pretty final for ordinary biology but…why DNA is the way it is…it’s a chemical fact, and the chemical fact depends on quantum mechanical facts and physics facts; physics facts depend on deeper physics facts. A model or example of how there could be underlying laws deeper than the laws we see that may or may not be final is string theory. Now, I’m not enough of a believer in string theory to say I know it’s the final answer to particle physics, but it’s an example of a theory. 

Not a believer but one of the inventors!

Yes, yes, that’s right… I didn’t say I wasn’t a believer—I said I wasn’t enough of a believer to say I know, OK? That’s a different statement. Yeah, I think it’s probably right but that’s not the point; the point is it’s an example of a theory which, like DNA, can be put together in many different ways. There are many, many possible ways a string theory—a particular example of string theory—gets put together. Those possibilities are like the possibilities of putting together a DNA, so the string theory becomes a theory from which you cannot expect a unique explanation for why the proton is 1,800 times more massive than the electron or why any given contingent fact is a contingent fact. String theory is an example of a law which does not determine the laws as we see them—it determines a set of rules for what possible laws can exist. It is an example—maybe the only example, maybe a unique example—of a set of principles in a set of equations which allow many, many possibilities. If those possibilities are sprinkled about in different contingent behaviours in different parts of the universe, then we might have an explanation for why the universe was put together—our corner of the universe!—so that we can exist here. 

Do the discoveries made by the Large Hadron Collider provide support for any theory or make any theory more likeable? 

The only idea they make more likeable is the idea that the laws of physics as we know them are contingent facts. That’s the only thing they make more likeable. Everything else they make less likeable. 

I understand you’ve recently introduced the holographic principle. Does this only apply to the edges of black holes, or do you think it may apply to other domains of reality as well? 

I think the consensus now is that it can be applied to everything. 

To everything? 

Yes. Look, what was the statement? The holographic principle states that if you take a region of space, a well-defined region of space, everything taking place in that well-defined region of space can be described by a theory which illustrates the region’s boundary. As if…this is a metaphor; there’s no real film out there…but as if everything was taking place on a film at the boundary of that region, everything inside it being a sort of holographic image. You can make that region bigger and bigger and bigger until it becomes the whole universe, and then you could say the whole universe is describable by a theory having to do with the very boundary of the universe, that it’s like a hologram of the universe’s boundary. I won’t try to explain the mathematics of it right now but…yeah, I think that most people—well, not most people, but most of the physicists I know and have a lot of regard for—think the holographic principle is more than just a black hole. It’s a description of any region of space, even outside the boundaries of the whole universe. The universe is in some sense a hologram. But this is still a work in progress. This is not something we fully understand. 

But if the principle applies to the whole universe, in a way it introduces something called perspectival reality, where reality depends on perspective. 

That has been true for many, many years. 

Well, in a sense it has been true for…

…a hundred years. Or more! Einstein said that simultaneity depends on the observer’s state of motion. He was talking about certain aspects of reality being dependent on…

That’s exactly the point! Those were ‘certain aspects of reality’ whereas in this case…

It’s just more aspects. Surprising things that we didn’t expect but…

I assume you’ve noticed that some of these things scientists formulate spill over into the general public’s awareness, and the general public then begins to realise…

Yeah, that something crazy is happening. 

‘All reality is illusion!’ Or, ‘It’s all projection!’

I would be very wary of making broad philosophical… I’m not opposed to philosophy! My friend Dick Feynman didn’t like philosophy.

Why? 

Look, he was as philosophical as anybody. He just liked to play with people. 

Because he asked philosophical questions, questions which by all means…

Deeply philosophical questions! I think a lot of us don’t like it when philosophers…there aren’t very many who do this, but when philosophers tell us what is and isn’t science and how to do science. Karl Popper was a guy who told scientists how they should be doing science…. So that was annoying. And Dick didn’t like that kind of thing. I think he understood as much as anybody that physics is a deeply philosophical subject and that physics can inform philosophy. It’s when it goes the other way—when you think of philosophy informing physics, telling physics how it ought to be done—that scientists or physicists are somehow unhappy. My statement was that I would be very wary, at least at the present time, of drawing deep philosophical lessons from the holographic principle. We don’t understand it well enough; we’re still groping around it. We have very precise mathematical examples, but its deeper meaning… I don’t think we know it. Perhaps in the end, when it’s all understood, the philosophical implications will be very profound—maybe they will say the universe is in some sense an illusion. But just as likely they’ll say something else! I don’t feel like an illusion, do you? 

Sometimes I do, yes. 

Me too.

You do?

Yeah, sometimes. 

If I may ask, when do you feel like an illusion? 

Oh! Sometimes I have this weird sense of ‘Yes, I am an illusion!’ but it’s got more to do with my hormones than…

An illusion of whom? As in someone is dreaming you?

Well, I didn’t mean that. Right… The holographic principle, according to some people, says the world is an illusion, but I don’t think the implication is an illusion of somebody.

But at least it’s an illusion for somebody. 

I wouldn’t say that. No, I don’t want to use the idea that it’s an illusion. I think a better statement is that there are two parallel mathematical descriptions of the same thing. A hologram has two sides to it—and in some sense two equivalent sides. There’s the description of the film the hologram is coded in, and that film, if you were to look at it, just looks like a bunch of random little light spots and dark spots, and its mathematics is just basically the mathematics of a very complicated interference pattern. On the other hand, exactly the same information can be crystallised into a three-dimensional image by showing some light on it. Which one is more real? Is the piece of film more real than the thing of which it is an image? The thing of which it is an image is just as real as the piece of film. Which one is which? I don’t know; I don’t think it’s necessary to say at this point. Is true reality the reality at the surface of the universe that describes this holographic theory, or is true reality the reality of the actual thing that you normally think about as the universe? We have two mathematical descriptions of the same thing, and one of them is just more precise than the other. Holographic description is more precise, but which one is more real?

How does time fit into the holographic description? 

Not comfortably. Time is just time. 

What’s that? What do you mean by ‘Time is just time’? 

The holographic idea has not much changed our idea of time; it has changed our idea of space a great deal. It has not changed our idea of time very much. Now, that could be temporary! I hope it’s temporary because everybody wants to have a deeper understanding of time. The fact is that the holographic idea has not had a great deal to say about it. So at the present, time is time—no big change.

But as a scientist, can you also imagine time as a holographic reality? Meaning, in layman’s terms, that perspective could determine whether time is a kind of one-directional arrow or the simultaneity of the whole?

Yes, you could certainly imagine it. What’s more, there are even some ideas and theories along those lines. I don’t think they quite hold together yet. Look, I think the right thing to say is that you should keep watching, if you’re interested in these kinds of questions. Because it’ll change—the idea about how time fits into the holographic principle is going to change. I’m not a very experienced physicist, but then again I suspect I’m as experienced as anybody alive—I’ve been doing this subject for more than fifty years. Very few people can say that, so I’m experienced in that sense. And experience tells me: Expect surprises. Chances are pretty good that a large fraction of the things I’m saying will be true. Something will come along and flip it on its side and say that the lesson you were trying to draw from it is a completely wrong lesson. If I knew how to tell you what surprises are coming, they wouldn’t be surprises. And even if they were surprises, I knew them; I would publish them and I wouldn’t tell you. 

But wouldn’t you agree that the nature of time is still one of the deep unsolved questions? 

Absolutely. But again, it’s one of those questions you’re not absolutely sure is a good question. 

Whether it’s a scientific question? 

Yeah. Time is there. Will there be a deeper explanation of it? I don’t know. Maybe it’s just there! My feeling is there’ll be something deeper, but maybe it’s just one of these things which is there. It may have a deeper explanation, but it may be totally unavailable to us. Another question is: How much of nature can we penetrate? How much of nature can a human mind penetrate? My own feeling is the human mind is a very flexible organ that can adapt itself to a very, very wide range of questions as long as they’re real questions and as long as it’s possible to answer them. But which of the many questions we ask… ‘What is consciousness?’ Is that a real question? Can you imagine an explanation of consciousness? 

I think we still don’t know whether it’s a real question in your sense, in the scientific sense.

That’s right… ‘Was the universe created by an intelligence?’ Questions of that nature. Make up the answers! Maybe they’re real questions. On the other hand, I do believe that a human being is just a physical system made up of physical molecules and so forth—and how to connect that with my sense that I’m conscious? I just don’t know! I can’t even imagine how to find out. 

I assume you don’t think that thoughts consist of protons and electrons? 

Thoughts? 

Thoughts. 

In some way yes and in some way no. 

In which way no? 

It seems to me a thought transcends the physical things that make it up. On the other hand, I do believe that all there is in nature…is nature! And nature consists of physical objects that make it up. 

So you can’t accept the idea that some things do not exist in time and space.

I can’t accept it on the one hand. On the other hand, in a certain sense I do think that thoughts and consciousness and those things… I don’t see how we can express them and understand them in terms of ordinary physical realities. That’s what I mean when I say we don’t know if some questions are real questions—real questions in the sense of scientific questions. 

Werner Heisenberg apparently found some inspiration for his thinking reading Plato.

I didn’t know that. 

I wonder what you think of Plato’s statement about time being a moving image of eternity. 

I like another one better—I think it was Einstein who said time is what makes things happen. I don’t understand what Plato said. Maybe it doesn’t translate well from the Greek. 

In a way I see a certain analogy there to the image of two perspectives in time—from a certain perspective all time could be considered simultaneous. That would be a kind of eternity. But for me the idea that all events in time are simultaneous is utterly strange.

That’s right. But perhaps this is because you evolved as a certain kind of… Evolution played an enormously large role in the way we think about reality. We think about reality as three-dimensional—not seven-dimensional, not four-dimensional. But it’s worse than that; we can’t even think or at least visualise…any normal human being cannot visualise a five-dimensional space, seven-dimensional space. You can’t even really visualise two-dimensional space! If you close your eyes and try to think about two-dimensional space…well, two-dimensional space is a surface. 

It’s a plane, right? 

Yeah, it’s easy to visualise, right? Now, close your eyes and visualise a plane. Let’s go down one dimension—let’s visualise a one-dimensional space. 

Well, that’s a line. 

Close your eyes. Can you visualise a line? 

Well, I can draw a line in my mind. 

Yeah, but when you think about it, don’t you see that line as being a line through three-dimensional space? 

Yes, I agree. 

Don’t you agree that it’s a surface in three-dimensional space? 

Sure. 

So you can’t even get away from three dimensions to view two dimensions. Why is that? What’s special about three? Is it something special in physics? Nope. It’s your neural architecture that evolved… Darwinian evolution! Because you live in a three-dimensional world, the last thing you want is a mind that sees in four or five dimensions—you’d probably trip over your feet and fall off a cliff. We were wired in a certain way, and that certain way was wired for a certain class of phenomena. Classical physics, for example. We didn’t evolve a natural instinct for quantum mechanics; we didn’t evolve a natural instinct for five-dimensional worlds. We evolved a neural architecture which fits together with the world of throwing stones and… I forgot how we got on to this; something made me think of this, something you mentioned. I don’t remember what…but it certainly had to do with how we view nature and…time! Yeah, we were talking about time. I suspect that the way we see time—as a flow and an evolution—is in part…because if we didn’t see time this way, we would be very ineffective spreaders of our own genes; we would not be good evolution machines. I think probably to some extent evolution created the sense of time we perceive in our heads. So really we can’t think of time without thinking of going from one step to another step to another step. Perhaps there were people born throughout history who had this picture of time as just a big long time where everything happens sort of…simultaneously? Is that what you were saying? Yes, let’s call it that, from one perspective. And those people were wired in such a peculiar way they didn’t survive past their first birthday. So it’s very hard to tell which of our prejudices about the way the world works—time, space, all those things—are consequences of our evolutionary history and which are really built into the phenomena themselves. However, what allows us to escape from that cycle is abstract mathematics—we formulate time as a one-dimensional axis, we write equations with it, we solve those equations and then we do experiments and judge whether those experiments were adequately and correctly described by our mathematical model of time. And it seems to work! So we escape from our evolutionary constraints through the use of abstract mathematics. 

Do you think the whole universe, megaverse, multiverse or whatever could be like an arrow flying towards some target? Could there be a target, a goal that somehow determines the events occurring on the way towards it? Does this make any sense?

It could. Again, this may be one of those questions that’s not a question, or not a good scientific question. But I can imagine that the universe was set in motion by an agent for a purpose. Why not? I can imagine it! I can also imagine that’s not the case, that it was just a random event that took place at some instant and started the whole thing going in some totally meaningless way. 

So you can also imagine a random beginning. 

Meaningless at least from the point of view that my mind can give it meaning. Perhaps there are meanings—meanings and purposes—that a human being, a human mind, cannot even conceive. But if so, I can’t conceive of them. Do the universe and its evolution have a purpose—to somebody or something, or some agent? Who knows! I certainly don’t, and I don’t see how it’s possible to answer that question. Am I curious about it? Of course. Do I ever expect to have an answer to it? No. So I tend to think this is one of those questions which sound like an awfully good question but for which it’s impossible to imagine an answer, a conclusion. That separates the scientific questions from the non-scientific questions. 

Couldn’t such a deep scientific question be answered in a deep meditation? 

I don’t believe that. I personally don’t believe that, no. Different people who meditate simply come to different conclusions, and there’s no way to test them. I think that has a lot to do with your brain’s wiring and your particular hormonal state at a particular instant in time. I think spiritual things, things of that nature, have more to do with biology, the biology of brain. But it’s just my opinion! I am an atheist, more or less, but I am not an ideological one. It’s not an ideology I don’t like—Christianity or religion in general—nothing to do with that. I’m curious, and the religious answers don’t satisfy my curiosity. As soon as somebody tells me God made the world, I want to know who made God. And I want to know: Is God made of atoms? Does God satisfy the rules of quantum mechanics? And it seems to me more questions are opened up than answered. It just doesn’t answer the question! 

But if you assume thoughts transcend their own biological basis, couldn’t you suppose that…

I said it seems to me that they do. But at the same time I cannot tell if that even means anything. I used the word ‘transcend’! But I was using it not to say one thing or the other; I was using it to express my own confusion. I said ‘they seem to’, ‘it seems to me’. But I am a product of my own evolution, and I am a product of my own hormones, and I am a product of my own biology; and how much of that is simply a trick of my own mind that was created for evolutionary purposes and which somehow is useful? I don’t know. 

So you believe some purposes exist. Something was built for some purpose. 

I certainly believe that. 

Eyes were built for the purpose of seeing, the brain was built for… 

In some sense, yes. But I am also a very strong believer in the selfish gene hypothesis. 

Really?!

Yes. I may hate it—I don’t like being just the vehicle for my genes; I feel that I am more!—but it fits together with my instincts as a scientist, my instincts as a physicist. It’s a hypothesis which seems to explain an awful lot. You may hate it but…

Well, it’s not very complimentary. 

It’s not very complimentary at all! But science was not intended to make us feel good. And it does explain a lot! So given that it explains a lot I take it very seriously, yeah. Why were you so surprised that I take it seriously? 

Because I think it’s mythology; to a large extent it’s pure mythology repeating the ancient… It’s a story that imitates ancient models of story-telling. 

But all science is a story! How about Darwin? Is Darwin a story? Sure Darwin is a story! But Darwin in my opinion was one of the great theories of nature. 

He was a great observer, and to a limited extent he made a wonderful narrative—no doubt about that. 

But that’s what a wonderful narrative is—it explains so much! It explains so much which previously had no explanation at all. Unless you’re religious. I’m not. 

I’m interested in knowing whether you think it’s possible some things exist outside time and space. Does this possibility make sense to you as a physicist and a scientist? 

I don’t think I’d go so far as to say such things are meaningless, but I think we’d have to be much more precise about the nature of those things, what we mean by them, before I’d want to say that. 

Recently a guy who studies ancient philosophy said, ‘Well, it’s very easy—two plus two exists outside space and time.’ But maybe that answer is too easy.

No, no… I think that’s an example of mathematical existence. The numbers exist, and many other mathematical things exist in a sense which a mathematician would call existence. They don’t exist in space and time. It seems to me a different kind of existence, and we’d probably be making a mistake to conflate them and think of them even using the same term. Perhaps we shouldn’t use the same term. Three exists, electrons exist, you exist… They may be such different kinds of existence that we’re abusing language by using the same term. But why, I’m getting in over my head! I don’t know the answer to these things; you don’t know the answer. We keep learning more and more about the world, and at any given stage, there’s always some part of it beyond the curtain. The curtain opens up, we see more and more of the stage…and there’s always more behind it that we cannot see and don’t know. And what’s more, it’s always been the case that every time we open a little more, we discover that what we were thinking about the part we saw was wrong! Or inadequate. Or incomplete. So it’s so hard to guess the outcome of the things we seem to be learning at any given time. Very frustrating! You know what the only thing worse than not discovering all the answers would be?

Discovering all the answers.

Yes. Then there wouldn’t be anything left to do.

Questions by Arnis Rītups