The Joy of x

Frank Wilczek on the Strong Force, Quarks and Dark Matter

The theoretical physicist Frank Wilczek explained what holds atomic nuclei together, and he is still pushing at the limits of what the standard model can tell us.

Frank Wilczek of the Massachusetts Institute of Technology is responsible for some of the greatest accomplishments in theoretical physics over the past hundred years, including an explanation for the strong nuclear force and key contributions to our understanding of quantum chromodynamics. This week, Wilczek talks with host Steven Strogatz about how he made those discoveries, what is still missing from the standard theory, and a possible explanation for dark matter. This episode was produced by Dana Bialek. Read more at Production and original music by Story Mechanics.

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Frank Wilczek: When I took this call I was thinking, well, they’d just say, “Congratulations, you won a Nobel Prize, goodbye.”

Steve Strogatz: Yeah.

Wilczek: But it wasn’t like that at all.

Steve Strogatz (narration): From Quanta Magazine, this is The Joy of x. I am Steve Strogatz. In this episode, Frank Wilczek.

Wilczek: They described this process. There were various Swedish friends who extended their congratulations and so it was —.

Strogatz: You’re saying  —.

Wilczek: So, it was about 20 minutes, 20 minutes —.

Strogatz: They’re all on the phone, they’re jumping in?

Wilczek: And this whole time I was, yeah, the whole time I was on the phone — we talked 20 minutes — I was dripping wet, naked because I had just gotten out of the shower. But, you know, I was kinda — I didn’t want to interrupt the conversation, so we just kept going. And my wife sort of dried me off.

Strogatz: [LAUGHS]

Strogatz: Frank Wilczek is one of the world’s great physicists. He’s especially renowned for his work on theories of the strong force, which is the force inside the atomic nucleus that holds the protons and the neutrons together. His work on his force has to rank as one of the biggest achievements in all of 20th-century physics and, you know…

You’ll hear in my conversation with Frank that I’m a Frank fanboy. Just listening to this amazing physicist describe his remarkable achievements, and the scientific mysteries that he has helped unravel, and the ones he’s trying to unravel right now, you know, I’m just like an awestruck teenager talking to a celebrity.

Wilczek: Well, I was born in New York City, on sort of the outskirts of New York City in Queens, and I was born at the time of the Cold War, and my father was kind of a technician.

Strogatz: Oh, yeah?

Wilczek: And my father, really, and mother were blighted by the Depression.

Strogatz: Oh, really?

Wilczek: You know, they had to go to work very early, then complete their education and so forth. But my father, as a technician, was involved in the early days of television and radio, so we had a lot of pioneering electronics, vacuum tubes, and televisions that were two-inch diagonal, and things like this.

Strogatz: Two-inch diagonal TV? Am I supposed to picture that that was kind of like a state-of-the-art miniaturized TV at one time?

Wilczek: Yeah, that was — I think, you know, before TVs became a commercial product that was widely used, there were sort of these pilot projects.

Strogatz: Uh-huh. How could you even fit a tube in a TV like that? Were these transistors? What was in the back of that thing?

Wilczek: No, no, no, it was —.

Strogatz: It was tubes.

Wilczek: It was a little — yeah, it was tubes, all right.

Strogatz: But they must have been very tiny. I mean, I remember when the TV repairman would come to our house — because I used to, as a kid I used to look at the back of the TV, and it was all glowing and yellow light coming out of there. And sometimes a guy would show up with a big, like, a briefcase sort of thing, and he’d put a new tube in there.

Wilczek: Well, I mean the back end of this was much bigger, but the screen was very tiny.

Strogatz: Oh, okay. The back end, I see, it’s a two-inch screen but it’s got a big back end, okay, okay.

Wilczek: Yeah, it had a big back end.

Strogatz: Okay. All right, I got it. So, when you say —.

Wilczek: In fact, what we had around the house was mostly components.

Strogatz: Really.

Wilczek: Almost nothing ever worked. I mean, it was just components from the shop —

Strogatz: Was there a basement or some kind of a room with —?

Wilczek: Yeah.

Strogatz: Yeah?

Wilczek: Yeah, there was a basement.

Strogatz: But in Queens, how do you have rooms in there?

Wilczek: My father was also —.

Strogatz: I mean this was a house or a —?

Wilczek: Well, we didn’t, we didn’t. I mean the house, it was — yeah, it was a very special environment we had. So, I grew up with that, and I grew up, as I said… At the time of the Cold War and the New York City Public School System, there was a lot of emphasis on technology, and that was in the news, and scientists were still thought to be very important for the national interests. They still had this kind of aura of having produced the atom bomb.

Strogatz: Yep.

Wilczek: Won the war and all. So, it was a very stimulating environment for someone like me, and that’s what I picked up on. But actually, my interests early on were more — were not so well defined. They had a big admixture of philosophy and logic and thinking about what the — how minds worked.

Strogatz: Really? This is you as a young teenager now.

Wilczek: I was also —.

Strogatz: Is that what you mean?

Wilczek: Yeah, preteen and teenager. And as a preteen I was trained in the Catholic church.

Strogatz: Yeah.

Wilczek: So — and I took that very seriously.

Strogatz: Uh-huh.

Wilczek: And so, I had this kind of idea that the world had some hidden structure, and that it had some hidden meaning. I kind of, as I learned more about science, I kind of — well, not kind of, I did get disillusioned with that, because I didn’t think it did justice to —. I mean, I didn’t think the texts and the dogmas did justice to what I was learning. But somehow, the residual was that I retained a lot of interest in philosophy and mathematical logic, and things like that.

Strogatz: Well, I’m curious about this. It’s hard for me to think about quite what you mean when you say a preteen thinking about philosophy and logic. Were you arguing with classmates? Were you reading Thomas Aquinas, what was going on?

Wilczek: No, I was reading Bertrand Russell a lot. Yeah, Bertrand Russell was a big hero. We used to go to a bookstore where they had a big selection of Dover books.

Strogatz: Oh great, yeah.

Wilczek: Yeah. And there were a couple of books on mathematical logic that really caught my fancy, and — yeah. Yeah. So, I was pretty precocious so —.

Strogatz: I guess so.

Wilczek: Yeah, I did — yeah.

Strogatz: But was your — so I’m taking it that your dad being a technician, that he wasn’t exactly academic. He probably knew a lot of things.

Wilczek: No, no, not at all. In fact, he didn’t even finish high school.

Strogatz: Okay.

Wilczek: He was going to night classes as I was growing up, to get his degree, and he took home books about calculus. He was learning calculus, and I was reading the books at the same times he was trying to understand them, but he didn’t like that, actually.

Strogatz: Really?

Wilczek: He thought — you know, yeah, no, he felt very self-conscious about this, you know, this kid —.

Strogatz: Yeah.

Wilczek: Competing with his father and — yeah.

Strogatz: Now, you haven’t mentioned much about your mother yet, so was she —?

Wilczek: Well, my mother was extremely supportive, extremely supportive —.

Strogatz: Now, you never mentioned any brothers or sisters, so I take it you’re the —.

Wilczek: I had a younger brother.

Strogatz: Oh, you did.

Wilczek: But he was such — you know, five and a half years younger. And so by the time — you know, I left home to go to college when I was 15.

Strogatz: Oh, my God.

Wilczek: Skipping grades was not terribly rare.

Strogatz: Yeah.

Wilczek: It was… To skip several grades was fairly rare but to skip any individual grade, there was a regular process and —.

Strogatz: Okay. Huh, okay. So, then what happened? So, you’ve got this — well, you mentioned math, and I sort of cut you off. You said math and logic, those were interests of yours. That must have been —.

Wilczek: Yeah, just things with a mathematical flavor. But especially in the early days, I really, really liked mathematical logic, because I had a feeling that I — I mean, I could understand it. It was — you know, geometry is axiomatic at some level, and algebra is axiomatic at some level, but mathematical logic is really axiomatic.

Strogatz: Yes, it is.

Wilczek: And I felt I could understand it completely, and I also just liked the kind of calculations, and I liked — I always liked doing logic puzzles and things like this.

Strogatz: It’s hard to believe but Frank was just a young graduate student, only 21 years old, when he started working on the strong force. At the time, one of the biggest, grandest unsolved problems in all of fundamental physics.

Wilczek: So, there are four basic interactions in nature. There’s gravity, which has had a profound, precise mathematical theory since Newton. And then general relativity takes it to another level. And there’s electromagnetism, which had a beautiful theory starting with Maxwell — the Maxwell equations, which we still use. But then in the 20th century, when people started to study inside of atoms, so at a very —. It was found in particular… In atomic nuclei, it was found that those forces were no longer sufficient. It needed additional forces. And there’s one called the strong force and one called the weak force.

Strogatz: Yeah.

Wilczek: The weak force is responsible for certain kinds of radioactive decays, and let’s just leave it aside for the moment.

Strogatz: Okay. Yep.

Wilczek: The strong force is the one that holds atomic nuclei together, and people —. It became sort of the top item of the agenda of physics, I would say, starting in the 1930s, to understand the strong force, because once quantum mechanics was in place, atoms and molecules were solved in principle. Of course, in practice it was a different matter, but in principle they were solved, and the frontier of fundamental mystery moved inward to the nucleus.

Strogatz: Let me just ask you something at this point, if I could just interrupt for one second, because it’s something any high school kid learning that protons are positively charged would have wondered about. You know that we’re told that there’s neutrons and protons in there. This is the old way that I learned in the —

Wilczek: Yes, that’s right.

Strogatz: Growing up in the ’60s, and —.

Wilczek: Yeah.

Strogatz: It seems like, well, that can’t work, because the protons will push each other apart, they’re going to repel each other.

Wilczek: That’s why you need an extra force.

Strogatz: Yeah.

Wilczek: That’s how it was inferred that there had to be a force that was more powerful than electromagnetism but of short range, so that it could lead to attraction —

Strogatz: Okay.

Wilczek: And pull things very strongly into compact nuclei but doesn’t exert effects significantly outside. So, there’s no significant long-range interaction between different nuclei, and the electrons don’t feel the strong force at all, so.

Strogatz: I see. So, the inference at that time, I don’t know, maybe even back in the ’30s, was there has to be something holding the protons together —

Wilczek: Yes.

Strogatz: In the nucleus. But it can’t be something that extends out to great distance, or we would have noticed it in other effects, I guess.

Wilczek: That’s right, exactly. I mean it turns out that once you put in quantum mechanics, just electromagnetism and Maxwell’s equations does a great job in atomic physics, but it leaves the question you asked totally hanging in the air: What holds the nucleus together when electromagnetism wants to blow it apart?

Strogatz: Okay.

Wilczek: So, people, you know, gave this thing a name, called it the strong force, but had no idea what it was about. So, they set about studying it, and it’s not easy to study, of course, because the distances involved are much smaller than atoms, and how do you study things that small? But basically, the technique was to shoot protons at atomic nuclei, or electrons or photons, the scattering experiments to probe inside. So, you have things penetrate, and come out, and give some information about what they’ve gone through.

Anyway, to make a long story short —.

Strogatz: Right.

Wilczek: Over several decades, very partial understanding of the strong interaction accumulated that was — it got very, very messy. It soon became clear that protons and neutrons could not be regarded as elementary particles. They have internal structure, so that they don’t obey simple equations, that was one thing that became clear. Another thing that became clear is that protons and neutrons are representative of a much larger family of particles called hadrons that also exhibit the strong interaction and participated, but they’re unstable.

So, although it wasn’t really articulated this way, I think it was widely understood, if only subconsciously, that protons and neutrons were more analogous to molecules. They should be made of something — they’re complicated objects, extended objects that shouldn’t be expected to behave simply and in fact, don’t.

Strogatz: That’s interesting, that’s very interesting.

Wilczek: Yeah. Right.

Strogatz: I mean ’cause I’m almost… So, I have kids who are now going into college; one is in college, one is going. And so, I had to, like all parents, work on their high school and middle school science and math with them, and I feel like I remember them, even in today’s education, being taught about protons and neutrons. But I don’t know that they were really taught about hadrons as this broader family.

Wilczek: No, well, it’s not extremely useful knowledge but — for most people. But it’s very important if you want to study cosmology or high energy physics or, you know —

Strogatz: Yeah.

Wilczek: A variety of advanced subjects, but it’s not — even for professional chemists, say, or solid-state physicists, it’s not really essential to go beyond protons and neutrons.

Strogatz: Okay.

Wilczek: Okay. But if you are determined to get down to fundamentals —

Strogatz: Yeah.

Wilczek: Then protons and neutrons clearly aren’t the last word.

Strogatz: Okay.

Wilczek: That became very clear.

Strogatz: Uh-huh. So, now we’re at, what, this is like now 1950 or ’60?

Wilczek: Yeah. Hadrons began being discovered in the 1930s. At first it was just a dribble but then as people built accelerators to examine things more systematically, it rapidly blossomed into tens and then hundreds of unstable hadrons. And then it became clear that it was more like molecules —.

Strogatz: Okay.

Wilczek: Or more precisely, like you have atoms, and atoms can exist in different energy levels.

Strogatz: Yeah.

Wilczek: So, it’s more like that. You have arrangements of a few building blocks which can exist in various states of excitation. The difference is that in the strong interaction, the difference in energy is so much that it reflects itself according to E = mc2, as a difference in mass. So, the different excitations represent particles with different mass.

Strogatz: Ah, nice. Oh, good.

Wilczek: But really — yeah, but it’s not so different from — I mean, in retrospect, this wasn’t so clear at the time, but it’s clear in retrospect. So, the next big thing that helped clarify it was called the quark model, which was basically that, realizing that you should think of all these hadrons as being built out of a few simpler objects. And the objects, sort of by adventures in building models, their properties became a little better defined, they’re called quarks. But the properties of quarks seem — that were required were very peculiar and so —.

Strogatz: Yes.

Wilczek: So, even the people who invented quarks didn’t really believe in it. Like, they somehow thought that these were mathematical abstractions.

Strogatz: Yeah.

Wilczek: So, at first, as the concept of the quark was kind of empirical, you know, it was like a cookbook recipe of, you know, it wasn’t very clear exactly what it was or what the forces were. And that was kind of the state of things in the 1960s. And then there were all kinds of experiments. The key experiments, it turned out, were some experiments done at the Stanford Linear Accelerator.

Basically, this was taking pictures of the interior of protons. When you take pictures using light and lenses, you get fairly direct images.

Strogatz: Yeah.

Wilczek: But if you want to take pictures on an atomic scale, say, you use x-rays and you get x-ray diffraction patterns, and you have to do quite a bit of image processing to get from that to the geometric structure of what you’re seeing. So, for instance, it was a great step to go from the x-ray diffraction pictures of DNA molecules to the double helix, to figure out what you were looking at, you know, based on —.

Strogatz: Exactly the example I was thinking of, too. That’s probably the most famous x-ray diffraction picture of all time. Yeah.

Wilczek: Yeah. Now — and these experiments were sort of in that spirit, but at more extreme energies — and it introduces a new element, because inside protons, things move really, really fast, so you have to take not only things with good spatial resolution, but very good time resolution.

Strogatz: Wow.

Wilczek: And so that requires high energies and also big changes in energy. It turns out, to get that kind of resolution. And so, some fancy image processing, which was not very well understood at the time, but you know, properly interpreted — was giving snapshots of protons, and showing that inside were indeed much simpler objects which had some of the properties that people thought quarks should have. Anyway.

So, it became clear that trying to formulate the theory of strong interaction in terms of protons was totally wrong, that you had to be really thinking about the behavior of these quarks, and what the forces were between them. And they had this very unusual property, now, that you knew that quarks were real because they’ve got pictures of them. You had to take seriously their unusual property, that inside the proton, they don’t seem to interact very strongly with each other, but you can’t pull them apart.

So, the force seems to be weaker at short distances, and then becomes stronger at long distances, which is very unusual behavior, no other force had anything like that and that’s where we came in.

Strogatz: Oh, well, let’s just make sure —.

Wilczek: Yeah.

Strogatz: I just want to underscore that point in case that slipped by quickly. So, like, if I try to jam two electrons close together, they will push each other — repel each other more and more intensely the closer they get.

Wilczek: That’s right.

Strogatz: Not so with the quarks, you are saying. If they’re packed inside of a proton, at some point, they sort of don’t notice each other very much.

Wilczek: That’s right. So, their force weakens as they get closer together, and that’s what’s called asymptotic freedom. It has two elements, it has a spatial element, which we just talked about, that the force gets weaker at short distances. It also has a property that’s deeply related mathematically and in the understanding of quantum physics, which is that the force gets weaker also at high energies. So, at either short distances or high energies, the forces get weaker.

Strogatz: Hmm. What does that mean here, high energies? What would that — I don’t really know what you’re saying.

Wilczek: Well, so typically, you do scattering experiments, where you, say, bash protons into each other. First, you accelerate them, and then you bash them into each other.

Strogatz: Yep, yep.

Wilczek: So, basically, the statement is that, as you bash them at higher and higher energies, the interactions become sort of more basic interactions between the individual quarks, rather than the protons as a whole recoiling against each other and exerting stronger forces at larger distances. If you push harder, you penetrate to shorter distances, the forces are weaker, and things become much, much easier to understand and analyze, it turns out.

Strogatz: Uh-huh, okay, good, thank you, okay, got that.

Wilczek: Okay. So, that’s what we were working on, trying to understand how it was possible that you could have a force that gets weaker at short distances and in the framework. I mean, of course you could — if your standards are low enough, it’s easy enough to write down a force law like that. But if you try to implement that in the framework, where you’re obeying the rules of relativity and quantum mechanics —.

Strogatz: Yeah.

Wilczek: So, in quantum field theory, it’s much more constrained. The possible force laws are much, much more constrained. And so, we analyzed systematically what was possible.

Strogatz: When you say we, you mean you and your advisor, I guess.

Wilczek: Yeah, my advisor is David Gross, who is, you know, a very, very distinguished physicist. At that time, I was 21 and he was 31.

Strogatz: Nice. No, I like the picture.

Wilczek: And I thought of him as this really old guy.

Strogatz: He was.

Wilczek: You know, infinitely wise and experienced, but he was — you know, he was just getting tenure and very dynamic, and that’s what led me to him. He was extremely dynamic. When we began to work together, I was not even a graduate student in physics, I was a graduate student in mathematics.

Strogatz: I didn’t know that.

Wilczek: I came to Princeton as a graduate student in mathematics.

Strogatz: Really, huh.

Wilczek: I wasn’t at home, sort of, in pure mathematics. I didn’t — so I was looking around for other things to do, so I went to all kinds of things. I went to biology lectures and computer science lectures. And I got the sense that very exciting things were happening in physics, that new methodologies were appearing. You know, now what we call renormalization group, gauge theories, those were bright, shiny new subjects with opportunities for mathematical investigation that really appealed to me.

So, I don’t remember exactly the process, but somehow, we settled on this particular problem as one that could be investigated fruitfully. And once the calculations got going, they were sufficiently complicated that we often settled down in his office and worked through some of them. And then as the result emerged, that we found one particular class of quantum field theories and only one —.

Strogatz: Yeah.

Wilczek: That satisfied this requirement, then it got really serious.

Strogatz: I guess so.

Wilczek: Because this kind of theory also was something that you could conceivably think of as a possible theory of how quarks interact at a fundamental level, and what the equations were exactly, because we had a very restricted class of theories that had the necessary properties. So, basically, only one, if you put together some very crude clues with constraints. And so, within a few weeks, we went from groping around and, sort of, in this formal problem of “could you find a theory that accounted for this behavior?” to a very specific candidate theory, not only for that behavior but for what the strong interactions are, period.

Strogatz: So, let me see if I — let me try to recapitulate in my own words, if you’ll allow me.

Wilczek: Yeah.

Strogatz: Yeah. Just to see if I’m getting it. I think I got it. That at first, you think of it almost as a beautiful math exercise, that “I’m looking for a theory that will satisfy things that are” — you know, this is the great thing in creativity and physics and in other parts of certain mathematical sciences, that you cannot violate special relativity. You cannot violate quantum mechanics. You have these nice constraints, except that they make your job hard, because you can’t just try any old thing. You have to be consistent with these known very powerful parts of physics, and then you ask, what quantum field theories that respect relativity and all the other known constraints can be asymptotically free?

Wilczek: Yes.

Strogatz: And then you find one example, or one family of examples, and then you think, maybe this is actually how nature does it, with quarks.

Wilczek: Yes, that’s right, yeah. It was a miracle. I mean, really, even in retrospect, it was a gift from heaven that there was, sort of, such a unique solution. It was extremely highly leveraged. So, it gave us, you know, the whole theory. And you know, of course, at first it wasn’t clear that it was true.

Strogatz: Right.

Wilczek: It took many years to get really decisive experimental evidence.

Strogatz: Okay. Well, wow. So, this discovery of a way to explain the asymptotic freedom and then develop — and a theory that I suppose gave you a lot more than just that. I mean, it must have predicted many other things.

Wilczek: Well, that was the thing. I mean, discovering that basic property was by no means enough to establish the theory.

Strogatz: Yeah.

Wilczek: You needed a much, much more precise set of tests. And so that began a very intensive period, where we worked out testable consequences. You know, it turns out although the force gets weaker, it doesn’t suddenly turn off. And you can predict exactly how it changes at short distances, and how it changes at high energies. So, you get a lot of quantitative tests of the basic ideas, but they’re not — they all involve kind of fancy image processing to extract from the raw data.

Strogatz: Okay.

Wilczek: But it does get simpler, and this is important. This is kind of skipping ahead in the history 10 or 15 years, but it does get much easier, it turns out, when you work at sufficiently high energies. I mean, the theory indicates that it should get simpler at high energies, but I don’t think any of us anticipated how much simpler the experiments would look.

Because what happens at extremely high energies is, you get this phenomenon called jets, which is basically — let me talk about it concretely in the case of a quark. If you produce a very, very energetic quark, it can’t stay a quark, because individual quarks always call forth extremely strong forces and attract — sort of spark the vacuum and produce anti-quarks that they bind to, and other quarks that they bind to. So individual quarks are never observed.

But what happens is sort of like a lightning bolt, if you produce a quark that’s moving rapidly —

Strogatz: Yeah.

Wilczek: It produces a whole ionization track of other quarks and anti-quarks and gluons —

Strogatz: Oh, neat.

Wilczek: That then congeal into the particles we actually see.

Strogatz: Literally out of nothing. I mean, really out of the vacuum? Is this what you’re saying, they spark out of the vacuum, so to speak, with particles and antiparticles?

Wilczek: Yes, yes, yes, yes.

Strogatz: That’s amazing.

Wilczek: Yeah, it is but it’s kind of like — it’s E = mc2 run backwards, and you know, classic… In the early days of nuclear energy and atomic bombs and so forth, E = mc2 was thought of as a recipe for getting lots of energy out of mass.

Strogatz: Right.

Wilczek: Out of converting mass into energy. But in this domain, it’s just the opposite. You produce enormous amounts of energy, out of which you can produce many particles.

Strogatz: Amazing. I’m still saying it’s amazing.

Wilczek: So that’s what happens.

Strogatz: Uh-huh.

Wilczek: So, from a very energetic quark you produce many, many particles. And the thing that’s another gift from heaven about this, strongly related to asymptotic freedom, is the fact that in this process of a quark turning into a lot of other particles, the total energy and the total momentum are conserved.

So, you have a lot of particles moving in the same direction. That’s what called the jet. And if you add up their total energy and add up their total momentum, it reflects accurately the energy and momentum of the quark that initiated the event. So, although you don’t see quarks directly, you do see these jets which reflect their properties in a way that’s quite tangible.

Strogatz: Nice.

Wilczek: And so, you can test the theory very precisely by comparing — you know, instead of — because the theory is about quarks and gluons.

Strogatz: Right.

Wilczek: You don’t see quarks and gluons, but you see jets and you can predict the angles at which they come out with different probabilities, the energies and momentum. You can predict a lot, and check it, and that’s how the theory got verified.

Strogatz: After the break, the Nobel Prize, and how a strong interaction mystery led Frank to a new theory of dark matter.


Strogatz: Well, so let’s shift gears, if it’s okay with you, to the emotional or human side of this, because I feel like, you know, you’ve given us a really nice, clear description of what you found, its significance, but I want to picture the 21-year-old —.

Wilczek: Well, first of all, I was very inexperienced, so I didn’t — you know, I thought, okay, so today we’ve solved the strong interaction, tomorrow we’ll solve the weak interaction.

Strogatz: Oh my God.

Wilczek: And then the next day we’ll solve —.

Strogatz: That’s funny.

Wilczek: We’ll solve quantum —. There were still lots of loose ends in fundamental physics.

Strogatz: Yeah.

Wilczek: I also wasn’t convinced completely that the theory was right. I mean, it was a very, very bold hypothesis and I thought of it that way from the start.

Strogatz: Yeah.

Wilczek: I mean, the theory was all about quarks and gluons, but no one had ever seen a quark, no one had ever seen a gluon.

Strogatz: Sure.

Wilczek: We didn’t really understand how — once the forces got strong, we didn’t really understand how they behaved. So, the theory was really a house of cards.

Strogatz: Okay.

Wilczek: And the data was very sparse.

Strogatz: Yeah.

Wilczek: There wasn’t — you know. So, I wasn’t at all confident that it was correct, and so, you know, celebration would have been premature.

Strogatz: Well, what about the uniqueness, though, of the family that could satisfy the constraints? That must have been cause for some celebration.

Wilczek: Well, yeah, that was, of course, wonderful, and I could get a thesis and very rapidly ascend the academic ladder based on that.

Strogatz: But that was all it was? There wasn’t a feeling like this is a mathematical miracle?

Wilczek: Well, no. I felt — well, I felt, and I told David very early on, that if this holds up, we’ll get a Nobel Prize for it.

Strogatz: Yeah, and guess what.

Wilczek: And he said, nah. But to me it was a big if, if it holds up.

Strogatz: Yeah.

Wilczek: We were able to analyze the behavior at infinitely high energy, so to speak, or infinitely short distances, but experiments aren’t done at infinitely high energies or infinitely short distances, and I — you know, it could have been that the hints were not asymptotic.

Strogatz: Sure.

Wilczek: That somehow, we were misinterpreting intermediate scales as asymptotic, and we were just barking up the wrong tree.

Strogatz: But I’m curious, I mean I haven’t talked to any Nobel Laureates on this podcast yet, so you’re the unfortunate first victim. I mean a lot of us on the outside would love to hear anything about that. Like did it — how it affected you at the time, since then, going to Sweden, anything. You know, give me some dirt! Give me something.

Wilczek: Well, a lot of things happen when you win a Nobel Prize, and of course, it’s different in each case. But I can tell — I mean probably the most amusing thing is what happened at the very beginning when I got the phone call.

Of course, I knew when the prize was going to be announced and I really would lose sleep over it.

Strogatz: Really.

Wilczek: Because, yeah, because —.

Strogatz: I shouldn’t be chuckling. I can’t even imagine what this is like.

Wilczek: It was — you know, I felt — well, I felt — I hoped it was a matter of time but, you know, you can’t be sure until it actually happens. But yeah, it’s just an empirical fact I was not able to sleep, and I knew that the call would come at 6:00 Eastern, or the announcement would be made at 6:00 Eastern, and you know, I kept peeking at the clock, and it got to be 5:00.

Strogatz: Yeah.

Wilczek: And I said, “Look, look, Frank, you’re not sleeping, you might as well get up, take a shower —.

Strogatz: Okay.

Wilczek: “So, you’ll be ready in case — just in case, you’ll be ready. So, I went to the shower, and then very shortly afterwards — so, you know, 5:01 or 5:02 — my wife came into the shower with our mobile phone and said, “Someone’s calling from Sweden,” or “Someone with a Swedish — she seems to have a beautiful Swedish accent — is calling.”

Strogatz: Yeah.

Wilczek: “And here’s the phone.” So, I turned off the shower. I was taking a shower and turned off the shower and picked up the phone, and sure enough, that was the call.

Strogatz: Right.

Wilczek: And, but okay, then that came to an end, and the next thing is, I wanted immediately to call my parents, because this —.

Strogatz: Oh, great, they were still alive.

Wilczek: Enormously meaningful to them. Yes. Because they — you know, they had really grown up in very disadvantaged circumstances.

Strogatz: Oh, what a great story.

Wilczek: And been so supportive and invested so much in my success, emotionally and otherwise. And so, it was a very special thing to be able to call, and I called, and my father was furious. He said, “Don’t you know what time it is? Don’t —? What are you —? Whatever it is you’re selling, I don’t want to buy it.”


But then I told him and, you know, it was really quite something.

Strogatz: What exactly did your dad do in reaction, after you told him what it was, and I’m sure he knew about the Nobel Prize, what that meant?

Wilczek: Oh, well.

Strogatz: But what, was he hysterical? What was his reaction?

Wilczek: No, no, he — but he was obviously very moved. He was at a loss for words, I would say.

Strogatz: He was?

Wilczek: Yeah, and my mother, too.

Strogatz: Oh, very sweet.

Strogatz: Something I love about Frank’s work is that he uses beauty as a criterion for scientific theories. He looks for theories that have beautiful equations, symmetrical, elegant equations that follow certain constraints and broad principles. And what’s so really astonishing about this criterion is that, sometimes, like in Frank’s case, it actually works. These beautiful theories will map onto what nature actually does, the way the world actually is.

Wilczek: It’s not like it was in Newton’s day or Maxwell’s day, where you put together an array of phenomena and found mathematical equations which describe them. It’s much more the opposite, that you look for attractive equations and then see if they apply to the world. And quarks and gluons are a very interesting case, because even today we — well, the theory tells us that they shouldn’t exist as particles in the conventional sense. They are not things that you can isolate and study.

But the equations tell you that they do have this kind of very tangible existence as jets and so you can see their traces in the physical world, and you can certainly see that the equations they participate in are the correct equations that describe the physical world even though they’re not particles in the conventional sense. So, you sort of learn what particles are by — or learn what the ingredients of the world are — by interacting with the world, and making models and mathematical guesses and seeing what there is.

Strogatz: Hmm, really interesting point of view here. I mean because, like, you drew a — I don’t know where you would put the line, but you said you thought in Newton and Maxwell’s era it was different, and I —.

Wilczek: The balance was different. I mean it’s —.

Strogatz: The balance was different.

Wilczek: I wouldn’t say — I would be hard pressed to say precisely, quantitatively, so to speak, or in terms of mathematical logic, what the difference is. But you can feel it, that in the 19th century and even in the early 20th century, the level of interpretation between the concepts that appear in the equations and their ultimate implications in the world was much more direct than it is now. And so, you would study experiments, you would codify them into equations and refine the equations somewhat, and that was it. I mean now that’s much too hard. The clues are much too scattered and diverse and partial, so you have to work the opposite way. You make guesses for what the equations might be that are consistent with broad principles, and simple and beautiful, and then work out their consequences and see if it matches things in the world.

Strogatz: Even though Frank made this big discovery about the strong force and asymptotic freedom and quarks there was still something mysterious about the strong interaction, something he hadn’t figured out yet.

Wilczek: The laws have this property of being almost reversible when you — in time. So, that is, if you take a movie of fundamental processes and run it backwards in time you get a valid process.

Strogatz: Yeah.

Wilczek: So, you can’t tell if you’re running it backwards and forwards in time, almost.

Strogatz: Almost.

Wilczek: It turns out that’s not quite true. There are certain obscure processes and weak interactions that don’t have that property, but it’s very small. So, it’s not a fundamental principle, and you’d like to understand how can it be accurate and not fundamental. And again, it’s a story of strong constraints, what’s possible within the symmetries and structure of the standard model and relativity and quantum mechanics is very restricted. And it turns out that there are two possible interactions within the standard model that could violate time reversal symmetry, and one of them is observed, and the other interaction, which is allowed by general principles, doesn’t happen.

And so that’s a possibility, that’s sort of standing between us and a fully satisfactory understanding of why the laws so accurately obey this principle of time reversal symmetry. Accurately, but not quite precisely.

Strogatz: Let’s see if I’m getting you here. So, you’re saying there is this other possible thing.

Wilczek: Yeah, another possible interaction that would have —.

Strogatz: Another possible interaction —.

Wilczek: Fouled things up.

Strogatz: You say it would violate time reversal symmetry?

Wilczek: Yes.

Strogatz: But it’s allowed by the theories.

Wilczek: Yes. It’s allowed, otherwise, by the general principles, right.

Strogatz: But you say it’s not observed in nature?

Wilczek: So, general principles tell you that there is the interaction, and if you sort of put a numerical value to it, you would think the interaction should be like one in appropriate units, but it turns out the experimental limit is more like 10-10.

Strogatz: Oh.

Wilczek: So, it’s very, very tiny. Something is missing in our understanding.

Strogatz: Really?

Wilczek: Yeah.

Strogatz: So, you’re saying even in — am I right in saying even in the standard model, there is this important thing missing, or —?

Wilczek: Yes.

Strogatz: Or something — there is more to do on the standard model, you’re saying, maybe?

Wilczek: Well, there is — this tininess of this interaction is observed, but not explained by the — so, it’s consistent with the standard model, but it’s not explained by the standard model, why it’s so small. So, it appears within the context of the standard model to be a coincidence that it’s so small.

Strogatz: Uh-huh.

Wilczek: It’s roughly comparable, I would say, to the equality of inertial and gravitational mass that was a coincidence in Newton’s theory of gravity, that led Einstein to general relativity, or it was an important part of getting to general relativity.

Strogatz: Right.

Wilczek: So, it’s like that, it’s a coincidence. It’s not a contradiction with the standard model, but it’s something that the standard model does not explain.

Strogatz: So, in the case of Einstein, it was a deeper understanding that led to why those are equal.

Wilczek: Right.

Strogatz: And you’re thinking maybe there is a successor to the standard model —.

Wilczek: Yes.

Strogatz: Where this will appear naturally.

Wilczek: Yes. We have a candidate theory.

Strogatz: Ah.

Wilczek: Of that.

Strogatz: Okay.

Wilczek: Where you introduce an extra symmetry, it’s called Peccei-Quinn symmetry, for the physicists who first introduced it, but they didn’t really realize how it worked in detail. The main consequence of this symmetry, as a practical matter, is that there is a new kind of particle that it implies, that’s called an axion.

Strogatz: I see, I see.

Wilczek: I got to name it because I —.

Strogatz: You got to name it.

Wilczek: Yes, because —.

Strogatz: Because this axion is the prediction, then, of this newer theory.

Wilczek: That’s right. Yeah. And basically, the theory is a kind of theory of evolution of this coupling. See, within the standard model, it’s just a number that could have been 1, but turns out to be less than 10-10. Within the theory, that number gets promoted into a field, a quantum field that can change with time. And within the theory, if you put it into the context of cosmology, you find that the field settles down to a very small value over time.

Strogatz: Oh.

Wilczek: So, it starts out like 1 but the dynamics allows it to relax and it does relax to a very small value.

Strogatz: Oh.

Wilczek: Not quite zero, actually, but a small value. So that’s the theory of evolution that explains why this coupling is small, but it has a most unexpected and remarkable consequence, which is that it doesn’t settle down to quite zero as I mentioned, and although numerically the value is very small, if you ask how much energy is implicated in the deviation, in the residual oscillations of this field, it has the right kind of energy density and the right properties to make the dark matter of the universe.

Strogatz: Uh-oh.

Wilczek: So, this is something we didn’t anticipate when we were making the theory, but it just falls out.

Strogatz: Oh boy.

Wilczek: It just falls out. So, it’s very tempting to think that this is the dark matter of the universe.

Strogatz: Oh, you’re saving the best for last here.

Wilczek: Yeah. So that it’s made out of these axion particles and it’s become a very active field in — well, field, I’m sorry, a little joke.

Strogatz: Yeah, you can —.

Wilczek: It’s become a very active area of research to try to detect this cosmic background.

Strogatz: So, there’s something really deeply mysterious going on in the heavens that is being given the name dark matter. It’s matter that’s having a gravitational effect on stars and galaxies but it’s not shining, it’s dark. We don’t know what the heck it is and there is a lot of it. In fact, it’s most of the matter in the universe, much more than the stars and other stuff that we can see. So, this is a gigantic question mark, and so what’s so incredibly stunning about what Frank is saying is that, based on beautiful, elegant physics, he has a prediction for what the dark matter is made of and why it’s there.

Now, his theory might be wrong, but he may have solved the mystery of the dark matter. And where did this come from? It wasn’t from studying astronomy, it wasn’t from looking carefully at the galaxies, it was from the quest for elegance, that Frank is thinking about what’s the next generation of physics theories based on mathematical harmony and it leads him to this thing that totally serendipitously, not planned, would account for dark matter. So, this is another case of science being led to something unexpected, unanticipated through the quest for mathematical beauty.

Next time on The Joy of x: computational physicist Sharon Glotzer reveals how order emerges from disorder except when it doesn’t.

Sharon Glotzer: At some point this oil pump blew up all over me.

Strogatz: But wait a second, what happens when an oil pump blows up on you? I never had that happen to me.

Glotzer: I don’t know, there was just oil all over me. I was just like covered in oil. And so, I am standing there, like in the middle of the basement, and so I could look up, like through the skylights above or up to the ceiling, and so Gene Stanley is walking up the stairs. And he sees me, and he says, “You look like a theorist, come up and talk to me later.”

Strogatz: The Joy of x is a podcast project of Quanta Magazine. We’re produced by Story Mechanics. Our producers are Dana Bialek and Camille Peterson. Our music is composed by Yuri Weber and Charles Michelet. Ellen Horne is our executive producer. From Quanta, our editorial advisors are Thomas Lin and John Rennie. Our sound engineers are Charles Michelet and at the Cornell University Broadcast Studio, Glen Palmer and Bertrand Odom-Reed, who I like to call Bert.

I’m Steve Strogatz. Thanks for listening.


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