PRL Top Ten: #1

NUMBER ONE

A Model of Leptons
(S. Weinberg, Phys. Rev. Lett. 19 (1967) 1264), 4602 citations.

This is the tenth in a series of articles by James Riordon. The first article appeared in the November 2002 issue.

Photo Credit: Jessica Clark APS News correspondent James Riordon chats with Steven Weinberg in his University of Texas office in Austin.

The most-cited paper on our list of the top ten, highly cited Physical Review Letters established the link between the electromagnetic and weak interactions, and laid one of the most important cornerstones of the Standard Model of particle physics. Although Steven Weinberg published the paper in 1967, the electroweak theory attracted relatively little attention until Gerard 't Hooft (at the time, a PhD candidate at the University of Utrecht) showed that the theory was renormalizable. Weinberg shared the 1979 Nobel Prize for Physics with Sheldon Lee Glashow (a co-author of the tenth most cited PRL) and Abdus Salam for "their contributions to the theory of the unified weak and electromagnetic interaction between elementary particles, including, inter alia, the prediction of the weak neutral current."

In March, APS News correspondent James Riordon visited Weinberg in his ninth-floor office at the University of Texas at Austin, where he holds the Josey Regental Chair of Science in the Departments of Physics and Astronomy. An excerpt of that interview follows.

APS News: Did you recognize the extraordinary significance of your 1967 paper at the time that you were writing it?

S. W. : I had the idea that the weak interactions were transmitted by a gauge field like the electromagnetic field, but that the gauge invariance was broken, spontaneously broken, and that's why the particle that transmits the force, the W-particle, is heavy. I thought that idea was very important and that's what I was really excited about. I presented that idea in the paper, but mostly in the form of a specific example. But I wasn't that excited about the example, this SU(2) x U(1) example. It was just the first one I thought of. I imagined that there might be other examples, but this was an illustration of the way this idea would work out. What I didn't know, what I couldn't have known at that time, was that nature had chosen this example to be the real world. And I recall that for some years afterwards, especially during the period when some experiments seemed to be going against the theory, I wasn't convinced that this was the right model. But I was convinced of the underlying idea that the weak interactions are very similar to the electromagnetic forces, with the difference being made by the phenomenon of spontaneous symmetry breaking.

APS News: Your paper took on much greater significance after 't Hooft proved that the theory was renormalizable in 1971. Why didn't you immediately set about checking the renormalizability of the electroweak theory back in 1967?

S. W.: I thought that the theory was renormalizable, and I tried during this period to prove it (in work published later), but I had a hang-up which prevented me from doing what 't Hooft did. There are essentially two approaches to quantum field theory. One is the quantum mechanical operator method. The other is Feynman's path integral method. All the reading I had done used the operator method. I knew about the path integral method, but it looked to me like a way of doing things with a lot of hand waving, things I knew how to do perfectly well by direct calculation in the operator formalism. So it just seemed pointless. What I didn't realize is that the path integral formalism opens up a lot of ways of doing calculations that really aren't available in the operator formalism. In fact, without it you could never prove that the theory was renormalizable. It was because of 't Hooft's work that I then learned the path integral method. I have used it often since then, and of course teach it, and it's in my treatise on quantum field theory, although I still start with the operator method. Anyway, I assigned the problem of proving that the electroweak theory is renormalizable to a student, and the student was not able to do anything with it either. Also, I was writing a book on gravitation and cosmology, and it had taken over much of my time. And I had gotten very much involved with arms control.

One of the problems with path integrals, this is getting into detail and you may not want to get into this, but maybe I could just mention it for the record: Feynman in his book with Hibbs on the path integral method, which I had looked at, explains it terms of a picture of QED that was obsolete at the time. [In their picture], you treat radiation as a field and you treat the electrons and positrons as particles rather than as quanta of a field. So in that book, and in other places I was exposed to the path integral method, it was always explained in those terms?that you have electron and positron paths, real particle trajectories, plus an electromagnetic field. I didn't want to think that way. I thought then, and I think now, in terms of a more unified picture in which electrons, like photons, are quanta of fields and what you want to study is the way that fields change with time, not particles. Because of that association, I was very strongly prejudiced against the path integral method.

Although I tried during that period [to solve the problem using the operator method], it only made the problem seem more difficult. I just couldn't see my way out of it. Of course as soon as the news of 't Hooft's work came to us, I dropped everything else, and a number of other theorists dropped everything else.

I remember that there was a small private meeting on the anniversary of Oppenheimer's death every year for a while at Princeton. In previous years, most of the discussion was about Regge poles and other things I wasn't following, and I felt sort of out of things. After 't Hooft's work was known, I was asked to talk about the electroweak theory. I gave a 40-minute talk and I said, "Well that's all I have time to talk about." Some members of the audience said "No,
no! This is what we have to hear about. This is the important thing." So I went on and I felt pretty good, because, as I say, up to that point I had felt rather far out of the center of things at these meetings.

That was a very exciting period. The '70s were the last golden age of particle physics. There have been golden ages before, but that was a great time when suddenly everything was coming together and experiments were coming together with the theory. We haven't really had such a period since the '70s.

We need another golden age. I think that's the way progress is made. I think of the progress of physics a little bit like a logjam in a river. The logs are jammed and nothing is moving, and every once in a while you're able to pick one log out and things move a little bit, then they get stuck again. At a certain point, you pick out a crucial log and all the logs start flowing downstream. The 70s were a period when everything was flowing beautifully, then it jammed up again and we haven't really been making much progress since then. It isn't that I think we're departing from some correct way of doing physics; it's just that after a period of success things get hard again. And they're really hard now.

APS News: You mention in your '67 PRL that many attempts had been made previously to unify the electromagnetic and weak interactions. Why did you decide to tackle a problem that had evaded so many others?

S. W.: This did not come out of my sitting down and saying "Let's unify the weak and electromagnetic forces." What I was trying to do was apply ideas of broken symmetry to the strong interactions. In particular I imagined a kind of gauge theory, a Yang-Mills theory, of the strong forces in which there was a spontaneous symmetry breaking which split the particles that transmit the force, so that some of them would be much heavier than others. And what I was really thinking of was that the rho meson, which is a negative parity, spin-one particle and the A1 particle which is a positive parity, spin-one particle, have really the same mass but they get split by this spontaneous symmetry breaking.

The idea went nowhere partly because when I implemented the ideas, it turned out that the rho meson would have to have zero mass, which was obviously not true. The rho meson is a particle with a mass of about 750 MeV, which is not small at all by the standards of strong interaction physics. I was just going nowhere with it, and I was very frustrated. Then it suddenly occurred to me that this was the solution not of the strong interactions but of the weak interactions and that the thing analogous to the A1 was the W particle and the thing analogous to the rho was the photon, and it did have zero mass. So my work on the electroweak theory came directly out of work on the strong interactions.

APS News: In recent years, it appears that you have spent more time writing papers about cosmology.

S. W.: Yes, I've gone over completely to cosmology. Particle theory, the kind of cutting edge particle theory where you're trying to make the next big step, is now mostly in the hands of the string theorists. And I think probably they're heading in the right direction, but progress is slow and difficult. It's incredibly demanding, requiring mathematics beyond anything I ever learned. I tried during the 80's to work actively in string theory, and I actually published a few papers, but they weren't very important. I realized that I could sacrifice everything in my working life and try to keep up with the younger people in that area who have the mathematical background I don't have, or I could go into something I was already pretty familiar with.

I'd written a book on gravitation and cosmology, and I've written articles of some importance on cosmology, where very important things are happening. Cosmology [these days] is fantastically exciting. So it was an obvious choice. I think I can go on making some interesting contributions in cosmology, whereas I never could have done in string theory.

APS News: You've published extensively, but you rarely have coauthors on your papers. Why is that?

S. W. : Yes, I don't collaborate very well. I had one collaborator I really liked working with: Ben Lee. He died tragically in an automobile accident in 1977. I wrote a few papers with Ben, including one on cosmology that is very widely cited, and I think is quite important. But generally speaking, I don't like collaborating, partly because I have very strong views about how articles should be written. Even though in collaborations we each do a fair share of the actual work, I'm always the one who writes the article because I don't like the way that other people write.

Also, I don't work well with students because my ideas are usually very ill formed until they crystallize, and then it's obvious what to do. For example in the 1967 paper, I was dithering around with applying the idea of broken gauge symmetries to the strong forces, and if I'd had a student it would have been going nowhere because it was obviously a crazy idea. It wasn't working, and then suddenly everything clicked and I saw it was the answer to the weak forces. Well once that happened I didn't need the help of a student anymore - I just wrote the paper, it was easy.

I talk to people a lot, but usually it's picking their brains in areas where they're experts and I'm not. It's interesting that you pointed it out because I've always known that about myself; that I 'm not fond of collaborating.

APS News: You're asked about religion a lot. Why is that?

S. W. :Well, I've always been interested in religion. I tend to make provocative statements about religion, and then I get asked about them. Like the statement at the end of The First Three Minutes where I say that the more the universe seems comprehensible the more it seems pointless. I was more recently involved in a debate where I made another remark that people keep coming back to me about: With or without religion, good people will do good things and bad people will do evil things; but for good people to do evil things, that takes religion. These things seem to circulate.

APS News: You are one of the most famous living physicists, due to both your research and your popular writing. How does your celebrity status affect your work?

S. W.: It's an embarrassing thing to ask me about. Well, you said it, I didn't say it. I think physics is a very healthy field; I think most areas of science are. The working scientists - and I certainly know this is true in physics - working scientists don't have much respect for past achievement when it comes to deciding what they themselves are going to read or do.

A lot of the papers I've written in the past decade, I would say, are pretty unimportant. I wrote them because I was trying to learn a certain field, and I discovered a few odds and ends that hadn't been done or hadn't been done right. So I did them right, or I thought there's a way of explaining something that's better. I didn't regard these papers as particularly important myself. And despite whatever reputation I might have, I think the world of physics hasn't regarded them as important either. If I write an important paper, I think it'll be paid attention to. And if I write papers that are mostly important to me as a form of self-education, then the world will probably not get excited about them. And I think that's just the way it should be.

I'd like to think that even without [the '67] paper, I'd be regarded as a pretty good physicist. I've done a lot of things apart from the unification of the weak and electromagnetic forces. There's a whole area of physics that people are actively pursuing now, the use of what's called effective field theories, to solve problems in strong interaction physics, including even nuclear physics, that comes out of a paper I wrote in 1979, which in turn grew out of work I did in the 1960s. It has nothing to do with the weak interactions, and I'm very proud of that paper. I think it sparked that whole line of development. There are other papers of mine that I'm very proud of that have nothing to do with electroweak unification.

APS News: You are hoping to make another major breakthrough?

S. W. : I'd like to do something important again, sure. As you get older, so many things surround you, the list of people you have to write recommendations for becomes longer and longer. You spend more of your time on things like that. And now I've gotten involved in defense issues again. I testified before the U.S. Senate Committee on Foreign Relations about the administration's Nuclear Posture Review, I'm giving a talk at the American Physical Society in this area, and I've written articles for the New York Review of Books on it. So I've gotten involved in that again. Maybe I'm making the same mistake I made from '67 to '71, allowing myself to get involved in politics. But it's a way of keeping alive.

APS News: Thank you for your time.

S. W.: Nice talking to you.