The First Steps Toward Unity-- My Harvard Days

[Ed. note: In 1979, Sheldon Glashow received the Nobel Prize for physics. But in 1954, he was still a brash and irreverant new graduate student at Harvard. Here, he remembers those heady early days.] To be perfectly honest, I went to Harvard for graduate school largely because Harvard had admitted me—Princeton University had been more choosy. (Since then, I have rarely had occasion to visit Princeton.) What I knew of the place was simply this: 1) It had a snotty reputation. 2) The undergraduates threw great parties. 3) Harvard was near Boston, not quite New York but a lot more of a city than Ithaca, where I had been an undergraduate at Cornell. Harvard was Julian Schwinger’s stomping grounds, and Schwinger was as godlike to me as Michael Jackson is to my children. Precocious, brilliant, and chain-smoking, Schwinger was one of the greatest theoretical physicists, a founder of modern quantum field theory. His lectures, presented in the voice of a radio announcer, were masterpieces of pedagogical organization. He often ended his lectures with a chalkboard full of rapidly scrawled formulas (which had to be copied because there never was a textbook) and poised himself for a quick getaway. A shy man who did not welcome questions, he would disappear while his students were still ascribble. A Team Effort Toward the end of my second year I decided to pop Schwinger the question, that is, I wanted to ask to become one of his graduate students. When I screwed up my courage, I was dismayed to discover that 10 of us had the same thought. Schwinger interviewed us together. He gave us each a problem and told us not to return until we had solved it. His mistake was that he gave us all the same problem. Of course we collaborated and returned a few days later with the solution in our collective hands. In the end, he accepted us all. Among Schwinger’s 10 new students were Danny Kleitman, an old buddy, and Charles Sommerfield. Charley had the most miserable taste in movies. If he liked a flick, we avoided it. If he hated it, it was a must! We loved him all the same. Schwinger gave Charley a problem in electrodynamics, Danny a problem in the strong force, and I inherited the weak. Three forces for three friends. Today at the Harvard physics department all of the graduate students are provided with office space. Many of the offices enjoy a superb view of the Charles River and the Boston skyline. This is not the way things were when I was a graduate student. We theorists were relegated to tha windowless warren of basement offices known as B-24. There wasn’t much of a Boston skyline in those days, anyway. With no windows, there was no indication of the passage of time. Day and night, winter and summer, were all the same. We had at our disposal, desks, chairs, antique mechanical calculating machines, and a dusty and neglected collection of useless books and journals. Each of us was expected to produce a magnificent contribution to human knowledge, our thesis. Then and only then would we be released. It was to B-24 that Sommerfield took his problem of quantum electrodynamic and Kleitman his puzzle on the pattern of the strong force. I repaired to that subterranean world to pursue Julian Schwinger’s tantalizing idea that the weak force might somehow be two aspects of the same phenomenon. It was a time for great discoveries, and a time for me to learn a lot more physics. Enrico Fermi had already proposed that the weak force is mediated by a charged particle, what came generally to be called the intermediate vector boson, or boson. The trouble was that--the idea of the particle did not seem to be very useful. Back in the 1950s, there simply was no acceptable theory of the weak force, only a crude and qualitative description of the observed phenomena. The intermediate vector boson hypothesis was founded only upon a perceived analogy, imperfect but suggestive, between the weak interactions and electromagnetism. There was one big hint from experimental data, however, that such an analogy might have something to do with reality. Both forces displayed a kind of universality. The electromagnetic force acts indiscriminately between two electrons, two protons, or one of each. The weak force is just as promiscuous, acting between either leptons or hadrons or both. Only a very few grown-up physicists believed in Schwinger’s fancy of unifying the weak and electromagnetic interactions. (I, of course, as his novice graduate student, had little choice in the matter.) After all, the weak interactions were so very weak compared to electromagnetism. This meant that the W-boson had to be very, very heavy; about a hundred times heavier than the proton. And most physicists were unwilling to contemplate an elementary particle with such great mass. Faith Versus Observation Another apparent obstacle came from experiments. If the W-boson were to be related to the photon, which mediates the electromagnetic force, then it had to have the same spin as the photon. But the experimental results seemed to show that this could not be the case. We learned later that the key experiments had been done wrong. Yet, if Schwinger or I had taken these data too seriously, I would not have written a thesis about the intermediate vector boson and, probably, would not have won the Nobel Prize. The idea of unifying the weak and electromagnetic forces was too tantalizing to give up. Sometimes (but not too often) faith in one's own ideas should override mere observation. We did have one promising theoretical notion. In 1954, C.N. (Frank) Yang and Robert Mills invented a generalized principle of gauge invariance that led to a form of gauge theory that was richer and more elaborate than electromagnetism alone. Their idea was that the redefinition of fields that constituted a gauge transformation could mix up the identities of neutrons and protons. This implied the existence of new particles whose exchange mediates new forces. To do this, the particles carrying the forces must be three in number. Schwinger sensed that a Yang-Mills theory could form the basis of a unified theory of weak and electromagnetic interactions. One of the triplet of gauge bosons was the photon, he suspected. The other two were oppositely charged intermediate vector bosons. But there was a serious problem with the idea. According to Yang and Mills’ mathematics, all three of these particles had to be massless and we knew this was not true for the W-boson. Neither Schwinger nor I had the foggiest idea of what produced the W-boson’s mass. It was a most nettlesome question, and it probably led Schwinger to abandon his quest for an electroweak synthesis. I didn’t have that option; I had a thesis to write. (Little did I dream that the problem of the boson mass would be figured out by my old redheaded high school buddy Steven Weinberg.) At the time, the detailed properties of the weak force were just being revealed by careful studies of nuclear beta decay. Strange particles were being produced and examined at accelerators, and they showed mysterious properties. For example, the universality of the weak force did not seem to extend to the decay of these particles. I began to wonder if it was possible to construct a theory that led to meaningful calculations. In the midst of these cogitations, a great bombshell hit. I learned in the New York Times, of all places, of the experimental discovery of parity violation. It looked as if my thesis was going to be detoured— maybe over a cliff! Until then, physicists had believed that the laws of physics did not distinguished between right and left-handedness; therefore the laws were identical in the real world and a mirror-image world This geometrical principle is known as space-reflection symmetry, and its consequence is the law of parity conservation. Everyone believed it except Tsung Dao Lee of Columbia and Frank Yang, then at Princeton. They even went so far as to spell out experiments to test whether the weak force conserves parity. When the relevant experiment was done, it showed that the electrons produced in beta decay were more often left-handed than right-handed. Not only did the weak force violate parity, but it did so as much as it possibly could. There I was, writing my thesis on the weak force while the rules of the game were being changed with abandon. It was like trying to surf in a hurricane. The next development was the construction of what is called the V-A theory of weak interactions. It was the invention of Richard Feynman and Murray Gell-Mann at Caltech and, independently, of Robert Marshak and George Sudarshan at the University of Rochester. By asserting that the neutrino is intrinsically a left-handed particle, it explained some aspects of parity violation. But it was also compatible with a weak force mediated by intermediate vector bosons. What’s more, experiment showed that various other possible types of weak force, types that could not be explained in terms of intermediate vector bosons, were not present. It was an encouraging portent. I became convinced that a unified electroweak theory was possible. I laid out all the arguments in my thesis and concluded, “It has been our declared viewpoint, an attractive if not overly plausible one, that the photon and the [charged W-boson] comprise a family . It was not a great thesis. I had not quite risen to the challenge and constructed a fully acceptable unified theory of weak and electromagnetic forces. But I had come across some hints that Schwinger’s idea might be right. Over the next dozen years, working in Denmark, California, and back at Harvard, Glashow was able to prove that this “attractive if not overly plausible" idea was in fact correct. And for his role in the unification of the electromagnetic and weak forces, he shared the Nobel Prize with Steven Weinberg and Abdus Salam. This account of one period in the physicist’s life is reprinted from Interactions by Sheldon L. Glashow with Ben Bova. Copyright © 1988 by Sheldon Glashow. Published by Warner Books, Inc., New York.

The First Steps Toward Unity-- My Harvard Days

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