My Life as a Quant (10 page)

Reality, of course, was different. I was friendless at Penn, surrounded by married nine-to-fivers, a sharp change from the casual gregariousness of the graduate-student life that I had grown accustomed to. Philadelphia was disagreeably unsafe and deserted compared to Morningside Heights. I was a married person unaccompanied by his spouse, a little too old and attached for the company of single graduate students and a little too young and uncoupled for the domesticity of the married faculty. I had little social life and spent most of my time alone.

Postdoc life was an atavism, a relic of a time long past. Postdoctoral research positions had been created to provide a brief interlude between being a graduate student and becoming a professor. But Americans' post-Sputnik view of science as the moral equivalent of war had produced a bubble of young scientists that, now tenured, occupied all the available faculty positions; they would not retire in less than thirty years. Faculty need students, and so aspiring physicists continued to enter the head of the PhD pipeline, but when they emerged at the tail, there was almost nowhere to go. Postdoc positions temporarily filled the void. They lasted about two years and paid little. They worked well for universities, though, for they got to sample a new bunch of young research physicists each year, and could pick an exceptional one for the rare faculty position that became available.

It was less pleasant for the average postdoc. You started each two-year position in the fall with a one-year grace period, during which time you tried to begin, complete, and publish some interesting research, so that by the fall of the second year you could apply for another postdoc position in another lab or department somewhere else in the world. Mitchell Feigenbaum, famous for his contributions to chaos theory, described it aptly: “These two-year positions made serious work almost impossible. After one year you had to start worrying about where you could go next.” If you were unlucky enough to have only a one-year postdoc position, which was not uncommon, then you had no respite; you had to start applying for your next job as soon as you began the current one. The only way out—other than abandoning academic physics entirely—was to write a paper so acclaimed that you would be offered one of the few faculty positions available.

Some of my PhD friends, passionate and desperate enough to stay in physics even if they were paid nothing, became “freebies,” people who had found no job anywhere and then asked for and received a desk and rudimentary research facilities with no pay at a top-ranked institution. Their aim was to get into a stimulating environment, make good contacts, and then do a piece of work that would get them a paying job. One friend went so far as to turn down a paid postdoc job at a secondtier institution in order to become a freebie at Harvard, where he then pulled off some research that landed him a subsequent paid position at a first-rate place, SLAC.

At the University of Pennsylvania, now without a PhD advisor, I had to take my own road, and so I began to look for something new to work on. I had spent most my graduate student years working on high-energy phenomenology, comparing other people's theories with other people's experiments. It was useful and interesting, but not as visionary as the physics I had imagined doing. Trying to be ambitious, I began to study the so-called Lee model, an idealized and therefore analytically soluble theory of particle interactions that was the subject of an early paper by T. D. himself. I hoped that it would form the basis for a deeper understanding of quark forces. I spent most of my first semester at Penn trying vainly to master the field. But I found it difficult to concentrate; I was restless from the lack of friends, tense from the strains of a geographically divided marital life, and tired from all the back-and-forth driving—I would go to New York on Friday nights and return to Philadelphia early on Monday mornings. Some weekends I was just too weary to make the trip and remained alone in Philadelphia, killing time and feeling half-resentful.

My first semester passed without much significant progress. The tenured professor who had hired me had his own problems, and provided ambivalent encouragement. He seemed disspirited by the competitiveness of physics. Once, inviting me over to his home for dinner, he spoke about how “we” had to resign ourselves to not having achieved something great. His wife quickly pointed out to him that it was still premature to include me in that “we.”

I felt time racing by. The same professor tried to guide me into working on a problem in his area, the algebra of weak and electromagnetic currents, but it was so far from my interests that it became almost repellent to me. What was the point of being in physics if you could not pick your own problems? By May 1974, at the close of my first academic year, I was heading for trouble. In three months I would have to start my next job search, and I had not published a paper; worse, I was not even involved in anything that could conceivably lead to a publication. I developed a visceral understanding of the meaning of “publish or perish,” and made darkly foreboding comments to my friends and acquaintances about where I was headed.

Life wasn't all bad, though. Three good things did happen that year, all extracurricular. I spent many evening hours in my Philadelphia bedroom learning to juggle three tennis balls. I started running more seriously than I had before, tagging on to a cadre of dedicated graduate-student long distance runners who trained every day at noon on the university's famous Tartan track, site of the Penn Relays. I remembered when Roger Bannister broke the four-minute mile, and now, temperamentally overenthusiastic to the point of stupidity, I ran a single mile against my stopwatch as fast as I could, several times a week, determined to remain ignorant of the benefits of warming up or running longer distances more slowly. Every few weeks I stopped to let my shin splints heal, and it took me several more years to acquire the patience to train rather than simply run as quickly as I could. Finally, I learned to play the recorder in a small group at a West Philadelphia arts center, enjoying my capacity to make even low-grade music in harmony with others.

My first academic year of employment passed, and Eva and I spent a month together in the summer of 1974 at the Aspen Center for Physics, where I was awed by the casual proximity of so many famous physicists. Aspen was popular; research space and apartments were in such short supply that some of the senior physicists had bought houses there in which they could spend the entire summer. The junior postdocs were allotted only a few weeks. We hiked in the mountains once or twice a week and swam at the Hotel Jerome pool where the women all seemed to have crocheted their own bikini tops—tops that they removed as soon as they sat by the pool. Each day I tried systematically to learn more about the theoretical structure of the increasingly topical Yang-Mills gauge theories of weak and electromagnetic interactions. I carefully worked my way through
New Yorker
writer and physicist Jeremy Bernstein's pedagogic introductory article in the
Reviews of Modern Physics
and, since he was an Aspen regular, I sometimes wandered over to ask him questions. Working, hiking, talking physics, listening to music at the Aspen Music Center, playing volleyball to let off steam—this was what academic physics was supposed to be like, but my enjoyment was tempered by my year without publication, which made me feel that regular summers in Aspen and the partaking of its pleasures would not be part of my destiny.

June passed quickly, and in July I traveled to Cape Town. My mother, like Stephen Hawking, had been ill with amyotrophic lateral sclerosis for several years, and each year I went home to see her. But while Hawking miraculously seemed to stabilize, my mother went steadily downhill in the 1970s, growing worse each year, first losing the ability to move her arms, her hands, and then her legs, until she finally began to have difficulty holding her head up or swallowing. It was a perpetual mystery to her that no one knew how to cure her ailment. In August I returned to New York for one month, continuing to read about gauge theories in our apartment or in the Columbia library while Eva continued her thesis work. As an academic, you could work (or not work) wherever you liked. It was freedom, but now, with one year as a postdoc gone and the future looming, it sometimes felt like the freedom to fail.

In September 1974 I returned to dreary Philadelphia, where I could no longer avoid pondering whether to try to seek another postdoctoral position for the fall of 1975, I had no new accomplishments to report on my résumé, something which was going to make job applications difficult. I began to think seriously of ceasing to “do physics,”
1
and to steel myself for the shame it involved. It seemed like my time had come. Then suddenly something good happened: A really interesting physics puzzle came along, and the techniques necessary to resolve it were closely related to those I had used in my PhD thesis.

Al Mann, one of the senior experimentalists at Penn, had been part of an international collaboration to scatter high-energy, muon-type neutrinos off protons at the CERN
2
particle accelerator in Geneva. Mann's experiment was very similar to the deep inelastic electron-proton scattering I had analyzed for my thesis, the difference being that he fired neutrinos rather than electrons at the proton target. According to what was then known about weak interactions, one expected the incoming neutrino to turn into a
single
negatively charged muon
3
when it collided with the proton. The proton would in turn then shatter into a debris consisting of many other protonlike particles, as illustrated in
Figure 3.1(a)
. This was not exactly what happened, however. Searching through their data, Al and his collaborators discovered a number of so-called
dimuon
events in which there were two muons, one negatively and one positively charged, among the final products of the collision. These were just the sort of “Swiss watch” collisions that Feynman had described, in which experimental anomalies could lead to the discovery of new particles or the new forces that produce them.

Figure 3.1
Neutrino-proton collisions. (a) A normal collision that produces a single negatively charged muon. (b) A hypothetical collision that produces a neutral heavy lepton, which in turn decays into two muons and a muon neutrino. (c) A hypothetical collision that produces a single negatively charged muon plus a charmed quark, which in turn decays into a positively charged muon, a neutrino, and a strange quark.

The puzzle was the cause of the two muons. There were (at least) two possible explanations, each involving the production of a hypothetical new particle. The first hypothesis was that the incoming neutrino had turned into a new
neutral heavy lepton
,
4
which then, because of the weak force, decayed into the two muons observed by Mann and his collaborators. This is illustrated in
Figure 3.1(b)
. The second hypothesis was that the incoming neutrino had, as is normal, turned into the one negatively charged muon, but that among the proton debris was a new so-called
charmed
quark that was required to exist in the standard theory of weak and electromagnetic interactions, and that this charmed quark then decayed via the weak force to produce a second positively charged muon, as illustrated in
Figure 3.1(c)
.

Which of these new particles—the hypothetical neutral heavy lepton or the hypothetical charmed quark—was the progenitor of the dimuons? The test would lie in the distribution of their speeds. The relative speeds of the positively and negatively charged muons would depend on whether the heavy lepton or the charmed quark had been their parent. In the former case, both muons arose from the decay of the heavy lepton and, because both had a similar origin, they tended to emerge with similar speeds; in the latter, the positively charged muon arose from the decay of a charmed quark and would have a very different speed. Much as each brand of water gun produces its own characteristic spray of water, so the different particles, when they decay, produce their own characteristic dimuon distribution.

Together with my colleagues Lay Nam Chang and John Ng, I began to investigate the properties of the dimuon distribution that resulted from heavy lepton production in order to compare it with the muon speeds reported by Mann and his collaborators. It was a classic phenomenology problem, the comparison of theory and experiment, closely related to my thesis work, and so I knew how to calculate the distributions of the final speeds and angles of muons. Lay Nam, John, and I checked each other's analytical calculations, and I wrote the computer program to evaluate the distribution of the muons. Suddenly I was involved again, and it was simply thrilling to be working on something new and relevant in close proximity to the experimentalists. I entered a period of great mental stimulation which resuscitated me. Spontaneously, I began to rise early in the morning; as soon as I awoke I rushed into work to calculate and program. I participated in long, intoxicatingly buoyant arguments and discussions on blackboards, where Lay Nam, John, and I took turns scribbling and talking, one of us seizing the chalk from the other. Rivalries and self-doubt disappeared as we pushed forward, working keenly late into the night.