The Last Man Who Knew Everything

by David N. Schwartz

  FIGURE 21.1. Fermi at the blackboard during a publicity photo shoot. The equation for the fine structure constant alpha directly above his head is incorrect—most likely Fermi’s idea of a joke. Courtesy of Argonne National Laboratory.

  Over time, however, as the invitations piled up, so did the graceful but firm letters declining them. Segrè suggests that Fermi might have begun to appreciate how little time he had left. Fermi may not have known that he would die so soon, but he certainly knew that, in general, physicists’ achievements tend to come early in life. He wanted to do as much as he could as fast as he could, while he still had the energy and mental acuity to do so.

  IN THE SUMMER OF 1945, WALTER BARTKY, A UNIVERSITY OF Chicago astronomer, replaced Arthur Compton as dean of physical sciences. Bartky had the idea for new, interdisciplinary research institutes modeled on the Met Lab and Los Alamos. Endorsed by University of Chicago president Hutchins, Bartky invited Fermi to become head of an institute devoted to nuclear science. Though Fermi was enthusiastic about the new institute, he refused Bartky’s invitation to be the director, fearing that an administrative role would constrain his opportunities to pursue his own research. He suggested Allison for the role, and Allison accepted, seeing it as an opportunity to build a world-class scientific research institution, building on Fermi’s enormous reputation. Many of Fermi’s Manhattan Project colleagues eventually joined, and by 1950, the Institute for Nuclear Studies boasted some thirty-four senior scientists and a number of more junior staff, many of them either current or future Nobel laureates.

  FIGURE 21.2. Fermi at the unveiling of a plaque at the University of Chicago commemorating the tenth anniversary of CP-1. University president Robert Maynard Hutchins is unveiling the plaque. From right to left, Fermi, Walter Zinn, Farrington Daniels, Robert Bacher, and William W. Waymack. Courtesy of Argonne National Laboratory.

  Fermi may not have wanted to direct the new institute, but in a December 1945 letter to Bartky he detailed his thoughts on its research agenda. He wanted to use high-energy particle beams to explore the force holding together the atomic nucleus. Yukawa’s theory that the force was conveyed by a particle, the “mesotron,” was his starting point. Hitting a nucleus with enough energy might stimulate the creation of these particles, which would then be used to probe the nucleus.

  Fermi urged Bartky to build a cyclotron capable of accelerating relatively heavy protons to a level of sixty million electron volts (MeV)—enough, Fermi believed, to create the conditions for mesotron production. Eventually, a souped-up cyclotron, capable of a then-astonishing 450 MeV, occupied most of Fermi’s experimental time when it came on line in 1951.

  Fermi noted that during the five years it would take to build the cyclotron, cosmic-ray research could provide clues as to the behavior of mesotrons. This research would not require a cyclotron. Some of these rays were extremely high energy. Studying them could be useful in exploring the nuclear force in the absence of the cyclotron.

  At the time he wrote this letter, Fermi was unaware that his Italian colleagues Conversi, Pancini, and Piccioni, working with makeshift equipment in a dingy Rome basement, had demonstrated conclusively that the mesotron was not the particle predicted by Yukawa, the manifestation of the field that holds the nucleus together. Their work had not yet reached the United States. The particle that did seem to match Yukawa’s particle was discovered in 1947, a bit more than a year after Fermi’s letter. It was originally called the pi-meson, a clumsy name that Fermi shortened to pion. The particle that his colleagues in the Roman basement studied eventually became known as the mu-meson, or muon, again a shortened name proposed by Fermi. In the context of what was known at the time, however, Fermi’s letter was a clarion call for research into the increasingly complex world of subatomic particles, a compelling agenda for the infant institute.

  LIFE AT THE INSTITUTE WAS COLLEGIAL AND INFORMAL, A FAR CRY from the formality and hierarchy of the Met Lab under Compton. Office doors were typically open, and Fermi encouraged researchers to wander the halls and find out what their colleagues were doing. Fermi might have gone home for lunch, but just as often he would pick up colleagues and wander over to the faculty club, or he might have lunch with students at Hutchinson Commons. This was his favorite place to grab a hamburger and Coke during the Met Lab years, challenging his younger colleagues with thought-provoking questions like “How thick does the dirt on the windows of the Commons have to get before it begins to fall off the window?”

  Wandering the halls was one of Fermi’s ways of finding out what was going on in the world of physics. He had stopped reading professional journals sometime before the war, relying instead almost exclusively on gossip with colleagues and institute visitors about interesting results and discoveries, challenging himself to figure out how the results were obtained. He also used these discussions to interest his colleagues in his current enthusiasms. One such enthusiasm was a concept called “spin-orbital coupling,” the notion that the spin of a particle or a set of particles within the nucleus could affect the way those particles orbited within the nucleus. In late 1949 and early 1950, he tried to get his young graduate student Richard Garwin interested in the concept, but Garwin showed no interest, so Fermi wandered down the hall to where Maria Mayer, his old friend from Columbia days, was working on her own project.

  Mayer later vividly recalled the moment. She was working on why certain specific “magic” numbers of particles inside a nucleus resulted in particularly stable arrangements, using a model of the nucleus arranged in proton and neutron “shells,” similar to the electron shells surrounding the nucleus that determine the chemical properties of the various elements. She was getting nowhere. Fermi and Mayer chatted for a while, Fermi politely tolerating Mayer’s chain-smoking (he didn’t smoke and generally frowned upon people smoking in his office). They chatted about physics and the shell problem for several hours and then they were interrupted by a knock on the door. Fermi had received a phone call in his office. As he got up to leave, he casually asked, “What about spin-orbit coupling?” Mayer was thunderstruck: “Yes, Enrico, that’s the solution.” Fermi, always cautious, replied, “How can you know?” However, she immediately saw that spin-orbit coupling explained the problems with which she was struggling and within two weeks she successfully incorporated it into one of the first comprehensive theories of the nuclear shell model. Years later she would describe it with an analogy to a floor full of couples arranged in concentric circles dancing a waltz. Some circles are moving around the room in a clockwise fashion, others, counterclockwise; the couples are also each spinning themselves, some clockwise and some counterclockwise. In her analogy, each particle in the nucleus is like one of those couples; the overall effect of the relationship between their orbit and their spin influences the stability of the nucleus.

  In 1963, her work was rewarded with a Nobel Prize, shared with Wigner and the German physicist J. Hans D. Jensen, both of whom also elaborated the shell model of the nucleus. She always credited Fermi’s generosity with helping her make the crucial breakthrough. Fermi had been generous indeed. Having seen the connection between the nuclear shell model and spin-orbit coupling, he could easily have done the work and published it himself. Instead, he offered the idea up and stood back, letting Mayer do the rest and take the credit. Mayer wanted to include him as a coauthor of the paper, but Fermi rejected that outright, explaining that because he was more famous, most people would assume he had done all the work, which was clearly not the case. It was a generosity that characterized his later years and reflected a more mature Fermi, comfortable in his stature as one of the world’s preeminent physicists.

  The institute was exciting not only because of the staff or the easy collegiality that Sam Allison and Fermi set by example but also because it was a magnet for visitors from around the world. Many of the most celebrated physicists would visit and some would be invited to present at the institute’s biweekly theoretical physics seminar, organized by Maria Mayer’s husband
, Joseph. These seminars covered a huge range of topics, all of which were cutting-edge science. Hans Alfven elaborated his theory of the intergalactic magnetic fields that Fermi believed were responsible for the high energy of cosmic rays. Feynman spoke about his work on liquid helium and its strange properties. Other visitors from the University of Illinois and the Institute for Advanced Studies at Princeton presented on topics of their choice. Most of the seminars, however, featured institute staff. Murray Gell-Mann presented on numerous occasions, as did Valentine Telegdi, Gregor Wentzel, and many others. In 1952 Fermi presented work on pion-nucleon scattering experiments he had just begun conducting at the newly operational cyclotron. These seminars ensured a continuing intellectual ferment within the institute community and kept everyone, including Fermi, abreast of developments in the world of physics. The seminars always started at four thirty in the afternoon. Staff members knew and visiting speakers were informed that they needed to finish by six o’clock. That is when Fermi would excuse himself and, irrespective of where the discussion stood, return home for the evening.

  Gell-Mann recalls with some frustration Fermi’s tendency, if he disagreed with a speaker, to ask questions with the persistence of a terrier chewing on someone’s ankle. If Fermi raised his hand and said, “There is something here I do not understand,” the speaker was in for trouble and, if he had seen Fermi in action before, knew it. One senses that Fermi enjoyed disagreeing with Gell-Mann the same way he enjoyed disagreeing with his old friend Teller. It may have reflected an actual disagreement, but it was also a sign of respect.

  IT WOULD TAKE FIVE YEARS FOR FERMI TO ACHIEVE HIS DREAM OF a major particle accelerator. Fermi was a consultant to the project and introduced a simple invention to remotely control the placement of the accelerator’s target without breaking the vacuum of the ring in which the protons traveled. Sam Allison oversaw the project, Herb Anderson took day-to-day responsibility for its development, and John Marshall assisted. The accelerator ended up growing from ninety-two inches to its eventual one-hundred-seventy-inch size, with a magnet weighing twenty-two hundred tons, and involved close, if sometimes fractious, collaboration among General Electric, Westinghouse, and the US Navy’s Office of Naval Research. It also involved a new technology, varying the radio frequency of the cyclotron voltage to maintain strict synchronization with the beam itself, in what is now called a synchrotron. The new technology was challenging to the electrical engineers constructing the machine, and Fermi regularly advised them on methods to overcome these challenges.

  Because Fermi had no operational responsibility for the cyclotron’s construction, he faced a lengthy period during which he could not conduct high-energy particle experiments. What would Fermi do while he, and everyone else, waited for the new accelerator to be built? He wasn’t exactly the kind of person to bide his time. Though none of his postwar work had quite the lasting significance of the work he did before the war, he kept himself quite busy, in both experimental work and in theory. His pre-cyclotron research focus was on exploiting the neutron sources provided by CP-3 at Argonne. He also began a five-year study of cosmic rays and their origins and in the process developed a close professional and personal connection with one of the most unusual and important astrophysicists in the world.

  AT ARGONNE, A NEW PILE, CP-3, HAD SUPERSEDED CP-2. IT WAS a fine source of neutrons that could be exquisitely controlled. For the next few years, Fermi undertook a series of important studies of neutron collisions using CP-3, collaborating mainly with Leona Libby but also with Herbert Anderson, Albert Wattenberg, and a number of younger graduate students. With Fermi’s old colleague, Walter Zinn, the new director of the lab, it was the old Fermi team working together again, as they had under the stands of Stagg Field in 1942, this time on projects selected solely on the basis of scientific curiosity.

  Unburdened of his bodyguard-driver, Fermi enjoyed driving himself everywhere with characteristic, prewar gusto. Wattenberg was a frequent passenger on the forty-minute commute to Argonne. He recounts a harrowing moment when Fermi raced successfully to get across a railroad track before being blocked by an oncoming train. What Fermi did not realize was that there was a second track, obscured by the first train, and on that track was a second train going in the other direction. They missed the second train by a matter of a few feet. Pulling off to the side of the road so that the two of them could recover from what must have been a heart-stopping near miss, Fermi turned to his young colleague to reassure him. “This is why it is very important for you to be with me; my time may be up. But yours isn’t.”

  At Argonne, Fermi and his colleagues bombarded some twenty-two elements with neutrons—slow, fast, and moderate in speed—to study how neutrons diffracted within solid material. They used a wide range of techniques invented by Fermi, including a “neutron mirror,” to measure the angular reflection of neutrons at various energies. Wattenberg recounts another characteristic Fermi moment. A group was looking for Fermi at Ekhart Hall, where the physics department was located, and someone told them that Fermi was in the machine shop in Ryerson Hall next door. They found him in discussion with one of the machinists, a fine and reliable tool and die maker with whom Fermi worked closely. Fermi was explaining to him how to use grit to produce the right kind of polish on a neutron mirror. The mystified machinist asked Fermi how to know if he had done the job properly. “I’ll hold the mirror up,” Fermi replied, “and if I can see my eyelashes it will be okay.” Not an especially rigorous standard for a high-precision instrument, particularly, coming from a Nobel Prize–winning experimental physicist, but it was characteristic of Fermi’s rough-and-ready approach.

  These experiments, which continued through much of 1947, established an important base of information about how neutrons diffract through a wide variety of substances and were quite influential in setting the stage for research reactors to be used as test beds in materials science industry as well as in biological and medical research.

  IN HIS 1945 LETTER TO WALTER BARTKY, FERMI MENTIONED THE importance of studying cosmic rays. The more these rays were studied, the more fascinating physicists found them. Some of these rays pounded the earth’s atmosphere with almost unimaginable energy, colliding with atoms in the earth’s atmosphere and creating a wide range of subatomic particles in the process. The universe, it turned out, was itself an enormous particle accelerator and, if a physicist was sufficiently diligent and armed with the right type of detectors, cosmic rays could be studied at very high energies, far higher than those achievable in man-made accelerators.

  After the war, scientists debated the origins of these high-energy rays. Some, like Edward Teller, believed that they originated in the sun and that solar processes heretofore not understood were responsible for them. Others believed that they originated deeper in the universe, in interstellar space. There seemed to be no clear way to decide between these two propositions. These questions interested Fermi deeply, as did another question: How did they get their almost inconceivable energy? What could possibly accelerate these particles to such high speeds?

  Fermi first began to think about these issues in 1947–1948, largely at the instigation of Teller. Fermi—perhaps, as he joked later, simply to find a way to disagree with his old friend—decided to argue that they come from deep space, well beyond the solar system. Thinking perhaps of how earth-bound accelerators push charged particles to ever higher energies, he constructed a theory to explain the high energies of cosmic rays. Fermi hypothesized that the presence of large-scale interstellar magnetic fields could explain the speeds with which cosmic-ray particles showered the earth. The Norwegian astrophysicist Hans Alfven visited Chicago in 1948 and gave a lecture arguing for the existence of these interstellar magnetic fields. Fermi liked the lecture enormously, because it added weight to his own theory of cosmic-ray acceleration, although Alfven always doubted the existence of fields strong enough to account for cosmic-ray energies.

  Fermi soon found another foil for his musings on the origins of
cosmic rays, with one of the most fascinating physicists of the twentieth century—Subrahmanyan Chandrasekhar. Born in India, “Chandra,” as Fermi came to call him, wrote his undergraduate thesis on the relationship between “Compton scattering”—the photon experiments that won Arthur Compton his Nobel Prize in 1927—and Fermi-Dirac statistics. Chandrasekhar won a doctoral fellowship to Cambridge University and did post-doc work under Max Born at Göttingen and Niels Bohr at Copenhagen. In short, he had as strong an academic pedigree as any major physicist working in Europe at the time.

  While at Cambridge, Chandrasekhar was the first to predict that, for a star over a certain mass that runs out of nuclear fuel and goes cold, the gravitational force would be sufficient to overwhelm the degeneracy pressure implicit in Fermi-Dirac statistics, and the star would have no choice but to collapse upon itself to a point-like “singularity.” This mass came to be known as the Chandrasekhar limit, that is, the limit beyond which a white dwarf star would collapse into what John Wheeler later dubbed a “black hole.”* However, Chandrasekhar’s work was initially ridiculed publicly by the powerful astronomer Sir Arthur Eddington, who made a point of ruining the young scholar’s career. Chandrasekhar had no choice but to leave England, and, in a move of striking prescience, the University of Chicago hired him in 1937. He won the Nobel Prize for his work in 1983 and remained in Chicago until his death in 1995.