Enrico Fermi in America Free

A few days after Fermi’s arrival in New York, the news of the discovery of fission was brought to America by Niels Bohr. Partly in association with Leo Szilard, who had immediately realized the possibility of a chain reaction and its implications, Fermi promptly went to work experimentally on that possibility. Only one paper on that work appeared in the open literature, by Fermi, Szilard, and Herbert Anderson. Published in Physical Review, the paper concludes that, on average, about 1.5 neutrons are produced per fission, which might be sufficient for a chain reaction. Subsequent work was described in 16 secret reports (now accessible in Fermi’s collected papers ⁴ ) that document this early phase of the Manhattan Project.

Even in the excitement of the fission experiments at Columbia, Fermi did not abandon theoretical physics. In January 1941, he published his celebrated calculation on the stopping power of gases. In retrospect, the motivation for this work looks curious: Fermi wanted to propose an alternative to the conventional explanation of certain cosmic-ray effects in the atmosphere, based on the hypothesis of an unstable meson! The existence of the pion would not be demonstrated for another six years.

As the secret activity in Chicago expanded, Fermi had to make frequent trips from New York before moving there with his family. From April 1942 until September 1944, he was the key figure of the “Metallurgical Laboratory,” the secret project on the Chicago campus aimed at developing a nuclear reactor and, ultimately, the atomic bomb. This work culminated on 2 December 1942 in the start-up of the world’s first functioning reactor, or “pile,” a graphite–uranium assembly erected under the grandstand of the university’s football field. Today, with Stagg Field long gone, this historic location is marked with an abstract statue by Henry Moore.

Although Fermi’s style emerges unmistakably from any report that bears his name, there is a marked difference between his many secret reports (now publicly available) on his Met Lab work and the earlier reports from Columbia. At Columbia, Fermi had directed a small group of physicists and participated personally in even the most menial tasks. The experiments were on a small scale, and Fermi, through his pioneering work in Rome in the 1930s, was already the accomplished master of the relevant theory. In Chicago, by contrast, the project assembled a large group of scientists and engineers from various fields. Fermi had to assign the execution of experiments to different subgroups. He called it “doing physics by telephone.” But he generally evaluated the data himself, because he preferred his knowledge to be firsthand. His leadership was greatly enhanced by his marvelous ability as a lecturer. He instituted a lecture series on neutron physics. Later, at Los Alamos, he gave a more extensive course on the subject, complete with homework problems.

On the 10th anniversary of the first chain reaction, Fermi published an extensive paper ⁵ on the first pile. Nothing in that paper conveys the drama of the event. In fact, two weeks after the pile first went critical, Fermi wrote tersely: “The activity of the Physics Division [of the Met Lab] in the past month has been devoted primarily to the experimental production of a divergent chain reaction. The chain-reacting structure was completed on December 2 and has been in operation since then in a satisfactory way.”

In September 1944, after witnessing the start-up of the Manhattan Project’s first plutonium production pile at Hanford, Washington, Fermi moved to Los Alamos for the rest of the war. He witnessed the explosion of the first atomic bomb on 16 July 1945 in New Mexico’s Jornada del Muerte desert. Characteristically, he dropped a few bits of paper at the moment the blast wave reached him and noted the distance they were blown as a rough measure of the “gadget’s” yield. His estimate agreed closely with that obtained by the more sophisticated official measurements.

At Los Alamos, Fermi was in charge of a division (aptly called F-Division) that concerned itself with special projects. Edward Teller worked in that division on the prospects for a hydrogen bomb, the so-called Super. Perhaps Fermi’s postwar opposition to the building of such a device was rooted in the technical knowledge he gained during this period. In the summer of 1950, Fermi and Stanislaw Ulam investigated the possibility of initiating an explosive thermonuclear reaction in a mass of deuterium. They concluded that ignition would not propagate in such a scheme. To my knowledge, their report is still classified.

With the Japanese surrender a week after the bombing of Nagasaki in August 1945, the original mission of the Los Alamos laboratory had been fulfilled and the unprecedented galaxy of scientists assembled on the mesa began to disperse. They were anxious to return to their natural academic habitat, but with a new attitude. The wartime effort had ushered in the era of Big Physics—large-scale equipment and massive financial support.

Fermi, together with a group of other brilliant senior scientists (including Szilard, Teller, Willard Libby, Cyril Smith, Harold Urey, and Joseph and Maria Goeppert-Mayer) and their junior wartime associates (among them Anderson, Robert Christy, John and Leona Marshall, Darragh Nagle, and chemists Nathan Sugarman and Anthony Turkevitch) accepted offers from the University of Chicago. Some kind of package deal was involved. University president Robert Hutchins realized the immense potential of this package, and found the means to launch three research institutes: the Institute for Nuclear Studies (now named after Fermi), the Institute for the Study of Metals (now named after James Franck), and the Institute for Biophysics.

As soon as he returned to Chicago, Fermi took up his academic research and teaching. But experimental facilities in the physics department were minimal at that time. So Fermi undertook to do neutron physics, his old interest, by exploiting the intense neutron flux from the CP-3 reactor built during the war at the Argonne laboratory outside Chicago. Nine remarkable papers, all but one produced in collaboration with Leona Marshall (née Woods), came out of this research over a period of two years. All those papers have the hallmarks of Fermi’s style: extreme economy of technical means, efficiency of execution, and self-contained theoretical discussion, formulated in the most elementary possible terms.

After those neutron physics investigations, Fermi’s personal participation in experiments came temporarily to an end. He returned to what he considered to be his main vocation, theoretical physics, focusing on entirely novel topics. In 1946–47, some of the most exciting results were coming from cosmic-ray physics, primarily from experiments in Europe. Ettore Pancini, Oreste Piccioni, and Marcello Conversi in Rome reported an extraordinary anomaly: Negative mesotrons (the old name for what were thought to be mesons) stopped in carbon were not appreciably absorbed by carbon nuclei, as was expected if they were indeed the mesons postulated by Hideki Yukawa. Instead, they decayed in about 10⁻⁶ seconds. Fermi and Teller, and independently Victor Weisskopf at MIT, gave convincing arguments that this surprisingly long survival could not be explained away in terms of some anomaly in the process of slowing down in carbon. Shortly thereafter in Bristol, Giuseppe Occhialini, Cecil Powell, and their colleagues, while examining cosmic-ray tracks in photographic emulsion, discovered that the mesotron (now called the muon) was, in fact, the decay product of a slightly heavier particle (now called the pi meson), which was indeed the Yukawa particle.

Very soon, accelerators at Ernest Lawrence’s Berkeley laboratory were producing artificial pions; the era of high-energy physics had begun. It was decided to equip the new Chicago Institute for Nuclear Studies with a 450-MeV synchrocyclotron. Starting up in the spring of 1951, it was for a few years the highest-energy accelerator in the world! Fermi contributed to the project in many ways. He calculated the orbits of pions from the production target to the experimental area, using the MANIAC electronic computer at Los Alamos. (Fermi quickly realized the potential of electronic computing and became a fluent programmer.) He also built a small electrical cart with which one could readily move the pion production target around the cyclotron, exploiting the accelerator’s high magnetic field as the stator for the cart’s motor. He was quite proud of this device, which came to be known as “Fermi’s trolley.” He also devised a simple way to measure the intensity of the internal beam by the temperature increase of a metal target in the beam.

Fermi’s first major theoretical paper after the war concerned the origin and acceleration of cosmic rays. He suggested that a galactic magnetic field played the key role in the acceleration mechanism. This paper was stimulated by Teller, often Fermi’s favorite sparring partner, who had argued that cosmic rays originate in the Solar System. Another remarkable paper was one that Fermi wrote in collaboration with C. N. Yang, entitled “Are Mesons Elementary Particles?” It had generally been assumed that pions have the same relation to nucleons as photons do to electrons—that is, the relationship of a field to its source. But Fermi and Yang advanced the bold hypothesis that the pion is a bound state of a nucleon–antinucleon pair. This hypothesis, not readily verifiable nor very useful in itself, paved the way for the later radical ideas of nuclear democracy and bootstrapping in particle theory. Nowadays, we picture the pion as a bound state of a quark–antiquark pair.

The next theoretical problem that Fermi attacked was estimating the probability that a given number of pions would be produced in the collision of a proton with a nucleus. Discarding dynamical considerations, he based his deductions solely on statistical arguments. Notwithstanding Fermi’s legendary versatility, I would say that he had a deeper feeling for statistical methods than for any other subject. As always, Fermi was fully aware of the limitations of this simplified model; he meant it to serve only as a guideline. Thus he resented it when experimental departures from these rough predictions were raised as serious objections.

Fermi produced several outstanding textbooks while at Chicago. The first of these was Nuclear Physics , ⁶ an extensive set of lecture notes compiled by his students Jay Orear, Arthur Rosenfeld, and Robert Schluter. It is a classic, a compendium of simple (or at least seemingly elementary) solutions to all the relevant nuclear problems of its time. The second was Elementary Particles , the written version of his 1950 Silliman Lectures at Yale University. ⁷ Fermi also planned to produce an American version of a high-school physics text that he had published much earlier in Italy. But because of frequent disagreements with the publisher’s educational expert, Fermi abandoned the project.

Once the Chicago synchrocyclotron began operating routinely, Fermi returned to experimentation. He worked primarily on the interaction of pions with protons. Some measurements had been done slightly earlier at Columbia’s lower-energy Nevis cyclotron. Fermi undertook these experiments in close collaboration with Nagle and Anderson. This work lead Fermi and his collaborators to two outstanding discoveries: the first, that the nucleon has an excited state , with an excitation energy of some 180 MeV; the second, that the pion–nucleon interaction obeys charge independence, a symmetry already sketchily known from nuclear physics and characterized by a conserved quantum number called isotopic spin. The excited state manifests itself as a prominent resonant peak in the energy dependence of the pion–nucleon scattering cross section.

When the first indications of this resonant peak were reported, Keith Brueckner wrote a theoretical paper anticipating the Fermi group’s most striking data by a few days. ⁸ As Anderson commented in volume 2 of Fermi’s collected papers:

Even in the midst of all this experimental excitement, Fermi took time to delve into theoretical questions in very different areas of physics. During the fall of 1952, he began meeting with Subrahmanyan Chandrasekhar once a week for two hours to discuss a variety of astrophysical problems. Their meetings led to two major joint papers on galactic dynamics, one entitled “Magnetic Fields in Spiral Arms,” and the other entitled “Problems of Gravitational Stability in the Presence of a Magnetic Field.” The first paper bears the stamp of Fermi’s power to obtain estimates by simple means, and the second demonstrates Chandrasekhar’s analytic virtuosity.

Fermi’s interests and contributions during his postwar Chicago period ranged even farther than one can deduce from his publications. Goeppert-Mayer, who would share the 1963 Nobel Prize in Physics for formulating the shell model of nuclear structure, acknowledged that she was put on the right track by a single crucial question raised by Fermi (who, characteristically, did not refer to this fact in his own published discussion of the model).

Fermi did not read the journals regularly. Instead, he often asked experts to bring him up to date on some topic of current interest. He had a considerable interest, both theoretical and technological, in superconductivity. It was he who got Berndt Matthias, then on the Chicago faculty, interested in high-temperature superconductors by raising at lunch the question, Would it not be enormously important to have superconductors at, say, liquid hydrogen temperature?

In my opinion, Fermi’s greatest contribution to physics in his Chicago period lay in his teaching. The Fermi spirit lives on through his students. Here is the list of his PhD students at Chicago, in roughly chronological order: George Farwell, Anderson, Wattenberg, Harold Agnew, Goeffrey Chew, Marvin Goldberger, Jack Steinberger, Owen Chamberlain, Richard Garwin, T. D. Lee, Uri Hasber-Schaum, Orear, John Rayn, Schluter, Rosenfeld, Horace Taft, and Jerome Friedman. The list, including both theorists and experimenters of great eminence, speaks for itself. Four of them became Nobel laureates.

Fermi generally spent his summers at Los Alamos. But in the summer of 1949, he taught three courses at Chicago. He once taught the University of Chicago College’s introductory physics courses, even though (or perhaps because) he abhorred the humanistic approach to undergraduate science teaching that prevailed at Chicago during the Hutchins presidency. The idea, as Fermi saw and decried it, was “to discuss how Galileo thought, but not to teach what he thought about.” The table on page 41 displays the impressive range of courses he taught at the University of Chicago.

Fermi’s legendary classroom teaching was the fruit of careful preparation. He seemed to derive pleasure from the act of teaching, without regard for the result. He never showed annoyance at a student’s failure to grasp on the first try (or even the second) what he was trying to explain. On the contrary, if Fermi had to repeat an explanation, his pleasure appeared to be doubled.

His style of lecturing was not, however, entirely above criticism. It differed radically from his private approach to working problems. In class, he often chose to discuss general problems in terms of specific examples, with all constant factors carefully adjusted to be of order unity and thus expendable. In his own calculations, generally performed on 2 × 3 ft. Drafting sheets—far from the proverbial back of an envelope—all factors were carefully kept. He delighted in giving simple derivations of results thought by others to require elaborate calculations. But he occasionally sidestepped certain topics for which he couldn’t find a very elementary argument.

His lectures had an almost hypnotic effect. In class, many of the students thought they had understood everything, but subsequently often felt empty-handed. I found most fascinating the talks by Fermi that covered notions familiar to me. It was like seeing an accustomed landscape from the viewpoint of a soaring eagle—all the important points stood out with remarkable clarity.

In several of his courses, Fermi handed out mimeographed notes before each lecture. They contained mostly equations, with very little accompanying text. He explained that he did this because he personally was unable to listen and take notes at the same time. He had hardly ever taken notes during his student years at the University of Pisa. Some of the mimeographed Chicago notes (on quantum mechanics, for example) were subsequently published by the University of Chicago Press. In my opinion, they do Fermi’s memory a disservice: Presenting the equations without Fermi’s comments is like showing a skeleton in place of a full-length portrait.

Fermi’s way of teaching and thinking about quantum mechanics deserves special mention. His attitude was entirely pragmatic. Quantum mechanics is acceptable because its predictions agree with experiment. He once said that “the Schrödinger equation has no business agreeing so well.” Nothing else counted. He devoted no time to such topics as the quantum theory of measurement. He was immune to the “Copenhagen spirit,” both by temperament and by educational background. He was completely self-taught in quantum mechanics, an outsider to the Göttingen-Zürich-Copenhagen founders’ circle. Fermi drew a firm line between physics and philosophy. Although he was endowed with remarkable analytic powers, Fermi often affected an aversion to abstract mathematics.

It was not easy to know Fermi intimately, in the sense of understanding his deeper motivations. Professionally, he was always accessible, but he stayed aloof on the personal level. When I knew him at Chicago, he did not seem to develop bonds of friendship with colleagues at the university. Herb Anderson and Leona Marshall were perhaps the only exceptions. Fermi avoided gossip and rarely expressed his opinions, high or low, about the capacities of others. All this gave him an air of modesty that belied his full awareness of his own capabilities.

In my opinion, Fermi divided physicists into three categories: (1) those few from whom he could learn something (this category, in the 1950s at Chicago, included only the young Murray Gell-Mann); (2) people who had the courage to contradict him (nuisances, he thought, inasmuch as he was almost invariably right); and (3) people who accepted his opinions almost automatically and thereby qualified as efficient assistants.

Fermi was completely devoted to physics. He appeared to have very few outside interests. He did enjoy physical activities like tennis and mountaineering. But I suspect that was mostly for the sake of maintaining mens sana in corpore sano. He was a scientist of absolute integrity and total dedication, with an incredible gift for efficiency. He was a very clear thinker, but not an exceptionally quick one—compared, say, to Teller or Lev Landau. He solved simple and difficult problems at the same steady pace. In his dealings with other physicists, he displayed reserve and modesty. He didn’t like to throw his weight around. One day he needed an oscilloscope owned by somebody outside his own group. He asked one of his associates to go and fetch it—but he added, “Don’t tell him that it’s for me.”

Fermi rarely made mistakes when he was talking about physics. A public mistake was a painful experience for him. The story is told that once, when writing on the blackboard in front of a class, he realized that he had gotten a certain factor wrong. He faced the audience to make some interesting remarks and, at the same time—without interrupting his delivery—he wiped out the wrong formula with his left elbow. Another story tells of a student who pointed out that Fermi had written a c in the numerator rather than in the denominator where it belonged. “Who told you,” responded the great teacher, “that I use c and not 1/c for the velocity of light?”

Fermi had very regular working habits and a frugal lifestyle. He usually came to work before 8 AM, either walking or biking when weather permitted. He had already been working for several hours at home. He was totally secure in his own physics talent and almost never displayed jealousy of another scientist. The only exception was Einstein. More than once, Fermi expressed annoyance at the attention Einstein received from the press.

Fermi also disliked what he considered upper-class mannerisms. Once he commented that Robert Oppenheimer “was born with a golden spoon in his mouth.” I was at lunch with Fermi on the day the Oppenheimer security clearance case broke. “What a pity that they attacked him and not some nice guy like Bethe,” he remarked. “Now we have all to be on Oppenheimer’s side!” Fermi’s testimony at the Gray Board hearings was, as one would expect, sober and not damaging to Oppenheimer.

Fermi claimed to have a rather poor memory. So he compiled for himself an “artificial memory,” an encyclopedic collection of notes, summaries, calculations, numerical data, and so forth. He displayed hardly any of the characteristics often attributed (rightly or wrongly) to Italians. But he did, in my opinion, possess one Italian quality that’s all too rare among American intellectuals: a total absence of psychological complexes.

He was perfectly well integrated into American life, preferring to be called “Enrico” rather than something like “Egregio professore” or “Herr Geheimrat.” He participated in the students’ social life, going to their parties and inviting them to his home for square dancing. Although he never lost his Italian accent, his English was excellent.

His ability to improvise order-of-magnitude estimates on the spot was legendary, and he would sometimes exercise it under surprising circumstances. William Zachariasen, a distinguished colleague, was in the hospital recovering from a heart attack. When Fermi came to visit, Zachariasen complained that he was being given too little to eat—only 1500 calories a day. “Willy, you are a great reader of detective stories, are you not?” asked Fermi.

“Yes I am,” replied Zachariasen.

“Willy, how long does it take a corpse to cool to room temperature?”

“Four or five hours,” was Zachariasen’s answer.

After some thought, Fermi concluded, “Then you cannot survive on 1500 calories a day!”

Fermi’s humor could be quite sarcastic. One of the favorite pastimes of physicists at Los Alamos was fishing. Segrè enjoyed it and urged him to come along. Fermi showed no interest, so Segrè tried to convince him of the intellectual merits of fishing: “You see Enrico, it’s not so simple. The fish are not stupid; they know how to hide. One has to learn their tricks.” Fermi replied, “I see, matching wits!”

Lee tells the story that Fermi, at some point, decided to teach group theory to his private seminar. He took out his index cards on the subject and started first to discuss Abelian groups, then Burnside’s theorem, and next the notion of a group’s Center. Only much later did he get to the group concept itself. Some of the students expressed confusion at this seemingly erratic approach. “Group theory is nothing but a compilation of definitions,” replied the master. So he was simply following the index at the end of Hermann Weyl’s book in alphabetical order.

Fermi looked at his surroundings with a physicist’s eyes. Once at a seminar, to answer a question from Bill Libby about mixing in the ocean, he quickly derived an equation describing the phenomenon. The equation had only one parameter, the wavelength λ of surface waves. For this, Fermi promptly inserted a numerical value of 200 meters. “Enrico, isn’t it more like 600 meters?” asked somebody in the audience. “Maybe so,” he replied. “But it was certainly 200 meters when I last crossed the Atlantic.”

One of Fermi’s greatest assets was his wife, Laura, a woman of considerable intellect and great charm. During the frequent parties in their home in Chicago, she managed to make the younger crowd, especially the Europeans overawed by the presence of the master, feel perfectly relaxed. Her book, Atoms in the Family, shows us the great physicist from a unique vantage point. ⁹  

The material presented here partly overlaps my essay “Enrico Fermi,” in the book Remembering the University of Chicago: Teachers, Scientists and Scholars, E. Shils, ed., U. Of Chicago Press (1991).

Valentine Telegdiwas the Enrico Fermi distinguished Service Professor of Physics at the University of Chicago before moving to the Swiss Federal Institute of Technology (ETH) in Zürich, from which he is now retired. He divides his time between CERN, in Geneva, and Caltech, in Pasadena, California.