My purpose is to celebrate the centenary of Enrico Fermi’s birth rather than to make a scholarly contribution to biography. Fermi was born in Rome on 29 September 1901 and died in Chicago on 28 November 1954.

Having one’s name permanently attached to an important physical concept or unit bestows a kind of immortality. It is hard to imagine any discussion of modern physics in which Fermi’s name does not come up at least once, in terms such as fermion, Fermi gas, Fermi momentum, Fermi temperature, Fermi surface, Fermi coupling, Fermi transition, and Fermi length (1 F = 10−15 m). Still, because Fermi died so young—and so long ago—he has become, for the current generation of physicists, a somewhat mystical figure.

No one in the history of modern physics was more versatile than Fermi. His contributions in pure theory and concrete experimental work were equally great. He could, with equal ease, solve abstract problems or design and build, with his own hands, astonishingly useful experimental tools. He was also an exceptionally lucid expositor and an active and patient thesis supervisor. Through the influence of his eminent students, in Italy and the US, Fermi effectively revolutionized the training of physicists.

biography of Fermi 1 1. E. Segrè, Enrico Fermi: Physicist, U. of Chicago Press , Chicago (1970). I confine myself here to Fermi’s American years, thus omitting many of his greatest glories. I had the privilege of being a junior colleague of Fermi’s at the University of Chicago from 1951 until his untimely death in the fall of 1954. For a more balanced picture, I refer the reader to Emilio Segrè’sof Fermiand the articles by Hans and Henry Bethe (page 28) and Silvan Schweber (page 31) in this issue.

Physics Today, Physics Today 0031-9228 46 1 1993 44 cancer that had escaped early detection. He was only 53 years old. (Roy Glauber’s photo essay on page 54 of this issue shows Fermi in the Alps in the summer of 1954, still unaware of his fatal illness.) Fermi brought his uranium-fission team from Columbia University to Chicago in April 1942. (See the article by Albert Wattenberg in1 1993 44 https://doi.org/10.1063/1.881378 January 1993, page 44 .) Returning from Los Alamos after the war, he joined the Chicago faculty in January 1946 as Arthur Holly Compton’s successor. Fermi died in the university’s Billings Hospital six weeks after being admitted. A physically strong man, he was felled by a multiply metastasizedthat had escaped early detection. He was only 53 years old. (Roy Glauber’s photo essay on page 54 of this issue shows Fermi in the Alps in the summer of 1954, still unaware of his fatal illness.)

US several times. In 1930, he had taught a course in quantum electrodynamics at the famous University of Michigan summer school, a course that led to his celebrated—and still very readable—Reviews of Modern Physics article on the subject. 2 4, 87 (1932). 2. E. Fermi, Rev. Mod. Phys., 87 (1932). https://doi.org/10.1103/RevModPhys.4.87 thermodynamics. The course lecture notes, edited into book form by Lloyd Motz, 3 3. E. Fermi, Thermodynamics, Dover , New York (1956). Before emigrating to America in January 1939, Fermi had already visited theseveral times. In 1930, he had taught a course inat the famous University of Michigan summer school, a course that led to his celebrated—and still very readablearticle on the subject.(See Schweber’s article.) In 1936, he was a visiting professor at Columbia University, where he taught a course onThe coursenotes, edited into book form by Lloyd Motz,are still in use today.

It was during this 1936 visit that physics department chairman George Pegram first offered him a permanent appointment at Columbia. Fermi was not particularly critical of Mussolini’s fascist regime prior to the dictator’s promulgation of anti-Jewish laws in 1938. But those new racial laws applied to his wife, Laura. So he decided to leave Italy and discreetly inquired of Pegram whether the position was still available. Fermi’s trip to Stockholm to receive the Nobel Prize in December 1938 was a convenient way to emigrate unobtrusively with his whole family.

Bohr brings the news Section: Choose Top of page ABSTRACT Bohr brings the news << Los Alamos The era of high energies An excited state The teacher Fermi’s personality REFERENCE CITING ARTICLES 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 4. E. Amaldi et al. , eds., Collected Papers of Enrico Fermi, vols. 1 and 2, U. of Chicago Press , Chicago (1962). 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 aand 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, the paper concludes that, on average, about 1.5are produced per fission, which might be sufficient for aSubsequent work was described in 16 secret reports (now accessible in Fermi’s collected papers) 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. chain reaction, Fermi published an extensive paper 5 20, 536 (1952). 5. E. Fermi, Am. J. Phys., 536 (1952). https://doi.org/10.1119/1.1933322 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.” On the 10th anniversary of the firstFermi published an extensive paperon 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 theDivision [of the Met Lab] in the past month has been devoted primarily to the experimental production of a divergentThe chain-reacting structure was completed on December 2 and has been in operation since then in a satisfactory way.”

Los Alamos Section: Choose Top of page ABSTRACT Bohr brings the news Los Alamos << The era of high energies An excited state The teacher Fermi’s personality REFERENCE CITING ARTICLES 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.

The era of high energies Section: Choose Top of page ABSTRACT Bohr brings the news Los Alamos The era of high energies << An excited state The teacher Fermi’s personality REFERENCE CITING ARTICLES 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. textbooks while at Chicago. The first of these was Nuclear Physics , 6 6. E. Fermi, Nuclear Physics, A Course Given by Enrico Fermi at the University of Chicago, J. Orear, A. Rosenfeld, R. Schluter, eds., U. of Chicago Press , Chicago (1950). 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 7. E. Fermi, Elementary Particles, Yale U. Press , New Haven (1951). 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. Fermi produced several outstandingwhile at Chicago. The first of these wasan extensive set ofnotes 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, the written version of his 1950 Sillimanat Yale University.Fermi also planned to produce an American version of a high-schooltext that he had published much earlier in Italy. But because of frequent disagreements with the publisher’s educational expert, Fermi abandoned the project.

An excited state Section: Choose Top of page ABSTRACT Bohr brings the news Los Alamos The era of high energies An excited state << The teacher Fermi’s personality REFERENCE CITING ARTICLES 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. theoretical paper anticipating the Fermi group’s most striking data by a few days. 8 86, 106 (1952). 8. K. Brueckner, Phys. Rev., 106 (1952). https://doi.org/10.1103/PhysRev.86.106 When the first indications of this resonant peak were reported, Keith Brueckner wrote apaper anticipating the Fermi group’s most striking data by a few days.As Anderson commented in volume 2 of Fermi’s collected papers: + p cross section. Brueckner had seized upon isotopic spin as being an essential element in the pion–nucleon interaction. Arguing that the dominant state was one with total angular momentum 3/2 and isotopic spin 3/2, all the features of the experiments could be understood at once. It took hardly more than a glance at Brueckner’s paper for Fermi to grasp the idea. Twenty minutes after he left the experimental room to work through the idea by himself in his office, he emerged with this happy conclusion: “The cross sections will be in the ratio 9:2:1,” he announced. He [was referring] to the π+ elastic, π− charge-exchange, and π− elastic processes, in that order. 4 4. E. Amaldi et al. , eds., Collected Papers of Enrico Fermi, vols. 1 and 2, U. of Chicago Press , Chicago (1962). In fact, Fermi [read] the preprint of Brueckner’s paper the very day he found the astonishingly high πp cross section. Brueckner had seized upon isotopic spin as being an essential element in the pion–nucleon interaction. Arguing that the dominant state was one with total3/2 and isotopic spin 3/2, all the features of the experiments could be understood at once. It took hardly more than a glance at Brueckner’s paper for Fermi to grasp the idea. Twenty minutes after he left the experimental room to work through the idea by himself in his office, he emerged with this happy conclusion: “The cross sections will be in the ratio 9:2:1,” he announced. He [was referring] to the πand πprocesses, in that order. 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?

The teacher Section: Choose Top of page ABSTRACT Bohr brings the news Los Alamos The era of high energies An excited state The teacher << Fermi’s personality REFERENCE CITING ARTICLES 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. 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 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 introductorycourses, even though (or perhaps because) he abhorred the humanistic approach tothat prevailed at Chicago during the Hutchins presidency. The idea, as Fermi saw and decried it, was “to discuss how Galileo thought, but not to teachhe thought about.” Theon page 41 displays the impressive range of courses he taught at the University of Chicago. 1946 Winter Nuclear Structure Spring Electrodynamics I Autumn General Physics I Special Problems in Physics 1947 Winter General Physics II Special Problems in Physics Spring General Physics III Special Problems in Physics Research in Physics Autumn Quantum Mechanics and Atomic Structure I 1948 Winter Quantum Mechanics and Atomic Structure II 1949 Winter Nuclear Physics I Spring Nuclear Physics II Summer Mathematical Physics II Quantum Mechanics and Atomic Structure II Nuclear Structure I 1950 Winter Quantum Mechanics and Atomic Structure II Autumn Nuclear Physics 1951 Winter Physics of Solids Spring Nuclear Particles Autumn Thermodynamics and Statistical Physics II 1953 Winter Nuclear Physics II Spring Nuclear Particles 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.