ENRICO FERMI (1901–1954)

(On the Anniversary of His Death)

Bruno Pontecorvo

Among the scientists of our time, the great Italian physicist Enrico Fermi occupies a special place. In our age, when narrow specialization in scientific research has become a common phenomenon, it is difficult to point to so universal a physicist as Enrico Fermi was. He made a major contribution to the development of theoretical, experimental, and even technical physics. It is not surprising that Soviet physicists, like physicists throughout the world, felt so keenly the loss of Fermi, the first anniversary of whose death is being marked on November 28 of this year.

Fermi was born in Rome on September 26, 1901, into the family of an ordinary office employee. If one may speak of vocation, then, without doubt, Fermi was born a physicist. Although neither in his family nor among those around him did anyone encourage him to pursue science, Fermi, while still a boy, showed an exceptional interest in mathematics and physics. Without outside help, he enthusiastically read and mastered the contents of a number of books on physics and higher mathematics. The varied “assortment” of physics books that Fermi read as a boy included, alongside casual books—such as, for example, one of the old courses in physics and mathematics, written in Latin—also works such as the widely known physics course by Khvolson.

But Fermi did not seek explanations of what was happening around him only in books; he tried independently to analyze the phenomena that interested him. Often toys led him to the solution of scientific problems. Thus, for example, entirely on his own Fermi developed the theory of the top, of the rainbow, of the vibrating string, and so on.

When Fermi was admitted as a student to the University of Pisa in 1918, he already knew classical physics. How deeply he had mastered this field of science at that time may be judged from his

words spoken in 1934: “When I entered the university, I knew classical physics and the theory of relativity just as well as I do now.”

At the university the professors could give him nothing new: already at that time Fermi understood the problems of physics better than his teachers. Fermi benefited greatly from contact with his talented fellow student Franco Rasetti. In particular, their joint discussion of questions of theoretical physics helped develop Fermi’s exceptional didactic abilities, which later became apparent.

During this period Fermi devoted himself to an in-depth study of quantum physics, which at that time was still unknown in Italy. Even before receiving his degree (in 1922), Fermi wrote several theoretical papers in the fields of classical mechanics, statistical mechanics, and the theory of relativity. His dissertation (corresponding to our diploma thesis), however, was an experimental study in X-ray spectroscopy.

After receiving his degree, Fermi went abroad for a short time (Germany, Holland). At that time Fermi was not sure of himself. In Italy there were no physicists with whom he could compare himself, and the young Fermi did not have the self-confidence that is so necessary for creative work. As Fermi himself related, he finally acquired such confidence thanks to the well-known physicist Ehrenfest, who did not fail to tell him that he had the gift of a major physicist. The moral support that Ehrenfest gave him during his trip to Holland had even greater significance in Fermi’s life than his meetings abroad with such brilliant young theoretical physicists as Pauli and Heisenberg, who, unlike Fermi, had had the good fortune to study under such great scholars as Sommerfeld and Born.

During his teaching activity in Florence, Fermi published (1926) his famous paper[^1] on the statistical mechanics of particles obeying the Pauli principle. This work laid the foundations of what is known as Fermi—Dirac statistics. As is well known, the principal significance of Fermi—Dirac statistics lies in the fact that it provided the key to understanding the properties of electrons in metals. But the other applications of Fermi statistics are also very numerous, as is illustrated by the multitude of expressions that have entered the scientific literature, such as Fermi gas, fermion, the Fermi nuclear model, the Thomas—Fermi atom model, Fermi moments of nucleons in the nucleus, etc.

After the discovery of the statistics that bears his name, Fermi became well known first outside Italy, and only later—strange as it may seem—in his homeland.

In 1928 he was invited to the chair of professor of theoretical physics at the University of Rome, and, when he was only 27 years old,

He was elected a member of the Royal Academy of Italy. In 1929 Fermi was elected a corresponding member of the Academy of Sciences of the USSR. Subsequently he was elected a member of many academies of sciences throughout the world.

Fermi created the Italian school of modern physics. Many of his pupils, such as Rasetti, Amaldi, Segrè, Wick, Racah, Rossi, Ferretti, Bernardini, Cocconi, became widely known physicists.

The indelible mark left by Fermi on the development of Italian physics may be judged from the fact that at the present time, almost 20 years after he left his homeland, there is successfully working there a group of fairly well-known young physicists who are continuing the tradition of modern high-quality research created by Fermi himself.

In the period from 1930 to 1938, for the first time in this century, thanks to Fermi, foreign physicists were drawn to the Italian center of research. These physicists, among whom were Bethe, Bhabha, Bloch, London, Peierls, Placzek, Teller, Uhlenbeck, took part in the seminars of the Roman Institute of Physics together with a small group of Italian scientists, one of whom—Majorana—Fermi regarded as the greatest theoretical physicist of our time. Fermi’s seminars were conducted in an unconstrained atmosphere and always gave much to their participants.

Fermi was a born teacher. His university lectures on quantum mechanics, atomic physics, mathematical physics, thermodynamics, and his favorite course on geophysics were distinguished by great clarity and rigor of exposition; this, however, was not the result of special preparation for the lectures (Fermi almost never prepared for them), but was explained by the teacher’s profound knowledge and exceptional clarity of mind. In the final analysis, the quality of the lectures was a reflection of his independent work, begun while still a schoolboy, when he tried to grasp and understand various phenomena of nature.

In physics, in Fermi’s opinion, there is no place for confused thoughts: the physical essence of any truly intelligible question can be explained without the aid of a blackboard for writing down complicated formulas. The correctness of this opinion was illustrated by Fermi’s remarkable ability to be understood by listeners of the most varied level.

Fermi always emphasized the enormous importance for students of a good preparation in classical physics, and he himself liked to give lectures on elementary physics. The general course in mathematical physics delivered by Fermi in Rome was something like an encyclopedia containing elements of electrodynamics, potential theory, relativity, the propagation of heat, diffusion, and elasticity; he very much objected to a course in mathematical physics of the monographic type.

It is impossible to draw a line between Fermi the physicist and Fermi the man. Fermi taught his students and collaborators not only physics in the direct sense of the word. By his own example he taught them to love physics passionately, as well as to understand the spirit and ethics of science. Fermi persistently emphasized the exceptional moral responsibility of the scientist when publishing scientific work; in particular, he was intolerant of the frequently encountered tendency of experimenters to overestimate the accuracy of their measurements.

For Fermi, the interests of science were always above personal interests. He paid no attention to questions connected with his own priority. He always emphasized the contribution of collaborators to his research. Enrico Fermi was unusually simple and modest. The limits of this article, whose author had the good fortune to study with Fermi and to work under his direction, do not permit a full characterization of this remarkable scientist and man.

The captivating clarity of thought characteristic of Fermi’s lectures also distinguishes all his books (Fermi wrote seven of them), both survey works and original ones. Some of his books are well known in the Soviet Union. However, his two-volume Course of Elementary Physics for secondary schools and the splendid Introduction to Atomic Physics (which served as a textbook of theoretical physics at the University of Rome) are almost unknown in the USSR.

Fermi wrote his books in the same way as he delivered his lectures—extremely clearly and, it seemed, with minimal effort. Some physicists remember how, when they were students, they studied from his book Molecules and Crystals while the author was still writing it. Every morning, between 6 and 8 o’clock, Fermi carefully wrote on the odd-numbered pages of a notebook, leaving the even-numbered pages blank for corrections. However, when the manuscript of the book was ready for printing, the number of corrections turned out to be literally negligible.

It also surprised me that Fermi could write while hardly resorting to other articles or books. In general, Fermi read little, and still less did he buy books on physics after graduating from the university; he preferred to work out a given question himself rather than find a ready-made answer.

Fermi also spent comparatively little time on scientific journals, although he was always excellently informed about what was happening in the world of physics. This was achieved by “extracting,” in Fermi’s own expression, information in direct conversation with other physicists. I recall an incident that well illustrates yet another trait characteristic of Fermi—the ability to give advice to people working even in narrow areas of applied physics with which Fermi himself was little acquainted. In 1942 I happened to meet Fermi in Chicago. At that time I was working in the field of applying nuclear physics to the prospecting of deposits

oil (neutron logging and gamma logging). Since Fermi had no information concerning logging, he, of course, began to “draw” it out of me. Soon he himself was already giving me advice and expressing numerous ideas that served as the basis for further, long-term work in this field.

Among Fermi’s numerous theoretical works that appeared after the publication of his work on statistics and up to 1934, when he began to work in the field of nuclear physics, one should note the Thomas–Fermi method^2 (1928)—the application of Fermi statistics to the determination of the mean electric potential in an atom; the theory of the hyperfine structure of spectral lines^3 (1933); and his reformulation of quantum electrodynamics^4 (1932), which is a brilliant example of a clear treatment of a difficult question.

As his “debut” in the field of nuclear physics, Fermi published in 1934 his famous theory of β-decay—a classical work^5 based on Pauli’s assumption that in the β-process an electron is emitted simultaneously with a neutrino. Important in itself, this work also became the prototype of modern theories of the interactions of elementary particles. Many physicists at present believe that the interaction found by Fermi between the nucleon field and the field of the electron–neutrino pair is a special case of a more general interaction among any four fermions—the so-called Fermi interaction, whose extremely small intensity is determined by the magnitude of the “Fermi constant.”

Despite the fact that Fermi’s research activity before 1934 was theoretical in character, the hidden experimentalist occasionally awoke in him. There was, for example, the following incident: one day Fermi received the proofs of his book Molecules and Crystals; one of the photographs, which showed the alternation of intensities in the molecular spectrum of nitrogen, did not satisfy the author of the book. Fermi immediately found a suitable free spectrograph (this was at the Rome Institute of Physics, which at that time was in effect a spectroscopy laboratory) and, turning into an experimentalist, made the good photograph needed for the book.

Fermi carried out his first major experimental works in the field of nuclear physics (1934). These works^6, ^7, for which Fermi received the Nobel Prize, were preceded by two events: Rasetti’s trip to Germany for the purpose of studying nuclear experimental techniques, and the discussion, at a seminar of the institute under Fermi’s direction, of Rutherford’s classic book on radioactivity.

Immediately after Frédéric and Irène Joliot-Curie discovered the phenomenon of artificial radioactivity, Fermi came to the conclusion that neutrons, since they have no charge, should especially

effectively producing radioactive elements, and with his characteristic energy he began the systematic bombardment with neutrons of almost all existing elements. There is no need to recall here all the striking results of Fermi’s experiments—the production of more than sixty radioactive elements; the discovery of the slowing down of neutrons and of their high probability of capture in such elements as cadmium and boron; “groups” of neutrons, and so on. All these brilliant and wholly unexpected discoveries were published as brief communications in the Italian journal Ricerca Scientifica, which, thanks to Fermi, was transformed from a completely unknown publication into a journal of international importance. In the course of only one year the spectroscopy laboratory became a first-rate, though small, laboratory of nuclear physics.

The Roman laboratory was indeed small. The total number of scientific workers and mechanics working with Fermi barely reached ten. Each year, on average, one or two students received diplomas in physics, despite the fact that in the Faculty of Physics and Mathematics the professors included Fermi, Rasetti, Volterra, Levi-Civita... The small number of graduates was explained by the unpromising prospects awaiting young Italian physicists at that time.

As for the funds needed for research work, the fascist government, which so generously assisted big industrialists, proved rather stingy when it came to funds for science. Once, in order to save money, Fermi decided that standard electrical plugs should be made in the laboratory workshop; he spent two days with a mechanic, trying to find a convenient way of making them, but after that he had to abandon his proposal as uneconomical.

Fermi’s participation as a performer in experimental work was always direct; he not only directed it, but also liked to work with his own hands. In particular, Fermi was a good glassblower. Fermi’s direct and daily participation in the work he directed was possible only because he persistently and stubbornly refused to hold administrative positions. Few people know that he was never the head of the laboratories in which he worked.

In the laboratory Fermi always preserved unchanging calm. It was said that in 1942, when the first nuclear reactor built by him was approaching critical conditions, Fermi broke the general tension with the well-known phrase: “Let’s go have lunch.” Almost ten years before this, when in the Roman Institute of Physics an increase in radioactivity caused by neutrons was unexpectedly discovered, due to the presence of hydrogen-containing substances, Fermi cooled the ardor of his coworkers with the same phrase: “Let’s go have lunch.” By the end of lunch Fermi had already explained the discovery (the Fermi effect) as a phenomenon of neutron slowing down and remarked:

“How foolish that we did not predict this earlier.” About a year later, the foundations of that area of physics which today bears the name “neutronics” had been so clearly formulated by Fermi that some of his articles, in particular the papers “On the Motion of Neutrons in Hydrogen-Containing Media”^8 and “Absorption and Diffusion of Slow Neutrons,”^9 today, almost 20 years after publication, are the best introduction to a science of equal interest to physicists and engineers.

In experiments carried out in Rome in 1934–1935, the bombardment of uranium by neutrons caused the formation of a number of radioactive elements, among which, in Fermi’s opinion, there was also an element with atomic number 93. As became clear subsequently, these elements were in fact fission products, and, although transuranic elements are formed when uranium is bombarded, Fermi’s report of element 93 was incorrect—the only mistake in the course of his long and glorious research career. It should be noted that this did not impede the development of the investigations that led to the discovery of fission. However, Fermi was very distressed by the publication of the work on element 93.

At the Rome Institute of Physics, Fermi received the nickname “the pope,” by which all his colleagues and friends addressed him not only in Rome but throughout the world. This nickname meant that Fermi (in the field of physics!) was infallible, just as the head of the Catholic Church—the Pope of Rome—is considered infallible in matters of religion. Fermi, of course, remained “the pope” even after the incident with element 93.

During his stay in Rome in the period of the fascist dictatorship, Fermi preserved his unshakable honesty, even while being in the utterly corrupt Fascist Academy of Italy. In particular, in the Academy and in the universities he always fought boldly for the recognition of scientific achievements, rather than services to the fascist state, as the criterion in the appointment of university professors.

However, moving in the very narrow circle of university professors, to whom the world of the heroic anti-fascist Italian working class was entirely unknown, Fermi showed no interest in politics.

Later Fermi had occasion to express his antipathy to fascism more directly. In 1938 he was awarded the Nobel Prize for his research work on the properties of neutrons. After the introduction of the anti-Semitic fascist laws, he and his family went from Stockholm, where he had gone to receive the prize, directly to New York, although these laws did not directly affect him (Fermi’s wife was an Italian of the Jewish faith; he himself was a Catholic).

Subsequently, by his own example, Fermi made a significant contribution toward dispelling the very widespread in capi—

tical countries, the opinion that “Italian” and “fascist” are synonyms. And it is no accident that today in Italy it is precisely the neo-fascist press that does not consider it shameful to insult the memory of a man of whom the entire Italian people has the right to be proud as one of its finest sons.

In the United States Fermi accepted the post of professor of physics at Columbia University. There he created (1939) a quantitative theory of the ionization energy losses of charged particles, taking into account the polarization of the substance through which these particles pass[^10]. From this theory, subsequently verified by experiment, it follows that the stopping power of substances depends on the degree of their condensation (the Fermi density effect).

Immediately after the discovery of fission by Hahn and Strassmann, Fermi understood what revolutionary possibilities might follow from this phenomenon. Independently of the group of experimenters working under Joliot-Curie, Fermi experimentally proved[^11] that several neutrons are emitted in fission—a circumstance that makes a chain reaction possible.

From that time on (1939), all of Fermi’s activity for several years was devoted to obtaining atomic energy from uranium: he achieved this in December 1942[^12]. Fermi called the first nuclear reactor a “pila,” which in Italian means something composed of many similar layers, just as a voltaic pile—the first source of a long-lasting direct current—is called in Italian Volta’s “pila.” In the Soviet Union, where the world’s first power station operating on atomic energy has been built, it is clear to everyone that Fermi’s “pila” has no less historical significance than Volta’s “pila.”

It is impossible here to give even a remote idea of the colossal work carried out by Fermi in the field of atomic energy. One can only hope that many of Fermi’s unpublished works, which have historical significance, will soon appear in print. The works carried out by Fermi together with Anderson on the slowing down and diffusion of neutrons in graphite are an example of experimental and theoretical mastery. Many scientific terms current in this field bear Fermi’s name: neutron “age” according to Fermi, the Fermi thermal column, and others. Here one should also recall Fermi’s method for determining the critical dimensions of a reacting medium in experiments performed with a relatively small amount of uranium-containing material (Fermi’s exponential experiment). The experiment, the description of which can be found in all books devoted to nuclear reactors, is so simple that today it is difficult to imagine any other approach to the problem under consideration.

Fermi possessed exceptional physical intuition; he always found the simplest approaches to the solution of the most complex

practical problems. As for investigations of a fundamental character, the great problems chosen by Fermi always became simple, although such simplicity, of course, appeared only after he had brilliantly solved them.

After the war Fermi accepted the position of professor of physics at the University of Chicago. Using the reactor he had built as a source of neutrons, he opened a new chapter in the field of nuclear physics—neutron optics, a number of whose important questions are well described in his book Lectures on Atomic Physics, translated into Russian. In the same book one may also find a discussion of the fundamental problem of the neutron-electron interaction, to whose solution Fermi devoted an ingenious experiment[^13] (1947).

The founder of nuclear science, Rutherford, said that pupils do not allow him to grow old. This statement is true of the majority of teachers worthy of that title. As for Fermi, to the end of his life he was younger in spirit than any of his pupils or colleagues. And to the end of his days Fermi remained a student, always full of a passionate desire to acquire new knowledge.

At about the age of fifty, Fermi, who had at his disposal a number of reactors for fundamental research in the extremely interesting field that he himself had created, decided completely to change the direction of his activity and devoted himself to the study of high-energy particles. In particular, he was attracted by one of the central problems of modern physics—the problem of the meson-nucleon interaction. His work[^14] (1953) on the scattering of positive and negative π-mesons of different energies by protons opened a new chapter of experimental and theoretical physics.

In the works on the scattering of π-mesons by hydrogen, Fermi’s personality as an outstanding theorist and experimenter stands out with particular vividness. That he participated in these works not only as a leader but also as a direct performer is evident, if only from the fact that he was responsible for the construction of such details as the internal target of the synchrocyclotron, controlled remotely.

In the works on π-mesons, as in other works, the indelible mark left by Fermi was expressed not only in their content, but also in special methodological approaches, in new scientific expressions, and even in extremely apt notations. Incidentally, Fermi was of the opinion that the question of simplicity of notation is of paramount importance in theoretical physics.

It is impossible to form an idea of all the labor that Fermi invested in theoretical works solely from those of them that were published: only an insignificant part was selected for publication.

...a considerable part of all work. That is why there is not a single theoretical paper by Fermi that is not outstanding.

Among Fermi’s theoretical papers in the field of high energies, two occupy a special place: those concerning the so-called Fermi mechanism of acceleration of primary cosmic rays and the theory of multiple meson production. Both are based on an idea as simple as it is astonishing.

At the basis of the explanation (1949)15 of the mechanism of acceleration of primary particles in cosmic rays lies the following argument, based on the principle of equipartition of energy. Let us consider collisions of microparticles with moving macroscopic bodies. Although in a single collision particles may lose or gain energy, in the final account there is a tendency toward statistical equilibrium, and this means that particles on average are accelerated in collisions with macroscopic bodies. In Fermi’s theory the charged particles are deflected by magnetic fields associated with the conducting gas, and in the end tend to acquire an energy equal to the energy of the moving gas as a whole.

In the theory of multiple particle production16 (1950), the process of collision at very high energies is considered with the aid of statistical and even thermodynamic methods. The theory, which was considerably extended and improved by Soviet physicists, found reflection in one of the recent articles in this journal17.

All of Fermi’s works are characterized by extreme concreteness. His theories were all created in order to explain, say, the behavior of a definite experimental curve, the “strangeness” of a given experimental fact, and so on.

In the twenties of this century, when the basic principles of physics were undergoing a fundamental upheaval, it must have been extremely difficult for the young Fermi, without teachers, to find his bearings. It is possible that the traits inherent in Fermi—concreteness, hatred of uncertainty, exceptional common sense—while helping him in the creation of many fundamental theories, at the same time, under these conditions, did not allow him to create such theories and principles as quantum mechanics, the uncertainty relation, and the Pauli principle.

Recently one of the participants in a meeting of the Academy of Sciences of the USSR devoted to the peaceful use of atomic energy, a well-known Soviet physicist, said: “It is a pity that Fermi is no longer here; sooner or later he would have attended one of our meetings, and without doubt we, Soviet physicists, would not have missed the opportunity to shake his hand.”

These words well express the feeling of admiration of Soviet scientists for the great physicist, whose glorious name will forever remain in the memory of physicists throughout the world.

References

1. Zeits. f. Physik 26, 902 (1926).
2. Zeits. f. Physik 48, 73 (1928).
3. Zeits. f. Physik 82, 729 (1933).
4. Rev. Mod. Phys. 4, 87 (1932).
5. Nuovo Cimento 11, 1 (1934).
6. Proc. Roy. Soc. 146A, 483 (1934).
7. Proc. Roy. Soc. 149A, 522 (1935).
8. Ricerca Scientifica 7, 13 (1936).
9. Phys. Rev. 50, 899 (1936).
10. Phys. Rev. 56, 1242 (1939).
11. Phys. Rev. 56, 284 (1939).
12. See Uspekhi Fizicheskikh Nauk 32, 54 (1947).
13. Phys. Rev. 72, 1139 (1947).
14. Phys. Rev. 91, 155 (1953).
15. Phys. Rev. 75, 1169 (1949).
16. Progr. Theoret. Physics 5, 570 (1950).
17. S. Z. Belenky and L. D. Landau, Uspekhi Fizicheskikh Nauk LVI, no. 3, p. 309 (1955).