by Gino Segrè
The biggest surprise for Fermi was meeting Einstein, who was spending twenty days in Leiden visiting Ehrenfest. This forty-five-year-old man took a fancy to Fermi. Einstein, himself an outsider of sorts, engaged Fermi immediately. There was a meeting of the minds, a shared interest in quantum physics and statistical mechanics. The obviously pleased Fermi tried to make light of his meetings with Einstein and the interest shown in him by the world’s greatest physicist. In the letter to Persico, he described Einstein as a “very nice person despite his wearing a wide-brimmed hat that gives him the air of a misunderstood genius. He has been taken by a great liking for me that he cannot help telling me about every time he meets me (pity that he’s not a beautiful girl).” In this case, Fermi overcame his usual embarrassment about attention paid to him. His parenthesized notation may have alluded to his lack of success in finding a girlfriend who would accord him similar admiration.
Fermi’s warm reception was not the only reason the Leiden stay was happier than the Göttingen one. Fermi had matured personally and intellectually in the two intervening years. Many papers in Fermi’s Collected Works, published in 1962, include introductions by fellow physicists who knew him at the time each paper was written. Looking back as a sixty-year-old, Persico commented on a 1924 Fermi paper, saying it had “the characteristics of the more mature style of Fermi: a fundamental idea, at the same time simple and clever, is applied to several concrete problems of physical importance with the help of mathematical methods of sufficient approximation, but not better than warranted by the underlying physical hypotheses.” No better description of Fermi’s style in theoretical physics was ever written.
In that 1924 paper, “The Theory of the Collisions between Atoms and Electrically Charged Particles,” Fermi demonstrated that if one knew the effect of electromagnetic radiation on an atom, one could also deduce the result of that atom’s collision with a charged particle. The varying electric field the particle produced could be described analogously to how radiation was treated. The paper highlighted Fermi’s interest in all aspects of physics, the formidable arsenal of tools at his fingertips, and his ease in moving from the very practical to the very formal.
Nevertheless, in spite of the growing recognition of Fermi’s breadth, depth, and creativity, he still lacked an academic position in Italy. And he needed one.
7
FLORENCE
To obtain an academic appointment that matched his ambitions, Fermi briefly considered emigrating, but his attachment to Italy was such that he continued his attempts to advance in its university system. The milieu was not conducive since most Italian senior physics professors remained mired in nineteenth-century physics, unaware of or unwilling to accept the innovative ideas of relativity and quantum theory.
Fortunately a few Italian senior physicists were forward-looking and well positioned. Antonio Garbasso was one of them. Like Orso Corbino, he had succeeded in politics as well as in physics. He was both Florence’s mayor and a physics professor. This meant, however, that he spent most of his time in his majestic Palazzo Vecchio mayoral office and not in a physics laboratory.
Garbasso and Corbino identified Fermi, Enrico Persico, and Franco Rasetti, all three in their early twenties, as the most likely carriers of the torch toward modernity in physics. After graduation, Persico was taken on as Corbino’s assistant and Rasetti as Garbasso’s. They now had to find a position for Fermi, recognized as the most promising of the three young physicists. There was an opening as a lecturer in Florence. Although the position was neither prestigious nor well paid, Garbasso and Corbino thought it would suit Fermi until he could obtain a real professorship. Both urged him to accept the appointment, which would also reunite him with his friend Rasetti. Fermi agreed.
At the time, Italian universities had no campuses, so departments were housed in buildings throughout the host city. This meant Florence students had to scramble, since the physics and chemistry departments were miles apart. The physics institute was located, with symbolic if not practical intent, near the villa that Galileo had returned to after his confrontation with the Inquisition and where he spent the last decade of his life under virtual house arrest. Situated amid a beautiful grove of olive trees on the Arcetri hillside, a few miles from downtown, the institute commanded a stunning view of the city. This made up for its lack of amenities, the principal one being no heating: the building’s wintertime average daily temperature was in the low forties Fahrenheit.
During his first two years in Florence as Garbasso’s assistant, Rasetti lived in a small room adjacent to the institute. With a bed, a desk, and a sink, it was sufficient for his needs. The wife of the janitor who took care of the institute would cook him simple meals. But in 1924, Rasetti’s father died, and his mother decided she wanted to be near her only child. She sold her Pisa house and bought an apartment in Florence large enough to also house her son.
The timing was propitious. On his arrival in Florence at the end of 1924, Fermi moved into Rasetti’s room in Arcetri. The facilities would be as simple as those in his Pisa dormitory and the meals no better, but he was feeling cheerful after his stay in Leiden. Physics was stimulating and Fermi was brimming with ideas. Besides, Rasetti had survived living there; he would not be more demanding than his friend.
Though Fermi and Rasetti were maturing as physicists, they still relished wicked practical jokes like the ones they had indulged in during their student days in Pisa. Rasetti described one that took place at Arcetri. Having gathered about thirty geckos in the surrounding fields, he and Fermi released them in their little dining area just before the janitor’s wife came in with their lunches. Mayhem ensued and the two physicists roared with laughter, somewhat muted as they saw their lunches spill on the floor.
Fermi and Rasetti’s outdoor activities were also continuations of their Pisa ones: skiing, hiking, and playing tennis. Tennis remained a favored sport of Fermi’s. Other players would comment that though his style was not particularly graceful, his tenacity often made him able to wear down more accomplished opponents.
The two young physicists saw little of Garbasso. He came to Arcetri from the Palazzo Vecchio only three times a week to deliver lectures. Rasetti’s limited tasks and Fermi’s light teaching responsibilities left the two friends with time to conduct whatever research they could manage with the limited equipment at hand and the tiny budget available to them. Fermi had been working exclusively on problems in theoretical physics for the more than two years since his Pisa graduation. The possibility of conducting experimental research again intrigued him, particularly if he could do so with Rasetti and if it would serve to advance his knowledge of how quantum physics related to atomic structure.
Always an adept experimentalist, Rasetti was pursuing ideas in spectroscopy, a field once considered a backwater of physics and primarily of interest to chemists. Beginning in the 1850s, scientists had observed that an element, when heated sufficiently, emitted electromagnetic radiation. After further analysis, the radiation was seen to be different from element to element, but in all cases it consisted of distinct frequencies. This made spectroscopy an extremely valuable tool for chemical identification. Indeed, several new elements of the periodic table, most notably helium, were first detected by these means. This seemed, however, to have very little to do with atomic structure. Bohr would later say that it had been like trying to understand a butterfly by studying the colors in its wings.
That changed with Ernest Rutherford’s 1911 discovery that the atom was composed of a tiny central nucleus surrounded by electrons. Two years after that, Bohr introduced his planetary model of the atom in which electrons moved in orbits about the nucleus, much as the planets circle the sun. Instead of a gravitational force, the negatively charged electrons were held in orbit by their electric attraction to the positively charged nucleus.
A volcano in the landscape of quantum physics had been smoldering since 1900, when Max Planck, in what he would refer to as “an act of desperation,” introd
uced the concept of quanta. In order to reconcile basic principles of thermodynamics with experimental data, he had concluded that the energy contained in electromagnetic radiation was both absorbed and emitted in minute packets. The energy of a single packet was proportional to the radiation’s frequency. Planck called these packets quanta.
Bohr’s model of the atom had quanta emitted when electrons jumped from an orbit with higher energy to one with lower energy. The quantum’s energy necessarily was equal to the difference between the energies of the electron in its initial and its final state.
The novel idea introduced by Bohr was to extend the notions of quantum physics to electron orbits in hydrogen by having their radii follow a numerical sequence of n squared times R, where n was any integer greater than or equal to one and R was the average radius of the smallest orbit. Since the energies of each electron depended directly on their position in the atom, those energies also followed a sequence. The integer n that characterized a given electron’s energy was referred to as its principal quantum number.
Distinct electron orbits explained why hydrogen frequencies obeyed mysterious arithmetic rules. Measuring those frequencies, the field known as spectroscopy, became the primary tool for studying atomic structure. The butterfly wings had been meaningful after all. Bohr’s model of the atom was so spectacularly successful in explaining the frequencies emitted by heated hydrogen gas that physicists immediately believed it contained some deep truth. But hydrogen, with only one electron, is the simplest of all atoms; extensions of the theory to other elements were not as convincing. Despite a subsequent series of successes over the next decade, the Bohr model met a corresponding number of failures. It was becoming clear that some key concepts were missing.
Searching for clues in spectra became one of the central problems in physics. Elliptical rather than circular orbits were considered, corrections due to the theory of relativity were made, and changes in spectra by external electric or magnetic fields were analyzed. The hunt was on for a deeper understanding of the atom.
By the early 1920s, each electron in an atom was being assigned not one, but three quantum numbers. They were roughly thought of as corresponding respectively to the orbit’s size, its ellipticity, and its orientation with respect to an external magnetic field.
In early 1925, Fermi and Rasetti set out to examine a facet of this much larger subject. Rasetti made a specific proposal to his friend: they should carry out a set of investigations focusing on the polarization of light emitted by mercury vapor under the influence of an alternating magnetic field. He would provide the spectroscopic expertise and Fermi would be in charge of building the electric circuits.
The two obtained results notable enough to warrant being published in the prestigious foreign journals Nature and Zeitschrift für Physik, but they didn’t move the field significantly forward. However, they constituted “the first instance of an investigation of atomic spectra by means of radiofrequency fields,” a technique that was to receive numerous applications many years later, when radio frequencies became a more frequent tool in spectroscopy. The research was also distinctive because once again Fermi was showing his unique skill as both a theorist and an experimentalist.
On the whole, Fermi found the Florence position satisfactory, but he was eager to move on, win a competition for a professorship, and earn a decent salary. A professorship of theoretical physics still did not exist in Rome, so Corbino, supported by the Rome mathematicians, set out to establish one. It would be Italy’s first. Fermi knew of the backroom maneuvers, but since administrative matters were holding up the process, he decided in the fall of 1925 to enter the competition for a professorship in mathematical physics at the University of Cagliari.
He wrote to Persico in October that “given the uncertainties in Rome, I plan to compete because I think it is advisable to have a double-barreled rifle, even if I don’t find the idea of winding up on the Islands very pleasing.” “Islands” is a reference to Sardinia, where Cagliari is located. Sicily and Sardinia are often the first destination of young professors ascending the Italian academic ladder.
Volterra and Levi-Civita, two of the five professors on the Cagliari professorship selection committee, voted for him, but the other three were all physicists with little sympathy or interest in the field’s modern developments. The Cagliari position went to a man thirty years older than Fermi and undoubtedly less deserving. The committee’s three elderly members had felt obligated to reward their seasoned colleague rather than the young upstart. Their choice was even less defensible since while they were deliberating, Fermi was writing a seminal paper that would be considered a breakthrough in the world of physics.
The origins of the paper’s focus dated to 1924, when Fermi had been stymied by problems he had encountered while applying quantum ideas to notions of statistical mechanics. A year later, in 1925, after reading Wolfgang Pauli’s new Zeitschrift für Physik article on what came to be called the Exclusion Principle, Fermi was inspired.
The Exclusion Principle, for which Pauli was awarded the Nobel Prize in Physics in 1945, was an extraordinarily important advance in quantum physics. It put order into all the data accumulated in atomic spectra by answering the question of how many electrons could occupy an orbit. The principle postulated that no two electrons in an atom could have all their quantum numbers be identical. Fermi found almost immediately an innovative application for Pauli’s idea by extending it beyond the confines of the atom to the larger systems encountered in statistical mechanics. One could imagine a gas of electrons, or electrons moving freely in a metal. They, too, would have energies obeying quantum rules and following the Pauli Principle.
The notions Fermi introduced in doing this turned out to be the key to understanding a wide variety of disparate phenomena, from the difference between electrical insulators and conductors to the stability of white dwarf stars. With this paper, first published in Italy during the spring of 1926 and almost immediately afterward in the Zeitschrift für Physik, twenty-five-year-old Fermi entered into the company of the world’s elite physicists—the only Italian in that select circle. It also was a way of proceeding that was characteristic of all of Fermi’s theory work: take a clear physics notion, understand it in a way others had not, and apply it to one or more important physics problems.
Fermi’s paper was quickly appreciated in Northern Europe’s great physics centers. Many noteworthy applications of it followed, among which were Pauli’s explanation of previously baffling aspects of magnetism and Sommerfeld’s study of electric current flow in metals. Now even more afraid that his protégé might be lured away from Italy, Corbino redoubled his attempt to keep him. Under his steady prodding, a competition for a professorship in theoretical physics at the University of Rome was set for November 1926. Furthermore, two other Italian universities, Florence and Milan, also determined that such a position would be desirable. Three professorships in theoretical physics had been created in a single swoop.
Bringing Fermi to Rome was not without hurdles. The city’s university had two professors of physics. Rome’s second, Antonino Lo Surdo, was not in favor of Fermi joining its faculty. He viewed the young man’s possible arrival as a challenge to his standing and did not embrace the new generation, although he would have benefited from the freshness of its ideas. In mid-1920s Rome, while Corbino was looking forward to what the young would contribute to a new Italian physics, Lo Surdo was looking backward to the physics of yesterday and attempting to maintain the old guard’s entrenchment. Like other Italian physicists of that era, he refused to accept either modern developments or their proponents.
In some ways Lo Surdo was a conservative mirror image of the progressive Corbino. Born only four years apart, both were Sicilians and had taught in Messina. However, Lo Surdo was no match for Corbino, who, while maintaining collegial civility, easily outmaneuvered him. The competition for all three chairs in theoretical physics had Corbino and Garbasso on its adjudicating committee. As expected
, Fermi placed first, which gave him the position in Rome. Persico, placing second, went to Florence. And, with Persico’s position as Corbino’s assistant vacant, Rasetti transferred from Florence to Rome.
With the leverage of his new professorship, Fermi anticipated playing a major part in the overdue transformation of Italian physics research and teaching. Interest in science was slowly on the rise in Italy. Unfortunately, while there was progress on one front, Fascism was taking a toll on another.
In 1923 Italy inaugurated the Consiglio Nazionale delle Ricerche, or simply the Consiglio. It was founded largely as a result of the post–World War I realization that a flourishing economy and modern armed forces would require a country to have a solid scientific research base. Germany’s powerful umbrella organization, the Kaiser Wilhelm Gesellschaft, had been founded in 1911 and the United States’ National Research Council five years later. Italy moved to catch up by instituting its own variant. This was an auspicious start, particularly because the mathematician Vito Volterra, a man known for good judgment and impeccable honesty, was the Consiglio’s first president.
Volterra would not last long in that role. In the wake of the 1924 Matteotti murder, he had asserted his integrity and independence by joining twenty other senators in casting a vote of no confidence in Mussolini’s rule. The consequences of that vote were soon felt. With political loyalty trumping other considerations, Volterra’s influence waned. His position as head of the Consiglio was not renewed when it expired in 1926. More ominously, even Volterra’s presidency of the prestigious and supposedly independent Accademia dei Lincei was allowed to lapse when it expired, also in 1926.
