by Gino Segrè
Like other victims of totalitarianism, science was targeted by the regime’s heavy hand. Mussolini insisted on having a loyal party member replace Volterra as head of the Consiglio. Guglielmo Marconi, not a scientist but a distinguished inventor who had shared the 1909 Nobel Prize in Physics for his contribution to wireless telegraphy, fit the bill. This enthusiastic Fascist, who had joined the party immediately in the wake of the March on Rome, became the government face of Italian research in science. Marconi was, however, not an academic. This allowed Corbino to maintain his influence in university circles. Fermi, who continued to cocoon himself from politics, depended on him for guidance and also for protection.
While these political gyrations impacted Italian science and played out in the public domain, the greatest twentieth-century physics revolution—quantum mechanics—was on the brink of forever altering the scientific landscape.
8
QUANTUM LEAPS
The first glimpse of a decisive resolution to the ongoing quandaries of quantum physics came in June 1925. In Göttingen, twenty-three-year-old Werner Heisenberg was struck with a severe attack of hay fever. He retreated to the grassless North Sea island of Helgoland. In his words, as he was searching for a new way to attack the problems engulfing atomic structure, “There was a moment in Helgoland in which the inspiration came to me … It was rather late at night. I laboriously did the calculations and they checked. I then went out to lie on a rock looking out at the sea, saw the Sun rise and was happy.” Heisenberg had taken advantage of the white nights of Scandinavian summer and worked until greeting a new day.
Heisenberg had done away with electron orbits, replacing them with a set of abstract rules based on observable quantities in electron motion. Assisted by Göttingen’s senior theorist Max Born and his student Pascual Jordan, these ideas were soon extended into the full-blown theory that came to be known as matrix mechanics.
Its impact was not immediate, since the theory was cast in a novel mathematical formalism that almost all physicists had difficulty grasping. On the twenty-third of September, Fermi wrote to Enrico Persico, “My impression is that there hasn’t been much progress in the past few months despite Heisenberg’s formal results on the zoology of spectroscopic terms.” Franco Rasetti remembered being told by Fermi, “Now I’m trying to see what Heisenberg is trying to say, but so far I don’t understand it.” Though Fermi was respectful of the Göttingen school’s achievements, Heisenberg’s paper seemed to confirm his earlier belief that its physicists were overly reliant on abstract mathematical techniques. Fermi wanted a clear physical picture of what they were saying.
At first young Paul Dirac did not understand Heisenberg’s theory either, but he soon recognized its essence. In early November 1925, he submitted for publication a paper entitled “The Fundamental Equations of Quantum Mechanics.” The Göttingen trio, unaware even of Dirac’s existence, was stunned reading his paper: he had reached the same conclusions they had. As Born wrote in his memoirs, “This was—I remember well—one of the greatest surprises of my scientific life. For the name Dirac was completely unknown to me, the author appeared to be a youngster, yet everything was in its way perfect, admirable.”
Youngsters were indeed leading the rise of physics. By the end of February 1926, four remarkable papers, each written by a relative unknown, had appeared in the previous twelve months. Pauli, Fermi, Heisenberg, and Dirac had all rocked the quantum world. At twenty-five, Pauli was the oldest of the four. It is no wonder that Germans began referring to theoretical physics as Knabenphysik (boys’ physics).
However, the revolution was not entirely led by youth. In early January 1926, while on a ski vacation in the Swiss resort of Arosa, thirty-eight-year-old Erwin Schrödinger was busy with something other than schussing down the slopes or charming the mysterious mistress who had accompanied him.
In a paper he wrote immediately after returning from Arosa, Schrödinger reintroduced the electron orbits that Heisenberg had done away with, but he did so with a new way to visualize them. In Schrödinger’s version of quantum mechanics, the motion of an electron within an atom was guided by a so-called wave function. That unlocked the secrets of the atom for him.
Two months later, Schrödinger brilliantly showed that his theory, to which the name wave mechanics was given, was mathematically equivalent to matrix mechanics. In other words, every step of one had a mathematical analogue in the other. This meant there was only one underlying theory, though it apparently could be cast in two totally different forms. And Schrödinger had strong feelings about which one of the two was preferable.
He expressed them in a footnote to “On the Relation of the Heisenberg-Born-Jordan Quantum Mechanics to Mine.” In that footnote, Schrödinger wrote, “I was absolutely unaware of any genetic relationship with Heisenberg. I naturally knew about his theory, but because of the [to me] very difficult-appearing methods of transcendental algebra and because of the lack of Anschaulichkeit [clearness], I felt deterred by it, if not to say repelled.” The word “repelled” is an indication of Schrödinger’s adamant feelings.
Planck, Einstein, and most senior figures in theoretical physics agreed with Schrödinger’s assessment of the two theories, as did Fermi. Rasetti remembered his friend’s reaction to reading Schrödinger’s papers, a sharp contrast to the struggle he had seen Fermi undergo with Heisenberg’s matrix mechanics. Fermi “understood it and then he poured it into the brains of a few people around him.”
Schrödinger’s papers quickly led a number of physicists to find ways to apply the ideas and techniques he introduced. The results were startling and gratifying. The wizards worked their magic. Even the problems that had already been solved saw their answers recast and clarified by making use of Schrödinger’s wave functions. Dirac’s treatment of statistical mechanics, in another paper by this young virtuoso, was a case in point. He quickly arrived at and then extended the conclusions that Fermi had reached six months earlier.
Unaware of Fermi’s earlier work, Dirac published an article on the subject in the Proceedings of the Royal Society. Since his contribution was far from negligible, the resulting “Fermi-Dirac” statistics carries both their names. However, the identical particles it describes are known only as fermions. A large assemblage of them was called a “Fermi sea” and its edges a “Fermi surface.” For the first time in the scientific lexicon, Fermi’s name was used.
Dirac must have felt disappointed to discover that the Göttingen trio had independently obtained his main results about quantum mechanics and that Fermi had been the first to derive the statistical mechanics that a “Fermi gas” obeyed. But Dirac’s originality was unquestioned.
Heisenberg was frustrated at seeing wave mechanics favored over his matrix mechanics. But he sensed that some key ingredients were missing. Bohr, who perhaps had a deeper understanding of the problems in quantum theory than anyone else, agreed with him. More tremors were still to come.
To decipher what might still be lacking, Heisenberg moved to Copenhagen in the fall of 1926 to work with Bohr. They were undertaking a search for quantum mechanics’ true meaning. Working intensely, they formulated two new notions in 1927. Bohr’s complementarity principle, emphasizing the complementary nature of matter as both particle and wave, and Heisenberg’s uncertainty principle, establishing a limit on the simultaneous measurement of complementary variables, formed the basis of what came to be called the Copenhagen Interpretation of Quantum Mechanics. Its formulation brought to an end the second phase of the earthshaking change to physics.
The concepts Bohr and Heisenberg proposed were presented to the physics community during a prestigious conference held in Brussels during the month of October. Solvay Conferences had been taking place approximately every three years since 1911. They were intended to be meetings in which a few dozen of the world’s leading physicists would gather for a week to address a major current scientific topic. The selection for 1927 was “Electrons and Photons,” but those present knew the real
subject would be quantum mechanics, its concept revolutionizing physics’ foundations.
Significantly, no Italian was among the elite at the 1927 Solvay Conference, none deemed worthy of being invited to such an august gathering. One Italian, Enrico Fermi, would attend the next one, held in 1930. His presence there underscored Italy’s arrival on the international physics scene.
Another international physics conference was held in 1927, a month earlier than the Solvay Conference. The Volta Congress, unlike the Solvay Conference, was a one-time affair. The nominal occasion for the meeting was the hundredth anniversary of the death of Alessandro Volta, the discoverer of the electric battery. The unspoken reason was the Italian government—and Mussolini in particular—wanting to show the world that Italy belonged to science’s elite.
The meeting took place in Como, Volta’s birthplace. Located on the beautiful eponymous lake, it was a delightful setting and invitees were treated to elegant lodgings and memorable boat rides. The indefatigable Fermi even managed to take a few hikes beyond the handsomely terraced gardens and up the steep slopes surrounding the lake, reaching huts with glorious views of snowy Alps to the north and the Italian plains to the south.
To ensure that nothing was lacking in organization or trappings, Mussolini demanded that the Italian electricity companies, dependent as they were on government backing, provide massive funding for all aspects of the meeting. Corbino quietly but wisely gave an honest assessment of the proceedings, observing that “Italy should have exhibited more physics and less hospitality and that it should not deceive itself that sponsoring a conference was a substitute for scientific achievement.”
The invitation list was impressive. More than a dozen of the sixty-one individuals participating had already received Nobel Prizes, and several others would win the prize in years to come. Many hesitated before accepting the invitation. Arnold Sommerfeld, the highly respected senior professor in Munich, wrote to two eminent colleagues who had liberal views similar to his own, “I have serious reservations about attending because I assume the Italians will not forgo the opportunity of making it political and trotting out Mussolini.” After wavering, all three went to Como. However, Einstein refused, wanting no part of an occasion he sensed might be used to burnish Mussolini’s image.
The Volta Congress was notable in another respect. Though World War I had been over for almost a decade, feelings still ran high in many quarters. Since the war there had been no large international physics meeting that gathered together representatives from all the warring countries. The Volta Congress was the first. The leading physicists of Germany, France, Italy, England, Austria, the Netherlands, Belgium, Denmark, and the United States attended. In that sense it was a huge success. It was also a propaganda success for Italy. Physics was another matter.
Unlike the Solvay Conference, the Volta Congress had a broad agenda loosely tied to electrical and magnetic phenomena. This allowed for a very wide range of presentations, but the conference lacked the intense focus that made the subsequent Solvay meeting so famous. Besides Bohr’s first presentation of the Copenhagen interpretation to an international audience, none of the other more than fifty papers delivered was memorable.
Sommerfeld’s presentation, however, turned out to be influential for Italian physics because its main thrust was to emphasize the significance of Fermi’s recent work in statistical mechanics as the key to understanding the phenomenon of electric conduction in metals. The irony of Italian physicists having one of theirs achieve legitimacy through a foreigner’s approval was not lost on Rasetti: “Everybody began to realize that Fermi had achieved something very important. So that was really the revelation of Fermi in Italy. His reputation in Italy came back through Germany.”
When the week of meetings was over, the conferees were taken to Rome. On September 19, 1927, in Mussolini’s presence, they were officially welcomed from atop the Michelangelo-designed steps of the Capitoline Hill by Marconi, the current president of the Consiglio. The scene was unabashedly dramatic. Mussolini and Marconi appeared to reign from on high. Afterward, Il Duce accompanied them to a gathering at his residence, Villa Torlonia. He was delighted to have an occasion for strutting in front of an illustrious audience, bombastically proclaiming the past, present, and future greatness of the nation he led. Abhorring posturing and ostentatious displays, Fermi must have cringed.
Though Fermi could not have been pleased by the meeting’s political overtones, he was gratified to see the regard it afforded him. One souvenir of the time in Como was a photograph of him, Heisenberg, and Pauli sitting together smiling, the lake in the background. The three, an Italian, a German, and an Austrian, held the future of physics in their hands.
They knew they had already produced a revolution in physics. What they did not know was that without understanding the Pauli Principle, quantum mechanics, and Fermi-Dirac statistics, the world would not have been able to produce semiconductors, transistors, computers, MRIs, lasers, and so many of the other inventions that shape our life. In a very real sense we live in a world they created.
9
ENRICO AND LAURA
After Fermi’s appointment on November 7, 1926, as Rome’s professor of theoretical physics, he returned to the city of his birth, moving into the house on the city outskirts that had been constructed as his mother was dying. His father and sister now lived there. Maria had recently acquired a position teaching Italian literature at the same Rome high school attended by all the Fermi siblings. Sadly, Alberto Fermi was showing signs of a serious illness that would soon take his life. Maria and Enrico took turns sitting up with their father at night, but to no avail. He died on May 7, 1927, almost three years to the day since his wife had passed away.
That summer Enrico retreated to the Dolomites, just as he had done after his mother’s death. Once again he found long walks in the mountains to be restorative. And on this occasion his sorrow was mitigated by joy. Although there had been passing fancies before, this was the first time he found himself truly in love. The object of his affection was a beautiful nineteen-year-old Roman woman named Laura Capon whom he had met the summer before.
The Capons were one of Italy’s assimilated Jewish families that had risen to positions of relative prominence in the wake of Italy’s independence. Like many other such families, they were basically nonobservant and seldom went to synagogue, though usually they married within the faith. Their closest friends tended to be other Jews.
Laura’s father, fifty-four-year-old Augusto Capon, was a career naval officer. After Italy’s unification, the military and the academy were two careers to which many Jews gravitated. Having distinguished himself both by his intellect and his valor during World War I, Capon had risen to be the head of naval intelligence and would soon be made an admiral. Like many officers, he was also a fervent monarchist. With a happy family life and four children, of whom Laura was the second oldest, he was comfortably well off.
Laura had intended to spend the month of August 1926 with her parents and three siblings in Chamonix, the resort on the French side of Mont Blanc. These plans were disrupted by the shakiness of the Italian economy. Worried that Italy might succumb to the kind of inflation plaguing other European countries, Mussolini had placed restrictions on the export of Italian currency. In effect this necessitated the Capons’ staying in Italy.
An alternative plan was quickly formulated: the Capons would vacation in the Dolomites near their friends the Castelnuovos. Guido Castelnuovo, one of the Rome mathematicians who had befriended Fermi, was only a few years older than Capon. He was a fellow Jew and, like Capon, a Venetian by birth. The two families were close, particularly with these similarities and children of the same age.
In late July the Capons arrived in Santa Cristina, a small town in the glorious east–west valley known as the Val Gardena. Located about twenty miles from the main road to the Brenner Pass, it had already become one of Italy’s most desirable summer and winter resorts, with ample opportunities for
hiking, climbing, and skiing. The scenery is spectacular, with jagged peaks soaring over lush meadows and high plateaus. The church spires of small villages compete with mountain pinnacles to create a wonderland of beauty.
As soon as the Capons arrived, Laura went to see her good friend Gina, the Castelnuovo daughter closest in age to her. As Laura later recounted, Gina greeted her by saying, “We are going to have lots of fun. Even Fermi has written my mother asking her to find a room for him.” When Laura inquired who Fermi was, Gina replied, “You must know him, I am sure. He is a brilliant physicist: the hope of Italian physics, as my father says.” And that is how Laura and Enrico got together that summer.
Laura had met Enrico two years earlier, albeit fleetingly. The occasion was apparently not filed in the annals of ardor or in the annals of memorable encounters. She described the 1924 meeting as follows:
He shook hands and gave me a friendly grin. You could call it nothing but a grin, for his lips were exceedingly thin and fleshless, and among his upper teeth a baby tooth too lingered on, conspicuous in its incongruity. But his eyes were cheerful and amused.
Those gray-blue eyes may have peered at her differently two years later, as did her brown warm ones toward him.
During the weeks that followed the young vacationers went on day hikes along the region’s many inviting paths, pausing from time to time to admire the glorious views. The group typically included a mix of friends and siblings. Laura discovered that though the young physicist was conscious of his rising reputation, he was not pompous; it was fun to tease him and he took the ribbings gracefully. She did not think Enrico was especially handsome, but there was something she found compelling about him. It probably was not the Tyrolean jacket and loosely fitting knickerbockers he typically wore in the mountains.
