by Gino Segrè
Fermi returned to Ann Arbor in the summers of 1933 and 1935, catching on to spoken English, although his Italian accent would never disappear entirely. Laura remained in Italy both times. The new mother was reluctant to travel with her little one across the Atlantic, where she would sit in a strange house in Ann Arbor and not have the help of their tried and true nursemaid.
Returning to Rome, Fermi found that his rising fame as a theoretical physicist was beginning to attract visitors, most of them Rockefeller fellows from northern Europe, who spent appreciable amounts of time at Via Panisperna. Bethe was one of the first, arriving in February 1931. He was soon writing his Munich friend Rudolf Peierls, later Sir Rudolf, about the wonders of Fermi’s approach to physics: “His ability to summarize any problem is amazing; he can immediately tell whether or not a paper makes sense … and his judgment of the theoretical and experimental (!) literature is infallible.” Even after a short time in Rome, Bethe had recognized why Fermi was called the Pope.
Peierls, obviously intrigued, followed Bethe to Rome with his own Rockefeller fellowship a year later. Managing to extend his fellowship, Bethe joined Fermi again a year after that in the spring of 1932.
Via Panisperna had become a magnet for creativity and innovation in the field, much like Copenhagen under Bohr’s tutelage, Munich under Sommerfeld’s, Zurich under Pauli’s, and Leipzig under Heisenberg’s. Felix Bloch from Switzerland, George Placzek from Czechoslovakia, Edward Teller from Hungary, and numerous others came to Rome. They had all studied in more than one of those Northern European centers and sought to learn the less formal problem-oriented style advocated by Fermi. An added bonus was to spend time amid Rome’s wonders.
Foreigners were not the only physicists coming to Via Panisperna. Word spread in Italy, and young Italian aspiring theoretical physicists such as Giulio Racah, Giancarlo Wick, and Ugo Fano also came to Rome. Largely thanks to Fermi they had become intrigued by the prospect of physics as a career path.
Pilgrimages to the Eternal City had been taking place for centuries. This was a new kind of pilgrimage, one whose endpoint was an encounter with a new kind of Pope.
13
BOMBARDING THE NUCLEUS
By 1930, Enrico Fermi was internationally recognized and Rome was becoming a world center of physics. To establish further prominence, it was timely to focus on the most exciting field in physics: the atom’s nucleus. What transpired within the minuscule nucleus had largely been set aside, at least until the motion of the atom’s electrons around its central core was understood. The problems raised were now coming to the fore.
In a talk Orso Corbino gave in September 1929, the Padreterno had said, “The study of the atomic nucleus is the true field for the physics of tomorrow,” and he continued by asserting that to study physics “without an up-to-date knowledge of the results of theoretical physics and without huge laboratory facilities is the same as trying to win a modern battle without airplanes and without artillery.”
Fermi followed up Corbino’s envoi with a 1930 article in the Periodico, declaring that he, too, believed that studying the atomic nucleus was the foremost problem in the future of physics. He expanded Corbino’s assessment by asserting that more than “up-to-date knowledge” would be asked of theoretical physics and that “we should expect that it will be necessary to modify the laws valid for the atom before obtaining a satisfactory theory of nuclear phenomena.” The challenge was formidable.
The unexpected picture of the atom that Ernest Rutherford discovered in 1911 and Niels Bohr extended two years later did not attempt to explain the atom’s nucleus. It did, however, seem plausible to interpret it as saying that the simplest of all nuclei, hydrogen, was nothing but a single particle with positive electric charge. The particle’s mass, almost two thousand times as great as the electron’s, accounted for why most of hydrogen’s mass was concentrated in the nucleus. That positively charged particle was eventually given a simple name: proton.
But puzzles began right away. The next element in the periodic table of elements, helium, had a nucleus with two protons. But the helium nucleus’s mass was approximately four times the size of a proton’s. Something else had to be contained in it, but it was far from obvious what that might be. The same puzzle persisted all along the periodic table, with the imbalance between the number of protons and nuclear masses only growing.
Aside from the question of mass there was a glaring problem. What kept a nucleus together? The gravitational force was far too weak to make any difference. The only other known force, the electric one, explained how atoms were constituted: positively charged nuclei attracted negatively charged electrons. But rather than attracting protons to one another, electric forces caused repulsion.
Some other forces, unknown and powerful, had to be at work. As Fermi had written in one of his first publications, “These numbers show that the energy of the nuclear bonds is a few million times greater than those of the most energetic chemical bonds.” But neither Fermi nor anybody else in the next ten years had been able to obtain a clue as to why those bonds were so strong.
That wasn’t all. Even before he discovered the atomic nucleus, Rutherford had observed two kinds of decay in radioactive elements in which electrically charged particles were emitted, one negative and the other positive. In beta decay, an electron was emitted. In alpha decay, a positively charged particle was emitted. The alpha particle, far more massive than an electron, was ultimately identified as a helium nucleus. After the decay, the final nucleus differed from the initial one by having a different electric charge, plus one unit for beta decay and minus two units for alpha decay.
Since the existence of protons within the nucleus was well established, it was likely that two protons could unite inside a helium nucleus. But the presence of electrons in a nucleus’s interior was dubious. The main attempt to circumvent this puzzle was to conjecture that a proton inside a nucleus had found an electron and bonded tightly to it.
This possibility had its pros and cons. It would explain how it was that electrons were observed to be exiting from a nucleus: they were contained in it to begin with. It would also explain the extra mass seen in nuclei other than hydrogen. Since the electron’s and the proton’s electric charges were opposite in sign but equal in magnitude, a bound-together electron-proton pair would have mass but no electric charge.
On the other hand, the conjecture had more than its share of problems, including one based on Heisenberg’s uncertainty principle. But even putting that aside, how could an electron and a proton bind tightly together? And if they did, how could the electron free itself to form a beta ray? As if that wasn’t enough, there was another problem: energy was apparently not conserved during beta decay. As expected, the difference in a nucleus’s energy before and after it emitted an alpha particle matched the energy carried away by the exiting helium nucleus. The same was not true for beta decay.
All these interesting problems led Fermi in 1931 to organize an international weeklong meeting in Rome on nuclear physics to debate the issues. He was also hoping it would be an aid for him and the Boys to stay abreast of both experimental and theoretical developments in the field. Held at the Italian Royal Academy, it brought together leading experts from around the world. Underlining its importance and coincidentally the Academy’s, Mussolini attended the inaugural meeting, held on October 11. The Italian press gave ample coverage to the event, not failing to note that the proceedings demonstrated “the depth and universality of Italian thought.”
The gathering showed the Boys once again where the big challenges in nuclear physics lay, but it didn’t provide them with a road map for how they could enter the field. Rome didn’t even have equipment that was crucial for studying nuclear decays. That didn’t deter Fermi. He and Amaldi enjoyed constructing experimental apparatus, so late in 1931 they set about building at least one of the needed items: a cloud chamber. This table-sized instrument contains vapor that records, via the formation of drops, the passage through
it of electrically charged particles, alpha particles being prime examples.
Since the Via Panisperna Institute lacked what Amaldi and Fermi needed for building the chamber, they shopped in Rome in order to find just the right items, confounding clerks at the hardware stores with their strange assemblage of purchases. Fermi, a devotee of the do-it-yourself method, felt in his element by starting from scratch.
A cloud chamber is a delicate instrument. The one Amaldi and Fermi built could not compete with those built in more sophisticated laboratories. Their valiant attempt was, however, a useful lesson for them. It drove home to them the need for better experimental facilities and for further training in top foreign laboratories. Amaldi had already spent ten months in Leipzig and Segrè had left for Hamburg. Rasetti, the group’s senior experimentalist, now traveled to the Berlin-Dahlem Institute, where Lise Meitner and Otto Hahn were doing state-of-the-art nuclear physics research.
As 1931 came to an end, a giant jump in the field of nuclear physics was beginning to occur. The key was to make use of an intense source of radioactivity to produce a beam of alpha particles and then use that beam to bombard an appropriate target. The technique was first developed by the German physicist Walter Bothe and then perfected by the French chemist Irène Curie and her physicist husband, Frédéric Joliot, in their Paris laboratory. They employed a target made out of beryllium, element number 4 of the periodic table, and searched for what sort of radiation was induced in the beryllium after it had been bombarded.
Exposing a layer of paraffin wax to the radiation that had been produced in the target, Curie and Joliot observed a copious production of protons. They concluded that quanta of electromagnetic radiation, particles known as photons, were causing protons to be expelled from the paraffin.
In Cambridge, Rutherford’s right-hand man, James Chadwick, saw the note detailing the Paris experiment and reported its results to Rutherford. He was flabbergasted by his mentor’s immediate response, “I don’t believe it,” a reaction Chadwick described as “entirely out of character.” Rutherford and Chadwick had their own views on what caused those protons to be emitted: a particle whose existence they had been contemplating for years. It was massive and neutral: they had called it a neutron. When Chadwick now saw an opportunity to catch the evasive prey, he dropped all his ongoing research in order to do so. After two weeks of almost nonstop work, Chadwick proved that neutrons, not electromagnetic quanta, were responsible for knocking the protons out of the wax.
Curie and Joliot had misidentified what they had seen. In considering who should get the Nobel Prize for the discovery, Rutherford is reported to have said, “For the neutron to Chadwick alone; the Joliots are so clever that they soon will deserve it for something else.” Three years later, in 1935, Chadwick was awarded the Nobel Prize in Physics for his discovery.
Although the neutron’s discovery would eventually come to be seen as a watershed moment in nuclear physics and the true dawn of the subject, it took a while for it to become clear that was the case. Neutrons could account for the missing mass in nuclei, but there was a great deal of confusion about whether they should or should not be treated on a footing similar to protons. Were they elementary or were they composites? There was also no understanding of what kind of force could hold neutrons and protons together within the nucleus.
The first glimmers of hope in addressing this all-important issue came with three papers Heisenberg produced in late 1932 attempting to describe what such a force might be like. They provided a promising beginning on a subject that continues to this day. And Heisenberg was not the only one making such efforts; Majorana and others were as well. Chadwick and Rutherford had not been the only ones to have doubts about the Curie-Joliot report. In Italy, Majorana had shaken his head after reading it and had said to the other Boys, “They haven’t understood anything. The effect is probably due to protons recoiling after being struck by a heavy neutral particle.” None of them thought much about the remark, but a few weeks later they looked at Majorana with new respect.
Unfortunately, as was almost always the case with the hypercritical Majorana, he thought his own results were of no consequence. When Fermi asked Majorana if he might give at least a preliminary report on his work at a Paris meeting Fermi was planning to attend, the Gran Inquisitore had reportedly been furious, telling Fermi, “I forbid you to mention these things that are so stupid. I don’t want you to go around discrediting me.” Majorana was sadly showing signs of the paranoia and isolation that increasingly plagued him.
Fermi and the Boys tried to persuade Majorana to visit a few of the great European nuclear physics centers to gain some exposure to other theorists working in the field of nuclear physics and hopefully be convinced that his thoughts were not “so stupid.” In particular, Fermi suggested a stay in Leipzig where Heisenberg was a professor. With a grant, Majorana left for Leipzig in January 1933. There, as Fermi had hoped, he found his work appreciated by Heisenberg, who even managed to persuade Majorana that his contribution to the theory of nuclear forces needed to be published.
Majorana’s findings subsequently appeared in Zeitschrift für Physik, one of the last papers that foreigners sought to publish in this prestigious German science journal. The respect accorded to German science was being eroded by the diaspora of German scientists, many of whom were Jewish, fleeing Nazism’s far-reaching tentacles. The research that was useful to National Socialism’s racism was the distorted pseudoscience of eugenics. Basic science research continued, but soon that became reoriented toward weaponry and war.
14
DECAY
The Germany that Majorana found was very different from what it had been only a few months earlier. Adolf Hitler had become Chancellor on January 30, 1933. In an all-too-frequent scenario, political events were impacting the world of science. Laws were passed within two months giving Hitler total legislative and executive control; his dictatorship was firmly established. Another law was soon enacted that excluded Jews from civil service and restricted the number of Jewish students allowed in schools and universities. Göttingen’s twin pillars of physics, the experimentalist James Franck and the theorist Max Born, went into exile.
Einstein didn’t wait for the law that discriminated against Jews to be promulgated. After a prolonged visit to the United States, he had sailed back to Europe in March. When the ship docked in Antwerp, he went straight to the German embassy, renounced his German citizenship, and a few months later returned to the United States. He never went back to Europe.
Fermi, like most Italians, did not believe that the overt anti-Semitism sweeping Germany was a harbinger of what would soon be coming to his own country. Italy’s Jews were a far smaller proportion of the population than Germany’s and had risen to prominence in many positions of the government and the military after Italy’s unification. By and large they considered anti-Semitism a thing of the past in their country. Laura’s Jewish father, Admiral Capon, was not worried, nor was Laura. Fermi, too, put aside the recent developments in Germany and concentrated instead, as usual, on physics.
Fermi had benefited from the rise of Italian Fascism. His research group had been supported generously and his appointment to the Royal Academy had effectively doubled his university salary. Nonetheless, he was no fan of Fascism and also felt the intellectual void caused by the abrupt end of visits to Via Panisperna from the likes of Bethe, Bloch, Peierls, Placzek, and Teller. They were now searching for safe havens in the United States or the United Kingdom. Italy, with Mussolini firmly ensconced, was no longer appealing despite the excitement of physics there and the pleasures of exploring Rome.
Despite the deepening shadow of totalitarianism in Germany and Italy, the Solvay Conference was held as planned in October 1933, the seventh in the series typically held every three years in Brussels. As in the past, the Solvay Conferences convened a few dozen of the greats in physics to discuss a key topic. The choice in 1933 was “Structure and Properties of Atomic Nuclei.” Most of the st
ars of physics, including Fermi, attended.
In discussions of new theories and experiments, the neutron and what it might mean for nuclear physics was uppermost, but another subject was often debated as well: beta decay. The problem of energy seemingly not being conserved when a nucleus emitted an electron continued to be troubling and there was no solution in sight other than abandoning energy conservation or accepting Pauli’s problematic hypothesis of a mysterious undetected very light particle. While at the conference, a revolutionary idea of how to solve the decade-old problem of the missing energy in beta decay—one of the greatest mysteries in nuclear physics—began brewing in Fermi’s mind.
The elegant simplicity of the theory Fermi proposed a few months later is wondrous. Seventy years after its formulation Marvin Goldberger, a post–World War II student of Fermi’s, wrote, “Any physicist who has not read Fermi’s 1934 paper on beta decay should rush out and do so immediately … it is the very epitome of what a scientific paper should be. The problem is stated clearly, a solution is presented, and the results compared with experiment. No smooth talk, no pretension, no promise that this is the first of a long series, etc. Just the facts!” Goldberger could have added that most of the last line of his description of Fermi’s paper is also a good description of Fermi himself: “no smooth talk, no pretensions.”
Amaldi and Segrè were the first to hear about the new theory. During the 1933 Christmas vacation they had gone skiing along with the Fermis and others in the Dolomites. This winter excursion amid the majesty of soaring mountains provided the setting for a jovial gathering of congenial friends relishing the fresh air of this snowy wonderland.
