The task would hardly be boring. The truth is that Szilard lacked the resources for such work—access to a laboratory, a dedicated crew, sufficient financial support. “None of the physicists had any enthusiasm for this idea of a chain reaction,” he would remember. Rutherford threw him out. Blackett told him, “Look, you will have no luck with such fantastic ideas in England.761 Yes, perhaps in Russia. If a Russian physicist went to the government and [said], ‘We must make a chain reaction,’ they would give him all the money and facilities which he would need. But you won’t get it in England.” Soaking in his bath against the London chill, Szilard turned back to mapping the future. The opportunity to explore the elements systematically for surprises by bombarding them with neutrons passed him by.
* * *
It fell instead to Enrico Fermi and his team of young colleagues in Rome. Fermi was prepared.762 He had all on hand that Szilard did not. He saw as soon as Szilard that the neutron would serve better than the alpha particle for nuclear bombardment. The point was not obvious. One used alphas to generate neutrons (as the Joliot-Curies had done along the way to chasing down their positrons). Since not all the alphas found targets, the neutral particles were correspondingly that much more scarce. As Otto Frisch would write: “I remember that my reaction and probably that of many others was that Fermi’s was really a silly experiment because neutrons were much fewer than alpha particles. What that simple argument overlooked of course was that they are very much more effective.”763
Fermi was prepared because he had been organizing his laboratory for a major expedition into nuclear physics for more than four years. If Italy had been one of the hot centers of physical research he might have been too preoccupied to plan ahead so carefully. But Italian physics was a ruin as sere as Pompeii when he came to it. He had no choice but to push aside the debris and start fresh.
Both Fermi’s biographers—his wife Laura and his protégé and fellow Nobel laureate Emilio Segrè—assign the beginning of his commitment to physics to the period of psychological trauma following the death of his older brother Giulio when Fermi was fourteen years old, in the winter of 1915.764 Only a year apart in age, the two boys had been inseparable; Giulio’s death during minor surgery for a throat abscess left Enrico suddenly bereft.
That same winter young Enrico browsed on market day among the stalls of Rome’s Campo dei Fiori, where a statue commemorates the philosopher Giordano Bruno, Copernicus’ defender, who was burned at the stake there in 1600 by the Inquisition. Fermi found two used volumes in Latin, Elementorum physicae mathematicae, the work of a Jesuit physicist, published in 1840. The desolate boy used his allowance to buy the physics textbooks and carried them home. They excited him enough that he read them straight through. When he was finished he told his older sister Maria he had not even noticed they were written in Latin. “Fermi must have studied the treatise very thoroughly,” Segrè would decide, looking through the old volumes many years later, “because it contains marginal notes, corrections of errors, and several scraps of paper with notes in Fermi’s handwriting.”765
From that point forward Fermi’s development as a physicist proceeded, with a single significant exception, rapidly and smoothly. A friend of his father, an engineer named Adolfo Amidei, guided his adolescent mathematical and physical studies, lending him texts in algebra, trigonometry, analytical geometry, calculus and theoretical mechanics between 1914 and 1917. When Enrico graduated from the liceo early, skipping his third year, Amidei asked him if he preferred mathematics or physics as a career and made a point of writing down, with emphasis, the young man’s exact reply: “I studied mathematics with passion because I considered it necessary for the study of physics, to which I want to dedicate myself exclusively. . . . I’ve read all the best-known books of physics.”766
Amidei then advised Fermi to enroll not at the University of Rome but at the University of Pisa, because he could compete in Pisa to be admitted as a fellow to an affiliated Scuola Normale Superiore of international reputation that would pay his room and board. Among other reasons for the advice, Amidei told Segrè, he wanted to remove Fermi from his family home, where “a very depressing atmosphere prevailed . . . after Giulio’s death.”767
When the Scuola Normale examiner saw Fermi’s competition essay on the assigned theme “Characteristics of sound” he was stunned. It set forth, reports Segrè, “the partial differential equation of a vibrating rod, which Fermi solved by Fourier analysis, finding the eigenvalues and the eigenfrequencies . . . which would have been creditable for a doctoral examination.”768 Calling in the seventeen-year-old liceo graduate, the examiner told him he was extraordinary and predicted he would become an important scientist. By 1920 Fermi could write a friend that he had reached the point of teaching his Pisa teachers: “In the physics department I am slowly becoming the most influential authority. In fact, one of these days I shall hold (in the presence of several magnates) a lecture on quantum theory, of which I’m always a great propagandist.”769 He worked out his first theory of permanent value to physics while he was still a student in Pisa, a predictive deduction in general relativity.
The exception to his rapid progress came in the winter of 1923, when Fermi won a postdoctoral fellowship to travel to Göttingen to study under Max Born. Wolfgang Pauli was there then, and Werner Heisenberg and the brilliant young theoretician Pascual Jordan, but somehow Fermi’s exceptional ability went unnoticed and he found himself ignored. Since he was, in Segrè’s phrase, “shy, proud, and accustomed to solitude,” he may have brought the ostracism on himself.770 Or the Germans may have been prejudiced against him by Italy’s poor reputation in physics. Or, more dynamically, Fermi’s visceral aversion to philosophy may have left him tongue-tied: he “could not penetrate Heisenberg’s early papers on quantum mechanics, not because of any mathematical difficulties, but because the physical concepts were alien to him and seemed somewhat nebulous” and he wrote papers in Göttingen “he could just as well have written in Rome.”771, 772 Segrè has concluded that “Fermi remembered Göttingen as a sort of failure. He was there for a few months. He sat aside at his table and did his work. He didn’t profit. They didn’t recognize him.”773 The following year Paul Ehrenfest sent along praise through the intermediary of a former student who looked up Fermi in Rome. A three-month fellowship then took the young Italian to Leiden for the traditional Ehrenfest tightening. After that Fermi could be sure of his worth.
He was always averse to philosophical physics; a rigorous simplicity, an insistence on concreteness, became the hallmark of his style. Segrè thought him inclined “toward concrete questions verifiable by direct experiment.”774 Wigner noticed that Fermi “disliked complicated theories and avoided them as much as possible.”775 Bethe remarked Fermi’s “enlightening simplicity.”776 Less generously, the sharp-tongued Pauli called him a “quantum engineer”; Victor Weisskopf, though an admirer, saw some truth in Pauli’s canard, a difference in style from more philosophical originals like Bohr.777 “Not a philosopher,” Robert Oppenheimer once sketched him.778 “Passion for clarity. He was simply unable to let things be foggy. Since they always are, this kept him pretty active.” An American physicist who worked with the middle-aged Fermi thought him “cold and clear. . . . Maybe a little ruthless in the way he would go directly to the facts in deciding any question, tending to disdain or ignore the vague laws of human nature.”779
Fermi’s passion for clarity was also a passion to quantify. He seems to have attempted to quantify everything within reach, as if he was only comfortable when phenomena and relationships could be classified or numbered. “Fermi’s thumb was his always ready yardstick,” Laura Fermi writes. “By placing it near his left eye and closing his right, he would measure the distance of a range of mountains, the height of a tree, even the speed at which a bird was flying.”780 His love of classification “was inborn,” Laura Fermi concludes, “and I have heard him ‘arrange people’ according to their height, looks, wealth, or even sex appeal.”781
Fermi was born in Rome on September 29, 1901, into a family that had successfully made the transition during the nineteenth century from peasant agriculture in the Po Valley to career civil service with the Italian national railroad. His father was a capo divisione in the railroad’s administration, a civil rank that corresponded to the military rank of brigadier general. In accord with a common Italian practice of the day, the infant Enrico was sent to live in the country with a wet nurse. So was his brother Giulio, but because Enrico’s health was delicate he did not return to his mother and father until he was two and a half years old. Confronted then with a roomful of strangers purporting to be his family, and “perhaps,” writes Laura Fermi, “missing the rough effusiveness of his nurse,” he began to cry:782
His mother talked to him in a firm voice and asked him to stop at once; in this home naughty boys were not tolerated. Immediately the child complied, dried his tears, and fussed no longer. Then, as in later childhood, he assumed the attitude that there is no point in fighting authority. If they wanted him to behave that way, all right, he would; it was easier to go along with them than against.
In 1926, when he was twenty-five years old, Fermi was chosen under the Italian system of concorsos, national competitions, to become professor of theoretical physics at the University of Rome. An influential patron had seen to the creation of the new post, a Sicilian named Orso Mario Corbino, a short, dark, volatile man, forty-six when Fermi sought him out in 1921, the director of the university physics institute, an exceptional physicist and a Senator of the Kingdom. Since the old guard of Italian physicists resented Fermi’s rapid promotion, he especially welcomed the protection of Corbino’s patronage. Corbino found support for his efforts to improve Italian physics from the Fascist government of the bulletheaded former journalist Benito Mussolini, although the senator was not himself a party member.
In the later 1920s Corbino and his young professor agreed that the time was ripe for the small group they were assembling in Rome to colonize new territory on the frontier of physics.783 They chose as their territory the atomic nucleus, then finding description in quantum mechanics but not yet experimentally disassembled. Fermi’s tall, erudite Pisa classmate Franco Rasetti signed on as Corbino’s first assistant early in 1927. Rasetti and Fermi together recruited Segrè, who had been studying engineering, by taking him along to the Como conference and explaining the achievements of the assembled luminaries to him—by then, Segrè saw, Pauli and Heisenberg recognized Fermi’s talents and included him among their friends. The son of the prosperous owner of a paper mill, Segrè contributed elegance to the group as well as brains.
Corbino added Edoardo Amaldi, the son of a mathematics professor at the University of Padua, by frankly raiding the engineering school. The group quickly nicknamed Fermi “the Pope” for his quantum infallibility; Corbino, like Rutherford at the Cavendish, called them all his “boys.” Rasetti departed to Caltech, Segrè to Amsterdam, for seasoning. Fermi sent them out again in the early 1930s, after the decision to go into nuclear physics: Segrè to work with Otto Stern in Hamburg, Amaldi to Leipzig to the laboratory of the physical chemist Peter Debye, Rasetti to Lise Meitner at the KWI. By 1933, with a departmental budget above $2,000 a year, ten times the budget of most Italian physics departments, with a well-made cloud chamber and a nearby radium source and KWI training in the vagaries of Geiger counters, the group was ready to begin.
In the meantime, two months after the Solvay Conference, Fermi completed the major theoretical work of his life, a fundamental paper on beta decay. Beta decay, the creation and expulsion by the nucleus of highenergy electrons in the course of radioactive change, had needed a detailed, quantitative theory, and Fermi supplied it entire. He introduced a new type of force, the “weak interaction,” completing the four basic forces known in nature: gravity and electromagnetism, which operate at long range, and the strong force and Fermi’s weak force, which operate within nuclear dimensions. He introduced a new fundamental constant, now called the Fermi constant, determining it from existing experimental data. “A fantastic paper,” Victor Weisskopf later praised it, “ . . . a monument to Fermi’s intuition.”784 In London the editor of Nature rejected it on the grounds that it was too remote from physical reality, which Fermi found irritating but amusing; he published it instead in the little-known weekly journal of the Italian Research Council, Ricerca Scientifica, where Amaldi’s wife Ginestra worked, and later in the Zeitschrift fúr Physik.785, 786 With only minor adjustments Fermi’s theory of beta decay continues to be definitive.
The Comptes Rendus reporting the Joliot-Curies’ discovery of artificial radioactivity reached Rome shortly after Fermi returned from skiing in the Alps, in January 1934.787 “We had not yet found any [nuclear physics] problems to work on,” Amaldi reminisces. “ . . . Then came out the paper of Joliot, and Fermi immediately started to look for the radioactivity.”788 Like Szilard, Fermi saw the advantages of using neutrons. I. I. Rabi catalogues those advantages in a lecture:
Since the neutron carries no charge, there is no strong electrical repulsion to prevent its entry into nuclei. In fact, the forces of attraction which hold nuclei together may pull the neutron into the nucleus. When a neutron enters a nucleus, the effects are about as catastrophic as if the moon struck the earth. The nucleus is violently shaken up by the blow, especially if the collision results in the capture of the neutron. A large increase in energy occurs and must be dissipated, and this may happen in a variety of ways, all of them interesting.789
When Fermi began his neutron-bombardment experiments he was thirty-three years old, short, muscular, dark, with thick black hair, a narrow nose and surprising gray-blue eyes. His voice was deep and he grinned easily. Marriage to the petitely beautiful Laura Capon, the daughter of a Jewish officer in the Italian Navy, had encouraged him in methodical habits: he worked for several hours privately at home, arrived at the physics institute at nine, worked until twelve-thirty, lunched at home, returned to the institute at four and continued work until eight in the evening, returning home then for dinner. With marriage he had also gained weight.
He and his team of young colleagues occupied the south section of the second floor of the institute, sharing the space with Corbino and with the chief physicist of Rome’s Sanitá Pubblica—its health department—a generous soul named G. C. Trabacchi who lent Corbino’s boys some of the instruments and supplies they needed for their experiments (in return they cherished him, nicknaming him “Divine Providence”). Antonino Lo Sordo, a frustrated old-guard physicist, fended off the encroaching horde from an office at the north end of the floor. Corbino and his family lived above, the residence overlooking a private garden in back with a goldfish pond at its focus. The first floor served students; the basement held electrical generators and a lead-lined safe for the Sanitá’s gram of radium, worth 670,000 lire—about $34,000—in that year of its most historic use. Glass pipes passed through a wall of the special safe to carry radon, formed in the decay of radium, to a compact extraction plant, a modest refinery of glasspipe towers that purified and dried the radioactive gas. The residential upper story of the institute, contracted above the longer lower floors to make room at one end for the dome of a small rotunda, was roofed with tile. “The location of the building in a small park on a hill near the central part of Rome was convenient and beautiful at the same time,” Segrè recalls. “The garden, landscaped with palm trees and bamboo thickets, with its prevailing silence (except at dusk, when gatherings of sparrows populated the greenery), made the institute a most peaceful and attractive center of study.”790 A gravel path that shone white in the golden Roman sun led down to the Via Panisperna.
As usual, Fermi hewed the neutron experiments by hand. In February and early March he personally assembled crude Geiger counters from aluminum cylinders acquired by cutting the bottoms off tubes of medicinal tablets.791 Wired, filled with gas, their ends sealed and leads attached, the counters were slightly smaller than rolls of breath mints and
a hundred times less efficient than modern commercial units, but with Fermi to operate them they served. While he built Geiger counters he asked Rasetti to prepare a neutron source in the form of polonium evaporated onto beryllium. Since polonium emits relatively low-energy alpha particles, the resulting source emitted relatively few neutrons per second, and Fermi and Rasetti irradiated several samples without success.
At that point Rasetti, showing a surprising lack of eagerness for historic experiment, went off to Morocco for Easter vacation. Fermi cast about for some way to acquire a stronger neutron source. The rationale for using polonium in the first place, in Paris and Cambridge and Berlin as well as in Rome, had been that a stronger alpha emitter like radon also emitted strong beta and gamma radiation, which disturbed the instruments and interfered with measurements. Fermi realized suddenly that since he was trying to observe a delayed effect, he was measuring only after he removed the neutron source in any case—and therefore any beta and gamma radiation wouldn’t matter and he could use radon. Trabacchi had the radon to spare and willingly dispensed it; with a half-life of only 3.82 days it was perishable in any case and his glowing gram of radium continually exhaled a fresh draft.
To the basement of the physics institute on the Via Panisperna, in his gray lab coat, in mid-March, Fermi thus carried a snippet of glass tubing no larger than the first joint of his little finger. It was flame-sealed at one end and partly filled with powdered beryllium. He set the sealed end of this capsule into a container of liquid air. The radon, directed from the outlet of the extraction plant into the capsule, condensed on its walls in the—200 °C cold. Fermi then had to attempt quickly to heat and draw closed the other end of the capsule, without cracking the glass, before the radon evaporated and escaped. When he succeeded, he finished preparing the neutron source by dropping it into a two-foot length of glass tubing of larger diameter and sealing it into the far end so that it could be handled at a distance safe from dangerous exposure to its gamma rays. For all the tedious preparation its useful life was brief.
Making of the Atomic Bomb Page 29
