The Last Man Who Knew Everything

by David N. Schwartz

  The conversation ended with a discussion of secrecy. Wigner joined Szilard in pressing for secrecy about work on the chain reaction. Fermi respected Wigner as much as he respected anyone, but he remained opposed to any restrictions on publication, deferring to Pegram to make the final decision. Pegram opted for sending papers on the two experiments to the Physical Review, but with the extraordinary request that their publication be delayed while the community considered the implications of the secrecy debate. In particular, Joliot-Curie in Paris was resisting any attempts to keep chain reaction research under wraps.

  The next day Enrico Fermi met the US Navy.

  CHAPTER FOURTEEN

  FERMI MEETS THE NAVY

  YEARS LATER, IN WHAT SHE DESCRIBED AS A “FIT OF HOUSE-CLEANING enthusiasm,” combing through a filing cabinet that stored family papers, Laura Fermi found a copy of the letter Pegram sent to Admiral Stanford Hooper at the Office of Chief of Naval Operations. Dated March 16, 1939, it begins:

  Dear Sir:

  This morning I had a telephone conversation with Mr. Compton in the office of the Assistant Secretary of the Navy [Charles Edison, Pegram’s friend and son of the inventor], who has doubtless reported the conversation to you. It had to do with the possibility that experiments in the physics laboratories of Columbia University reveal that conditions may be found under which the element uranium may be able to liberate its large excess of atomic energy and that this might mean the possibility that uranium might be used as an explosive that would liberate a million times as much energy per pound as any known explosive. My own feeling is that the probabilities are against this, but my colleagues and I think that the bare possibility should not be disregarded and I therefore telephoned to Mr. Edison’s office this morning chiefly to arrange a channel through which the results of our experiments might, if the occasion should arise, be transmitted to the proper authorities in the United States Navy.

  Professor Enrico Fermi who, together with Szilard, Dr. Zinn, Mr. Anderson and others, has been working on this problem in our laboratories, went to Washington this afternoon to lecture before the Philosophical Society in Washington this evening and will be in Washington tomorrow. He will telephone your office and if you wish to see him will be glad to tell you more definitely what the state of knowledge on this subject is at present.

  Professor Fermi, formerly of Rome, is Professor of Physics at Columbia University. In December last he was awarded the Nobel Prize in Physics in 1938 for the work he did on the artificial creation of radioactive elements by means of neutrons. There is no man more competent in this field of nuclear physics than Professor Fermi.

  Professor Fermi has recently arrived to stay permanently in this country and will become an American citizen in due course. He is very much at home in this country, having visited here often to lecture at the University of Michigan, Stanford University and at Columbia.

  Professor Fermi will be staying tomorrow with Professor Edward Teller of George Washington University.

  Sincerely yours,

  George E. Pegram

  Professor of Physics

  Across Fermi’s copy of it, Pegram scrawled a note. “Dear Fermi—This may prepare the way for you a little better than Mr. Compton’s explanation to Adm. Hooper.”

  Laura was mystified by the letter, having never seen it before and blissfully unaware that her husband met with the Navy on the subject of a potential atomic bomb in March 1939. In what was to become a habit, he had not spoken to her of his initial research relating to the potential for a nuclear explosive. When she confronted him about it, he explained that he had kept it as a kind of insurance policy. In December 1941, when the Axis declared war on the United States, he felt he might need some proof of loyalty to his new country and set it carefully aside in a manila folder. If any authorities challenged him, he would pull it out.

  The interchange between Laura and her husband reveals much. Even as early as March 1939, Fermi was maintaining a certain veil of secrecy around his uranium work—this in spite of his disagreements with Szilard over the whole issue of secrecy. The secrecy issue was undecided, but with characteristic caution he decided to say nothing to Laura. The other point, perhaps more telling, is that at the moment the United States entered the war, the Fermis became, albeit for a short time, enemy aliens. Fermi himself was not disposed to take his importance in the war effort for granted and felt the need for documentary proof of his loyalty to the United States. He needn’t have worried. By the time the United States entered the war, he was one of the key players in the effort to build a nuclear weapon and few, if any, doubted his loyalty. At the time, however, he was keenly aware of his status as a foreign national of a potential enemy power and felt the need to have an insurance policy tucked away in his filing cabinet.

  Reading the letter from Pegram, Admiral Hooper may have wondered what all the fuss was about. Pegram had to tread carefully. The idea of creating an explosive out of uranium would have struck Navy technical staff at first blush as preposterous, and Pegram believed that overselling the concept would have resulted in doors being shut in Fermi’s face. The letter was designed in part to give the admiral, who may never have heard of Fermi, a sense of the man’s stature within the scientific community. Pegram also went out of his way to assure the admiral that he and his Navy colleagues would be able to understand this foreign national, that Fermi was “very much at home” in America and would make a good presentation to the presumably unworldly Navy staff. In retrospect, tentative though it was, Pegram drafted the perfect letter to initiate contact between the scientific community and the US government on the potential for an atomic bomb.

  March 17, 1939, was a cool day in Washington, and the high of forty-nine degrees suggests that the cherry trees around the tidal basin, a 1912 gift from the Japanese people, had not yet blossomed. The basin would have been visible from Navy headquarters, housed in an enormous and exceedingly ugly edifice on what is now beautiful parkland just north of the long, narrow reflecting pool on the west side of Constitution Mall. Fermi arrived to a tepid welcome in the nondescript board room and overheard an unenthusiastic staff member announce to the assembled group, “There’s a wop outside.”

  In the conference room, Hooper had assembled a range of technical experts from various offices in the Navy, officers responsible for ordnance, engineering, construction, and repair, as well as a team from the Navy Research Labs headed by Dr. Ross Gunn. Although it was a Navy meeting, a technical team from the Army had been included as a courtesy. Fermi spoke for about an hour providing an overview of the physics of nuclear fission, the potential for the development of nuclear energy, and the prospect of a weapon based on the fission of uranium. According to notes taken by Captain Garrett L. Schuyler, later chief of the Research Division of Ordnance, Fermi described the principles of slow-neutron fission and the importance of neutron emission from fission to create a chain reaction. In summarizing the experiments he and Szilard had just completed, Fermi concluded that “the excess in the number of released neutrons is not very great and has not yet been demonstrated absolutely beyond the possible limits of experimental error.” He added that new experiments were planned in the next months to make more definitive measurements and if “these experiments show more neutrons are released from the uranium atoms than are necessary to split them up, continuous release of energy in a mass of uranium is theoretically possible.” He gave a clear description of critical mass: “In the small samples used so far… the released neutrons are possibly not all effective because some will too rapidly escape; but in a sufficiently large mass of uranium, they necessarily will be all trapped and available in time.”

  At this point Captain Schuyler piped up with a question: “What might be the size of this critical mass?” Fermi smiled and gave an answer consistent with his strategy of downplaying the possibility of a nuclear weapon: “Well,” he replied, “it just might turn out to be the size of a small star.” Although Fermi might have been deliberately modulating the pr
acticality of a fission bomb, it is also true that no one—neither Fermi the pessimist nor Szilard the optimist—actually knew what the critical mass might be. Not nearly enough was known to even begin such a calculation.

  Fermi turned to the issue of uranium isotopes. He explained to the assembled military scientists that natural uranium consisted of a mixture of the two isotopes, about 99.3 percent of which was uranium 238 (U-238) and 0.7 percent, uranium 235 (U-235). On the basis of theoretical work done the previous month at Princeton by Bohr, Wheeler, and Czech émigré George Placzek, scientists now believed that the far rarer U-235 isotope was responsible for the fission reaction. The only clean way to build a fission weapon would be to separate the two isotopes, but at this point no one knew how to do this.

  In summary, Fermi made it clear that it may be possible to unlock atomic energy through fission and that this possibility, with all its attendant uncertainties, should be brought to the attention of the military.

  Had he witnessed his colleague’s performance, Szilard would have been disappointed. Fermi was disinclined to stir the Navy into a frenzy of action, and he didn’t. A natural reticence to make bold statements about science before he had clarified the facts for himself, combined with a feeling that the less said about the terrible possibilities the better, drove him to impart enough information for the military to decide on next steps without recommending any specific course of action. Years later, Szilard dismissively suggested that nothing came of the meeting.

  This was not entirely true. Fermi’s lecture fired up the Navy’s head of research Ross Gunn, who immediately saw that uranium could provide a source of energy for submarines. He launched a long, frustrating, but ultimately successful effort to develop nuclear-powered naval vessels. Before acting, however, he had to find out more about this odd, understated little man with the strong Italian accent. He called Merle Tuve, the Carnegie Institution physicist who was one of the cosponsors of the January conference at which Bohr and Fermi had presented. “Who is this man Fermi? What kind of man is he? Is he a Fascist or what? What is he?” Tuve assured Gunn of Fermi’s impeccable credentials. That was enough for Gunn. Unfortunately for the Navy scientist, he could not get his own project off the ground until after the war, when national priorities had shifted away from the Manhattan Project.

  Though the briefing did not exactly spur the Navy into action, it did result in a check to Columbia for $1,500 for continued research into fission. Who knew? Might there be something to this bizarre, science fiction idea of a new explosive based on nuclear fission? It seemed worthwhile, for a small expenditure, to keep tabs on the research being done at Columbia.

  Fermi was sailing in uncharted waters when he arrived on Constitution Mall in March 1939. He came bearing outlandish ideas, ideas that an institutionally conservative military was unprepared to accept. The military was not accustomed to funding private scientific research. In retrospect, it is impressive that he received even a small grant for his experiments.

  BACK IN NEW YORK, FERMI BEGAN TO SETTLE INTO LIFE IN HIS NEW country.

  The family home at 450 Riverside Drive was one of a row of apartment buildings overlooking the Hudson River, built in the early 1900s to house Columbia faculty. It was reasonably comfortable, but in the winter the wind howled up from the river and up the hill on West 116th Street. Walking the children up the hill or back down against the wind was not one of Laura’s favorite activities. In spite of this, she gradually adjusted to life in a new city, making the best of her situation.

  Fermi plunged into classroom work. He taught three courses that spring term, including a course on geophysics, one of his favorite subjects, with a group of rather fortunate undergrads, as well as higher level courses on quantum mechanics and applied quantum mechanics.

  He was also getting to know other members of the Columbia faculty, people who would become close colleagues and friends over the next decade. Rabi was one of them. An irascible, punchy personality with a wicked sense of humor, Rabi had met Fermi during his years as a post-doc in Europe. In the early 1930s, Rabi began experimental work that eventually resulted in his discovery of the nuclear magnetic resonance effect that is the basis of today’s MRI scanners. Fermi and Rabi hit it off right away. Years later, Rabi would tell his biographer that aside from Einstein, he considered Fermi the greatest physicist he had ever known.

  Another fast friend was Harold Urey. Slightly older than Fermi, Urey won the Nobel Prize in Chemistry in 1934 for his isolation of deuterium, the isotope of hydrogen with a proton and a neutron in its nucleus. Urey befriended Enrico and Laura and spent a good deal of time selling them on the joys of living the American dream in the suburban town of Leonia, New Jersey, where Urey himself lived. Within a year he succeeded, and the Fermis became suburban Americans with front and back lawns and a makeshift workshop in the basement. Fermi never quite got the hang of suburbia—he and Laura were city folk at heart—and their front lawn was often the least well manicured on the block. It was, however, the beginning of a lifelong friendship with the Ureys.

  As the family adjusted to life in the States, making new friends and settling into what they hoped would be a quiet domestic life, nuclear fission and Szilard’s obsession with chain reactions continued to preoccupy Fermi.

  FERMI HAD TOLD HIS NAVY AUDIENCE THAT HE WAS PLANNING another experiment to clarify some of the uncertainties surrounding fission and the possibility of a chain reaction. For this experiment, Fermi and Szilard collaborated as principal partners. Given their radically different work styles, it is not surprising that this was also their last direct collaboration.

  Creating a fission chain reaction using natural uranium taken from the ground poses some real problems. Natural uranium is composed of two isotopes. U-238, which is extremely difficult to split, accounts for 99.3 percent of natural uranium. U-235, which readily splits and is ideal for creating a chain reaction, accounts for only 0.7 percent of natural uranium. Thus, a chain reaction, if feasible, would require many tons of natural uranium, if not separated, to have enough of the fissionable isotope U-235 needed for the reaction. Although no one yet knew how to separate the two isotopes (U-235 and U-238) from each other, Fermi’s Columbia colleague John Dunning doubted the feasibility of a chain reaction based on natural uranium and so pressed for first solving the separation problem, purifying natural uranium so that the relatively small amount of U-235 needed for a chain reaction could be isolated. Doubting that the techniques for isotope separation could be developed quickly, Fermi and Szilard preferred using natural uranium, instinctively sensing that a chain reaction could be developed with a large enough supply of it. After much debate, Fermi’s proposal won out. Fermi recognized that isotope separation would eventually need to be solved but believed that an initial demonstration using natural uranium would provide proof of the chain reaction concept.

  Beginning in April 1939, Fermi and Szilard, along with Anderson, modified the water tank experiment, filling the tank with a 10 percent solution of manganese sulfate, a substance that becomes radioactive in proportion to the neutrons that hit it. Into this solution they placed a matrix of fifty-two tin cans, two inches in diameter and two feet high. In the middle of the tank they placed Szilard’s neutron source.

  Using four hundred pounds of uranium oxide the ever-resourceful Szilard had “borrowed” in one of his famous sleights of hand from the El Dorado Radium Corporation, which owned significant deposits of uranium ore in Canada, they measured a 10 percent increase in the radioactivity of the manganese with the uranium oxide present inside the cans, confirming the results of the two previous experiments.

  Then they set about trying to calculate the ratio of neutrons emitted per neutron absorbed during fission, what they called the “reproduction factor.” A cascade of fission reactions, as Szilard originally envisioned, required an average reproduction rate greater than one—even the slightest amount greater than one would eventually work. The experiment resulted finally in a measurement of about 1.5 fast neutr
ons emitted for every neutron absorbed during fission. They reported that “a nuclear chain reaction could be maintained in a system in which neutrons are slowed down without much absorption until they reach thermal energies and are then mostly absorbed by uranium rather than by another element.” They also suspected that the water they used in the experiment was absorbing too many neutrons: “It remains an open question, however, whether this holds for a system in which hydrogen is used for slowing down the neutrons.” This was the first conceptual outline of what was to become the “pile,” the world’s first nuclear reactor in 1942.

  Anderson later noted several important points about the experiment.

  First, the measurements they took indicated that plain water would probably not be a good moderator for a fully functioning reactor, because hydrogen atoms in the water had a tendency to absorb slow neutrons into their nuclei, taking them out of the chain reaction.

  Second, the team became aware of the importance of a phenomenon known as “resonance absorption.” U-238, which makes up the vast majority of natural uranium, tends to absorb slow-ish neutrons without undergoing fission, thus taking them out of the chain reaction. Fermi estimated that this phenomenon was responsible for a 20 percent decrease in the average number of emitted neutrons, an estimate based as much on Fermi’s intuition as on the data. To solve this problem, Fermi decided that lumping uranium into smaller chunks would reduce the tendency of the fast neutrons emitted in fission to slow down and become absorbed by the U-238.