The Pope of Physics

by Gino Segrè

  Later, Fermi reminisced in a semi-humorous fashion about how, given that both graphite and uranium oxide are black, the Columbia University physicists “started looking like coal miners and the wives to whom these physicists came back tired at night were wondering what was happening.” The toll collector on the George Washington Bridge may also have been curious as to why a gentleman headed for well-to-do suburban Leonia was covered with black dust.

  At the end of September 1941, Fermi, Anderson, and the team had their results. The experiment worked but the result was disappointing: k was only equal to 0.87, clearly less than the k greater than one needed for a chain reaction. But it was close enough for Fermi to believe he might be able to reach his goal if he had more graphite and more uranium, purer materials, and a still larger pile. This was going to be a bigger job than his team could handle and would require a substantial input of money.

  In 1941 it probably would have been impossible to obtain the funds for a much bigger pile had it not been for an intervening set of nuclear physics discoveries made in Berkeley that past year. These indicated there might be another way to make a nuclear weapon, one more promising than separating U-235 from U-238. It involved creating a new substance, a transuranic with 94 protons in its nucleus, two more than uranium.

  The only way to obtain enough of it for a bomb, however, was to produce it in a Fermi-like pile undergoing a chain reaction. Far from being secondary, such a pile might be key to the war effort. Fermi would no longer pursue science for science’s sake. Obtaining a self-sustaining chain reaction inextricably tied him to the evolution of nuclear weaponry.

  24

  THE SLEEPING GIANT

  America was slow to awaken to the perils of the maelstrom engulfing Europe. U.S. isolationists, intent on avoiding enmeshment in battles fought on foreign soil, stayed firm in their convictions. The United States behaved like a sleeping giant, devoid of nightmares of being attacked and oblivious to the prospect of a mega-bomb.

  Because of NDRC’s small grant to Columbia, NDRC head Vannevar Bush was aware of the progress Fermi was making. But before committing additional funds to fission research, Bush sought advice from the National Academy of Sciences (NAS). He asked them to appoint a committee of experts to evaluate the field, with particular emphasis on possible applications for national defense. Arthur Compton was chosen to head the committee. He was an excellent choice even though nuclear physics was not his specialty. Universally liked and respected, he was a first-rate scientist and administrator. His 1927 Nobel Prize in Physics had been for work confirming features of quantum physics, but Compton was well versed in almost all of physics.

  His committee’s report, the first of three he prepared during 1941, was delivered to the NDRC on the seventeenth of May. It was sanguine about using nuclear fission as a significant way to generate power and concluded that fission might have military applications. But given the difficulty of separating U-235 from U-238, the report estimated it would take several years to make an atom bomb. A second report, delivered on July 11, said much the same. The consequence was that in the summer of 1941, the NDRC was heading for a policy of passive observation. As Arthur Compton wrote in his memoirs, “The government’s responsible representatives were thus very close to dropping fission studies from the war program.” Fission was not high on their agenda.

  That attitude began to change after committee members read a letter from Ernest Lawrence, the winner of the 1939 Nobel Prize in Physics for his development of the cyclotron. He was a committee member but had been unable to attend their July meeting. Dated July 11, the letter expressed Lawrence’s concern that the committee’s outlook was overly conservative. He wanted to let them know of a recent finding that signaled a promising way of building a bomb.

  Almost two years earlier, a young Berkeley associate of Lawrence’s, Edwin McMillan, had tried producing transuranics. Earlier research by the Boys and others was wrong, but McMillan had found a clever way to distinguish transuranics from fission fragments.

  McMillan knew that ordinary uranium, U-238, turns into radioactive U-239 after absorbing a neutron. He studied two of U-239’s decay modes that seemed particularly interesting. One had a half-life of twenty-three minutes, and the other, much slower, decayed in a little over two days. Both were examples of beta decay, in which the original nucleus emits an electron. More work was needed, but McMillan made a positive identification during the spring of 1940. The twenty-three-minute decay’s endpoint was a nucleus with 93 protons, a genuine transuranic. Since Neptune follows Uranus in the solar system, he named it neptunium.

  However, the longer decay mode was still a mystery. In the fall of 1940, McMillan, scheduled to leave California to work on radar research at MIT, turned the problem over to the twenty-eight-year-old Berkeley chemist Glenn Seaborg.

  Over the next nine months, Seaborg and a new team made a series of revealing discoveries about that long decay mode, confirming the suspicion that it was because the transuranic had decayed into a second transuranic, one whose nucleus had 94 protons. Pluto followed Neptune, so this one was given the name plutonium.

  Nuclear physics considerations suggested that, when bombarded by slow neutrons, a nucleus with 94 protons would undergo fission in the same way U-235 did. This raised an intriguing possibility: a second route to a super-bomb that did not call for the difficult practice of separating two chemically identical isotopes.

  During a long discussion with Emilio Segrè in December 1940, Fermi confirmed the correctness of this supposition and suggested they meet with Lawrence, who happened to be in New York, and with Pegram, in order to decide how to move ahead. They agreed that Berkeley’s laboratory should pursue seeing if element 94 underwent fission and if it was stable enough to be used in a bomb. Segrè was excited by the possibilities the project offered. In the next six months, his work with Seaborg’s team established that plutonium, with a half-life of twenty-five thousand years, was nearly stable, and that, like U-235, it underwent fission after capturing a slow neutron.

  This was the basis for Lawrence’s pressing July 1941 letter to the National Academy Committee: a second route to a bomb was within sight. Prophetically, its name was associated with Pluto, the Roman god of the underworld. The myth converted, in years to come, into a monstrous reality.

  Berkeley researchers had collected only microscopic samples of plutonium. The way to obtain more was through a self-sustaining chain reaction; that could be achieved if Fermi was able to build a critical pile. Realizing how pivotal this was, Arthur Compton became an energetic advocate for Fermi to achieve his goal.

  This was not the only information galvanizing American support for nuclear fission research during the summer of 1941. Marcus Oliphant, a prominent Australian nuclear physicist transplanted to England, was the forceful messenger of the additional news. Speaking to high-level American scientists, Oliphant told them that British research showed the route to a U-235 bomb was far less arduous than the NDRC had imagined. Given the succession of Axis victories, Oliphant felt that the very life of Britain might depend on having this weapon. He repeatedly made the point that the United States needed to collaborate with Britain to build it. The States, soon to be similarly threatened, could not afford the false sense of security they were enjoying.

  Oliphant was privy to the current status of British fission research because he was one of six elite physicists who made up the MAUD Committee, a government-appointed group formed in early 1940. Oliphant had instigated the committee’s creation because two German physicists he had recruited to his Birmingham department had given him alarming news. The top-level MAUD Committee had adjudicated its validity.

  The Germans were Rudolf Peierls and Otto Frisch, both Jewish refugees from Nazi Germany. In early 1940 they concluded it would not be possible to make a bomb out of U-235 using slow neutrons as a trigger; the device would heat up so much that it would come apart before fully detonating. Subsequently, in estimating the amount of U-235 that would be needed if f
ast neutrons were employed instead, they had come up with a frighteningly small number.

  As Peierls wrote in his memoir, “We estimated the critical size to be about a pound, whereas speculations concerned with natural uranium had tended to come out with tons. We were quite staggered by these results: an atomic bomb was possible, after all, at least in principle!” Startled, Frisch and Peierls realized that a bomb of this sort might not cost much more to build than a battleship. If so, there was an imminent threat from Germany.

  By the end of the spring of 1941, having studied Peierls and Frisch’s analysis, MAUD was prepared to file its dire conclusions. A complete draft was forwarded to Lyman Briggs and to Vannevar Bush on July 15. The recommendations were crisp and unequivocal: (1) developing a uranium bomb was feasible; (2) such a bomb was likely to be decisive in a war; and (3) building it should be given the highest priority. The committee also provided technical details about the amount of U-235 needed for a bomb, estimates of the cost for separating it from uranium ore, and the expected destructive power it would have. MAUD vigorously endorsed continued collaboration with the United States.

  In August 1941, Oliphant flew to the United States. There were official reasons for his trip, but uppermost in his mind was his wanting to know why there had been no response to the MAUD Committee’s draft from either Briggs or Bush, both of whom had seen it. The answer was that apparently neither had heard the report’s clarion call.

  Bush was waiting for confirmation of its contents. Briggs, whom Oliphant later described as “inarticulate and unimpressive,” had simply locked the report in his safe without showing it to his NDRC subcommittee on uranium. A government employee for more than forty years and now head of the National Bureau of Standards, Briggs had treated the MAUD report as nothing more than one of the many government documents that should be shelved. Much as he had handled the warnings from Leo Szilard and Eugene Wigner, he dismissed this, too.

  Samuel Allison, a University of Chicago physicist who had recently joined the Briggs subcommittee, first heard of bombs in the presentation Oliphant made to them. As he remembered, “I thought we were making a power source for submarines.” He was astonished by the prospect of a uranium bomb. Distressed and angry at the lack of attention to the MAUD report, Oliphant telephoned Ernest Lawrence, offering to come to California in order to present him with the committee’s findings. In early September, he briefed Lawrence, who was so impressed by what he heard that he asked Bush and Harvard’s president James Conant to meet Oliphant in Washington. Lawrence then called Compton, arranging to see him later that month in Chicago.

  Finally Oliphant had found someone who shared his trepidation and grasped the enormous implications of what he was imparting. By contacting Lawrence, he had chosen well. Lawrence, energetic and entrepreneurial, had won the 1939 Nobel Prize in Physics for his development of the cyclotron. He was at the time the United States’ most eminent nuclear physicist, someone Bush and Conant could not ignore. Even after meeting Oliphant, Bush and Conant remained skeptical of MAUD’s conclusions, but thought they were persuasive enough to warrant ordering a new National Academy of Sciences (NAS) review. Compton was once again its head.

  Up to this point, as he later admitted, Compton had been very cautious in involving Fermi because of security concerns. Fermi was not an American citizen. But Samuel Allison, Compton’s Chicago colleague and protégé, told him that Fermi was absolutely the man he needed to speak to first. When Compton had asked Allison who could give him a reliable estimate of how much U-235 would be needed for a bomb, Allison’s answer had come quickly, “No one can answer that question as well as Enrico Fermi.”

  Shortly afterward, in Fermi’s Columbia office, Compton asked him the question. Fermi promptly went to the blackboard and worked out, in Compton’s own words, “simply and directly, the equations from which could be calculated the critical size of a chain-reacting sphere.” Fermi then proceeded to estimate how much U-235 would be needed for a bomb. It was a more conservative number than that in the MAUD report, but an obtainable one.

  From New York, Compton—duly impressed by Fermi—traveled to Princeton to confer with Wigner about the relative merits of fast-neutron-induced versus slow-neutron-induced fission. After the technical discussion was over, Compton remembered Wigner urging him, “almost with tears, to help get the atomic program rolling. His lively fear that the Nazis would make the bomb first was the more impressive because from his life in Europe he knew them so well.” Wigner has said that getting the U.S. government to see the value of fission had “felt like swimming in syrup.” He continued to hope that America would see the risks it was running before it was too late.

  Wigner’s reaction was in sharp contrast to Fermi’s unemotional presentation. As Szilard wrote, “Even by the middle of 1942 Fermi thought that our work had no bearing on the war and that those who thought so were sadly mistaken.” Fermi had consistently refused to be an alarmist, acting only when the evidence was overwhelming. A conservative by nature, he did not like to jump to conclusions in matters of science or of politics. In many ways, this increased the weightiness of Fermi’s pronouncements.

  Compton was utterly convinced of the need to go forward by his own estimates and by all the arguments he had heard. The report he wrote urged undertaking the U-235 project at maximum speed. It did not mention the work on plutonium or on the pile, but Compton was already persuaded that it too was worth pursuing. He delivered the committee’s report to Bush on November 7, 1941. From that point onward, things moved quickly. The sleeping giant was finally stirring.

  On November 27, with MAUD’s findings officially transmitted to America and the NAS committee’s report in hand, Bush went to see the president to obtain the go-ahead on the nuclear fission project. Roosevelt quickly agreed, endorsing full exchange with Britain on technical matters. The president did, however, insist that policy matters regarding the use of the project’s results be restricted to a five-person group: Bush and Conant, the vice president, the secretary of war, and the army chief of staff.

  Having received presidential approval and assurance of funding, Bush set about finding the best way to produce weapons based on nuclear fission. He had already begun to lay the groundwork for this project by creating yet another organization that he would head: the Office of Scientific Research and Development (OSRD). It had what the NDRC lacked, the authority to plan and develop large engineering projects. The atom bomb would eventually become by far the largest such venture the OSRD undertook.

  On December 6, only nine days after his meeting with President Roosevelt, Bush summoned the uranium research leaders to Washington to tell them how he planned to organize the building of a fission bomb. Conant was appointed head of OSRD’s Section One (S-1), which included key leaders of groups attempting to separate U-235 from U-238. Compton was placed in charge of bomb development. Briggs joined S-1 and the Committee on Uranium ceased to exist.

  After the meeting, during the course of a lunch with Bush and Conant at Washington’s Cosmos Club, Compton pressed them about a topic not mentioned in the meeting: the building of a plutonium bomb. He had already asked Glenn Seaborg to come to Chicago to brief him about the feasibility of chemically extracting plutonium produced in a pile. Seaborg had been confident it could be done. Conant, a chemist, thought Seaborg was overconfident. And Bush, the engineer, was skeptical about being able to overcome the engineering problems that would surface in trying to create large self-sustaining chain reactions. Compton was nevertheless given the authority to proceed on the plutonium project.

  Everything changed radically the next day. The Japanese attacked Pearl Harbor on December 7, which President Roosevelt would call “a day which will live in infamy.” On December 8, the United States Congress declared war on Japan. On the ninth, Germany and Italy declared war on the United States. America thus abruptly came to terms with its vulnerability. In a matter of minutes most of the Pacific fleet and the American air defense force stationed in Hawaii had been
destroyed, and more than two thousand Americans killed. It had come as a complete surprise, with no warning. The sleeping giant had been rudely awakened.

  This surfaced fears in the American scientific enterprise of a German atom bomb, once the nightmare of only a few refugee physicists. Pearl Harbor would pale by comparison to the damage such a bomb would wreak. There could be no option other than a radical acceleration of the fission research program. It must have one aim in mind: a bomb. Bush and Conant apprised Compton that he had two weeks to formulate a research schedule leading to its development.

  An exhausted Compton met with them ten days later, on December 18. The timetable he proposed was that the United States should produce a chain reaction by the beginning of October 1942, a pilot plant for plutonium production by the end of 1943, and enough plutonium for one or more bombs by the end of 1944. The tasks were huge and the schedule breathtaking. Thanks to the potent and persuasive threesome of Compton, Bush, and Conant, America was poised to proceed. Within a space of a year, top-level governmental committees had been appointed, merged, and expanded in their responsibilities. They laid the infrastructure of an action agenda.

  In that same December 18 meeting, Compton suggested combining fission research being conducted at Columbia, Princeton, Chicago, and Berkeley under a single umbrella. The others agreed to the plan. In January, Compton met with leaders from the four university groups. He told them that with the four scattered across the United States, it would be too complicated to coordinate efforts and to enforce secrecy. Accordingly he asked key academic scientists to meet with him in Chicago to select a single site.

  On the agreed-upon day, Compton, sick with the flu, held the meeting—in his bedroom rather than his office. One of the most strategic decisions of America’s nuclear future was being reached bedside, amid throat lozenges. His illness did not deter Compton from being autocratic when needed; he announced Chicago as the selected site. It would be best: the university had agreed to support the mission, and Chicago’s central location made it easily accessible from both the East and West Coasts. The cover name for the center was the Metallurgical Laboratory, or more simply Met Lab.