by Gino Segrè
But he continued to be bothered that his chain reaction idea seemed frozen. In January 1939, having heard that Wigner, by then a professor at Princeton University, was in the school’s infirmary with a case of jaundice, Szilard decided to cheer him up by paying a visit. As soon as he entered the infirmary room, Wigner told him of reports about fission circulating in the Princeton physics department. Szilard found the news electrifying. Less than a month earlier he had written a letter to the British Admiralty suggesting that his chain reaction patent be withdrawn since nothing was moving. He promptly cabled them to disregard his recent letter.
Having caught the flu in Princeton, Szilard did not return to New York City until the end of January. By then the news was out: the meetings at George Washington University, convened by Gamow, had spread the word about fission. He also heard that Fermi was set on creating a chain reaction.
With his political sensitivities aroused, Szilard rushed to meet with the Polish-born Columbia physicist Isidor Rabi. Imploring him that anything to do with a chain reaction should be kept secret, Szilard recounted his fears that Nazi Germany would decipher its implications. The two sought out Fermi to share Szilard’s apprehensions. Fermi was consistent: his inner calm was unruffled. He had no qualms about publishing results since he thought the chance of obtaining a chain reaction was remote. When Rabi asked him how remote, Fermi replied ten percent. Although previously not so, Rabi was now alarmed. He retorted, “Ten percent is not a remote possibility if it means that we may die from it.”
The schism between Fermi and Szilard on how to treat information regarding a chain reaction was instantly exposed. It continued along the same lines and on many fronts in coming years. As Szilard recounts in his memoir, the men were both of a conservative bent, but had opposite views on how to proceed: “Fermi thought that the conservative thing was to play down the possibility that this may happen and I thought that the conservative thing was to assume that it would happen and take all the necessary precautions.”
Unable to settle their disagreement, Fermi and Szilard resolved to proceed independently for the time being and to keep each other abreast of any progress toward obtaining a chain reaction. Szilard nevertheless continued to try to convince Fermi and others in the United States not to publish any neutron research. Nothing should fall into the hands of the Third Reich.
Tensions were somewhat defused when Fermi and Szilard heard it was going to be harder to make a bomb with uranium than they had reckoned. That opinion had been reached by Bohr and articulated in answering a question posed to him by a thirty-three-year-old Czech theoretical physicist. Georges Placzek, who arrived in New York on Saturday, February 4, was on his way to a faculty position at Cornell University. He stopped in Princeton to speak with Bohr, with whom he had worked for years in Copenhagen. An expert in neutrons, Placzek was completely up-to-date. In fact, initially critical of the fission conjecture, he had been the one to suggest that Frisch look for the pulses within an ionization chamber.
Placzek was confused by a distinction between uranium and thorium that Frisch had very recently told him about. Both nuclei were similar in their response to neutron bombardment, but if a layer of paraffin was used to slow down the neutrons, only uranium underwent fission. After reflecting for a short while, Bohr was startled to realize the probable reason for the discrepancy. Ordinary uranium, like thorium, undergoes fission if the bombarding neutrons have energies greater than a million or so electron volts. But if the neutrons are slow, the only nuclei that undergo fission are those of U-235, the comparatively rare uranium isotope with 133 rather than the conventional 136 neutrons in its nucleus.
Bohr conjectured that a weapon built largely out of U-235 was likely to work and be extremely potent. However, since U-235 is only 0.7 percent of ordinary uranium ore, separating it out would pose an impossible task. Or at least, so he thought. The two uranium isotopes are chemically identical, as are any two isotopes of the same element, so no chemical procedure could distinguish and then separate them. As Bohr commented, obtaining enough U-235 for a bomb “can never be done unless you turn the United States into one huge factory.”
What Bohr could not foresee was that within a few years not the whole United States, but a significant fraction of it, would be converted into “one huge factory.” By the end of World War II more than a hundred thousand Americans had been employed in a secret project to build nuclear weapons.
22
THE RACE BEGINS
On the same day, March 16, 1939, that Germany invaded Czechoslovakia, Eugene Wigner went from Princeton to Columbia to meet with Fermi, Leo Szilard, and George Pegram. His mission was to urge them to deliver a warning to the U.S. government about the Nazis obtaining nuclear weapons. Trying to think of the best way to proceed, Pegram decided to call a friend who was an undersecretary of the navy. An appointment with navy staff was arranged for the next day in the office of Admiral Stanford Hooper, technical assistant to the Chief of Naval Operations.
Since Fermi was scheduled to be in Washington for other matters, the other three decided he was the optimal person to make the presentation. His expertise was impressive and just having won the Nobel Prize would be a definite asset. Fermi agreed to the task but was skeptical about its success. His premonition was deepened by overhearing how the desk officer announced him to the admiral: “There’s a wop outside.” Fermi’s accent and his appearance, sometimes described as swarthy, had labeled him.
Rather than dramatically conveying the German threat, as Szilard or Wigner doubtlessly would have done, Fermi was his low-key self. Without embellishment, he laid the facts before the navy officials. In retrospect, the group may have made a mistake in choosing Fermi since he tended to downplay dangers. Those present at the briefing apparently concluded that there was no cause for alarm. The meeting was treated as a courtesy call; no further action would be required.
Neither did Fermi see the need to act further. He was interested in the challenge of producing a chain reaction and not eager to deal with the extra complications of political or military involvement. Fermi only wished to proceed with experimenting. He was also happy to see that the experiments he and Anderson were planning to conduct were similar to the slow neutron ones he and the Boys had performed a few years earlier in Via Panisperna. Fermi was optimistic about their outcome; no extra retooling would be needed.
In their initial effort, Fermi and Anderson suspended a small spherical bulb in the middle of a large water-filled tub. The bulb contained a source of neutrons generated, as in Rome, by a mixture of radon and beryllium. The water was used to slow down the neutrons. Strips of a dependable neutron detector were placed at various distances from the bulb to see if the neutron count went up after uranium oxide was introduced. They soon detected a rise, exactly what was expected. Uranium’s presence in the water had led to the creation of extra neutrons.
A possible caveat to their results called for redoing the experiment with a lower-energy neutron source such as radium. There was no Divina Provvidenza at Columbia to go to for radium, as there had been in Rome, but Szilard saved the day. The enterprising Hungarian excelled at cultivating rich individuals who might finance his schemes. One such Maecenas lent him the funds needed to rent a gram of radium for three months.
With mutual dependency, Fermi and Szilard—an unlikely pair—joined forces. The radium Szilard rented and some uranium oxide he managed to borrow allowed him and Fermi to conduct a larger experiment. Theirs was not an ideal match. Fermi insisted on carrying out his share or more of the physical labor. Szilard disdained such efforts, hiring somebody to perform those tasks. Given their dissimilarities, it was predictable that the two directly collaborated only once.
The results of the experiment spurred Fermi and Szilard on to an expanded version of their present setup. The hopes of producing a chain reaction were curbed by George Placzek, who had led Bohr to conclude that only the rare U-235 underwent fission by slow neutrons. Placzek had also spent a year doing researc
h in Via Panisperna during the early 1930s and was eager to see Fermi again. Taking the train from Cornell, he described his current research to Fermi, who—in turn—showed him plans for the larger experiment he and Szilard were thinking of. Placzek told them it would not work. Water, with its hydrogen content (H2O), was necessary to slow neutrons down, but it also absorbed them. They couldn’t afford to have this happen if they wanted to obtain a chain reaction. Fermi and Szilard were at an impasse.
The lighter the element, the more effectively its nucleus slows neutrons. Since using hydrogen had proved unworkable, Fermi and Szilard considered the next elements on the periodic table. Helium was impractical and beryllium was dismissed as dangerous to use in large quantities. Lithium and boron were known to be strong neutron absorbers, so they were not candidates. Carbon came next. There were unknowns about how it interacted with neutrons but it might work, especially in the form of graphite, the smooth black substance running through a lead pencil.
It was going to be expensive to purchase the required amount of graphite, perhaps tens of thousands of dollars. Fermi and Szilard thought it was worth the effort but lacked the funds to do so. This time Szilard did not have a rich patron. Regrettably, their experimental program came to a standstill at the beginning of the summer of 1939.
Research on obtaining a chain reaction had come to a halt in the United States amid lingering fears that Germany was making strides toward producing an atomic bomb. There was no verification of German activity, one way or another, but the prospect was chilling. In France, Frédéric Joliot and his associates had independently performed the same fission experiment as Fermi and Szilard and submitted a note on their results to Nature. The race was public and the stakes were high.
Szilard was having no success in convincing American, French, and English scientists to withhold publication of fission-related research. Bohr, like Fermi, was insisting that secrecy should never be introduced in physics. The passivity of the United States was what concerned Szilard the most. He was stupefied that “between the end of June 1939 and the spring of 1940, not a single experiment was under way in the United States which was aimed at exploring the possibilities of a chain reaction in natural uranium.”
Meanwhile, during that summer of 1939, Fermi—accompanied by Laura, Nella, and Giulio—left New York to teach at the University of Michigan Summer School. Since his experimental program was stalled, he had decided to go back to being a theoretical physicist. This was neither the first nor the last time in his career he would make the switch.
Edoardo Amaldi was in Michigan as well, officially having come to the United States to study American facilities with the intent of building a cyclotron in Rome. Unofficially he had been looking for a position in America. The move would prove impossible even if he did find a position; his wife’s request for a passport had been denied. Italy was now limiting exits. A dispirited Amaldi went back to Italy in early October 1939 on an almost empty steamship, knowing that he would be the only one of the Boys still there. He was feeling more anxious about the future than ever before.
Franco Rasetti had happened to sail on the same boat to America as Amaldi, but his destination was Canada. Disgusted by Mussolini, he managed in 1939 to secure a professorship at Laval University in Quebec and departed from Naples with his mother on July 2. The voyage gave the two physicists a chance to talk. Unlike Amaldi, Rasetti decided he was no longer interested in fission. When asked in a 1982 interview why he had left the field and turned to cosmic rays, his reply was simple: “Because cosmic rays are free and everywhere.” Rasetti could pursue his quest in Canada, whereas Amaldi had to return to Italy’s oppressive atmosphere.
Obtaining a position in the United States would not have been a problem for Werner Heisenberg, also a lecturer that summer in Michigan—both Chicago and Columbia were wooing him—but he was adamant about wanting to remain in Germany. In small and informal gatherings, Enrico, Laura, and many others pressed him, emphasizing the Nazi regime’s ruthlessness and the anti-Semitism that had driven away so many of his colleagues. Heisenberg would not budge from his conviction that he had to go back to Germany.
In Heisenberg’s memoir, he recounts an argument Fermi made to him for staying in America. It is particularly interesting for what it says about why Fermi liked the United States so much. What Fermi apparently told Heisenberg was “This is a big and free country where you can live without being crushed by the weight of history. In Italy I was a great man: here I have gone back to being a young physicist, which is infinitely more exciting. Throw away the ballast of the past and begin again.”
In spite of his strong Italian roots, Fermi had never been inextricably wedded to his native country. As early as 1922, Fermi had stated that he was open to emigrating. When his disbelieving sister had asked, “Where to?,” Fermi shrugged his shoulders and answered, “Somewhere … the world is large.” Heisenberg was another matter. He had been a fervent adherent of the German youth movement, and his nationalism was deeply implanted. He was not seriously tempted to stay in the United States. His country needed him. His decision created scars and an abiding divide in the international scientific community.
Less than a month after Heisenberg’s return to Germany in August 1939, World War II began. Whether this complicated man actually wanted his country to win World War II or intentionally dragged his feet on the Third Reich war effort are questions that have never been resolved. Heisenberg was ambivalent about the atomic bomb, as were many scientists, including some who were working on it, but the tone he adopted in writing about the Allies’ use of it seemed disingenuous if not offensive. Heisenberg wrote that he “found it psychologically implausible that scientists whom I knew so well should have thrown their full weight behind such a project.”
Parts of Heisenberg’s book Physics and Beyond border on a rewriting of history. According to Heisenberg, while in Michigan, Fermi had asked him, “Don’t you think it possible that Hitler may win the war?” It is dubious that Heisenberg replied—as reported in his book—that it was not possible for Hitler to triumph, adding that “Hitler is irrational and simply shuts his eyes to anything he does not want to see.” Heisenberg was surely under surveillance, and his comments would have exposed him to tremendous peril.
One outcome is clear, though perhaps it would have been so even if Heisenberg had chosen another path. His contribution to physics after World War II was minimal. By contrast, Fermi’s influence and contributions as both a researcher and teacher were immense. He thrived by “being a young physicist.”
While Fermi focused on theoretical physics in Michigan, Szilard was redoubling his political activities. Along with Edward Teller and Eugene Wigner, Szilard sought ways of lessening the danger of Germany’s developing a nuclear weapon. All three Hungarians agreed that limiting Germany’s access to world supplies of uranium would be a strategic first step. There was little that could be done about the Czechoslovakian mines, already in German hands, but perhaps Belgium could be dissuaded from selling Hitler anything from its extensive uranium deposits in the Congo. Szilard remembered that Einstein was a close friend of Belgium’s queen; he had taken refuge there between leaving Germany and settling in Princeton. If Einstein asked her, maybe a ban on uranium sales would ensue.
Wigner and Szilard went on the sixteenth of July to inform Einstein about the recent physics findings and to ask if he would compose a letter to the queen. Learning of the possibility of a chain reaction, Einstein famously said, “Daran habe ich gar nicht gedacht” (“I hadn’t thought of that at all”). However, Einstein felt it would be more appropriate to go through diplomatic channels than to write directly to the queen.
That scheme changed almost instantly as a result of a meeting Szilard had with one of his influential friends. Alexander Sachs had worked on Franklin Roosevelt’s 1932 presidential campaign and had been a member of the National Recovery Administration before joining the Lehman Brothers investment firm in 1936. When Szilard described the situation to him, Sa
chs suggested that President Roosevelt be informed posthaste. If they obtained a letter from Einstein, Sachs would make sure it reached the president.
Szilard went back to Einstein, accompanied by Teller. With Einstein agreeing on the new plan, Szilard drafted a letter for him to be transmitted to Sachs. Dated August 2, 1939, the letter warns:
In the course of the last four months it has been made probable—through the work of Joliot in France as well as Fermi and Szilard in America—that it may become possible to set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power and large quantities of new radium-like elements would be generated. Now it appears almost certain that this could be achieved in the immediate future. This new phenomenon would also lead to the construction of bombs and it is conceivable—though much less certain—that extremely powerful bombs of a new type may thus be constructed.
Drafts of the letter had been written by Szilard and then rewritten by Einstein in German, his native tongue. The revisions went back and forth. When Szilard dictated the final version of Einstein’s letter into English, an innocent secretary at Columbia concluded Szilard was “a nut.” A letter from a Hungarian pretending to be Albert Einstein, writing to the president and talking about bombs? It stretched credibility and sanity.
The letter called for quick action. Its main recommendations were to secure uranium deposits, provide funding for research related to the use of uranium, and maintain a liaison between the physicists involved in this research and the government. The letter was delivered to Sachs on the fifteenth of August, but it was not until the eleventh of October that Sachs got to see the president. Fortunately Roosevelt’s reaction was not the same as the incredulous secretary’s. The president retorted, “Alex, what you are after is to see that the Nazis don’t blow us up.” Sachs replied, “Precisely.”
