Did a publication ban really slow the race to build an A-bomb, as Szilard had predicted? We now know how that race ended, but as Szilard feared, at its start the French disclosures did prompt several German actions when little private research and no government work were under way in America.
When Hahn and Strassmann bombarded uranium with neutrons in December 1938 in Berlin, they were unaware that the atom had split or “fissioned.” That word was coined by Otto Frisch, who, working with Lise Meitner, repeated the Berlin experiment and alerted Niels Bohr as he sailed for New York at year’s end. In early January, Hahn and Strassmann published their results under the misleading title “Concerning the Existence of Alkaline Earth Metals Resulting from Neutron Irradiation of Uranium,” because at the time, they were fascinated by the new elements they created, not by the extra neutrons they released. So when Joliot and his Paris colleagues reported in Nature that March on “Liberation of Neutrons in the Nuclear Explosion of Uranium,” their prestige highlighted the Hahn-Strassmann experiment.
Had all publications ceased then, as Szilard pleaded, the atom’s military potential may still have been overlooked for several critical months. But once Joliot’s “Number of Neutrons Liberated in the Nuclear Fission of Uranium” appeared in Nature in April, several governments outside the United States took direct actions.57
At London’s Imperial College, Nobel physicist George P. Thomson saw in Joliot’s second article a probable new power source, perhaps recalling his 1934 talks with Szilard. He thought a nuclear explosion unlikely, but with W. Lawrence Bragg, head of the Cavendish Laboratory since Rutherford’s death in 1937, Thomson quickly alerted the British government. Four days after the Nature article appeared, on April 26, Britain’s Ministry for the Co-ordination of Defence urged the Treasury and the Foreign Office to buy as much Belgian uranium as they could.
That spring, physicist Wilhelm Hanle described a “uranium burner” to a university physics colloquium in Göttingen, Germany. His superior wrote the Ministry of Education, which quickly appointed Abraham Esau to convene a conference on uranium. President of the German Bureau of Standards, Esau led an April meeting that urged buying all the uranium in Germany, banning exports, and negotiating radium contracts with the recently captured uranium mines at Jáchymov, Czechoslovakia.58
From Hamburg, physical chemists Paul Harteck and Wilhelm Groth alerted the War Office in Berlin to “the newest development in nuclear physics,” which “will probably make it possible to produce an explosive many orders of magnitude more powerful than conventional ones. . . . That country which first makes use of it has an unsurpassable advantage over the others.”59 By summer, Siegfried Flügge at the Kaiser Wilhelm Institute in Berlin concluded that uranium fission might create an “exceedingly violent explosion.”60 With Kurt Diebner, an army physicist and ordnance expert, heading up Germany’s uranium project at an office for nuclear research within the Army Ordnance Department, Germany was the first country with a military unit to study how nuclear fission might be used to make weapons.
In September 1939, Diebner was studying the rare isotope uranium235 as the likely source of fission, and physicist Werner Heisenberg described how the metal might be made to explode. By month’s end a handful of scientists working under Diebner and physicist Erich Bagge were studying two separate problems: how heavy water, which contains a double form of hydrogen, would act as a “moderator” to slow neutrons; and how uranium isotopes could be most efficiently separated.
In Russia during the spring of 1939, physicist Igor Tamm asked his students: “Do you know what this new discovery means? It means “a bomb can be built that will destroy a city out to a radius of maybe ten kilometers.”61 The French, too, realized where their research was leading. “Crude jokes were made,” Kowarski later recalled, “about whether we will get the Nobel Prize for physics or for peace first because we made a war impossible by discovering nuclear explosives, which obviously would make war impossible.”62
But in New York, Szilard feared just the opposite: that nuclear explosives would make war more possible and that Germany would make those explosives first. Niels Bohr said as much at the spring meeting of the American Physical Society when he surmised that a chain reaction or an atomic explosion could be created by bombarding a small amount of uranium235 with neutrons. If uranium235 could be separated and chain reactions begun, the New York Times reported in a science column, “the creation of a nuclear explosion which would wreck an area as large as New York City would be comparatively easy.” The Washington Post on April 29 headlined an account of Bohr’s speech “Physicists Here Debate Whether Experiments Will Blow Up 2 Miles of the Landscape.”63 Times science writer William L. Laurence called uranium235 the “philosopher’s stone” that would tap “the vast stores of atomic energy” and predicted that a tiny amount was enough to “blow a hole in the earth 100 miles in diameter. It would wipe out the entire City of New York, leaving a deep crater half way to Philadelphia and a third of the way to Albany and out to Long Island as far as Patchogue.”64
CHAPTER 14
“I Haven’t Thought of That at All”
1939
In the United States during the summer of 1939, nuclear research ceased almost everywhere but in Leo Szilard’s busy mind. Enrico Fermi went off to the University of Michigan at Ann Arbor to attend a theoretical physics conference and study cosmic rays, leaving Szilard in New York. “I still had no position at Columbia,” Szilard remembered, his three-month appointment as a guest researcher ended, “but there were no experiments going on anyway and all I had to do was to think.” Yet what Szilard thought about and shared with Fermi in four letters that summer became the basis for the world’s first successful chain reaction.
His summer at Columbia was an auspicious time for Szilard because by that spring he and Fermi had a clear understanding of what would not produce a nuclear chain reaction. Szilard had persuaded Fermi to begin a large-scale experiment with 500 pounds of uranium, and borrowed this amount from commercial suppliers. For the decisive experiment Szilard and Fermi again teamed up with Herbert Anderson to test if a chain reaction might begin when the neutrons escaping from uranium fission were slowed down, or “moderated,” by water. The physicists believed that if the neutrons moved slowly, they might have a better chance of hitting and splitting other uranium atoms. Working for several days around a circular pool of water, the three concluded that the uranium emitted more neutrons than it absorbed—a necessary step to start a chain reaction. But they could not make their uranium-water system create fission that was self-sustaining. Only later did they learn that many of the extra neutrons were not only moderated by the water but also absorbed by it, choking off the chain-reaction process.1
Working from his room at the King’s Crown Hotel, Szilard sent Fermi a draft on their inconclusive water-moderated uranium work, which they submitted to the Physical Review. If water were not an effective neutron moderator, what would be? “I started to think about the possibility of using perhaps graphite instead of water,” Szilard recalled. “This brought us to the end of June.”2
The Szilard-Fermi correspondence over the next two weeks captures both the understandings and the tensions between them. After receiving the draft article, Fermi replied on June 26 with clarifying language, making his first admission that “if it will prove possible to slow down the neutrons . . . a chain reaction will probably be maintained.”3
Fermi wrote Szilard on July 1. The cyclotron at Ann Arbor was out of order, he said, but when it was fixed, he wanted to repeat their last experiment on neutron absorption “because the results that we got seem to me rather crazy,”4 Writing to Fermi about “the trend of my ideas concerning chain reactions” on July 3, Szilard said, “It seems to me now that there is a good chance that carbon might be an excellent element to use [as a moderator] in place of hydrogen, and there is a strong temptation to gamble on this chance.” Fermi, a cautious experimentalist, must have bristled when he read this and what followed: �
��I personally would be in favor of trying a large-scale experiment with a carbon-uranium-oxide mixture if we can get hold of the material,” wrote Szilard. “I intend to plunge in the meantime into an experiment designed for measuring small capture cross sections for thermal [or slowed] neutrons,” in other words seeking a material that would absorb the fewest neutrons. The higher an element’s “cross section,” the more neutrons it would absorb.
Already thinking beyond his own plans, Szilard speculated that “if carbon should fail, our next best guess might be heavy water. . . .” Ordinary, or “light,” water, H20, has two parts hydrogen for each part oxygen. But hydrogen has a heavier isotope as well. Light water is composed of two lighter isotopes, H. Heavy water is composed of the heavier isotope, deuterium, whose symbol is D and is written D2O. Deuterium, they thought, would capture fewer neutrons than hydrogen. Already Szilard had “taken steps” to “obtain a few tons of heavy water.”5 That day, Szilard also wrote to Strauss about his work with Fermi and Anderson, concluding: “There is also a fifty-fifty chance that the matter may be of great importance from the point of view of national defense.”6
Two days later, Szilard wrote to Fermi again, sending a corrected value for his neutron-density calculations. His mind swung between algebraic calculations and practical business. Now “it seems that it will be possible to get sufficiently pure carbon at a reasonable price,” Szilard reported. A chain reaction might be created “if layers of uranium oxide” are arranged between carbon layers. Still probing all possibilities, Szilard concluded: “Pending reliable information about carbon, we ought perhaps to consider heavy water as the ‘favorite,’ and I shall let you know as soon as I can how many tons could be obtained within reasonable time.”7 The same day he wrote this, Szilard visited the National Carbon Company offices on East Forty-second Street, seeking a sixteen-inch-square graphite block.8
In his third letter to Fermi, written on July 8, Szilard urged starting a “large-scale” experiment “right away,” even before learning how well carbon absorbs neutrons. Szilard’s style was impatient and irrepressible.
Sorry to bombard you with so many letters about carbon. This is just to tell you that I have reached the conclusion that it would be the wisest policy to start a large-scale experiment with carbon right away without waiting for the outcome of the [neutron] absorption measurement which was discussed in my last two letters. The two experiments might be done simultaneously. The following can be said in favor of this procedure:
A chain reaction with carbon is so much more convenient and so much more important from the point of view of applications than a chain reaction with heavy water or helium that we must know in the shortest possible time whether we can make it go.
Szilard urged experimenting with “perhaps fifty tons of carbon and five tons of uranium . . . as a start.” The carbon and uranium should be “built up in layers” or stacked “in some canned form,” making assembly and cleanup relatively easy. Then Szilard reported telling Pegram about this plan, “and he seemed to be not unwilling to take the necessary action. I wonder whether you think it wise to proceed as outlined in this letter.”9
Szilard thought that Fermi’s reply on July 9 seemed to be a debate with himself. Fermi proposed a “homogeneous” sixty-to-one carbon-to-uranium mixture, in effect blending everything in a pot. This only convinced Szilard that Fermi was not serious about a chain-reaction experiment “because it was the easiest to compute.”10 In a letter to Anderson more than a week later Fermi confessed to “not having understood” Szilard’s proposal. “I think that the experiment is very important and should be performed.”11
On his own, Szilard proposed a joint experiment between Columbia’s physics department and his own Association for Scientific Collaboration12 and visited the National Carbon Company to ask about the purity of commercial graphite.13 “I have decided to prepare an experiment with all kinds of different materials . . . .” he reported to his friend Trude Weiss. “If things work out, I will be very busy.”14 But a disappointment followed within days when the navy, citing “restrictions” on government contracts, declined to support Szilard’s ideas for nuclear research.15
Writing to Fermi “in a hurry” on July 11, Szilard was by now sure that he knew just how to make a chain reaction. “During the second week of July,” Szilard later recalled, “I saw that by using a lattice of uranium spheres embedded in graphite, one would have a great advantage over using alternate layers of uranium and carbon.” This, he said, convinced him “from then on that there is a good chance of maintaining a chain reaction in a uranium-graphite system.”16
In fact, for nearly a year after the Anderson-Fermi-Szilard experiment with uranium and water, in the spring of 1939, there was almost no further work in the United States toward understanding fission or investigating the possibility of a nuclear chain reaction. University researchers were not accustomed to seeking federal support and scarcely tried, and the government scientists, though interested, failed to recognize the military significance of fission. It all just seemed too farfetched.
Not to Szilard, however, who still pursued private investors for his chain-reaction experiments. In the name of his Association for Scientific Collaboration, he met that month with a group of financiers in a suite at the Waldorf-Astoria Hotel, bringing along Bela “to restrain me when I begin to sound unrealistic.” But Bela could scarcely say a word as Leo outlined to the skeptical money men a grand scheme that would bring electrical energy to poor nations yet cost “practically nothing.” The machines to generate this power Szilard called “piles.” Szilard urged the businessmen and bankers to invest their millions in his association but also insisted that a majority of shares be controlled by the physicists. Understandably, no one in the suite reached for a checkbook. “The physicists are too naive in practical matters, and the bankers are too businesslike,” he complained to Trude. “I do not know how things will work out.”17
In Germany, meanwhile, things were working out just as Szilard had feared. That summer, Siegfried Flügge at the Kaiser Wilhelm Institute in Berlin published “Can Nuclear Energy Be Utilized for Practical Purposes?” He, too, assumed “the energy liberation should thus assume the form of an exceedingly violent explosion.”18
When Eugene Wigner came to New York from Princeton in early July, Szilard showered him with the calculations he had made for the carbon-uranium lattice. Wigner was quick to see that this might work. They were also quick to link this approach—the closest yet to a workable chain reaction—with recent news from Europe that German military expansion could easily overrun Belgium, whose colony in the Congo was then the world’s principal uranium source.
Wigner wanted to alert the Belgian government and suggested they seek advice from their former professor in Berlin, Albert Einstein.19 Wigner occasionally saw Einstein around the Princeton campus; to Szilard, who had worked closely with him in the 1920s and early 1930s, Einstein had reverted from colleague and counselor to a famous but remote scientist. Einstein knew the Belgian monarchs well—in his unpretentious way calling her “Queen” and addressing the royal couple as “the Kings”—so perhaps, Szilard suggested, he might alert the queen of the Belgians about the perilous importance of the Congo’s uranium. The two agreed that it was worth a try, and from his Princeton office they learned that Einstein was then at a cottage in Peconic, Long Island, owned by a friend named Dr. Moore.
Early on the morning of Wednesday, July 12, a clear and hot day, Wigner drove up to the King’s Crown Hotel in his 1936 Dodge coupe, and Szilard climbed in. The two drove out of New York across the new Triborough Bridge, passing the New York World’s Fair, whose theme “Building the World of Tomorrow” was symbolized by a 700-foot-high trylon, a tapered column rising to a point, representing “the finite,” and a 200-foot perisphere globe, representing “the infinite.”
Had Szilard and Wigner thought about it, their own drive that day had more to do with the “World of Tomorrow” than anything they passed on the fairgroun
ds. But their thoughts were fixed on finding Einstein’s cottage, a task demanding all their attention. First the two Hungarians confused the Indian names in their directions and drove to Patchogue, on Long Island’s south shore, instead of to Cutchogue, on the north. This detour cost them two hours, and once in Peconic, they drove around the tiny resort town asking vacationers in shorts and bathing suits the way to Dr. Moore’s cottage. No one seemed to know.
“Let’s give it up and go home,” Szilard said impatiently. “Perhaps fate intended it. We should probably be making a frightful mistake by enlisting Einstein’s help in applying to any public authorities in a matter like this. Once a government gets hold of something, it never lets go. . . .”
“But it’s our duty to take this step” Wigner insisted, and he continued to drive slowly along the village’s winding roads.
“How would it be if we simply asked where around here Einstein lives?” Szilard said. “After all, every child knows him.” A sunburned boy of about seven was standing at a corner toying with a fishing rod when Szilard leaned out the car window and asked, “Do you know where Einstein lives?”
“Of course I do,” said the lad, and he pointed the way.20
Szilard and Wigner were hot, tired, and impatient by the time they found the two-story white cottage. By contrast, the sixty-year-old Einstein was relaxed and genial; he had spent the early morning sailing in a small dinghy and now greeted his former colleagues wearing a white undershirt and rolled-up white trousers. Einstein bowed courteously as they met and led his visitors through the house to a cool screened porch that overlooked a lawn. There, speaking in German and sipping iced tea, Szilard and Wigner told Einstein about their recent calculations. They explained how neutrons behave, how uranium bombarded by neutrons can split or “fission,” and how this process might create nuclear chain reactions and nuclear bombs.
Genius in the Shadows Page 28