CHAPTER NINETEEN

ON A MESA

LAURA AND THE CHILDREN WERE THE FIRST FERMIS TO ARRIVE ON the dusty, high-security mesa that was quickly becoming the focus of the Manhattan Project. Her husband was still shuttling between Chicago, Oak Ridge, and Hanford and would not arrive until early September 1944. In midsummer 1944, she took the train, as instructed, from Chicago to Lamy, New Mexico, a town fifteen miles south of Santa Fe. Like all those destined for Los Alamos, she was ignorant of her final destination. Arriving at Lamy, she almost missed her ride into town. A young Army officer was eagerly looking for “Mrs. Farmer.” Laura was not aware of her husband’s code name and at first did not respond. After a few moments of thought, though, she asked if he was looking for “Mrs. Fermi.” The young man looked her over, figured she was indeed the person he was looking for, and brought Laura and the children to 109 East Palace Road in Santa Fe, where the cheerful Dorothy McKibbin dutifully checked her in and gave her and the children ID cards.

From Santa Fe, she was driven along the twenty miles of dirt road, narrow with frequent switchbacks, to the top of the mesa. She settled herself and the children into a modest apartment, on the second floor of a barracks built to Army specifications for standard issue housing. The Fermis could have insisted on more spacious housing along “Bathtub Row,” a street with private houses that was built for project VIPs, where Oppenheimer and his wife Kitty lived, next to Berkeley physicist Edwin McMillan and his wife, Elsie. Those houses were equipped with bathtubs rather than the showers that prevailed in more standard accommodations, hence the name of the street. In typically modest fashion, the Fermis decided not to pull rank and lived where the Army assigned them. Their downstairs neighbors were German theorist Rudolf Peierls and his wife Eugenia. Their old friends the Segrès were also nearby, having moved from Berkeley not long before.

This was the third major upheaval in five years for the Fermi family: first to New York, then to Chicago, and now to Los Alamos. Of the three, the move to Los Alamos was the most dramatic and the most disorienting for the upper-class woman from Rome and her children. They were enclosed in a compound where the highest security procedures prevailed, where the simple act of going into town (Santa Fe, in this case) to buy provisions was difficult and sometimes impossible, and where they knew almost nothing of what was going on around them. The children were not allowed to wander off-site and attended a small school on the grounds. There, Nella and Giulio joined other children of scientists and engineers in elementary school studies. Nella recalls these days as a great adventure; she and the Peierls’ daughter Gaby would sneak out of the compound and then check back in at the main gate, causing considerable consternation. For Nella, it was all very exciting to be set down in the middle of the New Mexican wilderness, with all sorts of important things going on, none of which she understood.

Certain aspects made life at Los Alamos bearable for Laura and the children. Familiar furnishings from the Leonia house, left behind on the assumption that the Fermis’ move to Chicago would be temporary, now arrived to fill the apartment and make it feel more like home. Many of the families on the mesa were of European origin and were old friends of the Fermis. It must have been pleasing to see the Bethes, and meeting up with the Segrès certainly reminded her of happier days in Rome. There were many others as well. Thrust together in the most extraordinary circumstances, they bonded and supported each other. Laura eventually found work helping the doctor in the Tech Area, the most sensitive section of the facility, where work on the bomb itself was being conducted. She was one of the first people to learn firsthand about the dangers of radiation poisoning. She socialized actively with other wives. Being one of the older women in the group, she was a bit of a mother hen to younger wives whose husbands—straight out of undergraduate school, in many cases—were drafted into the project.

The call for the Fermis to relocate to Los Alamos was perhaps inevitable, but it came specifically in response to a series of major crises in the project. In response to these crises, Groves and Oppenheimer decided to reorganize the Los Alamos project, and Fermi played a key role in that reorganization.

THE MAIN PROBLEM WAS A MATTER OF PHYSICS. THE WORK AT Los Alamos had always assumed that the “gun” method of assembly—shooting one subcritical chunk of metal, either uranium or plutonium, into another subcritical chunk at high velocity so that they would together form a critical mass—would be the most reliable way of creating a fission explosion. All the knowledge developed about U-235 suggested that the gun method would work for the uranium isotope. The initial studies of plutonium, conducted by Seaborg, Segrè, and others at Berkeley with material created in Lawrence’s cyclotron, suggested the same thing. Early on, physicists knew that “implosion”—that is, compressing a subcritical mass of either uranium or plutonium—would also achieve criticality and a fission explosion, but the challenge of actually executing an implosion was daunting.

However, as early as November 1942, doubts began to arise regarding the suitability of plutonium for use in a fission weapon. Those doubts only grew over time. Throughout the early part of 1944, Segrè had been working in a small shack located in a remote corner of the mesa, focused on analyzing the properties of the plutonium produced at Oak Ridge and Hanford. His research, which threatened to bring the entire plutonium project to a grinding halt, suggested that the plutonium produced in the heart of nuclear reactors was quite different from the plutonium produced in the cyclotron. Under the intense neutron bombardment over extended periods of time in the Oak Ridge reactor, some of the atoms of Pu-239 absorbed an additional neutron, creating the isotope Pu-240. That extra neutron threw the nucleus into turmoil, creating spontaneous fission. It was Fermi who, looking at Segrè’s data, suggested that Pu-240 was the culprit, prefissioning Segrè’s microsamples. At the power levels in the reactors at Oak Ridge and Hanford, Pu-240 could compose as much as 7 percent of the plutonium being produced. Spontaneous fission was going to be a major problem in using plutonium. As the news of Segrè’s findings was absorbed, it became clear that the only method of building a plutonium bomb would be the implosion method. A sphere of plutonium, even with 7 percent Pu-240, could be made sufficiently subcritical that spontaneous fission would not be a problem. If the subcritical sphere were then explosively compressed into a sufficiently dense sphere, it would become critical and a plutonium bomb would work.

That was, however, a pretty big “if.” In order to achieve an effective critical mass, the sphere would have to retain a perfectly spherical shape from the initiation of the implosion through to the moment of criticality. Otherwise, the fission reaction would pass through the sphere unevenly, resulting in a misfire. To maintain the plutonium in a perfect sphere throughout the implosion, the shock waves of the explosion would have to arrive across the entire surface of the subcritical sphere at exactly the same time. There was very little room for error.

To do this involved an unprecedented technical challenge. Imagine standing in a swimming pool, with one hand on a floating beach ball so that the ball is halfway submerged, the other holding a penny. When the water in the pool settles down and is perfectly calm, the penny is dropped. The penny creates a circular wave that travels on the surface of the water and strikes the beach ball. The front of the wave strikes the beach ball first, and then the rest of the wave breaks across the surface of the ball. A convex surface is striking a concave surface (or vice versa, depending on the point of view). The first moment they touch is at a single point on the surface of the ball closest to where the penny dropped. Blast waves behave like the waves created by the penny, emerging outward in circular fashion from the point of detonation. They begin to compress the subcritical sphere at the single point where the blast wave first touches the surface of the sphere, thereby deforming it almost immediately. Adding to this complexity is that the wave comes at the target sphere in three dimensions, unlike the wave in the pool. It is extremely difficult, but possible, to create simultaneous detonations all around the sphere at various equidistant points. However, the only way to make sure that the sphere retains its shape throughout the implosion is to shape the blast wave that emerges from each detonation so that it has the same shape as the sphere when it arrives there, microseconds after detonation. Scientists would need to reverse the shape of the wave and they would have to do it very quickly after the charge detonated. No one had ever done anything quite like this before. To do it at all, and with the requisite accuracy, would require an enormous amount of scientific and engineering brilliance.

The Los Alamos reorganization involved the redeployment of personnel from the plutonium gun project to the implosion project. Research into implosion, which was under the auspices of a low-priority group headed by Navy officer William S. “Deak” Parsons, was transferred to a high-priority group headed by a flamboyant, Russian-born physical chemist named George Kistiakowsky, on secondment from Harvard. As part of the shake-up Oppenheimer named Fermi associate director of Los Alamos with overall responsibility for research and theory and for all special problems related to nuclear physics. This was an honorific title that gave him little administrative authority but that allowed him to poke his nose into issues as needed and as might interest him. He was also given direct responsibility over a special division—F Division, F for Fermi—under which a variety of projects not subsumed in other divisions were grouped. These included theoretical and experimental work on the “water boiler” project and on the “Super” project.

The water boiler, a project particularly close to Fermi’s heart, was a high-intensity reactor that used powdered uranium enriched to 14 percent U-235 and mixed into ordinary water. Its location at a remote site code-named Omega in a canyon off one side of the mesa was an ideal place for Fermi to keep experimentally active, particularly when so much of his time was spent helping with other scientists’ projects. The water boiler ran at low power, but even so was sufficiently reactive, owing to the enriched uranium, that the water’s tendency to absorb neutrons could be ignored. The liquid was contained in a sphere one foot in diameter, with instrumentation surrounding it to measure neutron production and absorption, as well as control and safety features. Later configurations of the water boiler ran with increasingly enriched uranium. The water boiler was useful in the study of the critical mass of uranium. It also produced refined studies of neutron production, one of which, conducted by Fermi’s old friend Bruno Rossi, determined how quickly “prompt” neutrons emerge from fission reactions. Fermi’s Omega site team included L. D. P. King, Herb Anderson, and a young woman named Joan Hinton. King ran the project for Fermi. A Purdue-trained physicist, he worked closely with Fermi during these next few years. A graduate of Bennington and the University of Wisconsin, Hinton also became a daily colleague of Fermi at the Omega site. Segrè describes Joan as “very athletic,” perhaps because she could clamber down into the ravine that had been chosen to locate the water boiler. She was quite sympathetic to left-wing causes, and in 1948 as the revolution in China came to a close, she moved there and lived out the rest of her life under communist rule. For the time being, though, she was eager to serve as Fermi’s assistant.

The Super was the fusion bomb (hydrogen bomb) project that had preoccupied Edward Teller practically every moment since early 1942 when Fermi first suggested the possibility. To his enormous frustration, Oppenheimer could not get Teller to work on anything else, either at Berkeley or at Los Alamos. Oppenheimer judged the likelihood of a real breakthrough on the Super too low to devote significant resources to it, but he wanted to find a way to keep the creative Teller happy and gainfully occupied, so he worked directly with Teller on the project. After the reorganization, Teller would become Fermi’s problem.

A MAJOR PROBLEM PHYSICISTS HAD TO CRACK WAS THE MATTER OF critical mass. Given the extraordinary expense of producing U-235 and Pu-239, the project leadership required more than vague estimates. They required accurate calculations, based on theoretical considerations that were being explored for the first time. Fermi’s old friend Hans Bethe, the head of the Theoretical Division since the outset of Los Alamos, had been thinking deeply about this problem, as had many others, including, for example, a youngster from Queens, New York, named Richard Feynman, who had already annoyed military security with his penchant for breaking into locked safes and leaving “guess who?” notes. Bethe and company had been helped enormously by the arrival of a team from Britain, including Rudolph Peierls and his young protégé Klaus Fuchs who had been part of a parallel project, run by the British government since the spring of 1940, to explore the possibility of fission weapons.

The story of the British project, known by the code name “Tube Alloys,” is fascinating and in many ways mirrors the Manhattan Project, although it started earlier and made significant progress before the US and British governments revealed to each other what they had been working on. As part of this project, German refugees Rudolf Peierls and Otto Frisch did important theoretical calculations regarding the critical mass of uranium 235 and, although the approximately one kilogram mass they calculated was actually too small for a true critical mass, their work indicated that the problem was not intractable. Fermi’s old student Bruno Pontecorvo was also involved, working on a plutonium production reactor project run by the British at Chalk River in Ontario, Canada. Pontecorvo had several meetings with Fermi in Chicago before Fermi arrived in Los Alamos, and they discussed various aspects of reactor design.

Peierls spent time in Rome before the war and came to know Fermi well at Los Alamos. They worked together and lived one below the other in the cramped apartments built by Groves’s team. Their wives also hit it off and became fast friends. Peierls admired Fermi greatly, but he was also a subtle and observant critic. He noted that Fermi seemed deliberately to choose problems that could be radically simplified, that when he came to a stage in a problem where complex mathematics would be required to move forward, he “generally left them. He didn’t choose to go beyond that.” Peierls concedes that for Fermi “the range of things that seemed simple to him covered very many things which were complicated to all of us until he explained them,” but when a problem seemed like it would involve more work than he felt it was worth, he lost interest. This critique rings true.

By the time Peierls came to Los Alamos under the cooperation agreement between the Manhattan Project and Tube Alloys, he had in tow a younger colleague named Klaus Fuchs. Fuchs was a member of the German communist party who left Germany in 1939 and moved to England. He and Peierls worked together, and Peierls brought the younger physicist with him to Columbia, and then to Los Alamos. The two of them worked in the Theoretical Division together under Peierls’s old friend Hans Bethe. From this vantage point, Fuchs was ideally placed to pass vital intelligence to the Soviets, for whom he had begun to spy several years earlier.

The critical mass problem that preoccupied the Theoretical Division was amenable to brute force calculations. In an era prior to the ready availability of electronic computers, the most effective way of doing these calculations was to rely on slow, simple mechanical calculators, operated by teams of young women, called “computers,” who sat at their desks for eight-hour shifts of mind-numbing work. They were overseen by the exuberant young Feynman. An undergraduate at MIT before he was chosen by Oppenheimer for Los Alamos, Feynman had never before met Fermi. Feynman was mightily impressed with the Italian émigré, not because of Fermi’s reputation, which mattered little to him, but because of Fermi’s ability to interpret the results of calculations. Many years later he remembered an early encounter with Fermi:

We had a meeting with him, and I had been doing some calculations and gotten some results. The calculations were so elaborate it was very difficult. Now, usually I was the expert at this; I could always tell you what the answer was going to look like, or when I got it I could explain why. But this thing was so complicated I couldn’t explain why it was like that. So I told Fermi I was doing this problem, and I started to describe the results. He said, “Wait, before you tell me the result, let me think. It’s going to come out like this (he was right), and it’s going to come out like this because of so and so. And there’s a perfectly obvious explanation for this—” He was doing what I was supposed to be good at, ten times better. That was quite a lesson for me.

Feynman later engaged Fermi in an hour-long argument about a technical issue related to the water boiler, and when Fermi finally conceded that Feynman was right, the younger physicist regarded this as a sort of triumph.

The respect between the two physicists was mutual. As a mark of that respect, Fermi naturally felt at ease teasing him. At Los Alamos Feynman one day picked up the phone. It was Fermi at the other end. He had just read a report that Feynman had produced and explained to Feynman that he considered the research too trivial to merit publication. He claimed the results were obvious even to a child. Feynman countered, “Only if that child is Fermi.” To which Fermi replied, “No, even an ordinary child.”

THE CRITICAL MASS PROBLEM MAY HAVE BEEN DAUNTING, BUT the most challenging technical problem Fermi worked on involved calculations for the implosion device. Kistiakowsky’s team understood that high-explosive “lensing” would be required. Lensing is a technique that changes the shape of a blast wave through high-explosive material in the same way that an optical lens changes the shape of a light wave, by slowing it down. High-explosive material of differing densities through which the blast wave would travel at differing speeds, resulting in the proper shape of the wave just as it reached the subcritical plutonium sphere, would push the entire sphere inward at exactly the same time.

Lensing required technical expertise in many areas. Kistiakowsky was perhaps the greatest expert in the world on high explosives—he loved blowing things up—but that was not sufficient. Other expertise was needed, particularly in the physics of optics. Luis Alvarez knew a great deal about optics from his work prior to the war and was drafted into the project. So did Ed Purcell from Harvard, another optics specialist. A Hungarian Jewish mathematician, however, would be the central figure to do the calculations required to structure the high-explosive charges around the plutonium sphere. His name was John von Neumann.

Von Neumann is regarded by many as one of the greatest mathematicians of the twentieth century. Born in Budapest, he went to high school with the other Los Alamos Hungarians, Szilard, Teller, and Wigner. They all considered him the smartest of the bunch. By the age of eight, young “Johnny” was able to multiply eight-digit numbers by eight-digit numbers in his head, far faster than anyone could do it on paper. He had an idiot savant’s ability to calculate, but he was no idiot. He was highly social, at ease in groups, and a great storyteller. He was even shorter than Fermi and had an impish, mischievous look about him, which he reinforced with colorful but crude jokes. He also had an explosive temper and would erupt with anger frequently, certainly more frequently than the normally placid Fermi.

He emigrated in 1933 when the Institute for Advanced Studies at Princeton offered him a tenured position. By the time the war started, he had contributed to virtually every area of mathematics and had, while dabbling in physics, published major work giving a formal mathematical basis for the quantum work of Heisenberg and Dirac.He joined the war effort early on and worked on conventional explosive shock waves before arriving as a consultant at Los Alamos. Fermi knew of his work, but the two had never met. At Los Alamos they were thrown together frequently, for long stretches. Fermi quickly recognized the Hungarian’s superior mathematical ability but always tried to outdo von Neumann when it came to calculating. He rarely succeeded. Bethe, Fermi, and von Neumann could often be found sitting together in a quiet room inside the throbbing heart of the Theoretical Division, challenging each other to solve complex integral equations related to pressure waves. Sometimes Oppenheimer would join them. Von Neumann usually left these other three brilliant physicists in the dust.

Von Neumann’s mathematical abilities never ceased to amaze Fermi. Years later, returning from summer work at Los Alamos after the war, he regaled colleagues over lunch at the University of Chicago faculty club with a story of how von Neumann masterfully solved a particularly thorny mathematical problem. As Fermi’s young physics department colleague Courtney Wright recalls, Fermi observed of his own role in solving the problem: “You know, I felt like the fly who sits on the plow and says, ‘We’re plowing.’”

Not every calculation was done on paper or in von Neumann’s head. Like the work on critical mass, work on the implosion device required a variety of mechanical calculators operated by the “computers.” Fermi enjoyed using them himself, so much so that one of the first things he did upon arriving at Los Alamos was write to Pegram at Columbia to send him the calculator he left behind when he moved to Chicago. Fermi also used the newest wave of IBM mechanical calculators that were driven by punch cards. This experience left a mark on him and inspired him after the war to become one of the first physicists to use computers to simulate physical interactions. For von Neumann, the IBM machines inspired him in another direction and led him after the war to design the first programmable, fully electronic computer.

FERMI SOON SETTLED INTO A FAIRLY INTENSE BUT REGULAR ROUTINE. After his traditional simple breakfast prepared by Laura, he would walk or bicycle to the highly secured Technical Area where the daily work on the bomb took place. The mornings were his alone, and he concentrated on any particular physics problems that were bothering him. He also tried to keep abreast of the myriad administrative duties involved in managing the wide range of scientific efforts under his supervision. The afternoons were for others and he opened his office door to all. It soon became clear that if a physicist or an engineer had a difficult problem to solve, approaching Fermi would almost always lead to a quick, clear solution. Segrè recounts a moment when there was a problem with a particular electric circuit. Fermi analyzed the problem, listed the characteristics of an electronics tube that would solve the problem, and a few hours later a tube with those characteristics had been found, inserted into the circuit, and the problem was solved. He was pulled into one meeting after another, to give advice and counsel.

He also began to give lectures in physics to anyone who cared to attend. These became a regular series. The younger staff members particularly appreciated having the opportunity to break away from their work to hear one of the greatest physicists in the world lecture on neutron physics.

Frequently he would climb down into the ravine to the Omega site, working with King, Anderson, Hinton, and others on the water boiler. He also now had responsibility for Teller and the Super. Teller had mastered the fusion equations that were required for the work, but no one could figure out how to keep the assembly of fission and fusion devices together long enough to produce a true fusion explosion. These and other less vital, but still pressing, technical issues preoccupied Fermi at work. But Los Alamos was not just about work.

Even with the punishing work schedule at Los Alamos, Sundays were for leisure, and Fermi took active advantage of them. The surrounding Jemez Mountains were ideal for long, strenuous hikes with his colleagues and friends. As in his youth, he would plan out the excursion, lead it, and walk ahead of most of the others. Geoffrey Chew, a twenty-year-old physicist from George Washington University who worked in the Theoretical Division, was a tall, athletic young man. He recalls that he was one of the few who could keep up with Fermi. He doesn’t recall talking physics at all or in fact talking about much of anything except the beautiful austere Southwestern landscape. During the winter it snowed on the Jemez range, giving Fermi an opportunity to go skiing. He was joined by some of his former European colleagues who had learned to ski when they were young, notably Hans Bethe and Niels Bohr.

Fermi eagerly ventured off-site in other directions as well and explored the Bandelier forest, about ten miles southwest of the Los Alamos mesa, where old Pueblo Indian ruins could be found at the end of long, inspiring hikes. He also tried to pick up the art of trout fishing in the many streams at the base of the ravines surrounding the mesa. To the amusement of those who had labored to learn the difficult skill of fly fishing, he insisted on using live bait, usually worms, arguing that it was more humane to give fish live bait for their final meal. Segrè once pressed him on this and Fermi explained he could not see the point of fly fishing. Segrè patiently lectured Fermi on how it was harder to catch a trout with a fly lure, that it took real skill to put the fly down on the stream in such a way as to fool the fish into thinking it was an insect. Fermi grinned and said, with only mild irony, “I see, so it’s a battle of wits!”

FIGURE 19.1. Fermi skiing with colleagues in the mountains around Los Alamos. Courtesy of Los Alamos National Laboratory.

The social life at Los Alamos was also active. In general, those with white badges—the several hundred scientists who were cleared to know anything and everything about the project—socialized among themselves, and those with blue badges—everyone else—socialized among themselves. It was easier that way. The white badge parties were frequent and lively. Sometimes they were held at the cramped apartments of the scientists, although the Oppenheimers, who lived on Bathtub Row, also hosted parties in their more spacious digs. Sometimes there were grander affairs, held either at Fuller Lodge, a larger log cabin–style structure that served as the white badge mess hall, or even at the much larger general mess hall for the entire community. Neither Enrico nor Laura were big drinkers, but they enjoyed these parties and hosted many of their own, at which they led guests in the types of parlor games Enrico always tried to win.

Chew tells the story of one party he and his young wife hosted, which the Fermis attended. Chew suggested a party game involving the passing of a scissors around a circle; you passed it “closed” if your legs were crossed and “open” if your legs were not crossed. A few people knew the secret, while others had to guess as they watched. Round and round the scissors went and with each cycle Fermi grew more and more agitated because he could not guess the rule. Laura picked up the rule fairly early, and finally, sensing her husband’s growing frustration, she leaned over and explained it to him. He was so upset that the two made excuses and left the party early. The Chews were mortified, but the incident seemed to have no lasting effect. After the war Chew went to Chicago and was one of Fermi’s graduate students.

Some of the bigger parties involved square dancing, which was an entirely new experience for the Fermi family. Bernice Brode, the wife of physicist Robert Brode, who developed the fuse mechanisms for the first fission weapons, was one of the leaders of the square dance group, which met frequently throughout the period. In later years she recalled that when the Fermi family initially came to the sessions where she and others taught newcomers, they just sat and watched, presumably terrified at the thought they might have to execute these complex dance patterns. Eventually, Laura and Nella joined in, but Enrico still sat on the sidelines, studying the moves:

He said in his mild and reasonable voice he would let me know when he was ready to join a square, and one could almost see his mind watching and remembering. Then one evening he came up to me and said, “Well, I think I am ready now, if you will be my partner.” He offered to be head couple, which I thought most unwise for his first venture, but I could do nothing about it, and the music began. He led me out on the exact beat, knew exactly each move to make and when. He never made a mistake then or thereafter. I wouldn’t say he enjoyed himself, for he was so intent on not making a mistake, which the best of us did all the time. Although I congratulated him, I also kidded him and admonished him to relax and have fun. He laughed tolerantly, but I knew he could continue to dance with his brains instead of his feet.

He eventually learned to enjoy square dancing, so much so that it became a feature of the many parties hosted by the Fermis in Chicago after the war.

BY MARCH 1945, THE PLUTONIUM PRODUCTION REACTORS AT Hanford were producing at full speed, the giant facilities at Oak Ridge were enriching uranium on an industrial scale, and shipments were arriving at Los Alamos with increasing frequency. Much work had been accomplished regarding critical mass in uranium, most notably the famous “tickling the dragon” experiments that took place alongside the water boiler in the Omega site. A subcritical slug of enriched uranium was dropped through another subcritical block of enriched uranium with a hole bored through it just large enough to accommodate the slug being dropped. For a brief instant the whole apparatus was supercritical, not quite long enough to explode, but long enough to give high confidence that estimates of U-235 critical mass were accurate.

The implosion device, however, required more work. One critical experiment consisted of carefully controlled implosions of aluminum spheres. The configuration of the high-explosive charges around the sphere, designed by Kistiakowsky’s team guided by the calculations of Bethe’s Theoretical Division, proved sufficiently promising to begin manufacturing a sphere of plutonium metal. To the consternation of Robert Bacher, who headed the group responsible for weapons physics, Fermi’s attention turned now to the issue of the “initiator,” the device that would provide the fast neutrons at exactly the moment of criticality. Bacher was a respected experimentalist who would go on to a long and distinguished career after the war. Fermi and Bacher had enormous mutual respect, but Bacher felt that Fermi was becoming a bit of a nuisance. Fermi had been playing with a number of different initiator concepts, all of which Bacher considered hare-brained. In later years, Bacher, who remained friendly with Fermi in spite of his annoyance over the initiator issue, described it this way:

I think Fermi began to be very worried about the fact that this terrific thing that he’d sort of been the father of was going to turn into a great big weapon. I think he was terribly worried about it.… I think he [Fermi] was worried about the whole project, not just the initiator. But focusing on the initiator was the one thing that he thought he could look at. The thing really might not work.

And I think he also felt an obligation to take something that was as hare-brained as this was and try to find a way in which it really wouldn’t work. So he did look into every sort of thing, and I think every second day or so for a period, I’d see him and he’d come up or he’d see Hans [Bethe] and come up with a new reason why the initiator wouldn’t work.

Perhaps, as Bacher surmised, Fermi finally understood the enormity of the project and was looking for ways to demonstrate that the problem was impossible to solve. Or perhaps Fermi was struggling with the admittedly difficult technical problem of how to get enough neutrons out of the initiator to create a full explosive chain reaction before the initiator was destroyed. Regardless, Bacher and Oppenheimer decided to give the initiator assignment to Niels Bohr and his son Aage, who arrived as residents of the mesa during late 1943/early 1944. Bacher believed Fermi would accept a design approved by the Bohrs, and he was right. The Danish father and son team thought about the problem for a few days and came up with an elegant solution, dubbed “the urchin,” a spherical version of the original neutron sources that Fermi had used in Rome that would sit inside the plutonium sphere and would be triggered under the enormous pressure of the implosion, releasing between ten and one hundred neutrons before being destroyed. Those neutrons would be sufficient to initiate a full explosive fission throughout the plutonium sphere. Fermi, confronted with this elegant solution, conceded that it would probably work.

Bacher was not a psychiatrist and it is difficult in any case to speculate on Fermi’s inner life, given his profound reticence. Yet Fermi may well have been experiencing some deep level of psychic stress. He had climbed on board the Manhattan Project express train early on and had helped stoke the engine for more than five years. In fact, he was the best coal man the train had and that train was now bearing down on its destination. Perhaps somewhere deep inside, he was feeling a growing sense of panic that he was responsible for launching a physics project that would result in terrible consequences. Of course, it might also have reflected a concern that the project on which his advice and judgment and physics expertise largely relied might fail in the end, with unimaginable personal embarrassment for him. We have seen a certain reluctance all along, in his expressions of doubt that the project would work, in his lukewarm presentation to the military in March 1939, and in his decision not to spend the summer of that year devoted to fission, that fateful summer when his erstwhile colleague, Leo Szilard, petitioned Einstein to write a letter to FDR urging work on fission weapons. Now, in early 1945, his mind latched on to the idea that the initiator could not in principle be made to work. If this were true, then the United States would not be able to make nuclear weapons, but then neither would the Germans. If this were true, a lot of money would have been spent demonstrating that the demonic device was technically impossible. The fact that Bohr and his son came up with an initiator that would work in some subtle way let Fermi off the hook, but it left the project intact. Fermi would have one last chance to try and stop the express train from reaching its destination, but for now, at least, he stopped fighting it and returned to supporting it.

THE WAR WITH GERMANY EFFECTIVELY ENDED IN LATE APRIL 1945; on May 2, the Soviet flag flew over the Reichstag. The Allies’ most important European enemy had been comprehensively defeated and with it, the fear that Heisenberg and his colleagues would beat the Allies to the atomic bomb.

The Germans had come nowhere near building an atomic weapon, though, as Jeremy Bernstein conclusively demonstrates, they tried hard to do it. Their decision initially to pursue a heavy water model doomed the project from the start. Heisenberg later encouraged suggestions that he had deliberately chosen that route knowing that it would delay the project, but the consensus today among historians is that this explanation was somewhat self-serving. In addition, when the Germans turned to graphite moderation, the graphite they used was filled with impurities that altered the material’s ability to serve as a moderator. To cap it off, they grossly overestimated the critical mass of uranium. In the end, they were never even able to achieve a self-sustaining chain reaction.

THE END OF THE WAR IN EUROPE PROFOUNDLY CHANGED THE NATURE of the Manhattan Project. In the eyes of many of the scientists involved, including Fermi, the main justification for the project had evaporated. Germany had lost the race to build the first nuclear weapon. Looking to the future, no one took seriously the threat of a possible Japanese atomic bomb. Groups of scientists at the Met Lab, most importantly James Franck and Leo Szilard, now believed that the Manhattan Project should be slowed or altogether stopped, and that certainly the bomb, if developed, should not be dropped on Japan. The scientists at Los Alamos were aware of the growing discomfort of the Chicago scientists and some of them expressed misgivings as well.

Thinking within the political and military leadership, however, was not tending in the same direction. Groves wanted to push the weapon through to completion, believing it was a quick way to end the war against Japan and also wanting to measure the weapon’s effectiveness. Those few around the new president, Harry Truman, who knew about the project—most notably, Secretary of War Henry Stimson—shared Groves’s perspective. So did the president himself, who only learned of the project the day he assumed office.

Against this background, Secretary of War Henry Stimson called four key scientists to Washington in late May: Arthur Compton, Fermi, Lawrence, and Oppenheimer. They were to advise the “Interim Committee,” newly established to provide the president with high-level political and strategic advice on the future of the whole nuclear project. Members of the Interim Committee, chaired by Stimson, included a small group of the government’s highest level civilian and military leadership.

The meeting, held at Stimson’s Pentagon office, started around ten o’clock in the morning, continued over lunch, and ended late in the afternoon. Compton later recalled the meeting for his memoir. The conversation covered the current status of the project, estimates of the weapon’s effectiveness, whether, and if so when, and how to let the Soviets know of the project, how to manage the still secret technology in the postwar world, and, most importantly, how to use the weapon to end the war with Japan.

Over lunch, Compton and Lawrence both advocated exploring a demonstration, inviting the Japanese political and military leadership to view an explosion. The thinking was that the experience would be so dramatic that the Japanese would quickly sue for peace. Oppenheimer strongly disagreed. He wanted the bombs to be used against targets in Japan. He could not imagine a demonstration that would be sufficiently dramatic to persuade the recalcitrant Japanese to surrender. Nor did he believe the weapon would be significantly less humane than conventional bombing, which had already leveled Japan’s great cities, including Tokyo, and killed some two hundred thousand people.

Oppenheimer’s opinion counted; however, the political leadership attending were already leaning heavily in favor of using the bomb against Japan, either to end the war quickly or to make subsequent invasion easier. The quickest way to do this, in their opinion, would be to use the bomb in a dramatic fashion against major cities involved in the Japanese military-industrial effort. It was also clear that only one or two bombs would be ready for use in the immediate future and they opposed using one simply for a demonstration. If the Japanese did not respond constructively to the demonstration, perhaps only one weapon would remain in the arsenal. If the demonstration proved to be a dud, that would only make matters worse.

Fermi limited his participation to an estimate of the amount of enriched uranium that would be required for future research after the war—about twenty pounds initially, and half a ton in the next phase of work.

What Stimson thought of Fermi, or Fermi of Stimson, is not recorded. Fermi had been working at the heart of the Manhattan Project since its inception but had never been this close to the highest levels of political power. He had encountered important Americans before, but none of Stimson’s stature. Stimson, a product of Andover and Yale and the ultimate Establishment man, must have been bemused by this short, unprepossessing Italian immigrant with a thick accent who, by all accounts, was essential to the project’s success. As the meeting adjourned, Stimson instructed the scientists to prepare a short report to the Interim Committee with advice on the use of the bomb against Japan. It is unclear why Stimson bothered to do this, because the very next day presidential adviser and future Secretary of State James Byrnes reported to the president the Interim Committee recommendation that the United States drop the bombs on Japanese targets. Stimson may have been concerned about the growing consensus among scientists in Chicago and Los Alamos that the bomb should not be dropped and ordered this report from the nation’s top scientific leaders in order to undercut the developing consensus. As it happened, the four scientists, not knowing of Byrnes’s report to the president, canvassed colleagues at their respective labs. Compton’s job was perhaps the most difficult. He asked James Franck, whose moral authority was respected by most of the scientists at Chicago, to prepare a report. Franck was already concerned about how the bomb might be used in the aftermath of the German surrender. He drafted a letter, eventually signed by Szilard and Seaborg, among others, reflecting the views of many in the Met Lab that either a demonstration use or a decision to keep the very existence of the bomb secret would be preferable to using the bomb against Japan. While still leaning toward the option of a demonstration, Compton did not add his signature. Franck personally brought the letter to Washington and gave it to Stimson. It is not clear whether the four scientists who met with the Interim Committee ever saw a final copy.

Oppenheimer scheduled a meeting in mid-June 1945 at Los Alamos with the other three who had briefed the Interim Committee to continue discussions and to prepare the recommendation that Stimson had requested. The meeting began on June 15, 1945, and produced three reports. One relatively uncontroversial report recommended funding the postwar atomic research at a level of $1 billion annually. The third recommended that the Manhattan Project under Groves be extended for the duration of the war to continue oversight of nuclear technologies. It was the second report, however, that historians remember. It addressed the immediate issue of the bomb’s use. Just 350 words in length and presented under Oppenheimer’s signature on behalf of the four scientists, it recommended that the allies be apprised of US progress in atomic weaponry; noted that, though opinions differed among the scientific community, they themselves leaned toward immediate military use to end the war as quickly as possible; and observed that as scientists they recognized that they had no special competence in the political, social, or military aspects of the issue of atomic weapons.

How Oppenheimer crafted this “consensus” view is not exactly clear. Most accounts are based on Compton’s recollection, published in his 1956 memoir. Both Compton and Lawrence advocated demonstration, in opposition to Oppenheimer, at the May 1945 meeting in Washington. Compton returned to Chicago, where he faced active and increasingly frustrated lobbying by Franck and others who opposed using the bomb against Japan. Even Lawrence, a perennial hawk on defense issues, was somewhat fraught and continued to prefer a demonstration, as he argued at the Washington meeting. Oppenheimer was a forceful advocate of using the bomb, though, and skillfully drafted language that Compton and Lawrence could sign on to in spite of their reservations. Compton reports that Lawrence was the final holdout, but reluctantly went along with the consensus.

Oppenheimer’s secretary, Anne Wilson Marks, was interviewed in 1983 and tells an entirely different story. Soon after the June 15, 1945, meeting, Oppenheimer revealed to her that Fermi was the last holdout of the four. According to Marks, Fermi leaned heavily in the direction of the Franck Report, arguing not only against demonstration but also that the bomb should simply not be used, that it should be kept secret for as long as possible. This accords with views expressed later in his life, as he considered the question of whether to proceed with work on the hydrogen bomb. In his view of human nature, warfare was a permanent aspect of human life. Eventually another war would be fought and men would use these terrible weapons against each other. The entire project should therefore be kept secret as long as possible. Oppenheimer reported to his secretary that it had taken Oppenheimer till five o’clock in the morning of Sunday, June 17, 1945, to persuade Fermi to agree to the “consensus” recommendation on use. There is no particular reason to doubt her memory of the conversation.

Why did Fermi relent? We can only speculate. He maintained public solidarity with Oppenheimer and the others through the rest of his life. Oppenheimer was an effective, energetic spokesman for any position he promoted, and he was certainly in favor of military use of the weapon against the Japanese. It may be that he actually changed Fermi’s mind or perhaps Fermi felt it was more important for him to show solidarity with the man charged with running the project than it was for him to press his own perspective on the matter. This is generally consistent with Fermi’s view of himself as a scientist with only limited expertise in political matters. Finally, it may be that he was still sensitive to his position as a foreign-born national who only recently became a citizen of his adopted country. It might have been more important to underscore his loyalty to his country than to express any personal political or moral qualms.

In the end we cannot know his private motivations, only that he came around to the consensus view. He never spoke or wrote about this decision, never indicated anything but solidarity with Oppenheimer on this issue. We would not even know about his reservations were it not for journalist Peter Wyden’s interview with Anne Wilson Marks some thirty-eight years after the event.

Szilard, increasingly alarmed that the government might use the bomb militarily against Japanese targets without an initial demonstration, organized his own petition, signed by some seventy Met Lab scientists, and sent it off to Groves. Groves stamped it “secret” and placed it in a drawer.

The irony in all of this, of course, is that the recommendations of the group led by Oppenheimer hardly mattered at all. Neither did the Franck Report. Nor did the Szilard petition. The president and those closest to him had already come to their decision, based on political and military considerations, to use the weapons against Japanese cities. The only issue was the selection of which cities to bomb. Stimson, who had traveled in Japan as a youth, struck the magnificent cultural capital Kyoto off the list of potential targets, but all other cities were fair game.

Scientists have long fretted about their role in the decisions of July and August 1945. Could they have been more forceful? Could they have been more persuasive? They needn’t have worried. The decision makers in Washington were not listening to them, one way or the other.