4. Germany’s nuclear projects

Scientists and statesmen in Europe and the United States were largely unaware of Japanese efforts to build a bomb, and they allowed themselves to be unaware because they were unconcerned at the prospect. The Japanese, the ‘little men’, were regarded with contempt in the West: they were able imitators of the inventions of others, it was said, but incapable of developing new, large-scale technologies on their own. This was not the view held of the Germans. Despite losing dozens of distinguished scientists as refugees from Nazism, Germany in the 1930s retained some of the best theoretical and experimental physicists in the world. Otto Hahn remained. Kurt Diebner led physics research at the Army Weapons Bureau and in the early years of the war took charge of uranium research at the Kaiser Wilhelm Institute in Dahlem. Paul Harteck, the physical chemist who predicted a powerful nuclear explosive in 1939, was at Hamburg; Carl Friedrich von Weizsacker, whose father, Ernst, was the second-ranking official in the German Foreign Office, was at KWI, as were Erich Bagge and Horst Korsching (both of whom specialized in isotope separation), and the Nobel Prize winner Max von Laue, famous for his work on X-rays— though, as it turned out, someone who would evade weapons research during the war.14

Above all, Germany had Werner Heisenberg. He was one of the world’s great theoretical physicists, the man James Chadwick had called ‘the most dangerous possible German in the field because of his brain power’. Heisenberg had taken his doctorate at Munich with Arnold Sommerfeld, finishing the degree in 1923, when he was 22 years old. The previous year he had heard Niels Bohr speak at Gottingen, and had been captivated by the Dane’s depth of understanding, his brilliantly discursive presentation, even the famous mumbling that rendered his words barely audible. Heisenberg had asked Bohr a sharp question, and Bohr invited him for a walk following the lecture. ‘My real scientific career began only that afternoon,’ Heisenberg would write. Thus inspired, Heisenberg began postdoctoral work at Gottingen with Max Born. It was Born who coined, in 1924, the term ‘quantum mechanics’ to describe the nature of matter, especially at the atomic and subatomic levels. It was a way of knowing that would lead to a new understanding of the structure of atoms: they were made of particles, and electrons spun in orbits around their nucleii. Electrons might jump from one orbit to another, gaining (if jumping away from the nucleus) a quantum, or fixed measure, of energy, losing a quantum if jumping toward the nucleus. Bohr worked most imaginatively on the new physics, and it was thus to him that Heisenberg came, in March 1924, in the hope of extending his own investigations. Within five days of arriving in Copenhagen, Heisenberg had been invited to stay the year.

Bohr recognized Heisenberg’s quality. The men talked together for hours each day, often as they walked through the park surrounding Bohr’s institute. It was here that Heisenberg devised his uncertainty principle, which held, broadly speaking, that sure knowledge about a particle’s position in space erased certainty about its momentum, and vice versa. There was something finally unknowable (even if predictable) about the atom. So much for direct causation: ‘In the strict formulation of the causal law—if we know the present, we can calculate the future—it is not the conclusion that is wrong but the premise,’ wrote Heisenberg. Bohr was not altogether convinced, and neither was Albert Einstein—‘God does not throw dice,’ he scoffed—but the uncertainty principle came gradually to gain widespread acceptance and dramatically altered the landscape of quantum mechanics. Perhaps it also became a metaphor for Heisenberg’s life, or more particularly for his ethical position on serving the German state, come what may. If the smallest structures in the universe could not be known or understood in their totality, frozen in time and seen in full as if on a slide under a microscope, how was it possible for a human being, infinitely more complex than an atom, to know for sure what was right? If Einstein was wrong, and God did in fact throw dice, what kind of moral assurance could one expect from men? In November 1933 Heisenberg learned by telegram that he had won the Nobel Prize for physics. Students at the University of Leipzig, where he was now teaching, honored him with a torchlight march through the streets to his home. Heisenberg also thought it necessary to reassure the local head of the Nazi Students League of his support for the Fuhrer now in charge in Berlin.16

By the time Germany attacked Poland in 1939, Heisenberg and the others had taken a strong interest in the release of energy by nuclear fission and knew a good deal about it. Hahn, of course, knew more than practically anyone. Other German scientists grasped at least the rudiments of nuclear science; Harteck and Groth had written to the War Office in April 1939 that a nuclear explosive might be possible. Carl Friedrich von Weizsacker, at the KWI, was close to Heisenberg and had talked to him about fission. Heisenberg’s own path seemed to lead inexorably toward a bomb. He discussed it with colleagues in Germany; outside Germany its presence in his conversation was implicit. In summer 1939 Heisenberg came to the United States for a series of meetings with American and emigré scientists. Heisenberg avoided talking about the prospect of war, though he would do so if others insisted. The Americans avoided talk of atomic bombs. Wherever Heisenberg went—Cal-Berkeley to visit Robert Oppenheimer, the University of Chicago for Arthur H. Compton, to the University of Rochester, Purdue, and Columbia—his friends urged him to stay in the United States. He was pressed especially at the University of Michigan in Ann Arbor, where he arrived in late July. The Dutch physicist Samuel Goudsmit was there, as was Enrico Fermi, and Fermi’s former assistant Edoardo Amaldi, who was hoping to get permanently out of Italy, and a graduate student named Max Dresden, who served as bartender at a party one Sunday afternoon. Half a century later, he recalled the conversation. Fermi, Goudsmit, and others doubted that a scientist could ‘maintain his scientific integrity and personal self-respect in a country where all standards of decency and humanity had been suspended’. Heisenberg disagreed, arguing that his reputation in Germany would compel even the worst government to see scientific matters his way. Fermi dismissed this as Panglossian: ‘These people have no principles,’ he said of the Nazis, and ‘will kill anybody who might be a threat... You have only the influence they grant you.’ In the end, Heisenberg reverted to the argument he made everywhere that summer: he was a loyal German—loyal, that is, to an organic Germany, not some temporary German government—and German science needed him. He would go back. Things could not be that bad.17

Heisenberg’s relationship with the Nazi state was complicated. While he had not welcomed Hitler’s arrival in early 1933, he was relatively sanguine about the resiliency of the polity in the long run and the continued independence and efficacy of German science. When the eminent physicist Erwin Schrodinger resigned in protest from the University of Berlin in September 1933, Heisenberg got angry with him, ‘since he was neither Jewish nor otherwise endangered’. Of the Nazi regime he wrote a month later: ‘much that is good is now... being tried, and one should recognize good intentions.’ In 1938 Heisenberg, assigned to the army, almost went to war over the Sudetenland grab; he was spared by the Allied capitulation at Munich. He was no subversive, no ethical hero. At best, he hoped to temporize with the regime so he could get on with his scientific work.18

In fairness—and it is necessary to bend over backwards to find much sympathy for him—Heisenberg had a frightening encounter with the most rabid representatives of the Nazi apparatus. In the late 1920s two disgruntled scientists, Philipp Lenard and Johannes Stark, once well regarded but now increasingly left behind by the new thinking in theoretical physics, began writing and speaking on behalf of what they called deutsche physics. They insisted that true physics sprang naturally from Aryan soil and was thus deeply connected with ‘purely’ German culture. The dangerous opposite of deutsche physics was ‘Jewish physics’, which in the form of relativity theory and quantum mechanics truckled to the devil. Science, proclaimed Lenard, was not international in scope but instead ‘conditioned by race, by blood’. One need not, it turned out, be Jewish to be accused of practicing Jewish physics. When in 1935 Heisenberg was being put forward as successor to his old teacher Arnold Sommerfeld at Munich, Stark and Lenard intervened, demanding that Hitler’s government prevent the high-level appointment of a scientific heretic. In the summer of 1937, Stark placed an article in the SS newspaper Das Schwarze Corps, in which he attacked Heisenberg as one of several weisse Juden (white Jews). The appointment to Munich was held up while Heisenberg fought back. Using a faint family connection, he enlisted the help of Heinrich Himmler, head of the SS. Himmler demanded that Heisenberg make his case, and Heisenberg did so, in writing and in several harrowing sessions with Gestapo interrogators in Berlin. In the end, a full year after the publication of Stark’s ‘white Jews’ article, Himmler exonerated Heisenberg ‘precisely because you were recommended to me by my family’, and because, as Himmler wrote to Gestapo head Reinhard Heydrich, Heisenberg seemed a ‘decent person’ whom they ‘could not afford to lose or to silence decisively’. Heisenberg had to agree not to mention in his classes the names of Jewish scientists, and he gave up the effort to move to Munich, remaining at the University of Leipzig until called to the KWI in 1942.19

Thus, along with his dedication to his country and its science, his identification in good part of German science with himself, and his belief that Nazism might not be so terrible in the long run or at least susceptible to his influence, Heisenberg might have felt in a perverse way grateful to Himmler for having exonerated him, or at least for having protected him from the poisons of Lenard and Stark. As David Cassidy has written, for Heisenberg ‘remaining in Germany was apparently worth almost any price, as long as he could continue to work and teach’. The pleas of his colleagues in America during the summer of 1939 only made him more determined to return home. While he abhorred the racist parochialism of the deutsche physics advocates, their rhapsodizing concerning the uniquely glorious properties of German culture was not entirely without attraction for him. Werner Heisenberg was no Nazi. But in the name of Germany, and for physics itself, he was more than willing to continue his work, even to the benefit of the Third Reich.20

His remaining assured that German science would make a serious exploration of the uses of nuclear power. The Germans were quick off the mark. Fission was discovered by Hahn and Strassmann, then confirmed by Meitner and Frisch, in December 1938; Bohr reported on it in Washington in late January. Hahn and Strassmann then further clarified the process leading to a chain reaction (‘There could then simultaneously be a number of neutrons emitted,’ they wrote in January), and three French physicists, led by Frederic Joliot, confirmed the news of a chain reaction in uranium in a letter to the journal Nature on 7 April. Two weeks later, and two days after the Joliot letter had been published, Harteck and Groth wrote to the Reich War Office about the possibility of a nuclear explosive. And on 29 April, the Reich Education Ministry (REM) convened a meeting of experts at its headquarters in Berlin to discuss the findings to that point. Scientists here spoke mainly of constructing a nuclear reactor, a ‘uranium burner’ as the Germans liked to call it, out of which discussion came the decision to end the export of uranium, most of it available from the mines at Joachimsthal. (This step, along with an alarming report of the 29 April meeting given to a British scientist by the German chemist Paul Rosbaud, got the attention of British scientists, and ultimately several in the United States.)21

For all the loyalty German scientists like Heisenberg felt for the German state, and for all the apparent totality of the Nazi regime, physicists resisted a centralized nuclear project. Or perhaps it simply did not occur to them at first to combine efforts. Though out of the REM meeting there came the Uranium Club, or Uranverein, a group of scientists united in the cause of making fissions, as in the United States the scientific habit of independent research persisted, and as in Japan there was more than one government agency interested in directing the work. The REM, which had called the 29 April meeting, proceeded under the leadership (it will be recalled) of the physicist Abraham Esau to capture as much uranium as it could with an eye toward building a reactor. Meanwhile, Harteck and Groth’s provocative letter to the Army Weapons Bureau prompted the bureau’s explosives expert, Kurt Diebner, to initiate a parallel course of research, evidently unbeknownst to Esau and the REM. With the invasion of Poland in September, Diebner’s project gained the upper hand. When the Uranverein was summoned to the War Office on 16 September, Esau was not on the list of invitees; he learned of the conference, he later sniffed, ‘quite by chance’. The scientists in attendance were told that German intelligence had discovered that uranium research existed in other countries. Was it likely to lead to weapons? If so, Germany would need to accelerate its nuclear work. It was too soon to predict outcomes, the scientists replied— more research was needed, and Heisenberg, who was not at the meeting, would have to be enlisted in it.22

In the aftermath of the Berlin meeting, the Weapons Bureau ousted Peter Debye, the Dutch-born head of the KWI Physics Institute, and replaced him provisionally with Diebner. (Debye decamped to the United States, where he told journalists of the German military’s plans for his former professional home.) Dahlem thereafter became headquarters for German uranium research and strivings toward a burner. But the KWI, as one physicist privately complained, was now full of Nazis, and thus not immediately attractive to every scientist pursuing nuclear physics. Paul Harteck remained in Hamburg, where he built a primitive uranium pile using dry ice as a moderator. He competed for the uranium oxide necessary for the experiment with Heisenberg, who had stayed in Leipzig to work on a reactor of his own. In early 1940 Baron Manfred von Ardenne, not a physicist but an intellectually agile scientific entrepreneur, found a bountiful stream of funds from the Reich Post Office, which was headed by a friend of his father. At Post Office laboratories in Berlin-Lichterfelde, Ardenne designed his own reactor and worked on separating isotopes. Diebner tried in vain to coordinate these efforts.23

The researchers lacked neither imagination nor enthusiasm for their task. In the years following the war—in fact, from the moment German physicists learned of the bombing of Hiroshima—Werner Heisenberg, assisted by his colleague Carl Friedrich von Weizsacker, cultivated a myth that he and others had conspired to subvert research toward a German atomic bomb. Opposed to Hitler’s murderous regime and to the moral enormity of nuclear weapons in Hitler’s hands or anyone’s, Heisenberg had slowed his work deliberately and pointedly failed to pursue leads that he suspected might provide breakthroughs in decoding the science of the bomb. In a September 1941 meeting in Copenhagen with the revered Niels Bohr, Heisenberg claimed he had asked, albeit somewhat clumsily, whether Bohr thought it possible that physicists everywhere might refuse to work on the bomb, as he implied he himself would do. Heisenberg also passed Bohr a drawing of the reactor he was working on. According to Heisenberg’s subsequent, rueful account, Bohr misunderstood him to say that he hoped Bohr would use his influence to get the Allies alone to cease bomb research. Bohr in any case bridled, concluded that Heisenberg was, wittingly or otherwise, promoting Nazism, and thereafter refused to trust the man who had once been his closest scientific confidant. Heisenberg returned in frustration to Leipzig.24

Already primus inter pares among German nuclear scientists, Heisenberg was to become even more central after July 1942, when he replaced Diebner as director of the KWI Institute of Physics. Thus, his ethical position on nuclear weapons, and on a German bomb in particular, has undergone exacting historical scrutiny and has generated enormous controversy since 1945. Mark Walker has divided commentators into two camps: the ‘apologists’, who accept Heisenberg’s version of the meeting with Bohr and thus proclaim his innocence, even his nobility in quietly resisting the demands of the Nazi state; and the ‘polemicists’, who insist that Heisenberg’s version whitewashes the truth of his own complicity with Nazism—that the German failure to build an atomic bomb had nothing to do with deliberate subversion and everything to do with Heisenberg’s incompetence.

While this is an argument worth having, at least fora while, certain judgments seem finally irresistible. First, the divisions in the German scientific community, even within the Uranverein, made success in building a bomb problematical. If the most bilious attacks by the advocates for deutschephysics had faded by the early 1940s, the various sites of reactor building in particular frustrated any coordination of effort. Scientists competed for limited resources, especially uranium oxide and heavy water. What is lauded as academic freedom and scientific independence in peacetime comes to resemble an unaffordable luxury of disorganization in times of all-out war, as the Americans would discover. To an extent, Heisenberg’s appointment to the KWI in mid-1942 focused the effort to build a uranium reactor, and the center of nuclear research overall was the Virus House, built on the grounds of the KWI Institute of Biology and Virus Research. Still, scientific jealousy prevented any synchronization of the investigation. The ousted Kurt Diebner retained funding from the Army and resumed his reactor work in the Berlin suburb of Gottow. Experiments to separate isotopes and create chain reactions also continued in Berlin itself and in Munich.25

The lack of coordination among laboratories was never remedied by the German government, which had its own disjointed relationship with nuclear science, and here is a second reason why the German program failed. Some in the regime were suspicious of nuclear physics because of its association with Einstein and other prominent Jews. Hitler wanted weapons, certainly, but he never understood the science and technology that produced them. When the distinguished Max Planck, president of the Kaiser Wilhelm Society, approached the Fuhrer in May 1933 to argue that Jewish scientists could contribute to the state and should not be driven off, Hitler became so apoplectic that Planck simply got up and left. (Einstein reported that Hitler had threatened during his tirade to throw Planck, who was 75, into a concentration camp.) Reichsmarschall Hermann Goring, head of the Reich Research Council, ridiculed the hypothetical scientist who felt he must proclaim ‘his discoveries to the world, as though they are too much to hold in his bladder one moment longer’. He complained that ‘we can’t read the papers that these scientists publish—or at any rate I’m too feeble to’. (Goring soon thereafter relinquished management of the Research Council.) Bernhard Rust, the Minister of Education involved in the quest for fission since the first meeting of scientists in April 1939, was uninterested in nuclear research, preferring instead to rewrite history textbooks and purge Jews from universities.26

Albert Speer, who had once considered mathematics as a career, became the chief architect of the Third Reich and, in 1942, the Minister of Armaments. Only after this second appointment did he learn of the prospect of an atomic bomb. In June 1942 Speer and a trio of military representatives were briefed by Heisenberg, Hahn, and several others on the progress in nuclear research. Heisenberg talked about the ‘uranium machine’, his favorite nuclear subject, and could not help complaining about the lack of support for nuclear research by Rust’s ministry and the probability that the American scientists were by now ahead of the Germans as a result. Speer asked Heisenberg about atomic bombs. ‘His answer was by no means encouraging,’ Speer remembered. ‘He declared, to be sure, that the scientific solution had already been found’—indeed, Heisenberg had believed this as early as September 1941—‘and that theoretically nothing stood in the way of building such a bomb’. But the technology was lacking and would remain so for at least years, even if the project suddenly received full government support; Speer subsequently heard a timetable of three to four years. The session left Speer doubtful that a bomb could be built in time to win the war. He was additionally wary of raising Hitler’s hopes unduly, acquainted as he was with the Fuhrer’s ‘tendency to push fantastic projects by making senseless demands’, and so reported Heisenberg’s news to Hitler ‘only very briefly’. Hitler had previously heard more exciting accounts from others, who offered what Speer termed ‘Sunday-supplement’ versions of the bomb’s possibilities. But Hitler did not press Speer for details, and government sustenance for nuclear research remained inconstant, with occasional bursts of enthusiasm failing to overcome a patchwork system of administration and a good deal of official ignorance.

The progress and regress of the war also shaped the German nuclear program. From 1939 until sometime in 1942, most in Germany were confident of victory. The Germans won virtually all the battles, in Europe and Africa, on land and sea. They had invaded the Soviet Union (where the gradual turning of the tide was either not yet fully perceptible or hotly denied) and attacked Great Britain with war planes. Much of the German population celebrated the Third Reich’s triumphs; Heisenberg was at worst resigned to them and more likely content, and he especially hoped for the destruction of communism in the East. If what the German military was doing seemed to be working, there was no need to pursue such chimerae as atomic bombs. More ordinary weapons would finish the job. Once the war seemed to turn against the Germans—with the Allied victory at El Alamein in October and November 1942, or more clearly the German surrender at Stalingrad on 2 February 1943—the argument concerning nuclear weapons, oddly, remained much the same: while other new weapons, such as V-i and V-2 rockets, might be within reach and increasingly necessary as the German military situation grew dire, atomic bombs were beyond German imaginings or budgets.

The deterioration of Germany’s military position was brought about in part by the increased ability of Allied bombers and commando teams to strike in German-held territory or inside Germany. High on British and American target lists were weapons research and manufacturing facilities, and prominent among these were sites of nuclear investigation. The British, Norwegians, and Americans attacked the mammoth heavy-water plant at Vemork, Norway, destroying a ton of heavy water through sabotage in February 1943, hitting it with bombers the following November, and finally sending a last shipment of heavy water destined for Germany to the bottom of a deep Norwegian lake on 20 February 1944. RAF attacks on Hamburg and Kiel during July 1943 forced the removal to Freiburg of an ultracentrifuge that was (slowly) enriching fissionable Uranium 235, and raids that November on Frankfurt destroyed the factory responsible for producing uranium metal. In February 1944 the RAF hit Berlin, not for the first time. The Americans had encouraged the attack, unabashedly seeking to kill Heisenberg and Hahn, who were known to work at the KWI. While the raid only broke the windows of the Physics Institute, it burned out the Institute of Chemistry, in which Hahn had been working on fission. The scientists present stayed safe in a bunker built for them by Speer, but the attack was so devastating that one scientist suspected it had been conducted using nuclear weapons and authorized the examination of bomb craters and debris with Geiger counters. Soon the labs were disassembled and moved south.28

The relocation, to Hechingen, Tailfingen, and Haigerloch in the southwest not far from Freiburg, offered only temporary respite from Allied harassment. Heisenberg set to work on a new reactor in a Haigerloch cave previously used for storing wine, but supplies of the uranium cubes needed for the machine were very limited; the pile fell well short of criticality. Meanwhile, the Americans, who remained fearful that German science would yet produce a nuclear weapon, took steps to locate and dismember the German program. General Leslie Groves, the man in charge of the American Manhattan Project, working with the Office of Strategic Services, the US intelligence arm during the war, created in the fall of 1943 a special unit to chase down the German secret. ‘We had to assume’, wrote Groves, ‘that the most competent German scientists and engineers were working on an atomic program with the full support of their government and with the full capacity of German industry at their disposal.’ The group adopted the codename Alsos—to Groves’s horror, for alsos is Greek for ‘grove’. The work began with a largely unprofitable probe into Italy, then the dispatch to Switzerland of an extraordinary mission by the OSS agent Moe Berg, an enigmatic, multilingual, former baseball catcher with a creditable throwing arm and a deep knowledge of the game. Berg also had a basic understanding of nuclear physics. Berg’s assignment—given to him, apparently, by the OSS rather than by Groves directly—was to attend a physics lecture to be given in Zurich by Heisenberg in late December 1944. Berg carried a pistol in his suit pocket. If Heisenberg uttered a single sentence indicating that Germany was close to having an atomic bomb, Berg was to render him ‘hors de combat’, then and there, with a well-aimed gunshot or two. Heisenberg’s lecture proved to be sufficiently general in scope as to save his life.29

By that time, Alsos had mounted a systematic effort to uncover German scientific progress. Under the direction of Col. Boris Pash, who had previously tried to demonstrate that Robert Oppenheimer had been associated with communists or was one himself, and Samuel Goudsmit, the well-regarded Dutch physicist who had attended an Ann Arbor garden party with Heisenberg in July 1939, Alsos attached itself to the vanguard of Allied forces that moved into Paris in late August 1944. There they met with Frederic Joliot, who had disappointingly little information about the German bomb project and made clear his recently found dedication to communism. Mission members sampled river water in Holland as that nation was liberated, reasoning that radiation might be detected in the runoff from a German reactor; the samples were negative. (Goudsmit hurried to The Hague, his boyhood home, hoping for news of his parents, from whom he had had no word since early 1943. Among the broken window glass he found his high-school report cards. He later found an SS murder list revealing that his parents had been gassed at Auschwitz on his father’s seventieth birthday.) When Strasbourg fell in mid-1944, Pash and his team discovered at the university’s Physics Institute a handful of physicists, who were unforthcoming, and the scientific papers of Carl Friedrich von Weizsacker, examination of which indicated that the Germans were well behind the Americans in their quest for an atomic bomb.30

Having thus far captured no reactors or leading scientists, the Alsos team pressed on. By April 1945, with the outcome of the war no longer in doubt, the French had been given a German occupation zone that included Hechingen and Haigerloch. Neither Pash nor his handlers had any intention of allowing any other nation to grab people who might provide valuable nuclear intelligence, so Pash hastily assembled a flying column including a pair of tanks and a handful of trucks and jeeps and rushed into Haigerloch on the morning of the 22nd. There he found the Germans’ uranium burner. Many of its uranium cubes had been taken off to a nearby barn and concealed under the hay—shadows of H. G. Wells—but a German scientist told Pash where they were hidden. Pash had the reactor blown up. On the 23rd, the Americans proceeded to Hechingen, where they found and detained Erich Bagge, Weizsacker, Max von Laue, and their colleague Karl Wirtz, then went on to Tailfingen and arrested Otto Hahn. (‘I have been expecting you,’ he said, when they arrived.) Heisenberg, the Americans’ ‘target number one’, remained at large. Worried about his family as the Western Front crumpled, Heisenberg had left Hechingen on 19 April on his bicycle, heading for home in Urfeld, some 150 miles away. Pash and a small contingent of soldiers, having bluffed then shot their way through German lines, caught up with Heisenberg on 3 May. The quantum physics pioneer was sitting on his veranda. He asked the Americans in and introduced them to his wife and children. Pash allowed Heisenberg to collect a few things, then took him, by armored car and jeep, to occupied Heidelberg. Along with the five men taken earlier, and four others including Karl Diebner and Paul Harteck, Heisenberg was detained, from 3 July 1945 until 3 January 1946, at Farm Hall, a British intelligence ‘safe house’ near Cambridge.31

The Alsos mission revealed perhaps the single most important reason why the Germans failed to build an atomic bomb. For all the manifest brilliance of Heisenberg and his fellow scientists, and notwithstanding the limits of German resources and heightened pace of Allied attacks on German facilities after 1942, the Germans lost the first nuclear arms race because they did not fully grasp the science and technology required to build an atomic bomb. It was Heisenberg, the most eminent of the atomic scientists, who made two fundamental miscalculations. First, misunderstanding the fission process, he dramatically overestimated the amount of enriched U-235 needed to sustain a chain reaction, believing it to be a ton or several tons, rather than the 56 kilograms actually needed. As Jeremy Bernstein has demonstrated, even after the Farm Hall Germans got word of the Hiroshima bombing, Heisenberg failed to understand the physics of U-235. Attempts to refine enough uranium to produce its reactable form in the amount Heisenberg thought necessary proved time-consuming and frustrating. Second, the equally frustrating pursuit of many gallons of heavy water was the result of Heisenberg’s belief that it was the only possible moderator of a nuclear chain reaction. The Germans had tried experiments using graphite as a moderator; these had proved unavailing. But this was because the Germans had used industrial graphite contaminated with boron, a substance that, as Bernstein puts it, ‘soaks up neutrons like a sponge’. The Allies would understand the problem and demand pure graphite from their manufacturers. It was graphite that worked perfectly as a moderator in the atomic pile superintended by Enrico Fermi in a University of Chicago squash court in 1942.32

German scientists got a good deal of the bomb’s physics right: they experimented, for example, with creating a transuranic element that might be easier to use for a chain reaction than U-235 (plutonium, element 93, would be the basis for the Nagasaki bomb), seemed at times to grasp the proper scale of the bomb (Heisenberg may have told Albert Speer and others, in June 1942, that a bomb the size of a pineapple would be sufficient to destroy a city), and appeared to understand the difference between running a reactor and constructing a bomb (principally the speed of the chain reaction). But the miscalculation of the chain reaction’s critical mass and mistakes made in choosing a moderator for the reaction fatally undermined the Germans’ bomb project. These errors offer the simplest, and in this case the best, explanation of the German failure.

There is one thing more to be said. The mistakes of the German nuclear physicists during the war were in part the result of the enforced insularity of German physicists, pariahs to most of the rest of the world by their association with a murderous aggressor state. The Germans had left, and had been expelled from, the republic of science. They were expelled, wrote the American physicist Philip Morrison after the war, because, unlike those who planned and built the American bomb, ‘they worked for the cause of Himmler and Auschwitz, for the burners of books and the takers of hostages’. Brilliant as they were, Heisenberg, Diebner, Hahn, and the others were trapped inside a hardened, darkened bubble, unable to see or hear what was going on in the scientific community outside Germany, beguiled by the echoes of their own voices bounced back at them by the bubble’s inner surface. The community’s self-reflexiveness was made worse by the rivalries within it. Heisenberg, Diebner, and Manfred von Ardenne pursued their own nuclear projects and conferred only occasionally, and the separate scientific communities that formed around these men were thus even smaller and more limited in their knowledge than a single Uranverein would have been. At Los Alamos, New Mexico, where scientists from the United States and Europe devised and assembled the world’s first atomic bomb, efforts by General Groves to ‘compartmentalize’ the work process were frustrated by the scientists’ need to talk to each other, to solve problems across labs, tasks, and academic disciplines. Groves wanted each scientist to ‘know everything he needed to know to do his job and nothing else’; our people, he wrote, must ‘stick to their knitting’. Robert Oppenheimer, the scientific leader of the project, placated Groves as much as possible while encouraging his charges to exchange ideas through lectures and seminars. The information-sharing drove Groves wild, but it proved invaluable.33

Samuel Goudsmit came to the point another way. Along with Boris Pash, Secretary of War Henry Stimson’s aide General George Harrison accompanied Goudsmit and the Alsos team to Hechingen, where together they entered the office of the departed Heisenberg. ‘The first thing they saw’, Goudsmit recalled, ‘was a photo of Heisenberg and myself standing side by side.’ The two physicists had a complicated relationship. Once colleagues and friends, they had fallen out, for obvious reasons. At Ann Arbor in the summer of 1939 Goudsmit had futilely urged Heisenberg to stay in the United States, and Heisenberg had in 1943 written a letter to the authorities asking consideration for Goudsmit’s parents, who had recently been sent to Auschwitz; the letter was less than forceful. However they felt about each other now, there the two men were in a photograph on Heisenberg’s abandoned desk. Harrison was suspicious and wondered if Goudsmit could be trusted. ‘I could have helped him out, I suppose,’ wrote Goudsmit, ‘but that didn’t seem quite the moment to explain about the international “lodge” of the physicists.’ Carl von Weizsacker used a different metaphor: ‘We physicists formed one family,’ he claimed. Perhaps, he said, the family ought to have had ‘disciplinary power over its members’—but was ‘such a thing really at all practicable in view of the nature of modern science?’ Yes, Goudsmit would have said. Weizsacker had violated family rules, and was therefore cast out.34

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