Allied troops disassembling the German experimental research reactor at Haigerloch.
In Germany, Paul Harteck, a professor of physical chemistry at the University of Hamburg and an army consultant, and his assistant, Wilhelm Groth, wrote Erich Schumann, the physicist who headed the Heereswaffenamt (the Army Weapons Office of the Army Ordnance department), in April 1939: “We take the liberty of calling to your attention the newest developments in nuclear physics, which, in our opinion, will probably make it possible to produce an explosive many orders of magnitude more powerful than the conventional ones.” They observed that the “country which first makes use of it has an unsurpassable advantage over the others.”
At about the same time Schumann received the letter from Harteck and Groth, the Reich Ministry of Education was contacted by Göttingen University physicist George Joos, after attending a physics colloquium presentation by Wilhelm Hanle on a “uranium burner.” Joos also wished to alert the German government to the potential of atomic fission as a source of energy. To look into such matters, the ministry’s Reich Research Council chartered a group of physicists, designated the Uranverein (Uranium Club).
Meanwhile, although Harteck did not receive a response until August, which prompted a second letter, Schumann had turned the first letter over to one of his principal staff members, Kurt Deibner, a member of the Nazi Party with a Ph.D. in physics and, according to one historian, “a reasonably competent expert on both explosives and nuclear physics.” In September, after the outbreak of war, Army Ordnance moved to take over the Uranium Club.
On September 25, Germany’s most renowned nuclear physicist, Werner Heisenberg, was summoned to Berlin, as part of the mobilization for war, to join the Uranverein. One of a small group of young men who led the quantum revolution in physics, he had received his doctorate at the age of twenty-two and became a full professor of theoretical physics at the University of Leipzig in October 1927, just a few months short of his twenty-sixth birthday. Earlier that year, in a twenty-seven-page paper, On the Perceptual Content of Quantum Theoretical Kinematics and Mechanics, he formulated the uncertainty principle, which stated that it was impossible to precisely measure both the location and the velocity of an electron. That principle became a key component of quantum mechanics, along with Niels Bohr’s complementarity principle and the Max Born–Wolfgang Pauli statistical interpretation of Erwin Schrödinger’s wave function. Five years after his revolutionary paper, Heisenberg was awarded the Nobel Prize in Physics.
When Heisenberg arrived the next day at the Uranverein, he found a number of his colleagues waiting for him. Harteck was there, along with fission codiscoverer Otto Hahn. Hans Geiger, who had given his name to the device capable of detecting radioactivity, was also in attendance, along with Carl Friedrich von Weizsäcker and Walter Bothe. The research interests of the twenty-seven-year-old von Weizsäcker, son of the secretary of state for foreign affairs, spanned both nuclear theory and astrophysics. He had explored the nature of the nucleus as well as the origins of the universe. Bothe, whose work had included investigating the makeup of cosmic rays, was the leading nuclear experimental physicist remaining in Germany and head of the Institute of Physics at the Kaiser-Wilhelm Institute for Medical Research in Heidelberg, where he found refuge after being booted out of the University of Heidelberg for his lack of Nazi ardor. But that did not stop the army from “requesting” his appearance at the Uranium Club meeting, or Bothe from complying.
A debate ensued as to whether the club should help build a fission bomb. One attendee at the meeting, Deibner’s assistant Erich Bagge, recalled that Bothe ended the discussion with the pronouncement, “Gentlemen, it must be done.” Geiger chimed in, arguing that “if there is the slightest chance that it is possible it must be done.” Neither appears to have considered an atomic bomb in the hands of Adolf Hitler to be a terrifying prospect.
After the Berlin session concluded, the attendees headed back to their institutes. Research agendas were assigned by Diebner and Bagge, under the authority of the Army Weapons Office. The office proceeded to take control of the government-sponsored Kaiser-Wilhelm Institute for Physics in the Berlin suburb of Dahlem, with Diebner assuming command, while Dutchborn director Peter Debye headed to the United States on a forced leave of absence that would last a lifetime. The Uranium Club became the War Office Nuclear Physics Research Group.
Heisenberg returned to Leipzig and produced a secret two-part technical report for the weapons office. The Possibility of the Technical Acquisition of Energy from Uranium Fission explored the prospects and means for exploiting physics theory to develop military hardware. In part one, dated December 6, 1939, he concluded that a reactor (or “pile”), in which fission could be controlled to produce energy rather than an explosion, was technically feasible. He investigated the use of different moderators, which were needed to slow down the neutrons so that they would not be absorbed by U-238 but would fission the U-235 instead, and concluded that graphite and heavy water were best. (In heavy water 99 percent or more of the two hydrogen atoms in each molecule of water has been replaced by deuterium, an isotope of hydrogen with an extra neutron in its nucleus.) He also explored different configurations of uranium and moderator. Additionally, uranium could be used as the basis for a bomb of tremendous power if it could be highly enriched so as to significantly increase the proportion of the rare U-235 isotope while reducing the proportion of U-238.
In the second part of his report, delivered on February 29, 1940, Heisenberg was more skeptical of the promise of nuclear fission. He omitted any mention of fission as the basis for a bomb, noting the engineering difficulties involved. One problem was Germany’s lack of technical capability to enrich uranium by separating U-235. In addition, while Germany did possess a large supply of uranium ore owing to its seizure of Czechoslovakia’s Joachimsthal mine, it lacked the means required to process it on an industrial scale into uranium oxide and then into the necessary metal plates, cubes, and powder. Adding to Heisenberg’s caution was his conclusion that graphite would not make an appropriate moderator, leaving only heavy water, which Germany did not possess.
But such skepticism did not prevent Heisenberg and other German scientists and institutions from exploring the path to the development of reactors and bombs. Between 1939 and the end of 1941, staff members of sixteen universities and institutes produced secret technical reports on various aspects of atomic energy. Included were the Berlin-based Kaiser-Wilhelm institutes for physics and chemistry and the Heidelberg institute for medical research. Faculty attached to the physics institutes from universities in Göttingen, Cologne, Hamburg, Giessen, and Vienna also made contributions. And one company, the Linde Ice Machine Company, contributed a solitary technical paper (a patent) concerning the process for producing heavy water. Among the more prolific authors were army consultant Harteck, whose group worked on the separation of isotopes, von Weizsäcker, and Bothe.
Of particular importance were papers by von Weizsäcker, Bothe and Peter Jensen, and Fritz Houtermans. Calculations by von Weizsäcker’s assistants had supported Heisenberg’s conclusion about the futility of using graphite as a moderator, and a January 1941 paper by Bothe and Jensen, The Absorption of Thermal Neutrons in Electrographite, seemed to provide further confirmation of that judgment, leading the Germans to focus exclusively on heavy water as a moderator.
In a July 1940 paper, The Possibility of Obtaining Energy from U238, von Weizsäcker built on the discovery that after a U-238 atom captures a neutron, it decays in an average of twenty-three minutes to element 93, now known as neptunium. He argued that neptunium could be substituted for the hard-to-obtain U-235 as the key ingredient of an atomic bomb. While von Weizsäcker was wrong, in that neptunium decayed within less than three days into the longer-lived plutonium, such work opened up the alternative plutonium path to the atomic bomb—thanks to Fritz Houtermans.
Houtermans was an unlikely member of the uranium project. He grew up in Vienna, the son of a half-Jewish mother, with communist politics. He had been a classmate of Oppenheimer’s during the American’s time in Germany. His reputation as a theoretical physicist, partly the result of his coauthorship of a paper on energy production in stars, meant that he had no trouble finding employment in the Soviet Union when he fled Germany in the mid-1930s. In 1937 he was arrested by the Soviet secret police when the Stalinist purges reached the University of Kiev, his employer. After a thirty-month stay in the Soviet prison system, he was returned to Germany, where the Gestapo promptly locked him up, suspecting that he was a Soviet spy. Houtermans was able to get in touch with physicists such as Max von Laue, the deputy head of the Kaiser-Wilhelm physics institute, and was quickly released.
Von Laue also found him a job with Manfred von Ardenne’s laboratory, Institut A. Von Ardenne was a “gifted inventor” whose income came from obtaining contracts to do scientific work for a variety of clients, including the post office. His laboratory’s staff included several members trying to develop techniques for separating isotopes. In August 1941, Houtermans completed The Question of Starting a Nuclear Chain Reaction, reporting that a reactor using natural uranium as a fuel could produce plutonium, which could then be removed by chemical means and used as an explosive.
Despite the attention devoted to reactor and bomb development, the papers by the German scientists revealed a number of critical misunderstandings, omissions, or errors. The papers did not contain any calculation of the critical mass of a U-235 bomb, the recognition that a U-235 bomb would depend on fast neutrons (although Heisenberg understood this to be the case), or an equation for the internal multiplication of neutrons with respect to time—the latter being critical to attaining a chain reaction. In addition, the paper by Bothe and Jensen mistakenly confirmed Heisenberg’s conclusions about graphite’s inappropriateness as a moderator because they failed to understand that while industrial graphite would not work, highly purified graphite would.
Along with such theoretical investigations, the group undertook experimental work. In July 1940, construction began on a small laboratory near the Kaiser-Wilhelm Institute for Biology and Virus Research in Berlin-Dahlem. Ultimately, it would consist of a six-foot-deep circular pit lined with brick and a wooden laboratory barracks about twenty feet long. Named the “Virus House” to keep the curious away, it became, in October 1940, a facility for exploring the workings of a uranium reactor.
In the fall of 1941 the Nazi blitzkrieg had appeared on the verge of producing a quick victory, but the winter brought serious setbacks to Hitler’s armies. With all available resources being diverted to bring about a successful end to the war, long-term projects were considered a luxury. Schumann, the Army Weapons Office research director, sent a letter to the directors of the various institutes involved in the uranium project, notifying them of a meeting scheduled for December 16. He noted that “given the present personnel and raw materials shortage, the nuclear power project requires resources that can only be justified if there is certainty that an application will be found in the near future.”
After the meeting, during which Heisenberg, Bothe, Hahn, Harteck, and others delivered papers, Schumann reported to the head of the weapons office, Gen. Emil Leeb, and requested a decision on the army’s future role in the project. By late January 1942, the office decided to cede control of the effort, and the Kaiser-Wilhelm Institute for Physics returned to its traditional place in the Kaiser-Wilhelm Society, where it would stay until April.
Ceding control did not mean that the office had completely lost interest. In February, it issued a long report titled The Production of Energy from Uranium, a detailed description and analysis of the uranium project’s work through the end of January. Its first chapter briefly reviewed the potential employment of atomic energy for reactors and bombs. The report maintained that using plutonium rather than uranium would make it easier to build a bomb, and suggested that the critical mass for a plutonium bomb was in the range of 22 to 220 kilograms. However, either route would involve a long-term effort requiring “a very large isotope separation plant or the successful extraction of [plutonium] in large quantity from a reactor.” It was an effort, the report concluded, that should be undertaken, given its importance for both the economy and the German military.
The continued interest of the weapons office in the subject was also in evidence when a three-day conference sponsored by the office opened on February 26. Its first day overlapped the one-day meeting the weapons office cosponsored with the Reich Research Council and held at the House of German Research in Berlin-Steglitz. After the opening address by Erich Schumann, titled “Atomic Physics as a Weapon,” the attendees at the council meeting heard another seven, relatively nontechnical lectures by the key scientists involved in the atomic energy program. Hahn, as might be expected, talked about fission of the uranium nucleus, while Bothe addressed the results of research on energy production. Klaus Clusius lectured on the enrichment of uranium isotopes, and Harteck addressed the issue of heavy water. According to the manuscript version of Heisenberg’s lecture, “The Theoretical Basis for Energy Production from Nuclear Fission,” he reported that fission could produce “an explosive of unimaginable force.” He also stressed the importance of pure U-235 in producing a chain reaction, and of employing alternative means to obtain it, including uranium enrichment and the development of a reactor. In addition, he endorsed the use of plutonium as a nuclear explosive, citing von Weizsäcker’s work.
Very few of the high-level dignitaries invited to attend the meeting—including armaments chief Albert Speer, interior minister Heinrich Himmler, air force chief Hermann Goering, Hitler aide Martin Bormann, field marshal Wilhelm Keitel, and navy commander-in-chief Erich Raeder—actually did so. It certainly did not help that mistakenly enclosed with the invitation was the agenda for the army’s three-day conference, consisting of twenty-five highly technical topics, rather than the simpler agenda for the council’s meeting. That the actual council meeting featured a lunch of “experimental” food such as assorted deep-frozen and enriched dishes, baked or fried in synthetic fats, probably also deterred attendance. Himmler did sent a short note, thanking education minister Bernhard Rust for “your kind invitation” while informing him that “unfortunately” his duties prevented him from attending.
Not long after Heisenberg completed his lecture at the research council’s conference, the army-sponsored meeting at the Kaiser-Wilhelm physics institute opened. Over the following three days virtually all of the project’s scientists presented papers. Technical papers included Bothe’s report on the measurement of nuclear constants, Weizsäcker’s description of a new theory of resonance absorption in a reactor, and reports on the behavior of fast neutrons in uranium. Hahn and Strassmann focused on the creation of an isotope of neptunium. But the greatest attention was devoted to the development of a reactor.
In April, Abraham Esau, head of the physics section at the research council, had, in the wake of the army’s action, persuaded education minister Rust to restore his control of the uranium project, including the Kaiser-Wilhelm Institute of Physics. But in May, faced with complaints about the education ministry’s inadequate support for fundamental research, Speer obtained Hitler’s approval for a reorganization of the research council, and the appointment of Goering to head it, a change that took place in June. Esau continued as head of the physics section and eventually became Goering’s deputy for “all questions of atomic physics.”
Just a few days earlier, on June 4, Heisenberg and several of his colleagues, including Hahn and Harteck, assembled in the Kaiser Wilhelm Society’s Harnack House to brief Speer and the three military heads of weapons production. When, after the lecture, Speer asked Heisenberg about the feasibility of atomic bombs, Germany’s top physicist told him that while the scientific problem had been solved, the “technical prerequisites for production” were such that it would take years to achieve the goal. Speer, willing to think big, asked for requests for funds and materials. The scientists asked for only several hundred thousand marks and some small amounts of steel, nickel, and other priority metals, and resisted the armaments chief’s suggestions that they take a couple of million marks and correspondingly more material. The scientists’ reaction, and their explanation that such resources could not be utilized at the time, led Speer to assign the project a lower priority than a number of other projects, including Wernher von Braun’s missile program.
The project did receive approval for construction at the Kaiser-Wilhelm Institute for Physics, which Heisenberg assumed command of on July 1. With the construction funds, the institute built a bunker to house Germany’s first major large nuclear reactor, allowing an expansion of Virus House reactor experiments. At the same time, the theoretical efforts of the scientists and their institutes continued. From the beginning of 1942 through August 1943, over seventy-five additional technical papers dealing with reactor operations, the production of heavy water, isotope separation, and a variety of additional topics were prepared.
But all the work and all the experiments did not result in any renewed optimism. In a July 8, 1943, letter, Rudolph Mentzel of the Reich Research Council informed a member of Goering’s staff that while “the work has progressed considerably in a few months [it] will not lead in a short time towards the production of practically useful engines or explosives.” However, he added a silver lining: “enemy powers cannot have any surprise in store for us.”