W hile awaiting the arrival of more uranium and heavy water in 1940, Heisenberg’s Leipzig and Berlin research teams tested paraffin and regular water as possible moderators, substances that would slow the fission neutrons enough to escape capture by U-238 and thus fission U-235. Each fission, set off by the absorption of a single neutron, produces on average two or more neutrons. Each of these released neutrons, if sufficiently slowed, could go on to produce more fissions of the rare U-235 isotope in a piece of natural uranium, each of which would in turn produce more neutrons; an energy-producing chain reaction would occur in a self-sustaining critical reactor.
Heisenberg’s research teams tested the two moderator candidates in a nuclear pile, alternating layers of moderator and small amounts of natural uranium oxide — what they called for security reasons preparation 38, ^Og. 1 With only a general notion of the best layer configuration, the scientists had to find the exact geometry and amounts of material through trial and error. The Berlin team tried alternating horizontal layers of powdered preparation 38 and moderator in a cylindrical aluminum tank, 1.4 meters in height and diameter, that was immersed in water in the “Brunnengrube” (well hole) — a water pit dug inside the Virus House. Completed in October 1940, the Virus House stood under cherry trees, an insulated wood-frame house containing water and electrical connections to the nearby Kaiser Wilhelm Institute for Biology. The water in the 2-meter-deep Brunnengrube absorbed and reflected escaping neutrons from the containers lowered into it on chains by a crane spanning the 3-meter-wide pit. 2
Through mid-1942, under the able direction of Wirtz and Fischer, the Berlin (B) group performed five experiments using paraffin and uranium oxide or metal powder. Heisenberg and his Berlin coworkers reported their findings in detailed technical reports to army research: none of the models worked. Most of the neutrons emitted by a small source in the center of the pile were absorbed by the contraptions rather than multiplying in fission. 3
In the spring of 1942, the four Leipzig (L) experiments yielded quite a different result: the Germans’ first positive neutron multiplication. 4 But after this promising start, they never managed to achieve a chain reaction. Fermi’s Chicago group surpassed the Germans within months.
After first trying paraffin and water as moderators in early 1941 (model L-l), Dopel, who was closer to the power source than was the Berlin team, switched to precious heavy water, arranged in concentric spherical aluminum-lined shells, alternating with uranium-powder shells. After Paschen and Dopel placed the concentric shells in two aluminum hemispheres, they bolted the ball (radius about 40 centimeters) shut, then winched it into a water tank in the basement of the Leipzig Physics Institute. On inserting a weak neutron source through a connecting tube into the center of the ball, they measured the neutron flux as a function of radius. Model L-2 proved a dud. The source neutrons were all absorbed within the sphere, mainly, Heisenberg calculated, by the considerable aluminum. 5
To confirm this result and the findings by others that powdered uranium metal oxide, 38-metal, was superior to preparation 38, Heisenberg and his Leipzig team turned to metal powder and heavy water late in 1941. But only enough metal powder was available in Leipzig to make one spherical shell surrounded on both sides by heavy water. Careful measurements of this model, L-3, reported in early 1942, indicated a much smaller loss of source neutrons. 6 Heisenberg and Dopel were convinced that one more layer of uranium metal, a total of 755 kilograms, with 164 kilograms of heavy water, would yield a genuine multiplication —proof at last that Heisenberg’s “machine” really would work. 7
But the dangers of 38-metal and water had been overlooked. When water (heavy or not) and uranium meet, they produce flammable hydrogen gas. According to a formal report filed by Dopel, one day in December 1941 institute technician Paschen and his apprentice, F. Zumkeller, were pouring the powder into one of the hemispherical shells when heavy water somehow leaked in. An enormous flame suddenly shot out of the sphere, singeing the ceiling and burning Paschen’s
hand so severely that he could not work for nearly a month. 8 Dopel himself, his wife standing by with a fire extinguisher in hand, gingerly poured the remaining powder into the sphere for the L-3 measurements.
More 38-metal finally reached Leipzig in early 1942, and sometime in late spring, pile model L-4 began multiplying neutrons at the rate of 13 percent. “A simple expansion of the layer arrangement described here would thus lead to a uranium burner,” the ecstatic Heisenberg team coolly reported to Army Ordnance. 9 “With that,” wrote Heisenberg and Wirtz, “[we] proved the possibility of an independently working, energy-producing uranium burner.” 10 Nuclear fission research was no longer just a politically useful or theoretically interesting exercise; the likelihood of controlled — and even uncontrolled — fission suddenly became very real indeed.
Years later, Heisenberg reportedly recalled: “It was from September 1941 that we saw an open road ahead of us, leading to the atomic bomb.” 11 The project was at that time reporting on model L-2 and probably only just beginning model L-3. Moreover, every German effort to extract from natural uranium the fissionable isotope U-235, the explosive material needed for a bomb, had so far proved a failure. A reactor and a bomb were still far beyond reach.
But in August 1941 Fritz Houtermans of Manfred von Ardenne’s independent Berlin research institute obtained an extremely important result: a theoretical confirmation of the plutonium alternative. 12 The implication for what Houtermans called “the theme of our work” was clear from Weizsacker’s and Heisenberg’s earlier reports. Once a natural uranium reactor was finally up and running, the neutrons absorbed by the plentiful U-238 isotope would lead through beta decay of the nucleus to the fissionable, unstable daughter nucleus neptunium (Eka Re), which decays in a matter of days into the stable yet highly fissionable element plutonium. Since plutonium differs chemically from uranium, it can be easily extracted from a uranium reactor. Although Houtermans did not say so, a working reactor would produce fissionable material that could be readily separated and used either for a mobile energy-producing machine or for a new “irresistible” offensive weapon. The first of the two Allied bombs dropped on Japan in 1945 was fired by the separated uranium isotope U-235, the second by reactor-generated plutonium.
Although Heisenberg may not have met Houtermans at that time, he apparently learned of the new possibility in that period. A letter from Heisenberg to his professors’ club colleague Hermann Heimpel, dated October 1, 1941, strongly suggests that Heisenberg did perceive an open
road to a deadly explosive and that he was already mindful of the possible consequences.
Thanking Heimpel for a copy of his book Deutsches Mittelalter (German Middle Ages), Heisenberg wrote: “I really liked the passage in your book about the mind-set of the middle ages in contrast to our epoch. In this connection it suddenly came to me that such a transformation could occur once again in the near future. For perhaps we humans will recognize one day that we actually possess the power to destroy the earth completely, that we could very well bring upon ourselves a ‘last day’ or something closely related to it.” 13
Heisenberg discussed the newly opened road with his trusted Berlin institute staff. Sometime in August or early September, they determined that Heisenberg should discuss the turn of events with Niels Bohr in German-occupied Denmark. A September lecture series on astrophysics at a German propaganda institute in Copenhagen, in which Heisenberg had agreed to participate, provided a splendid opportunity. On September 15, 1941, Heisenberg traveled to occupied Copenhagen for the official purpose of delivering a lecture at that institute, along with Carl Friedrich von Weizsacker and other German scientists, and with the unofficial intention of meeting with Bohr.
Heisenberg probably met with Bohr on the evening of September 16. 14 Their meeting is still shrouded in controversy and questions. Although the official circumstances of the trip and some of its immediate consequences are well documented, the only indication of the content of their meeting comes from postwar accounts by the participants, their colleagues, and colleagues of their colleagues. Given the intense feelings and tensions of the early postwar period, the veracity of all these reports is questionable. Most of the German accounts were offered in defense of Carl Friedrich’s father, an official of the German Foreign Office, whose subdivision administered the German cultural propaganda institute in Copenhagen and who was tried and convicted at Nuremberg in 1948. These accounts are obviously products of the aims for which they were written. Rightly suspecting Gestapo surveillance of their meeting, Bohr and Heisenberg themselves did not dare commit any of their discussions to paper at the time.
Given the setting of the meeting (German-occupied Denmark), the occasion (Heisenberg’s lecture in a propaganda institute), and the topic (nuclear fission, controlled and otherwise), it may be little wonder that Heisenberg’s visit greatly disturbed his former mentor and that Heisenberg felt he had failed to communicate with Bohr. But Bohr and Heisenberg had been close friends and colleagues for nearly twenty years. If
Bohr came away from their meeting in distress, it was because Heisenberg must have said something distressful.
In a carefully worded postwar statement about the visit, apparently an early draft prepared for the Weizsacker trial (it differed from the less informative official defense exhibit that he submitted), Heisenberg again recalled his realization in 1941 that the production of an explosive, an atomic bomb, was now a real possibility. 15 Heisenberg remembered that his most important talk with Bohr occurred one evening as they strolled along a tree-lined path in the large and secluded Fadledpark, just behind Bohr’s institute. The Danish professor and his former assistant had often talked together while walking along these quiet paths. Aside from its other attractions, the venue offered the advantage of escaping whatever bugs the Gestapo had installed in the institute — discussing secret nuclear research was treasonous for the German and life-threatening for the Dane. The boyish-looking Heisenberg recalled opening the discussion with the taller and more distinguished-looking Bohr by asking whether Bohr believed that “as a physicist one has the moral right to work on the practical exploitation of atomic energy.”
An obviously startled Bohr responded by asking whether Heisenberg believed that atomic energy could be exploited in this war. “Yes, I know that,” Heisenberg answered. However, he claimed that he meant only a machine. Because of the technical difficulties involved, he told Bohr, a bomb could not be produced before the war was over.
Unraveling Heisenberg’s postwar account of this meeting, his intentions, and Bohr’s reactions requires a much fuller appreciation of the background of the meeting than was available then or since. 16 Again, our sources allow only speculation on crucial points. Heisenberg’s remembered question on morality is a case in point, for available sources give no evidence that he had ever raised it before. But it was also not quite so certain until then that a reactor could actually be constructed, that it would soon be within reach, and that, once working, it would easily provide the material for an atomic bomb. Since Bohr had served for years as a father figure to his youthful charges, especially to Werner, and had often debated philosophical and ethical concerns with his younger colleague, it seems reasonable that Heisenberg might have turned to Bohr when faced with an ethical dilemma in his research. But during the past few years, particularly during the SS affair, Werner had sought ethical advice on science in the political arena not from Bohr but from his German academic elders, Max Planck and (perhaps) Max von Laue, both of whom were in Berlin and more accessible than Bohr. Though in semiretirement, Laue was still vice director of the Berlin Kaiser Wilhelm
Institute, as he had been since the days of Einstein. Yet among the scant surviving records there is no indication that Heisenberg or his colleagues approached Planck or Laue about the morals of nuclear research or that they even fretted over them.
Aside from any advice that he may or may not have been seeking, Heisenberg probably had other aims in seeing Bohr. In a letter to B. L. van der Waerden after the war, Heisenberg seems to suggest that he was attempting to stave off an Allied crash program on nuclear research. 17 However untenable and naive such an aim may have been — if indeed that is what his letter was implying — it is not ruled out by a consideration of Heisenberg’s prewar trip to the United States two years earlier and especially by the circumstances surrounding his visit to Denmark.
Heisenberg’s 1939 trip to the United States occurred within several months of the publication of the basic Bohr-Wheeler theory and the French confirmation of neutron multiplication in atomic fission. As indicated earlier, on at least two later occasions Heisenberg recalled discussing the possibility of nuclear explosives with Fermi in the United States — a discussion that may have occurred with others as well. 18 As reported years later by Heisenberg, both participants had expressed a ready willingness at the time to engage in fission research for their respective governments. Booking back on this conversation in 1942, when his uranium pile first went critical, Fermi could have felt no assurance that Heisenberg was not working as hard as possible on nuclear energy — which, in fact, he was.
Nor, for his part, could Heisenberg be sure in September 1941 that the Allies were not doing just the same. According to Heisenberg’s recollection, Bohr apparently told him matter of factly that “the physicists of all nations would be put to work on war research.” 19 Elisabeth Heisenberg claims that throughout the war her husband “constantly tortured himself” with the thought that the better supplied Allies might develop and use the bomb against Germany. 20
By the fall of 1941, when Heisenberg visited Bohr, the German Reich had reached its zenith. Most of continental Europe was under Nazi occupation, the German army was plunging into Soviet Russia, and an end of the war may have seemed in sight. If the war ended soon with the German army in place, or if it bogged down at that point —as had World War I in the trenches of France —it was easy to suppose that the United States, which had not yet entered the war, would have enough time and resources to build a weapon, which the Allies would surely use on Germany. At least one secret German report in early 1942 indicated
that the Germans somehow knew about secret American pile research —and they now knew where that research could lead. 21
After the war, Heisenberg wrote that he learned just after the September visit that Bohr was in contact with the Allied scientists. The Gestapo had intercepted a secret message from Bohr to British scientists and had delivered it to Heisenberg. Heisenberg probably suspected as much even before his visit. Perhaps, as his postwar letter to van der Waerden suggests, Heisenberg was trying to avert an Allied crash program and an ultimate nuclear attack on Germany by letting the Allies know through Bohr that the Germans — who believed throughout the war that they were ahead of the Allies — were still a long way from constructing an explosive.
Whatever Heisenberg’s aims and intentions, his understanding of Bohr’s frame of mind in German-occupied Copenhagen and of how he himself would be perceived in Denmark was woefully incorrect and misguided. He had last seen Bohr in 1938. Two years later, the German army overran the frightened Danish kingdom practically without firing a shot. With the German Reich well entrenched in Denmark and across most of Europe by September 1941, it must have been cold comfort for Bohr to hear Heisenberg’s remembered amoral qualifier about the prospect for nuclear weapons: “At this point, it is certainly only a question of the exploitation of energy in machines; the production of bombs would probably require such an enormous effort that the war would be at an end before they could be made.” 22 Even as Heisenberg himself remembered it, only time and effort stood between him and the bomb, and Bohr could regard neither to be insurmountable in September 1941. Nor could he have been very pleased even with the much sooner prospect of a new energy source to power the German economy and to drive German ships and submarines around the world.
Until the autumn of 1943, the German occupation forces maintained the fiction that they had no intention of nazifying Denmark — they did not want a rebellious populace that would drain their forces from other invasions. For the most part German commanders left Danish Jews alone, and the German army and occupation authorities were under strict orders to avoid offending the Danes as much as possible 23 Bohr, his institute, and his cyclotron were also undisturbed, apparently, according to Bohr, on encouragement from Carl Friedrich’s father in the Foreign Office. 24 The Germans allowed Bohr’s institute to function as normally as possible, with continued Rockefeller Foundation funding. 25 But such niceties could not disguise the fact that proud Denmark had
been reduced to a colony of the Nazi Reich and that it was being subjected to incessant propaganda by its occupiers. Even a German propaganda expert had few illusions about Danish resentment: “A feeling of quiet rage prevails here, which only comes to the fore when the Danes believe themselves alone and unobserved.” 26
Several months before Heisenberg arrived in Denmark, just days before Carl Friedrich and a party of German scientists arrived for a visit, Danish communists and other anti-German Danes had been summarily arrested and deported to Germany, an action that incensed the Danes further toward any Germans. Meanwhile, Carl Friedrich himself had already made the German scientists thoroughly unwelcome. During a visit to Bohr’s institute the previous March, Weizsacker had reportedly insulted Bohr by bringing the head of the local German Culture Institute to meet him. 27
The newly opened Culture Institute was a propaganda arm of the Culture Division in the German Foreign Office. 28 In March 1941, Carl Friedrich had spoken there and elsewhere in Copenhagen to Germans and local sympathizers. According to surviving Reich Education Ministry records, Weizsacker’s trip was so successful that afterwards the German occupation office in Denmark requested a return visit by Weizsacker in the fall, this time accompanied by Professor Dr. Heisenberg. 29 Weizsacker was in contact with Heisenberg right after the visit and probably informed Heisenberg of the plan —if he was not already privy to it. 30 Certainly by mid-July Heisenberg knew that Carl Friedrich and the Culture Institute were planning to demonstrate support of German sympathizers among Danish scientists by organizing a conference on astrophysics to be held at the end of September 1941.
On July 22 Weizsacker wrote to the German Academic Exchange Service to confirm himself, Heisenberg, and several other German scientists as invited speakers and — aware of the lingering opposition to Heisenberg in the government —to argue for Heisenberg’s participation. After conferring with Heisenberg and Weizsacker in early August, the Reich Education Ministry scheduled the conference for September 18-24. Heisenberg, claiming personal commitments, would be in Copenhagen September 15-21. 31 But whether naive or malevolent, Carl Friedrich again insulted the Danes by cordially inviting them to attend his and Heisenberg’s lectures on solar physics and cosmic rays —in the odious German Culture Institute. 32 Already imbued with “quiet rage,” Bohr and his colleagues did not appreciate the scientists’ participation in a crass propaganda campaign. Heisenberg later surmised that his meet
A Copenhagen Interpretation 441
ing with Bohr “did not have the intended effect [because] Bohr evidently disapproved of my taking part in an astrophysics conference at the ‘German Culture Institute .’” 33
Although it is not clear from the available documents which came first the plan to speak with Bohr about fission or the plan to speak at the cultural propaganda institute —the timing of the latter seems to precede Houtermans’s results and the perceived open road to nuclear weapons. If so, this raises a question: What was Heisenberg doing in occupied Copenhagen in the first place? If he did decide to go there even before he contemplated approaching Bohr about nuclear research, it might be argued that he wanted to assure himself that Bohr and his institute were unmolested by the German occupation. But surely Weizsacker had already determined that, and others could have kept Heisenberg apprised of the situation. Instead, the most likely answer seems to be that Heisenberg was indeed joining his friend and colleague Carl Friedrich in a conscious or unconscious propaganda effort instigated by the Foreign Office subdivision under Carl Friedrich’s father. Heisenberg either readily joined Carl Friedrich in carrying out the effort or allowed himself to be drawn into it, probably as a way of proving his reliability to regime officials. There does not seem to be any other compelling reason for him to have visited occupied Copenhagen before or after the dilemmas of nuclear research became acute.
This interpretation is supported by further Reich Education Ministry documents. In his postwar affidavit, Heisenberg argued (as he would in other such cases) that his lecture at the Culture Institute was the smallest compromise possible to gain permission to visit Copenhagen. Although some opposition to his touring abroad did exist, it was not insurmountable. In particular, the REM office of Dr. Wilhelm Fuhrer, earlier prominent in blocking Heisenberg’s Munich appointment, had to approve all foreign travel, and Fuhrer had no intention of approving Heisenberg’s travel anywhere unless pressured. During a hastily arranged meeting with the REM in early September, Fuhrer demanded that party headquarters pass final judgment on whether or not to allow the Leipzig professor out of the country. The party quickly consented after the senior Weizsacker’s Foreign Office suggested that the trip could be used as a test case of Heisenberg’s suitability for future propaganda lectures . 34 For Heisenberg, his lecture trip to Copenhagen could be seen as a minor personal victory; for the regime, it would be a test of the professor’s reliability as a precondition for future exploitation; for the Danes, it was nothing more than gross propaganda. Bohr’s wife, Margrethe, never
wavered in her opinion of the episode: “No matter what anyone says, that was a hostile visit!” 35 Bohr and Heisenberg were never as close thereafter as they had been before the war.
What of the “intended effect” that Heisenberg had hoped to achieve by his meeting with Bohr? If the effect was an Allied moratorium, or even a joint boycott of applied nuclear research, the visit could not have achieved any such thing, even if Heisenberg had been well received in Copenhagen. Isolated in occupied Denmark, Bohr had only little inkling of the progress in nuclear research on either side of the war, and because of this, Allied scientists probably would not have accepted his assessment of a boycott offer from German scientists. Weizsacker, for instance, reported in March 1941 that “concerning the more technical questions [Bohr] knew a great deal less than we.” Perhaps this and the German scientists’ actions on their September trip are the origin of the view expressed in postwar Copenhagen that the Germans were actually on a spying mission. 36 No wonder Bohr was so thoroughly disturbed to learn from Heisenberg six months after Weizsacker’s first visit that German scientists already saw an open road to an atomic bomb.
Nevertheless, he still seemed skeptical two years later. Responding in 1943 to hints from British physicist James Chadwick (discoverer of the neutron and Bohr’s secret liaison with the British nuclear research team), Bohr wrote (in halting and deliberately cryptic English): “Above all I have to the best of my judgment convinced myself that in spite of all future prospects any immediate use of the latest marvelous discoveries of atomic physics is impracticable.” 37
Several months after Bohr’s letter to Chadwick, the German occupation turned uglier. In one of the most spectacular rescue operations of the war, the Danish underground conveyed the part-Jewish Bohr, his family, and virtually the entire Jewish population of Denmark to neutral Sweden, just ahead of a planned roundup and deportation of Jews to the German death camps. A British plane flew Bohr to England, where he met immediately with Chadwick. The little-known summary report of a British nuclear committee meeting shortly thereafter describes Chadwick’s impressions: “Chadwick . . . says that Heisenberg has visited Bohr in Copenhagen. He also says that he himself has been in communication with Bohr within a month or so, and that Bohr believes that there are no military possibilities. He thinks that perhaps Bohr has been sold this idea by Heisenberg.” 38 If he felt he had been led astray by Heisenberg, he would have thereafter resented Heisenberg’s wartime visit all the more. For his part, Heisenberg was pleased to receive the imprimatur of the Foreign Office as a traveling spokesman for Germany.
During the months following his meeting with Bohr, Heisenberg revealed his own reactions to the lately proven potentialities of fission research. This occurred in the context of a shift in the institutional and political framework of the uranium project in the wake of Germany’s changing fortunes at the front. By the end of 1941 the German Blitz had run its course. The invasion of Russia, begun in June, was bogged down outside Leningrad and Moscow by December. Previously confident of total victory by Christmas, Germany had squandered most of her raw materials. As the predicament became a crisis during that bitterly cold winter, Hitler took over as operational commander. For the first time, he ordered the full mobilization of the German economy in support of the war effort, along with the full exploitation of occupied territories. Germany would now use every means at her disposal to wage “total war.” By April 1942 the Reichstag would vote Hitler absolute dictatorial powers. As a result of this new state of affairs, Erich Schumann, head of army research, informed the uranium research directors in December 1941 that henceforth the Army Ordnance Office could support their efforts only “if a certainty exists of attaining an application in the foreseeable future.” 39
Ironically, just as the United States was entering the war and was launching a crash program to build the bomb, Schumann was calling the German scientists to Berlin for a meeting to decide whether their efforts were worth continuing at all. During that meeting, on December 16, 1941, which Heisenberg probably attended, the uranium researchers agreed to prepare a comprehensive report on their progress and on the prospects of their research for General Emil Leeb, the head of Army Ordnance, and Schumann agreed to call a conference of all uranium researchers for the end of February 1942 to evaluate the status of the project.
The only available copy of the 144-page memorandum to General Leeb, dated simply “February 1942,” lacks a title page and authors’ names. Heisenberg probably did not help to write it, but some of the wording and ideas appear in his reports and lectures at that time. Apparently well aware of Allied research, the scientists’ recommendations are clear: “In the present situation preparations should be made for the technical development and utilization of atomic energy. The enormous significance that it has for the energy economy in general and for the Wehrmacht in particular justifies such preliminary research, all the more in that this problem is also being worked on intensively in the enemy nations, especially in America.” 40 The authors considered time no longer a problem, for recent reactor experiments in Leipzig and
Berlin implied that “success can be expected shortly.” But the building of a nuclear weapon for the Wehrmacht depended on the development of new isotope-separation techniques or the generation of the new element plutonium in the first working reactor. The report concluded that progress toward achieving a working reactor was being hindered less by scientific problems than “by problems pertaining to the acquisition of materials” — problems that, the researchers felt sure, the military sponsors would not find difficult to solve. 41
The army chose to ignore the scientists’ qualified optimism. Leeb and Schumann slashed uranium research funding, reduced activities to Diebner’s army laboratory in the Gottow suburb of Berlin, and abandoned research altogether at the Kaiser Wilhelm Institute for Physics, returning the institute to its sponsoring society. The scientists clung nevertheless to their three-day conference on nuclear technology at the institute, scheduled to start on February 26.
Recognizing a sudden opportunity to regain control of uranium research after two years, Abraham Esau of the rival Reich Research Council scheduled a separate, conflicting series of nontechnical lectures for the opening day of the Army Ordnance conference at the Council’s House of German Research in the neighboring suburb of Berlin-Steglitz. With the army withdrawing from research and leading nuclear researchers eager for a new sponsor, Esau lined up an impressive panel of scientists—-Heisenberg, Hahn, Bothe, Geiger, Harteck, and Clusius — to deliver short lay lectures on nuclear energy development. Their audience was to be the top echelon of army, government, and SS officers — Himmler, Goring, Bormann, Speer, Keitel, Raeder, and others. But fate was unkind to Esau. Apparently through error Esau’s secretary enclosed the wrong list of lectures with invitations to the dignitaries. 42 Instead of receiving Esau’s list of eight nontechnical lectures, they received the list of reports for the rival Army Ordance conference and found themselves invited to hear twenty-five technical talks on such arcana as neutron diffusion lengths, enriched isotopes, and neutron multiplication and absorption factors in the latest Leipzig model. 43 Most of the dignitaries declined to attend.
With REM chief Rust as chair, military researcher Schumann opened the session in the cozy lecture hall of the former private mansion with “Nuclear physics as a weapon.” Hahn followed with “The fission of the uranium nucleus,” after which Heisenberg presented his favorite subject, The theoretical foundations for energy acquisition from uranium fission.” 44 While emphasizing reactor construction, Heisenberg mentioned the possibility of weapons development, but as in the Leeb
memorandum, he neither discouraged government support for atomic research nor encouraged the government to expect a weapon in the foreseeable future. Heisenberg was still walking a fine line.
Heisenberg did attest, however, that a uranium “machine” could soon be built to generate enough power to drive battleships and submarines. Moreover, enough pure U-235 would constitute an “explosive of totally unimaginable power.” But he hastened to add that separating U-235 from a block of raw uranium was difficult and required sophisticated techniques that were still unavailable. An alternative route to weapons lay through the uranium machine: “As soon as such a machine is running, the question of the acquisition of an explosive material . . . also receives a new significance.” A working machine, he told his audience of minor bureaucrats, would produce the more easily separable plutonium, a substance as explosive as U-235. 45
The implication of Heisenberg’s argument seemed to be that if the regime would leave scientists alone, ideologically and professionally, while at the same time supporting and protecting them through reactor research, it would help itself progress toward a more distant but no less cherished goal — the development of a powerful new explosive. Whether or not Heisenberg believed that this goal could actually be reached before the war ended, at the very least the regime could be assured in a few years of a vast new energy source to power the German economy. As one historian writes: “Tailored to both his audience and the times, Heisenberg’s talk illustrated clearly and vividly the warlike aspects of nuclear power.” 46 While true, that was only half the story: from another perspective, it also illustrated the extent to which Heisenberg, the young man who had once thrilled at the precipices of mountain peaks, was willing to flirt with the catastrophic consequences of atomic research for the sake of what he believed would be beneficial to himself and to German science.
Heisenberg’s renewed commitment to this dangerous strategy of enticing Nazi bureaucrats with the potentialities of nuclear energy in order to gain personal and professional advantages was not the effort of a loner. It coincided closely with a major campaign launched by the German Physical Society, aimed at Reich officials, to encourage more material support for physics education and research. 47 With Germany now headed for total war, the society’s main argument was the familiar one: physics and physicists were making tangible contributions to the war effort. Heisenberg fully intended his February 1942 Berlin lecture as a contribution to this campaign to make warfare serve physics by demonstrating how physics could serve warfare.
Two months after he delivered his Berlin talk, Heisenberg’s colleague, Wolfgang Finkelnburg, now vice president of the German Physical Society, congratulated him for his efforts: his lecture to the Reich Research Council and the press reports about it seemed to be having a “satisfactory effect. I have received various inquiries from party officials with questions about the war relevance of theoretical physics and especially about the relevance of your work.” 48 Heisenberg concurred: “In general, the interest of the highest officials in modern physics now seems to have become quite great.” 49 A month earlier, the authorities had suddenly confirmed their interest in Heisenberg and his physics. At the end of April 1942 one of Himmler’s two promises to Heisenberg was at last fulfilled: Werner received a call to Berlin to succeed Debye in the directorship of the Kaiser Wilhelm Institute for Physics and to assume a concurrent professorship in theoretical physics at the University of Berlin. 50
As the leaders of the Reich, unbeknownst to Heisenberg or to anyone else, prepared their final assault on humanity — the “final solution” agreed on at the Berlin-Wannsee conference in January 1942 — Heisenberg and his theoretical physics basked in the prospect of imminent rehabilitation and full recognition by some of those same leaders. The fine line had become a tightrope.
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