Physics in Perspective

, Volume 17, Issue 1, pp 33–54 | Cite as

Werner Heisenberg and Carl Friedrich Freiherr von Weizsäcker: A Fifty-Year Friendship*



This paper follows Werner Heisenberg and Carl Friedrich von Weizsäcker during their fifty-year friendship from 1926, when they first met in Copenhagen, to Heisenberg’s death in Munich in 1976. The relationship underwent profound changes during that period, as did physics, philosophy, and German society and politics, all of which exerted important influences on their lives, work, and interactions with each other. The nature of these developments and their impact are explored in this paper.


Werner Heisenberg Carl Friedrich von Weizsäcker the Weizsäcker family cosmic rays nuclear physics philosophy of physics physics in the Third Reich the German uranium project during World War II West German science policy 

Werner Heisenberg was a Nobel Prize physicist at the forefront of developments in quantum physics. Carl Friedrich Freiherr von Weizsäcker, his former student and assistant, pursued interests ranging broadly across physics, philosophy, and politics. Their friendship spanned a fifty-year period that encompassed profound shifts within each of those areas.

The friendship began as an acquaintance, became a student–teacher relationship, grew deeper, became strained, turned distant, and finally emerged into the mature relationship of long-time colleagues and acquaintances. Each was highly influential on the other—Heisenberg guided his friend and pupil into the intricacies of quantum physics; Weizsäcker provided his friend and mentor perspective and guidance drawn from his complementary skills in philosophy, politics, and people.

The two men first met in Copenhagen in December 1926. Heisenberg, then the university assistant to Niels Bohr, was also a professional-level pianist who often played at the time in evening concerts for the Copenhagen aristocracy. Among the attendees that December evening were Weizsäcker’s mother and his father, then an official in the German embassy. When they learned that the pianist that evening was a German physicist, their precocious fourteen-year-old son (who had read about him in an astronomy magazine) initiated what would become a life-long relationship with the then twenty-five-year-old assistant.1

The Weizsäcker family transferred back to Berlin in March 1927, the same month that Heisenberg submitted his paper on the uncertainty principle for publication. As Weizsäcker tells it, Heisenberg, traveling from Copenhagen to his home in Munich the following month, stopped in Berlin to change trains and invited Weizsäcker to join him for the taxi ride between stations. During the ride Heisenberg intrigued the philosophically inclined Weizsäcker with his newly developed uncertainty relations and his startling conclusion: “I believe I have disproved the law of causality.”2

Heisenberg left Copenhagen in November 1927 to accept a call to Leipzig as professor of theoretical physics. There he established a world-renowned center for research in theoretical quantum mechanics. Although Germany’s youngest full professor, Heisenberg had reached the pinnacle of his profession, but in personal relations his capacities were still quite limited. Despite success in work and teaching, his letters to his parents reveal his frustrating isolation in Leipzig, far removed from his youth-movement friends in Munich. They also reveal a budding friendship with Weizsäcker. It blossomed in June 1928 when the professor invited “the young Weizsäcker,” as he called him, to join him on a two-day hiking tour in the countryside near Leipzig.3 The trip was a huge success. Weizsäcker returned to Berlin to complete his Gymnasium studies and to dream of a career in physics and philosophy. Heisenberg was now a popular guest in the Weizsäcker family home. Reveling in the hospitality of young Weizsäcker, Weizsäcker’s parents, and his three younger siblings, Heisenberg engaged in numerous musical recitals, Sunday outings to the Grunewald, dancing, swimming, sailing, and endless chess matches.4 There were also long hiking tours alone with Weizsäcker, and even a hay-fever-induced return trip together to Helgoland where in 1925 Heisenberg had made the initial breakthrough to quantum mechanics. Heisenberg’s sense of loss following the tragic death of his father in November 1930—the professor of middle and modern Greek philology had contracted typhoid on a research trip to Greece—seems to have intensified their attachment. Through young Weizsäcker, the socially isolated professor became a trusted and accepted member of a cultured aristocratic family and a participant in the higher social and cultural world outside of physics. “Only that night in Pappenheim [an earlier outing with his youth group] and being together with the Weizsäckers have given my life any meaning at all,” he wrote his mother in 1932.5

As an avid member and group leader within the post-war Romantic German youth movement, relationships for Heisenberg took on a much deeper meaning than mere friendship. Reacting with a sense of betrayal at the older generation for having lost the war and the monarchy and repelled by the “superficiality” of the post-war social democracy, Heisenberg and his comrades strove for more “genuine” relationships with those whom they regarded as friends. Drawing upon the Romantic tradition, such relationships were to be built upon a passionate sense of honesty, commitment and trust, resulting in a mutual bonding of souls that gave meaning to their lives within the “decadent” world in which they lived. Such bonding, intended to last a lifetime, served for Heisenberg as the model for all of his personal relationships in this period and especially for those with Carl Friedrich and the Weizsäcker family, who apparently shared such views to some extent. However, the precise nature of these relationships cannot be discerned from the currently available sources.6

Heisenberg’s comrade Carl Friedrich completed his studies in 1929 at the humanistic Bismarck Gymnasium in Berlin. While Heisenberg carried the message of quantum mechanics and the Copenhagen Interpretation to the United States and around the world, Weizsäcker attended the University of Berlin that summer. For the winter semester 1929–30, the now seventeen-year-old Weizsäcker headed to Leipzig to study theoretical physics with his friend and mentor Professor Dr. Heisenberg. Quantum-theoretical physics, Weizsäcker later explained, provided him the basis for pursuing philosophical issues, which interested him even more.7

At that time, Heisenberg had just completed the second of two fundamental papers co-authored with his long-time colleague Wolfgang Pauli on relativistic quantum field theory.8 Despite their progress in uniting fields and particles in this work, the theory was plagued by infinities that did not correspond with reality. The infinities arose from the interactions of charged particles, treated as infinitesimal points, with fields. Only lower approximations to these interactions at lower energies yielded reasonable results; the higher approximations diverged. Such a situation severely limited quantum measurements of electromagnetic fields and seemed to set a high-energy threshold for the application of quantum mechanics to collisions of elementary particles.9

How to deal with these infinities became a topic of much research during the 1930s. They continued to plague quantum electrodynamics as well as other field theories until (for quantum electrodynamics) the advent of renormalization in 1947.10 The appearance of infinities occupied a central place in Heisenberg’s papers and letters in 193011 and in intensive discussions among Heisenberg, Bohr, Pauli, and Paul Dirac during the Sixth Solvay Congress, held in Brussels in October of that year.12 Perhaps a new quantum electrodynamic analysis of the energy limits on measurability as embodied in Heisenberg’s uncertainty relations might illuminate the problem and offer a possible solution. Returning to Leipzig from Brussels, Heisenberg assigned Weizsäcker the task of examining an instrument using quantum electrodynamics that he himself had twice mishandled in the past: the gamma-ray microscope. This hypothetical microscope utilized a gamma-ray photon to measure the position of a free electron. Despite Heisenberg’s faulty analysis of the microscope in Copenhagen where, under Bohr’s critique, he had to correct his initial use of it as an argument for the uncertainty principle (he had earlier mishandled the working of an optical microscope during his doctoral examination in Munich), the gamma-ray microscope remained for Heisenberg a strong argument for the uncertainty principle.13

Weizsäcker’s surer handling of the problem in quantum electrodynamics yielded the anticipated affirmation of the uncertainty principle as well as a reaffirmation of the measurability problem in quantum electrodynamics. It also yielded Weizsäcker’s first major published paper, in 1931, bringing him to the attention of the leaders of his profession.14 From then on, Weizsäcker was a frequent participant in the advanced discussions in Heisenberg’s Institute for Theoretical Physics (a unit of the Physics Institute headed by Peter Debye), on institute outings to Bavaria, sometimes with the aging “boys” of Heisenberg’s youth-movement group, and on skiing vacations with Bohr at the youth group’s Bavarian ski hut. During one eight-day Bavarian outing in February 1935, Heisenberg, Weizsäcker and other Leipzig physicists engaged in intensive discussions with Bohr regarding ways to handle Dirac’s “hole” theory in quantum electrodynamics, in which positrons behaved as holes in a “sea” of negative-energy electrons. This led to a series of papers from Leipzig on the subject, although none on this topic from Carl Friedrich von Weizsäcker (figure 1).15
Fig. 1

Heisenberg, Felix Bloch, Niels Bohr, and Weizsäcker on the Transjoch Peak during an Austrian ski trip, Easter 1932. Source: Emilio Segrè Visual Archives, American Institute of Physics

As Heisenberg’s physics made progress, his halcyon days with the Weizsäcker family came to an abrupt end in 1932. The family suddenly barred the thirty-year-old professor from their home after he naively professed his love for Carl Friedrich’s younger sister, Adelheid von Weizsäcker. Adelheid was at that time a sixteen-year-old Gymnasium student. Heisenberg may even have asked for her hand in marriage. Her staggered suitor could not comprehend the swift negative reaction he received from her family, who, in addition to the impropriety, may have felt he had betrayed their trust. Adelheid’s brother was caught in the middle. Left with little choice if he wanted to continue in Leipzig, Weizsäcker dutifully served as diplomatic intermediary to Berlin and as an emotional support for his devastated friend and mentor. In July 1932, Heisenberg began referring to “young Weizsäcker” as “Karl-Friedrich” (an alternate German spelling) in letters to his mother, perhaps suggesting that Carl Friedrich had become more than just a student. Professors rarely referred to students by first name.16

That summer Heisenberg returned from a very lonely trip to Ann Arbor for a summer school in theoretical physics to face an icy visit to his apartment from Adelheid’s mother. Heisenberg’s mother advised him to break completely from the Weizsäcker family. His response revealed not only his fragile emotional state but also his even closer attachment to “Karl-Friedrich.” Success and fame in science were not enough for him, he wrote his mother, “rather there are moments when I enter into the sphere of serious things… This world is very near to me in Karl-Friedrich or in Adelheid or in any other member of this family, and it has already had a deep meaning for my life… It is therefore important to me that I am able to see K. F. at least once a day.”17

Heisenberg would not relent regarding Adelheid. She did not know what to do. Her brother could only mediate. Heisenberg finally agreed to stay away from Adelheid until she had completed her Gymnasium studies. After she did so in 1934, Heisenberg accompanied her on several outings to the countryside—with Carl Friedrich as chaperone. Her parents apparently now considered her, at eighteen, a young adult. Heisenberg even insisted on visiting her at the family home in Bern, Switzerland, where her father was now at the German embassy. The matter did not reach a conclusion until as late as March 1936 when Adelheid’s father finally informed his wavering daughter that he would not approve of her marriage to the Leipzig professor.18 That opposition was enough for Adelheid to end the relationship. She later married a nobleman who, as a German infantry officer in the coming war, was killed in action on the Eastern front, leaving Adelheid alone in East Prussia with two small children. (Perhaps it was only fitting that, years later, one of Heisenberg’s sons married one of Adelheid’s daughters.)

As Heisenberg’s personal life collapsed in frustrated affections in 1932, bringing down his physical health with it, his and Weizsäcker’s physics suddenly experienced a revival. The so-called miracle year 1932 brought the discoveries of the neutron, deuteron, positron and artificial nuclear transmutation. The next year brought the appearance of cosmic-ray showers—the creation of many particles and photons from the collision of a single high-energy particle with matter. While the positron turned attention to Dirac’s hole theory, the neutron attracted Heisenberg to nuclear physics. Following a Bavarian ski vacation with Bohr in February 1932, Heisenberg presented the first of three papers setting forth the neutron-proton model of the nucleus.19 “The basic idea,” he wrote Bohr, “is to shove all fundamental difficulties onto the neutron and to do quantum mechanics in the nucleus.”20 Despite the turmoil of their personal lives, Heisenberg and Weizsäcker had plenty of exciting physics to keep them occupied.

As he had during the 1920s, Heisenberg was especially inclined to find in physics an escape from the world around him. While Heisenberg’s work focused on the neutron-proton model, then on the implications of hole theory, Weizsäcker worked on the absorption of cosmic rays in magnetic materials, for which he calculated the dependence of energy loss on conductivity. The study was a bit “lacking,” Heisenberg told his mother. But he hastily attributed the difficulty to the problem not to his pupil.21 The “lacking” paper, which did not fully examine the connection with conductivity, was enough for Weizsäcker’s doctoral dissertation, which he formally presented in June 1933 under the direction—not of Heisenberg—but of Leipzig theorist Friedrich Hund. Heisenberg, as first evaluator of the dissertation, awarded him the grade of “very good,” one step below “excellent.”22

A year later, Weizsäcker, at Heisenberg’s suggestion, was again examining the theoretical limits of quantum electrodynamics at high energies, such as those observed in cosmic-ray showers. He made the startling discovery that, theoretically, there should be no energy limit, thus no breakdown of the theory at high energies.23 This stood in stark contradiction with available data at the time regarding so-called cascade showers, the build-up of numerous electrons and photons through a series of individual electrodynamic creation processes. The problem was soon found to lie with the experiments, not with the theory.24 But that did not remove the apparent breakdown of theories for more penetrating particles. These were soon associated with fields other than the electromagnetic field. Beginning in 1936, Heisenberg began arguing that “explosive” cosmic-ray showers (now called multiple processes), unlike cascades, entailed a single creation event involving the newly discovered weak nuclear field. Such events, he argued, presaged a hopeful new revolution in quantum physics, comparable to the quantum revolution of the 1920s. In the end, neither the predicted revolution nor the role of the weak field proved accurate.25

With his doctorate in hand, Weizsäcker was now eligible to serve as Heisenberg’s university assistant. But Weizsäcker wanted a half-year break from Leipzig (and Heisenberg) at Bohr’s institute in Copenhagen before assuming the assistantship in summer 1934. Heisenberg worked instead with his new students Hans Euler and Bernhard Kockel on hole theory and on the perceived quantum revolution lurking in explosive cosmic-ray showers. Returning to Leipzig, Weizsäcker switched research from Heisenberg’s cosmic rays to his mentor’s former interest, nuclear physics. He “habilitated” under Heisenberg’s direction in June 1936, which qualified him to occupy a professorial teaching chair (roughly equivalent to American tenure), with a research paper “On the Spin Dependence of Nuclear Forces.”26 But it too was found lacking. Apparently, Heisenberg had not carefully checked the paper’s contents. Pauli’s assistant Markus Fierz found an embarrassing list of errors in the published version, which provoked doubts from the ever-critical Pauli regarding Heisenberg’s assistant (if not Heisenberg himself).27

As the habilitation episode suggests, Weizsäcker often discussed physics with Heisenberg, but they were not close collaborators. Heisenberg coauthored several papers with Euler in this period, but none appeared with Weizsäcker. In fact, the two men jointly published only one scientific paper throughout their fifty-year acquaintance. This was a paper in 1947 on fluid turbulence and the origin of spiral galaxies, a topic that had arisen during their British captivity together at Farm Hall in 1945.28

In a radio address celebrating Weizsäcker’s sixtieth birthday in 1972, Heisenberg noted that Weizsäcker’s interests extended far beyond physics to the disciplines of philosophy and politics.29 Both of these were also evident in the interaction between Heisenberg and Weizsäcker during those early years, at times providing the basis for greater collaboration than did physics.

As Weizsäcker joined the other students in their work discussions in Leipzig, Heisenberg noted, “he grew unusually animated whenever our physical problems impinged on philosophical or epistemological questions.”30 One of the most significant of those problems centered on the philosophical implications of quantum mechanics, particularly for the principle of causality. As in Weizsäcker’s remembered taxi ride, the challenge to causality, specifically classical Kantian causality, arose from the fundamental uncertainties in the simultaneous measurements of the initial conditions for the motion of a particle according to the uncertainty principle. This resulted in indeterminacy in the particle’s future motion that found expression in the statistical nature of quantum predictions. Heisenberg himself promoted philosophical discussion of the issue in several talks before audiences of philosophers and in correspondence with the physicist–philosopher Moritz Schlick of the Vienna Circle.31 In an address in 1930 to a conference on exact epistemology (published in the journal Erkenntnis a year later), Heisenberg told his audience, “I hope to have made understandable to you… that the situation created by atomic physics really does make necessary a renewed discussion of the law of causality….. that the classical formulation of the law of causality has proved itself to be empty and physically non-applicable.”32

Such statements and their roots in quantum mechanics inspired the philosopher Grete Hermann, then a Göttingen doctoral student under Emmy Noether and a denizen of the neo-Kantian Leonard Nelson school in Göttingen. She sent a paper to Leipzig and Copenhagen in 1933 on an attempt to restore classical causality through a critical logical analysis of the Copenhagen Interpretation. Since causality is a Kantian a priori notion, she argued, it serves as an epistemological foundation of scientific research. Its rejection would mean the rejection of science itself. Both Heisenberg in Leipzig and Weizsäcker, then in Copenhagen, responded with nearly identical criticisms of her paper, bringing Hermann to Leipzig for the winter semester 1934–35. Weizsäcker was now back in Leipzig as Heisenberg’s assistant.33 Her visit led to long philosophical discussions among Hermann, Weizsäcker, and Heisenberg, and it resulted in Hermann’s doctoral dissertation, submitted in Göttingen upon her return in 1935.34 Heisenberg’s recreation of these remembered discussions filled an entire chapter of his recollected conversations.35 He continued these and other discussions about language and objectivity with Weizsäcker during their hiking tours and even at the Bavarian ski hut with Bohr and others in 1935. Hermann’s work also apparently influenced Heisenberg’s response to the challenge raised by Albert Einstein, Boris Podolsky, and Nathan Rosen to the Copenhagen Interpretation of quantum mechanics, in a then-unpublished manuscript that he sent to Pauli in 1935 (figure 2).36
Fig. 2

Heisenberg and Weizsäcker in Leipzig, 1935. Source: Max Planck Society Archive, Berlin

Yet, as Heisenberg portrayed it in his book, his interest tended to wane during the more intense Leipzig philosophical discussions, which he left to the other two. Weizsäcker later stated that Heisenberg simply regarded physics as more important than philosophy. “Well, you can do that when you are an old man,” he once told Weizsäcker. Said Weizsäcker, “He never liked the idea very much that I did too much philosophy when I was supposed to learn physics.”37 Years later, when I asked Weizsäcker about this he replied that, though Heisenberg was interested in philosophy, his interest did not extend much beyond what he needed for his physics and for his public addresses.38 With exciting developments in nuclear physics, hole theory, and cosmic-ray showers, physics, for Heisenberg, clearly took the upper hand.

On the night of January 30, 1933, Heisenberg was with friend Weizsäcker in Berlin looking out from a small student room onto the street below. A seemingly endless torch-light parade of Nazi storm troopers marched beneath them in celebration of Germany’s newly appointed Führer.39 Try as they might to avoid what Weizsäcker called “the unclean medium of politics,”40 politics increasingly intruded on their Leipzig existence from that day forward. Three months after taking power, the Nazi regime announced the dismissal of all Jewish civil servants, which included all professors and teachers, from their jobs in accord with a newly instituted law. Heisenberg and others attempted to reverse implementation of the law and to retain first-rate physicists in Germany. When that failed, he sought to find acceptable replacements for those who had lost their jobs.41 In late 1933, after receiving both the Nobel Prize for physics and the prestigious Max Planck Medal of the German Physical Society, Heisenberg took a leading role in the response of German physicists to the continued dismissals in Leipzig and elsewhere and to the anti-Semitic demagoguery of the “German Physics” (Deutsche Physik) or “Aryan physics” movement.42 In addition to the political guidance that he received in Berlin from the physics profession’s leaders, Max Planck and Max von Laue, Heisenberg closely consulted in Leipzig with a small circle of his Leipzig colleagues regarding university political affairs. Students were on the vanguard of the Nazi movement in the universities. But for personal guidance regarding the “external world,” Heisenberg relied heavily on friend Weizsäcker, the diplomat’s son. To his mother he wrote in October 1934: “I probably have nearly the same position as you regarding the external world. Only the friendship with Carl-Friedrich, who struggles with the surrounding world with his unique earnestness, leaves open to me a small entrance into that for me otherwise foreign territory.”43

After a failed attempt to halt a second round of Leipzig faculty dismissals in 1935, Heisenberg was stunned to find himself the target of Nazi demagogues attempting to control physics education through professorial appointments. Heisenberg was by then the leading candidate to succeed the famous Arnold Sommerfeld, one of the founders of quantum theory, in the Munich university chair for theoretical physics. Sommerfeld had retired in 1935. In January 1936 the Nazi party newspaper Völkischer Beobachter published an article by a physics student calling for the replacement of “Jewish physics” (the quantum and relativity theories) at universities by “Aryan physics” (classical physics). Heisenberg learned from his Leipzig dean that the article and similar publications “were expressly intended as attacks against me.”44

While Heisenberg gathered his resources to publish a response in the same party newspaper, he received more startling news. Just weeks before March 1936, when Adelheid’s father ended her further association with Heisenberg, Weizsäcker informed Heisenberg that he himself had become engaged and that he was planning to leave Leipzig as soon as possible. Heisenberg was devastated. He wrote his mother, “I have had for almost ten years no other person for myself but him; and he is now leaving, forever. I am not feeling well, even regarding my health.” Looking ahead to the prospect of required military service that summer at the same time as he prepared his potentially revolutionary paper on cosmic rays, the Nobel Prize winner lamented to his mother, “I have to have a lot of luck if anything more is to become of my life.”45

Weizsäcker left Leipzig for good even before he had passed his habilitation oral exams in June 1936. Max Delbrück, Lise Meitner’s assistant at the Berlin Kaiser Wilhelm Institute for Chemistry and the “house theoretician” for the radiochemistry group headed by Meitner and Otto Hahn, planned to visit Bohr’s institute in Copenhagen that summer. In January, Meitner inquired of Heisenberg whether Weizsäcker might want to substitute for Delbrück. Although he had already agreed to assist Heisenberg’s colleague Pauli that summer, Weizsäcker jumped at the opportunity to return to the German capital with his future bride.46 He arrived in Berlin in April 1936. With Delbrück’s planned return to Berlin imminent in August, the prospect arose that Weizsäcker might have to return to Leipzig after all. But, now habilitated, he could no longer serve as Heisenberg’s official assistant. Since no professorial position was then available in Leipzig, at best he could be a mere Privatdozent, a private lecturer. Heisenberg’s primary work had already long shifted from nuclear physics to applications of nuclear field theories to cosmic rays, which coincided more with Euler’s work. After Heisenberg selected Euler as his new assistant, Weizsäcker informed Heisenberg that he had no intention of returning. Instead, he asked his mentor to write a letter of recommendation to Peter Debye, the former Leipzig experimentalist who was now director of the soon-to-open Kaiser Wilhelm Institute for Physics in Berlin.47 The institute, after its completion in 1937 with the help of Rockefeller Foundation funds, focused on nuclear research. Weizsäcker joined the Kaiser Wilhelm Institute for Physics in October 1936 but continued to work closely with Meitner at the nearby chemistry institute on possible transuranic elements until the opening of the new building. Delbrück, meanwhile, headed instead to Caltech on a Rockefeller Foundation fellowship to work in the newly emerging field of molecular biology.48 Weizsäcker remained at the physics institute through the war years and beyond. Much of his prewar work at the institute focused on nuclear binding energies; fusion processes in the sun, partially paralleling Hans Bethe’s Nobel Prize winning work on solar fusion, and on the formation of the solar system and Milky Way galaxy.49

Despite Euler’s professional and even personal support, Heisenberg felt once again desperately alone in the Leipzig “wasteland” during his first winter without Weizsäcker, a winter during which he suffered continued humiliation by the Nazi regime. He began to slip into depression and even, from his description, into a nearly psychotic state.50 But Heisenberg’s mood suddenly improved in January 1937 when he met his future wife, the much younger Elisabeth Schumacher, the daughter of a Berlin economics professor. They were married three months later. Ten months after that Pauli congratulated his colleague for “pair creation”; Elisabeth had given birth to fraternal twins, the first of their seven children.51

Heisenberg and Weizsäcker did not meet again professionally until after the outbreak of war in September 1939. Army researcher Kurt Diebner and former Heisenberg student Erich Bagge summoned our two subjects under military orders to a late-September meeting of leading German nuclear scientists in Berlin. The purpose of this “uranium club” (Uranverein), as they called it, was to initiate and coordinate Germany’s wartime research on the practical application of a recent discovery at the Berlin Kaiser Wilhelm Institute for Chemistry, nuclear fission.52

The formation of the uranium club also marked the beginning of a more mature relationship between Heisenberg and Weizsäcker as colleagues and acquaintances. By then, both men had married, established families, and secured permanent positions. In addition, both had reached accommodations with the regime and with themselves that enabled them to remain in Germany and to work on fission for the German army at war. Although the two men often discussed their research, it was, as before, not a close collaboration. Nor did Heisenberg rely entirely on Weizsäcker’s skills in the areas of politics and philosophy. Nevertheless Weizsäcker at times did play a key role and exert a profound influence on Heisenberg’s most important outlooks during the three political and ethical phases of their lives after 1939: the war years, the post-war controversy over their war-time nuclear research, and the debates over West German nuclear policy.

A great deal has been written about Weizsäcker and Heisenberg in each of these periods.53 I will focus here mainly on the highlights. Working on nuclear fission under the Hitler regime at war might well have led to unimaginable consequences. Within weeks of the founding of the uranium club Heisenberg produced two secret theoretical reports to the German army’s Ordnance Bureau in which he told of the possibility of a nuclear reactor and, through the separation of uranium isotope U-235, the prospect of a nuclear explosive, “the explosive power of which exceeds that of the strongest available explosives by several powers of ten.”54

With the army’s interests duly awakened, the members of the uranium club scattered to their laboratories with assignments for intensive research to realize the predicted prospects. Heisenberg’s primary research remained in Leipzig where he researched various reactor configurations, but through the machinations of Weizsäcker and Karl Wirtz in Berlin he also worked closely with Debye’s Kaiser Wilhelm Institute for Physics after the regime had forced Debye, a Dutch citizen, out of the institute. In order to prevent the appointment of a Nazi functionary and in recognition of Debye as the only legitimate director, in 1942 they arranged for the assignment of Heisenberg, in a turn of phrase, as director “at” but not “of” the institute.55

At the beginning of the war, physics and physicists were still regarded as ideologically suspect. Thus, as Heisenberg described it years later in a remembered conversation with Weizsäcker, one motivation as well as rationalization for their uranium work was to demonstrate to the authorities the utility of modern physics for the war effort. In addition, they could argue, such utility could be used to help silence the ideologues (with young Weizsäcker’s diplomatic help), preserve physicists from the front, enable the survival of decent research in Germany, and permit leading physicists to travel abroad (with the senior Weizsäcker’s help).56

While the strategy yielded some positive results, it came under question as the possibility of an atomic bomb grew more likely. Otto Hahn’s Berlin team (now without Meitner) had discovered that the uranium isotope U-239, produced in a reactor from isotope U-238 by the absorption of a neutron, undergoes beta decay. In July 1940, Weizsäcker reported to Army Ordnance that the beta decay of U-239 should yield the new element 93 (uranium being element 92), today called neptunium, and that neptunium should be as fissionable as U-235. Thus the bombardment of the plentiful isotope U-238 by two successive thermal neutrons could result in fission: the first neutron produces U-239, which decays in 23 min into element 93; the second neutron fissions the newly formed neptunium. A month earlier an American research team had already published in Physical Review the discovery that neptunium decays with a half life of 2.4 days into the long-lived element 94, now called plutonium. In August 1941, German researcher Fritz Houtermans theoretically confirmed that plutonium is at least as fissionable as U-235 and that it could sustain a chain reaction yielding the explosive release of enormous amounts of energy. In addition, it promised to be more easily obtained than U-235 since it could be separated from uranium in a reactor by familiar chemical methods.57

Suddenly the road to an atomic bomb opened before the German researchers. It was no longer a mere theoretical possibility. Heisenberg engaged in intensive discussions with Weizsäcker and their colleagues at the Kaiser Wilhelm Institute for Physics about the situation, about whether and how they should proceed, and about the possibility of an Allied atomic bomb (which they regarded as very remote). Weizsäcker had recently returned from a visit to Copenhagen in German-occupied Denmark where he had met briefly with Bohr. All agreed that Heisenberg should talk with Bohr about the situation.

But for those whose loyalty was suspect, permission to travel outside the Reich, or even to occupied countries within the Reich, was difficult to obtain. Through pressure exerted on party bureaucrats, in part by Weizsäcker’s father in the Reich’s Foreign Office, Heisenberg and others were able to make a number of trips to German-occupied countries, usually for the official purpose of cultural propaganda.58 Among these trips, Heisenberg and Weizsäcker traveled, together with Max Planck, to Budapest in German-occupied Hungary in 1942 and, without Planck, to Copenhagen in September 1941. Supported by the senior Weizsäcker, the official purpose of the Copenhagen trip was to demonstrate the vibrancy of German science through a small conference at the German Scientific Institute, a propaganda unit in Copenhagen. The unofficial purpose was for Heisenberg to discuss the problem of nuclear weapons with Bohr.

Many analyses and a popular theatrical play have been written about the Copenhagen visit and the content of Heisenberg’s unrecorded conversations with Bohr.59 Whatever was actually said during Heisenberg’s private meeting with Bohr, the trip proved a disaster. Bohr received the impression that the Germans were working feverishly on nuclear weapons, and he later conveyed that impression to the Allies. Heisenberg later argued that he had been completely misunderstood.60

Misunderstood or not, Heisenberg and Weizsäcker returned to Germany and continued their work on fission, though with reduced resources and urgency after 1941. Meeting its match on the Eastern front, the Reich, now mobilized for all-out war, turned its support to more immediate weapons, while the demagogues of “Aryan physics” were nearly silenced. Both men began to pick up their earlier work once again. In 1942, Heisenberg published the first of a series of papers on his so-called S-matrix theory of particle scattering, which enjoyed a period of popularity after the war.61

At the same time, Weizsäcker accepted a professorship in German-occupied Strasbourg in 1942 and promptly ceased work on nuclear fission in favor of his main research interest, astrophysics. While he and other physicists regarded Weizsäcker’s appointment as another triumph over their ideological enemies,62 Weizsäcker surely regarded it also as a way once again to escape Heisenberg’s shadow. It became for Heisenberg, a year later, a most fortunate move. On October 14, 1943, in the depth of total war, a shaken Heisenberg wrote to his wife from Berlin that he had a long and disturbing conversation with Weizsäcker there about Germany’s war aims. Heisenberg found Weizsäcker’s support of “the new belief” not only “completely foreign” but “totally unbearable,” even to the extent that, he wrote, “I am evidently entirely unGerman.”63 The conversation caused a schism between the two physicists that lasted at least until the end of the war, and perhaps beyond.64 After the “catastrophe,” most Germans, including physicists, attempted to forget everything that had happened before 1945.65

Despite his revived research on particle physics, Heisenberg and his Berlin reactor team began pushing ahead once again in a race to complete the reactor before the end of the war. The project moved to southern Germany in 1944 when heavy Allied bombing made work in Berlin impossible. The self-sustaining reactor would serve as evidence that decent German physics had indeed survived during the Reich. Samuel A. Goudsmit and the American Alsos Mission, dispatched into Germany to halt German fission research, captured Heisenberg and Weizsäcker just before the end of the war and detained them together in American prisoner of war camps before sending them, along with eight other German nuclear scientists, to British captivity for six months at Farm Hall, an English country manor that served as a MI-6 safe house. The German project had never achieved a chain reaction.66

Thinking themselves far ahead of the Allies in fission research, the Farm Hall detainees were shocked to learn of the Allied atomic bombs dropped on Japan in August 1945. Suddenly the scientists were faced with explaining to themselves and to their countrymen that their failure to achieve the bomb, or even a reactor, was not the result of incompetence and explaining to the Allies that their work was not directed toward a nuclear weapon but only at achieving a nuclear reactor. Transcripts of secret recordings of their conversations made by British agents indicated that the politically adept Weizsäcker took the lead in helping Heisenberg and most of the others (except Max von Laue and perhaps Otto Hahn) to formulate a dual response. To the world, they emphasized in an official memo initially drafted by Heisenberg and Weizsäcker, that the material conditions of war had limited their work to the reactor: “It did not appear feasible at the time to produce a bomb with the technical possibilities available in Germany. Therefore the subsequent work was concentrated on the problem of the engine [reactor].”67 But among themselves they began to emphasize their moral scruples about nuclear weapons for the German cause. According to Max von Laue, Weizsäcker took the lead in formulating what Laue called a “Lesart,” i.e., an interpretation.68 “I believe the reason we didn’t do it was because all of the physicists didn’t want to do it, on principle,” Weizsäcker is quoted as saying. “If we had all wanted Germany to win the war we would have succeeded.” Although Laue and Hahn objected to this interpretation, Heisenberg echoed the sentiment to Hahn in a paraphrased comment in the transcripts: “he feels himself that if they had been in the same moral position as the Americans and had said to themselves that nothing mattered except that Hitler should win the war, they might have succeeded, whereas in fact they did not want him to win.”69

Weizsäcker took this even a step further, suggesting that it was the Americans who were in a difficult moral position, since they had built and used the bomb, while the Germans bore no responsibility, since they had done neither. This position, in almost identical wording, reappeared in an early history of the atomic bomb published in 1956 by Swiss journalist Robert Jungk.70 Acknowledging assistance from Weizsäcker, Jungk argued that the Germans deliberately maintained control of the fission project with the moral intent “to divert the minds of the National Socialist departments from the idea of so inhuman a weapon.”71 Jungk then published in the Danish and English translations of his book an excerpt from a letter by Heisenberg giving his account of the 1941 meeting with Bohr. The book and Heisenberg’s letter met with widespread criticism in the United States and with a series of recently released angry draft letters to Heisenberg from Bohr, which, however, were never sent.72

It may be already evident that the context of the discussion regarding Germany’s war-time failure to produce a bomb, or even a reactor, was heavily influenced on both sides by the post-war environment—the deepening Cold War, the US military’s drive for influence over American physics research, anger over German atrocities, guilt about building the bomb, and efforts by German scientists to put the best light on their war-time work in order to promote their own post-war status as well as the revival of German physics. While Heisenberg and Weizsäcker portrayed German research as highly competent and morally untainted by nuclear weapons, Goudsmit and others argued just the opposite.73

Heisenberg and Weizsäcker returned to Germany in 1946 with the other Farm Hall detainees, many of whom settled in Göttingen. Heisenberg became official director of the Kaiser Wilhelm Institute for Physics in Göttingen, which was renamed the Max Planck Institute for Physics. Weizsäcker became a member of the nuclear physics section, headed by Karl Wirtz, within the institute. While Heisenberg promoted to the Allies and to the new West German federal government his vision for the reconstruction of West German science and the need for peaceful nuclear reactor research, Weizsäcker found himself in demand in the United States for his astrophysics, even though he remained a controversial figure for his wartime work and his interpretations of it.

The founding of a cabinet level ministry for “atomic questions” in 1955 brought Weizsäcker increasingly into the “unclean medium of politics.”74 It began with his appointment as chair of the ministry’s Working Group on Nuclear Physics (Arbeitskreis Kernphysik). Supported by Weizsäcker‘s working group, the ministry’s decision in 1956 to fund construction of a reactor at Heisenberg’s Max Planck Institute proved to be another turning point in Weizsäcker’s career and in his relationship with his long-time friend and former mentor. The decision induced a fissioning of the institute itself whose Göttingen grounds were deemed insufficient for a reactor. As negotiations with the Bavarian state and industry proceeded over Heisenberg’s plan to move the institute to larger grounds just outside his beloved Munich, Wirtz abruptly took the reactor project with him in 1957 to the newly founded Center for Nuclear Research near Karlsruhe in southwest Germany. Meanwhile, Heisenberg had been attempting to satisfy Weizsäcker’s “wishes for independence” since 1954, but apparently without success.75 Weizsäcker headed in the opposite direction, north to Hamburg where, also in 1957, he accepted appointment as professor of philosophy. Heisenberg wrote, “I was sad to see my close and long collaboration with Carl Friedrich and Karl Wirtz coming to an end.”76 A year later he moved the institute, now called the Max Planck Institute for Physics and Astrophysics, to Munich—without Weizsäcker or Wirtz.

As the Cold War deepened during the middle and late 1950s and as criticism of the German war research mounted in the wake of Jungk’s book, Weizsäcker’s work and interests turned almost exclusively to nuclear politics. German scientists, disturbed by the reaction to Jungk’s book and by West German nuclear policy, needed to distance themselves from both past and present nuclear weapons. As chair of the federal working group, Weizsäcker led Heisenberg and sixteen other prominent German atomic scientists in opposition to the West German government’s intention to accept a NATO plan to equip the West German army with tactical nuclear weapons.77 He took the lead in drafting a public “Declaration of Eighteen Atomic Scientists,” known as the “Göttinger 18,” that upon release on April 12, 1957 rallied public support against the government plan and against the notion that tactical weapons would be any less devastating than strategic weapons. A small state like West Germany on the front line of the Cold War, they argued, could best defend itself by renouncing nuclear weapons. The scientists renounced any participation in the development, testing, or use of nuclear weapons.78 The declaration proved a success. West Germany embraced nuclear energy, but it renounced the building and possession of nuclear weapons, as the scientists urged. Nevertheless, the West German army began training to use such weapons, which the Americans would provide when needed (figure 3).
Fig. 3

Heisenberg and Weizsäcker in Munich, 1966. Source: Weizsäcker family

Buoyed by the success of the Göttinger 18, Weizsacker served as West German representative to the international Pugwash conferences on nuclear disarmament. In 1962, he led a call by prominent German Protestant figures for more government support of social programs and against the general “possession of atomic weapons under national sovereignty.”79 He also led the new federal Working Group on Fusion that fostered research on a fusion reactor at the Munich institute’s branch location in Garching. In order not to lose touch with Heisenberg or his physics, the independent Weizsäcker accepted an invitation to remain a full member of Heisenberg’s Munich institute, and he agreed to spend at least one month in residence there each year.80 In addition to astrophysics, the philosophy of physics and Heisenberg’s attempts to achieve a unified field theory became topics of discussion during Weizsäcker‘s visits, according to Heisenberg.81 Heisenberg did not mention politics or nuclear policy, which surely also gained their attention. During that time, Heisenberg, now an “old man,” continued an effort that he had begun in the depths of the war: to place his work within the context of a larger intellectual tradition.82 In his discussions with Weizsäcker, that tradition was increasingly identified as ancient Greek philosophy, at first Aristotelian philosophy, then, by the 1960s, Platonic philosophy.83

Perhaps sensing the end was near, in 1970 Weizsäcker resigned his Hamburg chair and moved south to Starnberg in Bavaria, not far from Heisenberg in Munich. There he headed a new “peace institute,” officially named the Max Planck Institute for Research on Living Conditions in the Technologic-Scientific World. Six years after the move Weizsäcker’s friend, colleague, and former mentor died in Munich on February 1, 1976. Heisenberg’s long-time friend, collaborator, advisor, and former student died at his home near Starnberg on April 28, 2007.



I am very grateful to Elise Crull, A. J. Kox, Mark Walker, Dieter Hoffmann, and Karl von Meyenn for generous assistance with information and sources, and to Karl von Meyenn for his comments and suggestions. I am also very grateful to Klaus Hentschel and Dieter Hoffmann for editorial comments on the earlier German version of this paper, and to Robert P. Crease and Peter Pesic for editorial comments on the current version.


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© Springer Basel 2015

Authors and Affiliations

  1. 1.Natural Science ProgramHofstra UniversityHempsteadUSA

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