The Mysterious Meeting between Niels Bohr and Werner Heisenberg

The Bohr-Heisenberg Meeting at Copenhagen: 70^th Anniversary September 15-21, 1941

In the fall of 1941, the course of history in the Second World War took a different pathway based upon a mere conversation between two men. At first glance, it would appear unlikely such an event could have such an effect, particularly since both men were intentionally vague with each other, each later maintained misunderstandings of the other’s intention, and to this day the conversation remains shrouded in mystery. The two men were civilians, not soldiers; they were scientists, not politicians; they met in Nazi-occupied territory, not in free lands where they could speak frankly. Lastly and most importantly, they were thinkers dedicated to the discovery of knowledge of the natural world, not the mass destruction of human life.

But when the participants in this uncertain conversation were Niels Bohr and Werner Heisenberg, then the import of the event becomes clearer. On the surface, the Dane of Jewish descent and the German Lutheran, separated in age by sixteen years, did not have much in common. But their lives were deeply intertwined with each other on personal, intellectual, and professional levels. They began as an internationally recognized physics professor and gifted student when they first met in 1922, the year Bohr became a Nobel laureate. But they became much more than that. Personally, Heisenberg virtually became another of Bohr’s sons and shared the intimacy of his family life. Intellectually, Bohr’s manner of thinking about physical problems in which he attempted to comprehend phenomena as a whole found balance, advancement and authenticity through collaboration with Heisenberg, whose fascination with and ability to find the music of mathematics located within physical events led to great scientific breakthroughs.

Professionally, the two men mapped and explored the quantum universe inside the atom. In 1927, Heisenberg staked his own claim to great scientific achievement when he published his “Uncertainty Principle,” which stated that the exact position of an electron within an atomic nucleus at a given time could not be known with certainty, but only statistically calculated within a probability. By that same year Bohr had developed his ideas on “complementarity” in physics, through which he incorporated Heisenberg’s physics within his own, and proposed that the seeming chaos of the quantum world and the order of the universe based upon classic physics did not refute each other, but complemented each other in a manner which we had yet to comprehend and explain. The two men were the fathers of quantum mechanics, and together they had carried out a revolution in the physics world during the 1920s. In 1932, Heisenberg joined Bohr as a Nobel laureate.

Bohr, his wife, & Heisenberg in Greece (1920s)

Science was traditionally seen by its practitioners as an international fraternity at the beginning of the twentieth century. Information, news of discoveries, papers and ideas flowed freely across national boundaries between scientists intent upon expanding the frontiers of knowledge. This was an idealized era in science; while not free of individual rivalries, nonetheless the atmosphere was not romanticized. Scientists viewed themselves as colleagues in the craft of pursuing higher truths, not as national rivals. This was the atmosphere of Ernest Rutherford, Max Planck, and above all, Albert Einstein. Niels Bohr also entered this idyllic prewar scientific atmosphere. In 1913, he published “On the Constitution of Atoms and Molecules,” a paper which brought him reputation and fame when he described how electrons jumped between orbits within the nucleus.

However, the instabilities of the international political system began to change this idyllic scientific atmosphere after 1914. The Great War saw scientists mobilize on behalf of their national governments to use science as a new offensive weapon, creating new and more devastating ways to kill. The Great War did not kill off this atmosphere of scientific cooperation. The German-born Einstein completed his General Theory of Relativity while employed at the University of Berlin in 1915; copies of his paper were smuggled to Allied scientists, and the British astronomer Arthur Eddington successfully arranged financing from his government during wartime to photograph a solar eclipse in 1919 to prove Einstein’s theory. Nonetheless, nationalist politics showed itself as the greatest threat to the idealized world of free inquiry and knowledge which scientists held dear.

Bohr and Heisenberg had inherited the benefits of this atmosphere in the 1920s, when together they explored the quantum revolution. However, the rise of the arch-nationalist, Adolf Hitler in Germany, ushered in a new and shocking suppression of scientific inquiry and knowledge. Even before Hitler achieved power in Germany, Heisenberg personally experienced opposition to the “new physics” by hostile German scientists who demanded a German or “Aryan Physics” which applied to natural Germans (supporters of this “Deutsch Physik” included Nobel laureates such as Johannes Stark). The revolutionary physics of relativity and uncertainty, led by Einstein, Bohr, and Heisenberg, was tied to impurity, deformity and Judaism by the Nazis in a manner similar to how they dealt with the modern art of cubism and Picasso. It should be noted that the new physics (along with Freudian psychology) had been condemned in the Soviet Union as well in the early 1920s for not conforming to the precepts of Marxist-Leninist ideology. But where Russia was seen as scientifically and technologically backward, Germany was the most advanced educational, scientific, and technological power in Europe.

After Hitler assumed the Chancellorship in January 1933, the Nazis soon banned all Jews from working for the German state or in professional capacities such as university professors. This meant that better than a hundred German physicists of Jewish descent, including Einstein, were soon displaced. An exodus of the world’s greatest scientific talent began to emigrate from Germany to receptive western nations. From his Institute of Physics attached to the University of Copenhagen in Denmark, Niels Bohr tried to assist many colleagues in finding new lives and employment.
Werner Heisenberg was not an anti-Semitic; indeed, he even tried to hire a Jewish colleague into an open position at his University of Leipzig in the 1930s. He did not join the Nazi party. However, he was a dedicated German nationalist. He participated in the military drills of his reserve unit. Most distressing to his international colleagues, he refused to follow the example of Einstein and leave Germany as a symbolic protest against the Nazi regime and its attitude towards scientific inquiry.

But advancements in the physics and political worlds were on a collision course. One of those forced to flee Germany due to Hitler’s policies was Lise Meitner, an Austrian Jewess formerly employed at the Kaiser Wilhelm Institute where she had collaborated with the chemist Otto Hahn. After her departure, Hahn continued to send to Meitner information on experiments he had conducted upon the element uranium. On a skiing holiday, while seated under Kungalv castle in Sweden on Christmas Eve 1938, Meitner and her nephew Otto Frisch successfully diagnosed and calculated that nuclear fission had taken place. Within two weeks, they had delivered to Niels Bohr their preliminary paper which he took with him to America. In January 1939, the news was made public at a physics conference at George Washington University. To leading physicists, the possibility of splitting the atom and releasing untold amounts of energy through nuclear fission was now within reach, along with the understanding that it was now theoretically possible to build an atomic bomb. In April 1939 the first “Uranverein,” or German “Uranium Club” was established. This was a relatively small group of forty or fifty German scientists assigned to work upon the problem of nuclear energy. This was the situation in September 1, 1939 when war came to Europe. On the day that Germany launched the invasion of Poland, the German Army Ordnance Office took over the German nuclear energy project to explore potential military applications. This second Uranverein was a military and state secret. From his posts as a professor at the University of Leipzig and Acting Director of the Kaiser Wilhelm Institute for Physics in Berlin, Heisenberg became the lead theoretician of the Uranverein, cooperating with professors spread across Germany to study and develop nuclear fission.

Initially, physicists across the globe calculated that any proposed atomic bomb project would be years in the making due to the tremendous technical difficulties, scientific problems, labor commitments, and financial resources required of the undertaking. In the meantime, however, the war moved quickly. Hitler and the Germans had invaded Norway, Denmark, Holland, Belgium, and France by the summer of 1940; from his physics institute in Copenhagen, Niels Bohr found himself living under the Nazi occupation. Only Britain fought on alone against Hitler for over a year, until in June 1941 Hitler double-crossed his ally Stalin. The German invasion of the Soviet Union was initially very successful, as Operation Barbarossa saw deep German penetration of Soviet territories. By September 1, 1941, Leningrad was completely cut off from the rest of the Soviet Union except by Lake Ladoga; the Soviet military teetered under the terrific German onslaught, and it appeared the Germans might take Moscow before Christmas. The United States officially remained a neutral nation. And the Uranverein had recently conducted experiments which hinted at the pathway to develop a self-sustaining nuclear fission reaction. The ability to create an atomic bomb appeared to be a key that might guarantee ultimate victory to whoever developed it first, with Hitlerite Germany in the lead.

It is against this historical backdrop of what appeared to be impending German victory and potential German bomb that Werner Heisenberg, Nobel Laureate, chose to arrange travel to attend a German-sponsored conference in Copenhagen in September, 1941. Bohr would not attend the conference as a protest against the Germans, but he was willing to personally see Heisenberg. Heisenberg’s purpose was to query his great mentor and friend, Niels Bohr, on a major issue troubling his mind: namely, should a physicist help build an atomic bomb for his country in wartime?

It is difficult for historians to reconstruct exactly what was said between the two men. Heisenberg was so eager he arrived four days early for the conference, but neither man could remember the exact date or place of the conversation. In later years Heisenberg recalled the conversation taking place on a long walk around the harbor in Copenhagen; Bohr recalled that it took place in his study. Heisenberg was a leader in a secret German nuclear program. To merely reveal the existence of such a program was treason, and within Hitler’s state Heisenberg would almost certainly lose his life. But for Bohr, this was countered by the fact that Heisenberg was a loyal German national, and thus working for the victory of the Nazi state.

The two men’s personalities came into play. Heisenberg, the mathematical pragmatist, thought that the political situation should be viewed realistically. After arriving, he sought out Bohr at his Institute, and at lunch with Bohr and his colleagues (living under Nazi occupation) he stated that the German occupations of the Western European nations were unfortunate. However, he expected a German victory, and insinuated that a German victory over Communist Russia was a preferable, likely outcome. Given these realities, Heisenberg seemed to think that his colleagues should come to the best terms possible with the Hitler regime.

This set up the private conversation that most likely took place on Wednesday evening, September 17. It seems likely Heisenberg desired to have this conversation outside, to avoid detection or recording by the Gestapo, and he assumed that Bohr was under Gestapo surveillance and might likely reveal their talk to others; he wished to protect both men by providing deniability of an exact discussion. Bohr was already wary, and potentially angered by Heisenberg’s seeming insensitivity to what Nazi occupation meant to his own country and colleagues. Heisenberg began the conversation by confirming his lunchtime views. (He apparently believed that his version of political realities, of Germany’s impending victory, might influence others to his way of thinking, but instead it hastened alarm in Bohr). Heisenberg then brought up the subject of atomic weapons in a vague manner; he alluded to working on the “uranium problem,” and to the resultant “grave consequences in the technique of war.”

Bohr responded by asking Heisenberg if such a weapon was truly possible. Heisenberg knew that Bohr had been more recently in contact with Allied physicists and had more clandestine opportunities to communicate with Allied physicists than he had within Germany. Bohr thus might presumably know about the progress of the Allied nuclear effort. Heisenberg then replied that a “terrific technical effort” was required, but left no doubt that he personally knew such a weapon was possible, and produced a drawing of a reactor. This was not a bomb, but a reactor would produce the uranium or plutonium which was necessary to build an atom bomb. But Bohr had never seen a reactor, and did not distinguish it from a bomb design.

Heisenberg apparently assumed that Bohr was not convinced of the feasibility of building a bomb, but this was mistaken. By handing over the drawing to Bohr, Heisenberg had committed a treasonous act. Heisenberg appears to have thought that this would indicate to Bohr that the Germans had not produced great progress in building an actual bomb. He claimed that he hoped to initiate, through Bohr, a conversation to the physicists in America to resist building a bomb. If physicists on both sides tacitly refused to work on nuclear fission, the world would be spared of the future reality of the bomb and its destruction. But Bohr suspected Heisenberg was trying to gain intelligence information for Germany on the Allied efforts by convincing him that Germany would soon have an atomic bomb. He did not fully understand that the drawing was of a reactor until he handed it over to the physicists at Los Alamos in late 1943, who identified it as a reactor, not a bomb.

Bohr abruptly ended the conversation. Although they parted amicably later that week, Bohr’s basic reaction was a mounting and sustained anger. To Heisenberg, his main reaction was a sustained despair. After the war, the two men were never able to reconcile the details of the conversation. They were unable to agree upon basic facts and what was precisely each said and heard, much less gain a clear understanding of the context, meaning, and implications each meant to convey.

Heisenberg & Bohr postwar meeting

The major point about this conversation was the differing perceptions of political realities and motivations these two men had. They were both skilled in intuiting the way nature worked on the subatomic level, grasping realities beyond human sight, and constantly thinking and theorizing how to determine true knowledge. Presumably, in speaking to each other, they might understand each other’s political positions and goals through these methods. Bohr claimed that his perception of reality was that Heisenberg was attempting to seduce him to the morally tainted German side by emphasizing the probability of German victory. Even worse, Bohr thought he might be attempting to bring Bohr to dishonor through divulging information on the Allied nuclear effort. Heisenberg claimed that this was not true at all. In his perception of reality, he was attempting to inform Bohr that the Nazis knew that nuclear fission was possible, but that he was in a position to neutralize this effort. He claimed that working through Bohr, he might get the Allied scientists to do the same. The international physics community, but tacit consent, might cooperate to spare the world of this horrendous weapon.

How did this change the nature of the Second World War? What Bohr heard from Heisenberg was that he was working to get Hitler a bomb. Recognizing the immense scientific asset that Bohr might be to the Germans (and fearing for his safety given his Jewish heritage), in 1943 the British smuggled Bohr out of occupied Denmark. Bohr traveled to America and was brought into the Los Alamos laboratory, where he showed the Manhattan Project scientists the drawing of the reactor Heisenberg had given to him in September 1941. Niels Bohr, the icon of nuclear physicists, seen as seeker of truth on a level with Einstein, brought with him the opinion that Hitler and the Germans were building a bomb, and gave his moral blessing on the Allied effort to accomplish the same goal. It arguably spurred and gave penance to Allied scientists who themselves had moral qualms over building such a destructive weapon. The Americans spurred their effort and resources to succeed. At the cost of over $2 billion, the employment of over 120,000 laborers during war, and through the mobilization and spurred motivation of the American scientists, they succeeded at Alamogordo with the Trinity shot, July 16, 1945.

But with the fall of the Third Reich in May 1945, it became abundantly clear to the Allies that the German nuclear effort had fallen far behind the Allied effort. How did this happen? And what of Werner Heisenberg, Bohr’s most accomplished student and collaborator? What is not in dispute is that the German nuclear effort fell by the wayside as a military priority less than a year after Heisenberg had his discussion with Bohr. By early 1942, pressures in the eastern war forced the Nazis to economically prioritize their effort, and the Uranverein was asked for specific estimates on its operations. Before leading Nazi figures in February 1942, Heisenberg lectured on the use of building a reactor, but did not emphasize the feasibility of a bomb. In the resulting bureaucratic reorganization, Heisenberg again had to appear in June 1942 before Albert Speer himself, the czar of the Nazi economy. Heisenberg emphasized the enormous financial and labor cost of any nuclear project during wartime Germany, stated that the weapon could not be quickly built, and gave contradictory statements on what was scientifically required. The Nazi political and military leaders drew the conclusion that an all-out nuclear effort would be too expensive, probably fail, that the scientists themselves were less than optimistic or certain of the effort, and that their energies would be better directed into more promising weaponry such as the V-1 rocket. On June 23, 1942, Speer informed Hitler of these positions, and the Nazi nuclear effort faded.

Did Heisenberg intentionally kill the project? While he hinted at this in the postwar years, he would never directly admit the charge, as he believed he would be blamed for losing the war by his countrymen. He maintained simply that the German physicists had been spared the moral dilemma of building an atomic bomb. After the war, Bohr and Heisenberg managed to reestablish a strained and civil relationship, but it was only maintained by their tacit agreement to disagree, and not revisit or attempt to overtly mend a disputatious past.

Unfinished draft of Bohr's recollections to Heisenberg, never sent

It is fitting that historians have not been able to reconstruct with complete precision the events, discussion, and human elements of the Bohr-Heisenberg meeting in September 1941. This confused conversation shaped and impacted the thinking of scientists regarding an atomic bomb on both sides. The meeting may be a parable of our efforts to know ourselves and our universe, our exterior and interior worlds, as we grasp towards the nature of truth. Two men who were masters at understanding the outside, physical world, and who knew each other extremely well, were unable to communicate their inner motives and desires in the political world of men. As men, the meeting was the ultimate lynchpin of their relationship and ideas.

Mirroring Heisenberg’s concept of Uncertainty and the unknowable careening path of an electron, we may never know with precision the thinking and goals of the two men, their interaction, and their intended ultimate results. Mirroring Bohr’s Complementarity, we may perhaps know enough of their contradictory accounts and perspectives to build an ordered interpretation of what happened, and to sense an important turning point in our time, but perhaps we do not have a perspective or knowledge lofty enough to adequately understand the full reality. The struggle to grasp realities, to comprehend fully, to miss possible alternatives, and the tragic aspects of misunderstanding in the history between Niels Bohr and Werner Heisenberg is what holds our fascination with their encounter today.

Dr. Keith Huxen is the Senior Director of Research and History at the National World War II Museum.

One Response to “The Mysterious Meeting between Niels Bohr and Werner Heisenberg”

Virginia Weinmann says:

This is a fascinating article. To think that it may have been the comments of Heisenberg that caused Nazi Germany to discontinue its efforts to produce an atomic bomb is a bit of history that is extremely important to the study of WW2. Equally important is the effect that Bohr’s comments, and the drawing that he gave to the Manhattan Project scientists, had on the stepped up effort of the USA to achieve an atomic bomb. Certainly the latter was a life changing matter to all those who would have been called on to invade Japan.

In the fall of 1993 (or 1994) in Colorado Springs, CO, my husband, John G. Weinmann, ambassador to Finland and also Chief of Protocol to the White House under George H. W. Bush, and I attended a conference where those who formerly were President of the USA-Bush, Prime MInister of Great Britain, Prime Minister of Canada, and President of the Soviet Union-Mikhail Gorbachev were on stage. Among their remarks each one of the above admitted that he had grossly misunderstood the position of his counterparts during various meetings and conversations while in office. This mimics the possible misunderstanding of Heisenberg and Bohr during their conversations in Copenhagen in 1941. How hard it is to have a meeting of the minds.

September 15, 2011 at 9:24 pm

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