Heisenberg in Copenhagen

Editor’s Note

In September 1941, Werner Heisenberg — then leading Germany’s atomic bomb project — traveled to Nazi-occupied Copenhagen and requested a private meeting with his former mentor, Niels Bohr.¹

It is unclear what drove him to make this trip. For generations, historians — as well as the individuals involved, relying on imperfect and shifting memories — have wrestled with Heisenberg’s intentions. Did he hope to secure Bohr’s collaboration in building a German atomic weapon? Was he gathering intelligence on possible Allied initiatives? Might he have been subtly notifying Bohr of the German project, intending to prompt the Allies to quicken their own efforts? Or was he searching for a pretext to hinder or undermine the Nazi atomic venture? Equally compelling is the question of why the German program collapsed. Was failure caused by infighting and scattered resources among competing factions, or by a fundamental error — a reliance on an incorrect estimate rather than a precise calculation — of a vital neutron diffusion parameter?

What transpired during their conversation, witnessed only by Bohr’s wife Margrethe, remains a mystery. The encounter left Bohr visibly shaken and destroyed a friendship that had flourished since the 1920s, when Heisenberg had been a frequent visitor to Bohr’s institute. Or was he trying to forge some kind of scientists’ pact to prevent the bomb’s development? The question of what exactly Heisenberg said that evening has haunted historians for decades.

Remarkably, Heisenberg wrote to his wife Elisabeth immediately after this emotionally fraught meeting, producing one of World War II’s most revealing — yet cryptic — personal documents. Written in the raw aftermath of the encounter, before time and reflection could reshape memory, the letter offers something postwar memoirs cannot: Heisenberg’s unfiltered thoughts while the experience still burned fresh in his mind. Yet even here, security concerns forced him to speak in hints and allusions rather than explicit statements. By examining what the letter reveals — and what it carefully conceals — and then contrasting Heisenberg’s nostalgic, defensive account with Bohr’s starkly different postwar recollections, we can begin to illuminate both the enduring mystery of that September night and its profound consequences for history.

A Meeting Soaked in Nostalgia and Tension

Heisenberg’s letter overflows with longing for his intellectual youth under Bohr’s mentorship. Copenhagen’s familiar bells, streets, and night skies evoke a cherished continuity now shattered by war-torn Europe. He describes tender evenings with the Bohr family — reading aloud together, sharing ordinary conversation, playing Mozart — as attempts to resurrect a friendship that once existed beyond politics. Yet even as Heisenberg reaches for this vanished intimacy, tension infiltrates his account. He observes that Bohr cannot “separate out thinking, feeling, and hating” when discussing political matters, framing this as emotional excess rather than moral clarity. Heisenberg admits to feeling profiled as Nazi Germany’s defender in conversations with foreigners, trapped in an uncomfortable role that transcends both physics and personal bonds.

The letter also conveys Heisenberg’s astonishment and regret. Bohr’s institute now avoids German gatherings; once-effortless scientific exchange has become “almost impossible.” Heisenberg expresses puzzlement at the “hatred or fear” he senses from what he perceives as a relatively comfortable Danish populace, revealing a profound disconnect in how he and they understood the occupation. Significantly, he treads carefully around certain subjects, deferring sensitive details for in-person explanation with Elisabeth — a clear signal that some topics were too dangerous or pivotal to commit to paper.

Bohr’s Incompatible Reality

Bohr’s postwar recollections present a jarring contrast to Heisenberg’s account. Living under surveillance and occupation, Bohr perceived Heisenberg not as a cherished former student seeking connection, but as inextricably tied to Germany’s war apparatus. Heisenberg’s oblique remarks about nuclear physics struck Bohr as deeply ominous — suggesting German confidence in victory and hinting at atomic ambitions. Where Heisenberg sensed a wounded but salvageable relationship, Bohr experienced rupture: a moral chasm opening between them as someone he had regarded almost as family appeared to normalize Nazi dominance and implicitly threaten civilization itself.

This fundamental asymmetry defines the tragedy of their encounter. Heisenberg clung to the possibility of personal loyalty transcending political division. Bohr, confronting occupation’s daily horrors and moral stakes, saw no such bridge remaining. Their shared history — years of collaboration, affection, and mutual respect — collided with incompatible present realities: what felt to Heisenberg like a painful but meaningful reunion registered for Bohr as an unbridgeable wound carved by war.

Consequences Beyond Comprehension

The precise content of their conversation remains shrouded in ambiguity and dispute. Yet what was said — and crucially, what each man interpreted the other to have meant — set in motion a chain of events with world-altering consequences. Bohr’s alarm at the meeting contributed to his decision to escape occupied Denmark in 1943. Reaching the United States, he became an advisor to the Manhattan Project, where his theoretical insight influenced Allied nuclear research. The atomic bombs developed by the project were later dropped on Hiroshima and Nagasaki, helping to bring World War II to a close and inaugurating the nuclear age.

Understanding the profound disconnect between Heisenberg’s letter and Bohr’s experience is essential to grasping why a conversation between two old friends became a hinge point in history. The document’s value lies not just in what it records, but in what it unwittingly reveals about the impossibility of sustaining personal bonds across the moral and political abyss of that era.

Michael Frayn’s theatrical work

Copenhagen, Michael Frayn’s theatrical work, brings this obscure wartime incident to the stage. The play scrutinizes each plausible scenario with care and nuance, emphasizing the intricate counterpoint between the two physicists: their divergent temperaments, professional trajectories, and methodological philosophies.

The drama’s resonance lies unmistakably in its weaving together of temporal strands—past, present, and future—into a unified narrative fabric. Even as we accelerate toward a horizon glittering with the potential of revolutionary technological progress, we are repeatedly pulled back by the unanswered queries of the previous century—queries so elemental that they, much like the figures in Copenhagen, appear to exist beyond time. How could a nation at the zenith of cultural and scientific achievement spawn the exterminatory regime of Nazi Germany? And, connected inextricably to that: How could Werner Heisenberg — a scholar of unparalleled brilliance, molded by the highest ideals of Western thought, and no adherent of Nazism—not only elect to stay in National Socialist Germany throughout its twelve-year reign, but also actively seek a distinguished academic appointment in Berlin during the war’s most intense period? This role positioned him to lead the German military’s atomic fission research at its most critical juncture.

In Copenhagen, the audience’s focus is fixed upon the triad of Bohr, Heisenberg, and Bohr’s wife, Margrethe. Here, Margrethe acts as an incisive, unforgiving examiner of Heisenberg’s equivocations, whereas the genial, at times fatherly, Bohr displays a consistent leniency toward his former associate. The drama tenderly reveals the profound, if desperately strained, mutual regard between these two vastly different individuals—a connection stretched to breaking point by the atrocities of their age.

In the end, Copenhagen employs ambiguity as a tool to demystify Heisenberg’s consequential voyage. It frames him neither as saint nor fiend, but as an exceptionally intelligent and refined human being, caught helplessly in the cataclysmic sweep of history—a tragedy for which he, in common with countless others, possessed no readiness.

We now present Heisenberg’s letter to his wife in its entirety.

Letter dated September, 1941

Bohr and his family are doing fine; he himself has aged a little, his sons are all fully grown now. The conversation quickly turned to the human concerns and unhappy events of these times; about the human affairs the consensus is a given; in questions of politics I find it difficult that even a great man like Bohr cannot separate out thinking, feeling, and hating entirely. But probably one shouldn’t separate these ever. Mrs. Bohr also was well, she asked me a lot about you and the children, especially about Maria. The pictures I will show to her tomorrow night, I have a nice enlarged photo of Maria which I had made for Mama. Later I was sitting alone with Bohr for a long time; it was after midnight when he accompanied me to the streetcar, together with Hans. (Boring)

Thursday night. I will take this letter with me to Germany after all and send it from there. From everything I have heard, the censorship would delay the arrival several days as well, so it makes no sense to me that a censor should read this letter. Unfortunately, you then have to wait for my letter for almost eight days. I for my part have not received any mail here either.- Yesterday I was again with Bohr for the whole evening; aside from Mrs. Bohr and the children, there was a young English woman, taken in by the Bohrs, because she can not return to England. It’s somewhat weird to talk to an English woman these days. During the unavoidable political conversations, where it naturally and automatically became my assigned part to defend our system, she retired, and I thought that was actually quite nice of her.

This morning I was at the pier with Weizsäcker, you know, there along the harbor, where the “Langelinie” is. Now there are German war ships anchored there, torpedo boats, auxiliary cruisers and the like. It was the first warm day, the harbor and the sky above it tinted in a very bright, light blue. At the first light buoy near the end of the pier we stayed for a long time looking at life in the harbor. Two large freighters departed in the direction of Helsinor; a coal ship arrived, probably from Germany, two sailboats, about the size of the one we used to sail here in the past, were leaving the harbor, apparently on an afternoon excursion. At the pavilion on the Langelinie we ate a meal, all around us there were essentially only happy, cheerful people, at least it appeared that way to us. In general, people do look so happy here. At night in the streets one sees all these radiantly happy young couples, apparently going out for a night of dancing, not thinking of anything else. It is difficult to imagine anything more different than the street life over here and in Leipzig.

In Bohr’s institute we had some scientific discussions, the Copenhagen group, however, doesn’t know much more than we do either. Tomorrow the talks in the German scientific institute are beginning; the first official talk is mine, tomorrow night. Sadly the members of Bohr’s institute will not attend for political reasons. It is amazing, given that the Danes are living totally unrestricted, and are living exceptionally well, how much hatred or fear has been galvanized here, so that even a rapprochement in the cultural arena — where it used to be automatic in earlier times — has become almost impossible. In Bohr’s institute I gave a short talk in Danish, of course this was just like in the olden days (the people from the German Scientific Institute had explicitly approved) but nobody wants to go to the German Institute on principle, because during and after its founding a number of brisk militarist speeches on the New Order in Europe were given.

With Kienle and Biermann² I have spoken briefly, they were, however, for the most part busy with the observatory.

Saturday night. Now there is only this one night left in Copenhagen. How will the world have changed, I wonder when I come back here. That everything in the meantime will continue just the same, that the bells in the tower of city hall will toll every hour and play the little melody at noon and midnight, is so weird to me. Yet the people, when I return, will be older, the fate of each one will have changed, and I do not know how I myself will fare. Last night I gave my talk, made a nice acquaintance too. The architect Merck who had built the Reich Sports Arena in Berlin is slated to build a new German school here in Copenhagen, and he came to my talk. On a joint trip aboard the streetcar we had a pretty good time conversing. I always enjoy people who are especially good at something.

Today at noon there was a big reception at the German embassy, with the meal being by far the best part of it. The ambassador was talking animatedly in English to the lady seated next to him, the American ambassador. When she left, I believe I heard her say to somebody: We will meet again, definitely at Christmas, unless something quite unexpected comes up. One has to take these diplomatic dinners in a humorous vein.

Today I was once more, with Weizsäcker, at Bohr’s. In many ways this was especially nice, the conversation revolved for a large part of the evening around purely human concerns, Bohr was reading aloud, I played a Mozart Sonata (a-Major). On the way home the night sky was star studded again.

By the Way: two nights ago a wonderful northern light was visible, the whole sky was covered with green, rapidly changing veils.

It is now a quarter of one am and I am rather tired. Tomorrow I will post this letter in Berlin, so you will receive it Monday most likely. In one week I will be with you again and tell you everything that happened to me. And then we all will be together for the winter in Leipzig.

Good night for now!

Yours Werner

1. With the official launch of Germany’s nuclear project—the Uranverein or “Uranium Club” — by the Army Ordnance Office (Heereswaffenamt) on September 1, 1939, Werner Heisenberg emerged as its chief scientific architect and a central organizer. ↩︎
2. Ludwig Biermann (1907–1986) was a German astrophysicist known for his research in plasma astrophysics. Hans Kienle (1895–1975), a prominent German astronomer and astrophysicist, played a significant role in 20th-century German astronomy. Kienle specialized in stellar physics and spectrophotometry and served as director at the Göttingen Observatory and later the Landessternwarte Heidelberg-Königstuhl. Importantly, Kienle influenced Biermann by encouraging him to concentrate on astrophysics.  ↩︎