A letter from the German physicist Werner Heisenberg to his wife, Elisabeth, about his meeting with Niels Bohr in Copenhagen in September 1941, which is printed below, was released by the estate of Elisabeth Heisenberg on June 5. Precisely what Heisenberg said to anger his old friend Bohr has been an open question ever since Heisenberg told his wife and friends that the meeting had gone awry.

About the subject there is no question: Heisenberg told Bohr that it would be possible to build atomic bombs, Germany had a program underway, and if the war lasted long enough it might be settled by these terrible new weapons. But why Heisenberg told him this, and why it left Bohr with an anger which was never dispelled, has for decades been the subject of heated debate, and is the core question, asked and re-asked, in Michael Frayn’s remarkable play about the wartime visit, Copenhagen. “Why did he come?” asks Bohr’s wife, Margrethe, in the play. “What was he trying to tell you?”

In February of last year, Bohr’s family released the text of eleven letters concerning the wartime visit. Most were versions of a letter to Heisenberg which Bohr composed initially in 1958 and then revised again and again until his death in 1962. The letters recorded for the first time Bohr’s sense of what had been said, but they contained no new information about Heisenberg’s role in wartime Germany and left unanswered the question Bohr tried to ask in the final versions of the letter he never sent: Who authorized Heisenberg to tell Bohr of this secret German military program?

The letter bearing on the visit written by Heisenberg to his wife, Elisabeth, was composed on three separate days during the week-long visit, and then mailed on his return to Germany. The letter contains nothing about his talk of bombs with Bohr—he promises that after his return he will tell his wife “everything that happened to me”—but it does clear up a number of details about the visit, reveals something of Heisenberg’s reading of events at a critical moment, and subtly changes our understanding of the tenor of their conversations. The new letter now takes pride of place as the earliest document recording the visit.

Among the details established or nudged toward clarity are some of the most basic: Where did the two men meet—on a walk along Copenhagen’s Langlinie, as recalled by Heisenberg’s friend and colleague Carl Friedrich von Weizsäcker, who accompanied Heisenberg to Copenhagen? in Bohr’s office at his institute, as Bohr told friends? or at Bohr’s house, as others believed? Was the critical conversation held indoors (Bohr’s memory) or out, to avoid possible eavesdropping by the Gestapo? Was Margrethe present at all?

The answers to other questions are only suggested. It has long been evident that Heisenberg failed to understand how much the war had changed things between the two men; in his letter Heisenberg is unmistakably surprised by the intensity of feeling aroused in Bohr by the war and the German occupation of Denmark. Other reports have told us how Heisenberg angered Danes by the things he said; in the letter it is clear that he felt compelled to “defend” Germany during the visit, and it is obvious that he was keenly aware that censors would be reading what he wrote.

The Bohr letters released last year record his anger at what he took to be Heisenberg’s attempts to rewrite history in the 1950s by suggesting that he had tried to avoid building a bomb for Hitler. What Bohr remembered was different—Heisenberg’s claim that there was an ongoing program, he was part of it, and success could not be ruled out. But when did Bohr grow angry, and just how angry did he get? Was the breach immediate and deep? The new letter suggests not.

For those unfamiliar with Heisenberg’s biography it is helpful to know that he had lived and worked with Bohr in the 1920s when the two men revolutionized physics. For a time he had been virtually a member of Bohr’s family, not quite but in some ways even closer than a son. The rise of Hitler in 1933 and the tightening grip of Nazi tyranny put a strain on their friendship, and the outbreak of war in September 1939 ended their once frequent visits. Heisenberg’s daughter Maria, asked after by Margrethe, had been born (with a twin brother, Wolfgang) in January 1938, less than a year after Heisenberg’s first meeting and whirlwind courtship of Elisabeth Schumacher. They were married in April 1937 in St. Annenkirche in Berlin-Dahlem, home of the Kaiser Wilhelm Institute where Heisenberg later presided as theoretical chief of the German atom bomb program. Heisenberg arrived in Copenhagen by train from Berlin on Monday, September 15 of that year, and left the following Sunday, September 21. He had received official permission for the visit in order to give a talk at the German cultural institute in Copenhagen.


In a memoir of her husband, Inner Exile, published nearly forty years after the visit, Elisabeth Heisenberg recalled his distress at the failure of the two men to understand each other. “So what was Heisenberg’s ultimate concern during these discussions with Bohr?” she asks.

The truth was that Heisenberg saw himself confronted with the spectre of the atomic bomb, and he wanted to signal to Bohr that Germany neither would nor could build a bomb. That was his central motive. He hoped the Americans, if Bohr could tell them this, would perhaps abandon their own incredibly expensive development. Yes, secretly he even hoped that his message could prevent the use of an atomic bomb on Germany one day. He was constantly tortured by this idea…. This vague hope was probably the strongest motivation for the trip. But they were not communicating…. The two men who had been such close friends parted deeply disappointed…. This troubled Heisenberg for the rest of his life.

Michael Frayn’s play and the long difficulty in establishing just what Bohr and Heisenberg said to each other in 1941 have exaggerated the importance of the talk in Copenhagen for understanding what happened to the German bomb program. The fact that Heisenberg apparently broke all the rules of security to tell his friend about the progress of German research suggests the depth of his ambivalence; but what he actually did is stronger evidence of where he stood. After his return from Copenhagen, he convinced German officials that the bomb project was too big, too expensive, and too uncertain for Germany in wartime, and the military abandoned the program completely by mid-1942, just as the American effort was getting underway.


Copenhagen, Tuesday night [September 1941 added in Elisabeth’s handwriting]

My dear Li! Here I am once again in the city which is so familiar to me and where a part of my heart has stayed stuck ever since that time fifteen years ago. When I heard the bells from the tower of city hall for the first time again, close to the window of my hotel room, it gripped me tight inside, and everything has stayed so much the same as if nothing out there in the world had changed. It is so strange when suddenly you encounter a piece of your own youth, just as if you were meeting yourself. I liked the trip coming over here too: In Berlin we had pouring rain, over Neustrelitz storm and rainshowers as if from buckets, in Rostock it cleared up, from Wenemünde on the sky was scrubbed clean, almost cloudless, but still a stiff north wind; so it remained until I arrived here. Late at night I walked under a clear and starry sky through the city, darkened, to Bohr.

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 can not separate out thinking, feeling, and hating entirely. But probably one ought not to separate these ever. Mrs. Bohr too 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 foto of Maria which I had made for Mama. Later I was sitting for a long time with Bohr alone; it was after midnight when he accompanied me to the streetcar, together with Hans [Bohr].

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 is somewhat weird to talk with 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 Biermann 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 again starstudded.—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 a.m. 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!

Your Werner

The original letter in facsimile, the German text, and the English translation printed above by Irene Heisenberg, wife of the physicist’s son Jochen, can be found on the Web at werner-heisenberg .unh.edu.

This Issue

August 14, 2003