To: The editors of SCIENCE 22 February 2002

To: The editors of SCIENCE 22 February 2002

In his report “Letters Aver Physicist Supported Nazi Bomb” (15 February, p. 1211) Adrian Cho repeats erroneous statements which, unfortunately, have found a wide distribution even though they have already been cast into doubt by several writers. The letters, drafted by Niels Bohr, never posted and recently released by the Niels Bohr Archive, do not contradict Heisenberg’s own description of his meeting with Bohr in 1941.

Heisenberg never claimed that “he intended to subvert the Nazi bomb program from within”. Nor is it true that “the Dane abruptly ended … their long friendship”. This can be seen not only from the warm tone used by Bohr in his unsent letters to Heisenberg but also from the fact that after the war the Bohr and Heisenberg families visited each other in their homes, and spent their vacations together in Greece. Bohr contributed an article to the Festschrift on the occasion of Heisenberg’s sixtieth birthday in 1961.

German Army Ordnance had set up, at the beginning of the war in 1939, a research program to study the feasibility of atomic bombs. Heisenberg and some other physicists were drafted by the army to join this program in order to ascertain that the Allies would not be able to surprise Germany with a new weapon of this type. By 1941 Heisenberg had come to the conclusion that bombs would be feasible in principle but technically so difficult that it would take many years to make them. This meant that they would not be available in the present war. Therefore, the then small international community of nuclear scientists might have a chance to reach an agreement not to build these weapons. Heisenberg decided that it might be helpful to discuss the critical situation with his old friend and mentor Niels Bohr with whom he had discussed so many tricky issues in the past. He risked his neck, the nuclear project was secret. Heisenberg used very involved language which, he hoped, Bohr would nevertheless understand. What heisenberg did not realize was that the war had changed their old intimate relationship. They were now on opposite sides in a war, and Bohr was suspicious of Heisenberg’s motives behind his unexpected visit. When Heisenberg mentioned the technical feasibility of nuclear weapons (doubted so far by Bohr) and added that he knew what he was talking about, Bohr assumed that Heisenberg was working on the construction of bombs. He ended the conversation before Heisenberg got a chance to explain the technical difficulties and to ask for Bohr’s opinion regarding long-range attitudes of the scientific community under these circumstances.

In Germany it was decided that bomb construction would not be feasible while the war lasted, and should not be attempted. A reactor for power production was given official support. The question of bomb building never arose for Heisenberg, and he was glad about that. Robert Jungk, in his book Brighter Than a Thousand Suns gave the wrong impression that Heisenberg refrained, for moral reasons, from bomb making.

Heisenberg and his closest associate, von Weizsäcker, wrote letters to Jungk criticizing his exaggerations while appreciating his engaged research. Jungk only publshed the laudatory part of Heisenberg’s letter.

Summarizing, Bohr’s drafted letters to Heisenberg do not aver Heisenberg’s support for a Nazi bomb.

SCIENCE letter 1, 22.2.02

Dear Etta Kavanagh, 13

March 2002

Thank you for your e-mail of 11 March and for the edited version of my letter. I am glad to answer your questions and give you some comments with respect to the changes proposed by you.

Here are some references regarding my assertion that several writers have cast into doubt erroneous statements made by Adrian Cho:

Michael Frayn, historical appendix to the published version of his play “Copenhagen”.

Thomas Powers, “Warum Heisenberg das tat, was er tat” (Why Heisenberg did what he did!). In: Supplement to the German edition of Michael Frayn’s play “Copenhagen”, Wallstein publishers, Goettingen 2001.

Cathryn Carson, “Reflexionen zu ‘Kopenhagen’” (Reflexions on ‚Copenhagen’“). In:

Supplement to the German edition of Michael Frayn’s play “Copenhagen”, Wallstein publishers, Goettingen 2001.

Helmut Rechenberg, „Dokumente und Erinnerungen zum Bohr/Heisenberg-Treffen im Jahr 1941.“ (Documents and Recollections on the Bohr/Heisenberg meeting in 1941). In:

Supplement to the German edition of Michael Frayn’s play “Copenhagen”, Wallstein publishers, Göttingen 2001.

Thomas Powers, “Heisenberg’s War”, Alfred A. Knopf Publishers, New York 1993.

With regard to the warm tone of Bohr’s unsent letters (recently released by the Bohr Archive) see Document 2 where Bohr says: “Dear Heisenberg, Margrethe and I send you many heartfelt congratulations on the occasion of your birthday, when you can look back on such a rich life’s work in the service of the physical sciences. In particular, I think of all you achieved in the years when it was our great pleasure to have you as a colleague at the Institute in Copenhagen, and I have written a little about the memories from that time in the Festschrift that you will receive on your birthday. …”

One might also quote from a letter which Bohr DID send to Heisenberg, then interned at Farm Hall, in October 1945: “Dear Heisenberg, It was a great pleasure on my birthday to receive your kind letter. … I need not say that I have often thought about you all in these years and now, when peace is restored, I hope that it shall not last long before we can meet again and take up our old collaboration. … With my kindest greetings and best wishes to you and your family and all common friends, Yours Niels Bohr”. This letter was published in the biography of Niels Bohr written by Ulrich Röseberg: Niels Bohr, Leben und Werk eines Atomphysikers (Niels Bohr, Life and Work of an atomic physicist), Akademie Verlag, Berlin 1987, new edition 1993.

“Festschrift” is a technical term also used in the English language. According to Webster’s International Dictionary it is a volume of essays contributed by students, colleagues and admirers to honor a scholar on a special anniversary.

My statement “The Bohr and Heisenberg families visited each other …” This I was told

by Prof. Martin Heisenberg, Werner Heisenberg’s son. Bohr’s visit in the Heisenberg

home in Göttingen after the war is documented by Bohr’s signature in the Heisenberg

guest book.

My statement “… spent their vacations together …”. This is documented by a photograph taken by Heisenberg in 1953 showing Niels Bohr with his smiling wife Margrethe, with Heisenberg’s wife Elisabeth sitting between them and Heisenberg’s daughter Maria above them, on the steps of a Greek temple. This photograph was shown last December in the exhibition during the Heisenberg centennial celebrations in Munich. It was also published by Michael Frayn in his article “Ich gab Heisenberg eine Chance sich zu verteidigen“ (I gave Heisenberg a chance to defend himself) in the newspaper Frankfurter Allgeine Zeitung of 18 February 2002.

How did Bohr seem suspicious? This is evident from his behavior, as described by himself in his unsent letters and by Heisenberg in his memoirs. Heisenberg had expected that he could speak to Bohr in a situation of mutual trust, as in the past. Bohr, however, saw his friend as a representative of the occupation power. He suspected that Heisenberg was sent by German government authorities in order to win him over to cooperation with the Germans. He broke off the conversation.

The first name of von Weizsäcker is Carl Friedrich.

Heisenberg’s and von Weizsäckers letters to Robert Jungk are discussed by the historian Cathryn Carson, see above reference.

My present affiliation and postal address is: Max Planck Institute of Physics, Werner Heisenberg Institute, Föhringer Ring 6, D-80805 Munich, Germany.

In line 10 of your text you misspelt [in brackets] the first name of Bohr. It is Niels, not Neils.

For the second paragraph of my letter I would prefer a slightly different placement for the reference to my own work under Heisenberg. As it is placed by you it comes somewhat unmotivated in the middle of the description of Heisenberg’s mission. Here is my suggestion:

“German Army Ordnance had drafted Heisenberg, in 1939, to study the feasibility of

atomic bombs so that the Allies would not be able to surprise Germany with them. By

1941, Heisenberg had found that bombs would be feasible but technically so difficult to

make that their construction would take many years. Therefore, the then small

international community of nuclear scientists might have time to reach an agreement

not to build these weapons. Heisenberg decided that he should discuss the critical

situation with his old friend Bohr, with whom he had solved so many tricky issues in the

past. He risked his neck in doing so, because the nuclear project was secret. This is

what Heisenberg told me when I asked him, in 1969, about his visit to Bohr in

Copenhagen in 1941. I worked under Heisenberg at the Max Planck Institute for

Physics. Heisenberg had not realized that the war had changed their formerly close

relationship. Bohr seemed suspicious of the motives behind Heisenberg's unexpected

visit. When Heisenberg mentioned the technical feasibility of nuclear weapons (doubted

so far by Bohr), adding that he knew what he was talking about, Bohr apparently assumed, according to Bohr’s unsent letters, that Heisenberg was working on the construction of bombs. He ended the conversation before Heisenberg could explain the true purpose of his visit.“

I have also read the intended response by Adrian Cho. Perhaps you could let him have my comments to his draft response, thereby giving him a chance to modify his statements.

Adrian Cho asks: “… did he [Heisenberg] not perforce intend to subvert Nazi ambitions to achieve such weapons?” My reply: Subversion of this kind was not necessary because the Nazis had given up such ambitions when, in 1942, they heard from Heisenberg that it would take several years to make such weapons.

Adrian Cho says: “Bohr’s letters explicitly and peremptorily state that in 1941 Heisenberg said that he had been working in full earnest on atomic weapons for 2 years.” My comment: That is what Bohr thought Heisenberg said because at that time Bohr did not know the difference between a reactor and a bomb. Heisenberg only meant to tell him that after two years of study he had come to the conclusion that bombs are feasible in principle, but very difficult to make, and reactors for power production were feasible (also this was new for Bohr at the time) and somewhat easier to make, and that he would only work on the latter. Bohr ended the conversation before Heisenberg had a chance to explain this fully.

Adrian Cho says: “… if Bohr and Heisenberg remained close, then one can only wonder how the two managed to go to their graves without hashing out their differences over the encounter that clearly meant so much to both of them.” My comment: That is indeed the tragical fact. Bohr, as the non-sending of his letters shows, did not want to hurt Heisenberg’s feelings. Heisenberg, on the other hand, did not want to insist when at their first encounter after the war, in 1947, he noticed that Bohr had recollections of their 1941 meeting different from his own. To clear up the existing misunderstandings, it would have taken a painful discussion of war conditions in Denmark and Germany and of technical details of Heisenberg’s work which, Heisenberg thought, neither Bohr nor he would enjoy. Heisenberg thought that both he and Bohr had decided not stir up any longer the spirits of the past. But Heisenberg suffered all his life from the thought that the misunderstandings had not been cleared up. He did not know that Bohr also continued to ponder about their 1941 meeting, as his unsent letters now show. Had they been sent, both men could indeed “hashed out their differences.”

Best regards,

Klaus Gottstein