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This letter, dating from approximately 1957, was released this year by Bohr's family. The letter is a key bit of evidence in the debate around a meeting between the two physicists, Bohr and Heisenberg, in September 1941. There is much speculation about why Heisenberg came to visit Bohr in Nazi-occupied Copenhagen and what transpired at the meeting itself.

In this letter, Bohr is responding to a Heisenberg's accounts of the meeting, which was published in a book on the atomic bomb, Brighter than a Thousand Suns. Bohr challenges Heisenberg's recollection, saying "I am greatly amazed to see how much your memory has deceived you." He also reproaches Heisenberg for his confidence in Germany's ability to win the war and his belief that an atomic bomb could be constructed, something Bohr thought insurmountable for the time being (1941).

Images of the original letter are available for viewing at
http://www.nbi.dk/NBA/papers/docs/facs01.html

The letter is in the handwriting of Aage Petersen, Bohr's assistant. Below is the English translation of the letter, followed by the original Danish.

English


Dear Heisenberg,

I have seen a book, “Stærkere end tusind sole” “Brighter than a thousand suns” by Robert Jungk, recently published in Danish, and I think that I owe it to you to tell you that I am greatly amazed to see how much your memory has deceived you in your letter to the author of the book, excerpts of which are printed in the Danish edition.

Personally, I remember every word of our conversations, which took place on a background of extreme sorrow and tension for us here in Denmark. In particular, it made a strong impression both on Margrethe and me, and on everyone at the Institute that the two of you spoke to, that you and Weizsäcker expressed your definite conviction that Germany would win and that it was therefore quite foolish for us to maintain the hope of a different outcome of the war and to be reticent as regards all German offers of cooperation. I also remember quite clearly our conversation in my room at the Institute, where in vague terms you spoke in a manner that could only give me the firm impression that, under your leadership, everything was being done in Germany to develop atomic weapons and that you said that there was no need to talk about details since you were completely familiar with them and had spent the past two years working more or less exclusively on such preparations. I listened to this without speaking since a great matter for mankind was at issue in which, despite our personal friendship, we had to be regarded as representatives of two sides engaged in mortal combat. That my silence and gravity, as you write in the letter, could be taken as an expression of shock at your reports that it was possible to make an atomic bomb is a quite peculiar misunderstanding, which must be due to the great tension in your own mind. From the day three years earlier when I realized that slow neutrons could only cause fission in Uranium 235 and not 238, it was of course obvious to me that a bomb with certain effect could be produced by separating the uraniums. In June 1939 I had even given a public lecture in Birmingham about uranium fission, where I talked about the effects of such a bomb but of course added that the technical preparations would be so large that one did not know how soon they could be overcome. If anything in my behaviour could be interpreted as shock, it did not derive from such reports but rather from the news, as I had to understand it, that Germany was participating vigorously in a race to be the first with atomic weapons.

Besides, at the time I knew nothing about how far one had already come in England and America, which I learned only the following year when I was able to go to England after being informed that the German occupation force in Denmark had made preparations for my arrest.

All this is of course just a rendition of what I remember clearly from our conversations, which subsequently were naturally the subject of thorough discussions at the Institute and with other trusted friends in Denmark. It is quite another matter that, at that time and ever since, I have always had the definite impression that you and Weizsäcker had arranged the symposium at the German Institute, in which I did not take part myself as a matter of principle, and the visit to us in order to assure yourselves that we suffered no harm and to try in every way to help us in our dangerous situation.

This letter is essentially just between the two of us, but because of the stir the book has already caused in Danish newspapers, I have thought it appropriate to relate the contents of the letter in confidence to the head of the Danish Foreign Office and to Ambassador Duckwitz.

Danish


Kære Heisenberg,

Jeg har set en bog “Stærkere end tusind sole” af Robert Jungk, der for nylig er udkommet på dansk, og jeg synes jeg skylder dig at sige, at det har forbavset mig meget at se, hvor stærkt din hukommelse har svigtet dig i dit brev til bogens forfatter, der i uddrag er aftrykt i den danske udgave.

Jeg husker selv hvert ord af vore samtaler, der jo fandt sted på en baggrund af yderste sorg og spænding for os heroppe i Danmark. Især gjorde det et stort indtryk både på Margrethe og mig og på alle på Instituttet, som I talte med, at du og Weizsäcker gav udtryk for jeres bestemte overbevisning at Tyskland vilde sejre og at det derfor var ganske tåbeligt af os andre at opretholde håbet om en anden udgang af krigen og stille os tilbageholdende overfor alle tyske tilbud om samarbejde. Jeg husker også ganske nøje vor samtale i min stue på Instituttet, hvor Du i vage vendinger talte på en måde der måtte give mig det bestemte indtryk, at man i Tyskland under din ledelse gjorde alt for at udvikle atomvåben og at du sagde at vi ikke behøvede at tale om enkeltheder, fordi du var så nøje inde deri og i de sidste to år væsentlig kun havde beskæftiget dig med sådanne forberedelser. Jeg hørte på det uden at sige noget, idet det drejede sig om en stor menneskelig sag, hvori vi trods vores personlige venskab måtte opfattes som repræsentanter for to på liv og død kæmpende sider. At min tavshed og alvor, som du skriver i brevet, kunde opfattes som forskrækkelse over dine meddelelser om at man kunde lave atombomben, er en helt ejendommelig misforståelse, der må skyldes den store spænding i dit eget sind. Fra den dag tre år før, hvor jeg blev klar over at langsomme neutroner kun kunde frembringe fission i Uran235 og ikke 238 var det jo indlysende for mig at man ved at skille uranerne vilde have en bombe med sikker virkning. Jeg havde endda i Juni 1939 i Birmingham holdt et offentligt foredrag om uranspaltningen, hvor jeg talte om en sådan bombes virkninger, men naturligvis tilføjede at de tekniske forberedelser vilde være så store at man ikke vidste hvor hurtigt de kunde overkommes. Hvis noget i min opførsel kunde tyde på forskrækkelse lå det derfor ikke i sådanne meddelelser, men i efterretningen om at man, så vidt jeg måtte forstå, i Tyskland så energisk deltog i et kapløb om at komme først med atomvåben.

I øvrigt vidste jeg dengang intet om hvor langt man allerede var kommet i England og Amerika hvad jeg jo først fik at vide da det året efter lykkedes mig at komme til England efter meddelelse om at den tyske besættelsesmagt i Danmark havde truffet forberedelse til min arrestation.

Alt dette er jo kun en gengivelse af, hvad jeg klart husker fra vore samtaler og som i den næstfølgende tid naturligvis var genstand for indgående drøftelse på Instituttet og med andre fortrolige venner i Danmark. En ganske anden sag er at jeg dengang og siden altid har haft det bestemte indtryk at du og Weizsäcker havde arrangeret symposiet i det tyske Institut, hvori jeg af principmæssige grunde ikke selv deltog, og besøget hos os for at forsikre jer om at vi ikke led overlast og prøve på enhver måde at hjælpe os i vor farlige situation.

Dette brev er jo for så vidt kun mellem os, men på grund af det røre, som bogen allerede har vakt i danske aviser, har jeg ment det rigtigt i fortrolighed at meddele brevets indhold til direktøren for det danske udenrigsministerium og ambassadør Duckwitz.


Sources:
http://www.nbi.dk/NBA/papers/introduction.htm
http://www.nbi.dk/NBA/papers/docs/d01orig.htm
http://www.nbi.dk/NBA/papers/docs/d01tra.htm
http://www.nbi.dk/NBA/papers/docs/cover.html

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