In 1941 Drs. Bohr and Heisenberg met in occupied Denmark. Ostensibly to renew old friendships and collegial relations torn asunder by the German occupation of Denmark.
At the time of their meeting the Russian army was in retreat as the German army swiftly moved within 200 miles of Moscow and had besieged St. Petersburg (Leningrad). Throughout Europe the laws of Germany applied to its conquered provinces such as Czechoslovakia, Austria and Poland.
In December 1938 and winter of 1939, two German physicists Otto Hahn and Fritz Strassmann working at the Kaiser Wilhelm Institute in Berlin, had made the most important experiment in history. Their exiled associate Lise Meitner –safely in Sweden– looked at Hahn's results and could make the only logical conclusion about what Hahn & Strassmann had accomplished.
They had with slow neutrons bombarded an isotope of uranium (U235) and succeeded in breaking the massive nuclei into two separate and less massive elements. The elements: barium and krypton – were the elemental evidence that these uranium atoms had been split. Lise Meitner and her nephew Otto Frisch called the outcome: fission. The term was taken from biology because it had been used to describe mitotic division of one cell into two daughter cells.
Never in history had such an experiment based on three previous theoretical suppositions ever come at such an unreassuring time. Few meetings between Nobel Prize winners ever had greater significance for world history. It is embedded in the tragedy of our times that the progress of experimental particle physics was inversely proportional to the regression in Europe's social and political conditions. With the occupation of Czechoslovakia came the Slovakian Ore mines where the Curries had first obtained their radioactive materials. And with the subsequent occupation of Denmark and Norway came German access to sources of heavy water an essential ingredient to slow down the movement of neutrons essential for fission.

 

 

Niels Bohr's letter

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 [1957].

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

 

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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

 

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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.

Draft of letter from Bohr to Heisenberg, never sent.
In the handwriting of Niels Bohr's assistant, Aage Petersen.
Undated, but written after the first publication, in 1957, of the Danish translation of Robert Jungk, Heller als Tausend Sonnen, the first edition of Jungk's book to contain Heisenberg's letter
FissionWomen of teh Year, 1946.

Sources of commentary:

Ruth Moore. Niels Bohr: The Man, His Science, and The World They Changed. Cambridge, Mass.: MIT Press, 1985 [1966]. Fission pp. 248-285, Heisenberg's wartime visit to Copenhagen, pp. 290-294.

Mines: The "deposit is claimed to be one of the largest uranium deposits in Slovakia. . . . of over 1.8 million pounds of ore, reserves amounting to 32.55 million pounds of uranium ore."

Uranium exists naturally as 92 % is U238 but can separated into rare U235 which is fissionable. That had taken an enormous amount of electricity and facilities. In the United States at Oak Ridge this required 80 square miles of metallurgical industrial engineering facilities to achieve sufficient U235 for make the Hiroshima bomb nicknamed, "Little Man."

Photographic background.

The photograph in the background is the first photographic record of the atomic age, taken by a cameraman on the morning of August 3, 1945 in Hiroshima beside a railway station.

The Niels Bohr papers (http://nba.nbi.dk/papers/docs/d01tra.htm)

Niels Bohr Archive (NBA), Blegdamsvej 17, DK-2100 Copenhagen Ø, Denmark

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Last updated: February 2002.