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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
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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.