With apologies for burdening those on the list who (understandably) have no
interest in the subject....but I couldn't let this pass without comment.
Unfortunately the Times' piece is a fairly uneven report of what the Bohn
archives actually contain.(A fact we tried to point out in a Letter to the
Editor, which was initially refused because it was too critical; hopefully
the scaled-back letter will be accepted for publication tomorrow.)  For those
interested in the issue, please check out the actual draft correspondence, at
www.nba.nbi.dk.  Also, below is an commentary on the subject from another
viewpoint.


================================================
by Klaus Gottstein

Historians, physicists, writers, playwrights and interested members of the
public have wondered for many years why Heisenberg did his trip to
German-occupied Copenhagen in the midst of the Second World War and what he
discussed there with his old friend and mentor Niels Bohr with whom he had
laid, in the 1920s, the foundations of quantum mechanics. Both of them were
Nobel Prize winners. They had not seen each other since the beginning of the
war in 1939. In the meantime, Heisenberg and some other German physicists had
been drafted by Army Ordnance to explore the feasibility of a nuclear bomb
which, after the discovery of fission and of the chain reaction, could not be
ruled out. How real was this theoretical possibility? Was it to be expected
that Germany's adversaries in this war would come forward with this new kind
of weapon? By 1941 Heisenberg, after two years of intense theoretical and
experimental investigations by the drafted group known as the "Uranium Club",
had reached the conclusion that the construction of a nuclear bomb would be
feasible in principle, but technically and economically very difficult. He
knew in principle how it could be done, by Uranium isotope separation or by
Plutonium production in reactors, but both ways would take many years and
would be beyond the means of Germany in time of war, and probably also beyond
the means of Germany's adversaries. (When Heisenberg heard about the
Hiroshima bomb, almost four years later while interned in Farm Hall, at first
he could not believe it.)  Heisenberg and von Weizsäcker were very glad about
this result. It meant that they were spared a difficult moral decision. They
were able to concentrate on the construction of a reactor for power
production, a goal easily compatible with their value system. If a bomb would
have been within reach, how could they have avoided being forced to build it
without sacrificing their lives as saboteurs? But what about the long-range
future? Was the construction of nuclear weapons unavoidable? Was it
conceivable that the then small community of nuclear physicists could come to
an agreement not to work on bomb construction? Heisenberg and his friend and
colleague von Weizsäcker decided that it would be helpful to have discussions
with Bohr, their father figure. In a kind of naiveté they did not realize
that their old cordial relationship with Bohr had been affected by the events
of the war. For Bohr his old friend Heisenberg was now a representative of an
enemy country, of the occupying power of his native Denmark, whose remarks
would have to be looked upon with suspicion.

Heisenberg managed to make the trip to Copenhagen in September 1941, using
the opportunity of a scientific conference arranged by the German Culture
Institute in Copenhagen, boycotted by Bohr. Heisenberg spent several days in
Copenhagen and probably saw Bohr several times, in Bohr's office,in Bohr's
home and on a walk. On the latter occasion when there was no danger of being
overheard by the Gestapo, Heisenberg undertook to broach the questions which
were the real reasons for his trip. What happened? What did he tell Bohr? All
we knew for certain so far was that Heisenberg returned in despair from his
talk with Bohr telling von Weizsäcker (who was with him in Copenhagen) that
the discussion had gone completely wrong.

Bohr never published an account of his meeting with Heisenberg in wartime
Copenhagen, but Heisenberg did in his memoirs. What is written above is the
gist of what he wrote and what he explained to friends and colleagues when
questioned. But his explanations were not accepted everywhere, particularly
not by some of his British and U.S. colleagues and by some later writers who
were convinced that Heisenberg did all he could to make the bomb for Hitler,
but failed, and after the war tried to white-wash himself. The situation
became even worse in 1956 when the American journalist Robert Jungk published
a book "Heller als tausend Sonnen" (Brighter Than a Thousand Suns) in which
he described, greatly exaggerating, Heisenberg's satisfaction with the
technical difficulties of bomb construction and the lack of enthusiasm for
overcoming these difficulties, as a secret plan to prevent, for moral
reasons, the construction of an atomic bomb for Hitler which otherwise he
could have built. Heisenberg, and particularly von Weizsäcker, wrote letters
to Robert Jungk in which, while appreciating Jungk's extensive research and
detailed accounts of the developments, criticized some of his generalisations
and exaggerations. In the Danish translation of his book, which appeared in
1957, Jungk published an extraction of Heisenberg's letter, but only the
laudatory part. It was known that Bohr took exception to Jungk's book which
he had read in the Danish edition. Jungk's book, unfortunately, did much to
harm Heisenberg's credibility, particularly as the wrong impression had
arisen in some quarters that Heisenberg had commissioned it. Heisenberg was
unaware of this.

Meanwhile, all kinds of rumours circulated about the "real" motives behind
Heisenberg's 1941 visit to Bohr. It was suggested that he wanted to do some
spying, to find out what Bohr knew about the nuclear efforts in the U.S. and
Grest Britain. It was suspected that Heisenberg wanted to enlist the support
of Bohr for the German project. On the basis of some conversations which
Heisenberg and von Weizsäcker had had with members of Bohr's institute there
was also the version that the real reason for Heisenberg's visit was the
intention to convince Bohr that Germany was going to win the war, that this
outcome was desirable, and that Bohr had better end his unwillingness to
cooperate with German authorities. It was generally held that the formerly
cordial relationship between Bohr and Heisenberg was severely disturbed, if
not severed, ever since. Few members of the international community knew that
they continued to have friendly relations after the war, visiting each other,
with their families, in their homes in Copenhagen and Göttingen, and that
Bohr wrote an article for the Festschrift to Heisenberg's sixtieth birthday
in 1961.

When it became known that the Niels Bohr Archive in Copenhagen held a letter
by Bohr to Heisenberg, written after the appearance of Jungk's book but never
sent, speculation concentrated on this document, to be published 50 years
after Bohr's death, i.e. in 2012, from which the solution of all the open
questions was expected. However, to end speculation, the Niels Bohr Archive
has, a few days ago, released 11 documents pertaining to Heisenberg's visit,
including the much-discussed unsent letter, preceded by an article by Aage
Bohr, published in 1967, on "The War Years and the Prospects Raised by Atomic
Weapons". The documents, with the exception of one letter written by
Heisenberg to Bohr, are unfinished drafts written by Bohr in the late 1950s
and early 1960s, addressed to Heisenberg, but never sent. As the director of
the Niels Bohr Archive, Finn Aaserud, points out, the documents have to be
viewed with caution. They were written 16 years or more after the event and
represent just drafts, not finished papers. Nevertheless, the contents of the
documents are interesting and, depending on the pre-established views and
opinions of the readers of today, surprising to a lesser or greater degree.
Here are some of the general characteristics of  the documents, with my
comments in brackets:

·    Bohr's tone in addressing Heisenberg is extremely cordial and friendly.
·    Bohr was still highly interested in clarifying Heisenberg's intentions
and motivations behind his 1941 visit. His sentences in Document 11 c "I have
long been meaning to write to you ..." and "I have written in such length to
make the case as clear as I can for you and hope we can talk in greater
detail about this when opportunity arises" are proof of this. (This is new
information. Heisenberg was under the impression that Bohr and he, having
differing recollections of their discussion, had come to the conclusion that
it would be best to let rest the spirits of the past. It is a pity that the
letter was not sent. Several opportunities for clarifying conversations were
missed at later meetings of Bohr and Heisenberg. It seems that Bohr was
afraid he might hurt Heisenberg's feelings by insisting too much on his
interpretation of the events.)
·    Document 1 contains the confirmation that Bohr and Heisenberg met
several times during Heisenberg's visit to Copenhagen in 1941: Bohr refers to
"our conversations" in the plural, and he mentions "our conversation in my
room at the institute" as well as the strong impression Heisenberg's remarks
made "on Margrethe and me". Since it is unlikely that Bohr's wife Margrethe
was present at the confidential conversation in Bohr's room in the institute
one may assume that Heisenberg's recollection is correct that he was also
invited to Bohr's home. Moreover, there is Heisenberg's and von Weizsäcker's
testimony that the critical discussion took place during a walk, to avoid
unwanted earwitnesses.
·    Bohr was, at the time of the visit in 1941, highly distressed by the
circumstances of Heisenberg's visit, his lecture at the German Culture
Institute and  his contacts with the German Embassy (more correct: Legation)
in Copenhagen. (Heisenberg had assumed Bohr would understand that without
such contacts he would not have obtained visa and permission to enter
occupied Denmark.)
·    Bohr understood and appreciated that one of Heisenberg's reasons for the
visit was genuine care: to see how Bohr and his institute fared under German
occupation and to be of assistance, if at all possible (Document 11 c).
·    For Bohr it was of central and sad significance that Heisenberg during
his visit expressed his conviction of a German victory whereas Bohr, as a
Danish patriot, had placed all his hopes in a German defeat. Since towards
the end of the war Heisenberg's conviction must have disappeared, Bohr
wondered whether Heisenberg, in retrospect, had forgotten or repressed his
earlier views. (Again, for Heisenberg, mentioning the prospects for a German
victory, was not central to his mission. At the beginning of the war he had,
in private, expressed the view that Hitler would lose the war like a
chess-player would lose a game into which he entered with one castle less
than his opponent. However, after the surprisingly fast defeats of Poland and
France, the occupation of large parts of Europe and the initial great
victories and advances in the Soviet Union, with the U.S. still neutral,
Heisenberg like most non-nazi Germans had come to the conclusion that a
German victory now seemed likely. They feared that a German defeat would mean
Soviet occupation of Europe which, even for anti-nazis, was considered an
even greater evil than German domination. Auschwitz and the full extent of
nazi crimes was not yet known, but Stalin's massacres were. The hope -
completely unrealistic as we now know but considered realistic at the time -
was that after a German victory the German army would get rid of Hitler and
his henchmen. The anti-nazi stance of many German generals, who later took
part in the assassination plot of July 20, 1944, was known to persons who,
like Heisenberg through the "Wednesday Society", were close to opposition
circles. For Heisenberg, it was part of his care for Bohr to think in sober
terms of the future and of Bohr's and his institute's survival. It would be
advisable to end opposition to a victorious Germany. It is obvious that
Heisenberg's assessment of Germany's chance to win the war must have changed
a few months later when the U.S. entered the war and the German army suffered
severe setbacks in Russia.)
·    Bohr mentions several times his reticence caused by his suspected
surveillance by German police. (There is no indication of an awareness by
Bohr that Heisenberg was under the same handicap. He had to be extremely
cautious in choosing his language. Mentioning to Bohr the existence of a
German nuclear programme and of his involvement in it, could be interpreted,
and probably was, treason punishable by death. In public conversations, also
in the cafeteria of Bohr's institute, he may have had to say things which did
not represent his opinion. This situation is well-known to people having
lived under cruel dictatorships.)
·    Document 6 says that Heisenberg "did not wish to enter into technical
details but that Bohr should understand that he knew what he was talking
about as he had spent 2 years working exclusively on this question." Bohr had
known about the possibility of nuclear weapons only in a very general way and
at that time still had held the opinion that the technical difficulties were
insurmountable. Therefore Heisenberg found it necessary to mention his two
years of investigations in order to convince Bohr that he was not "talking
moonshine". Bohr had been "doubtful looking" (Document 11 a).
·    Bohr wondered (and this is new information) who had authorized
Heisenberg to discuss with him military secrets. (Heisenberg had assumed Bohr
would understand that he spoke in his private capacity as Bohr's old friend
and colleague who, however, because of the delicacy of the subject discussed,
had to use very involved language. Bohr, on the other hand, could not imagine
that Heisenberg acted on his own initiative, without any special permission,
let alone orders. But this was so. Heisenberg had thought, naively, that Bohr
would be ready, as he always had been in earlier times, to discuss with him
possible solutions for complicated problems. He had lacked the sensitivity
for Bohr's patriotic feelings and misgivings under the changed circumstances
of war and occupation. On the other hand, it is justified to say that it took
great moral courage to talk to Bohr about implications of his secret work.
Heisenberg risked his neck.)
·    Constant German propaganda talks of the imminent use of "new weapons"
fortified suspicions by Bohr and his Danish colleagues that there was a
German nuclear bomb programme. Assertions by Jensen to the contrary were not
trusted though he himself was considered honest. But Jensen was working on
the reactor programme, and it had to be doubted that he was privy to all
aspects of the programme.
·    After Bohr's escape to Sweden and subsequent flight to Great Britain in
the autumn of 1943 "it was quite clear already then, on the basis of
intelligence reports, that there was no possibility of carrying out such a
large undertaking in Germany before the end of the war". (Document 11 b).
This is a remarkable confirmation of Heisenberg's own conclusion. It is also
interesting that these intelligence reports had no influence on the progress
of the Manhattan project.
·    Aage Bohr writes "After the outbreak of war and especially after the
occupation of Denmark we in Copenhagen were completely cut off from following
the allied nations' efforts in the field of atomic energy." Niels Bohr confi
rms this in Document 11 c. (Also Heisenberg knew that. How could he expect to
do some spying, as some writers have suggested?) In a footnote to his article
Aage Bohr assures the reader that no secret plan was submitted to his father
by Heisenberg "aimed at preventing the development of atomic weapons through
a mutual agreement with colleagues in the allied countries." Again, in
Document 11 c, this is what Bohr remembers. It is quite true, also according
to Heisenberg. It had indeed been Heisenberg's intention to get Bohr's
opinion on possibilities for such an agreement or on other ways out of the
impasse presented by the basic feasibility of atomic weapons. But Heisenberg
never had a chance to present his questions because of Bohr's reticence and
Bohr's unwillingness to continue the conversation when Heisenberg, as an
introduction, had told Bohr that atomic weapons were technically possible,
and that he knew it. He was not even allowed to add, as he had intended, that
the technology was very difficult and would take a long time, thereby giving
the small international community of atomic scientists a chance to use their
influence in the meantime. Bohr had stopped listening. This is admitted by
Bohr in Document 11 c where he writes "During the conversation, which because
of my cautious attitude was only brief ...".

Summarizing, one might say that Bohr's description of Heisenberg's visit, as
he experienced it under the strains of occupation, is complementary to, but
not incompatible with Heisenberg's recollections from Heisenberg's point of
view. As Hans Bethe has put it  in a recent interview quoted by James Glanz
in the New York Times, the two physicists talked straight past each other.
They talked with different sets of assumptions. But James Glanz is quite
wrong when he thinks that Heisenberg "portrayed himself after World War II as
a kind of scientific resistance hero who sabotaged Hitler's efforts to build
a nuclear weapon." Heisenberg did nothing of the kind. He always stressed how
content he had been that nuclear weapons did not seem to be feasible for
several years to come so that Hitler and his government made no efforts to
build them when this had become clear to them.