Baggott J. The Quantum Story: A History in 40 Moments
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to build a ‘super-bomb’? ‘Yes, it would be possible to make a bomb,’ Bohr told his Princeton
colleagues in April 1939, ‘but it would take the entire efforts of a nation to do it.’
Word spread quickly. In April, the Reich Bureau of Standards and the German War
Offi ce both established nuclear research projects. In August, Einstein wrote a letter to
American President Franklin Roosevelt which warned of ‘extremely powerful bombs of
a new type’. A few days later Lindemann wrote to advise Winston Churchill MP that
atomic super-weapons could not be expected to be available for many years.
Following the German invasion of Poland, Allied governments declared war on
Germany on 3 September 1939. The two German nuclear research projects were hastily
consolidated into a single Uranverein, or ‘Uranium Club’ under the auspices of German
Army Ordnance. Heisenberg received notifi cation of his call to serve on the Uranverein
on 25 September.
The fi rst war of physics had begun.
In the late summer of 1941 Heisenberg and his Leipzig assistant Robert
Döpel set up their second experimental nuclear reactor, named L-II, in an
aluminium sphere about 75 centimetres in diameter containing a little
over 140 kilos of uranium oxide and about 160 kilos of ‘heavy’ water
moderator, water in which the hydrogen atoms are replaced by heavier
deuterium isotopes. The results were negative but, after correcting for
the absorption of neutrons by the aluminium vessel, their calculations
suggested the merest hint of neutron multiplication of the kind neces-
sary ultimately to achieve a self-sustaining nuclear chain reaction.
The physicists sensed they were on the right track. This was little
more than intuition, a ‘gut feeling’, but Heisenberg later stated that: ‘It
was from September 1941 that we saw an open road ahead of us, leading
to the atomic bomb.’
Heisenberg probably did not doubt that German science was already
far ahead of anything that physicists might have done in Britain or Amer-
ica. This left him in a unique and, possibly, quite uncomfortable position.
It was his Uranverein colleague and friend Carl Friedrich von Weizsäcker
who urged him to consult his former mentor, Niels Bohr. But Heisen-
berg’s reasons for seeking such a meeting may have been quite complex.
Bohr had preferred to remain at his Institute for Theoretical Physics
in Copenhagen following the German occupation of Denmark in April
1940. Of Jewish descent, Bohr was considered a ‘non-Aryan’ by the
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the first war of physics
occupation forces but, like all eight thousand Danish Jews, was protect-
ed—at least temporarily—by the agreement reached with the Danish
government designed to preserve the fi ction that the Nazis were there
by invitation.
That Heisenberg and Weizsäcker were concerned for Bohr’s welfare
is not in doubt. Growing moral qualms about the work the Uranverein
was doing may also have played a part in Heisenberg’s decision to visit
Bohr in Copenhagen. In his memoir, written almost thirty years after the
event, Heisenberg recalled Weizsäcker’s proposal. ‘It might be a good
thing,’ Weizsäcker had said, ‘if you could discuss the whole subject with
Niels in Copenhagen. It would mean a great deal to me if Niels were, for
instance, to express the view that we are wrong and that we ought to stop
working with uranium.’
According to Heisenberg, his primary purpose was to seek guidance
from Bohr on the morality of working on scientifi c problems that could
have ‘grave consequences in the technique of war’. As Heisenberg’s
Uranverein colleague Peter Jensen later put it: Heisenberg, the high
priest of German theoretical physics, sought absolution from his Pope.
Or, perhaps, as German émigré physicist Rudolf Peierls later suggested:
‘[Heisenberg] had agreed to sup with the devil, and perhaps he found that
there was not a long enough spoon.’
Heisenberg looked forward with great eagerness to the prospect of
talking to his former mentor. To Heisenberg, and indeed to many other
physicists of his generation, Bohr had long been something of a father-
fi gure. As Heisenberg’s wife Elisabeth later wrote:
In Tisvilde, the beautiful vacation home of the Bohrs, he had played with
their children and taken them for rides on a pony wagon: he had gone for
long sailing trips on the ocean with Bohr, and Niels had visited him in his
ski cottage; together they had grappled with the problems of physics, and
he thought he could talk about anything with Bohr.
But there may have been other reasons for seeking a meeting. It is possi-
ble that, alarmed by reports of American efforts to build an atomic bomb
that had appeared in the Swedish press, Heisenberg and Weizsäcker may
have also wanted to discover what Bohr knew.
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Having decided to pay Bohr a visit, Heisenberg now faced a number
of practical hurdles. Although Germany dominated much of con-
tinental Western Europe, travel was restricted and the authorities
were initially reluctant to let Heisenberg make the trip. Weizsäcker
proposed a potential solution. He had already given several lectures
in occupied Copenhagen, the most recent being a lecture on the
philosophical implications of quantum theory at Bohr’s institute. At
Weizsäcker’s prompting, an invitation was issued to both Heisenberg
and Weizsäcker to participate in a symposium on astronomy, math-
ematics, and theoretical physics at the newly formed German Cultural
Institute in Copenhagen.
The proposal was initially rejected by the Reich Ministry of Education
but, after some arm-twisting by the German Foreign Offi ce (whose offi -
cials suggested that State Secretary Ernst von Weizsäcker—Carl Friedrich’s
father—might intervene), approval was granted provided Heisenberg kept
a low profi le and stayed only a few days.
Despite this proviso, Heisenberg arrived in Copenhagen early on Monday,
15 September, four days before the conference was due to start on the follow-
ing Friday. As he described it in a letter to his wife he composed at various
times during his trip, it was a journey into the recent past:
Here I am once again in the city which is so familiar to me and where a
part of my heart has stayed stuck ever since that time fi fteen years ago.
When I heard the bells from the tower of city hall for the fi rst time again,
close to the window of my hotel room, it gripped me tight inside, and eve-
rything has stayed so much the same as if nothing out there in the world
had changed. It is so strange when suddenly you encounter a piece of your
own youth, just as if you were meeting yourself.
Such was his eagerness to see Bohr that he made his way to Bohr’s
Carlsberg residence that fi rst night, walking through the darkened city
under a clear and starry sky. He was relieved to discover that Bohr and his
family were doing fi ne. Their conversation quickly turned to the ‘human
concerns and unhappy events of these times’. In his letter to his wife he
expressed some dismay that ‘even a great man like Bohr can not separate
out thinking, feeling, and hating entirely,’ but then added, ‘But probably
one ought not to separate these ever.’
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the first war of physics
But Heisenberg remained stubbornly insensitive to the perceptions and
feelings of his former colleagues. In occupied Copenhagen in September
1941, with much of Europe conquered by Axis forces, with the German
Army Group North just seven miles from Leningrad, with Kiev encircled
and the assault on Moscow about to begin, it was not diffi cult to conceive
a German victory. There was a certain startling inevitability about the
impending Nazi domination of Europe, and everything this implied.
Heisenberg, the pragmatist, had made his bargain long ago. And here
he now was, a representative of German culture, in Copenhagen at
the behest of the German Cultural Institute, promulgating what many
Danes may have perceived to be thinly veiled Nazi propaganda. Surely,
Heisenberg may have reasoned, in the face of the inevitability of Nazi
victory it was in the best interests of themselves and of physics for his
former colleagues in occupied countries to make their bargain too?
Bohr and his colleagues boycotted the formal conference proceed-
ings, but Heisenberg still sought them out. He visited Bohr’s Institute
and joined some of the physicists for lunch on a couple of occasions.
Among those present were Christian Møller and Stefan Rozental. They
later had bitter recollections of the discussion: ‘[Heisenberg] stressed how
important it was that Germany should win the war . . . the occupation of
Denmark, Norway, Belgium and Holland was a sad thing but as regards
the countries in Eastern Europe it was a good development because
these countries were not able to govern themselves.’
It was during Heisenberg’s second meeting with Bohr on the Wednesday
evening that he raised the issue of atomic weapons. Their subsequent rec-
ollections of this highly charged meeting were vague and contradictory.
Heisenberg remembers that they took an after-dinner stroll, principally
to avoid the risk of Gestapo surveillance. Bohr believed the conversation
had taken place in his study. It would have made sense that Heisenberg
would want to talk somewhere more secure, away from prying ears or
listening devices, because when he raised the question of the military
application of atomic energy with Bohr he was in principle committing
an act of treason.
The conversation got off to a poor start. And then it got rapidly worse.
Bohr had heard about Heisenberg’s insensitive remarks, and became angry
when Heisenberg not only defended Germany’s invasion of the Soviet
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164
Union but further argued that it would be a good thing if Germany were
to win the war. When Heisenberg fi nally raised the question of working
on an atomic bomb, Bohr was completely and utterly shocked.
As far as Bohr understood, he had already shown in 1939 that achiev-
ing an explosive nuclear chain reaction would ‘take the entire efforts of
a country’. Yet here was his friend and former colleague, with whom he
had shared some of the most thrilling moments of scientifi c discovery in
his life, explaining with some impatience that a bomb was possible and
that he was working on it for the Nazis. In a letter that Bohr composed to
Heisenberg long after the war, but which he never sent, he wrote:
. . . in vague terms you spoke in a manner that could only give me the fi rm
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.
Heisenberg may have even drawn a sketch to explain his work, though
this now seems quite doubtful. When Bohr produced this some years later
it appeared to be a sketch of a reactor, but whether Heisenberg intended
it or Bohr misunderstood, Bohr assumed it was a sketch of an atomic
bomb. Even worse was to come. Heisenberg appeared to be probing
Bohr for any information about an Allied bomb programme. Was this,
after all, an intelligence mission? On whose authority was Heisenberg
now acting?
After the war Heisenberg claimed he was trying—through Bohr—to
establish a commitment from nuclear scientists not to develop atomic
weapons. Whether this was really his intention or not, Bohr interpreted
his efforts as those of a brashly confi dent representative of an aggressive
occupying power bent on delivering the ultimate weapon to his masters.
In another letter to Heisenberg that was never sent, Bohr wrote:
It had to make a very strong impression on me that at the very outset you
stated that you felt certain that the war, if it lasted suffi ciently long, would
be decided with atomic weapons . . . You added, when I perhaps looked
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the first war of physics
doubtful, that I had to understand that in recent years you had occupied
yourself almost exclusively with this question and did not doubt that it
could be done.
Heisenberg later recalled Bohr’s observation that it would be hopeless
to try to infl uence the activities of physicists now working in various
countries, ‘and that it was, so to speak, the natural course in this world
that the physicists were working in their countries on the production of
weapons.’
Despite this exchange, they appeared to have parted amicably.
Heisenberg met with Weizsäcker on the Langelinie, the picturesque walk
near Copenhagen harbour. Heisenberg admitted: ‘You know, I’m afraid it
went badly wrong.’
Heisenberg and Weizsäcker attended the conference on 19 September.
It was attended by just fi ve local astronomers from the observatory in
Copenhagen. In his obligatory report on the visit, Heisenberg commented
that ‘our relations with scientifi c circles in Scandinavia have become very
diffi cult.’
After attending a lunchtime reception at the German embassy,
Heisenberg joined Weizsäcker for a last visit to the Bohr household.
This fi nal visit was free of political or scientifi c discussions. Bohr read
aloud and Heisenberg played a Mozart sonata.
If Heisenberg had really been trying to stop the development of an
atomic super-weapon, he had failed. If he had been seeking absolu-
tion, he had failed. But the manner of his failure was to have profound
implications. The wartime efforts of nuclear scientists in Britain and
America were driven by a deep-rooted fear of what the Nazis might do
with such a super-weapon. Against this background of fear, the inter-
pretation of the intent of the German physicists, especially of Heisen-
berg, and the determination of the Nazi military authorities provided
a critical moral justifi cation for the work that Allied physicists were
now doing.
In the end, it did not matter much precisely what Heisenberg had
really intended to say to Bohr. As a result of the meeting one of the most
respected and revered of nuclear physicists, a Danish half-Jew living under
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166
Nazi occupation, had been left with the fi rm impression that Heisenberg
was working earnestly to deliver an atomic bomb to Hitler’s arsenal.
Heisenberg had been unable to see all possible ends. His dangerous
Faustian bargain was beginning to have unexpected consequences.
American efforts to develop nuclear weapons were spurred by the discovery that just a few
pounds of pure uranium-235 would be required to support an explosive fast-neutron chain
reaction. For a substance as dense as uranium this was a critical mass about the size of a
golf ball. Roosevelt sanctioned the American bomb project in November 1941, just days
before the Japanese attack on the American fl eet at Pearl Harbor. When the Army took
control in September 1942, it became the Manhattan Project.
The following month, American physicist J. Robert Oppenheimer was appointed as
scientifi c director of a new weapons research laboratory to be established at Los Alamos in
New Mexico. A few months later the world’s fi rst nuclear reactor was successfully tested
in a squash court at the University of Chicago.
Bohr escaped from Nazi-occupied Copenhagen in September 1942 with the aid of
the Danish resistance and the British Secret Intelligence Service. He arrived at Los Ala-
mos in January 1943. Revered by many at Los Alamos, he became a father-confessor,
particularly for some of the younger physicists such as American Richard Feynman.
His September 1941 conversation with Heisenberg had left him in no doubt that, under
Heisenberg’s leadership, everything was being done in Germany to develop atomic
weapons. He provided a forceful and timely reminder of the threat of a Nazi weapon,
and everything this implied.
Any Los Alamos scientist wrestling with his conscience over what he was helping to
build would fi nd the moral justifi cation for it in Bohr’s own experiences. ‘He made the
enterprise seem hopeful,’ Oppenheimer observed after the war of Bohr’s role, ‘when many
were not free of misgiving.’
But, in truth, Heisenberg and other leading Uranverein physicists had shied away from
making a substantial bid for military funding for a large-scale weapons project when they
had the opportunity in June 1942. Citing the huge technological challenges that stood
in the way of a bomb, they bid for much more modest funding to support research on a
nuclear reactor. Though rather put out by the German physicists’ modest requests, Albert
Speer, Hitler’s Reichsminister for Armaments, accepted that an atomic bomb could not
be produced in time to affect the course of the war.
The German physicists failed to build even a working reactor. When Heisenberg was
captured by Allied forces in May 1945 and interned with other Uranverein physicists at
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the first war of physics
Farm Hall in Cambridgeshire, England, hidden microphones recorded their reaction to
the news of the Allied atomic bombing of Hiroshima.
The bombing of Hiroshima and Nagasaki in August 1945 was just one more catas-
trophe on top of a long list of utter catastrophes; one that brought to an end a long and
profoundly immoral war with such a powerful exclamation that it burned into the con-
sciousness of all who lived then, and all who have lived since. But in ending the war the
physicists had unleashed a primordial force upon the world, the threat of which would
endure long after the perpetrators of evil were gone. Through their efforts they had helped
to put the world in an even greater jeopardy.
‘In some sort of crude sense which no vulgarity, no humor, no overstatement can quite
extinguish, the physicists have known sin,’ said Oppenheimer in 1947, ‘and this is a
knowledge which they cannot lose.’
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PART IV
Quantum
Fields