Goudsmit, Bohr, and Heisenberg Michigan Center for Theoretical
Physics November 27, 2001 On September 15, 1941, Werner Heisenberg
and Carl Friedrich von Weizsäcker traveled to Copenhagen for a week-long
conference on physics and astrophysics. It was in form an exercise in expounding
the results of pure research. Heisenberg had developed an interest in cosmic
rays, and von Weizsäcker had contributed mightily to solving the problem
of stellar energy production. But form is not everything. Held at the
German Cultural Institute, a propaganda establishment of the Nazi forces
occupying Denmark, the conference proved an occasion for the most distinguished
physicists in the land to stay at home. Von Weizsäcker was not only a distinguished
theoretical physicist; he was the son of Ernst von Weizsäcker, State Secretary
under Ribbentrop: father and son figure in the letter signed by Einstein
and delivered by Alexander Sachs to Franklin Delano Roosevelt on October
1, 1939. Heisenberg was the leading figure in German nuclear research.
And September 1941 was the high water mark of Nazi expansion across Europe. On September 16, 1941, the day after
his arrival in Copenhagen, Werner Heisenberg visited Niels Bohr and discussed,
well, I dont know just what he discussed, but it wasnt chopped
liver. Bohr was shaken, and the relationship between him and Heisenberg
suffered irreparable damage. The reverberations of their talk spread slowly
but have continued to this day. From her refuge in Sweden, Lise Meitner
wrote to her former colleague Otto Hahn on June 27, 1945, If you could
have seen for yourself those who came here from the camps. A man like Heisenberg
and many millions with him should be forced to see these camps and the martyred
people. His appearance in Denmark in 1941 is unforgivable. At a
meeting in Copenhagen in 1963, Margarethe Bohr was talking with Samuel Goudsmit
and, pointing at Heisenberg and Weizsäcker, said That wartime visit
of those two was a hostile visit, no matter what people say or write about
it. Michael Frayns play Copenhagen returns to the visit
and has placed before a more numerous public questions that have brought
forth at least four books and a score of scholarly articles in the last
decade. Now that Copenhagen is going to play in Detroit, a perspective
from Ann Arbor may be in order. On condition that a suitable piano be made
available to him, Heisenberg lectured in Ann Arbor during the summers of
1932 and 1939, Bohr in 1933, and Heisenberg stayed at the home of Samuel
Goudsmit in 1939. My own interest lay originally in August Heisenberg,
the great Byzantinist who was Werners father, and then it took a detour
to the history of astrophysics, settling on the history of spectroscopy,
and the introduction of astrophysical theory to Michigan. The words spectroscopy
and theory and the Kelsey Museums Egyptian collection
brought me to Goudsmit. Today I shall talk about Heisenberg and the German
wartime nuclear project, and a bit more about Heisenberg and Goudsmit. On April 29, 1939, Niels Bohr was
in Washington, D.C., lecturing on the conditions for a chain reaction to
occur in U235. He concluded that such a reaction was possible, but he added
that obtaining 235 in sufficient quantity was practically impossible. The
question was still open in Germany, however. On the same day the Nazi government
established a secret uranium research project in Berlin and banned the export
of uranium from the Reich and its recently occupied mines in the former
Czechoslovakia. When Heisenberg arrived in Ann Arbor
to lecture in late July of 1939, the clouds of impending war had lowered
further. Toward the end of his stay, Heisenberg spoke with Enrico Fermi
at a party hosted by Otto Laporte. Both Fermi and Goudsmit called on Heisenberg
to seek refuge in the U.S., as did Heisenbergs other hosts that last
summer in America, offering him posts at Chicago and Columbia. Max Dresden
recounts the conversation: Heisenberg believed that with his prestige,
reputation and known loyalty to Germany, he could influence and perhaps
even guide the government in more rational channels. Fermi believed no
such thing. In his opinion, the governments of Nazi Germany and Fascist
Italy were not open to persuasion, although their subjects were open to
violence. For his part, Heisenberg argued that he needed to be available
to repair the damage at wars end. No argument moved him, even though
George Pegram of Columbia kept up the pressure all the way to dockside in
New York later in August. And in September, Heisenberg was duly drafted,
not to arms but to the Army Weapons Bureau. Formally Heisenberg headed a reactor
research group in Leipzig and served as advisor to a larger group in Berlin
until 1942. From 1942 he headed the main reactor research group at the
Kaiser Wilhelm Institute of Physics, first in Berlin and at wars end
in Bavaria. At the end of 1939 and on February 29, 1940, Heisenberg provided
a report to the Weapons Bureau On the Possibility of Technical Energy
Production from Uranium Splitting. Among others, he reached three
significant conclusions. First, a chain reaction in a reactor would stabilize
itself without control rods: in his reactor, U238 would absorb sufficient
neutrons from fissioning atoms. It is my understanding that this was not
true and Heisenbergs failure to produce a working reactor meant that
we speak of Chernobyl and not Berlin. His second conclusion dealt with
the critical mass for a U235 explosive, which he calculated at between the
tens and hundreds of metric tons. This calculation, done without benefit
of sufficient technical measurements, was off by orders of magnitude. There
is no evidence that he revisited these rough calculations until after he
accepted the fact that the bomb dropped on Hiroshima was a nuclear weapon.
Finally, he rejected graphite as a suitable material for slowing neutrons
in a reactor and settled upon heavy water, a commodity available in quantity
only in Norway. Heisenbergs failure to calculate,
or to re-calculate, the critical mass has occasioned both criticism and
praise: criticism for his excessive confidence in his own judgement, praise
for a supposed unwillingness to explore the possibilities of producing a
nuclear weapon. It must be said, however, that precise calculations of
the critical mass took time and accurate measurement of, for example, fission
cross-sections not well known at the time. In May 1939 Perrin in Paris
estimated that the critical mass in a solid sphere of uranium oxide was
forty tons. In June Siegfried Flügge in Berlin estimated that it would
be hundreds of tons. By December Rudolf Peierls, then in Birmingham, thought
it would be many tons. But by March 19, 1940, Otto Robert Frisch
and Peierls thought that one kilogram would suffice. A second calculation,
using new measurements of the fast neutron fission cross section of U235,
raised the figure to eight kilograms or four with a thick neutron-reflecting
outer layer. The MAUD report in July 1941 posited a critical mass of twenty-one
kilograms of U235. In October, Vannevar Bush told President Roosevelt that
the critical mass was about twenty-five pounds. I understand that the actual
figure derived at Los Alamos was fifty-six kilograms. We cannot fault Heisenberg
for his initial derivation, but we can wonder at his continuing faith in
a rough calculation. There is little about a nuclear bomb
and the path to build one in his first report. It is not such a document
as the text of the lectures given by Robert Serber at Los Alamos in early
April 1943. Serber began his presentation: The object of the project
is to produce a practical military weapon in the form of a bomb in
which the energy is released by a fast neutron chain reaction in one or
more of the materials known to show nuclear fission. You cannot get
much more explicit than that. Are we to conclude from the record that already
Heisenberg was shying away from providing Hitler a bomb and moving in the
direction of reactor research only? In 1941 the German projects continued
and were in fact ahead of the British and Americans for much of the year.
In August there were two important developments. Heisenberg was able to
report to the Weapons Bureau that his team confirmed neutron multiplication
as a result of fission. At the same time, Fritz Houtermans, who was employed
in a nuclear research project supported by the Post Office, reported that
a working reactor could produce plutonium, which would be a highly effective
explosive. And in September Heisenberg visited Niels Bohr. What Heisenberg told Bohr is hotly
disputed, and Michael Frayns play does not resolve the dispute. It
is said that Heisenberg gave Bohr a drawing of a reactor, but there are
strong reasons for doubting this, not the least of which is the witness
of Aage Bohr. Bohr did discuss such a drawing when he arrived at Los Alamos,
but its provenance remains unclear. Heisenberg later claimed that he raised
the question of the responsibility of scientists when faced with the prospect
of nuclear weapons, and it is also claimed that Bohr may not have heard
him rightly, but there is today little to help us understand just why Bohr
was so shocked and angered. There is, however, new evidence that may settle
the question. The Bohr family will release, before the end of the year,
eleven documents bearing on this meeting, including a letter Bohr wrote
to Heisenberg but did not send. This letter was apparently folded into
a copy of Robert Jungks book Brighter than a Thousand Suns,
published in 1958. Jungk was a Swiss newspaperman whose book rested upon
interviews with a number of scientists, including Weizsäcker. The books
claim, since retracted by Jungk, is that while the German scientists chose
from moral principle to build a reactor and not a bomb, it was the Americans
whose bomb offended the moral sensibilities of the world. It is said that
Bohrs letter responds to this with reference to the Heisenberg visit
in 1941. We must remember that this was a moment
of great success for the German armies in the east. The war was going well.
Stefan Rozental, who was present at Bohrs institute during Heisenbergs
visit, wrote that
[Heisenberg] stressed how important it was
that Germany should win the war. To Christian Moller, for instance, he
said that the occupation of Denmark, Norway, Belgium, and Holland was a
sad thing, but as regards the countries in East Europe, it was a good development
because these countries were not able to govern themselves. Mollers
answer was that so far we have only learned that it is Germany which cannot
govern itself. Whether Heisenberg discussed nuclear weapons with
Bohr and to what end he did, his manner must have gone far to explain Bohrs
rejection and his wifes disgust. By 1942 the situation was changing.
By the spring, for example, unbeknownst to both sides, a disparity between
the German and American projects appeared, and a gap in funding and staff
began to widen. Before this time, the Germans had had many more people
involved in the effort to construct a reactor (200 versus 50 at Chicago),
and the monetary resources to build one (about one million dollars) were
certainly present. However, by this time the Army Weapons Bureau sought
projects from which some usable results would flow in nine months. This
development jeopardized funding for Heisenbergs project, and during
the year he addressed the military representatives more directly. On February
26, he spoke on The Theoretical Basis for the Generation of Energy
from Uranium Fission." Heisenberg pointed out that U235 was
an explosive of completely unimaginable power. However, this explosive
is very hard to produce. Again, Isolating U235 would lead to
an explosive of unimaginable potency. If a reactor were built, however,
the reactor can also lead to the production of an incredibly powerful
explosive [plutonium]. In addition to the extraordinary power of
a nuclear weapon, a nuclear reactor would provide enormous advantages to
submarines. In early June, Heisenberg lectured Albert Speer and some military
officers. He spoke of prospects for a bomb, but he did not expect rapid
success. Speer provided a modest level of continuing funding. This was,
then, not necessarily a bomb project, but a project that would lead to one
in due course. This was what Heisenberg wanted.
He was now back in Berlin, head of the main project. The funding level
was adequate, as all the researchers believed themselves ahead of the allies
and felt that the war would end before their research brought them face
to face with the prospect of a bomb. Heisenberg had recognition for his
work and also full ideological rehabilitation. This was crucial, for in
1937 the SS journal Das Schwarze Korps had published an article on
Jewish physics that accused Heisenberg of being a white
Jew. It took a year and the intervention of Himmler, whose family
had connections with the Heisenbergs, to clear Heisenberg, although in a
pointed coda Himmler considered it best if in the future you make
a distinction for your audience between the results of scientific research
and the personal and political attitude of the scientists involved.
Until Speer guaranteed support and protection, Heisenberg had not enjoyed
full security. It is no surprise that Heisenberg visited not occupied Denmark
alone, but on behalf of the authorities he visited and lectured in ten occupied
countries in the course of the war. For example, in 1943, while visiting
the occupied Netherlands, Heisenberg said to Hendrik Casimir, Democracy
cannot develop sufficient energy to rule Europe. There are, therefore,
only two alternatives: Germany and Russia. And then a Europe under German
leadership would be the lesser evil. We must be clear that Heisenberg was
no Nazi. He was no supporter of Hitler, but he was a patriot, as he put
it:
we should conscientiously fulfill the duties and tasks that
life presents to us without asking too much about the why or the wherefore
.
He did not deny the acts of the Nazi regime, but he always held that once
the war was over (that is, after a German victory) things would change for
the better. It is difficult to condemn such naïve thinking. Even at the
very end of 1944, while on a visit to Switzerland, he commented to Gregor
Wentzel that it would have been so beautiful if [Germany] had won. But Germany did not win. In May 1945
Heisenberg, his team, and his unsuccessful reactor had moved to Bavaria,
where the allies caught up with them, and along with the allies, Samuel
Goudsmit of the Alsos mission, devoted to rounding up the German physicists
and assessing the state of their work on nuclear weapons. Heisenberg offered,
If American colleagues wish to learn about the uranium problem, I
shall be glad to show them the results of our researches if they come to
my laboratory. When Heisenberg inquired about the American nuclear
program, Goudsmit smiled and intimated that there had been more pressing
tasks during the war. Goudsmit was amused at Heisenbergs ego; Heisenberg
took Goudsmits comments about weapons research to an enemy subject
as the whole truth. Within a few weeks Heisenberg and
nine of the German researchers were interned at Farm Hall, near Cambridge.
The accommodations, while less than lavish, were well appointed, including
numerous listening and recording devices. The Farm Hall transcripts, which
were held in confidence until 1992, are among the most revealing private
documents of the history of the nuclear age. On August 6, as they learned
of the Hiroshima bomb, we follow their attempts to come to grips with the
event, to understand how the allies succeeded where they had not, and over
the next few days we see them adjusting their past, present, and plans for
the future in response to the bomb. The one whom the news most immediately
affected was Otto Hahn, who had a dreadful evening, compounded of equal
amounts of guilt, sorrow, and curiosity. On August 6th and the
succeeding day or two Heisenberg discussed with Hahn just how the bomb could
have been made. At first, Heisenberg refused to believe that it was a nuclear
weapon: All I can suggest is that some dilettante in American who
knows very little about it has bluffed them in saying: If you drop
this it has the equivalent of 20,000 tons of high explosive and in
reality doesnt work at all. Hahn responded, At any rate,
Heisenberg, youre just second-raters and you might as well pack up.
Later on Heisenberg and Hahn have intense discussions, in which Heisenberg
repeats his early errors about the critical mass and Hahn probes the extent
to which Heisenberg has made serious calculations and measurements. By
August 14, Heisenberg, making new calculations and working backwards on
the basis of news accounts of the actual bomb, is prepared to give a lecture
about the bomb that comes closer to reality. I do not pretend to understand the
lecture, but I do understand the annotations of Professor Jeremy Bernstein,
who occasionally throws up his hands and announces that he will return to
the explanations when the German physicists begin to make sense. They were,
of course, operating with very little assistance under difficult conditions,
but there are certain points that are stand out. First, according to Bernstein,
Heisenbergs August 14th lecture and the discussion among
the internees reveals nothing like the intellectual depth of a discussion
if you brought together ten of the best from Los Alamos. It is clear that
while the Los Alamos staff worked well as a team, the German physicists
did not. Second, you will search high and low in the Farm Hall transcripts
to find regret for the acts of the Nazi regime. The discussion of the Firma
Auer, which produced uranium, excludes mention of the slave labor responsible
for it. On July 18, Karl Wirtz opines that A man like Goudsmit doesnt
really want to help us; he has lost his parents. Paul Harteck responds,
Of course Goudsmit cant forget that we killed his parents.
Thats true too and it doesnt make it easy for him. Nobody
present brings up the possibility that it is not easy for them. Third, the German scientists, even
on August 6th, began to establish their history of the German
nuclear program, which scholars call the Lesart. The founder
appears to be von Weizsäcker, who commented I believe the reason we
didnt do it was because all the physicists didnt want to do
it, on principle. If we had all wanted Germany to win the war we would
have succeeded. Otto Hahn responded, I dont believe that
.
Nobody contradicted him. However, over time the Lesart grew: no bombs had
been built in Germany because of a conscious, principled choice. It was,
as Jungk concurred, the Americans whose failing was moral. Heisenberg subscribed
to the Lesart. On August 6 he stated, I would say that I was absolutely
convinced of the possibility of our making a uranium engine but I never
thought that we would make a bomb and at the bottom of my heart I was really
glad that it was to be an engine and not a bomb. One month later,
however, he wrote to P.M.S. Blackett that In wartime, naturally, these
results [the reactor] would have been followed by technical developments
which would have been aimed at a practical use of the energy. And
over the years, Heisenbergs memory developed to meet the requirements
of the moment, with respect to his work on the project, even to point of
claiming that he falsified his calculations. He was always clear about
having wished to preserve German science for the time after Hitler, so he
would use the military for the purposes of physics. He remained true to
the Lesart. In 1955 and again in 1974 he criticized Einstein for having
written the letter to Roosevelt, for having brought on the horror. But he was more critical of Goudsmit.
In this room Goudsmit and his colleague George Uhlenbeck are known for their
discovery of electron spin. After the mission to Europe was over, Goudsmit
published a book, Alsos, in which he criticized the German nuclear
scientists and their organization. This provoked a fierce response from
Heisenberg, and their dispute never really came to an end. Heisenberg accused
Goudsmit of misrepresenting certain aspects of the German understanding
of the physics, while Goudsmit was unwilling to let the Germans off the
hook for their claim of moral equivalency on both sides. Even at their
last meeting, in 1973, the wounds had not healed. And Goudsmit was not
the only American physicist with ambivalent feelings about Heisenberg.
At a function for Heisenberg during a visit after the war, John Wheeler
made sure to have a drink in one hand and a notebook in the other, so as
to be unable to shake Heisenbergs hand. Goudsmits feelings towards Heisenberg
had more than one basis. There may have been something personal. When
Goudsmit went to Copenhagen in the 1920s, Bohr set him upon the explanation
of the helium spectrum. After he failed to solve all of the problems, Goudsmit
returned to Holland to work with Zeeman and Bohr gave the problem to Heisenberg,
who solved it: it is mentioned in Heisenbergs Nobel Prize citation.
I do not believe that this was crucial, but it may have been an element.
Beyond that, Goudsmits views were international. The standards of
behavior of the international physics community called for an attitude that
was incompatible with allegiance to Nazi Germany. Goudsmit also understood
that the center of physics research had shifted to the United States, and
he was amused at Heisenbergs automatic assumption of German superiority.
From 1947 on Goudsmit was concerned with understanding just why America
had become so significant in the development of physics and equally concerned
with preserving the conditions that made for that position: it is even a
major point in his obituary of Heisenberg. Further, Goudsmit felt strongly
about freedom of inquiry and information in the U.S.; he saw the German
effort as an object lesson in what can happen under the control of an authoritarian
regime. His goal was to preserve those freedoms in an era of the growth
of big science and government funding. In his hands the story of the German
program was a means to a very specific end. Let us view the story from Heisenbergs
perspective. When Heisenberg was in Leiden in the 1920s, Ehrenfest likened
him to Newton, that not only had he invented a new mechanics, but
also had to invent a new mathematics to go with it. And Heisenberg
agreed with Ehrenfest. Now, however, in 1945, it was not possible to face
his nation and admit to failure, so instead the lack of a bomb had to be
the result of conscious acts. And he expressed his anger at Goudsmit for
having misled him in May 1945 about the allied program, so that he was caught
unawares before his colleagues in August. He insisted upon Goudsmits
retracting remarks about what the Germans did not know with respect to bomb
physics, for they reflected upon what he called his honor. Are we to make much of his thoughts
about politics during the war? His most severe critics have practically
turned him into a monster if not a Nazi. Perhaps we should understand his
assumptions by appreciating the circumstances in which he formed his views
of the world. His father, August Heisenberg, was one of the greatest Byzantinists
of his age, an editor of the leading journal in the field, holder of the
most prestigious chair. His work, on the Palaeologan era of Byzantium,
that is, on the thirteenth, fourteenth, and fifteenth centuries, was pioneering
and retains its value today. He much appreciated the organization, order,
and political organization of Byzantium, with an autocratic government poised
as the fulcrum between God and humankind, a leadership that treated the
state as a father rules his household, a notion of order and tradition that
mocked the changing realities of a shrinking empire. There were emperors
good and bad, but the order of society remained intact. We forget that
Werner Heisenbergs education also included his fathers field.
Later, his correspondents included medievalists, and he commented on the
similarities between aspects of the medieval order and his own era, on the
need for patriotism and obedience as a way of preserving order and tradition.
I do not claim that August Heisenberg formed his sons politics, but
we would be wrong to ignore his influence. Heisenberg and his colleagues did
not produce a bomb because they wanted not to produce one. They never got
far enough along to face the decision, or to face the decisions being
taken away from them. He was as unconcerned about the Americans as the
Americans were worried about the German project. Indeed, he may have been
the wrong man for the project. Hans Bethe commented on his failure to realize
that graphite could be a suitable moderator in a chain reaction. Fermi
and Szilard thought the problem through, got the boron impurities removed,
and achieved a chain reaction using pure graphite by the end of 1942. Fermi
worked on the graphite himself; Heisenberg relied too much on the preliminary
work of his colleagues, and his colleagues took his word as final. Bethe also remarked on Heisenbergs
failure to develop a definitive figure for the critical mass. In Bernsteins
view, Heisenberg revealed himself to be a very great physicist but
not a very good one. Peierls stated that Heisenberg was very casual
about numbers and did not worry about results that were incompatible. Fermi,
on the other hand, had that capacity. We all know the story of his accurate
calculation of the yield of the Trinity test, done on the spot with some
wadded papers. It was not a moral choice that hindered the German nuclear
projects. Rivalries between research groups, hubris about the Americans,
miscalculations and incomplete measurements, these, and not the Lesart,
were key. What happened to Heisenberg along
the path to glory did not come to him alone. The classic example is the
great conductor, Wilhelm Furtwängler. He could not bear to desert his homeland
and felt that he could do more within Germany than outside. Like Heisenberg,
he did what he could to protect his players from harm; like Heisenberg,
he found himself associated with a regime unlike any he had expected. And
there were others who took the same gamble. One of the most remarkable aspects
of Goudsmits story is the transformation of his views over the years.
After Heisenbergs death, it was Goudsmit who wrote the obituary for
the American Philosophical Society: I do not yet know whether he sought
the assignment. It is a remarkable literary document, gentle, generous,
and even self-critical. He wrote that It is unfortunate that we expect
a person who is outstanding in one area to be also a world-wise person in
all important phases of human relations. And in an autobiographical
lecture delivered to the Dutch Academy,
subconsciously, I was
disappointed when I realized that this great man was not any wiser than
the bulk of his colleagues. By looking at the examples of Nobel Prize winners
Lenard and Stark, I should have known that greatness in physics does not
mean anything outside that narrow domain. Heisenberg may have felt he had little
choice in 1939 but to continue his work in Germany. And in order to fulfill
that inner command, he may have felt capable of dealing with, and if necessary
surviving, the Nazi regime. He survived, but at a terrible cost. How could
he have known that You cannot sup with the devil even with a long
spoon.