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 don’t know just what he discussed, but it wasn’t 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 Frayn’s 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 Werner’s 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 Museum’s 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 Heisenberg’s 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 war’s 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 war’s 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 Heisenberg’s 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.

            Heisenberg’s 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 Frayn’s 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 Jungk’s 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 book’s 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 Bohr’s 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 Bohr’s institute during Heisenberg’s 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.  Moller’s 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 Bohr’s rejection and his wife’s 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 Heisenberg’s 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 Heisenberg’s ego; Heisenberg took Goudsmit’s 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 doesn’t work at all.”  Hahn responded, “At any rate, Heisenberg, you’re 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, Heisenberg’s 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 doesn’t really want to help us; he has lost his parents.”  Paul Harteck responds, “Of course Goudsmit can’t forget that we killed his parents.  That’s true too and it doesn’t 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 didn’t do it was because all the physicists didn’t want to do it, on principle.  If we had all wanted Germany to win the war we would have succeeded.”  Otto Hahn responded, “I don’t 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, Heisenberg’s 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 Heisenberg’s hand.

            Goudsmit’s 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 Heisenberg’s Nobel Prize citation.  I do not believe that this was crucial, but it may have been an element.  Beyond that, Goudsmit’s 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 Heisenberg’s 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 Heisenberg’s 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 Goudsmit’s 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 Heisenberg’s education also included his father’s 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 son’s 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 decision’s 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 Heisenberg’s failure to develop a definitive figure for the critical mass.  In Bernstein’s 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 Goudsmit’s story is the transformation of his views over the years.  After Heisenberg’s 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.”