‘Copenhagen’: Uncertainty in Life and in Science

Copenhagen by Michael Frayn. New York: Anchor, 2000. 132 pages.

Peter Frayn’s play Copenhagen, recently returned to the stage in America, speculates on what might have transpired during a meeting between Nobel laureates Niels Bohr and Werner Heisenberg in Copenhagen in September 1941, at the height of the German advance into Russia and just three months before America’s entry into the war. The power of National Socialist Germany was at its pinnacle, and the Germans had just been made aware, through Swedish sources, of U.S. plans to build an atomic bomb.

The meeting was at Heisenberg’s behest. As Germany’s leading theoretical physicist and head of the German Uranium Club, the organ which would assess the possible war uses of nuclear energy, he was the man best situated to advise his government on the creation of an atomic bomb. The older Bohr was not only a professional colleague of Heisenberg, but a close personal friend as well. The play ponders the possible reasons for Heisenberg’s visit, linking them to the failure of the Germans to develop the bomb.

The stage is set austerely, with but three actors in the roles of Heisenberg, Bohr, and Bohr’s wife, Margrethe. The set resembles a university physics seminar, with about two dozen members of the audience arranged in a semicircle around the circular center stage, as though participating in the seminar or sitting in judgment at a tribunal. The principals, Heisenberg and Bohr, orbit around the stage like electrons around the nucleus, Margrethe, who comments on the actions and words of her husband and Heisenberg. She is obviously disposed against the latter.

The action of the play encompasses the initial meeting of the two physicists in Copenhagen in 1941, another encounter in 1947, and finally an imagined meeting that takes place after all three characters have died. Margrethe, Bohr’s wife, is present in all scenes as interlocutor and commentator. Even after death they are unable to ascertain with certainty (thus, the uncertainty principle in human life) precisely what was said in Copenhagen in 1941, what was implied, and what was inferred. Did Bohr understand what Heisenberg intended to convey? Did Bohr misinform – intentionally or unwittingly – the Western Allies of Germany’s wartime plans?

As Frayn notes (Copenhagen, p. 96), dialogue plays an important role in Heisenberg’s own memoirs, because he wanted “to demonstrate that science is rooted in conversations.” In the play Margrethe says of her husband and Heisenberg: “The first thing they ever did was to go for a walk together … Walk, and talk. Long, long before walls had ears.”

Did Heisenberg ask to meet Bohr in order to confirm the reports concerning an American effort to build an atomic bomb? Did he want Bohr to disassociate himself from the American project? Did he want Bohr to dissuade the West from developing the bomb because he, Heisenberg, intended to discourage Germany from building the bomb? Did he tell Bohr that Germany would build only a reactor – an engine – and not a nuclear weapon? Or was he attempting to mislead Bohr about Germany’s real intentions?

Michael Frayn has based the historical background to his play on two major books – Thomas Powers’s Heisenberg’s War and Robert Jungk’s Brighter Than a Thousand Suns – each of which views Heisenberg more favorably than did Allied opinion in the first decades after the war.[1] Until the appearance of these books (as well as David Irving’s 1967 The German Atomic Bomb),[2] Heisenberg was treated with undeserved hostility and contempt by many of the physicists who had been involved in the U.S. Manhattan Project, some of whom were his former students or friends. On this, Frayn has Heisenberg comment: “When I went to America in 1949 a lot of physicists wouldn’t even shake my hand. Hands that had actually built the bomb wouldn’t touch mine.”

Because it presents Heisenberg in a favorable light, Copenhagen has drawn the particular ire of Paul Lawrence Rose, Professor of Jewish Studies and European History, as well as the director of the Center for Research on Anti-Semitism, at Pennsylvania State University.[3] Rose finds the play to be a travesty of scientific history, a white-wash of Heisenberg’s and Germany’s inability to make the bomb, and (of course) anti-Semitic. (Frayn says that the true inventors of the bomb, Otto Frisch and Rudolf Peierls, were Jews.) Heisenberg, in the play, also helps arrange safe-passage for boatloads of Jews, including Bohr and his wife, from Denmark to Sweden in 1943. Furthermore, Frayn asserts, to Rose’s dismay, that Churchill and Roosevelt were amoral power-wielders just like Hitler.

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Werner Heisenberg with two of his sons, in the late 1940s. Awarded the 1932 Nobel Prize for physics, he headed wartime Germany's atomic research program. His meeting with fellow Nobel laureate Niels Bohr in Denmark in September 1941 is the subject of a much-discussed new play, “Copenhagen.”

For years the Allies insisted that Germany had intended to build an atomic bomb but had failed because its scientists didn’t understand bomb physics, hadn’t made the proper calculations, had used the wrong materials, disagreed among themselves, and so on. Some American physicists accused German physicists, especially Heisenberg, of concocting a false story of moral scruples that constrained them from building so diabolic a weapon.

Samuel Goudsmit, a top physicist and occasional personal friend of Heisenberg, was appointed head of the Allied wartime mission (codenamed “Alsos”) charged with obtaining and evaluating scientific intelligence on the German uranium project. In its findings “Alsos” contemptuously dismissed what it called the moral pretensions of the German physicists, concluding that Heisenberg and his colleagues had never fully understood the fast-neutron reaction in the U-235 bomb, and that under the Nazi regime no such advanced research could have ever hoped to succeed.

Heisenberg disparaged the “Alsos” report, praising Irving’s study of the German effort instead:

I did not like the Goudsmit book, Alsos. It was not a good book. I felt that he wrote it for political propaganda. I can only say that Irving really has studied the documents much better than Goudsmit has. In Irving you get the facts practically correct. He has done very careful work.

Rose objects strenuously to Frayn’s attempt to establish a moral equivalency between the positions of Heisenberg and Bohr. Heisenberg, according to Rose, was a brilliant but weak man, whose shallow moral character allowed him to be easily corrupted by his nationalist German sympathies into colluding with Nazism. Most interesting, Rose has explicitly condemned Copenhagen for its revisionism:[4]

Thanks to the play’s chic postmodernism as well as the complexity of its idea, the subtle revisionism of Copenhagen has been received with a respect denied to such cruder revisionisms as that of David Irving’s Holocaust denial. Revisionism it is, nonetheless, and Copenhagen is more destructive than Irving’s self evidently ridiculous assertions – more destructive of the integrity of art, of science, and of history.

It was not until 1976, the year of Heisenberg’s death, that Samuel Goudsmit revised his earlier dismissal of his friend’s scientific abilities and moral concerns. The former head of Alsos wrote:

Heisenberg was a very great physicist, a deep thinker, a fine human being, and also a courageous one. He was one of the greatest physicists of our time, but he suffered severely under the unwarranted attacks by fanatical colleagues. In my opinion he must be considered to have been in some respects a victim of the Nazi regime. (“Copenhagen,” p. 110)

As portrayed in Copenhagen, Heisenberg again and again expresses his doubts as to whether scientists should cooperate with the state in developing weapons of war. As an individual and a loyal German Heisenberg was confronted by a moral dilemma. If he chose to thwart Germany’s development of the bomb, he might threaten the very existence of his country, since he knew the enemy was building a bomb. And indeed the preponderance of historical evidence suggests that Heisenberg chose to dissuade the German war office from building the bomb by providing spurious and exaggerated estimates of the materials and time required.

The New York Times reviewed Copenhagen just before the play opened on Broadway, with an emphasis on the staging and scientific content rather than the moral issues. The review particularly emphasized the “elegance and clarity” with which director Michael Blakemore presents the complexities of Heisenberg’s uncertainty principle (the more precisely you measure one variable, the less precise your measurement of the related variable can be), complementarity (perceiving something from two incompatible standpoints), Bohr’s quantum mechanics, and other advances in physics. The reviewer gives particular note to the ways in which these findings in physics are echoed in human behavior: for one, uncertainty surrounds the reasons and motives for Heisenberg’s position regarding a German attempt to build the atom bomb.[5]

In Copenhagen Bohr describes his complementarity principle thus:

Particles are things, complete in themselves. Waves are disturbances in something else. We must choose one of the two ways of seeing, but as soon as we do we can’t know everything about them.

In illustration of the principle of complementarity in life, during a heated discussion between the two principals Heisenberg says:

You thought I was trying to arm Germany with nuclear weapons. It was a war. You were absolutely entitled to kill me. Of course, this didn’t even occur to you because while I’m your enemy, I’m also your friend. I’m an enemy to mankind, but I’m also your guest. I’m a particle but I’m also a wave.

It must be stated that although a German patriot, Heisenberg never joined the Nazi Party, and always got along with his Jewish colleagues. “I am not a Nazi, but a German!” he often said. Because of his unprejudiced views on theoretical physics, he was accused by some Party members of being a “white Jew,” a gentile who took Einstein’s relativity theory seriously. In the drama Heisenberg expresses his attachment to Germany:

Germany is where I was born. Germany is where I became what I am. Germany is all the faces of my childhood, all the hands that picked me up when I fell, all the voices that encouraged me and set me on my way, all the hearts that speak to my heart. Germany is my widowed mother and my impossible brother. Germany is my wife. Germany is our children.

Tortured by the importance of his recommendations to the German government on whether or not to build an atomic bomb, Heisenberg is torn between his own personal reluctance and moral compunction about building the bomb, and his concern for his homeland and family if he recommends against it. In Frayn’s dialogue, Heisenberg looks back to the consequences of Germany’s defeat in 1918, including Communist uprisings and the Allied hunger blockade:

I have to know what I am deciding for them! Is it another defeat? Another nightmare like I grew up with? Bohr, my childhood in Munich came to an end in anarchy and civil war. Are more children going to starve as we did? [referring to the postwar British blockade] Are they going to have to spend winter nights as I did when I was a schoolboy, crawling on my hands and knees through enemy lines, creeping out into the country under the cover of darkness in the snow to find food for my family? And maybe I’m choosing something worse even than defeat. Because the bomb they’re building is to be used on us. On the evening of Hiroshima Oppenheimer said it was his one regret that they hadn’t produced the bomb in time to use on Germany.

When Bohr interrupts him to say that Oppenheimer also tormented himself afterwards, Heisenberg retorts:

Afterwards, yes. At least we tormented ourselves a little beforehand. Did a single one of them stop to think, even for one brief moment, about what they were doing? Did Fermi, or Teller, or Szilard? Did Einstein when he wrote to Roosevelt in 1939 and urged him to finance research on the bomb? Did you, when you escaped from Copenhagen two years later, and went to Los Alamos?

Bohr replies that at least he and the Los Alamos group weren’t supplying the bomb to a Hitler. To which Heisenberg responds:

You weren’t dropping it on Hitler, either. You were dropping it on anyone who was in reach. On old men and women on the street, on mothers and their children. And if you’d produced it in time they would have been my fellow countrymen. My wife. My children. That was the intention. Yes?

Bohr: That was the intention.

Much has been made of the comments by a number of Germany’s leading physicists (referred to as the Farm Hall transcripts) during their detainment in Britain for six months after the war. It was during that time that they learned that an atomic bomb had been dropped on Japan. Since they unquestionably assumed that their remarks were being recorded, the German scientists no doubt said one thing for the ears of the British and other things among themselves during their daily walks. Jeremy Bernstein has made a fair and objective analysis of the transcripts, concentrating mostly on scientific considerations. He does not believe that Heisenberg ever made the crucial calculations necessary to determine the critical mass of the bomb, although he obviously knew quite well that a bomb would require fast (U-235) rather than slow (U-238) neutrons. Of the man Heisenberg, Bernstein writes: “He had the first truly quantum-mechanical mind – the ability to take the leap beyond the classical visualizing pictures into the abstract, all-but-impossible-to-visualize world of the subatomic.”[6]

The main reason for Heisenberg’s visit to Copenhagen in 1941 appears to have been his hope that Bohr in the West, and he in Germany, would be able to discourage work on a bomb. In Germany Heisenberg argued that building a nuclear-fission bomb when the war was still raging would be beyond Germany’s technical capabilities. Moreover, he incorrectly informed the German war office that more than a ton of fissile material would be required.

Heisenberg chose to build instead an Uranmaschine (a nuclear reactor). He drew a simple sketch of the reactor for Bohr, but at the time the Dane apparently did not yet understand the difference between a reactor and a bomb. Bohr assumed that Heisenberg’s drawing was a bomb sketch and passed his opinion on.

In Copenhagen Heisenberg tells Bohr explicitly that he is not working on a bomb, but on a reactor. “A machine to produce power! To generate electricity, to drive ships!”

Theatrically, the most dramatic moment in the play occurs as Heisenberg, at Bohr’s urging, performs the crucial calculation for the critical mass of U-235 that would have given Germany the key to the bomb:

Bohr: Why are you confident that it’s going to be so reassuringly difficult to build a bomb with 235? Is it because you’ve done the calculation?

H: The calculation?

B: Of the diffusion in 235. No, it’s because you haven’t calculated it. You haven’t considered calculating it. You hadn’t consciously realized there was a calculation to be made.

H: And of course now I have realized. In fact it wouldn’t be all that difficult. Let’s see … The scattering cross-section’s about 6 × 10–24, so the mean free path would be … Hold on …

At this point an explosion, white light, and thunderous noise fills the stage, simulating the burst of an atomic bomb.

As to Frayn’s accuracy in depicting the principals in the play, Heisenberg’s son, Jochen Heisenberg, currently professor of physics at the University of New Hampshire, has criticized the playwright’s representation of his father: “You can’t try in a play to reproduce real people. There are many differences between how Heisenberg is presented and how the real person is. He was a rational person, not outwardly emotional. His emotions came through when he played music. That last part when his long monologue regrets the destruction of his country – my father would never have done something like that.”

On the other hand, Bohr’s grandson, Vilhelm Bohr, currently a researcher at the U.S. National Institutes of Health, called the play “a wonderful piece of drama, very exciting” and agreed that “some of the character of my grandfather comes through. In many ways it is accurate about my grandfather’s personality.”[7]

No relatives seem to have expressed themselves on the portrayal of Bohr’s wife, Margrethe, but to this reviewer she comes across as a querulous woman unable to conceal her disdain for Heisenberg.

The central question of whether Heisenberg willingly refused to calculate the amount of the U-235 isotope necessary to sustain a chain reaction, or whether he deliberately fudged his estimate to discourage the German war leaders, or whether he simply was unable to make the calculations, is not answered in Copenhagen. In response to Bohr’s direct question as to why he didn’t make the crucial calculation, Heisenberg answers simply but convincingly.

Frayn’s dialogue:

H: Why didn’t you calculate it?

B: Why didn’t I calculate it?

H: Tell us why you didn’t calculate it and we’ll know why I didn’t.

B: It’s obvious why I didn’t.

H: Go on.

M: Because he wasn’t trying to build a bomb!

H: Yes, thank you. Because he wasn’t trying to build a bomb. I imagine it was the same with me. Because I wasn’t trying to build a bomb. Thank you.

In several interviews after the war, Heisenberg stated explicitly that he and a few colleagues had calculated the critical mass quite accurately but chose not to inform the German government. In 1967 he stated: “The German physicists knew from their calculations how many kilograms were needed to build an atomic bomb – and these figures agreed well, as was shown after the war, with the American ones.”[8] And in 1970, in a letter to Ruth Nanda Anshen, the editor of his memoirs: “Dr. Hahn, Dr. von Laue, and I falsified the mathematics in order to avoid the development of the atom bomb by German scientists.”

To summarize the uncertainty surrounding Heisenberg’s wartime decisions and actions, Frayn has the German physicist say, somewhat sarcastically: “Everyone understands uncertainty. Or thinks he does. But no one understands my trip to Copenhagen.”

Ironically, paradoxically, it was Bohr who, in a small way, contributed to the bombs that were dropped on Nagasaki and Hiroshima. Heisenberg’s wartime activities contributed to no one’s death.

It is this reviewer’s opinion that some of the uncertainties about Heisenberg’s role in the German wartime nuclear research program can be removed by reconstructing the war situation and by questioning the basic assumption upon which U.S. physicists have doubted Heisenberg’s integrity and competence. U.S. scientists, led by Einstein in 1939, were the first to begin work on an atomic bomb, justifying the need for it on the assumption that the Germans were working, or would be working, on the bomb. Heisenberg repeatedly stated that at that time he and the Uranium Club were attempting to build a reactor – an engine. In the absence of any physical evidence of attempted bomb construction, or of any official German documents authorizing the building of a bomb, Heisenberg must be taken at his word.

Werner Heisenberg’s entire life was exemplified by excellence: in classical studies, in music (he was an accomplished pianist), and of course in theoretical physics. Before the war he was generally considered by his colleagues to be the most gifted mathematician in the field. His personal integrity has only been questioned out of political enmity over his alleged wartime role.

Professor Rose, in deriding Copenhagen’s subtle revisionism, speaks for many of Heisenberg’s (and Germany’s) critics. Their agenda, of course, is all too patent: to begrudge the physicist and the Germans their humanity, while obscuring the inhumanity of the Allied leaders and scientists. Years after the meetings with Bohr, Heisenberg all too charitably remarked of his detractors: “After a great war, history is written by the victors and legends develop that glorify them.”[9] By its unraveling a key such legend that glorifies the victors and smears the vanquished, Copenhagen, though a drama, gives substantive impetus to the revisionist quest for a fair and accurate picture of the Second World War.

Notes