What Is ‘Copenhagen’ Trying to Tell Us?

Gerald Holton is the author of, most recently, "Einstein, History, and Other Passions." He is Mallinckrodt professor of physics and professor of the history of science emeritus at Harvard University

In terms of science or the history of science, Werner Heisenberg’s visit to Niels Bohr, in September 1941 in German-occupied Denmark, was a nonevent. During the 1920s, physics had been changed profoundly as a result of the close interactions between these two great scientists. This time it was not. Nor did their 1941 encounter, at the height of the German army’s advances, have any effect on the course of the war or the ongoing work of the nuclear scientists on either side. As far as we know, whatever Heisenberg may have hoped to gain from insisting on this meeting did not occur.

But on this unfruitful base, the imagination has built castles. Michael Frayn’s play “Copenhagen” is one such construction, a drama of remarkable effectiveness in which the author has cleverly highlighted the scientific concept of uncertainty, associating it with the speculations of what may have actually happened during that meeting. The choreography of the actors’ movements as they pace along the circular stage, apparently meant to evoke the supposed motions of electrons in Bohr’s first atomic theory, adds to the theatrical experience. The dramatist’s device to start the action over and over again from the same point, as had been done so successfully in Max Frisch’s “Biography,” adds a hypnotizing element.

There are only three actors on the stage, representing plausible versions of Bohr, his wife Margrethe and Heisenberg. But they have become ghosts, recounting the extraordinary scientific breakthroughs in quantum mechanics, with asides about other remarkable physicists with whom they had collaborated or fought. Often they become suddenly unaware of one another’s presence, adding to the spooky quality of their interaction. But again and again these three return to two haunting puzzles. One is why the Germans never succeeded in building an atomic bomb. The other, perhaps connected with the first puzzle, is why Heisenberg had suddenly turned up at Bohr’s doorstep.

Margrethe: Why did he come? What was he trying to tell you?


Bohr: He did explain later.

Margrethe: He explained over and over again. Each time he explained it became more obscure. . . . I’ve never seen you as angry with anyone as you were with Heisenberg that night. . . .

Heisenberg: Now we’re all dead and gone, yes, and there are only two things the world remembers about me. One is the uncertainty principle, and the other is my mysterious visit to Niels Bohr in Copenhagen in 1941. Everyone understands uncertainty. Or thinks he does. No one understands my trip to Copenhagen.

The play’s intricacy on its many levels was rewarded this year with Broadway’s coveted Tony Award for the best play of the year. But by bringing together the three quite different worlds of science, history and theater, it is highly likely that much of the audience will confuse the play--a work of fiction--with a historical documentary--just as many think they know all about Galileo Galilei or J. Robert Oppenheimer from having read or seen the dramas about them. They tend to forget that the task of the poet or dramatist is, as Samuel Taylor Coleridge wrote in 1817, to create in the reader or viewer the “willing suspension of disbelief.” John Keats, at about the same time, also memorably celebrated what he called the “negative capabilities” of great authors, which he defined as their ability to remain “content with half knowledge” and capable of “being in uncertainties, mysteries, doubts, without any irritable reaching after fact and reason.”


These truths are especially relevant for “Copenhagen”: At the very heart of Frayn’s play is a piece of half-knowledge, one that can be traced chiefly to books by journalists Robert Jungk and Thomas Powers. In 1958 in “Brighter than a Thousand Suns: A Personal History of the Atomic Scientists,” Jungk published Heisenberg’s version of the meeting, which in Frayn’s play becomes the key event: the private conversation between Heisenberg and Bohr during their evening walk in 1941, which Bohr ended abruptly, deeply disturbed by something Heisenberg had said.

Heisenberg’s account, offered in a letter to Jungk that is reproduced in the book, was that he told Bohr that the researchers in his team, known as the “Uranium Club,” in 1941 “knew that one could produce atom bombs but overestimated the necessary technical expenditure at the time.” Still, he added, the physicists could have “decisive influence on further developments, since they could argue with the government that atom bombs would probably not be available during the course of the war.” What Frayn dramatizes is the implication that a moral dilemma preoccupied the German scientists. As Heisenberg’s letter put it, the discussion with Bohr during the evening walk “probably started with my question whether or not it was right for physicists to devote themselves in wartime to the uranium problem.” Heisenberg said Bohr was shocked by this train of thought, assuming “that I had intended to convey to him that Germany had made great progress in the direction of manufacturing atomic weapons.” Heisenberg added he was unable to “correct this false impression.”

To be sure, Heisenberg cautioned readers at the beginning of his letter that “I may be wrong after such a long time” about what occurred during the meeting and introduced the most controversial part of his recollection using the word “probably.” Nevertheless, Heisenberg’s account provided the key suggestion, that Jungk’s widely read book and Frayn’s play have vastly expanded on: that at least for Heisenberg, an impeding moral compunction may have existed for him and his colleagues at that time about working on atomic weapons.

Heisenberg’s letter was not the first time this idea had been launched. It had surfaced initially on Aug. 7, 1945, in a proposal by one of the German scientists detained in England at an internment facility at Farm Hall. Immediately after hearing of the existence and use of the atomic bomb by the Allies, it was proposed that “History will record that . . . the peaceful development of the uranium engine [reactor] was made by the Germans under the Hitler regime, whereas the Americans and the English developed this ghastly weapon of war.” Present at that time was Max von Laue, a great physicist, who was interned although he had been an outspoken opponent of the Nazi regime and had not engaged, like everyone else there, in nuclear energy research. Laue recorded his observation that right then and there, during that August 1945 conversation at Farm Hall, a version of history was fashioned: The German failure to produce a bomb was not the result of the numerous errors made during their research as well as the insufficient help and ultimate disillusion of the administrators with the whole project. Rather, the German failure was now to be presented as owing either to the unrealistic timetable for achieving it or because they didn’t want to do it at all. This latter reason again underlined the moral reluctance to do such work, as contrasted with the apparent eagerness of the Allies. (As Frayn recently indicated, Jeremy Bernstein’s “Hitler’s Uranium Club"--which contains von Laue’s letter--is an excellent source for understanding the ambitions of Heisenberg’s team and the reason for its ultimate lack of success.)

That story--that a supposed moral contrast existed between the scientists of the Axis powers and those of the Allies--was later greatly elaborated by some German scientists and given a prominent place in Jungk’s book, although the author himself eventually realized that he had been gravely misled, even, as he put it, verraten, betrayed, in order to propagate “eine legende.”

The legend might eventually have blown away for lack of sufficient credible evidence. In fact, one of Heisenberg’s prominent co-workers, C.F. von Weizsacker, was asked later by an interviewer from Stern magazine: “So, the idea was, we want to build the bomb, to have something in hand?” He answered, “Yes. Or at least, we wanted to come as close to the bomb as we could.” But, despite such incredible disclosures, the legend was given new, vigorous life by the widely distributed publications of Powers who, in his 1993 book “Heisenberg’s War” and in many articles, took the tale of moral compunction to its logical extreme: Now Heisenberg’s failure as a leader of the team during the war years was portrayed as an act of conscious sabotage--that Heisenberg understood all along what had to be done but, in the name of principle and moral virtue, secretly misled his co-workers and subverted the Germans’ path to developing an atomic weapon. As Powers put it: “He killed it.” Although Powers has acknowledged that his argument hasn’t persuaded any historians, Frayn confessed in the original postscript to “Copenhagen,” printed in the play’s first edition, that Power’s book “first aroused my interest in the trip to Copenhagen.”

Indeed, Frayn skillfully prepares the audience for the moving climax: Heisenberg accuses Bohr of having gone to Los Alamos. Frayn’s Bohr adds: “To play my small but helpful part in the deaths of a hundred thousand people,” whereas Heisenberg never contributed “to the death of one single solitary person.” At this, Heisenberg exults: “There’d be a place in heaven for me.” That sentiment is immediately reinforced by the tearful bravura soliloquy in which Heisenberg bemoans what the ferocious Allies had done to his beloved homeland during the war. The play ends on this note.

As the audience was leaving a performance which I attended in New York, I noticed tears in many eyes. At least for those playgoers, all “uncertainties” had undoubtedly given way to a “knowledge” of what really had happened on that day in September 1941 and to which side the moral victory belonged. The triumph of good fiction was palpable.


As to historical facts, the accounts of the famous meeting, given by Heisenberg and others, present only one side. We have not yet seen Bohr’s own reaction. But it does exist. In 1985 I was in Copenhagen at a meeting in honor of Bohr’s memory. There I was approached by Bohr’s son, Erik. He showed me a letter which he explained had been written by his father and found after Bohr’s death, folded in his copy of the 1958 book by Jungk. That letter, addressed to Heisenberg, took serious issue with Heisenberg’s published version of the 1941 meeting in quite firm language--so much so that Niels Bohr had apparently decided not to mail it.

When asked what should be done, I advised that the letter be kept in Bohr’s archives. Today the letter is there, part of the Bohr political correspondence file which his family has decided not to release to the public until 2012, marking 50 years since Niels Bohr’s death. Although the letter had not yet been embargoed when it was shown to me, it would be inappropriate for me to say more about it now. So, unless Bohr’s family decides otherwise, the world will remain with half-knowledge about what happened during that walk for perhaps a dozen more years.

But already there are some signs of change. One is the new version of the postscript of “Copenhagen,” which Frayn prepared for the recently published U.S. edition of the play. Frayn repeats that Powers’ book first aroused his interest, and he also agrees, as indeed everyone does who has studied the matter, that the German atomic researchers exhibited a “relative lack of zeal” compared to the Allies. The German scientists mistakenly underestimated the likelihood that the other side, fearful of a possible preemption by the Germans, would have the sufficient skill and wits to pursue the bomb project successfully. Hearing on Aug. 6, 1945, about the first atomic bomb, for example, the interned members of the Uranium Club felt it was a hoax, with Heisenberg saying, according to the Farm Hall Papers, “All I can suggest is that some dilettante in America who knows very little about it has bluffed them. . . .”

What does Frayn now think of Powers’ main contention, that Heisenberg “well knew how to make a bomb with far less [mass] but kept the knowledge to himself”? As he confesses in the new postscript, having at last understood the Farm Hall conversations thanks to Bernstein’s commentary in his book, Frayn now dismisses Powers’ main thesis with the cutting comment, “If he [Heisenberg] had kept the fatal knowledge . . . from anyone, as Powers argues, then it was from himself.”

But that raises an unsolvable problem for Frayn: If the play was initially structured in good part to reflect an idea of Powers which now has to be laid aside, what is the playwright to do? Frayn has made some small changes and corrections for the new version, fixing minor errors in the physics. But even though it is now acknowledged to be even further from historical reality, the body of the play must remain standing: After all, it continues to be a hugely successful work of fiction for the theater, honored with awards and accolades, even though part of the motivation behind it has disappeared.

Therein lies a curious irony--but one which Coleridge and Keats might well have understood and honored.