Copenhagen is a play by Michael Frayn, based around an event that occurred in Copenhagen in 1941, a meeting between the physicists Niels Bohr and Werner Heisenberg. It debuted in London in 1998 and in 2000 won the Tony Award for Best Play.
The play contains three characters: Heisenberg, Bohr, and Bohr's wife Margrethe. Its temporal and spatial location was purposely vague, suggesting that the three were spirits in some sort of timeless, ethereal realm, reflecting on past events and interpretations. The characters interact on stage without benefit of written stage direction or set design, and in many ways resemble the photons and particles they discuss in the play -- each colliding with the others emotionally and altering the others' paths by their own actions. The original production was staged "in the round," though a stylized, minimalized version of it, and gave the audience a sense of being scientific observers as the "particles" (actors) moved about in a contained space in front of them.
Bohr and Heisenberg were friends and had collaborated together and were influential in developing quantum theory before World War II. Heisenberg in 1941 was in charge of the German effort to develop atomic energy for practical purposes (possibly including an atomic bomb). Bohr's native Denmark was occupied by Germany, and he was under severe restrictions on what he could work on, and was in a position of potential danger because he was considered half-Jewish.
At one point in Heisenberg's visit with Bohr, they went outside for a walk so that they could speak without fear of being monitored by the Gestapo. The walk was however brief. The two formerly close companions were never again good friends. The play deals with why Heisenberg had gone to visit Bohr in an occupied country, the potential conversations between Bohr and Heisenberg, and what this visit ultimately signified as to Heisenberg's efforts on behalf of the German war efforts.
The question of why Heisenberg had come to Copenhagen to visit with Bohr is considered in the play, and it is the uncertainty that makes it interesting. Two concepts which Frayn uses as overarching metaphors in the play are the most famous scientific theories associated with Bohr and Heisenberg: Bohr's idea of complementarity (by which a photon can be regarded as both a wave and a particle, depending on how one looks at it) and Heisenberg's idea of the uncertainty principle (by which it is impossible to know the exact momentum and location of a particle at the same time). Frayn stresses the complementarity of language (by which an ambiguous statement can be seen in a number of ways) and the uncertainty of history (by which looking backwards at the past clouds our ability to fully understand it).
Bohr was later smuggled out of Denmark (it is insinuated in the play that Heisenberg may have had a role in this, likely saving his life), and went to work on the Allied effort to produce nuclear weapons at Los Alamos. By the time Germany lost the war in 1945, Heisenberg had only to almost construct the first self-sustaining chain reaction in a primitive nuclear reactor in a cave, without proper safety equipment. He made it into a zone of Allied occupation and was later moved to Farm Hall, Great Britain, where he and other German scientists were kept under observation as part of an Allied attempt to discover the full extent of the German bomb program.
The play was adapted as a television movie in 2002, with Daniel Craig as Heisenberg, Stephen Rea as Niels Bohr, and Francesca Annis as Margrethe Bohr. The movie substantially cuts down the script of the play, eliminating several recurring themes, and most of the material that established the community of scientists in Copenhagen. It also abandons the abstract staging of the theatrical version in favor of being set in the city of Copenhagen, in Bohr's old house.
Frayn has said the play was inspired by Thomas Powers' book Heisenberg's War: The Secret History of the German Bomb (Knopf, 1993).
Heisenberg historians remain divided over their own interpretations of the event, and the 1998 play put more attention on what had been a previously primarily scholarly discussion.
Much of the initial "controversy" resulted from a 1956 letter Heisenberg sent to the journalist Robert Jungk after reading the German edition of Jungk's book Brighter than a Thousand Suns (1956). In the letter, Heisenberg described how he had come to Copenhagen to discuss with Bohr his moral objections toward scientists working on nuclear weapons, but how he had failed to articulate this clearly before the conversation came to a halt. Jungk published an extract from the letter in the Danish edition of the book in 1956 which, out of context, made it look as if Heisenberg was claiming to have purposely derailed the German bomb project on moral grounds. (The letter's whole text shows Heisenberg was careful not to claim this.)  Bohr was outraged after reading this extract in his copy of the book, feeling that this was false and that the 1941 meeting had proven to him that Heisenberg was quite happy with producing nuclear weapons for Germany.
After the play inspired numerous scholarly and media debates over the 1941 meeting, the Niels Bohr Archive in Copenhagen released to the public all heretofore sealed documents related to the meeting, a move intended mostly to settle historical arguments over what they contained. Among the documents were the original drafts of letters Bohr wrote to Heisenberg in 1957 about Jungk's book and other topics. .
These drafts proved to be significant in several respects. First, the proved to be relatively consistent with Heisenberg's own recollections of the meeting  given to Jungk in 1956, meaning that the course of the conversation can now be fairly well established. Both Bohr and Heisenberg agree that Heisenberg started the visit by stating to Bohr that nuclear weapons were now conceivable. As Heisenberg wrote to Jungk,
Bohr confirms this by writing
Heisenberg repeated his convictions on the technical feasibility of building nuclear weapons. As Heisenberg recalled:
Bohr's draft letters are consistent with this:
[This point is of interest, because it is at odds with the view of some that Heisenberg's miscalculations had led him to conclude, erroneously, that atomic weapons were not feasible.] According to Bohr's later notes, Heisenberg then told Bohr that he had not come to discuss the technical aspects of the potential weapons:
Unfortunately, because of Heisenberg's concerns about being monitored -- his discussion of any aspects of the nuclear efforts to someone in an occupied country would have been illegal -- his discussion was cryptic. Indeed, Bohr's letters note that Heisenberg spoke "in vague terms" from which Bohr was only able to get an "impression" about Heisenberg's efforts. Unfortunately, Bohr's reaction to this information was one of shock, caused in part because the concept that nuclear weapons might be developed was new to him: "I had at that time no knowledge at all of the preparations that were under way in England and America." . This circumspect discussion, combined with Bohr's shocked reaction to it, apparently cut off the discussion between the two. Thus, the Bohr letters cannot resolve the question, posed by the Copenhagen play, of what Heisenberg had wanted -- but failed -- to convey to Bohr.
The Bohr letters are significant here, because they do eliminate some of the more sinister motives that had been postulated. For example, in a 1998 book, Heisenberg and the Nazi atomic bomb project: a study in German culture (Berkeley: University of California Press, 1998), Paul Lawrence Rose had speculated that Heisenberg had visited Bohr as "an intelligence-gathering mission." However, the release of Bohr's letters disprove that claim, showing that Heisenberg had not come to elicit information from Bohr, and did not even want to discuss the technical issues of building a bomb:
This shows that Heisenberg wanted to move the discussion into another area of nuclear weapons, not the technology. This passage also appears to counter the arguments of some Heisenberg critics (such as Rose and Bernstein) that Heisenberg's errors in 1940 had mistakenly led him to conclude that building nuclear weapons was not possible.
Finally, the 1957 Bohr draft letters, written 16 years after the meeting itself, suggest a conflict between Bohr and Heisenberg. However, Heisenberg's own letter to his wife, written on the eve of his departure from Copenhagen, provides no hint of a fracture. In it, he related his final evening with Bohr as very pleasant and unremarkable: "Today I was once more, with Weizsaecker, at Bohr's. In many ways this was especially nice, the conversation revolved for a large part of the evening around purely human concerns, Bohr was reading aloud, I played a Mozart Sonata (a-Major)."