When the author Michael Frayn spent two years writing Copenhagen, he had no idea how successful the play would become. He doubted that audiences would sit through a historical drama about a war-time meeting between Werner Heisenberg – head of Germany’s nuclear programme – and his old mentor Niels Bohr in the Nazi-occupied Danish capital in 1941. But Frayn’s efforts paid off. Audiences and critics alike have thrilled at the way the award-winning play probes the historical uncertainty that surrounds the encounter.

Was Heisenberg fishing for information about the Allies’ atomic plans – or was he trying to recruit Bohr for Germany’s bomb programme? Did Heisenberg want to suggest that the Germans were close to finishing a bomb so that the Allies would make peace with Hitler? Maybe he was simply seeking approval from Bohr for his own atomic work. There is also a moral debate: did Heisenberg know how to build a bomb, but decided not to – or did he want to build one, but got his calculations wrong?

Unfortunately, no-one was there to record or observe the encounter and we cannot know for sure what was said or implied between the two men. All we do know is that the pair dined together and took a short walk – and that the incident damaged Bohr and Heisenberg’s friendship forever. To piece together what happened, historians of science have had to rely on Heisenberg’s post-war recollections – which have been ambiguous and contradictory – along with scraps of evidence from secondary sources.

History and reality

Now, however, new light could be shed on the meeting following a decision by the family of Niels Bohr to release previously secret documents that he either wrote or dictated to others about the meeting. A total of 11 documents will be released, including various attempts that Bohr made to formulate letters to Heisenberg about what happened in 1941. Bohr was a stickler for writing down everything he worked on, and he may not have intended to send the drafts at all.

The documents have until now been locked up in the vaults of the Niels Bohr Archive in Copenhagen, where they have been accessible only to members of the Bohr family. Finn Aaserud, director of the archive, is part of a group who will transcribe Bohr’s notoriously difficult handwriting and translate the Danish into English. He plans to have the documents ready by the end of this year.

The decision to release the material was announced at a conference on drama and the history of science that was held in Copenhagen at the end of September, exactly 60 years after the war-time meeting. The Bohr family originally intended to release the documents in 2012 – 50 years after Bohr died – but they hope that doing so now will avoid any further “misunderstandings regarding their contents”.

One controversy concerns the suggestion, first proposed by the journalist Robert Jungk in his 1958 book Brighter Than a Thousand Suns, that Heisenberg submitted a secret plan to Bohr at the meeting. The plan, Jungk claims, was to prevent the development of an atomic bomb through a mutual agreement between German and Allied physicists. Jungk’s assertion, which Heisenberg repeated in several of his own post-war accounts of the meeting, implies that the building of a bomb was a moral issue for Heisenberg.

That no such plan came to pass could be interpreted as a failure on Bohr’s part. Indeed, the characters in Frayn’s play – Bohr, Heisenberg and Bohr’s wife Margrethe – debate the fact that Bohr later worked on the Manhattan atomic-bomb project, which led to the deaths of thousands of people, whereas Heisenberg did not contribute to the loss of a single life.

Aaserud, however, believes that the documents will confirm what Aage Bohr briefly noted in a paper in 1963 about his father’s war years. “According to Aage Bohr,” explains Aaserud, “Heisenberg did give Niels Bohr the impression that the Germans were working on an atomic bomb. But Heisenberg did not propose any secret plan for physicists on both sides to prevent such research by mutual agreement. This is Aage Bohr’s account of the matter, and to a large extent I agree with that.” Indeed, Aaserud is convinced that Aage Bohr had seen the documents when he wrote the paper.

Andrew Jackson, a theoretical physicist who chairs the board of directors of the Niels Bohr Archive, agrees that releasing the documents will help to clear the air. “It’s not just a question of respecting the interests of Bohr – but also those of his family,” he says. “They do not want Bohr’s reputation to be sullied by unwarranted uncertainty regarding his role during the war. They want to make it clear that there is nothing to hide.”

History and drama

Another person who has seen one of Bohr’s letters is Gerald Holton, a historian of science at Harvard University. Holton was originally shown the letter in 1985 by Bohr’s son, Hans, but refused to reveal any details because of the Bohr family’s embargo. Having read some of the material, Holton believes that Frayn’s play gives too much credence to the views expressed by the journalist Thomas Powers in his 1993 book Heisenberg’s War.

“Powers took the tale of moral compunction to its logical extreme,” noted Holton in the Los Angeles Times last year. “Heisenberg’s failure [to build a bomb] was [seen by Powers] an act of conscious sabotage – that Heisenberg understood what had to be done but, in the name of principle and moral virtue, secretly misled his co-workers and subverted the German [programme].” Other historians, such as Paul Rose from Penn State University, agree that there can have been no moral dimension to Heisenberg’s visit. After all, he was in Copenhagen as an official representative of the Nazi government and had made at least ten other high-level visits to German-occupied nations during the war.

Of course, this arcane dispute might not matter, were it not for the fact that audiences might leave the play – as Holton puts it – thinking that they have a “knowledge” of what really happened on that day in 1941. “People may end up questioning Bohr’s morality, while seeing Heisenberg as morally upright,” says Robert Marc Friedman, a historian at the University of Oslo.

But whatever the Bohr documents eventually reveal, they are unlikely to diminish the status of Copenhagen as drama. “What people say about their own motives and intentions is always subject to question,” writes Frayn in the preface to the published version of Copenhagen. “Thoughts and intentions, even one’s own, remain shifting and elusive. There is not one single thought or intention that can ever be precisely established.”