One day during the run of Michael Fryan's play Copenhagen, a curious letter arrived from a housewife in Chiswick. She enclosed a few faded pages of barely legible German which she thought might have some relevance to the mystery at the play's heart. They turned out to mark the start of a long and winding trail.The subject of Copenhagen is the strange visit that the German physicist, Werner Heisenberg, made to his former Danish colleague, Niels Bohr in 1941. The two old friends now found themselves on opposite sides in a world war, and Heisenberg could not explain to Bohr that he was running the Nazis' secret atomic programme. His intentions have intrigued and baffled historians, and the hitherto unpublished German documents which Celia Rhys-Evans now began to send Michael Frayn cast a remarkable new light on certain aspects of the story. The gradual emergence of these papers was followed with particularly close interest by the actor, David Burke, who was playing Niels Bohr, and who had happened to have a wide experience of documents of this sort. When it was all over David Burke and Michael Frayn sat down together, rather as Bohr and Heisenberg do in the play, to try to unravel the mystery, and, like Bohr and Heisenberg, to confront once again the eternal difficulty of knowing why we do what we do.