Copenhagen

I saw Michael Frayn's play Copenhagen (John Ziman's Physics World review here) last night, and I was very impressed. The principal subject is Heisenberg's September 1941 visit with Bohr in Copenhagen. At that time, the Bohr-Heisenberg collaboration that was so fundamental to the discovery of quantum mechanics was a decade and and a half in the past, Denmark was occupied by Nazi Germany, and Heisenberg was already at work on a German fission bomb.

Heisenberg's visit was risky - he and Bohr were both being watched by the Gestapo, and Bohr, who was half-Jewish as well the world's most prominent nuclear physicist, was also under watch. What Heisenberg apparently said to Bohr "is it moral for a physicist to work on applications of fission" would have gotten them both shot if the Gestapo had heard. In any case, Bohr apparently reacted with intense anger, but no answer. Bohr was smuggled out of Denmark just before the arrest of all Danish Jews (who were tipped off, apparently by a source in the German embassy - who himself may have had Heisenberg links, and also mostly escaped.) Bohr wound up at Las Alamos, and contributed to the bombs that destroyed Hiroshima and Nagasaki, while Heisenberg's bomb was never built.

This was a very difficult play for the actors, because many crucial question hinge on Heisenberg's understanding of the feasability of the fission weapon, especially the critical mass required for a fission weapon based on U235, which in turn depends on understanding the neutron diffusion equation. Moreover, there is much discussion of the heady moments of the nineteen twenties when Bohr, Heisenberg, and others struggled to create quantum mechanics. I think that the play captures both the thrill and much of the substance of those days. Both things require that the actors talk a great deal of physics - diffusion, cross-sections, fission, uncertainty, complementarity - and these are things actors usually don't know. I spoke to two of the actors afterwards, and this was something they worried about, but I assured them that they were right on, and they seemed pleased that a physicist thought so.

Though important, the physics is not the central point. That central point is the nature of moral choice and fate, and its contingency. Heisenberg comes off pretty well in this play, but there are no unblemished heroes here. Bohr was famous for the many drafts his papers required, an arduous task in those days before the word processor and the computer. The play too presents several drafts of the reality of the events depicted.

I should mention that the play was written after all the principals - Bohr, his wife Margarethe, and Heisenberg - were all dead, and it is in the form of a trialog among the three ghosts. I call them ghosts since they know and acknowledge that they are all dead.

I think it is a terrific play, especially for physicists and other fans of Bohr and quantum mechanics.