CASSIDY REPLIES: One of the most difficult and debated episodes in recent physics history concerns the involvement of German scientists in nuclear-fission research during World War II. The difficulty stems from the traumatic events, from the characteristic ambiguity of circumstances and behavior in that period, and from the realization that some admired participants were as imperfect as the rest of us.
The play Copenhagen succeeded in part by making a virtue of ambiguity. By magnifying the mystery of what Werner Heisenberg told or tried to tell Niels Bohr during his visit to Copenhagen, the play explores many possibilities. A primary purpose of my paper was to defuse much of the mystery in the play by briefly outlining the historical context. As Harry J. Lipkin notes, that visit occurred during a “window of time” in which the prospect of a German victory seemed quite real. I was not arguing, as Klaus Gottstein suggests, that Heisenberg was there to convince Bohr that a German victory was inevitable, but rather “that the seemingly inevitable German victory would not be so bad for Europe after all” when compared with a Soviet victory.
At the time of Heisenberg’s visit, the German army was indeed still in control of fission research. After the army withdrew almost entirely in early 1942 and Albert Speer assumed administrative authority, the project never fully recovered. I don’t find any slackening of Heisenberg’s fission research before 1942, and Gottstein seems to agree. But I did not equate “fission research” with “construction of a nuclear bomb,” as Gottstein suggests. All of the work until war’s end was directed toward reactor construction, a goal the Germans did not achieve. However, echoing Heisenberg’s vague reference from his letter of 1 October 1941, I did refer to a “march of events” (my words) that he apparently perceived as leading toward nuclear weaponry. I don’t see where I referred in this article to “Heisenberg’s ‘bomb work.’ ”
I also tried to show how, in hindsight, Heisenberg, as well as many other prominent Germans, had been used by the Third Reich for its own purposes. Obtaining such lessons is among the tasks and benefits of history. I explored this topic in my biography. 1 With encouragement from Max Planck, Heisenberg came to believe, long before the outbreak of war, that he personally must survive in Germany so that his students and decent German science could survive.
The war began nearly 7 years into the 12-year reign of the Third Reich. We cannot fully understand behavior during the war without first examining attitudes and responses established during the pre-war years. Some authors have reached what I find to be untenable conclusions because they neglected to consider the preconditions. Gottstein disagrees with my understanding of Heisenberg’s rationale, but we would have to work through the earlier years to determine exactly where we disagree.
My suggestion that Heisenberg might have consulted with Planck and Max von Laue also refers to the earlier years, specifically 1933 through 1936, when the three men frequently discussed how best to respond to the Nazi assault on physics and on academe in general.
I agree with many of Gottstein’s other comments, in particular, that Heisenberg’s invited travels “brought him and modern physics a level of esteem” in Germany, which is why he accepted the invitations.
Lipkin’s report of Bohr’s reaction to the visit and to Heisenberg’s subsequent remarks on the lack of progress in fission research may find support in Bohr’s unpublished letter to Heisenberg regarding Robert Jungk’s book. 2 However, as Gerald Holton reported, the Bohr family has sealed this letter until 2012.
