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the uncertainty principle

A play based on quantum physics may sound like a recipe for heady abstraction, but Michael Frayn's 'Copenhagen' - 'Kopenhagen' in the current rendition by Het Nationale Toneel - is anything but. Focusing on a pivotal moment in 20st century history, it creates a suspenseful debate between three people, which at the same time dramatically illustrates the uncertainty principle.

The backstory is a historical meeting between the physicists Niels Bohr and Werner Heisenberg in Copenhagen in 1941. In the 1920's Bohr had been Heisenberg's mentor and together they had produced a series of breakthroughs in quantum physics, including Bohr's principle of complementarity and Heisenberg's uncertainty principle. Since then, however, the outbreak of World War II had divided them: Bohr, who was half-Jewish, in occupied Denmark and Heisenberg in Nazi Germany (unlike many other scientists who fled to the U.S.).

By that time both were aware that their theoretical research could be applied to a new kind of weapon... the nuclear bomb. In fact, Heisenberg was leading a Nazi project to develop nuclear energy (and possibly a bomb). And Bohr would escape to the U.S. in 1943, where he worked on the Manhattan Project.

In this context Heisenberg suddenly showed up in Copenhagen to talk to Bohr. To avoid being monitored they went for a walk and had a ten minute conversation. No one knows what exactly they discussed, and their own post-war recollections differ. Heisenberg later wrote:

...it probably started with my question as to whether or not it was right for physicists to devote themselves in wartime to the uranium problem - as there was the possibility that progress in this sphere could lead to grave consequences in the technique of the war.

This is the moral dilemma at the heart of the play. Frayn adds the character of Margaretha, Bohr's wife, and has the three of them look back on the meeting in 1941. From a spirit-like perspective, in a minimalistic decor with a mushroom cloud-like canopy looming overhead, they try to reconstruct the conversation, and their own motives at the time.

Was Heisenberg developing a nuclear bomb for the Nazis, and did he try to get information from Bohr, either on the Allied nuclear program (about which Bohr knew nothing at that point) or about the practical problems of creating nuclear weapons? Or, on the contrary, was Heisenberg stalling the Nazi program by presenting false calculations? Or did he simply miscalculate? And what was Bohr's response, and how did it influence Heisenberg's decision?

They find that these questions can't be answered satisfactorily. In their speculation, the characters are like subatomic particles, bound to each other by powerful attraction, whose position in space can be located but not their momentum - in this case their motives. In dramatic terms, the uncertainty principle is like the Rashomon effect: there is no such thing as objective reality, merely an accumulation of subjective accounts of reality. Thus 'Copenhagen' has no real resolution, but as an exploration of the moral responsibilities of scientists it is a fascinating debate.

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