Copenhagen: Can We Ever Really Know?

Listener 29 September 2001.

Keywords History of Ideas, Methodology & Philosophy; Literature and Culture

The ‘Copenhagen model’, developed in the 1920s, remains the foundation of the quantum mechanics account of the atom which physicists use to this day. Two of the revolutionary developers were Dane Niels Bohr, then about forty, and German Werner Heisenberg, in his twenties. …

… Both were to receive Noble Prizes for their work. The model’s best known element is Heisenberg’s uncertainty principle which says – this is a loose interpretation – that an observer disturbs the phenomenon being examined. In the case of atomic level activity the disturbance is such that one cannot know precisely everything. That is also true socially. The very activity of trying to describe an event may change the event itself.

Playwright (and novelist) Michael Frayn explores this in his 1998 play Copenhagen. It is 1941, fifteen years later, and Denmark is occupied by Germany. Heisenberg goes back to Copenhagen to see Bohr and wife Margrethe (a feisty woman who worked with Bohr) but the purpose of his visit is unclear. They are under surveillance. They go outside for a walk but when Heisenberg broaches his chosen subject Bohr intemperately cuts him off short and they return in minutes. After the war their two accounts are irreconcilable, and Heisenberg seems to keep changing his story. Sixty years later historians still bitterly dispute over what actually happened.

The crucial issue is Heisenberg’s attitude towards the development of atomic weapons. That meant, since he was the only great German physicist left – for the rest, often Jews, had fled – what was to be the strategy of Nazi-ruled Germany? Did he come to his mentor to get a go-ahead? Perhaps he had not made up his mind, or perhaps had he already made the decision that he would focus on nuclear power, the course which he took? (He had almost got his nuclear reactor to the critical – self-sustaining – stage when the war ended, but it did not have quite enough uranium. Importantly a reactor uses different physical processes from a bomb, although it can make the material for one.) The reason why some historian’s reactions to Heisenberg’s intentions are so intense, are because they reflect on the decisions of those who made the American atomic bombs, eventually dropped on Japan.

Frayn, whose background is in philosophy, reenacts these events in a discussion among the three now-dead participants. Throughout the London production I was on the edge of my seat. (Perhaps not: sometimes I was in tears, and not sure where I was.) Heisenberg is torn between his commitment to his family, to his country, to his profession, and to his professional competence. Yet the wrong decision might help the Nazi regime. (There is no hint in the play he favoured fascism, except in the way that unthinking patriots can be blind to rather despicable national leadership.) An especially painful moment was when he reported that after the war those who made the American bomb would not even shake his hand – a reflection of the view that the anti-nuclear movement detested: our bomb is honourable but the enemy’s is evil

Some of the subsequent confusion arose out of Heisenberg’s unwillingness to admit he had made a mistake, and perhaps let his side down. He overestimated the uranium required to make a bomb, probably the major factor in Germany not proceeding. The other side also made a miscalculation, a proportionally bigger one, but it was an underestimate, and so the Allies went ahead. (Ironically, that slip was first made by Jews who had fled Nazi persecution.) It is very human – especially where there is a policy implication – to make a mistake which favours the outcome one wants. If it goes the other way, you rework and rework the calculations until you have eliminated all error, or got your preferred answer.

Thus the play is not so much about some crucial event in the history, but about the agonising moral dilemma that one man faced. Potential theatregoers need not be put off at having to follow the intricacies of the Copenhagen model. Those who saw the play, usually tell me they either understood the play’s exposition, or it did not matter. (Hopefully some entrepreneurial Auckland physicist will take the opportunity to give a public lecture. A citizen ought to have some understanding. Even for physicists at the time, there was bewilderment. Hence Einstein’s remarks that God ‘does not play dice’ and ‘He is subtle but not malicious.’ Physicists’ understanding of subatomic behaviour has since got murkier, although they may be near a new synthesis. I comfort myself with J.B.S. Haldane’s intuition that not only is the world weirder than we understand, but it may be a whole lot weirder than we can ever understand.) And over this dilemma presides the uncertainty principle. Can historians know what happened? Can even the participants?

This column may not seem to have much to do with economics. But dull would be an economist who was not gripped by one of the twentieth century’s greatest intellectual achievements. Or one who was not gripped by one of its great plays.