It isn’t often that the subject matter of a play on Broadway is science. “Copenhagen” opened on Broadway in 2000 after a run in London’s West End. Its focus was a 1941 meeting in Bohr’s Copenhagen house between Danish physicist Niels Bohr and his former protégé German physicist Werner Heisenberg. The meeting was an informal one so there is no record of the actual discussion, but when questioned later, the two parties had different memories of the dialogue. They did agree that Heisenberg initiated a discussion about the development of the atom bomb, but they differed in their recollection of Heisenberg’s motive. “Copenhagen” is all about the views of the two Nobel Prize winners and the conversation that could have taken place between them.
Bohr was already world renowned in 1941, having been awarded the 1922 Nobel Prize in physics for "for his services in the investigation of the structure of atoms and of the radiation emanating from them." He had worked with Ernest Rutherford, who had been at McGill before taking up a post in England where he carried out a famous experiment that led him to conclude that atoms consist of a nucleus of protons and neutrons around which electrons orbit much like planets around the sun. Bohr refined this theory by proposing that the electrons actually occupied “shells” at different distances from the nucleus, and that the properties of an atom were determined by the number of electrons in the outermost shell. Furthermore, electrons in lower shells could be excited by energy to jump into higher shells, and this energy could be emitted when they “relaxed” back to lower levels. Bohr had now provided an explanation for a mystery that scientists like Robert Bunsen had been coping with. Light emitted by stars or flames consisted only specific wavelengths, a phenomenon that now could be explained by the energy emitted when electrons transitioned from one shell to another.
A different kind of transition complicated Bohr’s life in the 1930’s, that of the Nazis coming to power in Germany. Jewish scientists were being expelled from their positions and were fleeing the country. With his Jewish heritage, Bohr decided he had to do something and opened the door of the Institute of Theoretical Physics in Copenhagen of which he was the head to the Jewish scientists.
By 1941 Werner Heisenberg was involved in the German nuclear weapons program that had been launched in 1939. Like Bohr, he was a Nobel Laureate, having won the Prize in 1932 for laying the foundations to quantum mechanics, a field of physics that deals with the motion and interaction of particles such as photons, protons and electrons that are smaller than atoms. Quantum mechanics explains how these can have both the characteristics of particles or waves, and as Heisenberg proposed with his “uncertainty principle,” either their position and speed can be measured, but both cannot be measured at the same time. These highly mathematical concepts are very difficult for a mere mortal chemist to understand but they do explain the energetics involved in the formation of chemical bonds and why some molecules are energetically more favourable than others. This is important when designing chemical syntheses.
Heisenberg had spent time at Bohr’s Institute in the 1920s and the two men had an amiable relationship. But why did Heisenberg choose to visit Bohr in Copenhagen in the middle of the war? Was it to pick Bohr’s brain about nuclear fission, an area of physics in which Bohr had become interested, was it to try to enlist Bohr to help with the German effort, or was it to let Bohr know that he had answered Hitler’s call so that he could sabotage the project from within? Later Heisenberg would claim that his remarks about the potential of building a bomb and its catastrophic consequences shocked Bohr who thought the project was not feasible. Bohr disagreed about this exchange and in later interviews explained that he had been very much aware of the energy released by the fission of Uranium-235 and what he was shocked by was Heisenberg happily working on producing a bomb. There is also the theory that Heisenberg had been instructed to give Bohr the impression the Germans were not close to developing a bomb so that he would transmit this information to the Allies who would then see no need to rev up their own effort while Germany speeded along. We will never know the actual gist of the conversation between two of the world’s leading physicists who had frequently cooperated but now found themselves on opposite sides of the conflict between the Nazis and the Allies. It is the uncertainty of what happened during that famous meeting that provides the fodder for “Copenhagen,” the play.
What is certain is that Denmark fell under German occupation and in 1943 Bohr received a message that he would be arrested because his mother was Jewish. He fled to Sweden where he convinced King Gustav to allow Jewish refugees into the country. Eventually over 7000 Danish Jews were given asylum in Sweden.
Bohr spent the last two years of the war in England and in the U.S. with frequent visits to Los Alamos where his contribution was prized by Robert Oppenheimer, the leader of the Manhattan project. After the war, he returned to Copenhagen and campaigned against the use of nuclear weapons for the rest of his life. Bohr advocated full disclosure of nuclear research between nations and is one of the few scientists who has an element named after him. Bohrium does not occur in nature and is only fleetingly produced via a nuclear fusion reaction.
A movie version of Copenhagen is available starring Daniel Craig of James Bond fame as Heisenberg. And if you are fond of watching The Big Bang Theory, as you should be, the animation that you see at the beginning of each segment is that of the Bohr atom!
