Now deceased, the spirits of Margrethe and Niels Bohr have a conversation. Margrethe asks her husband why he met with his onetime pupil and former friend Werner Heisenberg in Copenhagen in 1941. Heisenberg had explained his reason for the meeting but, over time, the reason has become obscured. The meeting was difficult to arrange; Denmark was under German occupation at the time, and Margrethe remembers her husband being remarkably angry with Heisenberg. Bohr believes that he “remained remarkably calm” (6). The meeting, however, was the end of their famous friendship.

Heisenberg, also deceased, considers the meeting. It is one of only two things the world remembers about him, the other being his uncertainty principle. Though Heisenberg and Bohr were incredibly close in the 1920s, Margrethe always viewed the German as slightly alien and unlikable. Bohr reminds her that Heisenberg a very great physicist. Bohr himself was the father of modern atomic physics; many great physicists of the era came to study under him in Copenhagen. His close relationship with Heisenberg was made difficult by the Nazis’ rise to power.

In September 1941 Heisenberg takes a train to Copenhagen. He is set to give a lecture on astrophysics but has memorized a message for his old friend and mentor.