Heisenberg and Bohr in Copenhagen

In 1924 Werner Heisenberg began studying with the director of the Institute of Theoretical Physics at the University of Copenhagen, Niels Bohr. In September 1941 Heisenberg returned to Copenhagen to meet his old professor. He later wrote: "Late at night I walked under a clear and starry sky through the city, darkened, to Bohr."

They met at Bohr's home and took a walk through Ny-Carlsberg together. Heisenberg had come to talk to Bohr about "whether or not it was right for physicists to devote themselves in wartime to the uranium problem". Heisenberg was 39 then. Bohr was 55. Bohr was under German surveillance so Heisenberg only felt safe speaking in the vaguest of terms.

Unfortunately, this led Bohr to misread Hiesenberg and take all his foggy speech about nuclear weapons to mean that the Germans were in fact close to having one. Heisenberg could see what Bohr was thinking, but was too afraid to speak directly to clear the matter up. They argued about it for years, who'd said what to whom and what they'd meant and how it was taken. Later, historians inherited the problem.

I still like to imagine them, teacher and student, strolling down a dark Copenhagen alley and whispering about nuclear technology like a pair of spies.