Pauling’s guide to quantum physics was Arnold Sommerfeld, whose institute at the University of Munich was a nerve center for new ideas about the structure of atoms. Short and slight, but still a commanding figure with his waxed moustache and dueling scar, Sommerfeld had a knack for turning out some of the best scientific minds of his day. He corresponded with all the leading physicists, was visited by many, and made his classes into exercises in cutting-edge thinking.

When Pauling arrived in Munich on his Guggenheim Fellowship in 1925, Sommerfeld’s institute was abuzz with news of a radically new approach toward understanding the atom that had been proposed by one of Sommerfeld’s former students, a young physicist named Werner Heisenberg. Conceived during a rapturous solo vacation on a windswept island in the North Sea, Heisenberg’s approach replaced all physical ideas about the atom with pure mathematics. His work caused a furor among traditional physicists, who thought it absurd to form a theory without a physical picture of the atom behind it.

But then, just as the Paulings were settling into a tiny Munich apartment, a seemingly new, very different approach was presented by one of Heisenberg’s critics, the Austrian physicist Erwin Schrödinger. The two competing theories were the subject of heated debate during the entire time Pauling was in Europe. But he quickly decided which one appealed to him most.