Chapter 4
HOW DOES AN ELECTRON DECIDE?
I
n September 1911, a young Dane just shy of his twenty-sixth birthday arrived in Cambridge to learn electron physics from J. J. Thomson. Niels Bohr was the son of a professor of physiology at the University of Copenhagen. His family, going back three generations, boasted schoolteachers, university professors, and ministers of the church. Bohr had written a doctoral thesis on the conduction of electricity in metals, assuming that electrons carried the current and that they rattled more or less freely about inside a conductor, rather as atoms of a gas might fly up and down a tube. The model didn’t work very well, and Bohr already suspected that something was fundamentally amiss with the idea of treating electrons, in nineteenthcentury style, as electrically charged billiard balls. Bohr in repose had a mournful look about him. His heavy
