Bohr had electrified the scientific world in 1913 with his bold theory of how atoms are constituted. Drawing on an idea proposed in 1900 by the German physicist Max Planck, he said that the electrons that orbit the dense central nucleus are constrained to specific orbits, able to jump between them only by emitting or absorbing light in discrete packets of energy called quanta. The theory won Bohr a Nobel Prize in 1922, but it was an ungainly, ad hoc mix of traditional physics and Planck's new "quantum" hypothesis. Bohr craved an explanation that got to the root of why atoms seemed to behave in this peculiar way. It couldn't be constructed from the traditional classical mechanics that had prevailed since Isaac Newton laid out its basic rules in the 17th century but demanded a new mechanics of quanta.