3. Magic wand

‘Magic wand’ refers to the ‘correspondence principle’ that Bohr devised and deployed to investigate the interface between quantum and ordinary (‘classical’) physics. The chapter covers various lines of work, some inspired by his approach and some independent of it, all of which confirmed its fertility. The mobilization of the international brotherhood of physicists for the Great War gave Bohr breathing space to develop the correspondence principle with the help of Hendrik Kramers, who had come to neutral Denmark to study with him, and in friendly competition with Arnold Sommerfeld, who made important formal extensions of the theory.

2. Productive ambiguity

‘Productive ambiguity’ begins with Bohr’s move to Cambridge in 1911 to work with J. J. Thompson on the electron theory and to publish an English translation of his thesis. He did not flourish in Cambridge, however, and moved to Manchester in early 1912 to study under Ernest Rutherford. He soon took an interest in the work of another researcher in the laboratory, Charles Galton Darwin, who was wrestling with the problem of how electrons in a nuclear atom interact with passing alpha particles. Consideration of Darwin’s problem prompted Bohr to discover the radical mechanical instability of the nuclear atom, a result for which his thesis and his philosophy had prepared him. He exploited the instability to develop his quantum atom. His several attempts to ground his invention in existing physics give a precious insight into his mind at work, into his way of entertaining several contradictory formulations of his thought at the same time.

1. A richly furnished mind

Niels Bohr was born in Copenhagen in 1885. ‘A richly furnished mind’ describes the Christian and Jewish elements of his childhood and upbringing, and his relationship with his fiancée Margrethe Nørlund. It also discusses the influence of the philosophers Kierkegaard and Høffding and the physicists J. J. Thompson, Lord Kelvin, Max Planck, and Albert Einstein on Bohr’s early work and enduring beliefs. This rich background prepared him for the analysis and results of his doctoral thesis of 1911, which exposed serious faults in the largely successful electron theory of metals. He concluded that the faults could not be fixed without fundamental departures from ‘ordinary physics’.

4. Enthusiastic resignation

‘Enthusiastic resignation’ describes Bohr’s work with Wolfgang Pauli and Werner Heisenberg. ‘Resignation’ refers to their realization that the electron orbits that had served as the basis of Bohr’s theory had only ‘symbolic’ value. They took the correspondence principle as a guide to translating symbols describing the orbits, like position and momentum, into symbols specifying the values of observable products of atoms, like the frequency and intensity of spectral lines. Heisenberg’s breakthrough in the summer of 1925, based on a particulate view of matter, provided a basis for a coherent description of the phenomena to which atomic electrons give rise. Almost simultaneously, Erwin Schrödinger found another route to the same mathematical solution, based on a wave picture of matter, which avoided discontinuity and made calculations easier. In answering Schrödinger’s challenge, Heisenberg invented the Uncertainty Principle and Bohr worked out a more general reconciliation of the quantum puzzles, which he called ‘complementarity’.

6. Elder statesman

Bohr worked briefly at Los Alamos, the centre of the American atomic bomb project, after fleeing to Sweden with most of Danish Jewry in 1943. As an ‘elder statesman’ he secured audiences with Churchill and Roosevelt to argue (invoking complementarity!) that if the Allies shared information about nuclear weapons with the Russians they might avoid a postwar arms race. He continued to argue this position in various forums after the war and received the first Atoms for Peace Award for his efforts. Back in Denmark, he used his prestige to advance scientific and cultural causes. In his last years, he took an increasingly strong interest in the history of physics as a means of acquainting a wider public with the difficult birth of the splendid science he and his collaborators had made.

5. The Institute

‘The Institute’ describes the work undertaken at Bohr’s headquarters in Copenhagen. It prospered under the ‘Copenhagen Spirit’, as Heisenberg called Bohr’s approach to quantum problems, and money from the Rockefeller, Carlsberg, and Rask-Ørsted Foundations and the Danish government. The Institute saw a decline in the early 1930s owing to Bohr’s doubts about certain productive ideas in nuclear physics that his philosophy seemed to rule out. Their productivity changed his mind and he raised money to build the large apparatus for experimental investigation of the nucleus. At the same time, he applied complementarity to biology and other domains outside of physics, to the dismay of Einstein, who continued to try to find ways to contravene what he called Bohr’s ‘tranquilizing philosophy’.