Niels Bohr

Niels Bohr was born in Copenhagen in 1885, son of a professor of physiology whose drawing room rang with argument about science — his father’s discovery of how blood grabs oxygen in the lungs is still taught as the Bohr effect. Niels and his younger brother Harald played serious football; Harald, a mathematician, was good enough to win an Olympic silver for Denmark in 1908, while the family joked that Niels would be just as famous — only in physics, not on the pitch. After a doctorate on the electron theory of metals he went to Cambridge in 1911 to join J.J. Thomson, found the great man had no patience for a newcomer’s doubts, and moved north to Rutherford’s Manchester — the luckiest relocation in modern physics.

Rutherford’s nuclear atom carried one fatal flaw: an orbiting electron should, by classical physics, radiate its energy away and spiral into the nucleus, yet atoms last forever. In 1913 Bohr proposed an outrageous cure: electrons may sit only in certain fixed orbits, emitting or absorbing light only when they jump between them, the energy of each jump matching the colour of the light exactly. The model reproduced the spectrum of hydrogen with stunning accuracy and settled an old puzzle: lines astronomers had credited to hydrogen really came from ionised helium. It set electrons in shells — two in the first, eight in the next, the numbers that leave helium content and neon complete — the skeleton the periodic table had only implied. Henry Moseley’s X-ray law, built on the same arithmetic, soon put the model to a hard test and gave the table its true order.

The Nobel Prize arrived in 1922, and that same year Bohr predicted that element 72 was not a rare earth but a heavier twin of zirconium; within months Coster and Hevesy found it in zirconium ore at his Copenhagen institute — paid for largely by the Carlsberg brewery’s foundation — and named it hafnium, from the city’s Latin name. His institute became the world capital of quantum mechanics, where he argued out the Copenhagen interpretation and his principle of complementarity and trained a generation of physicists. In 1943, as the Nazis closed on Denmark’s Jews, he escaped by fishing boat to Sweden, was flown on to Britain so high that he passed out for lack of oxygen, and later joined the atomic-bomb project under the alias Nicholas Baker, pleading afterwards for openness between nations. He died in 1962; his son Aage won the physics Nobel in 1975.