Niels Bohr

A Copenhagen childhood

Niels Henrik David Bohr was born on 7 October 1885 in Copenhagen, into a household where physics, philosophy, and physiology were dinner-table talk. His father Christian was a professor of physiology and a serious contender for the Nobel Prize; his mother Ellen Adler came from a wealthy Jewish family with deep roots in Danish banking. Niels grew up listening to arguments about Kantian epistemology one evening and respiratory physiology the next. His younger brother Harald became a celebrated mathematician.

Niels was athletic; he played football for Akademisk Boldklub in goal. The slow speech that would later frustrate American audiences was already present in school. His mind moved sideways, qualifying each clause, hunting for the exception. The same temperament would, decades later, let him hold two contradictory pictures of light in his head and declare them complementary rather than wrong.

Cambridge friction, Manchester revelation

In 1911 Bohr finished his doctorate and travelled to Cambridge to work with J. J. Thomson. The visit was a quiet disaster: Bohr’s English was halting, his thesis pointed out a flaw in one of Thomson’s calculations, and Thomson never quite found time for the polite Dane sitting in his laboratory. After a few months Bohr slipped north to Manchester, where Ernest Rutherford had just announced the nuclear atom.

The chemistry was different from the first hour. Rutherford was loud, plain-spoken, and instantly fond of the young man who arrived with a suitcase full of objections; Bohr later wrote that Rutherford treated him like a son. The central problem stared back at both of them: how could the tiny, dense nucleus, orbited by electrons, be stable for even a millisecond? Classical electromagnetism said the electrons should spiral into the nucleus in nanoseconds, radiating furiously as they fell.

Niels Henrik David Bohr (; ; 7 October 1885 – 18 November 1962) was a Danish theoretical physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922. He was also a philosopher and a promoter of scientific research.

The 1913 trilogy

Back in Denmark, Bohr made the leap. In three papers in Philosophical Magazine under the umbrella title “On the Constitution of Atoms and Molecules”, he postulated that electrons could only occupy certain stationary states whose angular momentum was an integer multiple of ħ. In those states they did not radiate. They emitted light only when jumping from one allowed orbit to another, with the photon’s frequency fixed by E₂ − E₁ = hν. From two unjustifiable axioms he reproduced the Balmer formula for hydrogen, including the Rydberg constant, to four significant figures.

Older physicists found the postulates outrageous. Rutherford asked, gently, how an electron knew which orbit it was going to jump to before it jumped. Bohr had no answer except that this was how the world worked. Einstein called the result “an enormous achievement” and admitted he had been considering something similar but had not dared.

The Institute on Blegdamsvej

In 1921 the Carlsberg Foundation funded a new Institute for Theoretical Physics on Blegdamsvej, with Bohr as director. For the next twenty years it was the gravitational centre of the new physics. Heisenberg, Pauli, Dirac, Gamow, Landau, Schrödinger: almost every young quantum theorist made the pilgrimage. The pattern of life was peculiar: blackboard sessions by day, ski trips and pingpong in the evenings, Bohr pacing his office at midnight, dictating to a postdoc who was forbidden to write anything down until the sentence was perfect.

Out of those years came the complementarity principle and what Heisenberg would later christen the Copenhagen interpretation: the wavefunction is the most complete description physics permits, measurement is part of the phenomenon, and wave and particle pictures are mutually exclusive but jointly necessary.

The debates with Einstein

From the 1927 Solvay Conference through to 1935, Bohr argued with Albert Einstein about the meaning of quantum mechanics. Einstein, who believed that “God does not play dice”, arrived at each conference with a fresh thought experiment designed to defeat the uncertainty principle. Bohr would spend the night thinking and produce a rebuttal that turned Einstein’s own relativity against him; the most famous, in 1930, used the gravitational redshift to defuse Einstein’s clock-in-a-box. Their disagreement never closed.

Escape across the Øresund

In April 1940 the German army occupied Denmark. Bohr, whose mother was Jewish, was tolerated for three years while he quietly helped refugee physicists pass through to neutral Sweden. In September 1943, warned that he was about to be arrested, he and his family slipped down to the coast and were rowed across the Øresund in a fishing boat. Bohr went on to Stockholm and lobbied King Gustaf V to give public asylum to Danish Jews, a move that helped save nearly all of them. From there he was flown to Britain in the empty bomb bay of a Mosquito; he passed out from lack of oxygen mid-flight and was revived on the runway.

At Los Alamos, under the assumed name Nicholas Baker, his contributions to the bomb itself were modest, but his contributions to its politics were not. He pressed Roosevelt and Churchill to share the weapon’s existence with Stalin in advance and to internationalize atomic energy. Churchill refused, and accused Bohr of “almost mortally dangerous” indiscretion.

After the war

Bohr returned to a restored Copenhagen and spent the rest of his life arguing for an Open World of free scientific exchange and arms control. He kept the Institute at the heart of the quantum conversation through the discovery of the neutrino and the early nuclear shell models. He died of heart failure at his home on 18 November 1962, aged 77. The Institute was renamed the Niels Bohr Institute in 1965 and still carries his name.

Niels Henrik David Bohr (; ; 7 October 1885 – 18 November 1962) was a Danish theoretical physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922. He was also a philosopher and a promoter of scientific research.