In the early years of the 20th Century, there came a generation of physicists that would change the world. Many of the men who unlocked the secrets of the atom were destined to be engaged in the race to exploit its destructive power, and most of them had egos to match their intellects. This is not to say that men like Rutherford, Einstein and Fermi were hubristic. All of them wrestled with their consciences and tried in their different ways to fulfil their responsibility to humanity. All of them were awed by the forces of nature that they strove to harness. Even so, there was really only one of the great atomic physicists who exhibited humility. Had Niels Bohr been as self-assured as his peers, history might have taken a different course.

Bohr was a Dane, brought up in Copenhagen as the eldest son of an academic family. He was in his mid-twenties when he came to Cambridge in 1911. His ideas were already profound, but his confidence in expressing them was not. Bohr’s speaking voice was notoriously quiet throughout his life. He was punctilious too, with a tendency to long-winded and laboured explanation of his point, often with diminishing conviction. He was not much better in writing, and never really believed that he could speak English properly. As a result, few people in those early years had the fortitude to hear Bohr out.

Atoms

JJ Thomson, discoverer of the electron and luminary of the Cavendish Laboratory, was not very good at listening to even the most eloquent of researchers. He had already lost Rutherford to Manchester, and soon cemented his reputation for blindness to genius by driving Bohr in the same direction. Quite how the diffident Dane survived his interview with the famously impatient Rutherford is not recorded, but brilliance must have recognised brilliance. The discrete nature of atomic spectra and the fundamental mathematics of electronic structure were charted by this pair. Soon they were leading players in the revelation of the nucleus too, and their mutual affection proved to be lifelong.

By 1922, Niels Bohr was a Nobel laureate and a revered theoretician in the field of atomic physics as well as the emerging science of quantum mechanics. Now back in Copenhagen and married with a young family, he became more assured socially and less inhibited by his communication difficulties, at least while he was among friends and peers. The old habits still came out, though, when in the company of strangers and particularly when he encountered aggressive and dismissive behaviour. Bohr matured into a loveable and lively man, but he was never to be much of an advocate.

Nucleii

The University of Copenhagen became an increasingly significant centre for nuclear research in the late 1930s, although the threat of Nazism was by then already keenly felt across Europe. Meitner and Frisch did pioneering experimental work there, investigating the fission of uranium nuclei by neutron bombardment. Bohr, in parallel with Wheeler at Princeton, postulated an explanation of their results through the propensity of uranium to undergo a chain reaction. Their conjecture was that a specific isotope of the element, present only in small amounts, was undergoing near-complete disintegration.

Within a year, further experiments proposed by Bohr proved that U-235 could sustain such a chain reaction. By now though, Denmark was becoming a dangerous place for the scientist, whose mother was Jewish. Just before the Nazi occupation of April 1940, Bohr left Copenhagen to reside briefly in Britain. Soon after that came the realisation that a critical mass of barely a kilogram of U-235 would make a bomb capable of incinerating a large city, and the demonstration that Pu-239 was an alternative isotope with a similar capacity. There was still a long way to go to build the atom bomb, either by refining the naturally-occurring uranium or by synthesising enough of the artificial plutonium. The course was set, however, and the justification for the development was the unthinkable prospect of Hitler possessing the technology first. Bohr, in spite of grave misgivings, consented to a move to Los Alamos.

In December 1942, Fermi at the University of Chicago demonstrated a sustained reaction in an atomic pile. America had already entered the war, and Bohr came to a characteristically reasoned conclusion that the Nazis were no longer the key opponents in the nuclear arms race. He was over-optimistic in his expectation of an imminent German surrender, but he was absolutely correct in his anticipation of the Cold War, precipitated by a Soviet development of the Bomb very soon after the cessation of hostilities. Bohr knew many capable Russian physicists, and could see that most of them had returned to their country of origin as a result of the Western European diaspora.

Politics

Bohr became convinced that a confrontation with Russia could only be averted by the promotion of international trust. He wondered what would happen if the status of Allied research was disclosed to Moscow. It was obvious, though, that suggesting the idea to Manhattan Project colleagues would land him in serious trouble. Typically, Bohr didn’t presume to evaluate the merit of the plan himself, considering instead that somebody better qualified should assess it. He confided in Viscount Halifax, the British Ambassador to Washington, and received a sympathetic reaction, encouraging him to develop and espouse his ideas. A meeting with Churchill was suggested, and Bohr set his hopes on it. He began to genuinely believe in a possibility that the development of the Bomb could be halted before it was used in anger.

The meeting took an inordinately long time to organise, so long in fact, that the opportunity was probably lost even before it took place. In May 1944, though, Bohr met Churchill in London. The visitor was kept waiting for most of the morning, before being afforded a peremptory 30 minutes. All of Bohr’s diffidence flooded back, and Churchill was completely unimpressed. Ironically, the British Prime Minister needed no persuasion of the imminence of a Cold War. Instead, he dismissed Bohr because he didn’t trust such an unconvincing and ineffectual speaker to make an accurate assessment of the receptiveness of the Russians towards co-operation.

Bohr did not interpret his treatment as a snub. He blamed the poor outcome on himself. Determined to try and redress this failure to prevent a ruinous conflict, he returned to the United States and bravely went to the overseers of the Manhattan Project with his views. For a while it seemed he was getting somewhere, and he received a cordial and sympathetic audience with Roosevelt, but both the Allies and the Russians were by now converging on the heart of Europe from opposite directions, and half a million people were engaged directly or indirectly in American nuclear weapons development with a budget exceeding a billion dollars. With hindsight, it seems that Bohr’s representations were entertained mainly in the hope that his reputation and essential pacifism might persuade fleeing German scientists to head West rather than East.

Bohr returned to his native Denmark in late 1945, a few months after the devastation of Hiroshima and Nagasaki. For the rest of his life, he took no further part in military research. He made a notable appeal to the United Nations in 1950, pleading for an open world in which war could not be prepared secretly and where mutual confidence might grow and spread. Before he died in 1962, the spectre that Bohr foresaw had become manifest in the Berlin Wall and the Cuban Missile Crisis.

Reflections

Might a prouder man have given Churchill pause for thought? Since Bohr might well have been pushing at a partially open door, it’s frustrating that such a brilliant mind should have proved so ineffectual in political advocacy. One might wonder, indeed, whether Bohr’s motivation for trying lay more in salving his own conscience than in any belief he might succeed. Einstein’s contrition for his early hawkishness is well recorded, so maybe Bohr experienced a similar guilt-trip?

Bohr was never any kind of hawk, though. There is nothing in his writings to suggest that he felt responsible for the Bomb. He never sought to judge others either, probably because he had no pretensions about his own importance. His natural modesty is evident from this excerpt from his obituary:

With his heavy build and large hairy hands, Niels Bohr looked more like a peasant than a scholar, but his huge head with deep-set eyes under bushy eyebrows is not easily forgotten, nor the sudden sunny smile that seemed to deprecate what he had said, lest he be taken too seriously

Niels Bohr left us a rare but valuable demonstration of humility coupled with intellectual prowess. In a world where scientists are sometimes feared, the unassuming humanity of one of the very greatest is perhaps a source of comfort in itself.