THERE was, however, one theoretical physicist who became an intimate of Rutherford, and with whom there was mutual admiration and something like love, paternal on Rutherford’s side, filial on the other. This was Niels Bohr. Towards the end of 1911, Bohr came diffidently into Rutherford’s laboratory office in Manchester. They had met briefly before, but this was the first time Bohr had obtruded himself; obtrude perhaps isn’t a good description of Bohr’s behaviour – punctilious, excessively sensitive, eager to show his homage to a great man. Bohr was a Dane, then twenty-six years old, tall, with an enormous domed head, and much more muscular and athletic than his cautious manner suggested.
He was in trouble. He had been having a bad time in Cambridge, having spent months in the Cavendish trying to extract some interest from J J Thomson. Thomson had been bright and polite, as he usually was, had invited Bohr to dinner at Trinity, but had failed to show interest in his ideas. Bohr had brought his latest paper (actually a work of much originality), but in spite of many tactful hints Thomson hadn’t shown any inclination to read it. It remained among the pile of manuscripts on Thomson’s desk waiting for the attention which they never seemed to get. Bohr persisted, having a good deal of gentle Nordic stoicism, but after months he gave up. He left the old luminary and went to see if he could get some sympathy from the rising one. (Incidentally, Thomson had the unfortunate distinction of losing for the Cavendish both Rutherford and Bohr, founders of modern physics.)
Rutherford liked Bohr at sight. Patiently he listened. It says much for his judgement not only of scientific ability, but of men, that he formed a high opinion of Bohr on the spot, and one that never wavered.
Rutherford listened. The explication took a long time and Rutherford was by temperament not at all a patient man. But he stuck it out, and that is another tribute to his judgement – and perhaps to his kindness, for he saw that the young man was unhappy.
In retrospect it is a nice scene. One has to remember Rutherford was loud-voiced and explosive and liked his own way in a conversation. With Bohr, even the young Bohr, he was unlikely to get it. For Bohr, though one of the deepest minds of his century, and the incarnation of altruism, was a talker as hard to get to the point as Henry James in his later years. One qualification sprang out of another. He had to dig down for the final, the perfect word, and, on not finding it, had pauses, minutes long, in which he reiterated a word which was clinging to his mind.
It didn’t help that he spoke with a soft voice, not much above a whisper. Further, he was speaking to Rutherford in a language not his own. None of that deterred him. He was the most enthusiastic of talkers, whispering away, as he was to do for the next fifty years. He very much preferred talking to writing. On paper he was equally labyrinthine, and that took even more time as he, in search of the perfect expression, made draft after draft.
Not many acts of kindness and good judgement have had more creative results than that of Rutherford. Einstein wouldn’t have needed encouragement: the young Bohr did. He stayed in Manchester, buoyed up by Rutherford’s zest and his gift for communicating that he was usually right. Within two years Bohr, with his characteristic mixture of cautiousness and daring, produced a theoretical equivalent of Rutherford’s nuclear atom, a theory as daring as it was original.
In Rutherford’s model of the atom, electrons orbited the central nucleus, held in by its electrical attraction, in much the same way as the planets are held in orbit about the sun by its gravitational pull. It explained his experiments neatly. Unfortunately, the laws of classical physics did not allow Rutherford’s atom to exist. According to the electromagnetic theory which Maxwell had built on the foundations laid by Faraday, an electrically charged particle produces radiation if it is diverted from a straight path. The electrons in Rutherford’s atom were in circular orbits, so they should have been radiating all the time. If they did so, they would be losing energy, and would have spiralled down into the nucleus in a fraction of a second. The atom would have collapsed on itself.
Rutherford was not perturbed: he was not a theoretician. It was Bohr who provided the theoretical backbone. Without contradicting Maxwell in the general run of physics, he simply asserted that when an electron is orbiting a nucleus it does not radiate. This made no sense in classical physics. But it worked. For Bohr was bold enough to include a second assumption which meant his new theory could explain the long-standing puzzle of the pattern of wavelengths – spectral lines – from hydrogen.
Planck and Einstein had shown, years before, that light travels with certain energies, in packets called ‘quanta’. The energy of a quantum is related to the wavelength of the light in question. Nineteenth-century physicists had found that each element produces a characteristic spectrum of light: it emits light of only particular wavelengths. In the twentieth-century view, this meant that each type of atom produces only light quanta of particular energy – but until Bohr’s theory of the atom, no one had any idea why.
Bohr’s second assumption was that electrons cannot orbit the nucleus in just any old orbit. The radius – and so the energy – of the permissible orbits was determined by a number that came out of Planck’s earlier work, a number known to physicists as Planck’s Constant. When an electron was in a permissible orbit, it circled around the nucleus without emitting light (or any other radiation). But an electron could spontaneously jump from one permissible orbit to another. As it did so, it either absorbed light (going ‘uphill’), or emitted light (coming ‘downhill’). Bohr calculated the permissible orbits for the simplest element, hydrogen, which has only one electron. He then worked out what energies were involved in an electron jumping from one permissible orbit to another. Assuming that this energy was converted into light, he calculated the corresponding wavelengths. He compared these to the known, and long puzzling, spectrum of hydrogen. The match was exact.
This was the first success of quantum theory in the field that classical physics had always regarded as its own: the physics of matter. From 1913 on, theoreticians knew the limits of classical physics on the very small scale. Bohr’s quantum description of the hydrogen atom explained in brilliant and precise detail the spectrum lines of hydrogen, previously a blinding mystery. The nineteenth century had accumulated beautifully observed spectra of many elements, all of which had been as incomprehensible as Etruscan. When Einstein heard of how the theory matched so strongly with the data of the spectrum lines, he said, with delight and wonder: ‘Then this is one of the greatest discoveries.’
Very soon Bohr, still a youngish man, became the father of atomic theory. He became director of the Institute for Theoretical Physics, designed for him and by him, in Copenhagen, a centre unlike any other in the history of physics. For theoreticians it was both duty and pleasure to attend there, just to hear Bohr talk – talk at considerable length, but also with questions tentative and probing, not sharp or witty, but moving circuitously and patiently towards a new truth. Tough-minded scientists, not over-given to respect, used to come back from Copenhagen and report – in that idyllic age of physics – that Bohr was doing it all.
They liked to call his method Socratic, but they didn’t know those ancient dialogues very well. It was really different in kind, the chief resemblance being that Socrates wrote nothing and Bohr surprisingly little. Historians of science are going to find a puzzle in identifying exactly what he did. There is one good biography, but nothing like the mass of literature written around Einstein, alive or dead.
Bohr’s personality hadn’t the effortless power of Einstein’s and he hadn’t the devil-may-care attitude and the emphatic tongue. He didn’t convey, as Einstein did, the immediate presence of moral experience. But he did suggest brooding wisdom and, above all, selfless concern. More than almost any creative man of the highest calibre, he didn’t have a hard ego off which others bounced as from a billiard ball. He was not, as Einstein was, impersonally kind to the human race; he was simply and genuinely kind. It sounds insipid, but in addition to wisdom he had much sweetness. He was a loving and beloved husband and father.
The ‘Copenhagen school’ was a creation of his personality as well as his intellect. He had certain extra advantages. It was a help that he came from a small neutral country, had no national prejudices and evoked none. An American or Russian Bohr, or even an English or French one, would have caused more superficial impediments. Also he had a peculiarly propitious family background. His father had been a professor of physiology at Copenhagen, his grandfather an academic also; and one of his sons now occupies Niels’ old position. They were a high-minded academic family, in some ways similar to those the English have been used to – but appreciably more liberal, more conscious of a sense of moral duty, and very much more cultivated. The Bohrs had all read widely in at least four languages, applied themselves to philosophy as well as to literature, become lovers of music and the visual arts. They were as educated as it is possible to become in this century of ours. Without any of them realizing it, some of this civilization spun off. A good many young men couldn’t help the vestigial thought that this was what the intellectual life ought to be; just as those did, though there were far fewer, who came close to Einstein, himself cultivated in a good Central European fashion, with strong creative feelings about books and music, feelings more positive than Bohr’s. In the clash which was to come a few years later in the late l920s, right at the climactic point of modern physics, it wasn’t Einstein’s classical clarity which prevailed, but Bohr’s delicate nosing his way among the contradictions of the natural world. Clash is too bleak a word for a debate, as profound as any in intellectual history, between two such deep-minded men. The disagreement was about ultimate things, and expressed the mysteries, as well as the triumphs, of the scientific world picture.