The Physicists

by C. P. Snow

  Einstein was not deeply interested in the details of particle physics. Max Planck had shown in 1900 that light is emitted in distinct ‘packets’ or quanta of energy (a concept essential for the later theory of modern physics, though undervalued by some historians of science), and Einstein’s 1905 paper on the photo-electric effect added the significant point that light remains in these packets as it travels. But after that he went off, single-minded, in his own cosmic thoughts. No one has ever been able to think more obsessively, and for longer, on a single issue. It was a supreme gift for his kind of abstract creation – perhaps for any other creation – and one he shared, to the same degree, with his greatest predecessor, Newton.

  Of course, he read what was being discovered about the structure of the atom. He said later, in his benign manner, that he had been impressed by the beauty of Rutherford’s experiments, done with the simplest of means, and going straight for physical reality. Einstein wished, so he said, that he had been able to write something about Rutherford in his own work, but there hadn’t been any adequate connection.

  That may not have been more than a handsome gesture to another great figure. Rutherford tried to reciprocate, announcing in a speech that the General Theory of Relativity, irrespective of whether it was valid, was a magnificent work of art.

  The qualification in that tribute has an eloquence of its own.

  Rutherford knew perfectly well that the General Theory was valid, but he didn’t believe that it added much to his own idea of physics. The two men never became close. They made speeches together when in 1933 Rutherford was leading a campaign to find aid for Jewish scientific refugees. But Rutherford didn’t make an attempt to attract Einstein to Cambridge. It wasn’t necessary: Einstein, as everyone knew, was receiving offers from all over America, Britain, Europe, and in his casual good-natured manner was tending to accept them all.

  The trouble between the two was caused, at least in part, by the disparity in the treatment of the great experimentalists as contrasted with that of the great theoreticians.

  In terms of popular esteem, experimentalists felt, and still feel, as Rutherford did with his usual horse-power, that they got an unfair deal. The names of theoreticians survived in intellectual currency: the names of experimentalists didn’t. Einstein provided the most vivid illustration.

  Before he was forty, with the General Theory still waiting for a conclusive experimental test, Einstein was the most famous scientist in the world – perhaps more famous than any scientist would be again. Whereas Rutherford had nothing but celebrity amongst scientists themselves. He might have taken the first great steps in elucidating the structure of the atom, but he wasn’t a national figure, not celebrated as a film star in the way that Einstein was. Experimentalists did all the ground work, and without the results of experiments there wouldn’t be any theory. Yet no experimentalist had ever caught the popular imagination. Inventors had, occasionally, witness Edison and Marconi, but never scientists doing experiments to discover the fundamentals of the universe.

  It seemed mysterious. It was as though popular opinion had somehow realized that science, the whole great enterprise, was a collective activity in which individual personalities, and individual achievements, didn’t much matter. If one practitioner didn’t make a discovery this year, someone else would come along and make it next year. And that was true at the height of scientific creativity. If Rutherford hadn’t proved the existence of the atom’s nucleus in 1911, no one doubts that someone would have performed the same experiments within a decade, probably less than that.

  A great scientist adds his own brick to the cumulative edifice. There are few human beings who have the capacity to be great scientists. It was once estimated that only one person in a million could do first-class original work, and that was drawing the line far below the Rutherfords. Nevertheless, if one scientist doesn’t add his appropriate brick, another will. H J G Moseley, one of the rising stars of English science, was killed at Gallipoli, aged twenty-seven. He had had time to leave an indelible mark on the text books: if he had lived, he would have been one of the very great. And yet, though he would have done splendid things, those things in due course were inexorably done by others, and the sand smoothed down over Moseley’s absence as though he had never been.

  Much of those sobering thoughts applies to theoreticians also – but not quite so bleakly. First of all, great theoreticians are even rarer animals than great experimentalists. That kind of conceptual skill is one of the most uncommon of all human gifts. Perhaps one in a hundred million is born with the potential to be something like Clerk Maxwell, and even that guess may be over-optimistic. As a consequence, their creations are not quite so quickly replaceable. The most incisive tribute to Einstein was made by Dirac, who doesn’t inflate his words. Dirac said first that if Einstein hadn’t published the Special Theory of Relativity in 1905, someone else would have done it within an extremely short time, five years or less. Several people, we now know, were already very near, Lorentz, Minkowski, Poincaré: that is exactly like the standard position in experimental physics.

  But, Dirac went on, the General Theory, which Einstein published in 1916, is an entirely different matter. Without it, it is likely that we should still be waiting for the theory today. That is one of the most striking things ever said of one great scientist by another.

  There was no injustice in Einstein’s transcending fame. Still it is possible that the mana of his personality encouraged it. He was so unlike the rest of humankind. Amusing, more than a bit of a deliberate clown; unshakable, supremely confident both in scientific and moral judgements. When he felt deeply, he was rather like an Old Testament prophet, or else a benign deity being patient with human stupidity and worse – but also like a benign deity who had considerable physical resemblance to a handsome and inspired golliwog. No one who knew him expected to meet anything like that again: and they were right.

  Whether Rutherford felt anything like that about him, we don’t know. In spite of the aura around Einstein, his scientific thinking was as direct as Rutherford’s own. But it might not have reconciled Rutherford to theoreticians in general, and to this one theoretician in particular, to be told so.

  4: The Quiet Dane

  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.

  5: The Gold
en Age

  FOR a period of ten years – which included the First World War – there was something like a muddle, as scientists argued about the Rutherford–Bohr atom. Not on the side of experimentalists. Rutherford went indomitably ahead, come war, come theorists (Bohr excepted, to whom he confided any startling result), come philosophers (dismissed under the general Rutherfordian heading of ‘those fellows’). Rutherford’s experiments, still masterly in their simplicity (brutally simple, the over-sophisticated felt), were having an earthquake-like success. In 1919, he started firing alpha particles at nitrogen atoms. Nothing much should have happened. A great deal did.

  As in his earlier experiments, the alpha particles came from radium. This time he was directing them down a tube filled with nitrogen gas. At the far end, he found he was detecting not just alpha particles, but also particles with all the properties of hydrogen nuclei. There was, however, no hydrogen in the tube. With his high-speed alpha-particle projectiles, Rutherford had actually broken them off the nuclei of the nitrogen atoms.

  The discovery of radioactivity had earlier shown that certain, rare types of atom could spontaneously disintegrate. Now Rutherford had shown that ordinary atoms were not indestructible. By knocking out a hydrogen nucleus (later called a proton) from the nucleus of nitrogen he had converted it into another element, oxygen. Rutherford had, to a limited extent, achieved the dream of the alchemists and changed one element to another.