The Physicists Page 6

  That scientific explosion was a time of triumph and hope. Though there were shadows just apparent in the world outside, the international scene between 1925 and 1929 to most people still seemed sunny enough. Nearly all the scientists, if they thought of such things at all, looked forward to a peaceful future. They tended to expect other human beings to be as free from class and racial tensions as they were themselves. If the work was going well, it was good to be alive.

  There was one oddity which didn’t attract much attention at the time. If it had come to mind, it would have been dismissed as trivial. Theoretical physics, at this high point, was very much a Jewish science. Heisenberg wasn’t Jewish, nor was Dirac or de Broglie. Almost all the other leading figures were. Bohr, the quintessence of Scandinavian virtue and the personification of Nordic manhood, had a Jewish mother. It would have seemed silly to wonder if this accidental fact about the physicists’ origins was going to have its consequences.

  6: The Clouds Gather

  THE 1930s saw a convulsion in Europe which the scientists did not anticipate, and which then disrupted many of their lives. They were forced into the greatest emigration of intellectuals since the collapse of Byzantium, and one far more dramatic and influential than that. Einstein, himself homeless, had to help make provision for Jewish scientists now deprived. Rutherford gave a lead to the English scientific community. Bohr arranged for Copenhagen to be a staging post, though one too near to the Reich for any long-term safety. The Göttingen faculty was broken up. Born found himself in Edinburgh; others were scattered round American or British universities. Hungarians reaching the United States included a wildly clever trio, Wigner, Teller, Szilard, all three to have an extra significance in a few years’ time. Hans Bethe arrived at Cornell. Men of high talent were grateful for comparatively humble posts.

  Towards the end of the decade, just before the beginning of the war, a recent power in the scientific world, the Physics School in Rome, had to cross the Atlantic. Their leader, Enrico Fermi, was not himself Jewish, but had a Jewish wife. He had been recognized very young as one of the physicists of the century, and the only one who could work on equal terms with the greatest in both theory and experiment. There had been no one like that for generations. The great physicists of recent years had been either superb experimenters or gifted theoreticians: the two talents had not been combined in the same individual. If Fermi had been born thirty years earlier, it was possible to imagine him discovering Rutherford’s nucleus, and then proceeding to Bohr’s theory of the atom. If that sounds like hyperbole, anything said about Fermi is likely to sound hyperbolic. As a professional scientist, not as a cosmic thinker such as Einstein or Bohr, he was one of the very greatest.

  The United States, because it was rich and because it was the most secure refuge (any Englishman had to advise German friends that these islands were uncomfortably near), received a high proportion of the Jewish scientists. It was the most significant influx of ability of which there is any record. America, of course, was already producing its native-born Nobel prize winners. The refugees made it, in a very short time, the world’s dominant force in pure science. They also helped create what was soon called the Jewish explosion, a burst of creativity in all fields, not only science. There already existed native-born, or effectively native-born, American-Jewish physicists of world class, like the dazzling Rabi, who was to win the Nobel prize for his work on the magnetic properties of atoms. The refugees gave the explosion a new dynamic.

  There were, needless to say, losses and tragedies in these transplantings. There must have been talent starved, most of which we shall never know. But physics, the physics of the 1920s, went on remarkably undisturbed. Quantum mechanics as welded together by Dirac was now established as a final intellectual statement. As an aside, Einstein’s argument with Bohr, whatever was happening to either of them, continued also undisturbed. At face value, quantum mechanics introduced uncertainty into the sub-atomic world: Heisenberg’s Uncertainty Principle stated explicitly that it is impossible to measure precisely the position and velocity of an electron (or other particle) at the same moment. If so, it is then impossible to predict exactly where it will be at any time afterwards. In other words, a physicist could send off two electrons in the same direction, at the same speed – as precisely as he could – and they would not necessarily end up in the same place. In the language of classical physics, the same cause had produced different effects. The principle of causality was violated.

  In the de Broglie–Schrödinger wave view of quantum mechanics, electrons are directed by a guiding wave. The intensity of the wave gives the probability of finding the electron. Even knowing the mathematical formulation of the wave precisely does not help the physicist predict an individual electron’s position. A wave travelling through a narrow slit spreads out on the far side, like ocean waves entering a harbour. When electrons are shot through a slit, most will travel straight through. But the occasional incident electron, no different from any of the others, will be sent ‘round the corner’ by the guiding wave. Again, causality is violated. Although all the electrons entering the slit are identical, they end up going in different directions on the far side.

  Since the guiding wave pattern determines the probability of an electron going in a particular direction, the physicist can say where the bulk of the electrons will end up, and which regions behind the slit will be avoided. So he actually can make predictions when dealing with a large number of electrons. Precise causality has become a matter of statistics – just as a gambler can’t predict he will throw a six when a die is rolled, but knows that if he rolls it often enough, a six will come up on one-sixth of the throws.

  Was the sub-atomic world understandable only in terms of statistical chance? Bohr was certain of that ground. But perhaps quantum mechanics was just a temporary formulation, an approximation to deeper laws as yet undiscovered. These deeper laws could then be as strictly causal as those of the old classical physics. So argued Einstein. ‘God doesn’t play at dice,’ he said with imperturbable conviction. Bohr, for once sharp-tongued, answered that he ought not to speak for what God (Bohr actually said Providence) could or could not do.

  That was almost the only brusque exchange in the whole controversy. It was conducted with maximum generosity by both men. It was a model for any profound disagreement at the highest level. They respected and admired each other: each had no doubt that he was right. Incidentally, it was the deepest exploration of how we know things that has so far been conducted, and ought to be part of any course in academic philosophy. The whole weight of scientific opinion was beginning to come down in support of Bohr. It was only Einstein’s transcendent authority that kept some of the argument open.

  The passionate excitement in the physics of those years, however, took place elsewhere. There were a series of discoveries, all experimental, which were the most dramatic since Rutherford had proved, a dozen years before, that atomic nuclei could be disintegrated. To almost everyone these new discoveries seemed of absorbing scientific interest, giving the first dim insights into atomic nuclei; but the interest was the interest of pure science, nothing else but that.

  The structure of the electrons in the outer part of the atom was now common knowledge amongst physicists. Whatever the philosophical status of quantum mechanics, it undoubtedly described how the electrons behaved. But little was known about the central nucleus. In the 1930s, physicists were to lay bare its mysteries with the same determination – and the same degree of success – as they had applied to the outer electrons in the previous two decades.

  The year 1932 was one of scientific revelations. Rutherford had once predicted, with one of his direct intuitions backed by good firm reasoning, that a third sub-atomic particle must exist. The lightweight, negatively charged electrons were now old friends. The atomic nucleus must contain much heavier, positively charged particles. Physicists called them protons. According to Rutherford, there must exist another particle, as heavy as the proton, but havin
g no electrical charge.

  Such a neutral particle would possess great carrying power once it was on the move. With no electrical charge, its motion could not be altered by the electrical charge of an atomic nucleus. It could be stopped only by direct collision. There actually was some evidence from disintegration experiments that such particles existed: but it was necessary to know what one was looking for. In Paris, the great tradition of the Curies continued in the hands of their daughter Irène. She had married the Curies’ assistant, Frédéric Joliot, and they combined their names, as they did their research. The Joliot–Curies had evidence for Rutherford’s neutral particle in their own experiments, but they had mistakenly interpreted it as a kind of penetrating radiation.

  But Chadwick knew what he was looking for. He calculated just what effects would distinguish a neutral particle from radiation. And then he set up experiments of classical beauty and simplicity to look for these effects. Like the Joliot–Curies, he shot alpha particles at a target of the light metal beryllium; and out came the mysterious ‘radiation’. But Chadwick intercepted it with paraffin wax. The ‘radiation’ hit the nuclei of hydrogen atoms within the paraffin wax, and ejected them at high speed. Now the nucleus of hydrogen is none other than a single proton. By his measurements on the ejected protons, Chadwick proved that what was hitting them was not radiation: it was a neutral particle, almost identical in mass to the proton.

  Chadwick, under a repressed façade, had the most acute of aesthetic senses and was an artist among the experimental physicists. He worked, night and day, for about three weeks. The dialogue passed into Cavendish tradition: ‘Tired, Chadwick?’ ‘Not too tired to work.’ And at the end, when he told his colleagues what he had done in one of the shortest accounts ever made about a major discovery: ‘Now I want to be chloroformed and put to bed for a fortnight.’

  The chargeless particle was named the neutron. It was at once clear that it must be a constituent of all atomic nuclei (apart from the single-proton nucleus of hydrogen). At last there was an explanation for the puzzle about atomic weights and atomic numbers. The relative weights of atoms are usually near whole numbers: the carbon atom is twelve times as heavy as the hydrogen atom, so its atomic weight is 12. Since electrons are very light, this means that the carbon nucleus is twelve times as heavy as the hydrogen nucleus which consists of a single proton. So the carbon nucleus is as heavy as twelve protons.

  Chemists, however, rank the elements by atomic number, the number of electrons an atom has. It is the electrons which govern an atom’s chemical behaviour. But each negatively charged electron ‘in orbit’ must be balanced by a positively charged proton in the nucleus. The atomic number thus automatically turns out to equal the number of protons in the nucleus. Carbon has an atomic number of 6, it has six protons in the nucleus. With the discovery of the neutron, it became obvious that the difference between its atomic weight of 12 and its atomic number of 6 was made up by six neutrons in the nucleus. They added the mass of six protons to the nucleus without adding any electric charge.

  The existence of the neutron also solved at a stroke the problem of isotopes – atoms of the same chemical element, but with different atomic weights. Among a hundred carbon atoms taken at random, ninety-nine have an atomic weight of 12, one is slightly heavier, at 13. Scientists call these isotopes carbon-12 and carbon-13 respectively. Since they have the same chemical properties, each must have six electrons, and a corresponding six protons in the nucleus, The difference in weight must depend on neutrons. Carbon-12 has six neutrons, carbon-13 has seven.

  Although the number of neutrons doesn’t affect an atom’s chemical properties, it – not surprisingly – does change the stability of the nucleus itself. You can add another neutron to carbon-13, to produce carbon-14, with eight neutrons to the six protons. This is too unbalanced. The nucleus is unstable. One of the neutrons spontaneously changes to a proton, emitting a high-speed electron in the process. This radioactive form of carbon is in fact most useful to archaeologists; the gradual decay of carbon-14 atoms enables them to calculate the age of organic remains. The radioactive properties of the different isotopes of the uranium atom were at the end of the decade to have a far more lethal significance.

  To return to 1932, though, it was clear that atomic nuclei must have a structure of their own – that is, they were complex entities, with a quite different complexity from the exterior electronic structure of the atom. There was no theory for this nuclear structure, but Bohr’s deep imagination was getting to its speculative work. It was too early, and there wasn’t enough quantitative detail, for any mathematical expression: but maybe one could begin to guess at primitive models.

  In that same year, 1932, atomic nuclei were, for the first time, split under man’s direct control. Rutherford and Chadwick had earlier split up nuclei by firing at them the fast alpha particles which naturally shoot out from radium. Now it was feasible to do it all artificially. The projectiles were protons, taken from ordinary hydrogen gas, and accelerated up to enormous speeds by electric fields. This was a process invented and developed by John Cockcroft: it was the beginning of Big Physics, and was to characterize particle physics in years to come – though the Cockcroft accelerator is tiny by the side of the gigantic feats of engineering of which it was the forerunner. The essential thing was, Cockcroft’s accelerator worked. Lithium nuclei were duly broken up by his accelerated protons. In about the only magniloquent gesture of a singularly modest and self-effacing life, Cockcroft walked with soft-footed games player’s tread through the streets of Cambridge and announced to strangers, ‘We’ve split the atom. We’ve split the atom.’

  It was still a scientific achievement, nothing else but that. The discovery of the neutron had come out of intuition, pure scientific thinking, and experiment. A year later, another particle was discovered, this time not predicted so much as already inscribed in quantum theory. Out of Dirac’s equations, it appeared that there must be a positive electron, identical with the familiar electron but carrying the opposite charge. There was a symmetry inherent in the natural world. The positive electron was soon identified in experimental fact, almost simultaneously but quite independently by different types of observation in America and England. Carl D Anderson got in first, and with justice had the priority. In England Patrick Blackett’s publication was a short head behind.

  The pressure of sensational results increased. In spite of the gathering political darkness, and the suffering of Jewish scientists in Germany, physics went on still remarkably undisturbed. The next year in Paris, the Joliot–Curies assuaged their chagrin over not recognizing the neutron by producing artificial radioactivity. By bombarding ordinary, stable isotopes of common elements with alpha particles they created new isotopes unknown in nature, and so unstable that they spontaneously broke up and emitted radiation just like the naturally occurring radioactive atoms.

  In Rome Fermi and his school carried that major discovery a decisive step further in 1934. To make isotopes with an unusual number of neutrons, they simply bombarded atoms with neutrons, in the hope that they would stick when they hit the target nucleus. Fermi decided to slow the neutrons down by sending them through paraffin. One of his gifts was inspired common sense, and he explained that the neutrons were more likely to stick in the nucleus they were hitting, the slower they moved. Though no one knew it, that apparently prosaic concept was going to have consequences far from prosaic.

  At that period, in the early 1930s, no one, and certainly none of the great physicists, had any notion of releasing the energies of the nucleus. It was possible, it had now been done frequently, to split the lightest nuclei. But everyone realized that the forces binding the more complex nuclei were of enormous strength. By bombarding these heavy nuclei, small bits could be knocked out: but to do more than that, to disintegrate a heavy nucleus and so trigger off what must be gigantic sources of energy, seemed beyond the realms of possibility.

  Those leaders of physics were far-sighted men. They were u
nusually positive in their view. They said as much. In a public lecture in 1933 Rutherford explained that this wonderful crescendo of discovery was getting nearer to the innermost secrets of nature, but that the world was not to expect practical application, nothing like a new source of energy – such as had once upon a time been hoped for from the forces in the atom. Now we had learned more, and it appeared to be beyond scientific capabilities. Bohr completely agreed. So did Einstein. It is hard to think of three wiser men being so much at one.

  Later, in 1934, Fermi bombarded uranium atoms with his slow neutrons. The results were puzzling. The nuclear scientists couldn’t agree on an explanation. An abnormal amount of radiation was being emitted. The natural interpretation was that some uranium nuclei had been collecting neutrons, and had been transmuted into elements unknown to nature – christened trans-uranic elements, for they would have heavier nuclei than uranium, the heaviest naturally present on earth. And these very heavy nuclei should be unstable: their radioactive breakdown could produce the copious radiation that Fermi was picking up. The achievement of new, artificial elements – a misinterpretation as it finally transpired – was actually announced in the Italian press with joyous fanfares. What should the new element or elements be called? Fermi, as usual cool-headed, remained somewhat sceptical about his own discovery, but began to believe in it. So did others. There were more random suggestions than had so far happened in any nuclear research. It was a pity, people thought later, that Rutherford, who had died shortly before, wasn’t on the scene. It was just the sort of problem that he might have seen straight through.