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 having 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 unusually 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.
One of the best chemists in the world, Otto Hahn, decided to repeat the Fermi experiments at the Kaiser Wilhelm Institute in Berlin. Not surprisingly, since Fermi and his colleagues were first-class experimenters, Hahn obtained the same results. Hahn did some careful chemistry on the end-products. The common isotope of uranium, uranium-238, has 92 protons and 146 neutrons in its nucleus. Trans-uranic elements would contain more of both, and have new chemical properties. But what Hahn was expecting to find was radium, on the rival interpretation that the neutrons were simply knocking fragments out of the uranium atom. A uranium atom that loses two alpha particles becomes radium-230.
But he found neither. To his own astonishment, and everyone else’s, what he did keep on finding was barium. And barium has a very much lighter nucleus. The common isotope has 56 protons and 82 neutrons; a total of 138 particles bound together in the nucleus, as compared to uranium’s 238. And all he could detect was barium. An impurity? But Hahn was one of the most meticulous of all chemists, and that was about as likely as if he had absent-mindedly slipped in some copper sulphate.
Once more suggestions proliferated, much talk, speculations getting nowhere.
When Hahn began to repeat the Fermi work, he had a collaborator called Lise Meitner. Lise Meitner was a respected and much loved physicist on the staff of the Kaiser Wilhelm Institute. She was Jewish, but of Austrian nationality and so, by some skilful covering up, had managed to keep her job. Then Hitler’s troops marched into Austria; overnight Lise Meitner’s nationality changed to German, and it was more than time to quit. Having good fortune, she managed to escape to Sweden, and it was there, in Göteborg, that she entertained her nephew, Otto Frisch, during the Christmas of 1938. Frisch was another high-class physicist and another refugee who had found sanctuary in Copenhagen. He arrived in Göteborg late at night, and didn’t see his aunt until the following morning.
They were an affectionate couple. Both were suffering exile and hardship. Still, the first thing they talked about was her latest letter from Hahn. Why could he detect nothing but barium? Frisch raised the conventional doubts: impurities? carelessness? Impatiently Lise Meitner brushed them aside. She had complete trust in her old chief.
They went for a walk in the winter woods. Each seems to have had the same thought, up to now inadmissible. Like everyone else, they had been living with an assumption. They had all taken it for granted that heavy nuclei couldn’t be split into two. Could that be wrong?
Nuclei seemed to be stable objects. Although the positively charged protons must repel one another, as all ‘like’ electric charges do, the presence of the neutrons glues the nucleus firmly together. Scientists had come to accept that there must be a nuclear force, in addition to the two forces then known – of gravitation and electromagnetism. In the big nuclei, the protons are repelling one another so strongly that there must be more neutrons than protons to keep the whole lot glued together. Even so, some nuclei of the really heavyweight kind – like radium – can’t contain all that electric force. Small fragments spontaneously break off. These consist of two protons and two neutrons – a bullet carrying off two units of electric charge and leaving the nucleus more stable. These bullets are the alpha particles, which Rutherford had harnessed to such good effect.
So even when nuclei were unstable, all experience showed that they didn’t break up. They simply emitted small fragments. Like all other physicists of the 1930s, Frisch and Meitner were carrying that assumption with them unquestioned. Now they alone, of all the physicists in the world, woke up to that assumption, and began to question it.
They sat down. It wasn’t comfortable in a Swedish Christmas time, but neither noticed that. Lise Meitner did some calculations. Although the structure of the nucleus was still a mystery, Bohr had proposed a model for it. With his great physical insight, Bohr had ignored all the complications – that nothing was known of the nature of nuclear force, for example. Two decades earlier his brilliantly simple model for the electrons in the hydrogen atom had paved the way for the correct, highly sophisticated quantum mechanical answer. Now he simply likened the nucleus to a drop of water. A water drop is held together by the attraction of the water molecules for each other; a nucleus is held together by the nuclear force between its constituents. The analogy is there. Let us not worry about the nature of the nuclear force. The electrical repulsion between the protons could be simply fitted to this model too.
Meitner carried on calculating, using Bohr’s liquid-drop model as her guide. Frisch followed her. In Bohr’s model the sums were quite simple. Almost at once they knew they had the answer. A heavy nucleus can indeed break into two halves. Imagine a water drop which is electrically charged to the limit of its extent to hold the charge. Water molecules can evaporate from the surface and carry off the excess charge – this is the equivalent of alpha-particle ejection from radium. Alternatively, the stresses within the drop can split it into two smaller drops. These are more tightly bound than larger drops. In the case of nuclei, the two small nuclei can contain the electric charges that made the parent nucleus unstable.
The neutrons that Fermi, and later Hahn, had fired at uranium nuclei had pushed them over the brink. The uranium nuclei didn’t accept the neutrons, to build up heavier, trans-uranic, elements. The neutrons didn’t just knock off small fragments. Under neutron bombardment, the uranium nuclei split into two smaller, lighter nuclei. The split need not be exactly half and half. A typical break-up would produce barium (with 56 protons) and the gas krypton (which has 36 protons). Here was the reason for Hahn’s strange discovery.
Frisch and Meitner did more sums, to check the release of energy. Those came out right. They had been out in the snow for three hours.
They were cautious, as they had to be. The result, in terms of pure science, was important but not earthshaking. Heavy nuclei could be disintegrated. It was going to deepen understanding of the nucleus. They had an intimation, though, that the result, in terms other than the purely scientific, might be momentous.
Lise Meitner went back to Stockholm after one of the more remarkable aunt-nephew reunions. Frisch returned to Copenhagen and reported to Bohr. Frisch, not usually an excitable young man, burst into the scientific explanation. Bohr, just about to take a trip to America, accepted the explanation within moments of Frisch beginning to speak. It was then that Bohr made his supreme comment: ‘Oh, what idiots we all have been. This is just as it must be.’
It shows the power of a received idea that so many of the best scientific minds in the world had scrabbled about for months, averting themselves from the simplest conclusion. However, they soon made up for lost time. By a loose tongue within Bohr’s entourage, the news was leaked as soon as his party arrived in New York. American laboratories repeated the experiment, confirming the results, measuring the energy discharge. Bohr was obliged to ensure that the prime credit went to Meitner–Frisch (whose letter to the science journal Nature wasn’t, in fact, the first published statement). With his incorruptible sense of justice, he exerted himself in getting the record straight, while he had more imperative matters to think about.
Physicists all over the world were in a ferment. Experiments everywhere. Gossip in newspapers. There were sceptics, but most scientists of sober judgement accepted that the discovery must mean that nuclear energy might sometime be set at large. The obvious thought was that this might lead to explosives of stupendous power.
Was this realistic? It would be so only if the neutron which split a uranium nucleus could bring about a chain reaction – a scientific term which soon became a common layman’s phrase. Each time a uranium nucleus split apart, it released energy as heat. But nuclear energy would never be a reality if one had to keep firing neutrons from some source at the uranium atoms to break them up. If, on the other hand, the uranium atom released neutrons as it split up, then these neutrons could go on and break up other nuclei. The neutrons from these disintegrations would trigger more, producing a chain of reactions that would carry on without outside help, liberating more and more heat, quicker and quicker. So far there was no sign of that. If there had been, Hahn’s laboratory, and a good many others, wouldn’t have been in a state to report the results: nor would a number of nice comfortable university towns.
Bohr got to work. So did a young colleague of his at Princeton, John Wheeler, a fine and strong-minded scientist who had the distinction of being the only person of Anglo-Saxon descent right at the centre of these first sensations. He and Bohr arrived at the answer with speed and clarity.
Obviously – and fortunately – most of the uranium nuclei were not being split. A small proportion were. These must belong to a particularly susceptible uranium isotope. Nuclear fission – this term for the splitting of a nucleus was just coming into use – happened not, in the stable, common nucleus of uranium (uranium-238), but in that of the much rarer isotope uranium-235. Both have 92 protons, but the neutrons number 146 and 143 respectively. Bohr, now feeling his way with certainty among nuclear structures, gave reasons for the nuclei of uranium-235 being fissile. It was a classical piece of scientific thinking. It was absolutely right. At this distance, it jumps to the eye as being right. But it was not immediately accepted. Fermi, who untypically made several misjudgements during this period, didn’t believe it. There were weeks of argument. It was March 1939 before the community of physicists were convinced that this uranium isotope could be disintegrated, emit neutrons, and, if accumulated in quantity, might start a chain reaction. Collect enough uranium-235, and there was the chance of an immense explosion.
There the pure science finished.