by C. P. Snow
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.
7: This Will Never Happen
THE pure science had produced the possibility. By the summer of 1939 it was known all over the scientific world.[1] Publication was open. The German physicists read the Bohr–Wheeler paper and the rest of the literature with, of course, as much realization as the Americans and English. So did Lev Landau in the Soviet Union, who ranked with Kapitsa as a leading Russian physicist. There was much troubled thinking.
Sensible people, certainly in Europe, took it for granted that war was coming, probably within months. It was now feasible at least in principle that explosives could be produced of a different order from any so far in human hands. Was this practicable? Could quantities of these fissile elements ever be made? If so, could it happen in the realistic future, that is within the duration of any foreseeable war?
With a few exceptions, scientific opinion was sceptical. There was plenty of commotion in the press, but, among the immediate prospects of war, these fears were dim and abstract. They did not penetrate to politicians anywhere, who were living, naturally enough, in the present moment, which was sufficiently threatening. Some scientists were blandly optimistic. It would take many years, some of them computed, to accumulate even a few grams of uranium-235. No one then knew how much was needed to make a bomb. But the guess was a quantity which was beyond present-day technological powers.
That wasn’t a scientific problem. Science had done its job. All the scientific knowledge was there and ready. If it could ever be applied, that would be a matter of engineering, in particular of abnormally difficult chemical engineering. The only way to separate the uranium isotopes from one another on an industrial scale would be to apply techniques similar to those that the chemical industry already used to separate and purify chemical compounds. In fact, if the discoveries of nuclear fission had taken place in a peaceful world, their future use would probably have been left to the great firms of the chemical industry – Dupont, ICI and so on. As it was, the ultimate production of the atomic bomb – as was also to be true of space travel – was not a scientific triumph, but an engineering one. In both cases, the science had been ready well before. In the event, scientists had to turn themselves into amateur engineers to play any further operational role.
That summer of 1939 a few scientists were apprehensive and far-sighted. In England, George Thomson (son of J J) and W L Bragg – both Nobel prize winning physicists – were advising the government to acquire the uranium ore in the Belgian Congo, if only as an insurance. In America the three Hungarian refugees, Edward Teller, Eugene Wigner and Leo Szilard, were campaigning for urgent action. All three had been close to the nuclear developments. All three were scientists of high class, and Wigner was already tipped for the Nobel, which he duly got. All three had inside knowledge of German science, and had much respect for it, even though so many of their old colleagues had been driven out. There was plenty of ability left, they knew, to solve the technological problem of a fission bomb, if the problem could be solved at all. The prospect of a fission bomb in Hitler’s control meant nothing short of doom.
On this they were agreed, though they were very different men with, on all other topics, very different opinions. Wigner was calm, judicious, ironic, temperate, mildly conservative: his sister was Dirac’s wife. Teller was dramatic, passionate, a man of the right (though more complex in his attitudes than popular accounts later suggested). Szilard was a man of the left, so far as he could be classified at all. He had a temperament uncommon anywhere, maybe a little less uncommon among major scientists. He had a powerful ego and invulnerable egocentricity: but he projected the force of that personality outwards, with beneficent intention towards his fellow creatures. In that sense, he had a family resemblance to Einstein on a reduced scale. He also had an unusually daring scientific imagination. In August 1939, while men as wise as Bohr still found it scarcely credible, Szilard didn’t doubt that the fission bomb could be made. That being so, it would be made. Incidentally, Szilard was a writer of interesting scientific fiction. He was the most active spirit among the Hungarian trio. It is likely, though, that Teller also believed that the bomb would be made.
What should they do? They were refugees in a foreign country. They were unknown, except in esoteric academic circles. They wanted to get to Roosevelt and warn him of the dangers. They decided to go to Einstein and persuade him to write a letter. Einstein was himself a refugee – he had been in the United States since 1930 – but he was the opposite of unknown. They duly went out to his summer retreat on Long Island and explained their thoughts. It hasn’t been stated, but the conversation must have been mostly in German. Einstein thought that they were completely right. The letter was drafted by Szilard. Einstein signed it.[2]
Then they indulged in some Central European elaboration. Not knowing how American politics worked, they resorted to finesse. Szilard had discovered someone who appeared to have the entrée to the President, an economist called Alexander Sachs. It would probably have been better to do what a simpler character such as Ernest Lawrence – the American physicist who won the 1939 Nobel prize for his improved particle acceleration – would have done, and use straightforward channels. Anyway, Sachs did deliver the letter to the President, though it took six weeks. Then there was an anticlimax. Nothing happened.
The romantic myth that Einstein was ultimately responsible for the atomic bomb has no foundation. It is true that much later he expressed some guilt about signing the famous letter, but that was taking an unnecessary burden upon his conscience. What is not in doubt is that he felt as strongly as the others that bitter necessity dictated that the bomb should be made. The threat of a Nazi bomb was enough. There were no moral qualms at that stage. Einstein had, for most of his life, been a pacifist. With the advent of Hitler he accepted that he had been wrong. He told old friends, who still clung to sweet optimistic dreams, that they were being foolishly unrealistic. Whatever military force meant, whatever the bomb meant, the anti-Hitler side must have it first.
That was the view, quite unqualified, of all who were not absolute pacifists (of whom in those scientific circles there were very few). It is desirable not to subtilize ethical attitudes after the event. There was no scientist or anyone else involved who didn’t believe that the work was necessary. That included Einstein and Bohr, who were among the loftiest and most benign spirits of our species. They don’t need to receive moral instructio
n from persons who did not live inside the situation.
The real impulse which led to the manufacture of the bomb came six months later. It was provided by two more refugees, Rudolf Peierls and Otto Frisch, the latter for the second time playing a decisive part. They were working in Oliphant’s physics department at Birmingham. With the death of Rutherford, the Cavendish stars had scattered all over Britain. At Birmingham Oliphant’s staff provided two of the major scientific contributions to the war. One was the invention by Randall and Boot of the cavity magnetron. This electronic device made it possible to generate intense, short-wavelength radio beams, which made the British radar far better than anything the Germans could achieve. It was the most valuable English scientific innovation in the Hitler war. The other was a paper of three pages, factual, succinct, accurately prophetic, by Peierls and Frisch.
They started with two acute clarifications. First, they accepted wholeheartedly what other physicists had been peculiarly hesitant about, namely the Bohr-Wheeler doctrine: it must be the isotope uranium-235 which had been disintegrated, and nothing else. Second, they were certain, knowing more of the latest chemical engineering than some of their colleagues, that it would be nothing like so difficult as had been generally assumed to separate this isotope from its natural intimate mixture with the far more abundant uranium-238, and produce uranium-235 in a relatively pure form.
From that, all else followed. It would need a certain amount of this isotope to set off cumulative disintegration, that is a chain reaction, which meant a nuclear bomb. They calculated what this amount would have to be, and came up with a startlingly small answer. It would need only about a kilogram (just over two pounds). This was called the critical mass. Smaller masses of uranium-235 are stable; larger amounts are not. To make a bomb, simply bring together two approximately equal parts, half a kilo each. As soon as they touch, the whole mass should explode with a force unequalled in human history. It was surprisingly simple. The reasoning was set down in about a thousand words and a few matter-of-fact calculations. It was convincing to anyone who could read scientific argument. It proved to be in all essentials correct. The estimates of quantities were just about right. The requirements for a fission bomb could be put in a couple of small suitcases. The concept of the bomb had been floating in the air. With those three typewritten pages, the practical manufacture got its first initiative.