The Physicists Part 2

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 JoliotCuries a.s.suaged 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 trans.m.u.ted 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 Inst.i.tute in Berlin. Not surprisingly, since Fermi and his colleagues were first-cla.s.s 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 Inst.i.tute. 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 Gteborg, that she entertained her nephew, Otto Frisch, during the Christmas of 1938. Frisch was another high-cla.s.s physicist and another refugee who had found sanctuary in Copenhagen. He arrived in Gteborg late at night, and didn't see his aunt until the following morning.

They were an affectionate couple. Both were suffering exile and hards.h.i.+p. 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 a.s.sumption. 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 a.s.sumption with them unquestioned. Now they alone, of all the physicists in the world, woke up to that a.s.sumption, 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 const.i.tuents. The a.n.a.logy 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 MeitnerFrisch (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 cla.s.sical 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 acc.u.mulated in quant.i.ty, 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 BohrWheeler 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 quant.i.ties 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 acc.u.mulate even a few grams of uranium-235. No one then knew how much was needed to make a bomb. But the guess was a quant.i.ty 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 s.p.a.ce 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 n.o.bel 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 cla.s.s, and Wigner was already tipped for the n.o.bel, 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, pa.s.sionate, a man of the right (though more complex in his att.i.tudes than popular accounts later suggested). Szilard was a man of the left, so far as he could be cla.s.sified 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 entree 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 n.o.bel 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 n.a.z.i 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 att.i.tudes 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 instruction 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 a.s.sumed 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 c.u.mulative 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 ma.s.s. Smaller ma.s.ses 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 ma.s.s 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 quant.i.ties 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.

It would require an immense industrial development. It was one thing to talk of separating the isotope on this scale, but a formidable job to do it. Britain might just conceivably have been able to try, in peace-time. But the country was at war, and still had to survive: which meant that an abnormal proportion of its resources had to be spent on radar, a device not only sensible but vital, and on bombing aircraft, which was not so sensible.

America was still not in the war. It took some time for the Peierls-Frisch memorandum to reach American scientists. It was carried over the Atlantic in August 1940 by c.o.c.kcroft, who talked in his quiet uninflected manner to American nuclear scientists. A good many were working on uranium projects, but there was not the urgency that was driving British scientists, who, in the good old Johnsonian mood, had their minds concentrated by the prospect of being hanged tomorrow. But it didn't take long for the Americans to be convinced that the uranium-235 bomb was feasible.

And American physicists had just discovered another isotope which could be used in a fission bomb. This was not an isotope of uranium. Edwin McMillan, with colleagues Philip Abelson and Glenn Seaborg, had achieved what Fermi thought he had done they had produced trans-uranic elements. Not surprisingly, these had unstable, radioactive nuclei. The new element beyond uranium was called neptunium because the planet Neptune is beyond Ura.n.u.s in the solar system. But it was the next element that was bomb material. Called inevitably plutonium, element number 94 has an isotope, plutonium-239, which can sustain a chain reaction of disintegrations. Making a plutonium bomb is not so much a question of separating isotopes, but of making sizeable quant.i.ties of an element that does not occur in nature. This, however, need not be any more difficult than separating the uranium isotopes; the plutonium bomb had strong advocates, too.

The Einstein letter hadn't produced much in the way of action. Now the entire US governmental scientific machine began to get to work. American energy was set free in its impressive abundance. The project was codenamed Manhattan. There was, of course, an element of fright communicated by the British. The PeierlsFrisch argument was only too convincing. It now seemed odds on that atomic bombs were makeable. What were the n.a.z.is doing?

The Manhattan project was a feat of technology and scientific administration. As has been said, the essential science had been done earlier. This was application on a gigantic scale. There were, in fact, scientific snags along the way, and plenty of puzzles on the frontiers of science and engineering. A number of the best scientists alive showed considerable versatility in attacking problems utterly different from anything they had met in an academic department. Fermi, who was able to apply his mind to almost anything on his death-bed he wished that he had given a little thought to politics was prepared to invent devices of extreme sophistication and occasionally, in an un-American fas.h.i.+on, of childlike simplicity. By common consent, he was the most valuable man around. But many others displayed talents which no one, including themselves, imagined that they possessed. At Los Alamos in New Mexico, which was the brain centre of the project, they lived a life remote but intense, certain that the job was imperative, not worried (such worry is swept away in war) by consequences. It was exciting to be living near a peak of technical achievement.

The chief scientific administrator, Robert Oppenheimer, was one of the most interesting figures in world science. Among a ma.s.s of very clever men, he was probably the cleverest. He was highly cultivated in the arts, and had an admirably organized and structured mind. He had genuine scientific talent, and could talk on equal terms with the greatest scientists in the place. Bohr, who was finally evacuated from Denmark via Sweden to London and Los Alamos, at the risk of his life, had a very high opinion of Oppenheimer's scientific gift. So had Rabi, the least soft of touches.

The curious thing was that Oppenheimer had no great scientific achievement to his name. This is hard to explain. He had lived through a period in which men with a tenth of his talent had made major discoveries. He was scientifically ambitious and would have liked real creative success more than anything in the world. He became a great figure: the achievement of Los Alamos made him famous and he deserved the fame. Nevertheless, one suspects he would have given all that away if he could have exchanged it for one single piece of work of the cla.s.s, say, of Pauli's Exclusion Principle. There was his tragedy, probably much more deeply wounding than the political misfortunes which later happened to him.

There were some other strangenesses about the population of Los Alamos. A high proportion were refugees, recent immigrants who had had time to be rapidly naturalized. This was partly, of course, because they included some of the best pract.i.tioners on earth: but there was another reason. Most native scientists, in America and even more in Britain, had been swept up in work which appeared, and was, more directly concerned with the Hitler war. For example, c.o.c.kcroft, who would have been peculiarly valuable at Los Alamos, was head of an English radar establishment. (He had to be extracted later to lead the British nuclear team in Canada.) Rabi was immersed in similar activities at the Ma.s.sachusetts Inst.i.tute of Technology, and so on for dozens of the top American and English nuclear scientists. Refugees were the main source of the available manpower of high cla.s.s. This may or may not have made a marginal difference when it came to disputes about the long-term political future of what they were doing. Refugees sometimes felt constrained. They wanted to accept the country which they hoped to make their own.

The dominance of refugees had some more farcical concomitants. Security procedures were thrown into a frenzy and at times displayed their dottier aspects. For example, Peierls and Frisch were never given places on Maud, the small British committee responsible for work on the nuclear bomb and so weren't able, in official terms, to discuss and explain their own work.

It was Fermi who took the first step into the nuclear age. Although no one now had any doubt that the bomb was possible, it was important to test that chain reactions could take place. Physicists needed to monitor, to measure, a nuclear chain reaction that went leisurely. Fermi achieved this with naturally occurring uranium, where the overwhelming amount of stable uranium-238 would prevent an explosion. His earlier intuition that slow neutrons were best at instigating nuclear fission was vital. This time he used blocks of graphite to slow them. In a disused squash court at the University of Chicago he built an edifice from six tons of uranium, fifty tons of uranium oxide and four hundred tons of graphite blocks: he called it a 'pile', because it was literally that. But in present-day terms it was the first nuclear reactor.

On 2 December 1942, Fermi withdrew the neutron-absorbing 'control rods'. The chain reaction began. Neutrons split the minority of uranium-235 nuclei; heat and more neutrons streamed from the disintegration. These neutrons shot out of the uranium block, but were slowed by the graphite, and so split more uranium-235 nuclei as they entered the next uranium block. Fermi's pile was not designed to produce nuclear power as such. It was a test. After making his measurements, Fermi took it apart again. Theoretically, at least, the path to the bomb was now clear.

Scientists at Los Alamos were certainly all confident that it would not be long before a bomb was ready. They had the euphoria of all concerned in an extraordinary enterprise. That was the overmastering emotion. Apprehensions about the putative n.a.z.i performance were lessening slightly though in official London the word still went round that the war was going well if we are safe from that which we mustn't talk about.

It would have horrified General Leslie Groves, the supreme administrator of the Manhattan project, to discover how badly his security system actually worked. It was nothing like so effective as security about the decoding techniques (the English called this process Ultra). Groves' iron rules certainly made communication between the people doing the job at times bizarrely complicated. Another result was to prevent any news of the operation reaching the Vice President of the United States and the Deputy Prime Minister of the United Kingdom: but a good deal of news reached hundreds of other people. This wasn't because of treachery or even gossip. Men like Groves underestimate the intelligence of their fellow citizens. Why were well-known scientists disappearing to unknown destinations? Why should Niels Bohr arrive in London and shortly afterwards get swallowed up in America? To scientists, it was all too obvious.

Niels Bohr was both unusually busy and unusually worried. After inspecting the diffusion plants where the uranium isotopes were separated, he had no doubt that the nuclear bomb was a certainty: and, what was more, not just a certainty for this war, but a feature of the world scene for ever. He was one of the most far-sighted of men, and he belonged to the world. He went to Los Alamos, anxious to help where he could, but deliberately not attaching himself formally to either the American or British contingents. He knew another certainty. It was taking America about four years to make the bomb: it wouldn't take long for the Soviet Union, or other industrialized societies with a strong enough purpose, to do the same. Nearly all scientists agreed. There are no secrets in science: and very few, and those short-lived, in technology.

From the moment it became known that the Americans were moving towards a fission bomb, the general guess was that it would take the Soviet Union perhaps five years to catch up some, more in touch with Soviet engineering physics, thought that was an overestimate. General Groves gave contemptuous snorts. He told his political masters that the United States had at least a twenty-year lead, probably much more. That was believed by those who wanted to believe, and produced some political dangers. General Groves was a singularly bad choice for his job.

Bohr, after characteristic reflection, decided that it was worth trying to avert or minimize the post-war perils which any sentient person could imagine. It would do no harm, and might do some good, to give the Soviet government an indication about the bomb. (We now know they were already informed. Bohr didn't know this, but he a.s.sumed that their scientists had made their own predictions from 1939 onwards, as had duly happened.) Even a tentative disclosure, Bohr thought, might make for international confidence.

Bohr revealed his thoughts to Halifax, the British Amba.s.sador in Was.h.i.+ngton, and received considerable sympathy, as he did from Felix Frankfurter. He was despatched to have a talk, with Churchill.

That encounter was one of the black comedies of the war. For some obscure reason, Churchill was strongly averse to seeing Bohr. It wasn't that he didn't come with the highest recommendations. Sir John Anderson, whom no one could think had pro-Soviet leanings, had already heard Bohr's case, and thought there was a lot in it. It perhaps wasn't irrelevant that Anderson had had a scientific education, had even done some research, and found it easy to believe the temporary nature of the Western lead. He also had great respect for Bohr. So presumably did Cherwell, who couldn't have been well disposed to the actual proposal, but knew all about Bohr and helped force the interview on Churchill. The President of the Royal Society had also insisted. After all, Bohr was one of the greatest men of the century.

After very long and discourteous delays, Bohr was granted a discourteous half-hour. It bore a resemblance, seen through a distorting mirror, to the meeting with Rutherford which got Bohr launched on his career. No doubt Bohr whispered conscientiously alone. This time, however, the other party wasn't prepared to listen, or apparently didn't trouble to understand what was being said. On the stroke of the half-hour Bohr was dismissed.

Bohr didn't suffer from offended dignity. But he was miserable. He had failed in what he believed to be his most important public mission.

Would Einstein have done better? Probably not, so far as the outcome went. There would have been a difference of tone. Einstein was not outfaced by any man alive, and there would have been some Jehovianic words spoken from his side of the table.

That meeting, if one can use an inappropriate word, took place in the summer of 1944, just before the invasion of Europe. As soon as the Anglo-American forces got a foothold in Germany, a mission was despatched to investigate what the German nuclear physicists had really been doing. The mission consisted of two excellent physicists, both originally Dutch, now American, Goudsmit and Uhlenbeck. Their report was pleasing but surprising. The German nuclear physicists had done remarkably little. As had been thought, Heisenberg had been in charge of a group, small but high-powered. The members were to be interrogated in England as soon as they could be tracked down. Anyway, that specific war-long anxiety was now wiped away.

So only the Americans, with their British affiliates, had been making the bomb. Bohr, nothing if not pertinacious, continued with his resolve. Brushed off by Churchill, he went back to American confidants, Frankfurter, Vannevar Bush (the first of presidential scientific advisers), J B Conant. They, too, had been trying to read the future, and were ready to support Bohr. It was arranged for him to explain his thoughts to Roosevelt.

There he got a very different response from Churchill's. It was warm, cordial, amiably sympathetic. With knowledge of what followed within three months, this now seems puzzling. It may have been just a politician's professional technique, but it appears more likely that the President was at least half impressed. He would, of course, have been carefully briefed by Bush and the others, and he had picked up more about Bohr himself than Churchill had. Churchill seems to have taken a violent personal dislike to Bohr about the only human being who ever did so.

It would be false to give the impression that the scientists at Los Alamos had any knowledge of these attempts to cope with the future. Bohr was much too punctilious and honourable to let slip any word of those discussions, though two or three of his senior colleagues, Fermi and Oppenheimer among them, had an intimation of what was being tried and agreed with it, though without much hope. Most of the Los Alamos population wouldn't have felt it as a personal concern. They knew that the project was soon going to succeed or fail. Failure was unthinkable, and yet some couldn't suppress the thought as the gigantic enterprise approached its climax.

The Manhattan project was now employing 500,000 people, directly and indirectly, and spending a billion dollars per year. The uranium isotopes were separated by two different processes at the beginning, no one knew which would be the more efficient. The first was a diffusion process. When the metal uranium reacts with fluorine, the compound formed uranium hexafluoride is a gas. A molecule containing uranium-238 is very slightly heavier than a molecule of uranium-235 hexafluoride, and as a result is slightly more sluggish. If uranium hexafluoride gas made from natural uranium which contains only 0.7 per cent uranium-235 is forced through a filter, the lighter uranium-235 will find it slightly easier to get through. So the gas on the far side is marginally enriched in the required isotope. Repeat the process, and the proportion of uranium-235 will rise a little more. To get 'weapons-grade' uranium containing 90 per cent of the rare isotope needed thousands of pa.s.ses through the filters. But, slow though it was, it was gradually acc.u.mulating the fissile material.

Running in parallel was separation by means of electric and magnetic fields. Uranium atoms were stripped of electrons in a vacuum. Now they were electrically charged, and they were susceptible to outside fields. Again the heavier uranium-238 was more sluggish, and uranium-235 could gradually be separated out.

And plutonium was now in 'commercial scale' production kilograms of a new element were being created. In huge reactors, uranium was bombarded by neutrons. The important isotope this time was the common uranium-238, which absorbed a neutron, then emitted two electrons from the nucleus and ended up as plutonium-239. With large quant.i.ties of plutonium to investigate, the scientists had found that it was indeed fissile something they had had to take on trust from the theoreticians at the beginning of the Manhattan project.

The bomb, or more exactly one uranium bomb and one plutonium bomb, should be ready by the late summer of 1945, a year ahead. There would be a test just before the bombs were despatched.

Thus very few at Los Alamos had any glimmer of the first results of Bohr's diplomacy. This was another piece of black comedy. Roosevelt and Churchill met at the second Quebec conference. Roosevelt surrendered without a struggle to Churchill's view of Bohr. He was on the verge of 'mortal crimes' an extraordinary Churchillian phrase. Churchill drew up his and Roosevelt's understanding. Nothing whatever about the project was to be communicated to anyone outside the circle of secrecy, certainly not to the French, above all not to the Russians. Bohr, and anyone under his influence, was to be kept under surveillance.

At one point Churchill was demanding that Bohr should be arrested. That was, however, too much for the President's advisers and Churchill's own, many of them shaken by this singular display. Possibly the only person who wasn't shaken was Admiral Leahy, who, with his habitual lack of judgement, was certain that the bomb would be a fiasco and wouldn't go off at all.

Why did Roosevelt and Churchill behave like that? Roosevelt was a sick man, and may not have felt capable of resisting Churchill in one of his obsessive nagging phases, prepared to go on grinding away in perpetuity. But Churchill? There has never been much of an explanation. He had always had a naive faith in 'secrets'. He had been told by the best authorities that this 'secret' wasn't keepable and that the Soviets would soon have the bomb themselves. Perhaps, with one of his surges of romantic optimism, he deluded himself into not believing it. He was only too conscious that British power, and his own, was now just a vestige. So long as the Americans and British had the bomb in sole possession, he could feel that that power hadn't altogether slipped away.

It is a sad story. Probably the result didn't make any real difference. Even if Bohr had prevailed, and there had been some attempt at international understanding, in practical terms everything would have gone on as it actually did in America, the Soviet Union, the United Kingdom, France, and in due course a good many other countries. There might have been a faint improvement in external civility, which is sometimes worth having. But the story remains a sad one, and something of a symbol.

Meanwhile the manufacture of the first bomb went on, the pace of sheer activity increasing. The Hitler war ended but there was no let-up. It was an illusion believed by many that there was a whole a.r.s.enal of bombs. That wasn't true for a long time. There was an a.s.sembly on a tower, not an actual bomb, for the test a plutonium device. Two bombs (one of each) which should be ready for use; one more plutonium bomb was in reserve. The rest consisted of threats.

By this time none of the scientists had doubts that the bomb would work: or at least no such doubt appeared in records or memoirs. Some political placemen, like the ineffable Leahy, added to their reputation for hard-bitten wisdom by continuing to regard the whole project as nonsense, a kind of long-haired hoax that wouldn't produce anything more lethal than a popgun.

Some of the scientists, though, had a different worry. They were sure that the bombs would be ready for use: but what would they be used for? Not many people seemed to have answered the question. The bombs had been made as an insurance against the n.a.z.is making them too and they hadn't needed to think further than that. Now the n.a.z.is were eliminated. A whisper spread that the American military were intending to use the bombs on j.a.pan.

Some of the American scientists had relatives in the forces who would be fighting if there was an invasion of j.a.pan. For them the ethical problem was simple: anything to get that war over. Just as the most charitable of Russians years later used to say, not lightly, that if they had possessed the bomb in early 1945, they would have dropped it on Berlin. They had lost too many men to have qualms. But most of the scientists were free to have such qualms. They hadn't access to diplomatic intelligence, which would have increased their misgivings. Still, it was enough to know that here was the climactic feat of applied science: it just couldn't be used for ma.s.s extermination without a thought. At the very least, there must be a demonstration. Warn the j.a.panese, drop a bomb in the sea. That would tell its own story. After that, consciences would be relatively clean.

Something like that was in fact proposed by Josef Franck, the leader of the Chicago group, another n.o.bel prize winner, a refugee from Gttingen and, like Born, another witness to the old German culture. He and half a dozen of his colleagues sent a statement to Was.h.i.+ngton. It had one vestige of a result. A small group of the Manhattan scientists were asked to give their opinion. This group consisted of Oppenheimer, Fermi, Ernest Lawrence, and the British n.o.bel prize winner A H Compton. They replied within a matter of days. Their opinions divided down the middle, two on each side. Oppenheimer and Fermi were in favour of dropping the bomb (actually the two bombs) without any preliminaries. Lawrence and Compton were against.

Probably nothing, or no representations from any man alive, could have stopped the bomb being used. As Einstein was to remark years later, there was a weird inevitability about it all.

The events that followed in July and August 1945 have often been described. The test at Alamogordo in the New Mexico desert went exactly according to expectation. If anything, the explosion was more powerful than predicted. It was one thing to have expectations, to believe in the certainties of reason: it was even more satisfactory to see them fulfilled as the most brilliant exhibition created by man. The scientists were jubilant, and they wouldn't have been human if they hadn't been. Fermi, with one of his Heath Robinson contrivances, was measuring blast by means of tin cans and pieces of paper. Someone more sardonic than the rest remarked that it was the most expensive dry run in scientific history.

The bombs were duly dropped. On 6 August Hiros.h.i.+ma was the target for the uranium-235 bomb; Nagasaki suffered the plutonium bomb three days later. Why was the second judged necessary? The question elicited comments, some cynical, some heart-wrung. There were utterances, in public and private, all over the physicists' world. The scientists have learned sin, said Oppenheimer. That was too rhetorical for what they truly felt. Many of them were searching for some effective action. Mark Oliphant not only made his speech about the death of a beautiful subject, but also was demanding that England and his own Australia should make the bomb themselves. Any country without it was helpless from now on. Others were campaigning for international control, as Bohr had urged on Churchill in their grotesque meeting.

However, those thoughts of August 1945 weren't to survive for very long. The future was to become not quite so apocalyptic. Physics hadn't been killed, and the beautiful subject stayed beautiful, though in forms as yet unimaginable. While applied physics, and the technology born out of it, was not to have ended with the bomb, but scarcely to have begun.

8: Nuclear Fusion.

WHEN the news of Hiros.h.i.+ma was first broadcast, a select a.s.sembly of German nuclear scientists (Heisenberg and Hahn amongst them) were in gentlemanly captivity in a Cambridges.h.i.+re country house. Their conversation was bugged. To begin with, they didn't believe the BBC report. This wasn't a fission bomb. It was some kind of bluff, designed to frighten the j.a.panese into making peace. After all, they, the Germans, hadn't found a way of making such a bomb. How could the Anglo-Americans have done so?

The mystery was the exact opposite. Why hadn't the Germans come nearer? The answer seems to be that, until late in the war, the German authorities, with whom decisions usually went much too high, often to Hitler himself, weren't prepared to devote resources to projects which wouldn't guarantee results within a couple of years. They wanted weapons for use next year, not in the dim future. Their engineering was still excellent, in many fields much better than that of the Anglo-Americans. They were producing the jet fighter, Me 262, by far the best fighter in the war, which didn't come into service until too late. Similarly with their final type of submarine. But they didn't expend any of that engineering skill on a nuclear bomb. That was too remote, and might as well be left to the scientists.

The scientists appear not to have had much access to high authority, or not much influence. Further, good as they were, as good as their counterparts in America, they didn't show themselves as flexible and adaptable. Apparently, though it seems inexplicable, they had no equivalent of the Peierls-Frisch calculations about the practicality of the bomb (the Germans were thinking in terms much more gigantesque). The German scientists didn't transform themselves into wartime engineers. They had no Fermi. If they had acquired him, it could have made a difference.

There gradually emerged a sweet romantic story, much to the credit of human nature, that the German scientists had deliberately held back. They wouldn't accept the moral responsibility of giving such bombs to a monstrous regime. It would be an intolerable crime. Better to pretend that the bomb wasn't feasible.

Well, it is a sweet story, but it happens to be utterly untrue. These were decent men: they were also dutiful men and, some of them, nationalistic Germans. Heisenberg had visited Bohr in occupied Copenhagen in 1941, and Bohr was certain that it was an attempt, not to inquire if Allied scientists had conscientious scruples, but whether they were setting about the job. From 1943 onwards, men as intelligent as Heisenberg knew that their country was fighting a desperate defensive war. If they lost, that was the end of Germany. Even under n.a.z.i rule, Germany was Germany. In comparable circ.u.mstances, American, English, Russian scientists would have felt that the evils of the regime counted for nothing against the evils of absolute defeat. They would have gone to the limit to make the bomb.

Nothing is known in the West of whether the Soviets had started their own nuclear project before the end of the war. As with the Germans, they were fighting a desperate war, and may not have been able to spare effort for longer-term enterprises. They certainly knew a good deal about what was happening in America. They had their legitimate sources of intelligence: and others, such as Klaus Fuchs, not so legitimate. When Stalin was told at Potsdam that the bomb was ready, it can't have come as a surprise.

Whether they had started before or not, they threw immense energy into catching up. The only genuine secret, as someone said, was that the bomb had been made. It wasn't hard, as Bohr and others had tried to impress upon the politicians years before, for another technological society to make it. Politicians, or some of them, still listened to General Groves and similar thinkers it would take a generation for the Soviets to possess their own bomb. It took four years. That figure had been about the average of the scientists' estimates.

The nuclear arms race was on. There was a sudden acceleration which made many thoughtful men lose what remained of their wits. It became likely that a different kind of nuclear bomb, many times more powerful than the fission bomb, could be developed. This was the hydrogen, or fusion, bomb.

In the Hiros.h.i.+ma and Nagasaki bombs, the heaviest of atomic nuclei uranium and plutonium broke up into smaller, more stable nuclei. The most stable of all nuclei, in fact, are intermediate-weight ones, like iron, which has 56 nuclear particles (26 protons and 30 neutrons). This means that one can take a different route to nuclear energy: join together 'fuse' the very lightest elements of all to make slightly heavier nuclei, and thereby generate energy. Before the war, astrophysicists had calculated that the sun and most other stars makes energy this way. At the sun's core, hydrogen nuclei (protons) get together in fours to make helium nuclei. The energy liberated is suns.h.i.+ne.

But it is also possible to release the energy of hydrogen fusion explosively, and it is far more efficient than the fission of uranium or plutonium. Should a hydrogen bomb be made, it would be a thousand times more powerful than the fission bomb. Such a bomb could annihilate the largest of cities, London, Chicago, Moscow. It would be the ultimate weapon.

Could it be made? Should it be made?