In the 1930s these experiments were going on in several places in Europe and America. The scientists carrying them out had several characteristics in common. Most of them were young for people who were advancing the frontiers of knowledge so dramatically, many in their twenties and early thirties. Most of them knew each other: they had passed through the same universities and attended the same international conferences. Sharing the same excitement about new discoveries that few others could comprehend, many of them became close friends, and would remain so for the rest of their lives. Also, a large proportion of them were Jewish, products of a humanistic, central European cultural background that fostered both intellectual interests and humane social concerns, and these were to leave the Continent with the rise of Nazism, mostly for Britain and America.
The analysis of one particular experiment provided a puzzle. Enrico Fermi in Rome had been bombarding minute quantities of elements with neutrons. This would knock away one or two particles to produce a slightly different substance. But when he and then Irene Curie in Paris did this with the heaviest element, uranium, two German chemists, Otto Hahn and Fritz Strassman, analysed the results and, in 1938, they found traces of barium apparently resulting from it. Now barium is not a slightly different kind of substance from uranium, but a very different kind. It weighs only about half as much; its atom has 137 particles, while a uranium atom has 238. A neutron cannot knock 101 particles out of an atom. Hahn and Strassman published their findings, and left it to others to provide an explanation.
Hahn also wrote about his findings to a former colleague of his, Lise Meitner, an Austrian and one of the few women to distinguish herself in the field of physics. She had to leave Germany because she was Jewish, and was now at the Nobel Institute in Stockholm. Hahn’s letter arrived at Christmastime 1938, when Dr Meitner was entertaining her nephew, Otto Frisch. He was also a physicist, and had worked in Germany until the advent of Hitler, and was now at the Niels Bohr Institute in Copenhagen.
The two pondered Hahn’s letter together during a long walk through the snow-covered countryside, and then Lise Meitner had an idea. She suggested that a uranium atom might be unstable in such a way that if a neutron were injected, it would split up into two roughly equal parts. They sat down on a log and tested the idea on the spot by working out the mathematics, and it seemed to come out right. Then Frisch designed and carried out an experiment to test the theory, and this seemed to confirm it.
They collaborated on a paper setting out this idea, and finished it in a series of telephone conversations when Frisch was back in Copenhagen. At one point Frisch asked an American biologist at the Bohr Institute what biologists call it when a cell divides spontaneously, and he said the word was ‘fission’. So Frisch described the splitting of the uranium atom as ‘atomic fission’, and he used this term in the paper, which was published in the British scientific journal Nature in February 1939.
Several physicists immediately spotted a possibility that was not mentioned in this paper. A uranium atom contains a lot of neutrons, and some of these would go flying off when the atom is split and split other uranium atoms in turn, in a chain reaction. When particles leave an atom, energy is released. The cumulative effect of a chain reaction could be a release of energy so rapid, and so great, that it would constitute a very powerful explosion.
Until this moment, atomic physics had been the most abstract of sciences, far removed from any practical application. Now, suddenly, it seemed that there might after all be a practical application. The US Government set up an Advisory Committee on Uranium to look into the possibility of a uranium bomb, and individual scientists explored the idea and carried out experiments.
In the summer of 1939 Otto Frisch was offered a teaching post at Birmingham University. He accepted, partly in order to place some more distance between himself and the Nazi regime, and he arrived in England just a few days before the outbreak of war. Peierls, an old friend and fellow adventurer on the frontiers of physics, was already at Birmingham. Frisch, a bachelor, stayed for a while in an uncomfortable boarding-house; then, as Peierls and his wife Eugenia had rented a large, three-storey house near the university, they invited him to move in with them, and he lived there for several months.
Frisch, having set off speculation about the possibility of a bomb that worked by uranium fission, examined the problem further. It was becoming clear that only one kind of uranium atom, which made up less than one per cent of the total, actually split. He concluded that this would make a fission bomb — or a super-bomb, as it was coming to be called — impossible. He was asked to write a contribution on advances in this area for the Annual Report of the Chemical Society, and he reported this in his paper. Peierls examined the possibility and came to the same conclusion. This was now the prevailing view in Britain and America, and the considered opinion of the US Government’s Uranium Committee: that a super-bomb was either impossible or else so very distant that there was no point in devoting resources to the prospect at present.
However, Frisch and Peierls found that their minds were still working on the subject, and would not stop. They talked about it in the evenings in the Peierls’ living-room.
The atom that fissions is an isotope of uranium called uranium 235. An isotope is a variant of an atom that is chemically indistinguishable, but has slightly more or slightly fewer neutrons. This particular isotope has three fewer neutrons — 235 particles altogether instead of 238; hence the number, u-235. What, Frisch asked, if you could isolate a quantity of uranium 235? Could it be done? He and Peierls worked out the mathematics of a u-235 chain reaction, and found to their surprise that the amount needed to create an explosion was not a few tons, as they had thought, but a few pounds. And they decided that it would be possible to isolate this amount of uranium 235. Frisch wrote later: ‘At that point, we stared at each other, and realized that an atomic bomb might after all be possible.’
They set out their reasoning and their calculations in a memorandum. The bomb would be made with uranium 235, and they made a suggestion as to how this could be separated from ordinary uranium. They calculated the critical size, the size of the mass of u-235 which would explode (underestimating it). They described the mechanism for exploding the bomb: two pieces of u-235 which together added up to the critical mass would be brought together with great speed. They described its effects, including radiation. Their key sentence was: ‘We have… come to the conclusion that a moderate amount of u-235 would indeed constitute an extremely efficient explosive.’ This memorandum is the first suggestion of how an atomic bomb could be built, and it is one of the historic documents of the twentieth century.
They took it to Mark Oliphant, a senior physicist at the university who was working on radar and was therefore in touch with the defence authorities. He fed it into the governmental machine. The result was the setting up of a committee, christened the Maud Committee, in April 1940 to explore the possibility of producing either power or explosives from nuclear fission. It came under the Ministry of Aircraft Production. Work was farmed out to scientists at Oxford, Cambridge and Liverpool Universities. Peierls and Frisch continued their work at Birmingham, but in the summer of 1940 Frisch moved to Liverpool, to work under James Chadwick, using Britain’s only cyclotron. Many of the scientists involved in this work were refugees from the Continent, simply because most British-born physicists were already working in military technology, mostly to do with radar.