By November 1938, Germany was in the grip of an anti-Semitic hysteria. In the national pogrom that became known as Kristallnacht, there were coordinated attacks on Jews, their businesses and places of worship, while tens of thousands of them were dispatched to concentration camps. In Berlin, festooned with swastikas, assaults were a spectator sport, respectable middle-class mothers on street corners holding up their babies to see the ‘fun’.7
On Monday 19 December, the city was in the grip of an Arctic cold snap. As midnight approached, the packed beer halls resounding with Christmas merrymaking, Otto Hahn was alone at his laboratory desk in the Kaiser Wilhelm Institute for Chemistry, contemplating a discovery that would soon shock scientists all over the world. Awaiting the return of his collaborator Fritz Strassmann, Hahn was describing his results in a letter to the Austrian-Jewish physicist Lise Meitner, a close colleague until only five months before, now exiled in Sweden. Two weeks later, Meitner was spending the Christmas vacation on the Swedish coast with her nephew Otto Frisch – a refugee from Nazi Germany working at Bohr’s Institute in Copenhagen – and she talked in detail about the new results described in Hahn’s letter. They were the first to understand that Hahn and Strassmann had observed the process that Frisch termed ‘nuclear fission’. According to Einstein’s equation E=mc2, the energy released by each cleaving of a nucleus would not deflect the path of a mosquito, but on the atomic scale the energy was huge. Every one of Hahn and Strassmann’s neutrons had less energy than a particle of visible light, but the energy released when one of them split a uranium nucleus was a billion times greater. With this level of return on energy investment, it might be possible – contrary to Rutherford’s dictum – for a nucleus to be a source of energy, rather than a sink.
Early in the New Year, Frisch returned to Copenhagen desperate to share his news with Bohr, who struck his forehead in disbelief: ‘Oh, what idiots we have been!’ he said. ‘Oh, but this is wonderful.’ Within a few days, Frisch had tested the explanation he and his aunt had devised and had found experimental evidence for it. A simple experiment showed that the energy released in the process really was as large as their calculations suggested. Everything added up, though for many scientists in the coming weeks the explanation seemed too far-fetched to be true.
A few days after speaking with Frisch about the new nuclear process, Bohr set sail for the United States, where Time magazine had recently named Hitler its Man of the Year. Bohr had arranged to work for a few months in Princeton, an hour’s train ride from New York, at the Institute for Advanced Study, where colleagues hoped he might collaborate with Einstein on the challenges of interpreting quantum theory.8 But those plans came to nothing: Bohr’s top priority now was to understand the fission process by thinking of nuclei as drops of liquid, an approach he and others had earlier pioneered. By virtue of his brains and charisma, he would soon become a leading player in the drama of fission, whose next act was to be set in the United States, which had now eclipsed the centuries-old dominance of Europe in physics.9
Bohr was too absent-minded to be a reliable keeper of secrets. In Copenhagen, he had promised not to divulge the Frisch–Meitner theory of nuclear fission until Nature published it, but he discussed it in detail with his close colleague Léon Rosenfeld during their voyage to New York. Within a few hours of arriving in freezing Princeton on 16 January, Rosenfeld shared the news at an evening gathering of scientists, who could scarcely contain their excitement.10 By the next morning, the news had spread via phone to hundreds of confidants.11 After Bohr had seen a copy of the fission paper in print, he agreed to talk about the theory ten days later at a meeting in Washington DC in an unscheduled presentation with Enrico Fermi, who had arrived from Europe shortly before, having fled Fascist Italy with his family.
This first public announcement of the discovery of fission was anything but polished – Fermi and Bohr scribbled hieroglyphics on a blackboard and struggled to explain the concept clearly in English. Afterwards, the New York Times reporter William Laurence rushed up to the speakers and blurted out: ‘Does this add up to an atomic bomb?’ Startled, Bohr looked up to the ceiling. Fermi, no less surprised, looked quizzical and after a long pause conceded that the Bomb may be twenty-five or even fifty years away.12 The physicists in the audience left the lecture theatre excited and ready to spread the word, but Laurence – believing it a ‘foregone conclusion’ that the Nazis would build the Bomb first – went home a ‘frightened man’. He might well have been gratified had he known that, within a week, a schematic diagram of a nuclear bomb would be on the blackboard of one of America’s leading theoreticians, Robert Oppenheimer.13
Since Bohr heard about the discovery of X-rays as a ten-year-old, he had hardly ever seen an experiment cause this much excitement. The American press did it full justice. Newspapers covered the initial announcement and, within a week, the New York Times pointed out that Bohr’s mentor Lord Rutherford may have been wrong to write off the idea of harnessing nuclear energy. ‘Romancers have a legitimate excuse’, the editorial declared, ‘for returning to Wellsian utopias.’14 It was, however, easy to miss such topics among the saturation coverage of America’s battered economy. Roosevelt had been re-elected by a landslide two years before but, struggling to secure a return to national prosperity, was now losing popularity.15
Nuclear fission also featured on the radio, which carried a special interview with Fermi, billed as the most recent Nobel Prize-winning physicist and a welcome new immigrant.16 In the Groundhog Day edition of CBS’s popular series Adventures in Science, the presenter Watson Davies could barely contain his excitement in his introduction: ‘The world may be on the brink of the release of atomic power.’ Fermi refused to play along – he spent the interview carefully considering his host’s optimism before finally deflating it.17 Inevitably, the press soon pointed out that nuclear bombs were no longer quite as unlikely as they had been led to believe. In the US, the suggestion first appeared on 11 February 1939 in the article ‘Atomic Bombardiers’ in the weekly Science News Letter.18 It warned that ‘scientists are anxious that there be no public alarm’ – it was best to ignore the ‘wild speculations’ of H. G. Wells, J. B. Priestley and the writers of the play Wings Over Europe: the world was not about to be ‘blown to bits’.19
The first new experimental insight into what was going on in nuclear fission came not from Fermi and his colleagues, but from a laboratory in Paris. Frédéric Joliot-Curie and his colleagues demonstrated that when a uranium nucleus fissions, a few neutrons are typically released at the same time, like a coconut splitting cleanly in two, but shedding a few splinters. No one understood the implications of this better than Leó Szilárd. He had finally settled in the United States and talked his way into a position at Columbia University alongside the steadier, more focused, more practical Fermi.20 Szilárd later said that when he heard of Joliot-Curie’s results, he saw straight away that a chain reaction might well be possible in uranium, with the neutrons from the fissioned nucleus possibly going on to split another one, releasing more neutrons that could split even more nuclei, and so on, rather like breeding rabbits. Huge amounts of nuclear energy would then be released. Szilárd later recalled: ‘All the things which H. G. Wells predicted appeared suddenly real to me.’21
Fermi began trying to set up a nuclear chain reaction, with the often unwelcome participation of Szilárd, who declared that experiments were not for him and hired a young scientist to do the work for him. It turned out that a chain reaction was most likely if uranium nuclei were bombarded by slow neutrons, much slower than the ones initially produced by fission. Fermi and his group – like other colleagues in Europe investigating these reactions – looked into ways of decelerating the neutrons, for example by making them collide with ‘moderators’ such as heavy water or graphite, whose molecules helped to slow the particles down. Heavy water was in extremely short supply in the US, so Fermi concentrated on the other option. Unlike his backseat driver, he was sceptical that the experiments woul
d soon yield useful results. Scared that the Nazis would now be able to develop a nuclear weapon, Szilárd began to campaign for nuclear fission researchers to keep their work under wraps (‘I invented secrecy,’ he later claimed22). He and two colleagues pleaded with physicists in Europe, urging them to self-censor their work on fission, but they were given a dusty response. Nor was Fermi going to abandon any time soon the centuries-old scientific tradition of publishing results at the first opportunity. He replied to Szilárd: ‘Nuts!’23
One morning in early February, during a five-minute walk across Princeton’s snow-carpeted campus, Bohr had one of his brainwaves.24 The upshot of the idea, worked out in detail with the American theorist John Wheeler, was that Hahn and Strassmann had engineered the fission not of all uranium nuclei but only of very rare ones containing 235 nuclear particles (each denoted 235U). Almost all of the nuclei in their uranium target – well over ninety-nine per cent – contained 238 particles (238U) and underwent no fission at all. Though widely disputed, if this reasoning was correct and a nuclear bomb were built from ordinary pure uranium, very few of its nuclei would fission and release energy, making it the dampest of squibs.
That winter, Bohr and Szilárd chewed over the possibilities of setting up nuclear chain reactions.25 The sight of these two loquacious men, both poor listeners, trying to converse in a foreign language, must have been richly comic: the ruminative Dane weighing up the dozens of possibilities; the Hungarian wanting to drive home only one. For both men, Wednesday 15 March was a red-letter day. The news that morning was terrible: the Nazi and Hungarian armies had marched into Czechoslovakia and ‘snuffed out’ its government, as the New York Times reported on its front page.26 Hitler pronounced the Munich Agreement dead, ending virtually all hopes of appeasing him, so it now required no imagination at all to visualise Fascist armies controlling every city on the European continent.
Soon after Bohr and Szilárd heard the reports, they met in Einstein’s office with John Wheeler and the theoretical physicist Eugene Wigner, a refugee from Hungary (Einstein himself was out of town). For hours, they talked over the possibility that nuclear fission could lead to the manufacture of enormously destructive bombs.27 Bohr and Wheeler explained that, if their theory was correct and only 235U nuclei undergo fission, nuclear bombs were simply not viable. ‘Then separate out the 235U’, Szilárd shot back, ‘and use it to make atomic bombs.’ Bohr told him he had nothing to worry about. To isolate enough of the extremely rare 235U nuclei would be a colossal industrial undertaking, requiring ‘the efforts of an entire nation’. Such an outcome was, Bohr assured him, ‘inconceivable’.28
When Bohr left the United States in early May and sailed for Europe, the ideas he and Wheeler had set out were still not widely accepted, though the consensus was heading in their direction. Bohr stood by his theory and was convinced that nuclear-fission bombs were not feasible. That stance was under fire again in early June, when Otto Hahn’s assistant Siegfried Flügge published the article ‘Can Nuclear Energy be Utilised for Practical Purposes?’ in the German journal Die Naturwissenschaften, the first of a few papers on the subject.29 He wrote that ‘the energy liberation should . . . assume the form of an exceedingly violent explosion’. Scientists soon began to take seriously the remote possibility that a fission bomb could be built and, later that month, virtually all scientists outside Germany and Italy accepted an unwritten voluntary agreement, unprecedented in fundamental physics, to self-censor new research on nuclear fission.30
Later that month, when Bohr visited physicists in Britain – including his friends at the Cavendish and G. P. Thomson at Imperial College – they almost certainly talked over the possibility of nuclear weapons.31 The implications were clear from a glance at the newspapers, full of anxious reports on the increasing political tensions in Europe.32 The bookshops were now selling Churchill’s new book Step by Step, a collection of his speeches, many of them warning of the dangers of appeasement and spelling out the need for Britain to rearm swiftly and improve its air defences. Now apparently vindicated, Churchill had won admiring reviews, The Times commenting that he ‘is clearly entitled to his triumph’ after his stance on rearmament.33
In the United States, concerns over the possibility of nuclear weapons were growing among the cognoscenti. An editorial in the July edition of Scientific American, entitled ‘Incomparable Promise or Awful Threat?’, pondered the question currently ‘worrying the physicists’.34 Should they terminate their fission experiments before the science is used to destructive ends? That would be ‘merely to abandon them to . . . war makers and conquerors’. The article ends by pointing out that the physicist is only a minor player in the great scheme of human events:
He cannot control his own discoveries, once they are given out, for he is far outnumbered. And if the human race won’t leave its new playthings alone, and gets badly hurt, that’s its own funeral. In a few years we may have the answer.
Predictably, Szilárd was now taking matters into his own hands, and considering how to take his warning to the White House. He was right to be alarmed. The Third Reich had become the first government in the world to think seriously about harnessing nuclear energy. In Berlin, several of its top military officials and a few civilian scientists, including Otto Hahn, met in secret to consider whether fission might have opened the way to nuclear bombs.35 Bohr knew nothing about the Nazis’ initiative and, even if he had, he would probably have been unperturbed, as he was still convinced it would take many years before nuclear weapons could be built. As he said in a public lecture soon afterwards, there was ‘no cause for alarm’.36
By mid-July, when Bohr and his family began their vacation at their summer home at Tisvilde on the Danish coast, Europe was on the edge of catastrophe. While the Bohrs were away, the news became even more alarming, culminating in Hitler and Stalin’s non-aggression pact, which also delineated their spheres of influence in north-eastern Europe. War now seemed inevitable.
Back in Copenhagen on 1 September, Bohr heard that war had begun after the German army had crossed the border into Poland. It was now only a matter of time before Britain and its Empire entered the conflict. A day later, in the Danish capital, the newspaper Politiken reported that Neville Chamberlain was taking steps to include in his War Cabinet a politician many believed had been out of government for too long: Winston Churchill.37
2
WORLD WAR II
AUGUST TO DECEMBER 1939
Churchill – nuclear weapons will not be ready for the war
‘I expect Lindemann’s view is right, i.e. that there is no immediate danger [that nuclear weapons will be developed], although undoubtedly the human race is crawling nearer to the point when it will be able to destroy itself completely.’
WINSTON CHURCHILL, 13 August 19391
On the first day of the war, the opening words of the editorial published that morning in Discovery magazine did not make reassuring reading: ‘Some physicists think that, within a few months, science will have produced for military use an explosive a million times more violent than dynamite.’2 The editor, C. P. Snow, was reflecting on the magazine’s article ‘Energy from Matter’ by the science-fiction specialist Douglas Mayer, about the latest nuclear-fission research. Snow concluded with a question: ‘Shall we have a Wellsian chaos with each nation dropping bouquets of uranium bombs in a policy of encirclement?’
Snow warned that ‘the power of most scientific weapons has been consistently exaggerated but it would be difficult to exaggerate this’. Laboratories in the United States, Germany, France and England ‘have been working on it feverishly since the spring’, he wrote, and President Roosevelt was well aware of the new developments. Snow’s conclusion was ominous:3
If it is not made in America this year, it may be next year in Germany. There is no ethical problem; if the invention is not prevented by physical laws, it will certainly be carried out somewhere in the world. It is better, at any rate, that America should have six months’ start . . . Suc
h an invention will never be kept secret . . . within a year every big laboratory on earth would have come to the same result. For a short time, perhaps, the US government may have this power entrusted to it; but soon after it will be in less civilised hands.
Such a conclusion would normally trigger a full-blown panic in the press, but it made no impact at all – on that day the country’s eyes were focused solely on the unfolding crisis in Europe. On the morning of 1 September, Churchill was chomping at the bit, refreshed after a working vacation in France.4 He did not have long to wait to get involved in the war effort – that afternoon he was summoned to 10 Downing Street, and a few hours later Neville Chamberlain offered him a place in the War Cabinet he was assembling, expecting hostilities to begin at any moment.5 London was on a war footing – sandbags on the streets, children evacuated, houses and streets blacked out in expectation of an aerial onslaught that the Home Office had predicted would lead to 2.5 million casualties in ten weeks.6 At 11 a.m. the following Sunday, Britain declared war on Germany, Chamberlain announcing his decision in an uninspiring radio broadcast. After the Commons met later that afternoon, Chamberlain summoned Churchill again and this time offered him the Admiralty, a job he had held during the First World War. ‘That’s a lot better than I thought,’ Churchill told Clemmie.7
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