The First War of Physics (4 page)

Placzek, having challenged Frisch in Copenhagen to go look for the telltale signature of fission, now found himself sitting at breakfast with Bohr and Rosenfeld in Princeton. ‘What kind of crazy thing is this big [rate] for both fast and slow?’ he exclaimed.

Bohr figured out the answer while walking from breakfast back to his office. The high rate of fission with slow neutrons must, he reasoned, be due to the rare isotope U-235, which makes up only a tiny proportion of naturally-occurring uranium. Bohr and Wheeler now worked out the details. There were two factors at play.

The balance between the force of repulsion of the protons in the uranium nucleus and the surface tension holding the nucleus together is much more delicate in U-235 than in U-238. The three extra neutrons in U-238 help to stabilise the nucleus, thereby increasing the barrier to fission. Faster, higher-energy neutrons are therefore required to get over the hurdle in U-238.

The second factor concerns the nature of the compound nucleus itself. Atomic nuclei exhibit a general preference for even numbers of protons and neutrons, behaviour that can be traced back to the quantum nature of their sub-atomic constituents. Adding an extra neutron to U-235 makes U-236, containing 92 protons and 144 neutrons, both even numbers. Adding an extra neutron to U-238 makes U-239, which contains an odd number of neutrons. This means that U-235 ‘accommodates’ an additional neutron, and so reacts more readily with it, than does U-238.

These two factors combined are enough to account for the significant difference in behaviour of the two uranium isotopes. To fission a U-238 nucleus requires fast neutrons. The much more vulnerable U-235 nucleus can be split with slow neutrons. This meant that in a bomb consisting of a mixture of U-235 and U-238 which relied on slow-neutron fission of U-235, the result would be a slow chain reaction. An atomic bomb based on a slow chain reaction would fizzle out long before it could explode.

The immediate prospect for a bomb diminished, though it did not disappear entirely. Of course, Bohr declared in discussions with his colleagues in April 1939, it
might
be possible to manufacture a bomb based on pure U-235. But this was a minor isotope, present in naturally-occurring uranium to only one part in 140, a miserly 0.7 per cent. Isotopes are chemically identical and so cannot be separated by chemical means. Physical separation would be required, relying on the tiny difference in the masses of the isotopes. Such physical separation on the scale required to build an atomic bomb – a scale presumed at this stage to be measured in tons – appeared profoundly impractical.

‘Yes, it would be possible to make a bomb,’ Bohr declared, ‘but it would take the entire efforts of a nation to do it.’

Hungarian conspiracy

There was no denying the novelty of atomic energy and the unprecedented scale of energy release promised by nuclear fission. Under any circumstances, the discovery of fission would have provoked interest among not just scientists, but also governments, the military establishment and business enterprises. But these were not any circumstances. Only a few short months had passed since Frisch and Meitner’s discovery, made as they scribbled their calculations sitting on a tree trunk in a wood near Kungälv, yet this was time enough to found a whole new physics. And this new physics was being elucidated just as preparations for war in Europe were taking shape.

French physicists Frédéric Joliot-Curie, Hans von Halban and Lew Kowarski
⁹
summarised the evidence for nuclear chain reactions in uranium in
Nature
on 19 March. Their subsequent report specifying the number of secondary neutrons released in each nuclear fission event appeared in the same journal on 22 April.
¹⁰
These publications provoked a number of breathless accounts in the popular press of an impending ‘super-bomb’. They also caused a flurry of activity in government ministries.

At a hastily-convened conference held on 29 April 1939, Abraham Esau, president of the Reich Bureau of Standards and head of the physics section of the Reich Research Council, recommended the establishment of a uranium research project under his leadership. He gathered a number of leading German nuclear physicists together, referring to them as the
Uranverein
(Uranium Club). He charged the Uranverein with the task of investigating the potential for atomic energy, urging that they at once secure all stocks of uranium in Germany and ban all future exports.

A few days earlier, on 24 April, young Hamburg chemist Paul Harteck and his assistant Wilhelm Groth had written to the German War Office, urging the military to take note of the new developments in nuclear physics. They wrote:

We take the liberty of calling your attention to the newest development in nuclear physics, which, in our opinion, will probably make it possible to produce an explosive many orders of magnitude more powerful than the conventional ones … That country which first makes use of it has an unsurpassable advantage over the others.

The letter was passed to the German Army Weapons Bureau, which subsequently initiated its own, rival, uranium research project under the leadership of physicist Kurt Diebner.

In Britain, Member of Parliament Winston Churchill grew increasingly alarmed by the summer’s press accounts of a possible ‘super bomb’. His concern was not that such weapons might be built first by Nazi scientists; it was rather that Hitler might seek to use the threat of a new secret weapon in his bargaining with Prime Minister Neville Chamberlain.

Churchill sought advice from his trusted scientific adviser and friend, Oxford physicist Frederick Lindemann, known to Churchill fondly as ‘the Prof’. Lindemann had visited him often in the period 1931–34, during his ‘wilderness years’ at Chartwell, the Churchill family home. Drawing on Lindemann’s advice, he wrote to Sir Kingsley Wood, Secretary of State for Air, on 5 August 1939 to advise him that such weapons would not be available for several years.

Nuclear physics research in the Soviet Union was conducted primarily (though not exclusively) at the Leningrad Physicotechnical Institute, commonly known as Fiztekh, which had been established in the early 1930s. The energetic and expressive physicist Igor Kurchatov headed the Institute’s nuclear department. Although the Soviet Union had many highly talented scientists, it had become increasingly difficult to maintain research programmes not directly supportive of the country’s efforts to achieve rapid industrialisation, and nuclear physics in particular was not seen to have any practical application. Even worse, as Stalin’s regime had become more isolated following the Great Purge of 1937–38 (in which 100 Soviet physicists are believed to have been among the seven to eight million citizens arrested), virtually all contact with Western nuclear scientists had ceased.

Soviet physicists had little choice but to follow developments through the pages of Western science journals. Once read, the new discoveries relating to fission were quickly repeated and extended in the Soviet Union. Kurchatov directed his colleagues Georgei Flerov and Lev Rusinov to measure the number of secondary neutrons formed by the fission of uranium. By 10 April they had confirmed that between two and four secondary neutrons are produced, thereby independently confirming the possibility of nuclear chain reactions. By June, Flerov and Rusinov had also indirectly confirmed Bohr’s suggestion that U-235 is primarily responsible for fission in uranium. At this stage the physicists did not see fit to bring these discoveries to the attention of the Soviet government or military authorities, or alert them to the potential for an atomic threat.

When Hitler had been appointed Chancellor in January 1933, Hungarian physicist Leo Szilard had packed his belongings into two suitcases, and
made ready to leave Berlin at a moment’s notice, ‘when things got too bad’. That moment arrived just a few months later. Szilard moved first to Vienna, then to London where he helped Lindemann to establish a fund to bring exiled scientists to Britain. He spent several years in London and Oxford anticipating the development of nuclear fission, chain reactions and atomic bombs,
¹¹
before emigrating to the US in early 1938. On hearing of the discovery of fission in uranium in January 1939, he borrowed $2,000 from a successful American inventor and persuaded the chairman of the physics department at Columbia University – where Fermi had recently arrived – to provide laboratory facilities.

Working with Fermi he independently verified the production of secondary neutrons and the possibility of a nuclear chain reaction, thus realising his worst fears. He had been thinking about the potential for releasing atomic energy for many years, and now confronted a real fear that Nazi Germany might be the first to build an atomic weapon. He sought commitments from his scientific colleagues to refrain from publishing their results openly in the scientific literature. Joliot-Curie refused.

Szilard voiced his growing concerns to fellow Hungarian émigré physicists Eugene Wigner and Edward Teller. Wigner had arrived in Princeton from Berlin in October 1930 to take up a temporary lectureship, which had become a permanent position in 1935. He had spent a couple of years at the University of Wisconsin before returning to Princeton in June 1938. Teller had left Göttingen in Germany first for Copenhagen, then University College London, then George Washington University in Washington, DC, where he had arrived with his new wife Mici in August 1935.

All three members of this ‘Hungarian conspiracy’ had had direct personal experience of the Nazi regime and understood precisely what it could be capable of. The news from Europe suggested that German expansionism might easily engulf Belgium, whose colony in Africa was a rich source of uranium ore. Wigner suggested that they alert the Belgian government to the danger.

Szilard remembered that his former colleague Einstein knew Elizabeth, Queen of the Belgians personally, and could perhaps approach her on their behalf. Shortly afterwards, on 16 July, Szilard, Wigner and Einstein met at Einstein’s holiday home on Long Island. This was the first that Einstein had heard of the possibility of nuclear chain reactions, and he enthusiastically agreed to help. Einstein dictated a letter in German, and Wigner wrote it down.

After having secured Einstein’s agreement, Szilard then found an alternative way to sound the alarm. He had been put in touch with Alexander Sachs, an economic adviser to US President Franklin Roosevelt. Sachs listened carefully to Szilard’s concerns, before concluding that this was surely a matter for Roosevelt himself. Sachs promised to provide a statement on the subject directly to the President.

Teller and Szilard held a further meeting with Einstein on 30 July, Teller later remarking that he had ‘entered history as Szilard’s chauffeur’ (Szilard had never learned to drive and did not own a car). Einstein agreed to the change of plan, and the three physicists then worked on a draft letter. The end result, dated 2 August 1939, was communicated to Sachs on 15 August but was not delivered verbally to Roosevelt until October.

The letter warned of ‘extremely powerful bombs of a new type’. It warned that Germany had banned the sale of uranium from mines in recently annexed Czechoslovakia and that American work on uranium was now being repeated in Berlin.

Declaration of war

At 4:40am on 1 September 1939 the German Luftwaffe attacked and destroyed the Polish town of Weilun, killing 1,200 people, mostly civilians. This was merely the first in a series of preludes to a full-scale German invasion.

Allied governments declared war on Germany on 3 September.

German Army Ordnance hastily consolidated the country’s two uranium research projects and issued call-up papers to selected nuclear scientists. On 16 September the scientists attended a secret conference
to establish the consolidated project and discuss some of the scientific problems they were likely to face. Among them were Diebner, Harteck and Hahn.

The paper by Bohr and Wheeler on the theory of nuclear fission and the importance of U-235 had recently been published in the American journal
Physical Review.
It had been widely and eagerly read by the German scientists. One of these – Erich Bagge – suggested that the Uranverein co-opt his professor at the University of Leipzig to investigate further the theory of nuclear chain reactions in uranium. This was Nobel laureate Werner Heisenberg, the country’s leading theoretical physicist, famous for his discovery of the uncertainty principle. Heisenberg received notification of his call-up from Bagge himself on 25 September.

The first war of physics had begun.

¹
  Particle accelerators had existed for some years prior to Lawrence’s development of the cyclotron in 1929. However, these earlier machines were
linear
accelerators, accelerating particles along straight lines by passing them through a series of plates to which a carefully controlled alternating high voltage was applied. Lawrence’s cyclotron was designed to accelerate particles along a
circular
path created by an electromagnet, promising greater efficiencies and higher particle energies.