The First War of Physics (3 page)

The most stable, common isotope of uranium contains 92 protons and 146 neutrons, giving a total of 238 altogether (written U-238). Radium nuclei contain 88 protons and can exist as a variety of radioactive isotopes with varying numbers of neutrons, the most common isotope containing 138 neutrons and 226 ‘nucleons’ overall. Transforming uranium into radium – as Hahn and Strassman were suggesting – appeared to require too big a leap, moving the target nucleus four places down the periodic table. This was much bigger than the small, incremental changes that were expected from neutron bombardment.

Late in 1938 Hahn had written to his sorely missed collaborator about these results, but Meitner urged caution. What Hahn and Strassman had found was quite simply unprecedented, and could not be explained by current theories of atomic nuclei.

Mass into energy

Frisch emerged from his hotel room after his first night in Kungälv to find his aunt already at breakfast. It was Christmas Eve 1938. This was an opportunity to set aside his gloom about developments in Germany and fears for the safety of his father. He wanted to talk physics, and he planned to tell Meitner all about a new experiment he was working on. However, he found that she was completely preoccupied. She was clutching a further letter from Hahn, dated 19 December, which contained news of further results from Berlin that were, if anything, even more bizarre.

Hahn and Strassman had repeated their experiments and concluded that the atoms they had created were not those of radium at all. They were, in fact, barium atoms. The most common isotope of barium has just 56 protons and 82 neutrons, totalling 138. This was a remarkable conclusion. The target uranium nucleus had moved not one, two or even four places down the periodic table: it had moved down an astonishing 36 places.
Bombarding uranium with neutrons had caused the uranium nucleus to split virtually in half.

‘I don’t believe it,’ Frisch declared, ‘there’s some mistake.’ But Meitner claimed that Hahn was just too good a scientist to have made a fundamental error.

Meitner had written back to Hahn a few days before, declaring that these results were ‘startling’, but going on to say: ‘but in nuclear physics we have experienced so many surprises, that one cannot unconditionally say: it is impossible.’

Frisch and Meitner’s animated discussion continued after breakfast. They set out from their hotel across the flood plain of the river, crossing the frozen river itself before entering open woods, Frisch on skis and Meitner on foot, all the time debating. How could a single neutron cause the uranium nucleus to fall apart so spectacularly? Of course, nobody really knew how a uranium nucleus would behave in such a reaction. All they could do was reach for analogies with other physical phenomena that were better understood, and hope for the best.

One of these analogies had been proposed ten years before by the Ukrainian-born physicist George Gamow and had been adapted by Bohr to describe nuclear reactions. Meitner now recalled it. In this model, the force binding an atomic nucleus together is imagined to act in much the same way as surface tension binds a drop of liquid. In this ‘liquid-drop’ model of the nucleus, a balance is maintained between the surface tension which holds it together and the force of repulsion between its positively-charged protons which threatens to tear it apart.

Both forces increase as the size of the nucleus increases, but the force of repulsion increases more rapidly, eventually overwhelming the surface
tension when the number of protons reaches about 100.
²
Perhaps in a uranium nucleus, with its 92 protons, this balance is very delicate. Maybe adding a single neutron is enough to cause the nucleus to distort, elongating and forming a narrow waist before splitting to form two smaller ‘drops’.

Frisch and Meitner sat on a tree trunk scrambling for pieces of paper on which to draw diagrams and scribble calculations. They quickly deduced that the sizeable positive charge of the uranium nucleus was indeed large enough to offset the surface tension. This suggested an image of ‘a very wobbly, unstable drop, ready to divide itself at the slightest provocation, such as the impact of a single neutron’.

But the nuclear fragments created by such a split would each carry away a sizeable amount of energy. Meitner estimated this to be about 200 million electron volts.
³
The fragments would be propelled away from each other by the mutual repulsion of their positive charges. Energy had to be conserved in this process. This was an unquestionable, fundamental law of physics that could not be broken. If they could not account for this energy then their idea would be worthless.

So, where could this energy have come from? Meitner recalled her first meeting with Albert Einstein, in 1909. She had heard him lecture on his theory of relativity, and had watched intently as he had derived his famous formula, E = mc
²
. The very idea that mass could be converted to energy had left a deep impression on her. She also remembered that the nuclear masses of the fragments created by splitting a uranium nucleus would not quite add up to the mass of the original nucleus. These masses differed by about one-fifth of the mass of a single proton, mass that had gone ‘missing’ in the nuclear reaction.

The sums checked out. It all fitted together. A neutron causes the uranium nucleus to split in two, converting a tiny amount of mass into energy along the way.
⁴

Nuclear fission

Frisch arrived back at Bohr’s institute in Copenhagen on 3 January 1939, and rushed to tell Bohr what he and Meitner had discovered. On hearing of their proposal Bohr immediately recognised its basic truth, declaring: ‘Oh what idiots we have all been! Oh but this is wonderful. This is just as it must be.’ He urged Frisch to publish what they had discovered as soon as possible and promised to keep the news to himself until Frisch and Meitner had established priority.

A new physical process demanded a new name. Frisch saw parallels between the contortions of the uranium nucleus, the ‘wobbly, unstable drop’, and biological cell division. On advice from a biologist, Frisch borrowed the term
fission
to describe the fragmentation of uranium nuclei in the paper he was hastily drafting with Meitner. Despite Bohr’s reservations about the name, it stuck.

Bohr turned his attention to preparations for a trip to Princeton University in the United States. He was intending to continue his debate with Einstein on the interpretation of quantum theory, a debate that had begun in 1927 and which proved to be one of the most important scientific debates of the twentieth century, if not the entire history of science. At issue was the role of uncertainty and probability in the behaviour of fundamental sub-atomic particles, to which Einstein had stubbornly refused to yield his insistence that ‘God does not play dice’. At stake was the ability of the human mind to comprehend the very nature of physical reality itself.

Bohr was joined on the trip by his son Erik and Léon Rosenfeld, a young protégé whom Bohr would frequently use as a ‘sounding board’, bouncing ideas off him as a way to sharpen his own thinking. They left for Gothenburg on 7 January, where they embarked on the MS
Drottningholm
, bound for New York. But the subject of discussion in Bohr’s stateroom – in which he had arranged to have a blackboard fitted – was not the interpretation of quantum theory, as he had originally intended. It was nuclear fission.

As Bohr crossed the Atlantic, Frisch was busy back in Copenhagen. Nuclear fission in uranium had been discovered through the careful identification of the chemical substances that had resulted from it. The scientists knew what substances they had started with, and they knew what they had finished with, and fission was proposed to account for the journey from start to finish. This was a bit like starting with the opening scenes of Shakespeare’s
Hamlet
, finishing with a stage strewn with corpses, and hypothesising about what had happened in between.

Frisch’s Czech colleague George Placzek was sceptical. If Frisch and Meitner were right, then surely the fission reaction would be expected to produce a tell-tale burst of energy that should be
physically
detectable. Frisch simply hadn’t thought of this. Within a matter of a few days he had retreated to his laboratory, devised and carried out a simple experiment and had found what he was looking for. He had found the tell-tale signature of fission.

But there was yet more to be discovered. Lighter elements in the periodic table tend to have equal numbers of protons and neutrons in their nuclei. But as the number of protons in the nuclei increases, so does the force of repulsion between them. Heavier nuclei therefore require an excess of neutrons over protons to create enough ‘surface tension’ to remain stable. If the uranium nucleus was splitting up to form lighter elements, then perhaps these were being formed with more neutrons in their nuclei than they could comfortably accommodate.

It was Frisch’s Danish colleague Christian Møller who suggested that if the newly-formed fission fragments spit out one or two additional neutrons in their turn (subsequently called
secondary neutrons)
, perhaps these could go on to break up more uranium nuclei, releasing more energy and creating more neutrons, and so on and on. The result would be a cascade, a
chain reaction
that could liberate nuclear energy on a large scale. Control the chain reaction and you would have a nuclear ‘reactor’.

An uncontrolled chain reaction would be a bomb of unprecedented destructive power.

Verification

The Bohrs and Rosenfeld disembarked at the Hudson River dock on 16 January 1939. They were met by a young Princeton University physicist called John Wheeler, who had worked with Bohr in Copenhagen in 1934 and 1935, and was looking forward to spending a few months with his former colleague.

Wheeler was joined at the dockside by Enrico Fermi and his wife Laura. On 10 December 1938 Enrico had collected a Nobel prize for his work on neutron bombardment, and the Fermis had ‘got lost’ on their way back from Stockholm. In truth, Fermi had sought to protect Laura from Mussolini’s Fascist state, which had introduced its own anti-Semitic laws a few months previously.
⁵
He had accepted the offer of a professorship at Columbia University and had arrived in New York on 2 January.

Bohr, ever mindful of the importance of establishing priority for his colleagues when a significant discovery had been made, said nothing to either Wheeler or Fermi about nuclear fission. But as Bohr left to spend a day catching up with Fermi, Rosenfeld – unaware of Bohr’s concerns – happily spilled the beans to Wheeler. The news spread quickly through the community of physicists in America, which by now included many European émigrés.

Hahn had contacted Paul Rosbaud about their experimental results on 22 December 1938, and Rosbaud had helped rush these into print. Hahn and Strassman’s paper on the neutron bombardment of uranium was published in
Die Naturwissenschaften
on 6 January 1939. Although Meitner had been part of the team that had worked on these problems and had continued to collaborate from her exile in Sweden, it was now politically unacceptable for Hahn to name her as a co-author. Meitner, in her turn, had remained reticent about her and Frisch’s interpretation of Hahn and Strassman’s results.

Frisch and Meitner’s paper on nuclear fission in uranium was published in the British scientific journal
Nature
on 11 February. Frisch’s paper reporting the results of his simple experiment to verify fission was published in
Nature
a week later, on 18 February. In an effort to ensure that Frisch and Meitner were properly credited with the discovery, Bohr himself published a short paper on the subject in
Nature
on 25 February.

Thanks to Rosenfeld’s indiscretion, and a subsequent official report on fission by Bohr at a conference at George Washington University, by the time these papers appeared, experiments repeating and verifying the results had already been carried out in America.

On the West Coast, 27-year-old physicist Luis Alvarez had found out about fission from an article buried in the
San Francisco Chronicle.
He immediately abandoned the barber’s chair in which he was sitting, cutting off the barber mid-snip, and ran to Berkeley’s Radiation Laboratory to spread the news.
⁶
His first encounter was with Philip Abelson, one of his own graduate students, who was within a week or so of making the discovery of fission for himself. Abelson quickly verified the results.

Alvarez also informed another young Berkeley professor, regarded by many as the American West Coast’s
wunderkind
of theoretical physics. The young professor ‘instantly pronounced the reaction impossible and proceeded to prove mathematically to everyone in the room that someone must have made a mistake’. He became convinced within minutes of being shown the experimental evidence, however, and within a few days a crude design for an atomic bomb had appeared on the blackboard in his office. His name was J. Robert Oppenheimer.

Nuclear fission was now not only an established scientific fact, it was fast becoming a new scientific discipline.

Uranium-235

By March 1939 research groups in America and in France had shown that, on average, between two and four secondary neutrons are released in each fission of a uranium nucleus – more than adequate to support a self-sustaining chain reaction. Growing concern about the possibility of an atomic bomb was, however, quickly dispelled.
⁷

Bohr hadn’t lost any time. There was clearly much work to be done in the newly-emerging field of fission physics, and Princeton was just as good a place to do this work as Copenhagen. He asked Wheeler if he would like to collaborate, and together they started work on a more detailed theory of the fission process. They were aided by some new experimental results from an apparatus that had been hastily set up in the attic of Princeton’s Palmer Laboratory. These results were initially quite puzzling.

The Princeton apparatus had been designed to discover how the rate of nuclear fission in uranium changes as the energy of the bombarding neutrons is varied.
⁸
It was found that the highest fission rates are obtained at the highest energies, with the rate falling as the energy of the neutrons falls, largely as expected. But then it was found that at low neutron energies the rate of fission increases once more.