PD Smith

Heart of the atom

Times Literary Supplement, 4 February 2005, p 32

Brian Cathcart, The Fly in the Cathedral: How a small group of Cambridge scientists won the race to split the atom (308pp. Viking/Penguin, 2004)

By PD Smith

In The World Set Free, one of HG Wells’s less well-known novels, written a year before World War One, a professor of physics tells a rapt audience about the wonders of radioactivity: “a little while ago we thought of the atoms as we thought of bricks, as solid building material, as substantial matter, as unit masses of lifeless stuff, and behold! these bricks are boxes, treasure boxes, boxes full of the intensest force.” In Wells’s fictional future, the secrets of radioactivity were unlocked in 1933, but by 1956 the treasure box had become a Pandora’s box as atomic bombs (a phrase coined by Wells) rained from the skies. Thankfully Wells’s prediction of a war fought between nuclear superpowers has not come true, but 1933 was indeed significant. In that year, a quixotic genius named Leo Szilard had a eureka moment while walking down Southampton Row in London, when he realised how the energy of the atom could be released by creating a self-sustaining chain reaction. Neutrons were the key; the existence of these atomic particles had been proven in 1932 by James Chadwick at the Cavendish Laboratory in Cambridge. And it was two of Chadwick’s colleagues who, in that same year, created the first machine to split the atom.

The Director of the Cavendish was Sir Ernest Rutherford and it had been his work on radioactivity, with chemist Frederick Soddy, that inspired Wells’s 1913 novel. Rutherford redefined the understanding of the atom. As Brian Cathcart says in The Fly in the Cathedral, he overturned the rather appealing notion that the atom resembled a plum pudding. According to this view “the sponge of the pudding was a spherical, permeable bulk of mysterious electrification and the electrons were the plums, speckled in large numbers through and around it.”

Instead Rutherford proposed that the atom was like a miniature solar system, with electrons orbiting a nucleus of other particles: “in relation to the atom, the nucleus was a mere speck in a cavernous void … it was like a fly in a cathedral.”

But the precise make-up of the atomic nucleus was still a mystery, and no one was more keen to catch this elusive fly than Rutherford. In 1919, using a process he called “artificial disintegration”, he had chipped away at the nucleus and identified one of its building blocks: the proton. But progress stalled and it took another eight years for Rutherford to dream up a high voltage machine that would generate a stream of atomic particles to shatter the nucleus. As he put it, “we are rather like children, who must take a watch to pieces to see how it works.” The era of particle accelerators was about to begin.

Cathcart’s excellent study tells the story of two Cavendish scientists who were in the vanguard of this revolution – Ernest Walton and John Cockcroft. While it is a story that has been ignored by most popular histories of science, for Cathcart, Cockcroft and Walton’s breakthrough “ranks among the most astonishing and unexpected scientific achievements of the twentieth century”. When Walton arrived at the Cavendish in 1927 from Trinity College, Dublin, he was 24 years old and “little better than a beginner in atomic physics”. Within six weeks, however, he was working at the cutting edge of physics — on the acceleration of particles by electrical means. Cockcroft, from Todmorden on the Yorkshire-Lancashire border, was 6 years older than Walton and an experienced electrical engineer. It was Cockcroft who first grasped the significance of a theoretical paper by the brilliant young Soviet physicist George Gamow. As a boy Gamow once smuggled communion bread home to test transubstantiation beneath his microscope. But in 1928 another mystery concerned him: how particles enter or leave the atomic nucleus. According to Gamow’s theory, protons carried less than half the positive charge of alpha particles, the bullet of choice for most physicists. Therefore, protons would “encounter less repulsion when they met the positively charged protective barrier of the nucleus”, and so might be the ideal particle to penetrate the nucleus using relatively low voltages.

Theory had never been the Cavendish’s strength. Rutherford once said that theorists “play games with their symbols, but we, in the Cavendish, turn out the real solid facts of nature.” But armed with Gamow’s theory, Cockcroft and Walton began working on the first of their accelerators in spring 1929.

The final machine, with its spark gap spheres and glass vacuum tubes, looked like something from Fritz Lang’s Metropolis. In Nicholas Mosley’s powerful novel about the science and philosophy of the period, Hopeful Monsters (1990), Cockcroft and Walton’s accelerator is memorably described as having “a bizarre appearance like something constructed as a prop for a modern ballet. It was like an outsize village pump crowned with a tin top hat: whoever worked it had to sit in a tea-chest lined with lead; this was to protect him from possible effects of radiation. But such was the excitement of the time that physicists did not worry much about radiation.”

After three years and four months of work, Cockcroft and Walton finally succeeded in disintegrating the atom: they fired protons at a lithium target and the nucleus split into two alpha particles. “No comparable nuclear reaction—nothing remotely like it—had ever been observed before. […] They were putting in protons, the lithium was essentially vanishing and what emerged were two alpha particles—nuclei of helium. They were not merely chipping bits off a nucleus; they had gone right to its heart.”

For the first time a man-made apparatus had shattered the atomic nucleus and for this both men shared the 1951 Nobel prize for physics. Rutherford was delighted: they had finally caught “that tiny fly buzzing about in the huge, empty cathedral of the atom.” Afterwards, much to Rutherford’s annoyance, newspaper headlines heralded an age of limitless energy. Though the disintegration of the lithium atom did indeed produce a relatively large amount of energy, only a modest 1 proton in 10 million entered a target nucleus. It was not until the heaviest element, uranium, was split in 1938 that scientists like Szilard realised that a new and terrible power source was finally within reach.

Cathcart has written a wonderfully lucid account of the origins of particle physics for the general reader. The Fly in the Cathedral offers a fascinating insight into the nuts and bolts of experimental (rather than theoretical) physics in the 1920s and 30s. He paints a vivid picture of research life at the Cavendish, from the “sadistic meanness” of its chief technician, Lincoln (“a man of Edwardian bearing, with a waxed moustache curled up in points on each side”), to the daily ritual of afternoon tea and penny buns. But the high point in the life of this remarkable scientific community were the meetings of the Cavendish Physical Society. For writer and scientist CP Snow, in his novel The Search (1934), they were “the essence of all the personal excitement in science; they were romantic…I had seen and heard and been close to the leaders of the greatest movement in the world.”