'Never believe', wrote the British physicist Jacob Bronowski, ‘that the atom is a complex mystery—it is not. The atom is what we find when we look for the underlying architecture in nature, whose bricks are as few, as simple and as orderly as possible.’ Reassuring words, perhaps, to a beginning student of physics, and logical too, for humans naturally seek to reduce large and complex matters to their essences. But the presence of atoms was neither demonstrated nor universally assumed until relatively recently. It is commonly said that the ancient Greeks postulated the existence of the atom, and it is true that the word atomos is Greek for ‘indivisible’, a coinage made by the philosopher Democritus around 430 bce. Both Plato and Aristotle, however, disparaged the notion of the atom, Plato contending that the highest forms of human society, including truth and beauty, could not be explained with reference to unseen bits of apparently inert matter. The Platonic-Aristotelean view largely held the field for centuries. In 1704, Sir Isaac Newton wrote (in Optics): ‘It seems probable to me that God in the beginning formed Matter in solid, massy, hard, impenetrable moveable Particles,’ which made the case for something like atoms, however ‘massy’ they might prove. A century later, the English chemist John Dalton posited the existence of atoms as hard and round as billiard balls, though these were particular to chemical elements and not, as Democritus had claimed, all like each other in composition.1
Undoing the atom was fundamentally the atomic inheritance of Ernest Rutherford, a New Zealander who came to study physics at Newton’s university, Cambridge, and its Cavendish laboratory, in 1895. ‘I was brought up to look at the atom as a nice hard fellow, red or gray in colour, according to taste,’ he would write. For a time, Rutherford found no cause to change his mind. He worked on radio waves at the Cavendish, then spent nine years at McGill University in Montreal, tracing atomic ‘emanations’ but not yet investigating the atomic structure itself. In the meantime, however, J. J. Thomson, one of Rutherford’s mentors, found in a closed glass tube evidence of particles with negative electrical charges that were themselves tinier than atoms; these would be called electrons, a name already long devised by the Irish physicist George Johnstone Stoney, who had posited though not demonstrated their existence. Using a similar tube, W C. Rontgen, working at the University of Wurzburg in Germany, produced an electrical discharge that yielded an odd glow. When he covered the tube with black paper and placed his hand between the tube and a screen, he could see faintly projected the bones of his hand. Rontgen called the phenomenon ‘X-rays’. (A startled and righteous assemblyman in New Jersey, apprised of the discovery, introduced legislation ‘prohibiting the use of X-rays in opera glasses’.) In 1896, the French physicist Henri Becquerel, inspired by Rontgen’s finding, decided to look for X-rays in materials that fluoresced—that is, absorbed light from one part of the spectrum and emitted light from another part. He wrapped a photographic plate in black paper, dusted it with a uranium compound, then left the plate in the sun. After a few hours, he wrote: ‘I saw the silhouette of the phosphorescent substance in black on the negative.’ Becquerel tried the experiment again, but, discouraged by a succession of cloudy days in Paris, and assuming the sun had caused the tracing on the negative, he closed the plate in a drawer. He was surprised, several days later, when he looked at the plate, to find that the silhouette effect had occurred even in the dark. It was not the sun, but something in the uranium, that had penetrated the black paper and left its ghostly image. Two years later, the French wife-husband team Marie and Pierre Curie discovered two new elements, polonium and radium, that gave off Becquerel’s mysterious discharge. They dubbed it ‘radioactivity’.2
Something was coming off, or out of, atoms. They were not themselves the smallest things, nor were they as solid and ‘massy’ as billiard balls. Thomson’s tiny electrons and the presence of radioactive emission demonstrated that. (Scientists would ultimately identify three types of radiations— alpha, beta, and gamma rays—with the betas being streams of electrons.) Over the first two decades of the twentieth century, Rutherford, who moved from Montreal to Manchester in 1910, ‘systematically dissected the atom’, as Richard Rhodes has written. He found that atoms, far from stable, might change themselves into another form of the same element they comprised (called an isotope) or another element altogether. He calculated that an enormous amount of energy came with radiation; if things went badly wrong, he said, ‘some fool in a laboratory might blow up the universe unawares’. And, on 7 March 1911, speaking before a general audience in Manchester, Rutherford announced that he had revised his notion of the atom’s structure: it had a central mass, or nucleus, around which spun electrons. Since electrons carried negative electric charges, the atomic nucleus must be charged positive. The force exerted by the electrons must be equal to that of the nucleus for the atom to remain stable.3
Rutherford did not work alone. At McGill he had teamed with Frederick Soddy, a chemist who, like Rutherford, would win a Nobel Prize, and also with the German Otto Hahn, who conjured with isotopes and would go on to do revolutionary experiments with the nucleus during the 1930s. In Manchester there was another German, Hans Geiger, builder of an electrical machine that detected radiation and clicked in its presence. He helped train James Chadwick, the Australian Marcus Oliphant, the Russian Peter Kapitsa, and the Japanese Yoshio Nishina—the latter two of whom would play leading roles in their nations’ nuclear-weapons programs. The great Danish physicist Niels Bohr considered himself Rutherford’s student, though he was Rutherford’s equal at refining ideas about the structure of the atom. (Curiously, a Japanese scientist named Hantaro Nagaoka suggested in 1903 that an atom resembled the planet Saturn, with the planet itself as a nucleus and the rings representing electrons orbiting it. Rutherford seems not to have known of Nagaoka’s vision, despite the two men having met in Manchester.)4
Rutherford concluded in 1919 that the nucleus of hydrogen, the first element in the periodic table, was a single, positively charged particle he called a proton. More complicated elements had more protons, and every nucleus of a single element had the same number of protons, which figure gives the element its atomic number. Rutherford and others, however, suspected that there was something more to the nucleus, for nuclei were evidently too heavy to consist only of protons. Suspicion was one thing, detection another. The other nuclear particles (to be called neutrons) were hard to find, as Laura Fermi wrote, because, unlike protons and electrons, they lack electrical charge, and because they ‘stay very much at home inside atomic nuclei, and it is very difficult to get them to leave’. It was James Chadwick who found the neutron, in experiments at the Cavendish in 1932. He reported his discovery before a group of physicists on 17 February, then said: ‘Now I want to be chloroformed and put to bed for a fortnight.’5
While a neutron likes to stay put, it is, as a result of its electrical neutrality, an ideal projectile with which to enter and explore the nucleus. Probing the nucleus with a neutron, especially the large nucleus of one of the heavier and less stable elements, instantly destabilizes it. This nucleus busting, this breaking of atoms, is called fission. It was first observed by Otto Hahn and Fritz Strassmann in a laboratory in suburban Berlin in late 1938, properly interpreted by Lise Meitner and her physicist nephew Otto Frisch at the year’s end (Hahn was inclined to resist the implications of his own experiment), confirmed experimentally by Frisch, then published in the February 1939 issue of the journal Nature, and even before that disclosed by Bohr at a meeting of the American Physical Society in Washington— from which excited physicists departed early in order to try the experiment themselves, and on which more later.
Holding together the protons and neutrons (the nucleons) is the strong nuclear force, which means that large amounts of energy are locked up inside the atom’s nucleus. When a projectile neutron strikes a target nucleus, the nucleus breaks apart, yielding two nearly equal halves, a burst of energy, and some its own neutrons. ‘These fly through the rest of the material,’ Bronowski explains, ‘and if the piece is large enough each neutron is certain to strike another nucleus and thus set off another burst of energy—and fire off still other neutrons to carry on the reaction.’ The materials most likely to sustain such a chain reaction (as it is called) are those with heavy, unstable, neutron-rich nuclei, particularly uranium and human-made plutonium. A gram of uranium, fully fissioned through such a chain reaction, produces enough energy to light 20,000 light bulbs for ten hours. A similarly fissioned pound of uranium makes as much energy as millions of pounds of coal. Near the culmination of this process comes the release of radiation in the form of beta particles and gamma rays.6
Certainly Ernest Rutherford, the nucleus around whom buzzed an electron cloud of other scientists, had not, despite his puckish comment about a fool in a laboratory blowing up the universe, set out to make a powerful explosive. Anyone claiming that the day of atomic power was dawning was ‘talking moonshine’, he wrote dismissively in 1933. The excitement of discovery was thus not tied to some cataclysmic result, and for this reason not circumscribed by the nation. Even during the First World War, Rutherford had stayed in touch with scientists throughout Europe, including those in Germany. When the war ended, cooperation redoubled; what the American J. Robert Oppenheimer called the ‘heroic’ days of atomic physics, the time of ‘great synthesis and resolutions’, occurred during the 1920s, when the world was at peace. In the great centers of interwar physics—Cambridge, Paris, Copenhagen, Gottingen—there was excitement about theory, tiny particles of matter and their puzzling behavior, and how to reconcile the evidence recorded on machines and with the eyes with what one knew, or thought one knew, about the way atoms worked. In 1914, the writer H. G. Wells published a novel called The World Set Free, in which the earth, forty years hence, was a place of atomic-powered cars and radioactive bombs made of an element Wells called ‘Carolinum’, which bored deep into the soil and fired off ‘puffs of heavy incandescent vapour and fragments of viciously punitive rock and mud, saturated with Carolinum, and even a center of scorching and blistering energy’. Leo Szilard, the Hungarian scientist who would become the Cassandra of the nuclear physics community during the 1930s and 1940s, at first regarded the book as entertaining fiction.7