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the place—we call Hiroshima as an episode in international history, not solely the consequence of wartime hatreds that marked the American-Japanese relationship in 1941-5. The result, I hope, is a serious, readable overview of one of the truly critical moments in the history of the twentieth-century world and all human history. ‘The scientists, helped by the engineers, had drawn a line across history so that the centuries before August 6, 1945, were sharply separated from the years to come,’ wrote the historian Margaret Gowing. ‘And though perhaps they did not contemplate the technical “escalation” of the next fifteen years nor think in terms of megatons and megadeaths, of weapons that could obliterate not a single town but half a country, they knew that the atomic age had only begun.’4 More than sixty years later, the Cold War is past, the danger of a nation-to-nation exchange of nuclear bombs or warheads seemingly diminished. Yet in an age of stateless terrorism and great power arrogance, where international norms and institutions appear helpless to prevent violence and nuclear materials go ominously missing, where there are no longer one or two nuclear nations but perhaps ten, we may wonder whether the world is safer from nuclear holocaust than it was in the bewildering days following that clear August morning in 1945.

ONE - The World’s Atom

'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

1. Dissecting the atom

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