“It is a profound and necessary truth,” Robert Oppenheimer would say, “that the deep things in science are not found because they are useful; they are found because it was possible to find them.”

Man-made death became epidemic in the twentieth century because increasingly efficient killing technologies made the extreme exercise of national sovereignty pathological. And it was evidently the discovery of how to release nuclear energy and its application to nuclear weapons that reduced the virulence of the pathogen. In a profound and even a quantifiable sense, the weapons that counseled caution these past seven decades at the level of deep nuclear fear served as containers in which to sequester the deaths they held potential, like a vaccine made from the attenuated pathogen itself.

Despite everything, across these past seven decades—nearly the length of my life—we have managed to take into our clumsy hands a limitless new source of energy, hold it, examine it, turn it over, heft it, and put it to work without yet blowing ourselves up. When we finally make our way across to the other shore—when all the nuclear weapons have been dismantled and their cores blended down for reactor fuel—we will find ourselves facing much the same political insecurities we face now. The bombs didn’t fix them and they won’t be fixed by putting the bombs away. The world will be a more transparent place, to be sure, but information technology is moving it in that direction anyway. The difference, as Jonathan Schell has pointed out, will be that the threat of rearming will serve for deterrence rather than the threat of nuclear war. I think of a world without nuclear weapons not as a utopian dream but simply as a world where delivery times have been deliberately lengthened to months or even years, with correspondingly longer periods interim during which to resolve disputes short of war. In such a world, if negotiations fail, if conventional skirmishes fail, if both sides revert to arming themselves with nuclear weapons again—then at worst we will only arrive once more at the dangerous precipice where we all stand now.

This vision of increasing disorder means that the universe is one-way and not reversible; the second law is the expression in physical form of what we call time. But the equations of mechanical physics—of what is now called classical physics—theoretically allowed the universe to run equally well forward or backward. “Thus,” an important German chemist complained, “in a purely mechanical world, the tree could become a shoot and a seed again, the butterfly turn back into a caterpillar, and the old man into a child. No explanation is given by the mechanistic doctrine for the fact that this does not happen. ... The actual irreversibility of natural phenomena thus proves the existence of phenomena that cannot be described by mechanical equations; and with this the verdict on scientific materialism is settled.”90 Planck, writing a few years earlier, was characteristically more succinct: “The consistent implementation of the second law ... is incompatible with the assumption of finite atoms.”

In 1922, the year his Nobel Prize made him a Danish national hero, Bohr accomplished a second great theoretical triumph: an explanation of the atomic structure that underlies the regularities of the periodic table of the elements. It linked chemistry irrevocably to physics and is now standard in every basic chemistry text. Around the nucleus, Bohr proposed, atoms are built up of successive orbital shells of electrons—imagine a set of nested spheres—each shell capable of accommodating up to a certain number of electrons and no more. Elements that are similar chemically are similar because they have identical numbers of electrons in their outermost shells, available there for chemical combination.

[Oppenheimer] was always an intensely, even a cleverly, private man, but late in life he revealed himself to a group of sensitive friends, a revelation that certainly reaches back all the way to his undergraduate years. “Up to now,” he told that group in 1963, “and even more in the days of my almost infinitely prolonged adolescence, I hardly took an action, hardly did anything or failed to do anything, whether it was a paper in physics, or a lecture, or how I read a book, how I talked to a friend, how I loved, that did not arouse in me a very great sense of revulsion and of wrong.”

Once out he returned to the laboratory until chaos retreated behind order again and the Kaiser’s government found time to direct that all Englishmen in Germany be interned for the duration of the war. The place of internment was a race track at Ruhleben—the name means “quiet life”—near Spandau. Chadwick shared with five other men a box stall designed for two horses and must have thought of Gulliver. In the winter he had to stamp his feet till late morning before they thawed. He and other interns formed a scientific society and even managed to conduct experiments. Chadwick’s cold, hungry, quiet life at Ruhleben continued for four interminable years. This was the time, he said later, making the best of it, when he really began to grow up. He returned to Manchester after the Armistice with his digestion ruined and £11 in his pocket. He was at least alive, unlike poor Harry Moseley. Rutherford took him in.

At Simon’s suggestion Peierls had written to Lindemann on June 2. Together at Oxford later in June they approached Lindemann in person. “I do not know him sufficiently well to translate his grunts correctly,” Peierls reported of the meeting. But he felt sure he had “convinced him that the whole thing ought to be taken seriously.”

A speck of U235 stuck to an operator’s coveralls was well worth searching out with a Geiger counter and retrieving delicately with tweezers. No essence was ever expressed more expensively from the substance of the world with the possible exception of the human soul.

By April 1945 Oak Ridge had produced enough U235 to allow a nearcritical assembly of pure metal without hydride dilution. The little bars arrived at the Omega site packed in small, heavy boxes everyone took pains to set well apart; unpacked and unwrapped, the metal shone silver in Frisch’s workbench light. Gradually it oxidized, to blue and then to rich plum. Frisch had walked in the snow at Kungälv puzzling out the meaning of Otto Hahn’s letters to his aunt; in the basement at Bohr’s institute in Copenhagen he had borrowed a name from biology for the process that made these small exotic bars deadly beyond measure; at Birmingham with Rudolf Peierls he had toyed with a formula and had first seen clearly that no more plum-colored metal than now lay scattered on his workbench would make a bomb that would change the world. At Los Alamos in Southwestern spring, dénouement: he would assemble as near a critical mass of U235 as anyone might ever assemble by hand and not be destroyed.

The committee and its Los Alamos consultants were not unmindful of the radiation effects of the atomic bomb—its most significant difference in effect from conventional high explosives—but worried more about radiation danger to American aircrews than to the Japanese. “Dr. Oppenheimer presented a memo he had prepared on the radiological effect of the gadget. ... The basic recommendations of this memo are (1) for radiological reasons no aircraft should be closer than 2½ miles to the point of detonation (for blast reasons the distance should be greater) and (2) aircraft must avoid the cloud of radio-active materials.”

The few explosions did not seem a miracle of deliverance to the civilians of the enemy cities upon whom the bombs would be dropped. In their behalf—surely they have claim—something more might be said about reasons. The bombs were authorized not because the Japanese refused to surrender but because they refused to surrender unconditionally. The debacle of conditional peace following the First World War led to the demand for unconditional surrender in the Second, the earlier conflict casting its dark shadow down the years. “It was the insistence on unconditional surrender that was the root of all evil,” writes the Oxford moralist G. E. M. Anscombe in a 1957 pamphlet opposing the awarding of an honorary degree to Harry Truman. “The connection between such a demand and the need to use the most ferocious methods of warfare will be obvious. And in itself the proposal of an unlimited objective in war is stupid and barbarous.”