The Making of the Atomic Bomb

“renunciation,” renunciation of the godlike determinism of classical physics where the intimate scale of the atomic interior was concerned. The name he chose for this “general point of view” was complementarity, a word that derives from the Latin complementum, “that which fills up or completes.” Light as particle and light as wave, matter as particle and matter as wave, were mutually exclusive abstractions that complemented each other. They could not be merged or resolved; they had to stand side by side in their seeming paradox and contradiction; but accepting that uncomfortably non-Aristotelian condition meant physics could know more than it otherwise knew. And furthermore, as Heisenberg's recently published uncertainty principle demonstrated within its limited context, the universe appeared to be arranged that way as far down as human senses would ever be able to see.

Emilio Segre, who heard Bohr lecture at Como in 1927 as a young engineering student, explains complementarity simply and clearly in a history of modern physics he wrote in retirement: “Two magnitudes are complementary when the measurement of one of them prevents the accurate simultaneous measurement of the other. Similarly, two concepts are complementary when one imposes limitations on the other.”

Carefully Bohr then examined the conflicts of classical and quantum physics one at a time and showed how complementarity clarified them. In conclusion he briefly pointed to complementarity's connection to philosophy. The situation in physics, he said, “bears a deep-going analogy to the general difficulty in the formation of human ideas, inherent in the distinction between subject and object.” That reached back all the way to the licentiate's dilemma in Adventures of a Danish Student, and resolved it: the I who thinks and the I who acts are different, mutually exclusive, but complementary abstractions of the self.

In the years to come Bohr would extend the compass of his “certain general point of view” far into the world. It would serve him as a guide not only in questions of physics but in the largest questions of statesmanship as well. But it never commanded the central place in physics he hoped it would. At Como a substantial minority of the older physicists were predictably unpersuaded. Nor was Einstein converted when he heard. In 1926 he had written to Max Bora concerning the statistical nature of quantum theory that “quantum mechanics demands serious attention. But an inner voice tells me that this is not the true Jacob. The theory accomplishes a lot, but it does not bring us closer to the secrets of the Old One. In any case, I am convinced that He does not play dice.” Another physics conference, the annual Solvay Conference sponsored by a wealthy Belgian industrial chemist named Ernest Solvay, was held in Brussels a month after Como. Einstein attended, as did Bohr, Max Planck, Marie Curie, Hendrick Lorentz, Max Born, Paul Ehrenfest, Erwin Schrodinger, Wolfgang Pauli, Werner Heisenberg and a crowd of others. “We all stayed at the same hotel,” Heisenberg remembers, “and the keenest arguments took place, not in the conference hall but during the hotel meals. Bohr and Einstein were in the thick of it all.”

Einstein refused to accept the idea that determinism on the atomic level was forbidden, that the fine structure of the universe was unknowable, that statistics rule. “‘God does not throw dice’ was a phrase we often heard from his lips in these discussions,” writes Heisenberg. “And so he refused point-blank to accept the uncertainty principle, and tried to think up cases in which the principle would not hold.” Einstein would produce a challenging thought experiment at breakfast, the debate would go on all day, “and, as a rule, by suppertime we would have reached a point where Niels Bohr could prove to Einstein that even his latest experiment failed to shake the uncertainty principle. Einstein would look a bit worried, but by next morning he was ready with a new imaginary experiment more complicated than the last.” This went on for days, until Ehrenfest chided Einstein — they were the oldest of friends — that he was ashamed of him, that Einstein was arguing against quantum theory just as irrationally as his opponents had argued against relativity theory. Einstein remained adamant (he remained adamant to the end of his life where quantum theory was concerned).

Bohr, for his part, supple pragmatist and democrat that he was, never an absolutist, heard once too often about Einstein's personal insight into the gambling habits of the Deity. He scolded his distinguished colleague finally in Einstein's own terms. God does not throw dice? “Nor is it our business to prescribe to God how He should run the world.”

6 Machines

After the war, under Ernest Rutherford's direction, the Cavendish thrived. Robert Oppenheimer suffered there largely because he was not an experimentalist; for experimental physicists, Cambridge was exactly the center that Oppenheimer had thought it to be. C. P. Snow trained there a little later, in the early 1930s, and in his first novel, The Search, published in 1934, celebrated the experience in the narrative of a fictional young scientist:

I shall not easily forget those Wednesday meetings in the Cavendish. For me they were the essence of all the personal excitement in science; they were romantic, if you like, and not on the plane of the highest experience I was soon to know [of scientific discovery]; but week after week I went away through the raw nights, with east winds howling from the fens down the old streets, full of a glow that I had seen and heard and been close to the leaders of the greatest movement in the world.

More crowded than ever, the laboratory was showing signs of wear and tear. Mark Oliphant remembers standing in the hallway outside Rutherford's office for the first time and noticing “uncarpeted floor boards, dingy varnished pine doors and stained plastered walls, indifferently lit by a skylight with dirty glass.” Oliphant also records Rutherford's appearance at that time, the late 1920s, when the Cavendish director was in his mid-fifties: “I was received genially by a large, rather florid man, with thinning fair hair and a large moustache, who reminded me forcibly of the keeper of the general store and post office in a little village in the hills behind Adelaide where I had spent part of my childhood. Rutherford made me feel welcome and at ease at once. He spluttered a little as he talked, from time to time holding a match to a pipe which produced smoke and ash like a volcano.”

With simple experimental apparatus Rutherford continued to produce astonishing discoveries. The most important of them besides the discovery of the nucleus had come to fruition in 1919, shortly before he left Manchester for Cambridge — he sent off the paper in April. Afterward, at the Cavendish, he and James Chadwick followed through. The 1919 Manchester paper actually summarized a series of investigations Rutherford carried out in his rare moments of spare time during the four years of war, when he kept the Manchester lab going almost singlehandedly while doing research for the Admiralty on submarine detection. It appeared in four parts. The first three parts cleared the way for the fourth, “An anomalous effect in nitrogen,” which was revolutionary.

Ernest Marsden, whose examination of alpha scattering had led Rutherford to discover the atomic nucleus, had found a similarly fruitful oddity in the course of routine experimental studies at Manchester in 1915. Marsden was using alpha particles — helium nuclei, atomic weight 4 — emanating from a small glass tube of radon gas to bombard hydrogen atoms. He did that by fixing the radon tube inside a sealed brass box fitted at one end with a zinc-sulfide scintillation screen, evacuating the box of air and then filling it with hydrogen gas. The alpha particles emanating from the radon bounced off the hydrogen atoms (atomic weight approximately 1) like marbles, transferring energy to the H atoms and setting some of them in motion toward the scintillation screen; Marsden then measured their range by interposing pieces of absorbing metal foils behind the screen until the scintillations stopped. Predictably, the less massive H atoms recoiled farther as a result of their collisions with the heavier alpha particles than did the alphas — about four times as far, says Rutherford — just as smaller and larger marbles colliding in a marbles game do.

That was straightforward enough. But then Marsden noticed, Rutherford relates, while the box was evacuated, that the glass radon tube itself “gave rise to a number of scintillations like those from hydrogen.” He tried a tube made of quartz, then a nickel disk coated with a radium compound, and found similarly bright, H-like scintillations. “Marsden concluded that there was strong evidence that hydrogen arose from the radioactive matter itself.” This conjecture would have been stunning, if true — so far radioactive atoms had been found to eject only helium nuclei, beta electrons and gamma rays in the course of their decay — but it was not the only possible deduction. Nor was it one that Rutherford, who after all had discovered two of the three basic radiations and had never found hydrogen among them, was likely to accept out of hand. Marsden had returned to New Zealand in 1915 to teach; Rutherford pursued the strange anomaly. He had a good idea what he was after. “I occasionally find an odd half day to try a few of my own experiments,” he wrote Bohr on December 9, 1917, “and have got I think results that will ultimately prove of great importance. I wish you were here to talk matters over with. I am