Making of the Atomic Bomb

by Richard Rhodes

  The solution, Bohr went on, is to accept the different and mutually exclusive results as equally valid and stand them side by side to build up a composite picture of the atomic domain. Nur die Fülle führt zur Klarheit: only wholeness leads to clarity. Bohr was never interested in an arrogant reductionism. He called instead—the word appears repeatedly in his Como lecture—for “renunciation,” renunciation of the godlike determinism of classical physics where the intimate scale of the atomic interior was concerned.496 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 Segrè, 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.497 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.”498 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 Born 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.”499 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 Schrödinger, 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.”500

  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.”501 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.”502

  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.503

  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.”504 Oliphant also records Rutherford’s appearance at that time, the late 1920s, when the Cavendish director was in his midfifties: “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.505

  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 stop
ped. 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.”506 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 detecting and counting the lighter atoms set in motion by [alpha] particles. . . . I am also trying to break up the atom by this method.”507

  His equipment was similar to Marsden’s, a small brass box fitted with stopcocks to admit and evacuate gases from its interior, with a scintillation screen mounted on one end. For an alpha source he used a beveled brass disk coated with a radium compound:

  Arrangement of Ernest Rutherford’s experiment: D, alpha source. S, zinc sulfide scintillation screen. M, microscope.

  The likeliest explanation for Marsden’s anomalous H atoms was contamination; hydrogen is light and chemically active and a minor component of the ubiquitous air. So Rutherford’s problem was basically one of rigorous exclusion. He needed to narrow down the possible sources of hydrogen atoms in his box until he could conclusively prove their point of origin. He started by showing that they did not come from the radioactive materials alone. He established that they had the same mass and expected range as the H atoms that recoiled from alpha bombardment of hydrogen gas in Marsden’s experiment. He admitted dry oxygen into the evacuated brass box, then carbon dioxide, and found in both cases that the H atoms coming off the radioactive source were slowed down by colliding with the atoms of those gases—fewer scintillations showed up on the screen.

  Then he tried dry air. The result surprised him. Instead of decreasing the number of scintillations, as oxygen and carbon dioxide had done, dry air increased them—doubled them in fact.

  These newfound scintillations “appeared to the eye to be about equal in brightness to H scintillations,” Rutherford notes cautiously near the beginning of the revolutionary Part IV of his paper.508 He went after them. If they were H atoms, they still might be contaminants. He eliminated that possibility first. He showed that they could not be due merely to the hydrogen in water vapor (H2O): drying the air even more thoroughly made little difference in their number. Dust might harbor H atoms like dangerous germs: he filtered the air he let into the box through long plugs of absorbent cotton but found little change.

  Since the increase in H atoms occurred in air but not in oxygen or carbon dioxide, Rutherford deduced then that it “must be due either to nitrogen or to one of the other gases present in atmospheric air.” And since air is 78 percent nitrogen, that gas appeared to be the likeliest candidate. He tested it simply, by comparing scintillations from air to scintillations from pure nitrogen. The test confirmed his hunch: “With pure nitrogen, the number of long-range scintillations under similar conditions was greater than in air.”509 Finally, Rutherford established that the H atoms came in fact from the nitrogen and not from the radioactive source alone. And then he made his stunning announcement, couching it as always in the measured understatement of British science: “From the results so far obtained it is difficult to avoid the conclusion that the long-range atoms arising from collision of [alpha] particles with nitrogen are not nitrogen atoms but probably atoms of hydrogen. . . . If this be the case, we must conclude that the nitrogen atom is disintegrated.”510 Newspapers soon published the discovery in plainer words: Sir Ernest Rutherford, headlines blared in 1919, had split the atom.

  It was less a split than a transmutation, the first artificial transmutation ever achieved. When an alpha particle, atomic weight 4, collided with a nitrogen atom, atomic weight 14, knocking out a hydrogen nucleus (which Rutherford would shortly propose calling a proton), the net result was a new atom of oxygen in the form of the oxygen isotope 017: 4 plus 14 minus 1. There would hardly be enough 017 to breathe; only about one alpha particle in 300,000 crashed through the electrical barrier around the nitrogen nucleus to do its alchemical work.511

  But the discovery offered a new way to study the nucleus. Physicists had been confined so far to bouncing radiation off its exterior or measuring the radiation that naturally came out of the nucleus during radioactive decay. Now they had a technique for probing its insides as well. Rutherford and Chadwick soon went after other light atoms to see if they also could be disintegrated, and as it turned out, many of them—boron, fluorine, sodium, aluminum, phosphorus—could. But farther along the periodic table a barricade loomed. The naturally radioactive sources Rutherford used emitted relatively slow-moving alpha particles that lacked the power to penetrate past the increasingly formidable electrical barriers of heavier nuclei. Chadwick and others at the Cavendish began to talk of finding ways to accelerate particles to higher velocities. Rutherford, who scorned complex equipment, resisted. Particle acceleration was in any case difficult to do. For a time the newborn science of nuclear physics stalled.

  * * *

  Besides Rutherford’s crowd of “boys,” several individual researchers worked at the Cavendish, legatees of J. J. Thomson. One who pursued a different but related interest was a slim, handsome, athletic, wealthy experimentalist named Francis William Aston, the son of a Birmingham gunmaker’s daughter and a Harborne metal merchant.512 As a child Aston made picric-acid bombs from soda-bottle cartridges and designed and launched huge tissue-paper fire balloons; as an adult, a lifelong bachelor, heir after 1908 to his father’s wealth, he skied, built and raced motorcycles, played the cello and took elegant trips around the world, stopping off in Honolulu in 1909, at thirty-two, to learn surfing, which he thereafter declared to be the finest of all sports. Aston was one of Rutherford’s regular Sunday partners at golf on the Gogs in Cambridge. It was he who had announced, at the 1913 meeting of the British Association, the separation of neon into two isotopes by laborious diffusion through pipe clay.

  Aston trained originally as a chemist; Röntgen’s discovery of X rays turned him to physics. J. J. Thomson brought him into the Cavendish in 1910, and it was because Thomson seemed to have separated neon into two components inside a positive-ray discharge tube that Aston took up the laborious work of attempting to confirm the difference by gaseous diffusion. Thomson found that he could separate beams of different kinds of atoms by subjecting his discharge tube to parallel magnetic and electrostatic fields. The beams he produced inside his tubes were not cathode rays; he was working now with “rays” repelled from the opposite plate, the positively charged anode. Such rays were streams of atomic nuclei stripped of their electrons: ionized. They could be generated from gas introduced into the tube. Or solid materials could be coated onto the anode plate itself, in which case ionized atoms of the material would boil off when the tube was evacuated and the anode was charged.

  Mixed nuclei projected in a radiant beam through a magnetic field would bend into separated component beams according to their velocity, which gave a measure of their mass. An electrostatic field bent the component beams differently depending on their electrical charge
, which gave a measure of their atomic number. “In this way,” writes George de Hevesy, “a great variety of different atoms and atomic groupings were proved to be present in the discharge tube.”513

  Aston thought hard about J. J.’s discharge tube while he worked during the war at the Royal Aircraft Establishment at Farnborough, southwest of London, developing tougher dopes and fabrics for aircraft coverings. He wanted to prove unequivocally that neon was isotopic—J. J. was still unconvinced—and saw the possibility of sorting the isotopes of other elements as well. He thought the positive-ray tube was the answer, but though it was good for general surveying, it was hopelessly imprecise.

  By the time Aston returned to Cambridge in 1918 he had worked the problem out theoretically; he then began building the precision instrument he had envisioned.514 It charged a gas or a coating until the material ionized into its component electrons and nuclei and projected the nuclei through two slits that produced a knife-edge beam like the slit-narrowed beam of light in a spectrograph. It then subjected the beam to a strong electrostatic field; that sorted the different nuclei into separated beams. The separated beams proceeded onward through a magnetic field; that further sorted nuclei according to their mass, producing separated beams of isotopes. Finally the sorted beams struck the plateholder of a camera and marked their precise locations on a calibrated strip of film. How much the magnetic field bent the separated beams—where they blackened the strip of film—determined the mass of their component nuclei to a high degree of accuracy.