6
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 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.”506 This conjecture would have been stunning, if true—so far radioactiveatoms 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.”509Finally, 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 aproton), 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.
Aston called his invention a mass-spectrograph because it sorted elements and isotopes of elements by mass much as an optical spectrograph sorts light by its frequency. The mass-spectrograph was immediately and sensationally a success. “In letters to me in January and February, 1920,” says Bohr, “Rutherford expressed his joy in Aston’s work,” which “gave such a convincing confirmation of Rutherford’s atomic model.”515 Of 281 naturally occurring isotopes, over the next two decades Aston identified 212. He discovered that the weights of the atoms of all the elements he measured, with the notable exception of hydrogen, were very nearly whole numbers, which was a powerful argument in favor of the theory that the elements were assembled in nature simply from protons and electrons—from hydrogen atoms, that is. Natural elements had not weighed up in whole numbers for the chemists because they were often mixtures of isotopes of different whole-number weights. Aston proved, for example, as he noted in a later lecture, “that neon consisted, beyond doubt, of isotopes 20 and 22, and that its atomic weight 20.2 was the result of these being present in the ratio of about 9 to 1.”516 That satisfied even J. J. Thomson.
But why was hydrogen an exception? If the elements were built up from hydrogen atoms, why did the hydrogen atom itself, the elemental building block, weigh 1.008 alone? Why did it then shrink to 4 when it was packed in quartet as helium? Why not 4.032? And why was helium not exactly 4 but 4.002, or oxygen not exactly 16 but 15.994? What was the meaning of these extremely small, and varying, differences from whole numbers?
Atoms do not fall apart, Aston reasoned. Something very powerful holds them together. That glue is now called binding energy. To acquire it, hydrogen atoms packed together in a nucleus sacrifice some of their mass. This mass defect is what Aston found when he compared the hydrogen atom to the atoms of other elements following his whole-number rule. In addition, he said, nuclei may be more or less loosely packed. The density of their packing requires more or less binding energy, and that in turn requires more or less mass: hence the small variations. The difference between the measured mass and the whole number he expressed as a fraction, the packing fraction: roughly, the divergence of an element from its whole number divided by its whole number. “High packing fractions,” Aston proposed, “indicate looseness of packing, and therefore low stability: low packing fractions the reverse.”517 He plotted the packing fractions on a graph and demonstrated that the elements in the broad middle of the periodic table—nickel, iron, tin, for example—had the lowest packing fractions and were therefore the most stable, while elements at the extremes of the periodic table—hydrogen at the light end, for example, uranium at the heavy—had high packing fractions and were therefore the most unstable. Locked within all the elements, he said, but most unstably so in the case of those with high packing fractions, was mass converted to energy. Comparing helium to hydrogen, nearly 1 percent of the hydrogen mass was missing (4 divided by 4.032 = .992 = 99.2%). “If we were able to transmute [hydrogen] into [helium] nearly 1 percent of the mass would be annihilated. On the relativity equivalence of mass and energy now experimentally proved [Aston refers here to Einstein’s famous equation E = mc2], the quantity of energy liberated would be prodigious. Thus to change the hydrogen in a glass of water into helium would release enough energy to drive the ‘Queen Mary’ across the Atlantic and back at full speed.”518
Aston goes on in this lecture, delivered in 1936, to speculate about the social consequences of that energy release. Armed with the necessary knowledge, he says, “the nuclear chemists, I am convinced, will be able to synthesise elements just as ordinary chemists synthesise compounds, and it may be taken as certain that in some reactions sub-atomic energy will be liberated.” And, continuing:519
There are those about us who say that such research should be stopped by law, alleging that man’s destructive powers are already large enough. So, no doubt, the more elderly and ape-like of our prehistoric ancestors objected to the innovation of cooked food and pointed out the grave dangers attending the use of the newly discovered agency, fire. Personally I think there is no doubt that sub-atomic energy is available all around us, and that one day man will release and control its almost infinite power. We cannot prevent him from doing so and can only hope that he will not use it exclusively in blowing up his next door neighbor.
The mass-spectrograph Francis Aston invented in 1919 could not release the binding energy of the atom. But with it he identified that binding energy and located the groups of elements which in their comparative instability might be most likely to release it if suitably addressed. He was awarded the Nobel Prize in Chemistry in 1922 for his work. After accepting the award alongside Niels Bohr—“Stockholm has been the city of our dreams ever since,” his sister, who regularly traveled with him, reminisces—he returned to the Cavendish to build larger and more accurate mass-spectrographs, operating them habitually at night because he “particularly detested,” his sister says, “various human noises,” including even conversations muffled through the walls of his rooms.520 “He was very fond of animals, especially cats and kittens, and would go to any amount of trouble to make their acquaintance, but he didn’t like dogs of the barking kind.”521 Although Aston respected Ernest Rutherford enormously, the Cavendish director’s great boom must ever have been a trial.
* * *
The United States led the way in particle acceleration. The American mechanical tradition that advanced the factory and diversified the armory now extended into the laboratory as well. A congressman in 1914 had questioned a witness at an appropriations hearing, “What is a physicist? I was asked on the floor of the House what in the name of common sense a physicist is, and I could not answer.”522 But the war made evident what a physicist was, made evident the value of science to the development of technology, including especially military technology, and government support and the support of private foundations were immediately forthcoming. Twice as many Americans became physicists in the dozen years between 1920 and 1932 as had in the previous sixty. They were better trained than their older counterparts, at least fifty of them in Europe on National Research Council or International Education Board or the new Guggenheim fellowships. By 1932 the United States counted about 2,500 physicists, three times as many as in 1919. ThePhysical Review, the journal that has been to American physicists what the Zeitschrift für Physik is to German, was considered a backwater publication, if not a joke, in Europe before the 1920s. It thickened to more than twice its previous size in that decade, increased in 1929 to biweekly publication, and began to find readers in Cambridge, Copenhagen, Göttingen and Berlin eager to scan it the moment it arrived.
Psychometricians have closely questioned American scientists of this first modern generation, curious to know what kind of men they were—there were few women among them—and from what backgrounds they emerged.523 Small liberal arts colleges in the Middle West and on the Pacific coast, one study found, were most productive of scientists then (by contrast, New England in the same period excelled at the manufacture of lawyers). Half the experimental physicists studied and fully 84 percent of the theoreticians were the sons of professional men, typically engineers, physicians and teachers, although a minority of experimentalists were farmers’ sons. None of the fathers of the sixty-four scientists, including twenty-two physicists, in the largest of these studies was an unskilled laborer, and few of the fathers of physicists were businessmen. The physicists were almost all either first-born sons or eldest sons. Theoretical physicists averaged the highest verbal IQ’s among all scientists studied, clustering around 170, almost 20 percent higher than the experimentalists.524 Theoreticians also averaged the highest spatial IQ’s, experimentalists ranking second.
The sixty-four-man study which included twenty-two physicists among its “most eminent scientists in the U.S.” produced this composite portrait of the American scientist in his prime:
He is likely to have been a sickly child or to have lost a parent at an early age. He has a very high I.Q. and in boyhood began to do a great deal of reading. He tended to feel lonely and “different” and to be shy and aloof from his classmates. He had only a moderate interest in girls and did not begin dating them until college. He married late . . . has two children and finds security in family life; his marriage is more stable than the average. Not until his junior or senior year in college did he decide on his vocation as a scientist. What decided him (almost invariably) was a college project in which he had occasion to do some independent research—to find out things for himself. Once he discovered the pleasures of this kind of work, he never turned back. He is completely satisfied with his chosen vocation. . . . He works hard and devotedly in his laboratory, often seven days a week. He says his work is his life, and he has few recreations. . . . The movies bore him. He avoids social affairs and political activity, and religion plays no part in his life or thinking. Better than any other interest or activity, scientific research seems to meet the inner need of his nature.525
Clearly this is close to Robert Oppenheimer. The group studied, like the American physics community then, was predominantly Protestant in origin with a disproportionate minority of Jews and no Catholics.
A psychological examination of scientists at Berkeley, using Rorschach and Thematic Apperception Tests as well as interviews, included six physicists and twelve chemists in a total group of forty.526 It found that scientists think about problems in much the same way artists do. Scientists and artists proved less similar in personality than in cognition, but both groups were similarly different from businessmen. Dramatically and significantly, almost half the scientists in this study reported themselves to have been fatherless as children, “their fathers dying early, or working away from home, or remaining so aloof and nonsupportive that their sons scarcely knew them.”527 Those scientists who grew up with living fathers described them as “rigid, stern, aloof, and emotionally reserved.”528(A group of artists previously studied was similarly fatherless; a group of businessmen was not.)
Often fatherless and “shy, lonely,” writes the psychometrician Lewis M. Terman, “slow in social development, indifferent to close personal relationships, group activities or politics,” these highly intelligent young men found their way into science through a more personal discovery than the regularly reported pleasure of independent research.529 Guiding that research was usually a fatherly science teacher.530 Of the qualities that distinguished this mentor in the minds of his students, not teaching ability but “masterfulness, warmth and professional dignity” ranked first.531 One study of two hundred of these mentors concludes: “It would appear that the success of such teachers rests mainly upon their capacity to assume a father role to their students.”532 The fatherless young man finds a masterful surrogate father of warmth and dignity, identifies with him and proceeds to emulate him. In a later stage of this process the independent scientist works toward becoming a mentor of historic stature himself.
The man who would found big-machine physics in America arrived at Berkeley one year before Oppenheimer, in 1928. Ernest Orlando Lawrence was three years older than the young theoretician and in many ways his opposite, an extreme of the composite American type.533 Both he and Oppenheimer were tall and both had blue eyes and high expectations. But Ernest Lawrence was an experimentalist, from prairie, small-town South Dakota; of Norwegian stock, the son of a superintendent of schools and teachers’ college president; domestically educated through the Ph.D. at the Universities of South Dakota, Minnesota and Chicago and at Yale; with “almost an aversion to mathematical thought” according to one of his protégés, the later Nobel laureate Luis W. Alvarez; a boyish extrovert whose strongest expletives were “Sugar!” and “Oh fudge!” who learned to stand at ease among the empire builders of patrician California’s Bohemian Grove; a master salesman who paid his way through college peddling aluminum kitchenware farm to farm; with a gift for inventing ingenious machines.534 Lawrence arrived at Berkeley from Yale in a Reo Flying Cloud with his parents and his younger brother in tow and put up at the faculty club. Fired compulsively with ambition—for physics, for himself—he worked from early morning until late at night.
As far back as his first year of graduate school, 1922, Lawrence had begun to think about how to generate high energies. His flamboyant, fatherly mentor encouraged him. William Francis Gray Swann, an Englishman who had found his way to Minnesota via the Department of Terrestrial Magnetism of the District of Columbia’s private Carnegie Institution, took Lawrence along with him first to Chicago and then to Yale as he moved up the academic ladder himself. After Lawrence earned his Ph.D. and a promising reputation Swann convinced Yale to jump him over the traditional four years of instructorship to a starting position as assistant professor of physics. Swann’s leaving Yale in 1926 was one reason Lawrence had decided to move West, that and Berkeley’s offer of an associate professorship, a good laboratory, as many graduate-student assistants as he could handle and $3,300 a year, an offer Yale chose not to match.
At Berkeley, Lawrence said later, “it seemed opportune to review my plans for research, to see whether I might not profitably go into nuclear research, for the pioneer work of Rutherford and his school had clearly indicated that the next great frontier for the experimental physicist was surely the atomic nucleus.”535 But as Luis Alvarez explains, “the tedious nature of Rutherford’s technique . . . repelled most prospective nuclear physicists. Simple calculations showed that one microampere of electrically accelerated light nuclei would be more valuable than the world’s total supply of radium—if the nuclear particles had energies in the neighborhood of a million electron volts.”536
Alpha particles or, better, protons could be accelerated by generating them in a discharge tube and then repelling or attracting them electrically. But no one knew how to confine in one place for any useful length of time, without electrical breakdown from sparking or overheating, the million volts that seemed to be necessary to penetrate the electrical barrier of the heavier nuclei. The problem was essentially mechanical and experimental; not surprisingly, it attracted the young generation of American experimental physicists who had grown up in small towns and on farms experimenting with radio. By 1925 Lawrence’s boyhood friend and Minnesota classmate Merle Tuve, another protégé of W. F. G. Swann now installed at the Carnegie Institution and working with three other physicists, had managed brief but impressive accelerations with a high-voltage transformer submerged in oil; others, including Robert J. Van de Graaff at MIT and Charles C. Lauritsen at Caltech, were also developing machines.
Lawrence pursued more promising studies but kept the high-energy problem in mind. The essential vision came to him in the spring of 1929, four months before Oppenheimer arrived. “In his early bachelor days at Berkeley,” writes Alvarez, “Lawrence spent many of his evenings in the library, reading widely. . . . Although he passed his French and German requirements for the doctor’s degree by the slimmest of margins, and consequently had almost no facility with either language, he faithfully leafed through the back issues of the foreign periodicals, night after night.”537 Such was the extent of Lawrence’s compulsion. It paid. He was skimming the German Arkiv für Elektrotechnik, an electrical-engineering journal physicists seldom read, and happened upon a report by a Norwegian engineer named Rolf Wideröe, Über ein neues Prinzip zur Herstellung hoher Spannungen: “On a new principle for the production of higher voltages.” The title arrested him. He studied the accompanying photographs and diagrams. They explained enough to set Lawrence off and he did not bother to struggle through the text.
Wideröe, elaborating on a principle established by a Swedish physicist in 1924, had found an ingenious way to avoid the high-voltage problem. He mounted two metal cylinders in line, attached them to a voltage source and evacuated them of air. The voltage source supplied 25,000 volts of high-frequency alternating current, current that changed rapidly from positive to negative potential. That meant it could be used both to push and to pull positive ions. Charge the first cylinder negatively to 25,000 volts, inject positive ions into one end, and the ions would be accelerated to 25,000 volts as they left the first cylinder for the second. Alternate the charge then—make the first cylinder positive and the second cylinder negative—and the ions would be pushed and pulled to further acceleration. Add more cylinders, each one longer than the last to allow for the increasing speed of the ions, and theoretically you could accelerate them further still, until such a time as they scattered too far outward from the center and crashed into the cylinder walls. Wideröe’s important innovation was the use of a relatively small voltage to produce increasing acceleration. “This new idea,” says Lawrence, “immediately impressed me as the real answer which I had been looking for to the technical problem of accelerating positive ions, and without looking at the article further I then and there made estimates of the general features of a linear accelerator for protons in the energy range above one million [volts].”538
Lawrence’s calculations momentarily discouraged him. The accelerator tube would be “some meters in length,” too long, he thought, for the laboratory. (Linear accelerators today range in length up to two miles.) “And accordingly, I asked myself the question, instead of using a large number of cylindrical electrodes in line, might it not be possible to use two electrodes over and over again by sending the positive ions back and forth through the electrodes by some sort of appropriate magnetic field arrangement.” The arrangement he conceived was a spiral. “It struck him almost immediately,” Alvarez later wrote, “that one might ‘wind up’ a linear accelerator into a spiral accelerator by putting it in a magnetic field,” because the magnetic lines of force in such a field guide the ions.539 Given a welltimed push, they would swing around in a spiral, the spiral becoming larger as the particles accelerated and were thus harder to confine. Then, making a simple calculation for the magnetic-field effects, Lawrence uncovered an unsuspected advantage to a spiral accelerator: in a magnetic field slow particles complete their smaller circuits in exactly the same time faster particles complete their larger circuits, which meant they could all be accelerated together, efficiently, with each alternating push.
Exuberantly Lawrence ran off to tell the world. An astronomer who was still awake at the faculty club was drafted to check his mathematics. He shocked one of his graduate students the next day by bombarding him with the mathematics of spiral accelerations but mustering no interest whatever in his thesis experiment. “Oh, that,” Lawrence told the questioning student. “Well, you know as much on that now as I do. Just go ahead on your own.”540 A faculty wife crossing the campus the next evening heard a startling “I’m going to be famous!” as the young experimentalist burst past her on the walk.541
Lawrence then traveled East to a meeting of the American Physical Society and discovered that not many of his colleagues agreed. To less inspired mechanicians the scattering problem looked insurmountable. Merle Tuve was skeptical. Jesse Beams, a Yale colleague and a close friend, thought it was a great idea if it worked. Despite Lawrence’s reputation as a go-getter—perhaps because no one encouraged him, perhaps because the idea was solid and sure in his head but the machine on the laboratory bench might not be—he kept putting off building his spiral particle accelerator. He was not the first man of ambition to find himself stalling on the summit ridge of a famous future.
Oppenheimer arrived in a battered gray Chrysler in the late summer of 1929 from another holiday at the Sangre de Cristos ranch with Frank—the ranch was named Perro Caliente now, “hot dog,” Oppenheimer’s cheer when he had learned the property could be leased.542 He put up at the faculty club and the two opposite numbers, he and Lawrence, became close friends. Oppenheimer saw “unbelievable vitality and love of life” in Lawrence. “Work all day, run off for tennis, and work half the night. His interest was so primarily active [and] instrumental and mine just the opposite.”543 They rode horses together, Lawrence in jodhpurs and using an English saddle in the American West—to distance himself, Oppenheimer thought, from the farm. When Lawrence could get away they went off on long recreational drives in the Reo to Yosemite and Death Valley.
A distinguished experimentalist from the University of Hamburg, Otto Stern, a Breslau Ph.D., forty-one that year and on his way to a Nobel Prize (though Lawrence would beat him), gave Lawrence the necessary boost. Sometime after the Christmas holidays the two men dined out in San Francisco, a pleasant ferry ride across the unbridged bay. Lawrence rehearsed again his practiced story of particles spinning to boundless energies in a confining magnetic field, but instead of coughing politely and changing the subject, as so many other colleagues had done, Stern produced a Germanic duplicate of Lawrence’s original enthusiasm and barked at him to leave the restaurant immediately and go to work. Lawrence waited in decency until morning, cornered one of his graduate students and committed him to the project as soon as he had finished studying for his Ph.D. exam.
The machine that resulted looked, in top and side views, like this:
The two cylinders of the Wideröe accelerator have become two brass electrodes shaped like the cut halves of a cylindrical flask. These are contained completely within a vacuum tank and the vacuum tank is mounted between the round, flat poles of a large electromagnet.
In the space between the two electrodes (which came to be called dees because of their shape), at the center point, a hot filament and an outlet for hydrogen gas work together to produce protons which stream off into the magnetic field. The two dees, alternately charged, push and pull the protons as they come around. When they have been accelerated through about a hundred spirals the particles exit in a beam which can then be directed onto a target. With a 4.5-inch chamber and with less than 1,000 volts on the dees, on January 2, 1931, Lawrence and his student M. Stanley Livingston produced 80,000-volt protons.
The scattering problem solved itself at low accelerations when Livingston thought to remove the fine grid of wires installed in the gap between the dees that kept the accelerating electric field out of the drift space inside. The electric fields between the dee edges suddenly began functioning as lenses, focusing the spiraling particles by deflecting them back toward the middle plane. “The intensity then became a hundred times what it was before,” Livingston says.544 That effect was too weak to confine the higher-speed particles. Livingston turned his attention to magnetic confinement. He suspected the particle beam lost focus at higher speeds because the pole faces of the magnet were not completely true, a lack of uniformity which in turn caused irregularities in the magnetic field. Impulsively he cut sheets of iron foil into small shims “having a shape much like an exclamation point,” as Lawrence and he would write in the Physical Review, and inserted the shims by trial and error between the pole faces and the vacuum chamber.545Thus tuning the magnetic field “increased the amplification factor . . . from about 75 to about 300”—Lawrence added these triumphant italics. With both electric and magnetic focusing, in February 1932 an eleven-inch machine produced million-volt protons. It had a nickname by then that Lawrence would make official in 1936: cyclotron. Even in the formal scientific report to the Physical Review on April 1, 1932, he was unable to contain his enthusiasm for the new machine’s possibilities:
Assuming then a voltage amplification of 500, the production of 25,000,000 volt-protons [!] would require 50,000 volts at a wave-length of 14 meters applied across the accelerators; thus, 25,000 volts on each accelerator with respect to ground. It does appear entirely feasible to do this.546
The magnet for that one would weigh eighty tons, heavier than any machine used in physics up to that time. Lawrence, now a full professor, was already raising funds.
* * *
In his graduate-student days in Europe Robert Oppenheimer told a friend that he dreamed of founding a great school of theoretical physics in the United States—at Berkeley, as it happened, the second desert after New Mexico that he chose to colonize.547 Ernest Lawrence seems to have dreamed of founding a great laboratory. Both men coveted success and, each in his own way, the rewards of success, but they were differently driven.
Oppenheimer’s youthful preciosity matured in Europe and the early Berkeley years into refinement that was usually admirable if still sometimes exquisite. Oppenheimer crafted that persona for himself at least in part from a distaste for vulgarity that probably originated in rebellion against his entrepreneurial father and that was not without elements of anti-Semitic self-hatred. Along the way he convinced himself that ambition and worldly success were vulgar, a conviction bolstered nicely by trust fund earnings to the extent of ten thousand dollars a year. Thereby he confounded his own strivings. The American experimental physicist I. I. Rabi would later question why “men of Oppenheimer’s gifts do not discover everything worth discovering.”548 His answer addresses one possible source of limitation:
It seems to me that in some respects Oppenheimer was overeducated in those fields which lie outside the scientific tradition, such as his interest in religion, in the Hindu religion in particular, which resulted in a feeling for the mystery of the universe that surrounded him almost like a fog. He saw physics clearly, looking toward what had already been done, but at the border he tended to feel that there was much more of the mysterious and novel than there actually was. . . . Some may call it a lack of faith, but in my opinion it was more a turning away from the hard, crude methods of theoretical physics into a mystical realm of broad intuition.
But Oppenheimer’s revulsion from what he considered vulgar, from just those “hard, crude methods” to which Rabi refers, must have been another and more directly punishing confusion. His elegant physics, so far as an outsider can tell—his scientific papers are nearly impenetrable to the nonmathematician and deliberately so—is a physics of bank shots. It works the sides and the corners and uses the full court but prefers not to drive relentlessly for the goal. Wolfgang Pauli and the hard, distant Cambridge theoretician Paul A. M. Dirac, Eugene Wigner’s brother-in-law, both mathematicians of formidable originality, were his models. Oppenheimer first described the so-called tunnel effect whereby an uncertainly located particle sails through the electrical barrier around the nucleus on a light breeze of probability, existing—in particle terms—then ceasing to exist, then instantly existing again on the other side.549 But George Gamow, the antic Russian, lecturing in Cambridge, devised the tunnel-effect equations that the experimenters used. Hans Bethe in the late 1930s first defined the mechanisms of carbon-cycle thermonuclear burning that fire the stars, work which won for him the Nobel Prize; Oppenheimer looked into the subtleties of the invisible cosmic margins, modeled the imploding collapse of dying suns and described theoretical stellar objects that would not be discovered for thirty and forty years—neutron stars, black holes—because the instruments required to detect them, radio telescopes and X-ray satellites, had not been invented yet.550 (Alvarez believes if Oppenheimer had lived long enough to see these developments he would have won a Nobel Prize for his work.) That was originality not so much ahead of its time as outside the frame.
Some of this psychological and creative convolution winds through a capsule essay on the virtues of discipline that Oppenheimer composed within a letter to his brother Frank in March 1932, when he was not quite twenty-eight years old. It is worth copying out at length; it hints of the long, self-punishing penance he expected to serve to cleanse any stain of crudity from his soul:
You put a hard question on the virtue of discipline. What you say is true: I do value it—and I think that you do too—more than for its earthly fruit, proficiency. I think that one can give only a metaphysical ground for this evaluation; but the variety of metaphysics which gave an answer to your question has been very great, the metaphysics themselves very disparate: the bhagavad gita, Ecclesiastes, the Stoa, the beginning of the Laws, Hugo of St Victor, St Thomas, John of the Cross, Spinoza. This very great disparity suggests that the fact that discipline is good for the soul is more fundamental than any of the grounds given for its goodness. I believe that through discipline, though not through discipline alone, we can achieve serenity, and a certain small but precious measure of freedom from the accidents of incarnation, and charity, and that detachment which preserves the world which it renounces. I believe that through discipline we can learn to preserve what is essential to our happiness in more and more adverse circumstances, and to abandon with simplicity what would else have seemed to us indispensable; that we come a little to see the world without the gross distortion of personal desire, and in seeing it so, accept more easily our earthly privation and its earthly horror—But because I believe that the reward of discipline is greater than its immediate objective, I would not have you think that discipline without objective is possible: in its nature discipline involves the subjection of the soul to some perhaps minor end; and that end must be real, if the discipline is not to be factitious. Therefore I think that all things which evoke discipline: study, and our duties to men and to the commonwealth, war, and personal hardship, and even the need for subsistence, ought to be greeted by us with profound gratitude, for only through them can we attain to the least detachment; and only so can we know peace.551
Lawrence, orders of magnitude less articulate than Oppenheimer, was also fiercely driven; the question is what drove him. A paragraph from a letter to his brother John, written at about the same time as Oppenheimer’s essay, is revealing: “Interested to hear you have had a period of depression. I have them often—sometimes nothing seems to be OK—but I have gotten used to them now. I expect the blues and I endure them. Of course the best palliative is work, but sometimes it is hard to work under the circumstances.”552That work is only a “palliative,” not a cure, hints at how blue the blues could be. Lawrence was a hidden sufferer, in some measure manicdepressive; he kept moving not to fall in.
To all these emotional troublings—Oppenheimer’s and Lawrence’s, as Bohr’s and others’ before and since—science offered an anchor: in discovery is the preservation of the world. The psychologist who studied scientists at Berkeley with Rorschach and TAT found that “uncommon sensitivity to experiences—usually sensory experiences” is the beginning of creative discovery in science. “Heightened sensitivity is accompanied in thinking by overalertness to relatively unimportant or tangential aspects of problems. It makes [scientists] look for and postulate significance in things which customarily would not be singled out. It encourages highly individualized and even autistic ways of thinking.”553 Consider Rutherford playing his thoroughly unlikely hunch about alpha backscattering, Heisenberg remembering an obscure remark of Einstein’s and concluding that nature only performed in consonance with his mathematics, Lawrence flipping compulsively through obscure foreign journals:
Were this thinking not in the framework of scientific work, it would be considered paranoid. In scientific work, creative thinking demands seeing things not seen previously, or in ways not previously imagined; and this necessitates jumping off from “normal” positions, and taking risks by departing from reality. The difference between the thinking of the paranoid patient and the scientist comes from the latter’s ability and willingness to test out his fantasies or grandiose conceptualizations through the systems of checks and balances science has established—and to give up those schemes that are shown not to be valid on the basis of these scientific checks. It is specifically because science provides such a framework of rules and regulations to control and set bounds to paranoid thinking that a scientist can feel comfortable about taking the paranoid leaps. Without this structuring, the threat of such unrealistic, illogical, and even bizarre thinking to overall thought and personality organization in general would be too great to permit the scientist the freedom of such fantasying.554
At the leading edges of science, at the threshold of the truly new, the threat has often nearly overwhelmed. Thus Rutherford’s shock at rebounding alpha particles, “quite the most incredible event that has ever happened to me in my life.” Thus Heisenberg’s “deep alarm” when he came upon his quantum mechanics, his hallucination of looking through “the surface of atomic phenomena” into “a strangely beautiful interior” that left him giddy. Thus also, in November 1915, Einstein’s extreme reaction when he realized that the general theory of relativity he was painfully developing in the isolation of his study explained anomalies in the orbit of Mercury that had been a mystery to astronomers for more than fifty years. The theoretical physicist Abraham Pais, his biographer, concludes: “This discovery was, I believe, by far the strongest emotional experience in Einstein’s scientific life, perhaps in all his life. Nature had spoken to him. He had to be right. ‘For a few days, I was beside myself with joyous excitement.’ Later, he told [a friend] that his discovery had given him palpitations of the heart. What he told [another friend] is even more profoundly significant: when he saw that his calculations agreed with the unexplained astronomical observations, he had the feeling that something actually snapped in him.”555
The compensation for such emotional risk can be enormous. For the scientist, at exactly the moment of discovery—that most unstable existential moment—the external world, nature itself, deeply confirms his innermost fantastic convictions. Anchored abruptly in the world, Leviathan gasping on his hook, he is saved from extreme mental disorder by the most profound affirmation of the real.
Bohr especially understood this mechanism and had the courage to turn it around and use it as an instrument of assay. Otto Frisch remembers a discussion someone attempted to deflect by telling Bohr it made him giddy, to which Bohr responded: “But if anybody says he can think about quantum problems without getting giddy, that only shows that he has not understood the first thing about them.”556 Much later, Oppenheimer once told an audience, Bohr was listening to Pauli talking about a new theory on which he had recently been attacked. “And Bohr asked, at the end, ‘Is this really crazy enough? The quantum mechanics was really crazy.’ And Pauli said, ‘I hope so, but maybe not quite.’ ”557 Bohr’s understanding of how crazy discovery must be clarifies why Oppenheimer sometimes found himself unable to push alone into the raw original. To do so requires a sturdiness at the core of identity—even a brutality—that men as different as Niels Bohr and Ernest Lawrence had earned or been granted that he was unlucky enough to lack. It seems he was cut out for other work: for now, building that school of theoretical physics he had dreamed of.
* * *
On June 3, 1920, Ernest Rutherford delivered the Bakerian Lecture before the Royal Society of London.558 It was the second time he had been invited to fill the distinguished lectureship. He used the occasion to sum up present understanding of the “nuclear constitution” and to discuss his successful transmutation of the nitrogen atom reported the previous year, the usual backward glance of such formal public events. But unusually and presciently, he also chose to speculate about the possibility of a third major constituent of atoms besides electrons and protons. He spoke of “the possible existence of an atom of mass 1 which has zero nucleus charge.” Such an atomic structure, he thought, seemed by no means impossible. It would not be a new elementary particle, he supposed, but a combination of existing particles, an electron and a proton intimately united, forming a single neutral particle.559
“Such an atom,” Rutherford went on with his usual perspicacity, “would have very novel properties. Its external [electrical] field would be practically zero, except very close to the nucleus, and in consequence it should be able to move freely through matter. Its presence would probably be difficult to detect by the spectroscope, and it may be impossible to contain it in a sealed vessel.” Those might be its peculiarities. This would be its exceptional use: “On the other hand, it should enter readily the structure of atoms, and may either unite with the nucleus or be disintegrated by its intense field.” A neutral particle, if such existed—a neutron—might be the most effective of all tools to probe the atomic nucleus.
Rutherford’s assistant James Chadwick attended this lecture and found cause for disagreement.560 Chadwick was then twenty-nine years old. He had trained at Manchester and followed Rutherford down to Cambridge. He had accomplished much already—as a young man, two of his colleagues write, his output “was hardly inferior to that of Moseley”—but he had sat out the Great War in a German internment camp, to the detriment of his health and to his everlasting boredom, and he was eager to move the new work of nuclear physics along.561 A neutral particle would be a wonder, but Chadwick thought Rutherford had deduced it from flimsy evidence.
That winter he discovered his mistake. Rutherford invited him to participate in the work of extending the nitrogen transmutation results to heavier elements. Chadwick had improved scintillation counting by developing a microscope that gathered more light and by tightening up procedures. He also knew chemistry and might help eliminate hydrogen as a possible contaminant, a challenge to the nitrogen results that still bothered Rutherford. “But also, I think,” said Chadwick many years later in a memoriallecture, “he wanted company to support the tedium of counting in the dark—and to lend an ear to his robust rendering of ‘Onward, Christian Soldiers.’ ”562
“Before the experiments,” Chadwick once told an interviewer, “before we began to observe in these experiments, we had to accustom ourselves to the dark, to get our eyes adjusted, and we had a big box in the room in which we took refuge while Crowe, Rutherford’s personal assistant and technician, prepared the apparatus.563 That is to say, he brought the radioactive source down from the radium room, put it in the apparatus, evacuated it, or filled it with whatever, put the various sources in and made the arrangements that we’d agreed upon. And we sat in this dark room, dark box, for perhaps half an hour or so, and naturally, talked.” Among other things, they talked about Rutherford’s Bakerian Lecture. “And it was then that I realized that these observations which I suspected were quite wrong, and which proved to be wrong later on, had nothing whatever to do with his suggestion of the neutron, not really. He just hung the suggestion on to it. Because it had been in his mind for some considerable time.”
Most physicists had been content with the seemingly complete symmetry of two particles, the electron and the proton, one negative, one positive. Outside the atom—among the stripped, ionized matter beaming through a discharge tube, for example—two elementary atomic constituents might be enough. But Rutherford was concerned with how each element was assembled. “He had asked himself,” Chadwick continues, “and kept on asking himself, how the atoms were built up, how on earth were you going to get—the general idea being at that time that protons and electrons were the constituents of an atomic nucleus . . . how on earth were you going to build up a big nucleus with a large positive charge? And the answer was a neutral particle.”
From the lightest elements in the periodic table beyond hydrogen to the heaviest, atomic number—the nucleus’ electrical charge and a count of its protons—differed from atomic weight. Helium’s atomic number was 2 but its atomic weight was 4; nitrogen’s atomic number was 7 but its atomic weight was 14; and the disparity increased farther along: silver, 47 but 107; barium, 56 but 137; radium, 88 but 226; uranium, 92 but 235 or 238. Theory at the time proposed that the difference was made up by additional protons in the nucleus closely associated with nuclear electrons that neutralized them. But the nucleus had a definite maximum size, well established by experiment, and as elements increased in atomic number and atomic weight there appeared to be less and less room in their nuclei for all the extra electrons. The problem worsened with the development in the 1920s of quantum theory, which made it clear that confining particles as light as electrons so closely would require enormous energies, energies that ought to show up when the nucleus was disturbed but never did. The only evidence for the presence of electrons in the nucleus was its occasional ejection of beta particles, energetic electrons. That was something to go on, but given the other difficulties with packing electrons into the nucleus it was not enough.
“And so,” Chadwick concludes, “it was these conversations that convinced me that the neutron must exist. The only question was how the devil could one get evidence for it. . . . It was shortly after that I began to make experiments on the side when I could. [The Cavendish] was very busy, and left me little time, and occasionally Rutherford’s interest would revive, but only occasionally.”564 Chadwick would search for the neutron with Rutherford’s blessing, but the frustrating work of experiment was usually his alone.
His temperament matched the challenge of discovering a particle that might leave little trace of itself in its passage through matter; he was a shy, quiet, conscientious, reliable man, something of a neutron himself. Rutherford even felt it necessary to scold him for giving the boys at the Cavendish too much attention, though Chadwick took their care and nurturing to be his primary responsibility. “It was Chadwick,” remembers Mark Oliphant, “who saw that research students got the equipment they needed, within the very limited resources of the stores and funds at his disposal.”565 If he seemed “dour and unsmiling” at first, with time “the kindly, helpful and generous person beneath became apparent.”566 He tended, says Otto Frisch, “to conceal his kindness behind a gruff façade.”567
The façade was protective. James Chadwick was tall, wiry, dark, with a high forehead, thin lips and a raven’s-beak nose. “He had,” say his joint biographers, colleagues both, “a deep voice and a dry sense of humour with a characteristic chuckle.”568 He was born in the village of Bollington, south of Manchester in Cheshire, in 1891. When he was still a small boy his father left their country home to start a laundry in Manchester; Chadwick’s grandmother seems to have raised him. He sat for two scholarships to the University of Manchester at sixteen, an early age even in the English educational system, won them both, kept one and went off to the university.
He meant to read mathematics. The entrance interviews were held publicly in a large, crowded hall. Chadwick got into the wrong line. He had already begun to answer the lecturer’s questions when he realized he was being questioned for a physics course. Since he was too timid to explain, he decided that the physics lecturer impressed him and he would read for physics. The first year he was sorry, his biographers report: “the physics classes were large and noisy.”569 The second year he heard Rutherford lecture on his early New Zealand experiments and was converted. In his third year Rutherford gave him a research project. His timidity again confounded him, this time almost fatally for his career: he discovered a snag in the procedure Rutherford had recommended to him but could not bring himself to point it out. Rutherford thought he missed it. Man and boy found their way past that misunderstanding and Chadwick graduated from Manchester in 1911 with first-class honors.
He stayed on for his master’s degree, working with A. S. Russell and following the research in those productive years of Geiger, Marsden, de Hevesy, Moseley, Darwin and Bohr. In 1913, taking his M.Sc., he won an important research scholarship that required him to change laboratories to broaden his training. By then Geiger had returned to Berlin; Chadwick followed. Which was a pleasure while it lasted—Geiger made a point of introducing Chadwick around, so that he became acquainted with Einstein, Hahn and Meitner, among others in Berlin—but the war intervened.
A reserve officer, Geiger was called up early. He fortified Chadwick with a personal check for two hundred marks before he left. Some of the young Englishman’s German friends advised him to leave the country quickly, but others convinced him to wait to avoid the danger of encountering troop trains along the way. On August 2 Chadwick tried to buy a ticket home by way of Holland at the Cook’s Tours office in Berlin. Cook’s suggested going through Switzerland instead. That struck Chadwick’s friends as risky. He again accepted their advice and settled in to wait.
Then it was too late. He was arrested along with a German friend for allegedly making subversive remarks—merely speaking English would have done the job in those first weeks of hysterical nationalism—and languished in a Berlin jail for ten days before Geiger’s laboratory arranged his release. 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.570 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.
Some of the experiments Chadwick conducted at the Cavendish in the 1920s to look for the neutron, he says, “were so desperate, so far-fetched as to belong to the days of alchemy.”571 He and Rutherford both thought of the neutron, as Rutherford had imagined it in his Bakerian Lecture, as a close union of proton and electron. They therefore conjured up various ways to torture hydrogen—blasting it with electrical discharges, searching out the effects on it of passing cosmic rays—in the hope that the H atom that had been stable since the early days of the universe would somehow agree to collapse into neutrality at their hands.
The neutral particle resisted their blandishments and the nucleus resisted attack. The laboratory, Chadwick remembers, “passed through a relatively quiet spell. Much interesting and important work was done, but it was work of consolidation rather than of discovery; in spite of many attempts the paths to new fields could not be found.”572 It began to seem, he adds, that “the problem of the new structure of the nucleus might indeed have to be left to the next generation, as Rutherford had once said and as many physicists continued to believe.”573 Rutherford “was a little disappointed, because it was so very difficult to find out anything really important.”574 Quantum theory bloomed while nuclear studies stalled. Rutherford had felt optimistic enough in 1923 to shout at the annual meeting of the British Association, “We are living in the heroic age of physics!” By 1927, in a paper on atomic structure, he was a little less confident.575 “We are not yet able to do more than guess at the structure even of the lighter and presumably least complex atoms,” he writes.576 He proposed a structure nonetheless, with electrons in the nucleus orbiting around nuclear protons, an atom within an atom.
They had other work. In hindsight, it was necessary preparation. The scintillation method of detecting radiation had reached its limit of effectiveness: it was unreliable if the counting rate was greater than 150 per minute or less than about 3 per minute, and both ranges now came into view in nuclear studies.577 A disagreement between the Cavendish and the Vienna Radium Institute convinced even Rutherford of the necessity of change. Vienna had reproduced the Cavendish’s light-element disintegration experiments and published completely different results. Worse, the Vienna physicists attributed the discrepancy to inferior Cavendish equipment. Chadwick laboriously reran the experiments with a specially made microscope with zinc sulfide coated directly onto the lens of the microscope’s objective, which greatly brightened the field. The results confirmed the Cavendish’s earlier count. Chadwick then went to Vienna. “He found,” write his biographers, “that the scintillation counting was done by three young women—it was thought that not only did women have better eyes than men but they were less likely to be distracted by thinking while counting!” Chadwick observed the young women at work and realized that because they understood what was expected of the experiments they produced the expected results, unconsciously counting nonexistent scintillations.578 To test the technicians he gave them, without explanation, an unfamiliar experiment; this time their counts matched his own. Vienna apologized.
Hans Geiger, among others, turned back to the electrical counter he had devised with Rutherford in 1908 and improved it. The result, the Geiger counter, was essentially an electrically charged wire strung inside a gas-filled tube with a thinly covered window that allowed charged particles to enter. Once inside the tube the charged particles ionized gas atoms; the electrons thus stripped from the gas atoms were drawn to the positively charged wire; that changed the current level in the wire; the change, in the form of an electrical pulse, could then be run through an amplifier and converted to a sound—typically a click—or shown as a jump in the sweep of a light beam on the television-like screen of an oscilloscope. The electrical counter could operate continuously and could count above and below the limits possible to fallible physicists peering at scintillation screens. But the early counters had a significant disadvantage: they were highly sensitive to gamma radiation, much more so than zinc sulfide, and the radium compounds the Cavendish used as alpha sources gave off plentiful gamma rays. Polonium, the radioactive element that Marie Curie had discovered in 1898 and named after her native Poland, could be an excellent alternative. It was a good alpha source and with a gamma-ray background 100,000 times less intense than radium it was much less likely to overload an electrical counter. Unfortunately, polonium was difficult to acquire. A ton of uranium ore contained only about 0.1 gram, too little for commercial separation. It was available practically only as a byproduct of the radioactive decay of radium, and radium too was scarce.
There was time in those years to recover from the bleakness of the war and get on with living. In 1925 Chadwick married Aileen Stewart-Brown, daughter of a family long established in business in Liverpool. He had been living at Gonville and Caius College; now he made plans for permanent residence. A year later, in the midst of house-building, when Rutherford asked him and another Cavendish man to take on part of the work of revising Rutherford’s old textbook on radioactivity, he fitted in the duty at night, working bundled in an overcoat at a writing table moved close to the fireplace of a drafty temporary rental. When the fire burned low he even pulled on gloves.
At the end of the decade the Rutherfords suffered a personal tragedy. Their daughter Eileen, twenty-nine years old and the mother of three children—she was married to a theoretician, R. H. Fowler, who kept up that end of physics at the Cavendish—gave birth to a fourth; one week later, on December 23, she was felled by a lethal blood clot. “The loss of his only child,” writes A. S. Eve, “whom he loved and admired, aged Rutherford for a time; he looked older and stooped more. He continued his life and work with a manful purpose, and one of the delights of his life was his group of four grandchildren. His face always lit up when he spoke of them.”579
Rutherford was elevated to baron in the New Year’s Honours List of 1931, the year he would turn sixty. A kiwi crested his armorial bearings; they were supported on the dexter side by a figure representing Hermes Trismegistus, the Egyptian god of wisdom who was supposed to have written alchemical books, and on the sinister side by a Maori holding a club; and the two crossed curves that quartered his escutcheon traced the matched growth and decay of activity that gives each radioactive element and isotope its characteristic half-life.580
Around 1928 a German physicist, Walther Bothe, “a real physicist’s physicist” to Emilio Segrè, and Bothe’s student Herbert Becker began studying the gamma radiation excited by alpha bombardment of light elements.581 They surveyed the light elements from lithium to oxygen as well as magnesium, aluminum and silver. Since they were concentrating on gamma radiation excited from a target they wanted a minimum gamma background and used a polonium radiation source. “I don’t know how [Bothe] got his sources,” Chadwick puzzles, “but he did.”582 Lise Meitner had generously sent polonium to Chadwick from the Kaiser Wilhelm Institutes, but it was too little to allow Chadwick to do the work Bothe was doing.
The Germans found gamma excitation with boron, magnesium and aluminum, as they had more or less expected, because alpha particles disintegrate those elements, but they also and unexpectedly found it with lithium and beryllium, which alphas in this reaction did not disintegrate. “Indeed,” writes Norman Feather, one of Chadwick’s colleagues at the Cavendish, “with beryllium, the intensity of the . . . radiation was nearly ten times as great as with any other element investigated.”583 That was strange enough; equally strange was the oddity that beryllium emitted this intense radiation under alpha bombardment without emitting protons. Bothe and Becker reported their results briefly in August 1930, then more fully in December. The radiation they had excited from beryllium had more energy than the bombarding alpha particles. The principle of the conservation of energy required a source for the excess; they proposed that it came from nuclear disintegration despite the absence of protons.
Chadwick set one of his research students, an Australian named H. C. Webster, to work studying these unusual results. A French team began the same study a little later with better resources: Irene Curie, Mme. Curie’s somber and talented daughter, then thirty-three, and her husband Frédéric Joliot, two years younger, a handsome, outgoing man trained originally as an engineer whose charm reminded Segré of the French singer Maurice Chevalier.
Marie Curie’s Radium Institute at the east end of the Rue Pierre Curie in the Latin Quarter, built just before the war with funds from the French government and the Pasteur Foundation, had the advantage in any studies that required polonium. Radon gas decays over time to three only mildly radioactive isotopes: lead 210, bismuth 210 and polonium 210, which thus become available for chemical separation. Medical doctors throughout the world then used radon sealed into glass ampules—“seeds”—for cancer treatment. When the radon decayed, which it did in a matter of days, the seeds no longer served. Many physicians sent them on to Paris as a tribute to the woman who discovered radium. They accumulated to the world’s largest source of polonium.
The Joliot-Curies had worked independently for the two years since their marriage in 1927; in 1929 they decided to work in collaboration. They first developed new chemical techniques for separating polonium, and by 1931 had purified a volume of the element almost ten times more intense than any other existing source. With their powerful new source they turned their attention to the mystery of beryllium.
Chadwick’s student H. C. Webster had progressed in the meantime, by the late spring of 1931, beyond recapitulation to discovery: he found, says Chadwick, “that the radiation from beryllium which was emitted in the same direction as the . . . alpha-particles was more penetrating than the radiation emitted in a backward direction.”584 Gamma radiation, an energetic form of light, should be emitted equally in every direction from a point source such as a nucleus, just as visible light radiates equally from a lightbulb filament. A particle, on the other hand, would usually be bumped forward by an incoming alpha. “And that, of course,” Chadwick adds, “was a point which excited me very much indeed, because I thought, ‘Here’s the neutron.’ ”585
With twin daughters now, Chadwick had become a family man of regular habits. Among the most sacred of these was his annual June family vacation. The possibility of finding his long-sought neutron was not sufficient cause to change his plans. It might have been, but he thought he needed a cloud chamber for the next step in the search, and the one immediately available to him at the Cavendish was not in working order. He found a cloud chamber in other hands; its owner agreed to help Webster use it when he had finished using it himself. Still assuming that the neutron was an electron-proton doublet with enough residual electrical charge to ionize a gas at least weakly, Chadwick wanted Webster to aim the beryllium radiation into the cloud chamber and see if he could photograph its ionizing tracks. He left his student to the work and went off on holiday.
“Of course,” Chadwick said in retrospect of the neutron he was hunting for, “they should not have seen anything” in the cloud chamber, nor did they. “They wrote and told me what had happened, that they hadn’t found anything, which disappointed me very much.”586 When Webster moved on to the University of Bristol, Chadwick decided to take over the beryllium research himself.
First he had to shift his laboratory to a different part of the Cavendish building, and that delayed him; then he had to prepare a strong polonium source. In the matter of polonium he was lucky. Norman Feather had spent the 1929–30 academic year in Baltimore, in the physics department at Johns Hopkins, and there befriended an English physician who was in charge of the radium supply at Baltimore’s Kelly Hospital. The physician had stored away several hundred used radon seeds; “together,” Feather remembers, “they contained almost as much polonium as was available to Curie and Joliot in Paris.”587 The hospital donated them to the Cavendish and Feather brought them home. Chadwick accomplished the dangerous chemical separation that autumn.
Irène Joliot-Curie reported her first results to the French Academy of Sciences on December 28, 1931. The beryllium radiation, she found, was even more penetrating than Bothe and Becker had reported. She standardized her measurements and put the energy of the radiation at three times the energy of the bombarding alpha particle.
The Joliot-Curies decided next to see if the beryllium radiation would knock protons out of matter as alpha particles did. “They fitted their ionization chamber with a thin window,” explains Feather, “and placed various materials close to the window in the path of the radiation. They found nothing, except with materials such as paraffin wax and cellophane which already contained hydrogen in chemical combination. When thin layers of these substances were close to the window, the current in the ionization chamber was greater than usual. By a series of experimental tests, both simple and elegant, they produced convincing evidence that this excess ionization was due to protons ejected from the hydrogenous material.”588 The Joliot-Curies understood then that what they were seeing were elastic collisions—like the collisions of billiard balls or marbles—between the beryllium radiation and the nuclei of H atoms.
But they were still committed to their previous conviction that the penetrating radiation from beryllium was gamma radiation. They had not thought about the possibility of a neutral particle. They had not read Rutherford’s Bakerian Lecture because such lectures were invariably, in their experience, only recapitulations of previously reported work. Rutherford and Chadwick alone had thought seriously about the neutron.
On January 18, 1932, the Joliot-Curies reported to the Academy of Sciences their discovery that paraffin wax emitted high-velocity protons when bombarded by beryllium radiation. But that was not the title and the argument of the paper they wrote. They titled their paper “The emission of protons of high velocity from hydrogenous materials irradiated with very penetrating gamma rays.” Which was as unlikely as if a marble should deflect a wrecking ball. Gamma rays could deflect electrons, a phenomenon known as the Compton effect after its discoverer, the American experimental physicist Arthur Holly Compton, but a proton is 1,836 times heavier than an electron and not easily moved.
At the Cavendish in early February Chadwick found the Comptes Rendus, the French physics journal, in his morning mail, discovered the Joliot-Curie paper and read it with widening eyes:
Not many minutes afterward Feather came to my room to tell me about this report, as astonished as I was. A little later that morning I told Rutherford. It was a custom of long standing that I should visit him about 11 a.m. to tell him any news of interest and to discuss the work in progress in the laboratory. As I told him about the Curie-Joliot observation and their views on it, I saw his growing amazement; and finally he burst out “I don’t believe it.” Such an impatient remark was utterly out of character, and in all my long association with him I recall no similar occasion. I mention it to emphasize the electrifying effect of the Curie-Joliot report. Of course, Rutherford agreed that one must believe the observations; the explanation was quite another matter.589
No further duty interposed itself between Chadwick and his destiny. He went fervently to work, starting on February 7, 1932, a Sunday: “It so happened that I was just ready to begin experiment [when he read of the Joliot-Curie discovery]. . . . I started with an open mind, though naturally my thoughts were on the neutron. I was reasonably sure that the CurieJoliot observations could not be ascribed to a kind of Compton effect, for I had looked for this more than once. I was convinced that there was something quite new as well as strange.”
His simple apparatus consisted of a radiation source and an ionization chamber, the chamber connected to a vacuum-tube amplifier and thence to an oscilloscope. The radiation source, an evacuated metal tube strapped to a rough-sawn block of pine, contained a one-centimeter silver disk coated with polonium mounted close behind a two-centimeter disk of pure beryllium, a silver-gray metal that is three times as light as aluminum.590 Alpha particles from the polonium striking beryllium nuclei knocked out the penetrating beryllium radiation, which, Chadwick found immediately, would pass essentially unimpeded through as much as two centimeters of lead.
The half-inch opening into the small ionization chamber that faced this radiation source was covered with aluminum foil. Within the shallow chamber, in an atmosphere of air at normal pressure, a small charged plate collected electrons ionized by incoming radiation and moved their pulses along to the amplifier and oscilloscope. “For the purpose at hand,” explains Norman Feather, “such an arrangement was ideal. If the amplifier were carefully designed, it was possible to ensure that the magnitude of the oscillograph deflection was directly proportional to the amount of ionization produced in the chamber. . . . The energy of the recoil atom producing the ionization could thus be calculated directly from the size of the deflection on the oscillograph record.”591
Chadwick mounted a sheet of paraffin two millimeters thick in front of the aluminum-foil window into the ionization chamber; immediately, he wrote in his final report on the experiment, “the number of deflections recorded by the oscillograph increased markedly.” That showed that particles ejected from the paraffin were entering the chamber. Then he began interposing sheets of aluminum foil between the wax and the chamber window until no more kicks appeared on the oscilloscope; by scaling the absorptions of aluminum compared to air he calculated the range of the particles as just over 40 centimeters in air; that range meant “it was obvious that the particles were protons.”592
Thus repeating the Joliot-Curie work prepared the way. Now Chadwick broke new ground. He removed the paraffin sheet. He wanted to study what happens to other elements bombarded directly by the beryllium radiation. Elements in the form of solids he mounted in front of the chamber window: “In this way lithium, beryllium, boron, carbon and nitrogen, as paracyanogen, were tested.”593 Elements in the form of gases he simply pumped into the chamber to replace the ambient air: “Hydrogen, helium, nitrogen, oxygen, and argon were examined in this way.”594 In every case the kicks increased on the oscilloscope; the powerful beryllium radiation knocked protons out of all the elements Chadwick tested. It knocked about the same number out of each element. And, most important for his conclusion, the energies of the recoiling protons were significantly greater than they could possibly be if the beryllium radiation consisted of gamma rays. “In general,” Chadwick wrote, “the experimental results show that if the recoil atoms are to be explained by collision with a [gamma-ray photon], we must assume a larger and larger energy for the [photon] as the mass of the struck atom increases.”595 Then, quietly, in what in fact is a devastating criticism of the Joliot-Curie thesis, invoking the basic physical rule that no more energy or momentum can come out of an event than went into it: “It is evident that we must either relinquish the application of the conservation of energy and momentum in these collisions or adopt another hypothesis about the nature of the radiation.” When they read that sentence the Joliot-Curies were deeply and properly chagrined.
The hypothesis Chadwick proposed adopting should come as no surprise: “If we suppose that the radiation is not a [gamma] radiation, but consists of particles of mass very nearly equal to that of the proton, all the difficulties connected with the collisions disappear, both with regard to their frequency and to the energy transfer to different masses. In order to explain the great penetrating power of the radiation we must further assume that the particle has no net charge. . . . We may suppose it [to be] the ‘neutron’ discussed by Rutherford in his Bakerian Lecture of 1920.”
Chadwick then worked the numbers to show that his hypothesis was the correct one to explain the facts.
“It was a strenuous time,” he said afterward.596 From beginning to end the work took ten days and he kept up his Cavendish responsibilities besides. He averaged perhaps three hours of sleep a night, labored over the weekend of February 13–14 as well, finished probably on the seventeenth, a Wednesday, the day he sent off a first brief report to Nature to establish priority of discovery. He titled that report, published as a letter to the editor, “Possible existence of a neutron.” “But there was no doubt whatever in my mind or I should not have written the letter.”597
“To [Chadwick’s] great credit,” writes Segré in tribute, “when the neutron was not present [in earlier experiments] he did not detect it, and when it ultimately was there he perceived it immediately, clearly and convincingly.598 These are the marks of a great experimental physicist.”
A young Russian, Peter Kapitza, had come up to Cambridge in 1921 to work at the Cavendish. He was solid, dedicated, charming and technically inventive and he soon made himself the apple of Rutherford’s eye, the only one among all the boys, even including Chadwick, who could convince the frugal director to allow large sums of money to be spent for apparatus. In 1936 Rutherford would attack Chadwick angrily for encouraging the construction of a cyclotron at the Cavendish; but already in 1932 Kapitza had a separate laboratory in an elegant new brick building in the Cavendish courtyard for his expensive experiments with powerful magnetic fields. As Kapitza had settled in at Cambridge he had noticed what he considered to be an excessive and unproductive deference of British physics students to their seniors. He therefore founded a club, the Kapitza Club, devoted to open and unhierarchical discussion. Membership was limited and coveted. Members met in college rooms and Kapitza frequently opened discussions with deliberate howlers so that even the youngest would speak up to correct him, loosening the grip of tradition on their necks.
That Wednesday Kapitza wined and dined the exhausted Chadwick into what Mark Oliphant calls “a very mellow mood,” then brought him along to a Kapitza Club meeting.599 “The intense excitement of all in the Cavendish, including Rutherford,” Oliphant remembers, “was already remarkable, for we had heard rumors of Chadwick’s results.” Oliphant says Chadwick spoke lucidly and with conviction, not failing to mention the contributions of Bothe, Becker, Webster and the Joliot-Curies, “a lesson to us all.”600C. P. Snow, who was also present, remembers the performance as “one of the shortest accounts ever made about a major discovery.” When tall and birdlike Chadwick finished speaking he looked over the assembly and announced abruptly, “Now I want to be chloroformed and put to bed for a fortnight.”601
He deserved his rest. He had discovered a new elementary particle, the third basic constituent of matter. It was this neutral mass that compounded the weight of the elements without adding electrical charge. Two protons and 2 neutrons made a helium nucleus; 7 protons and 7 neutrons a nitrogen; 47 protons and 60 neutrons a silver; 56 protons and 81 neutrons a barium; 92 protons and 146 (or 143) neutrons a uranium.
And because the neutron was as massive as a proton but carried no electrical charge, it was hardly affected by the shell of electrons around a nucleus; nor did the electrical barrier of the nucleus itself block its way. It would therefore serve as a new nuclear probe of surpassing power of penetration. “A beam of thermal neutrons,” writes the American theoretical physicist Philip Morrison, “moving at about the speed of sound, which corresponds to a kinetic energy of only about a fortieth of an electron volt, produces nuclear reactions in many materials much more easily than a beam of protons of millions of volts energy, traveling thousands of times faster.”602 Ernest Lawrence’s cyclotron, spiraling protons to million-volt energies for the first time the same month that Chadwick made his fateful discovery, fortunately proved to be adaptable to the production of neutrons. More than any other development, Chadwick’s neutron made practical the detailed examination of the nucleus. Hans Bethe once remarked that he considered everything before 1932 “the prehistory of nuclear physics, and from 1932 on the history of nuclear physics.”603 The difference, he said, was the discovery of the neutron.
Word of the discovery reached Copenhagen in the midst of preparations for an amateur theatrical, a parody of Goethe’s Faust, to celebrate the tenth anniversary of the opening of Bohr’s Institute for Theoretical Physics. The postdoctoral dramatists gave the new particle the last word. They had cast Wolfgang Pauli, a corpulent man with a smooth, round face and protuberant, heavy-lidded eyes who resembled the actor Peter Lorre, as Mephistopheles, Bohr as The Lord. Eclectically they cast Chadwick in absentia as Wagner and an anonymous illustrator drew him into the script, “the personification of the ideal experimentalist” according to the stage directions, balancing a vastly magnified neutron on his finger:604
In Copenhagen, as before in Cambridge, Chadwick reports his discovery briefly and succinctly:
The Neutron has come to be.
Loaded with Mass is he.605
Of Charge, forever free.
Pauli, do you agree?
Pauli steps forward to dispense his Mephistophelean blessing:
That which experiment has found—
Though theory has no part in—
Is always reckoned more than sound
To put your mind and heart in. . . .606
And a chorus of clowning, friendly physicists, Bohr’s brilliant young crew, dances out to sing a finale and bring the curtain down:
Now a reality,
Once but a vision.607
What classicality,
Grace and precision!
Hailed with cordiality,
Honored in song,
Eternal Neutrality
Pulls us along!
It was the last peaceful time many of them would know for years to come.