Making of the Atomic Bomb

  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. The Physical 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 i
n 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 negativel
y 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.