Blackett's War

by Stephen Budiansky

  In a scant half century since its founding in 1874, with a bequest from the Duke of Devonshire, the Cavendish had become the preeminent physics laboratory in the world, catapulting British science to the forefront of the exciting new fields of radiation and atomic physics. Its first four directors were, and still are, legendary names in the history of modern science. James Clerk Maxwell, the first director, formulated the basic laws of electromagnetism that laid the basis for modern electronics and communications technology. Lord Rayleigh, his successor, made pioneering discoveries in light and sound and was the discoverer of the element argon. Rutherford’s immediate predecessor, J. J. Thomson, had discovered the electron.

  Rutherford arguably surpassed them all. Born to a homesteading family struggling to make a go of life on the rough frontier of New Zealand, Rutherford combined brilliance, ambition, and apparently inexhaustible energy to rise to this preeminent position in British science. He had won a series of scholarships as a young student, culminating in the prestigious 1851 Exhibition scholarship that brought him to Cambridge. He swiftly made a name for himself with original work on radio waves, switching fields abruptly after the discovery of radioactivity in 1896; the next year, at age twenty-seven, he became a professor at McGill University in Canada, where he proceeded to carry out groundbreaking studies of the transmutation of elements via radioactive decay which won him the Nobel Prize in chemistry in 1908.

  Unlike virtually every other Nobel Prize winner in history, Rutherford then went on to make his greatest scientific discoveries after the work that won him the prize. Success never spoiled him; he was unstoppable. A colleague who had been a fellow student at the Cavendish with Rutherford was asked many years later if he and Rutherford had become friends back then. He replied, “One can hardly speak of being friendly with a force of nature.”5 Perhaps more to the point was what another colleague observed: Rutherford never lost “his genius to be astonished.”

  In 1911 he announced the results of some experiments that astonished everyone. The accepted model of the atom at the time pictured electrons evenly distributed through a cloud of positive charge. Rutherford had tried to probe the structure of this atomic space by shooting a stream of heavy alpha particles through a thin layer of gold foil and then measuring the small deflection of the particles from their course as they emerged on the other side. A thin layer of zinc sulfide spread on a glass plate served as a detector; alpha particles striking the coating created tiny, glowing tracks—“scintillations”—that could be seen through a microscope and individually counted. Looking for the tracks was tedious and difficult work. At one point, out of what his assistant thought an excess of experimental thoroughness, Rutherford suggested placing the zinc sulfide detector on the same side of the gold foil target as the alpha particle source. To his amazement, the detector glowed on that side, too: some of the alpha particles were rebounding directly off the target. “It was,” Rutherford said, “quite the most incredible event that has ever happened to me in my entire life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”6 He realized that the only way to explain such a ricochet was if the entire positive charge of the atom were concentrated in an extremely small space, creating an enormous repulsion force as the positively charged alpha particle passed near it. He had discovered the nucleus—and the structure of the atom itself.

  In 1919 Rutherford nearly equaled that monumental discovery in an experiment in which he bombarded nitrogen gas with a beam of alpha particles, chipping off a stream of protons from the target atoms. Rutherford was at the time serving on a committee of scientists exploring methods of detecting submarines underwater by sound—what would be the genesis of sonar—and he wrote Karl Compton, the chairman of the committee, a letter of apology explaining that he would be late for their next meeting, scheduled to take place in Paris: he had apparently just split the atom, and was in the midst of carrying out a second experiment to confirm the result. “If this is true,” he wrote Compton, “it is a fact of far greater importance than the war.”7

  The Cavendish was remarkable not just for its scientific preeminence but for its existing at all in the ivory-tower world of Cambridge, with its traditional disdain for the practical, much less the mechanical. The laboratory occupied a nondescript, three-story, gray stone Victorian building in a crooked medieval alley that ran behind one of the older colleges. Its facade, in the words of one observer, could have “graced any Scottish hotel.” The building bore no clue to its identity beyond a statue of its benefactor bearing the Latin inscription Magna opera Domini exquisita in omnes voluntates ejus: “The works of the Lord are great, searched out by all who have delight in them.” Inside, the building was even less prepossessing, “uncarpeted board floor, dingy varnished pine doors and stained plastered walls, indifferently lit by a skylight with dirty glass.”8

  The physicist Max Born, who would flee Germany and the Nazis for Cambridge in 1933, noted with amusement the British academic gentility that insisted on calling theoretical physics “applied mathematics”; by analogous reasoning, he suggested, the Cavendish should be called the Department of Applied Glass Blowing.9 The allusion to glassblowing was not a joke, though: every student and researcher at the lab was expected to master such practical hands-on skills, and it was part of the indoctrination of all new members of the lab to put in a stint in the “Nursery,” a course for newcomers held in a cramped attic room filled with bits of miscellaneous equipment, where they learned the basic techniques. There was clearly a sort of in-group pride among the physicists in possessing skills so different from their fellow rarefied academics. Patrick Blackett, in an essay he would write upon his departure from Cambridge—titled “The Craft of Experimental Physics,” it remains a classic exposition—underscored the satisfaction he and his fellow physicists took in being able to combine the mental and the manual in their daily work:

  The experimental physicist is a Jack-of-All-Trades, a versatile but amateur craftsman. He must blow glass and turn metal, though he could not earn a living as a glassblower nor even be classed as a skilled mechanic; he must carpenter, photograph, wire electric circuits and be a master of gadgets of all kinds; he may find invaluable a training as an engineer and can profit always by utilising his gifts as a mathematician.… The experimental physicist must be enough of a theorist to know what experiments are worth doing and enough of a craftsman to be able to do them. He is only preeminent in being able to do both.10

  Every year the research students at the Cavendish held a somewhat raucous celebratory dinner, the highlight of which was the performance of Gilbert and Sullivan songs and other familiar tunes with words reworked for the occasion. Published in a privately printed series of volumes (Postprandial Proceedings of the Cavendish Society), they were an affirmation of the comradeship of their world apart, often containing allusions to their experimental improvisations. To the tune of “Clementine,” one year they sang:

  In the dusty lab’ratory

  ’Mid the coils and wax and twine,

  There the atoms in their glory

  Ionize and recombine.11

  “Wax and twine” was not just a metaphor, and the wax referred to was indeed something of a legend in its own right: the physicists had discovered that the red sealing wax used by the Bank of England was the perfect medium for sealing vacuum apparatuses, and there was scarcely an experiment in the whole lab that did not use it (until another improvisation, plasticine, displaced it).

  The physicists were different in another way for being an unusually cosmopolitan group amid the generally still very insular society of British academia. The Cavendish regularly welcomed students and visiting scientists from around the world—Robert Oppenheimer from America, Peter Kapitza from Russia, Niels Bohr from Denmark among them—and everyone traveled and maintained a far-flung network of scientific contacts, in Germany, Denmark, Holland, Italy, France.

  But even the physicists and the Cavendish were not c
ompletely immune to the genteel charms of an ancient university, and there remained something quaintly civilized about the rhythms of life at the world’s preeminent experimental physics factory in the 1920s. For eight weeks a year, during university vacations, the doors of the Cavendish Laboratory were simply shut and locked; everyone was expected to go away and take a holiday, or catch up on some reading, or perhaps attend a scientific conference, but no work was done at the lab during these times, period. Every afternoon at 4 p.m. the researchers would drift down to a small room next to the library and gather for tea and cakes. (The tradition had been for the director’s wife to supply these herself but Lady Rutherford finally drew the line as the staff grew, and the scientists were forced to set up their own “bun fund” to pay for the cakes.) Starting hours at the Cavendish were loose and unenforced, but at 6 p.m. sharp the workday ended, a rule sometimes emphasized by a technician going from room to room pulling out plugs and snapping off switches. A visiting American physicist was amazed at the leisurely pace of life there; he confessed in a letter to J. J. Thomson after his visit that “a laboratory in this country in which nobody ever began work before 10 a.m. or worked later than six in the evening” would be viewed as a “terrible example of sloth and indolence.”12

  It was a gentler era altogether and Cambridge was still an Eden with its own stately pace and splendid isolation from the rough-and-tumble world. Students still wore academic gowns to class, in the library, when calling on senior members of the university, and when walking anywhere outside the college after dark (where a mortarboard cap was required, too, and being caught without this proper dress in town by a university proctor was a punishable infraction). Sport was still distinctly amateur, and an hour or two of tennis, cricket, rugby, bicycling, or running part of the daily afternoon ritual. Dinner every evening in the college halls was a formal and unhurried affair invariably including soup, fish, entree, roast, sweet, and savory; politics was rarely discussed.13

  “TO BE DEPOSITED, so to speak, on the shores of Cambridge just as the Cavendish Laboratory was rising under Rutherford’s inspired direction to great heights of eminence was luck indeed for me,” Patrick Blackett would later write. “So I owe much to the Royal Navy.”14 He had learned of his research fellowship shortly after his graduation in 1921 when he received a letter from the president of Magdalene inviting him to a dinner at college. There was a railroad strike on and he was at his parents’ home in Surrey but he got on his bicycle and pedaled to Cambridge, where he was told that both he and Kingsley Martin had been elected to fellowships by the college. “The world was at our feet,” Martin remembered thinking at the news, “an oyster to be opened with the sharp sword of Cambridge intellectualism.”15

  Blackett had the advantage, too, in this new world he had been deposited in, of making a striking impression, both physically and personally. Ivor Richards, a Cambridge don and literary critic who would become one of the most influential founders of the New Criticism movement, was Geoffrey Webb’s adviser and through him met his friend Blackett; years later Richards would vividly recall the day Blackett first stopped by his flat, which was at the top of a dilapidated building at the end of the same lane where the Cavendish stood. “Came a quick step … a tap on the door, and there entered a young Oedipus. Tall, slim, beautifully balanced and looking always better dressed than anyone.… But above was that mysterious intense and haunted visage.… The tragic mask, however, was highly mobile, alive indeed with intelligence, modesty and friendliness.” Solly Zuckerman, a zoologist who met Blackett a few years later, had a very similar reaction: “tall and strikingly handsome in the film-star mould,” yet “measured,” “not immediately forthcoming.… He often looked as though all the cares of the world were on his shoulders.”16 Late in Blackett’s life a colleague remarked to him that his official portrait as president of the Royal Society made him look very serious. “But I am a very serious man!” he replied.17

  Every new arrival at the Cavendish was conscripted to the tedious task of counting scintillations in Rutherford’s ongoing bombardment experiments. Blackett remembered hours spent in a dark room, letting his eyes adjust, and then straining to look for the faint tracks for a minute at a time as the experiment proceeded.18 But Rutherford also firmly believed in helping his students and research fellows learn to carry out their own research projects: it was why so many of the world’s top-rank physicists had gotten their start at the Cavendish under Rutherford. Even as he kept up his own experiments and growing administrative tasks, the director made the rounds of the laboratory every morning at 11 a.m., singing “Onward Christian Soldiers!” off-key as he came down the corridor, his voice booming as he asked questions, offered suggestions, and dispensed encouragement, or with alarming frequency erupted in unpredictable but mostly harmless rages that “his boys” learned to shrug off. “We are living in the heroic age of physics!” he declared at the annual meeting of the British Association for the Advancement of Science, and few of his protégés doubted it.19

  Within a year Blackett had published his first scientific paper based on his own research and was well on his way to the discoveries that would catapult him to the top ranks of physics, with a speed remarkable even by Cavendish standards. Blackett would later say that he never forgot this early lesson Rutherford gave him, of the importance of “choosing the really important problems and letting a young man get on with them.”20 Rutherford had suggested that Blackett try to adapt the cloud chamber—a device that, with characteristic enthusiasm, Rutherford had declared to be “the most original and wonderful instrument in scientific history”—to more precisely document the artificial disintegration of the nitrogen atom when it collided with an energetic alpha particle.21

  The cloud chamber had been invented twenty-five years earlier by another Cavendish scientist, C. T. R. Wilson. His lectures on light, which Blackett had attended, were legendary at the laboratory equally for their brilliant treatment of the subject and for Wilson’s comically abysmal presentation, delivered in barely audible tones with his back to the audience, his faint writing on the blackboard almost immediately wiped out by the eraser he held in his left hand as he worked his way across the board. In 1894 Wilson had spent a few weeks atop Ben Nevis, the highest mountain in Scotland, where a small observatory had been established, and he had become intrigued by “the wonderful optical phenomena” known as coronas and glories, produced when the sun shone through the clouds surrounding the hilltop. Hoping to reproduce these effects in the laboratory, he built an apparatus that could be filled with a mixture of air and water vapor. A piston allowed the volume to be suddenly expanded, lowering the temperature and causing the water vapor to become supersaturated; at that point the vapor would begin to condense into droplets, forming a visible cloud. But as Wilson said, “Almost immediately I came across something which promised to be of more interest than the optical phenomena which I had intended to study.” The condensation of supersaturated water vapor into visible droplets begins at so-called condensation nuclei, typically specks of dust. But when Wilson carefully removed dust from the air in the chamber, he noticed entire trails of droplets forming in the supersaturated vapor mixture. His great discovery was that it was charged particles that were causing the phenomenon; when he placed a radioactive source next to the chamber, a cloud trail tracing the path of the charged radioactive particles vividly appeared. He subsequently was able to produce beautiful photographs of the tracks of alpha, beta, and X-ray particles. Wilson would win the Nobel Prize for physics in 1927 for this work.22

  Rutherford at first gave the job of adapting Wilson’s apparatus to a Japanese student, who subsequently had to return home suddenly for family reasons. The project was passed on to Blackett. “So I found myself with a few bits of Schimitzu’s apparatus in an otherwise empty research room and told to get on with it,” Blackett recalled.23

  Blackett would become legendary among his fellow scientists for his ability to combine physical insight, mathematical understanding, a
nd extraordinary practical skills, and from this very first independent project of his own he displayed what one colleague would call his “remarkable facility” of “thinking most deeply when he was working with his hands.” Another colleague said: “He could use physical theory to design a piece of equipment and then draw it, make it and get it to work. I have known nobody who possessed this combination of talents in the degree that he did.”24

  The trick of capturing good images with the cloud chamber was to create a sharp, rapid expansion and then instantly take a photograph before the tracks faded. Blackett ingeniously solved the problems by designing an automatic device that mechanically linked the piston and the camera; to keep the chamber clear of old fading tracks, the radioactive source (a polonium deposit on the end of a copper wire) was shielded by a mechanical shutter also linked to the expansion bar, so that the alpha particles only entered the chamber at the moment sufficient supersaturation occurred, ensuring the tracks would be sharp.

  With this automated mechanism Blackett was able to produce a thousand photographs a day. He eventually had a staggering 23,000 images, each containing an average of eighteen alpha-particle tracks. A small number of the tracks made a sudden fork—a clear indication of a collision with a nitrogen or oxygen atom. But a grand total of eight out of the half a million tracks he had captured and pored over were the Holy Grail. These, as Blackett reported, were “of a strikingly different type. These eight tracks undoubtedly represent the ejection of a proton from the nitrogen nucleus.” The surprise was that instead of three tracks emerging—the ejected proton, the residual nucleus, and the alpha particle—there were only two; the alpha particle was apparently being captured in the reaction. The angle and momentum of the tracks allowed Blackett to calculate the mass of the new atom that had been produced in the process, and they left no doubt that it was an oxygen atom: “In ejecting a proton from a nitrogen nucleus the alpha-particle is therefore itself bound to the nitrogen nucleus.”25 Blackett had captured the first photograph of a nuclear reaction as it actually took place.