Collider Part 2

Because unification of all of the natural forces would have taken place at such high energies, the particles involved would be extremely heavy. Their ma.s.s would be a quadrillion times what could possibly be found at the LHC. Interacting with the Higgs, the Planck scale particles would tug its energy so high as to destabilize the Standard Model. In particular, it would render the weak interaction in theory much feebler than actually observed.

To avoid such a catastrophe, Dimopoulos and Georgi made use of auspicious mathematical cancellations that occurred when they constructed a supersymmetric description of a unified field theory. The cancellations negated the influence of higher ma.s.s terms and protected the Higgs from being yanked to unrealistic energies. One caveat is that the Higgs itself would be replaced by a family of such particles-charged along with neutral-including a supersymmetric companion called the higgsino.

If some of the low-ma.s.s supersymmetric companions are found, they would offer vital clues as to what lies beyond the Standard Model. They would reveal whether the MSSM or other extensions are correct, and if so, help tune the values of their unspecified parameters (the MSSM has more than a hundred). Ultimately, the findings could provide a valuable hint as to what string theory (or another unified field theory) might look like at much higher energies.

Because string theory has so many different possible configurations and its full energy could be well beyond reach, it is unlikely, however, that any LHC results would either confirm or disprove string theory altogether. At best, they would simply offer more information about string theory's limits and constraints. The experimental discovery of supersymmetry, for instance, would not validate string theory but might a.s.sure some of its proponents that they are on the right track.

One of the groups most desperately seeking SUSY consists of researchers trying to resolve one of the deepest dilemmas facing science today: the missing matter mystery. Astronomers are puzzled by unseen matter, scattered throughout the universe, that makes its presence known only through gravitational tugs-for example, through extra forces on stars in the outer reaches of galaxies. The dark matter mystery is one of the deepest conundrums in astronomy. Some researchers think the answer could be ma.s.sive but invisible, supersymmetric companion particles. Could a supersymmetric payload be the hidden ballast loading down the cosmic craft? Soon the world's most powerful high-energy device could possibly reveal nature's unseen cargo.

Resolving all of these mysteries requires the impact of high-energy collisions monitored carefully by sophisticated detectors to determine the properties of their ma.s.sive byproducts. Such methods have a long and distinguished history. The story of using collisions to probe the deep structure of matter began a century ago, with gold-foil experiments conducted in 1909. Naturally, the instruments used were far, far simpler back then.

Scientists at that time were trying to explore the inner world of the atom. Little was known about the atomic interior until collisions revealed its secrets. You can't crack open a coconut through the impact of a palm leaf; you need a st.u.r.dy mallet applied with vigor and precision. Revealing the atom's structure would require a special kind of sledgehammer and the steadiest of arms to wield it.

3.

Striking Gold Rutherford 's Scattering Experiments Now I know what the atom looks like!-ERNEST RUTHERFORD, 1911

In a remote farming region of the country the Maoris call Aotearoa, the Land of the Long White Cloud, a young settler was digging potatoes. With mighty aim, the boy broke up the soil and shoveled the crop that would support his family in troubling times. Though he had little chance of striking gold-unlike other parts of New Zealand, his region didn't have much-he was nevertheless destined for a golden future.

Ernest Rutherford, who would become the first to split open the atom, was born to a family of early New Zealand settlers. His grandfather, George Rutherford, a wheelwright from Dundee, Scotland, had come to the Nelson colony on the tip of the South Island to help a.s.semble a sawmill. Once the mill was established, the elder Rutherford moved his family to the village of Bright.w.a.ter (now called Spring Grove) south of Nelson in the Wairoa River valley. There, George's son James, a flax maker, married an English settler named Martha, who gave birth to Ernest on August 30, 1871.

Ernest Rutherford (1871-1937), the father of nuclear physics.

Attending school in Nelson and university at Canterbury College in Christchurch, the largest and most English city on the South Island, Rutherford proved diligent and capable. A fellow student described him as a "boyish, frank, simple, and very likable youth, with no precocious genius, but once he saw his goal, he went straight to the central point."1 Rutherford's nimble hands could work wonders with any kind of mechanical device. His youthful pursuits would prepare him well for his deft manipulation of atoms and their nuclei. With surgical dexterity, he disa.s.sembled clocks, constructed working models of a water mill, and put together a homemade camera that he used to snap pictures. At Canterbury, he became fascinated by the electromagnetic discoveries taking place in Europe, and decided to build his own instruments. Following in the footsteps of Gustav Hertz, he constructed a radio transmitter and receiver-research that would antic.i.p.ate Guglielmo Marconi's invention of the wireless telegraph. Rutherford showed how radio waves could travel long distances, penetrate walls, and magnetize iron. His clever undertakings would make him an attractive candidate for a new research program in Cambridge, England.

Coincidentally, in the year of Rutherford's birth, a new physical laboratory had been established at Cambridge, with Maxwell its first director. Named after the esteemed physicist Henry Cavendish, who, among other accomplishments, was the first to isolate the element hydrogen, Cavendish Laboratory would become the world's leading center for atomic physics. Its original location was near the center of the venerable university town on a narrow side street called Free School Lane. Maxwell had supervised its construction and planned out its equipment, making it the first laboratory in the world dedicated to physics research. Following Maxwell's death in 1879, another well-known physicist, Lord Rayleigh, had a.s.sumed the directors.h.i.+p. Then, in 1884, that mantle pa.s.sed to the extraordinary leaders.h.i.+p of J. J. (Joseph John) Thomson.

An intense intellectual with long, dark hair, wire-framed gla.s.ses, and a scruffy mustache, Thomson adroitly presided over a revolution in scientific education that allowed students vastly more opportunities for research. For physics students of earlier times, experimental research was merely the dessert course of a long banquet of mathematical studies-a treat that their tutors would sometimes only grudgingly allow them to partake. After satisfying their requirements with theoretical examinations of mechanics, heat, optics, and so forth, students would perhaps get a chance to sample some of the laboratory apparatus. At Cavendish, with its state-of-the-art equipment, these brief tastes would become a much richer meal unto itself. Thomson was pleased to take advantage of a new program allowing students from other universities to come to Cambridge, perform supervised laboratory research, write up their results in a thesis, and receive a postgraduate degree. Today we are accustomed to research Ph.D.s-it's the bread and b.u.t.ter of academia-but back in the late nineteenth century the concept was novel. Such graduate student a.s.sistance would help spark the revolution in physics soon to follow.

The new program began in 1895, with Rutherford one of the first invitees. He was funded through an 1851 scholars.h.i.+p offered to talented young inhabitants of the British Dominion (now the Commonwealth). His move from rural New Zealand to academic Cambridge would prove extraordinary not only for his own career but also for the history of atomic physics.

The moment Rutherford encountered his fate is a matter of legend. Reportedly, his mother received a telegram bearing the good news and brought it out to the potato garden where he was digging. When she told him what he had won, at first he couldn't believe his ears. After the realization sank in, he tossed his spade aside and exclaimed, "That's the last potato I'll dig."2 Bringing his homemade radio detector along, Rutherford sailed to London, where he promptly slipped on a banana peel and injured his knee. Fortunately, the country lad had no more missteps as he made his way through the smoky, labyrinthine city. Journeying north, he left the smoke for the fresh air of the English countryside and arrived at the hallowed jumble of colleges and courtyards on the River Cam. There he took up residence at Trinity College, founded in 1546 by King Henry VIII, where the arched Great Gate and legends of Newton's feats tower over nervous entering students. (Cambridge is organized into a number of residential colleges, of which Trinity is the largest.) From Trinity, Cavendish was just a short, pleasant walk away.

Along with Rutherford, the labs of Cambridge were soon filled with research students from around the world. Reveling in the cosmopolitan atmosphere, Thomson invited his young a.s.sistants for tea in his office every afternoon. As Thomson recalled, "We discussed almost every subject under the Sun except physics. I did not encourage talking about physics because the meeting was intended as a relaxation . . . and also because the habit of talking 'shop' is very easy to acquire but very hard to cure, and if it is not cured the power of taking part in a general conversation may become atrophied for want of use."3 Despite Thomson's efforts to help young researchers lighten up, the pressures at Cambridge must have been intense. "When I come home from researching, I can't keep quiet for a minute and generally get in a rather nervous state," Rutherford once wrote. His solution to his nervousness was to take up pipe smoking, a habit he would maintain for life. "If I took to smoking occasionally," he continued, "it would keep me anch.o.r.ed a bit. . . . Every scientific man ought to smoke as he has to have the patience of a dozen Jobs in research work."4 To make matters worse, many of the traditional students viewed the newcomers as interlopers. Taunted by some of his upper-crust colleagues as a yokel from the antipodes, Rutherford bore an extra burden. About one such mocker, he commented, "There is one demonstrator on whose chest I should like to dance a Maori war-dance."5 Thomson was a meticulous experimentalist and had been engaged for a time in his own explorations of the properties of electricity. Constructing a clever apparatus, he investigated the combined effects of electric and magnetic fields on what was known as cathode rays: negatively charged beams of electricity pa.s.sing between negatively and positively charged electrodes (terminals attached by wires to each end of a battery). The negative electrode produces the cathode rays and the positive electrode attracts them.

Electric and magnetic fields affect charges in different ways. Applying an electric field to a moving negative charge creates a force opposite opposite to the field's direction. In contrast, a magnetic field generates a force to the field's direction. In contrast, a magnetic field generates a force at right angles at right angles to the field's direction. Also, unlike electric forces, magnetic forces depend on the charges' velocities. Thomson found a way of balancing the electric and magnetic forces in a manner that revealed this speed, which he used to determine the ratio of the charge of the rays to their ma.s.s. Making the a.s.sumption that these rays bear the same charge as ionized hydrogen, he found the ma.s.s of the rays to be about ten thousand times smaller than hydrogen's. In other words, cathode rays consist of elementary particles much, much lighter than atoms. Repeating the experiment numerous times under a variety of conditions, he always got the same results. Thomson called these negatively charged particles corpuscles, but they were later dubbed electrons, a name that has stuck. They offered the first glimpse of an intricate world within the atom. to the field's direction. Also, unlike electric forces, magnetic forces depend on the charges' velocities. Thomson found a way of balancing the electric and magnetic forces in a manner that revealed this speed, which he used to determine the ratio of the charge of the rays to their ma.s.s. Making the a.s.sumption that these rays bear the same charge as ionized hydrogen, he found the ma.s.s of the rays to be about ten thousand times smaller than hydrogen's. In other words, cathode rays consist of elementary particles much, much lighter than atoms. Repeating the experiment numerous times under a variety of conditions, he always got the same results. Thomson called these negatively charged particles corpuscles, but they were later dubbed electrons, a name that has stuck. They offered the first glimpse of an intricate world within the atom.

Initially, Thomson's phenomenal discovery was met with skepticism. As he recalled, "At first there were very few who believed in the existence of these bodies smaller than atoms. I was even told long afterwards by a distinguished physicist who had been present at my lecture at the Royal Inst.i.tution that he thought I had been 'pulling their legs.' I was not surprised at this, as I had myself come to this explanation with great reluctance, and it was only after I was convinced that the experiment left no escape from it that I published my belief in the existence of bodies smaller than atoms."6 Meanwhile, on the other side of the English Channel, the discovery of radioactive decay challenged the notion of atomic permanence. In 1896, Parisian physicist Henri Becquerel scattered uranium salts over a photographic plate wrapped in black paper, and was astonished to find that the plate darkened over time due to mysterious rays produced by the salts. Unlike the X-ray radiation found by Roentgen, Becquerel's rays emerged spontaneously without the need for electrical apparatus. Becquerel found that any type of compound containing uranium gave off these rays, in a rate proportional to the amount of uranium, suggesting that the uranium atoms themselves were producing the radiation.

Similarly working in Paris, Polish-born physicist Marie Curie confirmed Becquerel's findings and, along with her husband, Pierre, extended them to two new elements she discovered: radium and polonium. These elements emitted radiation at a higher rate than uranium and diminished in quant.i.ty over time. She coined the term radioactivity to describe the phenomenon of atoms spontaneously breaking down by giving off radiation. For their monumental discovery of the impermanence of atoms through radioactive processes, replacing Dalton's century-old static concept with a more dynamic vision, Becquerel and the Curies would share the 1903 n.o.bel Prize.

Rutherford followed these developments with great interest. While his mentor Thomson was engaged in discovering the electron, Rutherford concentrated his attention on using radioactive materials as a source for ionizing gases. Somehow the emissions from uranium and other radioactive materials seemed to have the property of knocking the electrical neutrality out of surrounding gases, transforming them into electrically active conductors. The radiation seemed to perform the same function as rubbing dry sticks together and producing a spark.

Radioactivity ignited Rutherford's curiosity as well and launched him on a rigorous investigation of its properties that would revolutionize physics. From a novice keen on developing radio detectors and other electromagnetic devices, he would emerge from his experience an extraordinary experimentalist adept at using radiation to decipher the world of the atom. Using the property that magnetic fields steer differently charged particles along distinct paths, he determined that radioactive materials produce positive and negative kinds of emissions, which he named, respectively, alpha and beta particles. (Beta particles turned out to be simply electrons. Villard discovered gamma radiation, a third, electrically neutral type, shortly after Rutherford's cla.s.sification.) Magnetic fields cause alpha particles to spiral in one direction and beta in the other-like horses racing in opposite directions around a circular track. Testing the ability of each kind of radiation to be stopped by barriers, he demonstrated that alpha particles are more easily blocked than beta. This suggested that alpha particles are larger in size than beta.

In 1898, in the midst of his studies of radioactive materials, Rutherford decided to take time off for a matter of the heart. He headed briefly to New Zealand to marry his high school sweetheart, Mary Newton. They didn't return to England, however. Married life required a decent salary, he reasoned, so he accepted an offer of a professors.h.i.+p at McGill University in Montreal, Canada, that paid five hundred British pounds per year-respectable at the time and equivalent to about fifty thousand dollars today. The couple sailed to the colder clime, where Rutherford soon resumed his investigations.

At McGill, Rutherford focused on trying to uncloak alpha particles and reveal their true ident.i.ties. Replicating Thomson's charge-to-ma.s.s ratio experiment with alpha particles instead of electrons, he determined their charge-curiously finding it to be precisely the same as helium ions. He began to suspect that the most ma.s.sive products of radioactivity decay were just mild-mannered helium in disguise.

Just when Rutherford could use some help in unraveling atomic mysteries, a new sleuth arrived in town. In 1900, Frederick Soddy, a chemist from Suss.e.x, England, was appointed to a position at McGill. Learning of Rutherford's work, he offered his expertise, and together they set out to understand the process of radioactivity. They developed a hypothesis that radioactive atoms, such as uranium, radium, and thorium, disintegrated into simpler atoms a.s.sociated with other chemical elements by releasing alpha particles. Interested in medieval history, when alchemists tried to transform base materials into gold, Soddy recognized that radioactive trans.m.u.tation could lead to the fulfillment of that dream.

In 1903, soon after Rutherford announced their theory of trans.m.u.tation, Soddy decided to join forces with a noted expert in helium and other inert gases (neon and so forth), chemist William Ramsay of University College, London. Ramsay and Soddy conducted careful experiments in which they collected the alpha particles produced by decaying radium in a gla.s.s tube. Then, after the particles acc.u.mulated into a gas, they studied its spectral lines and found them identical to those of helium. Spectral lines are bands of specific frequencies (in the visible spectrum, particular colors) that make up the characteristic signature of an element when it either emits or absorbs light. For the emission spectrum of helium, certain violet, yellow, green, blue-green, and red lines always appear, along with two distinct shades of blue. Ramsay and Soddy found this fingerprint in what they observed, offering proof that alpha particles const.i.tute ionized helium.

Soddy would later coin the term "isotope" to describe when elements exist in two or more distinct forms with different atomic weights. For example, deuterium, or heavy hydrogen, is chemically identical to the standard form, but has approximately twice its atomic weight. Tritium, a radioactive isotope of hydrogen, has about three times the weight of the ordinary variety. It decays into helium-3, a lighter isotope of common helium. In what he called the Displacement Law, Soddy demonstrated how alpha decay causes elements to drop down two s.p.a.ces on the periodic table, as if sliding backward during a game of snakes and ladders. Beta decay, in contrast, causes a move one s.p.a.ce forward, to one of the isotopes of the element in the slot ahead. That's exactly what happens when tritium turns into helium-3 and moves forward in the periodic table.

Suppose you encounter a strange kind of marble dispenser with its contents s.h.i.+elded from view. Sometimes blue marbles pop out of the machine and it flashes once. Sometimes red marbles pop out and it flashes twice. What would you think is inside? You might guess that the interior is an even mixture of red and blue marbles, distributed hither and thither like plums in a pudding.

By 1904, physicists knew that atoms trans.m.u.ted by producing emissions of different charges and ma.s.ses, but they didn't know how all of these fit together. Thomson decided to venture a guess that positive and negative particles were distributed evenly-with the latter being much smaller and thereby freer to move. He hoped that the proof of his "plum pudding model" would be in the testing, but alas it would turn out to be plumb wrong-disproven, as fate would have it, by his former protege from New Zealand.

The next stage of Rutherford's life was arguably his most productive. In 1907, the University of Manchester, the northern English setting of Dalton's explorations, appointed him to a new position as chair of physics. Manchester's gain was a huge loss for McGill. By then, Rutherford had become a commanding presence, "riding the crest of a wave" of his own making-as he once boasted to his biographer (and former student) Arthur Eve.7 As helmsman, he ran a tight s.h.i.+p-recruiting some of the best young researchers, setting them challenging problems, and dismissing those who fell short. With a booming voice and a propensity toward fits of temper and yelling at equipment during stressful moments, the mustached, pipe-smoking professor could be an intimidating commander indeed. Moments of stress and anger would quickly pa.s.s, however, like the blazing sun behind calming puffy clouds, and no one could be friendlier, warmer, or more supportive. As helmsman, he ran a tight s.h.i.+p-recruiting some of the best young researchers, setting them challenging problems, and dismissing those who fell short. With a booming voice and a propensity toward fits of temper and yelling at equipment during stressful moments, the mustached, pipe-smoking professor could be an intimidating commander indeed. Moments of stress and anger would quickly pa.s.s, however, like the blazing sun behind calming puffy clouds, and no one could be friendlier, warmer, or more supportive.

Chaim Weizmann, a Manchester biochemist who would later become the first president of Israel, befriended Rutherford at the time and described him as: Youthful, energetic, boisterous; he suggested everything but the scientist. He talked readily and vigorously on every subject under the sun without knowing anything about it. Going down to the refectory for lunch I would hear the loud, friendly voice rolling up the corridor. . . . He was a kindly person, but he did not suffer fools gladly.8 Comparing Rutherford to Einstein, whom he also knew well, Weizmann recalled: As scientists the two men were strongly contrasting types-Einstein all calculation, Rutherford all experiment. The personal contrast was not less remarkable: Einstein looks like an etherealized body, Rutherford looked like a big, healthy, boisterous New Zealander-which is exactly what he was. But there is no doubt that as an experimenter Rutherford was a genius, one of the greatest. He worked by intuition and whatever he touched turned to gold.9 At Manchester, Rutherford had meaty goals: using alpha particles to crack open the atom and reveal its contents. Alpha particles, he realized, would be large enough to make ideal probes of deep atomic structure. In particular, he wanted to test Thomson's plum pudding model and see if each atom's interior was an evenly distributed mix of large positive and small negative chunks. To carry out his project, he was lucky enough to snag two prize catches: a precious supply of radium (for which he had vied with Ramsay) and the valuable services of German physicist Hans Geiger, who had worked for the former physics chair. Rutherford a.s.signed Geiger the task of developing a reliable way of detecting alpha particles.

The method Geiger pioneered-counting sparks that pa.s.s between electrodes on a metal tube when incoming alpha particles ionize a gas sealed inside, making it a conductor-became the prototype for what would become his most famous invention: the Geiger counter. Geiger counters rely on the principle that electricity travels around closed loops. Each time a sample emits an alpha particle, electricity rounds the circuit between the electrodes and the conducting gas-producing an audible click. Despite the utility of Geiger's innovation, Rutherford usually relied on a second means of detection: using a screen coated with zinc sulfide, a material that lights up when alpha particles. .h.i.t it through a process known as scintillation.

In 1908, Rutherford took a break from his research to collect the n.o.bel Prize in Chemistry for his work with alpha particles. He didn't stay away from the lab for long. Equipped with reliable detection techniques, he developed another project involving Geiger, in conjunction with an extraordinary undergraduate, Ernest Marsden.

Just twenty years old at the time (1909), Marsden had a background curiously parallel to Rutherford's. Like Rutherford, Marsden came from humble roots, with a father in the textile industry. Marsden's dad was a cotton weaver from Lancas.h.i.+re, the local English county. Rutherford started his life in New Zealand and ended up in England; for Marsden it would be the reverse. And each performed vital experimental research while still in their undergraduate years. In Marsden's case, he was just completing his course of study when asked to contribute his talents.

Rutherford recalled the simple query that led to Geiger and Marsden's monumental collaboration. "One day Geiger came to me and said, 'Don't you think that young Marsden . . . ought to begin a small research?' Now I had thought that too, so I said, 'Why not let him see if any alpha particles can be scattered through a large angle?' "10 Legendary for posing just the right questions at precisely the right time, Rutherford had a hunch that the possibility of alpha particles scattering backward from a metal would reveal something about the material. Although he was curious to see what would happen, he didn't necessarily expect a positive outcome. But given even the slightest chance of the particles bouncing off a hidden something, he felt it would be a sin not to try.

For certain types of sensitive measurements, particle physicists have to be like nocturnal cats on the prowl; they need to see well in the dark to spot the subtle signs of their prey. That's an area in which younger scientists can have an advantage-not just with better vision but also with patience. No wonder Rutherford and Geiger recruited a twenty-year-old for the alpha particle scattering experiment. Marsden was instructed to cover the windows, making the lab as dark as possible, and then to sit and wait until his pupils were dilated enough to sense every errant speck of light. Only then was he supposed to start taking readings.

Placing plates of various thicknesses and types of metal (lead, platinum, and so forth) near a gla.s.s tube filled with a radium compound, Marsden waited for alpha particles to emerge from the tube, hit the plates, and either pa.s.s through or bounce off. A zinc sulfide screen, acting as a scintillator, was positioned to record the rates and angles of any alpha particles that happened to reflect. After testing each kind of metal, recording the constellations of sparks his sensitive eyes could see, he shared the data with Geiger. They soon realized that thin sheets of gold offered the highest rate of bounces. Even then, the vast majority of alpha particles pa.s.sed right through the foil as if it were the skin of a ghost. In the rare cases of reflection, the majority took place at very large angles (ninety degrees or higher), indicating that something hard and focused within the gold was bouncing the alpha particles back.

Glowing with excitement, Geiger ran up to Rutherford. "We have been able to get some of the alpha particles coming backwards!" he reported. Rutherford was absolutely delighted.

"It was quite the most incredible event that has ever happened to me in my life," Rutherford recalled. "It was almost as incredible as if you fired a 15-inch sh.e.l.l at a piece of tissue paper and it came back and hit you."11 If Thomson's plum pudding model was correct, then alpha particles impacting a gold foil would be modestly diverted by the gelatinous mixture of charges within the gold atoms and bounce back at fairly small angles. But that's not what Geiger and Marsden found. Like champion sluggers in a baseball game, something within the atoms slammed back the projectiles at large angles only if they were within certain narrow strike zones; otherwise, they continued straight through.

In 1911, Rutherford decided to publish his own alternative to the Thomson model. "I think I can devise an atom much superior to J. J.'s," he informed a colleague. 12 12 His groundbreaking paper introduced the idea that each atom has a nucleus-a tiny center packed with positive charge and the bulk of the atom's ma.s.s. When the alpha particles. .h.i.t the gold, that's what batted them back, but only in the unlikely chance that they were right on target. His groundbreaking paper introduced the idea that each atom has a nucleus-a tiny center packed with positive charge and the bulk of the atom's ma.s.s. When the alpha particles. .h.i.t the gold, that's what batted them back, but only in the unlikely chance that they were right on target.

Atoms are almost completely empty s.p.a.ce. The nuclei const.i.tute but a minuscule portion of their volume-the rest is unfathomable nothingness. If an atom were the size of Earth, then a cross-section of its nucleus would be roughly the size of a football stadium. Rutherford colorfully described striking a nuclear target as akin to locating a gnat in the Albert Hall, a huge performance venue in London.

Despite their minuteness, nuclei play a critical role for determining the properties of atoms. As Rutherford surmised, the amount of positive charge in the nucleus corresponds to its place in the periodic table-called its atomic number. Starting with one for hydrogen, each atom's nucleus houses the positive equivalent of the charge of the electron multiplied by its atomic number. For example, gold, the seventy-ninth element, has a nucleus charged to the positive equivalent of seventy-nine electrons. Balancing out the central positive charge is the same number of negatively charged electrons-rendering the atom electrically neutral unless it is ionized. These electrons, Rutherford a.s.serted, are scattered in a sphere uniformly distributed around the center.

Rutherford's model was a great conceptual leap, but it left certain questions unanswered. Although it brilliantly explained the Geiger-Marsden scattering results, it didn't address many aspects of what was known about the atom at the time. For example, it didn't account for why the spectral lines of hydrogen, helium, and other atoms have distinct patterns. If electrons in the atom are evenly s.p.a.ced, how come atomic light spectra are not? And how did Planck's quantum concept and Einstein's photoelectric effect, showing how electrons can exchange energy via discrete bundles of light, fit into the picture?

Fortunately, in the spring of 1912, Rutherford's department welcomed a young visitor from Denmark who would help resolve these issues. Niels Bohr, a freshly minted Ph.D. from Copenhagen with an athletic build and a long face with prominent jowls, arrived at Manchester after half a year with Thomson in Cambridge. Bohr had written to Rutherford asking if he could spend some time learning about radioactivity. He had learned from Thomson about Rutherford's nuclear model and was intrigued about exploring its implications. While performing some calculations about the impact of alpha particles on atoms, Bohr decided to introduce the notion that electrons vibrate with only particular values of energy, multiples of Planck's constant. In a stroke, he forever painted atoms with the variegated coating of quantum theory.

After returning to Copenhagen in the summer of that year, Bohr continued his studies of atomic structure, focusing on the question of why atoms don't spontaneously collapse. Something must prevent the negative electrons from plunging toward the positive nucleus, like a meteorite hurtling toward Earth. In Newtonian physics, a conserved property called angular momentum characterizes the tendency of rotating objects to maintain the same rate of turning. Specifically, ma.s.s times velocity times...o...b..tal radius tends to remain constant-the reason why ballet dancers twirl faster when they tuck their arms closer to their bodies. Bohr noticed that by requiring an electron's angular momentum to be a multiple of Planck's constant divided by twice the number pi (3.1415 . . . ), he forced it to maintain specific orbits and energies. That is, electrons could reside only particular distances from an atom's nucleus, in discrete levels called quantum states.

Bohr's insight led to enormous strides in tackling the question of why atomic spectral lines are arranged in certain patterns. In his model of the atom, electrons neither gain nor lose energy if they maintain the same quantum state-a situation akin to an idealized, absolutely stable planetary orbit. Therefore, superficially, Bohr's picture treats electrons like little "Mercurys," "Venuses," and so forth, revolving around a nuclear "Sun." Instead of gravity acting as the central force, the electrostatic attraction between negative electrons and the positive nucleus does the job. At that point, however, the solar system a.n.a.logy ends, and Bohr's theory veers on a radically different course. Unlike planets, electrons sometimes "jump" from one quantum state to another, either toward or away from the nucleus. These jumps are instant and spontaneous, resulting in either the loss or gain of a quantum of energy, depending on whether the motion is to a lower or higher energy level. In line with the photoelectric effect, these energy quanta, later called photons or light particles, have frequencies equal to the energy transferred divided by Planck's constant. Thus, the particular lines of color in the emission spectra of hydrogen and other atoms are due to the luminous ballast flung away when electrons enact specific dives-generally the longer the dive, the greater the frequency. Indeed, Bohr's calculations matched up perfectly with the known formulas for the s.p.a.cings of hydrogen spectral lines-a stunning success for his model.

In the winter of 1913, Bohr wrote to Rutherford with his results and was disappointed to receive a mixed response. Ever the practical thinker, Rutherford found what he saw as a major flaw. He informed Bohr, "There appears to me one grave difficulty in your hypothesis, which I have no doubt you fully realize, namely how does an electron decide what frequency it is going to vibrate at when it pa.s.ses from one stationary state to another? It seems to me that you have to a.s.sume that the electron knows beforehand where it is going to stop."13 With his perceptive comment, Rutherford identified one of the princ.i.p.al quandaries involving Bohr's atomic model. How can you predict when an electron will abandon the tranquillity of the state it is in and venture to a new one? How can you determine exactly in which state the electron ends up? Bohr's model couldn't say-and Rutherford was bothered.

Only in 1925 would Rutherford's critique be addressed, and even then the answer was most perplexing. Bohr, by that time, had become the head of his own inst.i.tute for theoretical physics (now the Niels Bohr Inst.i.tute) in Copenhagen, where he hosted a stellar array of young researchers. One of the very brightest, German physicist Werner Heisenberg, who studied in Munich and Gottingen, developed a brilliant alternative description of electrons in the atom that, although it didn't explain why why electrons jumped, could accurately calculate the chances of their doing so. electrons jumped, could accurately calculate the chances of their doing so.

Heisenberg's "matrix mechanics" introduced a new abstraction to physics that confused many old-timers and revolted some of the prominent physicists who understood its implications-most famously Einstein, who argued vehemently against it. It draped a veil of uncertainty around the atom-and indeed all of nature on that scale or smaller-demonstrating that not all physical properties can be glimpsed at once.

Like many a rebellious youth, Heisenberg began his line of reasoning by abandoning many of the long held suppositions of his elders. Instead of treating the electron as an actual orbiting particle, he reduced it to a mere abstraction: a mathematical state. To represent position, momentum (ma.s.s times velocity), and other measurable physical properties, he multiplied the representation state by different quant.i.ties. His Ph.D. adviser, Gottingen physicist Max Born, suggested encoding these quant.i.ties in arrays called matrices-hence the term "matrix mechanics," also known as quantum mechanics. Equipped with powerful new mathematical tools, Heisenberg felt that he could explore the very depths of the atom. As he recalled, "I had the feeling that, through the surface of atomic phenomena, I was looking at a strangely beautiful interior, and felt almost giddy at the thought that I now had to probe this wealth of mathematical structures nature had so generously spread before me."14 In cla.s.sical Newtonian physics, both position and momentum can be measured at the same time. Not so in quantum mechanics, as Heisenberg cleverly demonstrated. If position and momentum matrices are both applied to a state, the order of their application makes a profound difference. Applying position first and then momentum generally produces a different result than applying momentum first and then position. The situation in which the order of operations matters is called noncommutative-in contrast with commutative forms of arithmetic such as addition and multiplication. While four times two is the same as two times four, position times momentum is not the same as momentum times position. This noncommutativity renders it impossible to know both quant.i.ties simultaneously with perfect certainty, a state of affairs Heisenberg later formalized as the uncertainty principle.

In quantum mechanics, Heisenberg's uncertainty principle mandates, for example, that when the position of an electron is ascertained, its momentum goes all fuzzy. Because momentum is proportional to speed, an electron can't tell you where it is and how fast it's going at the same time. Electrons are mercurial creatures indeed, not Mercurial-more like elusive quicksilver than an orbiting planet.

Despite the inherent uncertainty in quantum mechanics, as shown by Heisenberg, it offers accurate predictions of probabilities. So while it doesn't guarantee that a bet will pay off, at least it tells you the odds. For example, it tells you the chances that an electron will plunge from any given state to another. If the chances are zero, then you know that such a transition is forbidden. Otherwise, it is allowed and you can expect a line in the atom's spectrum with corresponding frequency.

In 1926, physicist Erwin Schrodinger proposed a more tangible alternative version of quantum mechanics, called wave mechanics. In line with a theory proposed by French physicist Louis de Broglie, Schrodinger's version imagines electrons as "matter waves"-akin to light waves, but representing material particles rather than electromagnetic radiation. These wave functions respond to physical forces in a manner described by a relations.h.i.+p called Schrodinger's equation. Subject to the electrostatic attraction of an atomic nucleus, for example, wave functions representing electrons form "clouds" of various shapes, energies, and average distances from the center of the atom. These clouds are not actual arrangements of material, but rather distributions of the likelihood of an electron's being in different points in s.p.a.ce.

We can think of these wave formations as akin to the vibrations of a guitar string. Because it is attached on both ends, a plucked guitar string produces what is called a standing wave. Unlike a rolling ocean wave heading toward a beach, a standing wave is constrained to move only up and down. Within such restrictions it can have a number of peaks-one, two, or more-as long as it is a whole number, not a fraction. Wave mechanics identifies the princ.i.p.al quantum number of an electron with the number of peaks of its wave function, offering a natural explanation for why certain states exist and not others.

Much to Heisenberg's chagrin, many of his colleagues favored Schrodinger's depiction over his-perhaps because they were accustomed to models of sound waves, light waves, and kindred phenomena. Matrices seemed too abstract. Fortunately, as the sharp-witted Viennese physicist Wolfgang Pauli proved, Heisenberg's and Schrodinger's descriptions are completely equivalent. Like digital and a.n.a.log clocks they're equally trustworthy instruments and can be relied on according to taste.

Pauli offered his own critical contribution to quantum mechanics: the concept that no two electrons can be in exactly the same quantum state. His "exclusion principle" inspired two young Dutch researchers, Samuel Goudsmit and George Uhlenbeck, to propose that electrons can exist in two orientations, called spin. Contrary to its name, spin has nothing to do with actual spinning, but rather with an electron's magnetic properties. If we imagine placing an electron in a vertical magnetic field (for instance, directly above a magnetized coil of wire), then the electron's own minimagnet could be aligned in the same direction as the external field, called "spin-up," or in the opposite direction, called "spin-down."

Ambidextrous by nature, an electron normally consists of an equal mixture of spin-up and spin-down states. How can a single particle have two opposite qualities at once? In mundane experience, compa.s.s needles can't point north and south at the same time, but the quantum world defies conventional explanation. Until an electron's spin is measured, quantum uncertainty dictates that an electron's spin is ambiguous. Only after a researcher switches on an external magnetic field does an electron reduce into a spin-up or spin-down orientation-a process known as wave function collapse.

If two electrons are paired and one is determined to be spin-up, the other automatically flips spin-down. This switching takes place even if the electrons are widely separated-an intuition-defying effect Einstein called "spooky action at a distance." Because of such unintuitive connections, Einstein thought a deeper, more straightforward theory would someday replace quantum mechanics.

Bohr, on the other hand, embraced contradictions. He reveled in unions of opposites-such as the notion that electrons are waves and particles at the same time, which he called the principle of complementarity. p.r.o.ne to enigmatic statements, he once said, "A great truth is a truth whose opposite is also a great truth." Appropriately, right in the center of his coat of arms he placed the yin-yang symbol of Taoist contrasts.15 Despite their philosophical differences, Einstein shared with Bohr the realization that quantum mechanics matches up incredibly well with experimental data. One sign of Einstein's recognition would be his nomination of Heisenberg and Schrodinger for the n.o.bel Prize in Physics, which Heisenberg was awarded in 1932 and Schrodinger shared with British quantum physicist Paul Dirac in 1933. (Einstein and Bohr were awardees in 1921 and 1922, respectively.) Rutherford would remain cautious about quantum theory, continuing to focus his attention mainly on experimental explorations of the atomic nucleus. In 1919, Thomson stepped down as Cavendish professor and director of Cavendish Laboratory, and Rutherford was appointed to that venerable position. During his final year at Manchester and his initial years at Cambridge, he focused on bombarding various nuclei with fast-moving alpha particles. Marsden had noticed that where alpha particles collide with hydrogen gas, even faster, more penetrating particles emerge. These were the nuclei of hydrogen atoms. Rutherford repeated Marsden's experiment, replacing the hydrogen with nitrogen, and much to his astonishment, hydrogen nuclei emerged from that gas too. Striking a fluorescent screen, the scintillations produced by the hydrogen nuclei were so faint and tiny that they could be seen only through a microscope. Yet they offered important evidence that the nitrogen atoms were releasing particles from their cores. As the discovery of radioactivity showed that atoms could trans.m.u.te on their own, Rutherford's bombardment experiments demonstrated that atoms could be altered artificially as well.

Rutherford coined a name for the positively charged particles found in all nuclei: protons. Some researchers wanted to call these "positive electrons," but he objected, arguing that protons are far more ma.s.sive than electrons and have little in common. When an actual positive electron was discovered, following predictions by Dirac, it ended up being called the positron. Positrons provided the first example of what is known as "antimatter": similar to ordinary matter but oppositely charged. Protons, on the other hand, are a key const.i.tuent of conventional matter.

A new type of particle detector, called the cloud chamber, aided Rutherford and his group in understanding the paths of particles, such as protons, after they are emitted from target nuclei. While scintillators and Geiger counters could measure the rate of emitted particles, cloud chambers could also capture their behavior as they move through s.p.a.ce, leading to an improved understanding of their properties.

Cloud chambers were invented by Scottish physicist Charles Wilson, who noticed during a hike up the mountain Ben Nevis that moist air tends to condense into water droplets in the presence of charged particles such as ions. The charges attract the water molecules and pull them out of the air, offering a vapor trail of electrically active regions. Realizing that the same principle could be used to detect unseen particles, Wilson designed a closed chamber filled with cold, humid air that displayed visible streaks of condensation whenever charged particles pa.s.sed through-similar to the jet trails etched by airplanes in the sky. These patterns can be photographed, providing a valuable record of what transpires during an experiment.

Although Wilson completed his first working model in 1911, it wasn't until 1924 that cloud chambers came into use in nuclear physics. That year Patrick Blackett, a graduate student in Rutherford's group, used such a device to record the release of protons in the trans.m.u.tation of nitrogen. His data wonderfully complemented Rutherford's scintillation experiments, presenting irrefutable evidence of artificial nuclear decay.

Protons are not the only inhabitants of nuclei. In another of his legendary successful prognostications, in 1920 Rutherford predicted that nuclei harbor neutral particles along with protons. Twelve years later, Rutherford's student James Chadwick would discover the neutron, similar in ma.s.s to the proton but electrically neutral. In a key paper written right after Chadwick's discovery, "On the Structure of Atomic Nuclei," Heisenberg introduced the modern picture of protons and neutrons const.i.tuting the cores of all atoms.

The nuclear picture helps explain the different types of radioactivity. Alpha decay occurs when nuclei emit two protons and two neutrons at once-an exceptionally stable combination. Beta decay, on the other hand, derives from neutrons decaying into protons and electrons-with the beta particles comprising the released electrons. As Pauli showed, this couldn't be the whole story because some extra momentum and energy couldn't be accounted for. He predicted the existence of an unseen neutral particle that came to be known as the neutrino. Finally, gamma decay represents the release of energy when a nucleus transforms from a higher- to a lower-energy quantum state. While in alpha and beta decay, the number of protons and neutrons in the nucleus alters, resulting in a different element, in gamma decay that quant.i.ty stays the same.

From Rutherford's historic techniques and discoveries, an idea was forged: using elementary particles to probe the natural world on its tiniest scale. Radioactive materials pumping out alpha particles offered a reliable source for early investigations of the nucleus. They were perfect for Geiger and Marsden's scattering experiments that proved that atoms have tiny cores. Yet, as Rutherford came to realize, exploring nuclear properties in a fuller and deeper way would require much higher energy probes. Breaking through the nuclear fortress would take a st.u.r.dy battering ram-particles propelled through artificial means to fantastically high velocities. He decided that Cavendish would build a particle accelerator-a project that he recognized would entail a certain amount of theoretical know-how. Fortunately, direct from Stalin's own fortress nation, a whiz kid would slip away and bring his cache of quantum knowledge to Free School Lane.

4.

Smas.h.i.+ng Successes The First Accelerators What we require is an apparatus to give us a potential of the order of 10 million volts which can be safely accommodated in a reasonably sized room and operated by a few kilowatts of power. We require too an exhausted tube capable of withstanding this voltage . . .

I see no reason why such a requirement cannot be made practical.

-ERNEST RUTHERFORD (SPEECH AT THE OPENING OF THE METROPOLITAN-VICKERS HIGH TENSION LABORATORY,.

MANCHESTER, ENGLAND, 1930).

The Soviet People's Commissariat of Education issued its coveted stamp of approval, permitting one of its most brilliant physicists, George Gamow, the opportunity to spend a year at Cavendish. He had almost missed the chance due to an odd medical mix-up. During his clearance check-up, his doctor inadvertently reversed the digits of his blood pressure, flagging him for heart disease. Once that error was cleared up, he got the green light. The Rockefeller Foundation generously offered to support his travel and expenses. Fellows.h.i.+ps funded through oil wealth weren't exactly the revolutionary means to success Lenin had antic.i.p.ated, but the Soviet Union at that time viewed the admission of one of its native sons to the world's premier nuclear physics laboratory as a triumph for its educational system.

It is lucky for the history of accelerators that Gamow managed to make it to England. His theoretical insights would offer the critical recipe for breaking up atomic nuclei and place Cavendish in the forefront of the race to build powerful atom smashers. Thanks in part to his contributions, and to the magnificent experimental work of his colleagues, Cavendish would become for a time the leading center for nuclear research in the world.

The Leningrad-trained physicist arrived in Cambridge in September 1928 and quickly found lodging at a boardinghouse. When a friend visited him soon thereafter he was astonished: "Gamow! How did you manage to get this house?"

At Gamow's perplexed look, his friend pointed to the name of the building. By sheer coincidence it was called the "Kremlin."

Several weeks after joining Cavendish, Gamow experienced one of its director's legendary bursts of temper. One day, without prior explanation, Rutherford called Gamow into his office. Red-faced, he started screaming about a letter he had just received from the Soviet Union. "What the h.e.l.l do they mean?" he bellowed as he shoved it at Gamow.

Gamow read over the letter. Written in scrawled English, it said: Dear Professor Rutherford,We students of our university physics club elect you our honorary president because you proved that atoms have b.a.l.l.s.

After Gamow patiently explained that the Russian word for atomic nucleus is similar to its word for cannonball, and that the letter was probably mistranslated, Rutherford calmed down and had a hearty laugh.1 Among the first items Gamow procured at Cambridge were instruments that were ideal for striking spherical projectiles and hurling them toward distant targets. It was a set of golf clubs-par for the course at a collision laboratory. Gamow's instructor in the sport was John Douglas c.o.c.kcroft, a young Cavendish researcher and avid golfer.

Born in Todmorden, England, in 1897, c.o.c.kcroft took a circuitous path to physics. His father ran a cotton-manufacturing business, but, like Rutherford and Marsden, c.o.c.kcroft opted for science over textiles. He began studying mathematics at Manchester University, but then World War I broke out and he joined the British army. Returning to Manchester upon the armistice, he switched to electrical engineering and found a job in the field. Finding that career path personally unfulfilling, he enrolled at St. John's College, Cambridge, and made his way to Rutherford's lab.

In golf, it can be frustrating when there's a hill right in front of the green, occluding the direct path to the hole. In that case, you need a bit of strategy to figure out what club to use and how hard to swing it in order to clear the barrier. Tap the ball with not enough force, and it's liable to fall short.

c.o.c.kcroft worked on a problem in nuclear physics that offered a similar kind of challenge. He wanted to hurl particles toward nuclear targets with the goal of exciting them to higher energy levels and possibly breaking them into subcomponents. If smas.h.i.+ng them together broke them up, seeing what came out would allow him and his colleagues to learn things about the makeup of an atom they couldn't learn in any other way. Blocking the path to the nucleus, however, was the barrier caused by the mutual electrostatic repulsion of positively charged particles and the positive nuclear charge. They naturally push each other apart-a formidable obstacle to overcome-like the north poles of bar magnets resisting being brought together, only far stronger.

Gamow knew just how to handle the issue theoretically. Plugging the parameters corresponding to protons and alpha particles (the particles radioactive atoms such as uranium give off) into his "quantum tunneling" formula, Gamow discovered that the former would need sixteen times less initial energy than the latter for the same probability of penetration. The choice was clear: protons offer much more economical projectiles. If protons could be induced to move fast enough, a few might pa.s.s through the force barrier around an atom and smash into its nucleus. What exactly would happen once they reached their targets was unknown, but, convinced by Gamow, Rutherford decided it would be worthwhile to try. It would be the one major step Rutherford took that was driven by theoretical predictions.

Already involved in planning the details of an atom smasher was an adept young experimentalist, Ernest Thomas Sinton Walton. Born in Dungaravan, Ireland, in 1903, he was the son of a traveling Methodist preacher. In 1915, Walton enrolled in a Methodist boarding school where he excelled in the sciences. Following his graduation in 1922, he became a student at Trinity College in Dublin, where he received a master's degree in 1927. Upon being awarded an Exhibition of 1851 scholars.h.i.+p to Cambridge, he joined the group at Cavendish and soon became one of Rutherford's trusted a.s.sistants.

In late 1928, Walton came across an extraordinarily innovative research paper by Norwegian engineer Rolf Wideroe that described his attempts to accelerate particles by means of a device called the ray transformer. Wideroe's mechanism combined several basic concepts in electromagnetic theory. It starts with the idea of an electromagnetic coil: a current-carrying wire wrapped in a loop that produces a magnetic field in its vicinity. If the wire has a changing current, then the magnetic field changes over time. Then, according to Faraday's law of induction, the changing magnetic field produces a second current in any wire that happens to be nearby. If that second wire is in a loop too, the setup is known as a transformer-a familiar system for transferring power from one wire to another. In a way, it's a.n.a.logous to the spinning of a bicycle's pedals giving rise to the turning of its wheels-with the chain representing the varying magnetic field connecting the two.

Wideroe's princ.i.p.al innovation was to replace the second wire with electrons accelerated through a vacuum-filled ring. These electrons would be removed from atoms and propelled through s.p.a.ce by what is called the electromotive force produced by the changing magnetic field. To keep the electrons moving in a loop, like race cars on a circular track, he envisioned a central magnet that would steer them round and round. Unfortunately, in trials of his machine at Aachen University in Germany, he found that "islands of electrons" built up in the tube, sapping the revolving electrons' energy. For some reason, the magnet couldn't keep the electrons moving smoothly, though he couldn't figure out why. The best Wideroe could manage given the turbulence was to get the electrons to circle around the loop one and a half times.

Frustrated by the problems with the circular track, Wideroe finally decided to abandon the project and turn to a different scheme. Borrowing a concept he found in a 1924 article by Swedish physicist Gustav Ising, he pursued the idea of a linear accelerator and built a small prototype, about one yard long. Rather than a ring, it used a pair of "drift tubes" (straight, isolated, vacuum-filled pipes) in which particles would be sped up by successive "kicks" of an electric field. These boosts were arranged rather cleverly-allowing the particles to be lifted up to higher speeds twice by use of the same voltage difference-something like the continuously ascending stairways in some of Escher's paintings. Just when the particles seemed to reach the top, there was more to climb.

Voltage, electric potential energy per charge, is a measure of how easy it is for particles of specific ma.s.s and charge to accelerate from one place to another; the higher the voltage difference, the greater the acceleration, all other factors being equal. In other words, voltage is a measure of how steep that staircase is-and how much of a boost it gives.

Particles began their journeys through a drift tube with high voltage (twenty-five thousand volts) at the entrance point and low voltage at the exit. This voltage difference caused them to speed up. When the particles were halfway through and already moving quickly, Wideroe tricked them by reversing the voltage difference, setting the formerly low voltage to high. Because they were already moving at high speeds it was too late for them to turn back. They rushed through the tail end of the first tube, across a gap, and then on to the start of a second tube, where the same voltage difference (once again, due to an initially high voltage and a final low voltage) accelerated them even farther. Because it used the same voltage difference twice, Wideroe's method doubled the impact of the boost, enabling a lower voltage source than otherwise required.

At the end of the second tube, Wideroe placed a photographic plate to record the streaks produced by the high-velocity particles as they impacted. Experimenting with pota.s.sium and sodium ions as the projectiles, he was able to run them through his device. The ions were made by stripping atoms of their outer electrons. The positively charged ions were then compelled by the voltage differences to accelerate through the tubes before hitting the plate. After collecting enough data, Wideroe incorporated his findings into a doctoral thesis for Aachen University. The thesis was published in a journal his Ph.D. adviser edited.

Fascinated by Wideroe's work, in December 1928, Walton proposed to Rutherford the idea of building a linear accelerator at Cavendish. Rutherford was keen to devise such a device that could look inside one of the lighter elements, such as lithium. (Lithium is the third element in the periodic table after hydrogen and helium and its atom is now known to have three protons and four neutrons in its core.) The next month, Gamow gave a talk that presented his barrier penetration formula to the group. c.o.c.kcroft was eager to apply this formula to the issue of penetrating the lithium nucleus with protons. Estimates showed that it would take several hundred thousand electron volts to do the job. By human standards, even 1 MeV (one million electron volts) is an extraordinarily tiny amount of energy-approximately one billionth of a billionth of a single dietary calorie (technically, a kilocalorie). Elementary particles obviously don't have to worry about slimming down-however for them it's quite an energizing burst!

Upon hearing these results, Rutherford called c.o.c.kcroft and Walton into his office. "Build me a one-million-electron volt accelerator," he instructed. "We will crack the lithium atom open without any trouble."2 Soon c.o.c.kcroft and Walton were hard at work building a linear accelerator that they would locate, when it was complete, in a converted lecture hall. They rigged up a straight tube with a specially designed high-voltage power supply, now known as the c.o.c.kcroft-Walton generator. It included a mechanism known as a voltage multiplying circuit that included four high-voltage generators stacked in a ladderlike formation twelve feet high. Capacitors (charge-storing devices) in the circuit helped oost a relatively modest input voltage to an overall voltage of up to seven hundred thousand. Propelled by this high voltage, protons would be accelerated by the electric forces through an evacuated tube and collide with nuclear targets on the other end-with any disintegrations recorded as sparks on a fluorescent screen placed inside the vacuum.