Collider Part 3

A c.o.c.kcroft-Walton generator, one of the earliest types of accelerator. This example is retired and located in the garden of Microcosm, CERN's science museum.

In 1931, Walton received his Ph.D. from Cambridge. With the Cavendish accelerator on the brink of completion, Rutherford certainly couldn't afford to lose one of its princ.i.p.al architects. Walton was appointed Clerk Maxwell Research Scholar, a position he would hold for an additional three years while continuing to work with c.o.c.kcroft and Rutherford.

Cavendish was far from the only player in the race to split the atom. Physicists around the world were well aware of what Rutherford was trying to do and hoped to unlock the nucleus themselves with their own powerful atom smashers. Aside from scientific interest, another motivation that would become increasingly important was antic.i.p.ation of colossal energy locked inside the atomic core. Einstein's famous equation for the equi-valence of energy and ma.s.s, E = mc E = mc2, indicated that if any ma.s.s were lost during a nuclear disintegration it would be converted into energy-and this could be formidable. In 1904, even before Einstein's finding, Rutherford had written, "If it were ever possible to control at will the rate of disintegration of the radio elements, an enormous amount of energy could be obtained from a small amount of matter."3 (In 1933, he would qualify this statement when, in a prediction uncharacteristically off the mark, he expressed the opinion that atomic power could never be controlled in a way that would be commercially viable.) (In 1933, he would qualify this statement when, in a prediction uncharacteristically off the mark, he expressed the opinion that atomic power could never be controlled in a way that would be commercially viable.) A highly innovative thinker who would become a key partic.i.p.ant in the development of nuclear energy was the Hungarian physicist Leo Szilard. In December 1928, while living in Berlin, Szilard took out a patent for his own concept of a linear accelerator. Like Ising and Wideroe, Szilard envisioned an oscillating (direction switching) electric field that would prod charges along. In his patent application, t.i.tled "Acceleration of Corpuscles," he described a way of lining up charged ions so they ride the crest of a traveling wave forcing them to move ever faster: With our arrangement, the electric field can be conceived of as a combination of an electric field in accelerated motion from left to right and an electric field in decelerated motion from right to left. The device is operated in such a way that the velocity of the accelerated ion equals, at each point, the local velocity of the field moving left to right.4 Curiously, Szilard never pursued his design. He developed patent applications for two other accelerator schemes that he similarly never followed up on. History does not record whether or not his patent applications were even accepted-conceivably the patent officers were aware of the earlier papers by Ising and Wideroe.

Around the same time as c.o.c.kcroft and Walton began to build their apparatus, American physicist Robert Jemison Van de Graaff developed a simple but powerful accelerator model that, because of its compactness and mobility, would become the nuclear physics workhorse for many years. Born in Tuscaloosa, Alabama, in 1901, Van de Graaff began his career on a practical track. After receiving B.S. and M.S. degrees in mechanical engineering from the University of Alabama, he worked for a year at the Alabama Power Company. He could well have remained in the electrical industry, but Europe beckoned, and in 1924, he moved to Paris to study at the Sorbonne. The great Marie Curie herself taught him about radiation-acquainting him in her lectures with the mysteries of nuclear decay. His savvy won him a Rhodes scholars.h.i.+p, enabling him to continue his studies at Oxford. There he learned about Rutherford's nuclear experiments and the quest to accelerate particles to high velocities. Oxford awarded him a Ph.D. in physics in 1928.

In 1929, Princeton University appointed Van de Graaff as a national research fellow at the Palmer Physics Laboratory, the center of its experimental program. He soon designed and built a prototype for a novel kind of electrostatic generator that could build up enormous amounts of charge and deliver colossal jolts. Its basic idea is to deliver a continuous stream of charge from a power source to a metal sphere using a swift, insulated conveyor belt. Van de Graaff constructed his original device using a silk ribbon and a tin can; later he upgraded to other materials. Near the bottom of the belt, a sharp, energized comb connected to the power source ionizes its immediate surroundings, delivering charge to the belt. Whisked upward, the charge clings to the belt until another comb at the top sc.r.a.pes it off and it pa.s.ses to the sphere. A pressurized gas blankets the entire generator, creating an insulated coc.o.o.n that allows more and more charge to build up on the sphere.

Using the Van de Graaff generator as an accelerator involves placing a particle source (a radioactive material or an ion source, for example) near the opening of a hollow tube, each situated within the sphere. The voltage difference between the sphere and the ground serves to propel the particles through the tube at high speeds. These projectiles can be directed toward a target at the other end.

Van de Graaff worked continuously at Princeton and later at MIT to increase the maximum voltage of his generators. While his prototype could muster up to eighty thousand volts, an updated model he presented at the American Inst.i.tute of Physics's inaugural banquet in 1931 stunned the dining guests (fortunately not literally) by producing more than one million volts. A much larger machine he a.s.sembled on flatbed railroad cars in a converted aircraft hangar in South Dartmouth, Ma.s.sachusetts, consisted of twin insulated columns, each twenty-five feet high, capped with fifteen-foot-diameter conducting spheres made of s.h.i.+ny aluminum. Its colossal power inspired the New York Times New York Times headline on November 29, 1933, "Man Hurls Bolt of 7,000,000 volts." headline on November 29, 1933, "Man Hurls Bolt of 7,000,000 volts."5 According to Greek mythology, Prometheus stole the secret of fire from the G.o.ds, offering humanity the sacred knowledge of how to create sparks, kindle wood, light torches, and the like. Yet even after that security breach, mighty Zeus reserved the right to hurl thunderbolts at his foes, illuminating the heavens with his terrifying power. Through Van de Graaff 's generator, even something like the awe-inspiring vision of lightning became scientifically reproducible, albeit on a smaller scale, ushering in a new Promethean age in which colossal energies became available for humankind's use. Given such newly realized powers, perhaps it is not surprising that many horror films of the day, such as Frankenstein Frankenstein (1931) and (1931) and Bride of Frankenstein Bride of Frankenstein (1935) most notably, offered sinister images of gargantuan laboratories sp.a.w.ning monsters electrified by means of colossal generators. (1935) most notably, offered sinister images of gargantuan laboratories sp.a.w.ning monsters electrified by means of colossal generators.

Why rely on expensive generators for artificial thunderbolts, if the real thing is available in the skies for free? Indeed, though lightning is, of course, highly unpredictable and extremely dangerous, several physicists of that era explored the possibility of using lightning itself to accelerate particles. During the summers of 1927 and 1928, University of Berlin researchers Arno Brasch, Fritz Lange, and Kurt Urban rigged up an antenna more than a third of a mile across between two adjacent peaks in the southern Swiss Alps near the Italian border. They hung a metallic sphere from the antenna and wired another sphere to the ground to measure the voltage difference between the two conductors during thunderstorms. During one lightning strike, more than fifteen million volts pa.s.sed through the device, according to the researchers' estimates. Sadly, during their investigations Urban was killed. The two survivors returned to Berlin to test discharge tubes with the potential to withstand high voltages. Brasch and Lange published their results in 1931.6 Lightning strikes, even of the artificial kind, are usually one-time-only affairs. Whenever great quant.i.ties of charge build up, it creates a huge voltage drop that maintains itself as long as the collected charge has nowhere else to go (if the apparatus is isolated or insulated, for example). Like cliff divers, particles plunging down the steep potential difference experience a force that accelerates them. But once they reach the ground, that's it-end of story.

However, as Wideroe pointed out in his "ray transformer" proposal, particles forced to travel in a ring, instead of a straight line, could be accelerated repeatedly each time they rounded the loop, building up to higher and higher energies. Although, after his initial experiments failed, Wideroe ceased working on the idea of a circular accelerator, his article inspired an extraordinary American physicist, Ernest Orlando Lawrence, to pursue this vital approach.

Lawrence was born in the prairie town of Canton, South Dakota, in 1901. His parents, Carl and Gunda, were both school-teachers of Norwegian descent. Carl was the local superintendent of schools and also taught history and other subjects in the high school; Gerda instructed in mathematics. Ernest was a cheerful baby, whom the neighbors across the street, the Tuve family, contrasted with their own colicky, crying boy born six weeks earlier, Merle.

Practically from birth Ernest and Merle were best friends. They would pull various pranks together, such as once dumping rubbish on another neighbor's porch. The neighbor happened to be home and s.n.a.t.c.hed Merle, before he escaped, through a hole in her fence. Meanwhile, Ernest managed to get away. They shared a code of honesty and tried not to fib, even when they were mischievous.

When the boys were only eight years old, they became interested in electrical devices. Practically all of their waking hours, aside from school and ch.o.r.es, were spent hooking up crude batteries into circuits, connecting these power sources to bells, buzzers, and motors, and testing which combinations worked best.

Tall and lanky, Lawrence earned the childhood nickname "Skinny," which he didn't seem to mind. His interests were as narrow as his build. Aside from tennis, he was little interested in sports and partic.i.p.ated grudgingly in athletic activities, mainly when prodded by his father. Nor, as a teenager, was he much inclined to go on dates and other social events. Rather, he buried himself in his studies so that he could graduate from high school a year early, while continuing to spend his free time, along with Tuve, a.s.sembling various mechanical and electrical apparatuses. To earn money for switches, tubes, and other radio equipment, he spent one summer working on an area farm-a job he detested. The farmer he worked for had a low opinion of his skills, complaining, "He can't farm worth a nickel."7 Despite his limitations in many areas beyond science, Lawrence's single-mindedness would prove a great strength. Like a magnifying gla.s.s on dry wood, whatever topics Lawrence's bright blue eyes did focus on would be set ablaze with his extraordinary energy and intuitive understanding. One of the first to recognize his talents was Lewis Akeley, dean of electrical engineering at the University of South Dakota, where he completed his undergraduate studies. Lawrence had transferred there in 1919 from St. Olaf College in Minnesota with the goal of preparing for a career in medicine, but Akeley steered him toward physics. Akeley was so impressed by Lawrence's phenomenal knowledge of wireless communication that he decided to experiment with a new teaching arrangement. For senior seminar he asked Lawrence, the only upper-level physics major, to prepare and deliver the lectures himself. While Lawrence was speaking, Akeley sat smiling as an audience of one, marveling that he was lucky enough to get to know perhaps the next Michael Faraday.

Meanwhile, Tuve was at the University of Minnesota and persuaded Lawrence to join its graduate program in physics. There, Lawrence found a new mentor, English-born physicist W. F. G. Swann, from whom he learned about the latest questions in quantum physics. Swann was a bit of a restless soul, an accomplished cellist as well as a researcher, who hated stodginess and valued creative thinking. Unhappy in Minnesota, he moved to Chicago and then to Yale, inspiring Lawrence to follow. Lawrence received his Ph.D. from Yale in 1925 and continued for three more years as national research fellow, working on new methods for determining Planck's constant and the charge-to-ma.s.s ratio of the electron. Along with another research fellow, Jesse Beams, he developed a highly acclaimed way of measuring extremely short time intervals in atomic processes. They showed that the photoelectric effect, in which light releases electrons from metals, takes place in less than three billionths of a second-lending support to the idea that quantum events are instantaneous.8 With his Ph.D. in hand, Lawrence finally found time to socialize, albeit in a manner unusual for a postdoctoral researcher. The daughter of the dean of the medical school, Mary Kimberly "Molly" Blumer, who was then only sixteen years old, needed a date for her high school prom. Word got out, and Lawrence agreed to be her escort. He was impressed with her quiet thoughtfulness, and after the prom he asked her if he could see her again. Politely, she said he could stop by, even though at that point in her life she understandably felt awkward being wooed by a man nine years her senior. Each time he visited her house, she would take any measure not to be alone with him-making sure her sisters accompanied them at all times. Sometimes she would even hide out in a family fis.h.i.+ng boat in the Long Island Sound and refuse to return to sh.o.r.e. It is a tribute to his perseverance that they would eventually get married.

Lawrence was courted in a different way by a rising star in the academic world, the University of California in Berkeley. Berkeley offered him an a.s.sistant professors.h.i.+p. When he turned it down in favor of remaining at Yale, Berkeley raised its offer to an a.s.sociate professors.h.i.+p-a rank usually reserved for more seasoned faculty. Lawrence then decided to accept, thinking Berkeley would offer quicker advancement and a greater chance of working directly with graduate students. Some of his stuffier colleagues at Yale were taken aback that he would even consider switching to a non-Ivy League inst.i.tution. "The Yale ego is really amusing," Lawrence wrote to a friend. "The idea is too prevalent that Yale brings honor to a man and that a man cannot bring honor to Yale."9 In August 1928, Lawrence drove out west in an REO Flying Cloud Coupe to a.s.sume his new position. After crossing the American heartland and reaching the rolling Berkeley hills, he took time to admire the beauty of the Bay Area and the exciting cultural jumble of San Francisco. The campus, dominated by a Venetian-style bell tower, offered a different kind of splendor. Although rooted in European architectural themes, it seemed refres.h.i.+ngly bright and modern-a far cry from pompous East Coast tradition.

Blessed with ample s.p.a.ce and support for his work, he resumed his studies of the precise timing of atomic processes. Then, barely seven months after he started, his research took an unforeseen turn. Around April Fools' Day of a year devastating for investors but auspicious for high-energy physics, Lawrence was sitting in the Berkeley library browsing through journals. Wideroe's article leapt to his attention as if it were spring-loaded. It was the diagrams, not the words, that he noticed at first-sketches of electrodes and tubes arranged to propel particles.

Of Wideroe's two accelerator designs, the linear dual-tube arrangement and the ringed "ray transformer" scheme, Lawrence found the latter more appealing. Lawrence realized immediately that a straight-tube setup would be limiting, providing only a few kicks before particles reached their targets. By curving the tubes into semicircles with electrifying gaps in between, and bending particle paths with a central magnet, he saw that he could jolt the particles again and again. He noted the fortuitous coincidence in magnetism that for a given particle steered in a circular loop by a magnetic field, if the field is constant the ratio of the particle's velocity to its...o...b..tal radius-a quant.i.ty known as angular velocity-similarly remains the same, even if the particle speeds up. Because angular velocity represents the rate by which an object travels around a circle, if it is constant then the object pa.s.ses the same point in equal intervals of time-like a racehorse pa.s.sing a grandstand precisely once a minute. This regularity, Lawrence, determined, would ensure that a voltage boost peaked at regular intervals (oscillating in the same rhythm as the orbits) could accelerate particles around a ring to higher and higher energies until they reached the level needed to penetrate a target nucleus. Lawrence realized the problem with Wideroe's design, and his pileup of electrons, was all in timing the voltage boosts.

Lawrence shared his design with Berkeley mathematician Donald Shane, who verified that the equations checked out. When Shane inquired, "What are you going to do with it?" Lawrence excitedly replied, "I'm going to bombard and break up atoms!"

The following day, his exuberance grew even greater when additional calculations confirmed that particles in his planned accelerator could continue to circle faster and faster no matter how far they spiraled outward from the center of the ring. As he strutted through campus like a proud peac.o.c.k, a colleague's wife distinctly heard him exclaim, "I'm going to be famous!"10 As he was accustomed to during childhood, Lawrence couldn't wait to run his idea past Tuve, who was then working at the Carnegie Inst.i.tution in Was.h.i.+ngton. Tuve was dubious about the scheme's practicality. Ironically, the best friends were becoming rivals-following separate tracks in the race to split the nucleus. Along with Gregory Breit and Lawrence Hafstad, Tuve was involved in efforts to crank Tesla coils-paired wound coils for which lower voltage in one induces high voltage in the second-up to ultrahigh energies. However, these devices were extremely hard to insulate and wasted a lot of energy. After Van de Graaff developed his high-voltage generators, Tuve recognized their promise and began constructing his own.

Because of doubts expressed by Tuve and other colleagues, Lawrence was at first hesitant to try out his concept, which he initially called the magnetic resonance accelerator and later became known as the cyclotron. It took some prodding by a noted scientist to get him going. Around Christmas 1929, he sat down for some bootleg wine (it was Prohibition) with German physicist Otto Stern-who was visiting the United States at the time-and outlined his scheme. Stern became excited and urged Lawrence to turn his design into reality. "Ernest, don't just talk any more," he urged. "You must . . . get to work on that."11 Placing himself in direct compet.i.tion with c.o.c.kcroft, Walton, Van der Graaff, Tuve, and other nuclear researchers, Lawrence hadn't a moment to spare to get his accelerator up and running. He took his first Ph.D. student, Niels Edlefsen, aside and inquired, "Now about this crazy idea of mine we've discussed. So simple I can't understand why someone hasn't tried it. Can you see anything wrong with it?"

Edlefsen responded that the idea was sound. "Good!" said Lawrence. "Let's go to work. You line up what we need right away."12 Under Lawrence's supervision, Edlefsen a.s.sembled a hodgepodge of materials found around the lab into a working prototype. A round copper box, cut in half, served as the two electrodes-wired to a radio-frequency oscillator that offered a cyclic voltage boost. Edlefsen encased the device in gla.s.s and centered it between the four-inch poles of a guiding electromagnet. Finally, he sealed all of the connections with sticky wax. Completed in early 1930, it wasn't very elegant. Nevertheless it was sufficient, after some tinkering, to get protons circulating-much to Lawrence's delight.

A thirty-seven-inch early cyclotron at the Radiation Laboratory, now the Lawrence Berkeley National Laboratory.

For energies high enough to break through nuclear barriers, Lawrence realized that he needed a bigger machine with a more powerful magnet. Fortunately an industrial executive, who also taught at the university, offered him an eighty-ton magnet that had been gathering dust in a warehouse about fifty miles south in Palo Alto. Built for radio transmission, it had been rendered obsolete by technological advances.

The generous donation prodded Lawrence to find a more s.p.a.cious setting suitable for a much larger accelerator. He got lucky again; in 1931, an old building on campus was about to be demolished and he was given permission to use it. Christened the Radiation Laboratory (and nicknamed the "Rad Lab"), it and its successor buildings would serve for decades as his dedicated center for research. It would eventually be renamed in honor of its founder and is now known as the Lawrence Berkeley National Laboratory.

Another pressing issue was how to move the colossal magnet to the lab. When yet another donor offered him funds to that effect, Lawrence scored a triple play in the tight budgetary age of the Great Depression. He finally had ample s.p.a.ce and equipment to construct a powerful machine.

In 1932, a banner year for nuclear physics, remarkable experiments around the world cast powerful spotlights on the murky inner workings of atoms. At Columbia University, chemist Harold Urey discovered deuterium, an isotope of hydrogen with approximately twice the ma.s.s of the standard version. James Chadwick's identification of the neutron, found through meticulous observations at Cavendish, explained why deuterium is twice as ma.s.sive as its similarly charged brother: the heavier isotope is bloated with extra neutrons. Speculations arose as to whether neutrons are particles in their own right, or alternatively protons and electrons somehow clumped together to make an electrically neutral particle.

A couple of different theories had been bandied about, and only experimentation could tell which of the theorists has guessed right. For example, beta decay is when a radioactive substance gives off electrons. Those electrons, some suggested, must be coming from neutrons breaking into protons and electrons. (We now know that it is the weak interaction that is mediating a transformation involving the quarks that form protons and neutrons, along with the electron and a neutrino.) Carl Anderson's discovery of the positron offered another possible explanation for the relations.h.i.+p between neutrons and protons. He found the positron in cloud chamber photographs taken at Caltech of positively charged cosmic rays (radiation from s.p.a.ce pa.s.sing through Earth's atmosphere) with the same ma.s.s as the electron. We now know a positron is the antimatter version of an electron, but at the time, Anderson wondered if the neutron is fundamental, and the proton an amalgamation of a neutron and a positron. Testing these alternatives would require precise measurements of the ma.s.ses of protons and neutrons, to see if one was sufficiently heavier than the other to accommodate an electron or positron. (Indeed, as we now know, the neutron is heavier, but is composed of quarks, not protons and electrons.) While Lawrence, along with Wisconsin-born graduate student M. (Milton) Stanley Livingston, toiled on the larger cyclotron, word came of victory in the race to split the lithium nucleus. The first to reach the finish line were c.o.c.kcroft and Walton, using the Cavendish linear accelerator. Walton recalled the moment of discovery when they finally bombarded the lithium target with protons and observed the stunning results: On the morning of April 14, 1932, I carried out the usual conditioning of the apparatus. When the voltage had risen to about 400,000 volts, I decided to have a look through the microscope which was focused on the fluorescent screen. By crawling on my hands and knees to avoid the high voltage, I was able to reach the bottom of the accelerating tube. To my delight, I saw tiny flashes of light looking just like the scintillations produced by alpha particles which I had read about in books but which I had never previously seen.13 After observing what surely looked like the decay of lithium, Walton called c.o.c.kcroft into the lab, who agreed with that explanation. Then they invited Rutherford to crawl into the chamber and check out the scintillations himself. They turned off the voltage, and he ducked inside. When Rutherford came out, he said: Those scintillations look mighty like alpha particle ones and I ought to be able to recognize an alpha particle scintillation when I see one. I was in at the birth of the alpha particle and I have been observing them ever since. 14 14 Uncharacteristically, Rutherford asked c.o.c.kcroft and Walton to keep the news a "dead secret" until they could conduct more measurements. As Walton explained in a letter to his girlfriend, Freda Wilson (whom he would marry in 1934): He [Rutherford] suggested this course because he was afraid that the news would spread like wild fire through the physics labs of the world and it was important that no lurid accounts should appear in the daily papers etc. before we had published our own account of it.15 c.o.c.kcroft and Walton ran the experiment further times using a cloud chamber to record the alpha particle tracks. (Recall that a cloud chamber is a box full of vapor for which charged particles pa.s.sing through create a visible misty trail.) Calculating the ma.s.ses before and after the collision, they confirmed that each lithium nucleus, with three protons and four neutrons, had been cajoled by an extra proton to break up into two alpha particles, each of two protons and two neutrons. They'd literally cut the lithium ions in half!

Moreover, the energy released during each hit corresponded precisely to the ma.s.s difference between the initial and final states, times the speed of light squared. Their experiment confirmed Einstein's famous formula. Satisfied with the accuracy and importance of their results, they published their findings in the prestigious journal Nature. Nature. For their exemplary work, c.o.c.kcroft and Walton would share the 1951 n.o.bel Prize for physics. For their exemplary work, c.o.c.kcroft and Walton would share the 1951 n.o.bel Prize for physics.

The news from Cambridge didn't dampen Lawrence's spirits. He had much to celebrate. For one thing, he had just married Molly and was on his honeymoon. His dogged persistence in romance as well as in science had finally paid off. The coy young woman had grown to love her awkward but accomplished suitor. They would have a large family together-four girls and two boys.

Another cause for Lawrence's optimism was his strong conviction that he was at the forefront of a new scientific era. Ultimately, he realized, cyclotrons could yield much higher energies than linear accelerators could muster, and would thereby be essential for future probes of the nucleus. He wasted no time in confirming c.o.c.kcroft and Walton's lithium results with an eleven-inch cyclotron. The larger device in the Rad Lab with the eighty-ton magnet was still under completion. When it was ready in March 1933, Lawrence bombarded lithium with protons and generated a bounty of highly energetic alpha particles-ricocheting back with impressive range. He also struck a variety of elements with deuterons, producing protons of Olympian stamina-some sprinting up to fifteen inches. By that point, he was more than ready to share his results with the physics community at large.

The Seventh Solvay Conference, held in Brussels during the last week of October 1933, was a milestone for discussion of the remarkable advances in nuclear physics. Among the scientific luminaries present were quantum pioneers Bohr, de Broglie, Pauli, Dirac, Heisenberg, and Schrodinger. The Parisian contingent included Marie Curie, along with her daughter and son-in-law, Irene Joliot-Curie and Frederic Joliot, each an esteemed nuclear chemist and future n.o.bel laureate.

From Russia came Gamow-the start of his permanent exile, as it turned out. Two years earlier, he had returned to his home-land by way of Copenhagen. Unhappy living under Stalin's iron thumb, he and his wife had attempted to escape across the Black Sea to Turkey but had been foiled by foul weather. Remarkably, an invitation by Bohr allowed both of them to slip into Belgium, where Gamow announced to his surprised host that they would never go back.

The Cavendish contribution to the meeting was impressive. Headed by Rutherford, it included c.o.c.kcroft, Walton, Chadwick, and Blackett. Finally, though Lawrence was the lone American attendee, his presence was vital, in as much that cyclotrons represented the future of nuclear exploration and that the United States would for decades be the princ.i.p.al testing ground for such devices.

c.o.c.kcroft delivered the conference's first talk, "The Disintegration of Elements by Accelerated Protons." Listening eagerly to his every word was Lawrence, keen to demonstrate the superiority of the cyclotron in handling the job. Perusing c.o.c.kcroft's handout, Lawrence noted a statement that "only small currents are possible" from the cyclotron and emphatically crossed it out. In the margin he wrote, "Not true," expressing his clear impatience with c.o.c.kcroft's a.s.sertions.16 When it came time for discussion, Lawrence was quick to respond. He presented an account of his own device and argued that it offered the best way forward to explore the nucleus. He also offered his own estimation of the ma.s.s of the neutron-controversially, much lower than Chadwick's value. Further experiments conducted by Tuve later that year would demonstrate that Lawrence was wrong; a mistake he would frankly acknowledge. The neutron turned out to be slightly bulkier than the proton.

After Solvay, Lawrence traveled to England and spent a pleasant couple of days at Cavendish. Rutherford warmly welcomed him and led him on a personal tour. After some heated discussions about lithium bombardment results, Rutherford said of Lawrence, "He's a brash young man but he'll learn."17 Lawrence tried to convince Rutherford to build a cyclotron at Cavendish. Chadwick and c.o.c.kcroft joined in the chorus, arguing that it was the only way for the lab to remain compet.i.tive. Rutherford would not budge. He had a preference for homemade equipment and was reluctant to import another group's idea. Moreover, he disliked trolling for funds and knew a cyclotron would be expensive.

Rutherford's reluctance cost him dearly. In 1935, Chadwick, frustrated with the lack of progress, departed for a position at the University of Liverpool where he began to solicit funds for a cyclotron. On a visit to Cambridge in the summer of 1936, he and his former mentor were barely on speaking terms. Around the same time, Australian-born physicist Mark Oliphant, another of Rutherford's proteges, was offered a position at the University of Birmingham, which he would a.s.sume the following year. Pressed by the loss of some of his top researchers, an embittered Rutherford finally agreed to let c.o.c.kcroft construct a cyclotron at Cambridge.

While Rutherford hedged, Lawrence was busy collecting funds to build an even larger cyclotron at Berkeley. Tremendously successful at fund-raising, he had no trouble continuing to expand the Radiation Laboratory's work. Oliphant, who would visit there and get to know Lawrence as well as Rutherford, explained the difference in their styles: "The Cavendish laboratory, under Rutherford and his predecessors, was always short of money. Rutherford had no flair and no inclination for raising funds. . . . Lawrence, on the other hand, had shrewd business sense and was adept at raising funds for the work of his laboratory."

Oliphant pointed out that Lawrence, who originally was on a premed track at university, had the savvy to foresee the medical applications of cyclotrons and how these could be used to draw funding. In a 1935 letter to Bohr, Lawrence wrote, "As you know, it is so much easier to get funds for medical research."

Unlike Rutherford, who suggested and personally supervised almost every experiment his lab undertook, Lawrence liked to delegate authority. He had exemplary management skills that impressed his benefactors in government and industry and enabled his lab to expand. As Oliphant noted: His direct approach, his self-confidence, the quality and high achievement of his colleagues, and the great momentum of the researchers under his direction bred confidence in those from whom the money came. His judgment was good, both of men and of the projects they wished to undertake, and he showed a rare ability to utilize to the full the diverse skills and experiences of the various members of his staff. He became the prototype of the director of the large modern laboratory, the costs of which rose to undreamt of magnitude, his managerial skill resulting in dividends of important scientific knowledge fully justifying the expenditure.18 On October 19, 1937, Rutherford died of a strangulated hernia. Having been raised to peerage six years earlier, he was buried with the honors accorded his position as "Right Honourable Lord of Nelson." His coat of arms reflected both his national and his scientific heritage: images of a New Zealand kiwi bird and a Maori warrior, along with a motto borrowed from Lucretius, "Primordia Quaerare Rerum (To seek the first principles of things)." Fittingly, his ashes were interred in a grave at Westminster Abbey next to the final resting places of Newton and Lord Kelvin. (To seek the first principles of things)." Fittingly, his ashes were interred in a grave at Westminster Abbey next to the final resting places of Newton and Lord Kelvin.

5.

A Compelling Quartet The Four Fundamental Forces

The grand aim of all science . . . is to cover the greatest number of empirical facts by logical deduction from the smallest possible number of hypotheses or axioms.-ALBERT EINSTEIN (THE PROBLEM OF s.p.a.cE, ETHER, AND THE FIELD IN PHYSICS, 1954, TRANSLATED BY SONJA BARGMANN) In 1939, Niels Bohr arrived at Princeton with a grave secret. He had just learned that n.a.z.i Germany was pioneering the methods of nuclear fission: the splitting of uranium and other large nuclei. The unspoken question was whether the powerful energies of atomic cores could be used by Hitler to manufacture deadly weapons. To understand fission better, Bohr was working with John Wheeler to develop a model of how nuclei deform and fragment.

Out of respect, Bohr attended one of Einstein's lectures on unification. Einstein presented an abstract mathematical model of uniting gravitation with electromagnetism. It did not mention nuclear forces, nor did it even address quantum mechanics. Bohr reportedly left the talk in silence. His disinterest characterized the spirit of the times; the nucleus was the new frontier.

Nuclear physics had by then become intensely political. The previous year, Otto Hahn, a German chemist who had a.s.sisted Rutherford during his days at McGill, along with Lise Meitner and Fritz Stra.s.smann had discovered how to induce the fission of a particular isotope of uranium through the bombardment of neutrons. When later that year Meitner fled the n.a.z.is after the Anschluss (annexation of Austria), she brought word of the discovery to her nephew Otto Frisch, who was working with Bohr. Bohr became alarmed by the prospects that the n.a.z.is could use this finding to develop a bomb-an anxiety that others in the know soon shared. These fears intensified when Szilard and Italian physicist Enrico Fermi demonstrated that neutrons produced by uranium nuclei splitting apart could trigger other nuclei to split-with the ensuing chain reaction releasing enormous quant.i.ties of energy. Szilard wrote a letter to Roosevelt warning of the danger and persuaded Einstein to sign it. Soon the Manhattan Project was born, leading to the development by the Americans of the atomic bomb.

The nucleus was a supreme puzzle. What holds it together? Why does it decay in certain ways? What causes some isotopes to disintegrate more readily than others? How come the number of neutrons in most atoms greatly exceeds the number of protons? Why does there seem to be an upper limit on the size of nuclei found in nature? Could artificial nuclei of any size be produced?

Throughout the turbulent years culminating in the Second World War, one of the foremost pioneers in helping to resolve those mysteries was Fermi. Born in Rome on September 29, 1901, young Enrico was a child prodigy with an amazing apt.i.tude for math and physics. By age ten he was studying the nuances of geometric equations such as the formula for a circle. After the tragic death of his teenage brother, he immersed himself in books as a way of trying to cope, leading to even further acceleration in his studies. Following a meteoric path through school and university, he received a doctorate from the University of Pisa when he was only twenty-one. During the mid-1920s, he spent time in Gottingen, Leiden, and Florence, before becoming professor of physics at the University of Rome.

Among other critical contributions Fermi made to nuclear and particle physics, in 1933, he developed the first mathematical model of beta decay. The impetus to do so arose when at the Seventh Solvay Conference earlier that year, Pauli spoke formally for the first time about the theory of the neutrino. Pauli explained that when beta rays are emitted from the radioactive decay of a nucleus, an unseen, electrically neutral, lightweight particle must be produced to account for un.o.bserved extra energy. He had originally called it a neutron, but when those heavier particles were discovered, he took up a suggestion by Fermi and switched to calling it by its Italian diminutive. Fermi proceeded to calculate how the decay process would work. Though, as it would turn out, his model was missing several key ingredients, it offered the monumental unveiling of a wholly new force in nature-the weak interaction. It is the force that causes certain types of particle transformations, producing unstable phenomena such as beta decay.

As physicist Emilio Segre, who worked under Fermi, recalled: Fermi gave the first account of this theory to several of his Roman friends while we were spending the Christmas vacation of 1933 in the Alps. It was in the evening after a full day of skiing; we were all sitting on one bed in a hotel room, and I could hardly keep still in that position, bruised as I was after several falls on icy snow. Fermi was fully aware of the importance of his accomplishment and said that he thought he would be remembered for this paper, his best so far.1 Fermi's model of beta decay imagines it as an exchange process involving particles coming together at a point. For example, if a proton meets up with an electron, the proton can transfer its positive charge to the electron, transforming itself into a neutron and the electron into a neutrino. Alternatively, a proton can exchange its charge and become a neutron, along with a positron and a neutrino. As a third possibility, a neutron can trans.m.u.te into a proton, in conjunction with an electron and an antineutrino (like a neutrino but with a different production mechanism). Each of these involves a huddling together and a transfer-like a football player approaching a member of the opposing team, grabbing the ball, and heading off in another direction.

In electromagnetism, two electric currents-streams of moving electric charge-can interact with each other by means of the exchange of a photon. Because the photon is an electrically neutral particle, no charge is transferred in the process. Rather, the photon exchange can either bring the currents together or separate them depending on the nature and direction of the moving charges.

In modern terminology, we say the photon is the exchange particle conveying the electromagnetic force. Exchange particles, including photons, belong to a cla.s.s of particles called bosons. The smallest ingredients of matter-now known to be quarks and leptons-are all fermions. If fermions are like the bones and muscles of the body, bosons supply the nerve impulses that provide their dynamics.

For the weak force, as Fermi noted, two "currents," one the proton/neutron and the other the electron/neutrino, can exchange charge and ident.i.ty during their process of interaction. Here Fermi generalized the concept of current to mean not just moving charges but also any stream of particles that may keep or alter certain properties during an interaction.

Just as ma.s.s measures the impact of gravity, and charge the strength of electromagnetism, Fermi identified a factor-now known as the Fermi weak coupling constant-that sets the strength of the weak interaction. He used this information to construct a method, known as Fermi's "golden rule," for calculating the odds of a particular decay process taking place. Suddenly, the long-established gravitational and electromagnetic interactions had a brand-new neighbor. But no one knew back then how to relate the new kid on the block to the old-timers.

Types of elementary particles.

To make matters even more complicated, in 1934, j.a.panese physicist Hideki Yukawa postulated a fourth fundamental interaction, similarly on the nuclear scale. Yukawa noted that while beta decay is a rare event, another linkage between protons and neutrons is much more common and significantly more powerful. Rather than causing decay, it enables coherence. To distinguish Yukawa's nuclear interaction from Fermi's, the former became known as the strong force.

The need for a strong force bringing together nucleons (nuclear particles) has to do with their proximity and, in the proton's case, their identical charge. Judging each other on the basis of charge alone, protons wouldn't want to stick together. Their mutually repulsive electrostatic forces would make them want to get as far away from each other as possible, like the north poles of two magnets pus.h.i.+ng each other apart. The closer together they'd get, their shared desire to flee would grow even greater. Then how do they fit into a cramped nucleus on the order of a quadrillionth of an inch?

Born in Tokyo in 1907, Yukawa grew up at a time when the j.a.panese physics community was very isolated and there was very little interaction with European researchers. His father, who became a geology professor, strongly encouraged him to pursue his scientific interests. Attending the university where his father taught, Kyoto University, he demonstrated keen creativity in dealing with mathematical challenges-which would propel him to a pioneering role in establis.h.i.+ng theoretical physics in his native land. At the age of twenty-seven, while still a Ph.D. student, he developed a brilliant way of treating nuclear interactions that became a model for describing the various natural forces.

Yukawa noted that while electromagnetic interactions can bridge vast distances, nuclear forces tend to drop off very quickly. The magnetic effects of Earth's iron core can, for example, align a compa.s.s thousands of miles away, but nuclear stickiness scarcely reaches beyond a range about one trillionth of the size of a flea. He attributed the difference in scale to a distinction in the type of boson conveying the interaction. (Remember that bosons are like the universe's nervous system, conveying all interactions.) The photon, a ma.s.sless boson, serves to link electrical currents spanning an enormous range of distances. If it were ma.s.sive, however, its range would shrink down considerably, because the inverse-squared decline in interactive strength over distance represented by Maxwell's wave equations would be replaced by an exponentially steeper drop. The situation would be a bit like throwing a Frisbee back and forth across a lawn and then replacing it with a lead dumbbell. With the far heavier weight, you'd have to stand much closer to keep up the exchange.

By subst.i.tuting nuclear charge for electric charge, and ma.s.sive bosons, called mesons, for photons, Yukawa found that he could describe the sharp, pinpoint dynamics of the force between nucleons-demonstrating why the interaction is powerful enough to bind nuclei tightly together while being insignificant at scales larger than atomic cores. All that would be needed was a hitherto unseen particle. If Dirac's hypothesized positrons could be found, why not mesons?

Nature sometimes plays wicked tricks. In 1936, Carl Anderson observed a strange new particle in a stream of cosmic rays. Because a magnetic field diverted it less than protons and more than electrons or positrons, he estimated its ma.s.s to be somewhere in between-a little more than two hundred times the ma.s.s of the electron. On the face of things, it seemed the answer to nuclear physicists' dreams. It fit in well with Yukawa's predictions for the ma.s.s of the exchange boson for the strong force, and physicists wondered if it was the real deal.

Strangely enough, any resemblance between the cosmic intruder and Yukawa's hypothesized particle was pure coincidence. Further tests revealed the new particle to be identical to the electron in all properties except ma.s.s. Indeed it turned out to be a lepton, a category that doesn't experience the strong force at all, rather than a hadron, the term denoting strongly interacting particles. (Lepton and hadron derive from the Greek for "thin" and "thick," respectively-a reference to their relative weights that is not always accurate; some leptons are heavier than some hadrons.) Anderson's particle was eventually renamed the muon, to distinguish it from Yukawa's exchange particle. Pointing out the muon's seeming redundancy and lack of relevance to the theories of his time, physicist Isidor I. Rabi famously remarked, "Who ordered that?"

True mesons would not be found for more than a decade. Not many nuclear physicists were contemplating pure science during that interval; much energy was subsumed by the war effort. Only after the war ended could the quest for understanding the world of particles resume in earnest.

In 1947, a team of physicists led by Cecil Powell of the University of Bristol, England, discovered tracks of the first known meson, in a photographic image of cosmic ray events. Born in Tonbridge in Kent, England, in 1903, Powell had an unlucky early family life. His grandfather was a gun maker who had the misfortune of accidentally blinding someone while out shooting-an action that led to a lawsuit and financial ruin. Powell's father tried to continue in the family trade, but the advent of a.s.sembly-line production bankrupted him.

Fortunately, Powell himself decided to pursue a different career path. Receiving a scholars.h.i.+p to Cambridge in 1921, he consulted with Rutherford about joining the Cavendish group as a research student. Rutherford agreed and arranged for Charles Wilson to be his supervisor. Powell soon became an expert on building cloud chambers and using them for detection.

In the mid-1930s, after c.o.c.kcroft and Walton built their accelerator, Powell constructed his own and actively studied collisions between high-energy protons and neutrons. By then he had relocated to Bristol. While at first he used cloud chambers to record the paths of the by-products, he later found that a certain type of photographic emulsion (a silver bromide and iodide coating) produced superior images. Placing chemically treated plates along the paths of particle beams, he could observe disintegrations as black "stars" against a transparent background-indicating all of the offshoots of an interaction. Moreover the length of particle tracks on the plates offered a clear picture of the decay products' energies-with any missing energy indicating possible unseen marauders, such as neutrinos, that have discreetly stolen it away.

In 1945, Italian physicist Giuseppe Occhialini joined the Bristol group, inviting one of his most promising students, Cesar Lattes, along one year later. Together with Powell they embarked upon an extraordinary study of the tracks produced by cosmic rays. To obtain their data they brought covered photographic plates up to lofty alt.i.tudes, including an observatory high up in the French Pyrenees and onboard RAF (Royal Air Force) aircraft. After exposing the plates to the steady stream of incoming celestial particles, the researchers were awestruck by the complex webs of patterns they etched-intricate family trees of subatomic births, life journeys, and deaths.

As Powell recalled: When [the plates] were recovered and developed at Bristol it was immediately apparent that a whole new world had been revealed. The track of a slow proton was so packed with developed grains that it appeared almost like a solid rod of silver, and the tiny volume of emulsion appeared under the microscope to be crowded with disintegrations produced by fast cosmic ray particles with much greater energies than any which could have been produced artificially at the time. It was as if, suddenly, we had broken into a walled orchard, where protected trees had flourished and all kinds of exotic fruits had ripened in great profusion.2 Among the patterns they saw was a curious case of one midsize particle stopping and decaying into another, appearing as if a slightly more ma.s.sive type of muon gave birth to the conventional variety. Yet a long line of prior experiments indicated that if muons decay they always produce electrons, not more muons. Consequently, the researchers concluded that the parent particle must have been something else. They named it the "pi meson," which became "pion" for short. It soon became clear that the pion matched the exchange particle predicted by Yukawa.

Around the same time, George Rochester of the University of Manchester detected in cloud chamber images a heavier type of meson, called the neutral kaon, that decays along a V-shaped track into two pions-one positive and the other negative. In short order, researchers realized that pions and kaons each have positive, negative, and neutral varieties-with neutral kaons themselves coming in two distinct types, one shorter lived than the other.

The importance of the discovery of mesons was so widely recognized that Powell received the n.o.bel Prize in lightning speed-in 1950, only three years later. Occhialini would share the 1979 Wolf Prize, another prestigious award, with George Uhlenbeck.

The Bristol team's discovery represented the culmination of the Cavendish era of experimental particle physics. From the 1950s until the 1970s, the vast majority of new findings would take place by means of American accelerators, particularly successors to Lawrence's cyclotron. An exciting period of experimentation would demonstrate that Powell's "orchard of particles" is full of strange fruit indeed.

While high-energy physicists, as researchers exploring experimental particle physics came to be known, tracked an ever-increasing variety of subatomic events, a number of nuclear physicists joined with astronomers in attempts to unravel how the natural elements formed. An influential paper by physicist Hans Bethe, "Energy Production in Stars," published in 1939, showed how the process of nuclear fusion, the uniting of smaller into larger nuclei, enables stars to s.h.i.+ne. Through a cycle in which ordinary hydrogen combines into deuterium, deuterium unites with more hydrogen to produce helium-3, and finally helium-3 combines with itself to make helium-4 and two extra protons, stars generate enormous amounts of energy and radiate it into s.p.a.ce. Bethe proposed other cycles involving higher elements such as carbon.

George Gamow, by then at George Was.h.i.+ngton University, humorously borrowed Bethe's name while applying his idea to the early universe in a famous 1948 paper with Ralph Alpher, "The Origin of Chemical Elements." Although Alpher and Gamow were the paper's true authors, they inserted Bethe's appellation to complete the trilogy of the first Greek letters; hence it is sometimes known as the "alphabetical paper."

Alpher and Gamow's theory of element production relies on the universe having originated in an extremely dense, ultrahot state, dubbed by Fred Hoyle the "Big Bang." (Hoyle, a critic of the theory, meant his appellation to be derogatory, but the name stuck.) The idea that the universe was once extremely small was first proposed by Belgian mathematician and priest Georges Lemaitre, and gained considerable clout when American astronomer Edwin Hubble discovered that distant galaxies are moving away from ours, implying that s.p.a.ce is expanding. Alpher and Gamow hypothesized that helium, lithium, and all higher elements were forged in the furnace of the fiery nascent universe.

Curiously enough, although they were right about helium, they were wrong about the other elements. While the primordial universe was indeed hot enough to fuse helium from hydrogen, as it expanded, it markedly cooled down and could not have produced higher elements in sufficient quant.i.ties to explain their current amounts. Thus the carbon and oxygen in plants and animals were not produced in the Big Bang. Rather, as Hoyle and three of his colleagues demonstrated, elements higher than helium were wrought in a different type of cauldron-the intense infernos of stellar cores-and released into s.p.a.ce through the stellar explosions called supernovas.

Gamow was flummoxed by the idea that there could be two distinct mechanisms for element production. In typical humorous fas.h.i.+on, he channeled his bafflement and disappointment into mock biblical verse: a poem t.i.tled "New Genesis."

"In the beginning," the verse begins, "G.o.d created radiation and ylem (primordial matter)." It continues by imagining G.o.d fas.h.i.+oning element after element simply by calling out their ma.s.s numbers in order. Unfortunately, G.o.d forgot ma.s.s number five, almost dooming the whole enterprise. Rather than starting again, He crafted an alternative solution: "And G.o.d said: 'Let there be Hoyle' . . . and told him to make heavy elements in any way he pleased."3 Despite its failure to explain synthesis of higher elements, the Big Bang theory has proven a monumentally successful description of the genesis of the universe. A critical confirmation of the theory came in 1965 when Arno Penzias and Robert W. Wilson pointed a horn antenna into s.p.a.ce and discovered a constant radio hiss in all directions with a temperature of around three degrees above absolute zero (the lower limit of temperature). After learning of these results, Princeton physicist Robert d.i.c.ke demonstrated that its distribution and temperature were consistent with expectations for a hot early universe expanding and cooling down over time.

In the 1990s and 2000s, designated satellites, called the COBE (Cosmic Background Explorer) and the WMAP (Wilkinson Microwave Anisotropy Probe), mapped out the fine details of the cosmic background radiation and demonstrated that its temperature profile, though largely uniform, was pocked with slightly hotter and colder spots-signs that the early universe harbored embryonic structures that would grow up into stars, galaxies, and other astronomical formations. This colorfully ill.u.s.trated profile was nicknamed "Baby Picture of the Universe."

The Baby Picture harkens back to a very special era, about three hundred thousand years after the Big Bang, in which electrons joined together with nuclei to form atoms. Before this "era of recombination," electromagnetic radiation largely bounced between charged particles in a situation akin to a pinball machine. However, once the negative electrons and positive cores settled down into neutral atoms, it was like turning off the "machine" and letting the radiation move freely. Released into s.p.a.ce the hot radiation filled the universe-bearing subtle temperature differences reflecting slightly denser and slighter more spread out pockets of atoms. As the cosmos evolved, the radiation cooled down and the denser regions drew more and more matter. When regions acc.u.mulated the critical amount of hydrogen to fuse together, maintain steady chain reactions, and release energy in the form of light and heat, they began to s.h.i.+ne and stars were born.

The creation of stars, planets, galaxies, and so forth is the celestial drama that engages astrophysicists and astronomers. Particle physicists are largely interested in the back story: what happened before recombination. The details of how photons, electrons, protons, neutrons, and other const.i.tuents interacted with one another in the eons before atoms, and particularly in the first moments after the Big Bang reflect the properties of the fundamental natural interactions. Therefore, like colliders, the early universe represents a kind of particle physics laboratory; any discoveries from one venue can be compared to the other.

The same year that Alpher and Gamow published their alphabet paper, three physicists, Julian Schwinger and Richard Feynman of the United States and Sin-Itiro Tomonaga of j.a.pan, independently produced a remarkable set of works describing the quantum theory of the electromagnetic interaction. (Tomonaga developed his ideas during the Second World War when it was impossible for him to promote them.) Distilled into a comprehensive theory through the vision of Princeton physicist Freeman Dyson, quantum electrodynamics (QED), as it was called, became seen as the prototype for explaining how natural forces operate.

Of all the authors who developed QED, the one who offered the most visual representation was Feynman. He composed a remarkably clever shorthand for describing how particles communicate with one another-with rays (arrowed line segments) representing electrons and other charged particles, and squiggles denoting photons. Two electrons exchanging a photon, for example, can be depicted as rays coming closer over time, connecting up with a squiggle, and then diverging. a.s.signing each possible picture a certain value, and developing a means for these to be added up, Feynman showed how the probability of all manner of electromagnetic interactions could be determined. The widely used notation became known as Feynman diagrams.

Through QED came the alleviation of certain mathematical maladies afflicting the quantum theory of electrons and other charged particles. In trying to apply earlier versions of quantum field theory to electrons, theorists obtained the nonsensical answer "infinity" when performing certain calculations. In a process called renormalization, Feynman showed that the values of particular diagrams nicely canceled out, yielding finite solutions instead.

Inspired by the power of QED, in the 1950s, various theorists attempted to apply similar techniques to the weak, strong, and gravitational interactions. None of the efforts in this theoretical triathlon would come easy-with each leg of the race offering unique challenges.

By that point, Fermi's theory of beta decay had been extended to muons and become known as the Universal Fermi Interaction. Confirmation of one critical prediction of the theory came during the middle of the decade, when Frederick Reines and Clyde Cowan, scientists working at Los Alamos National Laboratory, placed a large vat of fluid near a nuclear reactor and observed the first direct indications of neutrinos. The experiment was set up to measure rare cases in which neutrinos from the reactor would interact with protons in the liquid, changing them into neutrons and positrons (antimatter electrons) in a process that is called reverse beta decay. When particles meet their antimatter counterparts, they annihilate each other in a burst of energy, producing photons. Neutrons, when absorbed by the liquid, also produce photons. Therefore Reines and Cowan realized that twin flashes (in another light-sensitive fluid) triggered by dual streams of photons would signal the existence of neutrinos. Amazingly, they found such a rare signal. Subsequent experiments they and others performed using considerably larger tanks of fluid confirmed their groundbreaking results.

By the time of the confirmation of the final component of Fermi's theory-the prototype of the weak interaction-physicists had begun to realize its significant gaps. These manifested themselves by way of comparison with the triumphs of QED. QED is a theory replete with many natural symmetries. Looking at Feynman diagrams representing its processes, many of these symmetries are apparent. For example, flip the time axis, reversing the direction of time, and you can't tell the difference from the original. Thus, processes run the same backward and forward in time. That is a symmetry called time-reversal invariance.

Another symmetry, known as parity, involves looking at the mirror image of a process. If the mirror image is the same, as in the case of QED, that is called conservation of parity. For example, the letter "O," looking the same in the mirror, has conserved parity, while the letter "Q" clearly doesn't because of its tail.

In QED, ma.s.s is also perfectly conserved-representing yet another symmetry. When electrons (or other charged particles) volley photons back and forth, the photons carry no ma.s.s whatsoever. Electrons keep their ident.i.ties during electromagnetic processes and never change ident.i.ties. Comparing that to beta decay, in which electrons sacrifice charge and ma.s.s and end up as neutrinos, the difference is eminently clear.

The question of symmetries in the weak interaction came to the forefront in 1956 when Chinese American physicists Tsung Dao Lee and Chen Ning (Frank) Yang proposed a brilliant solution to a mystery involving meson decay. Curiously, positively charged kaons have two different modes of decay: into either two or three pions. Because each of these final states has a different parity, physicists thought at first that the initial particles const.i.tuted two separate types. Lee and Yang demonstrated that if the weak interaction violated parity, then one type of particle could be involved with both kinds of processes. The "mirror-image" of certain weak decays could in some cases be something different. Parity violation seemed to breach common sense, but it turned out to be essential to understanding nuances of the weak interaction.

Unlike the weak interaction, the strong force does not have the issue of parity violation. Thanks to Yukawa, researchers in the 1950s had a head start in developing a quantum theory of that powerful but short-ranged force. However, because at that point experimentalists had yet to probe the structure of nucleons themselves, the Yukawa theory was incomplete.

The final ingredient in a.s.sembling a unified model of interactions would be a quantum theory of gravity. After QED was developed, physicists trying to develop an a.n.a.logous theory of gravitation encountered one brick wall after another. The most pressing dilemma was that while QED describes encounters that take place over time, such as one electron being scattered by another due to a photon exchange, gravitation, according to general relativity, is a feature stemming from the curvature of a timeless four-dimensional geometry. In other words, it has the agility of a statue. Even to start thinking about quantum gravity required performing the magic trick of turning a timeless theory into an evolving theory. A major breakthrough came in 1957 when Richard Arnowitt, Stanley Deser, and Charles Misner developed a way of cutting s.p.a.ce-time's loaf into three-dimensional slices changing over time. Their method, called ADM formalism, enabled researchers to craft a dynamic theory of gravity ripe for quantization.

Another major problem with linking gravity to the other forces involves their vast discrepancy in strength-a dilemma that has come to be known as the hierarchy problem. At the subatomic level, gravitation is 1040 (1 followed by 40 zeroes) times punier than electromagnetism, which itself is much less formidable than the strong force. Bringing all of these together in a single theory is a serious dilemma that has yet to be satisfactorily resolved. (1 followed by 40 zeroes) times punier than electromagnetism, which itself