Massive

Peter Higgs packed some clothes and a folder full of research notes and went out to the car with his wife, Jody, and their six-month-old son, Christopher. He swung the suitcase in the back and had a long look at the road map. Satisfied with the directions, he pulled away, working north and east through the tree-lined streets and out toward the highway as the town eased itself to life beneath the spring morning sun.

It was March 14, 1966. Higgs, a physicist at the University of Edinburgh, had moved to Chapel Hill the previous year to spend his sabbatical at the University of North Carolina.¹ His work there had caught the eye of a prominent scientist, who invited him to give a seminar at Princeton's Institute for Advanced Study, one of the world's leading intellectual centers and the place where Einstein himself had spent much of his working life. The seminar was destined to be controversial: Higgs had proposed an idea that, if correct, could explain the origin of mass.

The trip turned out to be more than just another academic visit. It marked the beginning of a run of events that catapulted Higgs into the scientific limelight and set the stage for the greatest hunt in modern physics. Using multibillion-dollar machines occupying miles of underground tunnels, thousands of scientists have spent decades looking for the particle that formed the linchpin of Higgs's theory. Their mantra was simple: find the Higgs particle and the mystery of the origin of mass was solved.

For centuries, scientists had no idea that mass even had an origin, at least not in the modern sense of the phrase. The word "mass" described how much matter an object had, and matter was no more than a grand term for "stuff." A lump of rock had more mass than a loaf of bread (unless the baker was having an off-day), and that was that. The meaning of mass was so intuitive and tangible that no one seriously thought to question it.

Vague and incomplete notions of mass emerged in antiquity and were developed through the Middle Ages. Giles of Rome, a prominent theologian and one of the most influential thinkers of the late thirteenth century, took an important conceptual step when he distinguished between the dimensions of an object and the amount of matter it contained.² A block of ice, for example, clearly changed shape when it melted into water, evaporated into steam, condensed, and became frozen solid again. Yet the amount of matter remained the same, he said, whichever form it was in. The observation, which surely made for lively theological discussions about the Trinity, mirrors the modern definitions of volume and mass.

In the early fourteenth century, the Parisian philosopher Jean Buridan drew on the concept of mass when he described how throwing an object gave it an impetus that depended on how much matter it contained and the speed at which it was lobbed.³ The sixteenth-century German astronomer Johannes Kepler took things further, arguing that planets stayed true to their orbits and didn't hurtle around space like scattered snooker balls thanks to the inertia arising from their enormous masses.

Despite the valuable work of early philosophers and astronomers, the term "mass" was not used systematically until 1687, when Isaac Newton laid the foundations of classical mechanics in a great but wholly impenetrable work, the Principia.⁴ Newton said mass was a quantity of matter that arose from an object's volume and density. An object's mass governed its inertia, or how much it resisted being pushed around, and also how strongly it felt the force of gravity. With these definitions in place, Newton derived the basic laws of motion.

Newton had an intuitive grasp of mass and matter that went far deeper than he let on in the Principia. Worldly objects, he believed, were made up of countless tiny particles that were created by God and could never be destroyed. The particles came in different forms and sizes that stuck together to make different materials. All man could hope to do was fashion new shapes and forms from these conglomerations of vanishingly small particles.

Nearly twenty years after the Principia was published, Newton allowed himself to speculate on the nature of matter in his next great work, the more accessible Opticks: "It seems probable to me, that God in the beginning formed matter in solid, massy, hard, impenetrable, moveable particles . . . so very hard, as never to wear or break in pieces," he wrote.⁵

Newton's musings on matter were not so far off the mark. Today, scientists think of matter as being built up from a handful of particles that are almost indestructible. It took scientists more than half a century to identify the most basic building blocks of matter, which come together to form the innards of atoms. Variations of these give us the chemical elements of the periodic table: atoms that form metals, crystals, liquids, and gases and that intermingle to make an almost endless list of molecules.

Scientists call the ultimate building blocks of matter "fundamental" or "elementary" particles, and, by definition, they cannot be broken up into smaller pieces. The first was discovered in 1897 by J. J. Thomson at the Cavendish laboratory at Cambridge University.⁶ Thomson, like many other physicists of his time, was intrigued by the nature of glowing rays that appeared when a voltage was applied across glass tubes filled with low-pressure gases. The rays moved from the cathode, which is the negatively charged electrode, to the anode, which is positive. What the rays were made of was a mystery.

Thomson began a series of experiments to investigate these curious "cathode rays." In one, he used a 15-inch glass tube coated at one end with phosphorescent paint. Thomson modified the anode by creating a slit in it, so some of the rays coming from the cathode would pass through it, making a bright spot when they hit the phosphor. His masterstroke was to build into the glass vessel a second set of electrodes that the rays passed through. When Thomson linked the electrodes up to a battery, he found that the spot darted away from the negative plate and toward the positive one.

Further experiments showed that cathode rays were composed of streams of tiny, negatively charged particles. Thomson named them "electrons," a term introduced by the Irishman George Johnstone Stoney twenty years earlier, and suggested they were ubiquitous ingredients of all the atoms scientists knew. Emboldened by his discovery, Thomson proposed the "plum pudding" model of the atom, so called because it pictured atoms as positively charged balls of matter (the pudding) dotted with tiny negative electrons (the plums).

It turned out that Thomson's atomic pudding was not what Nature ordered .⁷ The idea fell apart when the New Zealand–born chemist and physicist Ernest Rutherford, based on his work with radium, announced the startling news that atoms were mostly empty. Instead, he said in 1911, almost all of an atom's mass was bundled up in a central, positive nucleus. Later that decade, Rutherford probed the nucleus more deeply and found evidence for a new kind of particle within, the positively charged proton.