Beyond the God Particle (16 page)

But from a purely mathematical point of view there is something very special about a world in which all particles are massless. This is a world of supreme symmetry. For example, there would be nothing to distinguish the muon from the electron in such a world—both would be exactly massless charged particles, and we wouldn't notice if we swapped all the electrons in a box with muons. When two systems have identical properties, we say they are
symmetric
to one another.

Symmetry is now known to be at the heart of our understanding of nature. In essence, we live in a world that is fundamentally governed by symmetry (and we have another book for you on that topic:
Symmetry and the Beautiful Universe
[Amherst, NY: Prometheus Books, 2007], chap. 4, note 4). But often the symmetries are hidden or appear to us as nonexistent or “broken” symmetries. This is the major lesson of the Standard Model that unifies all the forces of nature into a common logical framework.
The Standard Model achieves its unification by first imagining this supremely symmetric and un-marred world in which all particles exist without mass.
That's where we then see the unifying principles at work.

On the other hand, the real world of planets, stars, iPhones
^®
, and humans is a world of broken-down symmetries, as though we live among the ruins of some ancient civilization. Here we see an old pillar lying on the ground and over there the keystone of an arch half buried in the mud next
to a decapitated statue of Emperor Vespasian lying on its side. This is the world we encounter every day. It is a world in which things have mass and are different, as electrons are different in mass from muons. The masses of particles and atoms, and the lugubrious mass of a Jupiter, are all the symptoms of the broken symmetry of the Standard Model. The grand symmetries of the perfectly massless world of the Standard Model are hidden, just like the pinnacle of the great civilizations of ancient India, China, Central and South America, Persia, Greece, or Rome are hidden in history.

The analogy is striking when we realize that in the very earliest instants of the big bang these symmetries were fully in place and at work sculpting the future universe. In the first instants of creation all particles
were
massless, and the great vaulted towers of the symmetries of the Standard Model once stood aloft and uncorrupted. The universe expanded and cooled, and the symmetries fell into heaps of rubble, particles acquired mass, and the physics of our low-energy world of human perception emerged where the underlying Standard Model is hidden and hard to see. If by a licentious metaphor this symmetrical world was the Valhalla of Odin, then it was Götterdämmerung that broke down the symmetries and smashed Valhalla into ruins. And just as there was an agent of that event, Odin's daughter, the Valkyrie Brunhilde, so, too, is there an agent of the destruction of the symmetry of the Standard Model in the very early universe: the Higgs boson.

THE QUANTUM REALM

Another Götterdämmerung happened at the beginning of the twentieth century—the world of classical physics collapsed. Classical physics had evolved from the mists of history, to the new rational minds of Kepler, Galileo, and Newton, to Maxwell and Gibbs, and through to the end of the nineteenth century. Classical physics always involves descriptions of things that are
macroscopic
, involving collections of huge numbers of atoms. Some million, trillion atoms are contained in a single grain of sand. However, at the beginning of the twentieth century, the established and grandiose science of classical physics, with its precise predictions for the behavior of all things that are huge assemblages of atoms, like a four-hundred-year-old European monarchy, crashed down to the floor.

Through the newly refined and sophisticated experiments at the turn of the century, a revolution occurred, and the properties of individual atoms and those of the smaller particles the atoms contain came into view. The behavior of the individual atom itself turned out to be nothing like what Galileo and Newton had conceived. It was shocking and inexplicable to the scientists of the early twentieth century, who had been trained in the Galilean–Newtonian tradition of classical physics. A chaotic confusion emerged in a vast assortment of the data on atoms, but this gradually gave way to the desperate and intense efforts of the scientists to restore order and logic to this newly discovered realm. By the end of the 1920s, the basic logical framework of the new properties of the atom, which define all of chemistry and everyday matter, had been constructed.

And the logic seemed incomprehensibly illogical—but it worked and it survived many an onslaught by the doubters, including no less than the founding father of modern physics, Albert Einstein himself. Humans had begun to comprehend the bizarre new world of the smallest things, from atoms on down, that we now call
the quantum world
. The weird new quantum laws that now ruled the atom were primary and fundamental, and these new rules actually apply to everything, everywhere in the universe. We are all made of atoms and we cannot escape the implications of the surreal reality of the atomic domain, that nothing is solid, that atoms are mostly empty space, that “uncertainty” is now decreed and installed into the laws of nature.

Within this new quantum world, the concept of mass is also radically changed once we get down to the smallest denizens of nature, the “elementary particles.” A new burning aspect of mass rears its head, and the notion that it's only about the “quantity of matter” has to be written into our psyche. All of this new insight into elementary particles begins in the period of the development of the new particle accelerators, beginning in the postwar 1950s. These were the world's most powerful microscopes, and they began to reveal a new layer of matter. Particles that have lifetimes no longer than the time it takes for light to transit their tiny diameters, that are smaller than the atomic nucleus, glinted and sparkled into view in the detectors of the experimentalists.

THE EMERGENCE OF QUANTUM IDEAS OF MASS

The first new ideas about mass came from the minds of theorists and came initially from outside of particle physics. This derived from the new understanding of the world of ordinary materials through the lens of quantum theory, in particular, the phenomenon by which materials that are poor conductors of electricity, like lead or nickel, become
perfect
electrical conductors—“superconductors”—at ultra-low temperatures. Yes, we said “perfect”—superconductors have absolutely zero resistance to the flow of an electrical current! This is an astonishing and ghostly quantum behavior of aggregate matter.

Superconductivity was first observed in the laboratory in the early 1900s, and the first hints of a theory were given by Fritz London in the 1930s. But it was in the mid-1950s that superconductivity was finally explained in detail by a beautiful theory of John Bardeen, Leon Cooper, and Robert Schrieffer. This, and other work by Vitaly Ginzburg and Lev Landau laid a foundation for the new quantum ideas about mass.
³

Inside of a superconductor, like a small bar of ultra-cold lead, which can be easily constructed in a laboratory with good cryogenic equipment, the massless photon, the particle of light, becomes heavy—it acquires mass. We can actually, in principle, make photons stand still in a superconductor! It is as though one has created a mini-universe in which the vacuum state has been modified (it is filled with lead, or nickel or niobium, and is cooled to less than 2° above absolute zero temperature, that is, 2° Kelvin) and the quantum dynamics of this material causes a photon, the otherwise massless particle of light, to become a heavy particle. A superconductor allows us to become the architects of a little artificial universe in the lab, and it offers a switch that allows us to turn on or off a mass for an otherwise massless particle.

The mechanism of a superconductor can be described at different levels of detail, but it provides an insight into how mass, the quantity of matter of a particle, could be created by nature itself through quantum effects. The symmetries that are associated with the massless photon become hidden in a superconductor. The photon blends with the particles in the superconducting state to become something else, a new kind of photon with mass. This tells us that the properties of nature's
quantum vacuum
itself are inextricably wound up with the properties of particles and their masses.

Superconductivity is so well understood today that it has become an industrial tool. The enormous magnets of the Large Hadron Collider at CERN, and formerly those at the Fermilab Tevatron, use superconductivity to produce otherworldly strong magnetic fields at minimal cost in electricity. And, as a spin-off of the Fermilab Tevatron magnets, the powerful magnets used in MRI machines were born. Someday you may have a superconducting coffeemaker in your kitchen.

The underlying theoretical ideas of superconductivity were imported into particle physics by Jeffrey Goldstone, Giovanni Jona-Lasinio, Yoichiro Nambu, and others in the late 1950s to early 1960s.
⁴
The masses of elementary particles, at least the strongly interacting ones, the proton and the neutron, were beginning to look like a dynamical phenomenon, something that had to do with the vacuum of space itself.

IT'S ALL IN THE VACUUM

As weird as the quantum world can get, perhaps one of the strangest notions is that the vacuum itself is not empty, but rather, it is a complicated structure. The vacuum is a quantum state. We are pretty sure (not absolutely sure) that it is the state of lowest energy, called the “ground state.” And all quantum states, including the ground state, can have complex features—they are not empty. For example, the ground state of a hydrogen atom has an electron orbiting the proton in a spherical cloud-like wave—it is not empty.

So, too, we've just seen that the ground state of a superconductor contains a soup of particles that effectively give the photon a mass. Our vacuum's particular structure fundamentally and inextricably influences the properties of particles. Particles are now viewed as “excitations” of the vacuum—the concepts of the vacuum of space and time together with the elementary particles become welded into one. It is as if Shakespeare's Hamlet has as much to do with the other characters onstage as with the stage upon which they perform. (Shakespeare may have been the original quantum theorist.)

HOW CAN I ESCAPE THE VACUUM?

So, there's now a new complication we must dissect—the inseparable vacuum and its relationship to matter. These are not disjointed but are united—just as the brain is united with the body. But physicists have to dissect nature, and to do it they need a tool to turn one thing off while another thing is on.

In the quest of understanding mass, the place to start is to contemplate a particle that doesn't feel the effects of the vacuum structure. This is a particle, like the photon, that can have energy but has no mass at all—a particle that always travels at the speed of light—arriving instantaneously at any destination—and thus has no experience of the vacuum along the way.

As we noted, the world in which particles are all massless would be a world of profound and elegant simplicity, and simplicity in physics comes from
symmetry
, while the world in which we, the massive particles, actually live is one of broken-down symmetry. However, as ordinary particles approach the speed of light, they, too, begin to behave much like massless particles. If you hopped on a rocket ship that could travel at nearly the speed of light, you could take the trip from Earth to Andromeda, and the time elapsed on your wristwatch could become as short as you wish, depending upon how close to c, the speed of light, you can get. As you approach the speed of light you become much like a photon, experiencing no lapsing of time as you traverse the entire universe. You become an
approximately
massless particle yourself, as seen by a stationary observer at rest in the lab! And, by examining the behavior of
approximately
massless particles in the lab, any heavy particles traveling near the speed of light, we can glimpse the world where the symmetry is restored—the world of masslessness. The effects of the vacuum become decoupled from these near-to-the-speed-of-light particles. We can, therefore, by studying approximately massless particles, i.e., very energetic particles, at least in our mind's eye, restore the vaults and towers and walls of the ancient world of symmetry, much like archaeologists reconstruct a view of an ancient city.

Let's begin a mental journey into such a perfect world—a world in which there is no mass, a divine world of perfect symmetry, where particles travel always at the speed of light. Our journey toward understanding mass is about to become quite intriguing.

PARTICLE UTOPIA

It's essential we now draw some pictures. We begin by drawing a picture of the motion of a massless particle in space and time. That's what physicists did when they started asking these sorts of questions about mass. We're going to draw good old-fashioned pictures on a two-dimensional page of paper. This will be like a map, but it must somehow display the three directions in space and also the one direction in time.