Beyond the God Particle

by Leon M. Lederman

  Altogether our list of the tiniest known Russian dolls is somewhat long—it includes the 6 quarks, the 8 gluons, the 6 leptons, and the 4 electroweak gauge bosons (the photon and W+, W–, and Z0) and a not-yet-been-seen-but-surely-is-there graviton, the quantum of gravity—together with antiparticles, all of these comprise a complete list of all the known elementary particles (see figures A.35 and A.36 in the Appendix.) All of these are point-like, i.e., they have no discernible internal structure, so far as we can tell, and are what we mean by the “truly elementary particles.”

  You might then ask an obvious question in the spirit of Democritus and based upon the Russian doll experience: “OK, if there are so many of these ‘elementary particles,’ then what are quarks and leptons and all of these bosons, etc., made of?” To this we have no facts to offer—only theoretical speculation. We could quote the current hot theorist rock stars who say, “They're all made of strings.” But in a few decades, with another generation of rock stars, perhaps there will be another speculative theory. It may say that “they're all made of smithereens.” And maybe these ideas are right and maybe they're wrong—maybe we'll never know, no matter how many Discovery Channel documentaries about the Smithereen Theory are produced.

  As we've said, today we have an “almost complete list of elementary particles.” And that was the state of affairs on July 3, 2012. Things changed dramatically with the announcement on July 4, 2012, of a new object, which appears to be the Higgs boson. In fact, dozens of alternative theories about the Higgs boson and the Higgs mechanism were destroyed on July 4, as a veritable “mass extinction” of theories occurred. That's not a bad thing—it's progress (there's really no progress when science lapses into pure, almost religious, speculation about untestable things, like smithereens). So we now have the Higgs boson to add to our list. But what is the Higgs boson and why does it exist, and does adding the Higgs boson to our list make it complete? Many questions are raised by the Higgs boson. In short, with respect to the Higgs boson, “Who ordered that?”

  When we are at the level of the elementary particles, we are exclusively and deeply within the mysterious realm of quantum mechanics. Here we find that the nature of the phenomenon of mass itself becomes a more enigmatic mystery and a greater puzzle. It becomes more exciting as well—the multi-millennia-old idea of mass merely as a “quantity of matter”—a concept that we've been using since antiquity—starts to break down.

  NO MASS

  In particle physics, for the first time anywhere in science, we meet something radically new: there exist particles that have no mass. A truly massless particle, but one that has nonzero energy, is unprecedented anywhere else in nature. These particles, by our ancient and traditional concept of mass, would have absolutely zero “quantity of matter.” Yet massless particles exist—you can count them—they carry energy—so they do have “quantity of matter,” though they have no mass. With particle physics, mass evolves into a new concept that is different than the simple old one of “quantity of matter” that has served us so well since antiquity in describing big aggregate objects. The whole concept of mass starts to become intimately related to the forces, and especially the fundamental symmetries, that govern all of the elementary constituents of all the matter in the universe. In this sense there now emerges an enormous difference between large everyday things and the tiniest denizens of nature.

  So, to begin to understand what mass is at a deeper level, as a physical phenomenon of elementary particles, we must first understand what “masslessness” implies. What does it mean for a particle to exist but to have no mass? This is how Hamlet might have attacked the question.

  We focus on light, to coin a bad pun. Light is composed of particles called photons. These particles have rather unusual properties compared to things like marbles or billiard balls. Photons are “point-like,” that is, they have absolutely no internal size or structure, as far as we can see. Moreover, as we said, they are always moving, traveling at a well-defined speed called the speed of light, which physicists denote by the letter “c.” The speed of light is very, very large, about 300,000 kilometers per second (186,000 miles per second). In fact, it takes only a little more than a second for a photon to travel from the earth to the moon. Because the speed of light is so great, it took a long time for people to measure it from experimental observations, and they had to resort to a lot of tricks and develop new techniques to do it. Today we know the value of c very precisely.

  Photons were the first entities to reveal quantum physics: they also behave like particles and they also behave like waves—that is—they move in a wavelike manner, yet they also at the same time behave like particles—you can “count” them. This is a mind-numbing paradox, but it is true. This “dual” behavior of photons is called a quantum state, and all quantum states have it, and all things are quantum states. The wave-particle behavior is shared by all particles, electrons, quarks, muons, etc. This was the conclusion that became the bedrock of “quantum physics,” and if you are a poet (or artist, or musician, or lawyer, or statesman, etc.), we have a book all about this profound yet enigmatic business for you (see Quantum Physics for Poets [Amherst, NY: Prometheus Books, 2011], chap. 2, note 3).

  So, photons can behave like particles, despite their funny quantum waviness and their imperative to always travel at the speed of light. They do have something in common with marbles or billiard balls—each photon carries energy as it moves through the room at the speed of light. Photons can have a lot of energy, and then we call them “X-rays”; still more energy, and we call them “gamma rays,” as when they are the product of such things as radioactive disintegration or supernova explosions. Gamma rays readily will go “tick…tick…tick…tick” as they are counted in a Geiger counter. It's dangerous for living organisms to be exposed to too many X-rays or gamma rays because they tend to destroy biological tissue, such as DNA. But photons can have less energy, becoming the light we see, and with still less energy, will fade off into the far red scale of visibility, becoming warm, gentle infrared light emanating from a soothing fire in the fireplace on a cold winter night, finally becoming at the lowest energy scales microwaves and radio waves.

  The intriguing and novel thing about photons is that they each have absolutely no mass. As we have said, photons are massless particles. In fact, as far as what we have directly observed in the lab, photons are the only truly massless, freely moving particles (we expect there exist other massless particles, such as the hitherto unobserved particles of gravity, called “gravitons,” and the gluons that bind quarks but are trapped forever inside of hadrons, so we never get to see them freely moving as massless particles through space—see figure A.37 and surrounding text in the Appendix).

  “Hold on!” exclaims Katherine, who looks up from studying her law school notes in preparation for the bar exam, “Didn't that grandfatherly old man with long, bushy white hair and a pipe once say that energy is equivalent to mass? So, if a photon has energy, how can it have no mass? If it has no mass, doesn't E = mc2 tell us it has zero energy? How can a photon therefore exist at all if it always has zero energy?”

  Yes, Katherine, indeed the photon has no mass, but it does have energy. The photon defeats this apparent conundrum by never standing still—a photon always moves at the speed of light, and you cannot arrest a photon (in a real sense) and bring it into a stationary state of zero motion. The photon's very existence is a legal loophole in Einstein's relativity that permits massless particles to also have energy (and momentum), provided they always travel at the speed of light (see chapter 4, note 4). In fact, this goes to the core of Einstein's theory of relativity: no matter how fast we chase after a photon, it always moves away from us at exactly the same speed of light. You can never slow a photon down and place it on a balance scale to measure its mass, because its mass is zero and it always moves at c. “I see,” she replies, “photons have no mass, so they always travel at the speed of light. How clever is the fine print on the legal contract of n
ature! But why?”

  Photons are a sort-of exceptional case. Most particles have mass (in fact, all other known elementary particles at this time are massive particles, with the exception of the eternally trapped gluons and unseen gravitons) and thus, at least in principle, any elementary particle can be brought to a state of rest and will then have Einstein's famous amount of “rest energy,” E = mc2. But not the photon. The photon is a special particle that can never be brought to rest. Now, think about that for a moment. Isn't this interesting? Even if we are talking about a massive particle at rest, the formula for its total energy, at rest, is E = mc2, yet this involves c, the speed of light. The speed of light is intrinsically wrapped up in all of this phenomenon of mass. It is fundamental. We call c a fundamental constant of nature. It governs all properties of motion, whether we are at rest or moving near the speed of light.

  As we've seen when a massive object moves, relative to us, it acquires additional energy of motion, known in physicists’ jargon as kinetic energy. It acquires just a little kinetic energy if it moves slowly. But the kinetic energy becomes greater and greater as the particle moves faster and faster. And, as the massive particle approaches the speed of light, its total energy becomes infinite. So, in fact, no massive particle can ever travel at the speed of light, because it would require an infinite amount of energy to make it do so. The photon does it by being massless, but the photon can therefore never be at rest, and all photons travel at the speed of light.

  SYMMETRY IN MASSLESSNESS

  The existence of massless particles raises the interesting question: Why must mass exist at all? What kind of world would we have if all other elementary particles were massless?

  It certainly wouldn't be a very hospitable place in which to live. One of Einstein's results of special relativity is that time ceases to exist for objects that travel at the speed of light. That is, if a lowly photon carried a wristwatch and departed Earth from a flashlight heading for a distant galaxy, he would notice that he arrived at his destination instantaneously. The vast distances of intergalactic space present no problem for photons to traverse—from their perspective they do all trips instantaneously. No time would elapse at all on the photon's wristwatch in hopping from Andromeda to the Milky Way, or from Earth to UDFj-39546284, one of the most distant galaxies ever recorded.2 But, alas, for our little photon there would be no time to read a good book on the trip or to catch up with some zzz's. So, too, if we were all massless particles, we would always take zero time to do everything and anything, and we would always be on the go—at the speed of light. Our world would be completely devoid of experience as we know it. What kind of life would that be? Well, we wouldn't age. But unfortunately, a world without time is non-experiential. We wouldn't age, but we also wouldn't live.

  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