The Canon (17 page)

Yet these elements, too, can be found in fattened versions of themselves, called isotopes, the added bulk compliments of extra neutrons. Carbon, for example, exists in an eight-neutron isotope that is so unstable, and so predictably prone to jettisoning its eighth neutron, that archaeologists and paleontologists use its pace of neutron ejection, or decay, as a kind of clock to help them date ancient buried treasures, be they the caried teeth of a prehistoric king who had an obvious taste for sweets, or animal bones carved into the first dental tools, or the charred remains of the first dentist.

Among the nuclei of heavier elements, protons are usually outnumbered by neutrons, sometimes substantially so. Mercury's 80 protons, for example, are raised 1.5 to 1 by the slippery metal's 120 neutrons. But protons more than compensate for their minority status by their unshakable sense of self-worth. For while any type of atom may have a few neutrons more or less without losing its essential identity, its proton census is nonnegotiable, the most elemental element of an element. Proton content alone distinguishes one species of atom, one element, from another and therefore serves as the element's official atomic number. Gold holds 79 protons in its nucleus, and hence is given the atomic number of 79; while at slot 78 we find platinum, which, for all its 117 neutrons, is one proton shy of being, figuratively speaking, as good as gold.

So what grants protons their privileged status? If protons and neutrons are similarly proportioned, and equivalently responsible for the roughage in your broccoli floret or the buoyancy in your daughter's balloon, why is it proton tally alone that separates selenium, atomic number 34, an essential dietary nutrient that helps convert fats and protein into energy, from arsenic, atomic number 33, a highly toxic substance that is used to kill rats, weeds, and the occasional Roman emperor?

The answer is electric charge: a proton has it, while a neutron does not. The neutron, true to its name, is an electrically neutral particle, and if a neutron were to order a drink at the bar, as another MIT joke has it, and then ask how much it owed, the bartender would reply, "For you, no charge." The proton, true to a whimsical convention that dates back to Benjamin Franklin and his kite, is said to be a positively charged particle, while the atom's other electrically charged particle, the electron, is said to be negatively charged. The terms positive and negative are not judgment calls—a reflection of physicists' preference for one particle over another, or of the proton's capacity to improve property values while the electron leaves old car parts strewn on the lawn. The vocabulary could as easily have been reversed, and the proton designated as negatively charged and the electron pronounced positive, but they weren't, so let's not. What is important is that the charge of one counterbalances the charge of the other. An electron may be more than one thousand times lighter than a proton, but its charge is every bit of a match for that
of the nuclear giant. And well matched they are, for protons and electrons attract each other, just as opposites are legendarily said to do in the macroscopic community, although in that case the reaction all too often ends up requiring the intervention of other macroscopic units known as divorce attorneys.

But what exactly is this subatomic charge, this positive charge of the proton that attracts and tit-for-tats the negative charge of the electron? When you talk about a fully charged battery, you probably have in mind a battery loaded with a stored source of energy that you can slip into the compartment of your digital camera to take many exciting closeups of flowers. In saying that the proton and electron are charged particles while the neutron is not, however, doesn't mean that the proton and electron are little batteries of energy compared to the neutron. A particle's charge is not a measure of the particle's energy content. Instead, the definition of charge is almost circular. A particle is deemed charged by its capacity to attract or repel other charged particles. "A charge is an attitude; it is not in itself anything," said Ramamurti Shankar. "It's like saying a person has charisma."

Another way of defining charisma is "force of personality," which brings us to the reason why charged particles react to other charged particles. They are obeying the laws of electromagnetism, one of the four fundamental "forces" of nature. You've probably heard about these four fundamental forces, and you might know them by name: electro-magnetism, gravity, the strong force, and the weak force. But "force," like "charge," is one of those words that comes up so often in everyday conversation that its meaning seems deceptively self-evident, and it is rarely explained in the context of fundamentalism. What distinguishes a fundamental force of nature from the more familiar, frightening forces of nature, like hurricanes, earthquakes, Donald Trump's hairpiece?

A fundamental force is best thought of as a fundamental interaction, a relationship between two chunks of matter. It turns out that there are just four known ways that one piece of matter can communicate with another, four approaches to acknowledging the existence of a body other than one's own. Each of these interactions differs in strength and range, operates according to a distinct set of rules, and yields distinct results. They are not, however, mutually exclusive. For example, all bodies, no matter how minute, are gravitationally attracted to one another. Charged or neutral, spinning or sedate, masses make passes through the universal come-on of gravity. Yet difficult though it may be to believe if you've ever tried putting on a fake set of wings and then flapping your
arms while jumping from the roof of your house, gravity is by far the weakest of the four fundamental forces. It makes its impact felt only on relatively large chunks of matter like stars, planets, and chowderheads leaping from rooftops.

If you take a couple of electrically charged particles, on the other hand, they're gravitationally attracted to each other, sure. But, being charged, they're also under the sway of the electromagnetic force, which is, oh, about 10
⁴⁰
, or more than a trillion trillion trillion, times stronger than gravity. Depending on whether the particles are of the opposite or similar charge, the electromagnetic force will either pull them closer or push them apart, gravity be damned.

Scale and context always dictate which force will be with you. In one sense, for example, the strong force merits its swaggering codpiece of a name, for it is the strongest binder known in the universe, more than a hundred times stronger than electromagnetism. The rules of its engagement keep protons and neutrons glued together in the nucleus, overriding the electromagnetic repulsion that otherwise would send all those positively charged protons fleeing, one from the other. But the strong force operates only across the ludicrously short distances between and within the particles of the nucleus. As for the weak force, the prompter of neutron decay and the fussy obscurantist of the force quartet, its reach is also limited to nuclear dimensions.

Physicists propose that the four forces are really four manifestations of a single underlying superforce, and that when our universe was young, firm, and hot, the forces behaved as one, too; only with the inevitable aging and cooling and spreading of the cosmos did the single force fracture into four separate instruments. Scientists' quest to unite the four forces into a single equation, a Grand Unifying Theory succinct enough to fit onto one of those scratchy XXL Beefy T-shirts with the too high collar that nobody wants to wear, is an effort to discover the primal commonality underlying the current plurality.

Whether they succeed or not in tracing the math to glory, we live in a world of four fundamental forces, four distinct means of matter-to-matter communication; and whatever parleying occurs among particles, and the organisms constructed of those particles, occurs through one or more of the four. The ball you threw into the air, as it makes its way up and down, is responding to the lure of gravity. But what of the force that sent the ball flying in the first place? You the pitcher applied a force to it in the classic, Newtonian sense of the word, meaning you flexed your muscle and caused a stationary object to start moving. Through what fundamental force, though, did the particles of your
hand convey their message to those of the ball? You may be Ty Cobb, Pete Rose, or the decidedly Beefy-T David Wells, but sorry, it's not the strong force.

For the source of our sporty fling, and of the many other ways we seize the day and size it up with all five senses, we must look again to the atom's architecture, and the loves, snubs, and limits that keep it standing.

The electron, with a designated minus sign tattooed on its forehead, finds the positive proton terribly attractive, and wants to spend its time somewhere in the vicinity of one. Yet the electron is also in constant, twirling motion, and how grateful we should be for its vigor. You'd think that these oppositely charged particles would fall into each other's arms, that the electron, smitten by the Grace Kelly glow emanating from the nucleus, would simply dive toward the proton and not stop until it had reached its destination. You'd think that all atoms would collapse like popped bubble wrap, taking every one of us precious parcels down with them. But no, the tremendous momentum of the electron throws it into orbit around the nucleus, keeping it at a distance and on the fly, just as the angular momentum of the planets ensures that they continue wheeling around the sun to which they are gravitationally attracted, rather than plunging into its fiery depths like kernels into a corn-burning stove. Electrons can never stop to catch their breath. For one thing, they have no lungs. For another, if electrons did stop moving, you'd be able to tell both where the particles were and how fast they were—or rather, were not—going. Heisenberg said in no uncertain terms that you can't know both details about an electron simultaneously, so, oops, gotta dash. Electron speed changes depending on how excited the particles are: in the laboratory they can be propelled toward the velocity of light, but even on an ordinary day spent clouding around the atom, they race about at 1,370 miles per second—fast enough to circle Earth in 18 seconds.

Yet electron pace is hardly the sole determinant of an atom's configuration. The entire carousel flirtation between proton and electron is as vigorously supervised and ritualized as an antebellum courtship. Electrons cannot flit about wherever they please, but are confined to specific zones, or shells, around the protons to which they are so attracted. The shells are arrayed one inside the other, and each is able to accommodate a set number of electrons. The shell closest to the nucleus has room for just two electrons, the two subsequent beltways have space for eight negative particles apiece, while those farther out can manage eighteen or more electrons. Once a shell is filled, even the president and his
cloud of armored SUVs couldn't nose their way in. An electron also cannot travel in between shells, just as you cannot stand between two steps of a staircase. An electron can, however, switch from one byway to another, assuming there's room. Sometimes, if an atom is blasted with a beam of light, a few of its electrons may become stimulated and jump to vacancies in shells farther from the nucleus. But "jump," in the quirky subatomic subculture at the base of all being, does not mean "bound in a continuous motion from here to there"; it means "disappear momentarily from the shell I was in and reappear suddenly in the shell above me." This Houdini maneuver is the famed "quantum leap," for the electron is shifting from one permissible shell, or energy level, or quantum, to the next, without trying to ram through concrete barriers between lanes. The expression "quantum leap" long ago found its way into popular language, usually to mean something like "a really big change" or "a great jump forward," and though some people have griped that it's a misuse of language because the distance between electron shells is so vanishingly small, I'd say the criticism is misplaced. Quantity notwithstanding, a genuine quantum leap is qualitatively spectacular, a bit of
Bewitched
without the insufferable husband.

An atom's demand for electrons, and thus the number of orbital shells that surround it, emanates from its protons. As it happens, an atom is like Switzerland; it prefers to assume a neutral stance whenever possible. This preference requires that each of its protons, the comparatively massive, electrically charged, and imperious components of the nucleus, be paired with an electron. An atom of gold, with its 79 protons, requires 79 electrons to reach its favored state of neutrality. An atom of gold, then, is a snaggle-toothed hundred millionth of a centimeter of a beast, comprising a nucleus of 79 protons and 118 neutrons, and then, far, far from the dense, thumping heart, 6 cloudy shells, 6 probability pathways along which 79 electrons spin.

Yet even with all its intricacy, its swirling bazaar of particles, an atom of gold, like all atoms, is hollow, is nearly nothing, is emptier than a fraternity beer keg on Sunday morning. Why, then, do the two gold rings on my fingers—one my wedding ring, the other a gift from my husband in honor of our daughter's birth—feel reassuringly firm and enduring, slim, smooth circles that I never remove, yet which are palpably, as well as symbolically, not-me? Sometimes, in winter, my fingers shrink enough that the rings slide about and threaten to slink right over the knuckle and down the sink. But neither ring ever evinces the slightest inclination to fall right through the diameter of my finger like a hot
knife through butter, as hollow as my finger atoms and ring atoms may be. So what gives, or rather, what doesn't give?

Put simply, the answer is charge, this time of electrons. All atomic nuclei are surrounded by clouds of negatively charged electrons, and like charges repel. The electromagnetic force is second only to the strong force in its exertions, and so the repulsion is serious. "Electrons don't like being around other electrons," said Shankar. "Atoms keep a comfortable distance from each other because of their electrons. It is really the electromagnetic force that keeps you from falling through the floor."