Coming of Age in the Milky Way (44 page)

The four fundamental forces known to operate in nature today are here depicted in terms of characteristic interactions. In a typical electromagnetic interaction, a pair of electrons (symbolized
e
⁻
) exchange a photon. In the weak force interaction portrayed here, a neutron (n) decays into a proton (
p
) via the exchange of a weak boson; the event also converts a positron (
e
⁺
) into a neutrino (
v
). In a strong interaction, quarks
(q)
exchange a gluon. Gravitation involves the exchange of a graviton between any two massive particles (
m
).

The fermions that constitute matter, though notoriously numerous and varied, can all be classified as either
quarks
, which respond to the strong force, or
le
ptons
, which do not. Leptons are light particles; their ranks include the electrons that orbit atomic nuclei. Quarks are the building blocks of protons and neutrons: Three quarks make a nucleon.
^*
There are thought to be six varieties
each of leptons and quarks. Neither quarks nor leptons show any sign of having an internal structure, though their anatomy has been probed on scales down to some 10
⁻¹⁸
meter. This is to say that if a single atom were enlarged to the dimensions of the earth, any subcomponents of quarks and leptons would have to be smaller
than a grapefruit to have escaped detection. So quarks and leptons are the bedrock particles of matter, so far as we know.

The Building Blocks of Matter

Trios of quarks are thought to compose the nucleons—protons and neutrons—that in turn constitute the nuclei of atoms. According to this model, a proton consists of two “up” quarks, each of which carries an electrical charge of +⅔, and one “down” quark, which has a charge of -⅓ the total charge of the proton therefore is 4/3 - 1/3 = + 1. A neutron consists of two down quarks and one up quark; consequently its charge equals 0.

Every fundamental—meaning simple—event in the universe can in principle be interpreted by means of the standard model. When a child looks at a star, photons of starlight strike electrons in the outer atoms of the receptors of the child’s retina, setting off further electron interactions that convey the image to the brain; all this is the work of electromagnetism. The nuclear processes that produced the starlight are generated by the strong and weak nuclear forces at work inside the star. And gravitation is the force that holds
the star together and keeps the child’s feet (if only intermittently) on the ground.

Electromagnetic energy is generated by natural processes across a wide range of wavelengths, including gamma rays and X rays from gas falling into black holes, light from stars, microwaves from the cosmic background radiation, and radio from interstellar clouds.

The scientific accounts of how the various particles of matter behave under the influence of three of the four forces are known as relativistic quantum field theories. They are so called because they incorporate both the quantum precept and the special theory of relativity, in order to take into account such effects as increases in the mass of particles traveling at close to the velocity of light. Electromagnetism is described, with exquisite accuracy, by the theory of quantum electrodynamics, or QED. The strong force is described by quantum chromodynamics, or QCD. (The “chromo” comes from a quantum number, whimsically called “color,” that plays a role for quarks comparable to that played by electrical charge
in the affairs of electrons.) The weak force, as we will see, has recently come under the purview of the “electroweak” unified theory.

Gravitation remains the odd man out. Its workings are still described by Einstein’s general theory of relativity, which is a classical theory, meaning that it does not incorporate the quantum principle. This does not cause problems under most conditions, but relativity breaks down when it comes to extremely intense gravitational fields, like those inside a black hole or in the universe at the very beginning of its expansion. There the curvature of space goes to infinity, at which point the theory tips its hat and makes a graceful exit. There was, by the late 1980s, still no quantum theory of gravitation with which to supplement general relativity. One reason for this is that gravity is weak. Individual subatomic particles normally are so little influenced by the gravitational force exerted by their colleagues that gravity can be ignored. Another reason is that gravitational interactions are interpreted, through Einstein’s general theory of relativity, as resulting from the geometry of space itself. The “gravitons” thought to convey gravitation must therefore dictate the very shape of space, and for a theory to elucidate how they manage
that
is no simple matter.

Particle physics today is a house divided, and though the standard model gets results, few imagine that it represents the last word on the subject. The model is a crazy quilt, not a mándala. To fire it up on all cylinders requires inputting some seventeen separate parameters, numbers the values of which have been determined experimentally but whose fundamental significance is not yet understood. We know, for instance, that the electrical charge carried by an electron is equal to 1.6021892 × 10
⁻¹⁹
coulomb, and that the mass of the proton is 938.3 MeV, equal to 0.9986 the mass of the neutron, but nobody knows why these numbers are as they are and not otherwise. The roots of discontent with the standard model were described this way by Leon Lederman, the director of the Fermilab particle accelerator in Illinois:

So it was that physicists late in the century were still searching for a simpler and more efficient account of the fundamental interactions. The object of their quest went by the name “unified” theory, by which they usually meant a single theory that would account for two or more of the forces currently handled by separate theories. They were guided, to be sure, by experimental data and by the challenges immediately at hand—the theorist resembles, as Einstein said, an “unscrupulous opportunist,” more often trying to find a specific solution to an immediate problem than to write a grand explication of everything. But they were guided as well, as Lederman mentions, by the hope that their accounts of nature could more nearly approach the elegant simplicity and superlative creativity of nature herself.