High-energy physics, the science of the fundamental constituents of
matter and their interactions, is in a period of remarkable excitement
and promise. Recent research has brought us to the threshold of a new
level of understanding of the universe around us.
Experiments throughout the past decade have indicated the existence of two
classes of apparently elementary constitutents: the quarks, which make up
objects such as the proton, and the leptons, of which the electron is the
most familiar example. At the limit of present resolution, the quarks and
leptons have no spatial extent and no internal structure. That is, they
appear like geometrical points. All the known forms of matter may be
viewed as regular combinations of these building blocks. The leptons are
observed directly, but free quarks have not been observed. Nevertheless,
the indirect, circumstantial evidence for quarks is so overwhelming that
their physical reality seems unavoidable.
For half a century, physicists have
distinguished four fundamental forces: gravity, electromagnetism, the weak
interaction responsible for radioactivity, and the strong force that
binds atomic nuclei. Electromagnetism is itself the union of electricity
and magnetism, which until the nineteenth century were regarded as
distinct and unrelated phenomena. The unification of electricity and
magnetism led to an understanding of many phenomena involving light, a
form of electromagnetic radiation, and laid the foundation for our contemporary television and microwave-oven society. It also supplied crucial
clues to the theory of relativity, which brought together and gave new
meaning to the ancient concepts of space and time.
Unification of fundamental forces is again in the air today. We are now
confident that electromagnetism and weak interaction are two aspects of
the same underlying interaction. A unified theory of weak and
electromagnetic
interactions put forward by Steven Weinberg and Abdus Salam makes detailed
predictions that are in remarkable agreement with experiment. As the
unified theory of electricity and magnetism predicted the existence of
electromagnetic waves, the Weinberg–Salam theory predicted a new form of
radioactivity, known as the neutral-current interaction. The effects of
the neutral-current interaction were first detected in 1973. Since then,
many experiments at Fermilab and elsewhere have been devoted to the
study of its properties. Because the Weinberg–Salam theory is a quantum
field theory, it requires the weak and electromagnetic forces to be
mediated by quanta that are themselves elementary particles. The quantum
of light, the photon, has been known for 75 years, since it provided the
basis for understanding the photoelectric effect. According to the
Weinberg–Salam theory, the weak interactions are mediated by two species
of force particles, called W and Z. These intermediate bosons are extremely
massive, about 100 times the mass of the proton, so their detection
is beyond the reach of existing accelerators. The Fermilab proton-antiproton
collider project, intended to attain center of mass energies of
two thousand times the proton mass, will create an important facility for
the study of these objects.
There is another side to the unification story. It now appears highly
likely that the strong interactions result from an elementary interaction
among the quarks that is mediated by force particles called gluons. A
theory known as quantum chromodynamics (QCD) has emerged as a probable
description of these strong interactions. QCD still requires extensive
experimetal testing. Indeed, because of calculational difficulties, its
predictions have not been completely elaborated. The mathematical
structure of QCD is very similar to that of the unified theory of weak and
electromagnetic interactions. This resemblance leads us
(whether by supreme insight or by mere
hubris, it is too soon to know) to the audacious proposal of a "grand
unification" of the strong, weak, and electromagnetic interactions.
Nothing we know compels grand unification, but perhaps more important,
nothing is preventing it: we are free to take the leap!
Returning to QCD itself, it remains to devise ways of making quantitative
predictions and testing them experimentally. One important precept of
the prevailing view of QCD is that free quarks and free gluons cannot
exist. A search for free quarks, which can be carried out with the
Tevatron more incisively than before, thus takes on added significance.
Exuberance over the possibility of grand unification is tempered by the
recognition that some fundamental issues have not yet been addressed.
Among these is the question of why the mass spectrum of quarks and leptons
is as it is. There are two broad views of the spectrum problem. One is
that the ultimate unification of all the forces of nature, including
gravitation, will result in a definite prescription for the kinds of
fundamental particles that may exist. The contrary is that we have already
found too many quarks, leptons, and force particles—more than
thirty—for
them to be truly elementary. If this is so, higher-resolution
instruments such as the Tevatron and Collider may begin to reveal an
internal structure of the quarks and leptons we now perceive as
structureless.
Many questions still must be answered, and countless more have not yet
been phrased. What our recent discoveries imply for cosmic concerns about
the origin and ultimate fate of the universe is only beginning to be
understood. As we improve our instruments and extend the sweep of our
theories, let us delight together in the beauty of nature and recall with
humility that nature's possibilities are not limited by our
imagination.