Where We Are in Physics
Chris Quigg

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.

Published in Fermilab Annual Report,1979