Grand Unified Theory

A Grand Unified Theory (GUT) unifies, or interrelates in a single quantum field interaction, the three fundamental nongravitational forces: electromagnetism, the weak nuclear interaction, and the strong nuclear interaction. These three forces are each characterized by a coupling constant, which gives the strength of the interaction by a range over which the force acts (long-range like electro-magnetism, or short-range like the two nuclear forces), and by certain characteristic symmetries that are described by mathematical symmetry groups. A successful GUT would show how the three different coupling constants become identical at some very high energy, subsume the symmetries of the three individual interactions in a much larger symmetry group, and explain all of the masses, processes, couplings, decays, ranges, and other behaviors of all particles at lower energies—lower than the GUT unification energy. The current standard model of particle physics, though highly successful in other ways, does not do this. Further, there are strong indications that a more complete and adequate explanation, describing deep connections that have so far evaded our understanding, awaits a successful GUT model.

A GUT would express the fact that at the most fundamental level all nongravitational interactions, and all particles, quarks, electrons, and neutrinos, are intimately interrelated and, in fact, identical above the unification energy. Their difference at low energies is expressed in a GUT by saying that the symmetries characterizing the interactions at very high energies, rendering particles and forces identical or equivalent, are "spontaneously broken" below the unification energy. Such spontaneously broken symmetries are present in the underlying relationships characterizing the system, but are not expressed—are hidden—in a given equilibrium state of the system, such as that realized in the present state of the universe. Construction of a GUT theory is an essential step towards achieving total unification, which would also include gravity. Although promising and detailed progress has been made on a number of fronts, there was no adequate GUT as of 2002.

There are strong indications that all the basic physical interactions are intimately related and that they can be unified. In the mid nineteenth century, the Scottish physicist James Clerk Maxwell began realizing this intuition by unifying electrical and magnetic phenomena in his electromagnetic theory. In the 1970s, Sheldon Glashow, Stephen Weinberg, and Abdus Salam succeeded in developing an adequate electroweak theory, which describes how electromagnetism and the weak nuclear interaction are related, and how they are identical at temperatures above 1015 K (kelvin). This electroweak theory was confirmed in 1983 by the discovery of the W and Z massive bosons, which carry the electroweak force and which were predicted by the theory. A completely successful GUT would incorporate the strong nuclear interaction with this electroweak interaction in an analogous way at some higher temperature above 1027 K.

Part of the motivation for a GUT is the lack of explanation for many of the parameters and characteristics of the standard model, and of the universe itself. For example, there is no explanation for the baryon-anti-baryon asymmetry, which means that there is more matter in the universe than there is antimatter. There is also some positive experimental support for a GUT, including equality of the magnitude of the charges of the proton and the electron and the non-zero rest-mass of the neutrino. Furthermore, GUT candidates generically predict the decay of protons at some very slow rate, as well as the presence of monopoles and other topological defects, which are localized regions in which the vacuum energy is different from the rest of the universe (false vacuum). Observational limits on these phenomena are being used, and will continue to be used, to identify the most adequate GUT candidates.

From a cosmological point of view, the success of a GUT would mean that at some very early stage in the history of the universe—well before one second after the Big Bang, when the temperature of the universe was greater than 1027 K—the physics of the universe was characterized by just two interactions: gravity and the GUT interaction. The universe would have been much too hot for protons, neutrons, and electrons to exist, as they do at lower temperatures. As the universe expanded, it cooled. And, as it cooled below 1027 K, the GUT interaction split into the strong nuclear and the electroweak interactions. A short time later—still much less than one second after the Big Bang—when the temperature had plummeted below 1015 K, the electroweak interaction split further into the weak nuclear and the electromagnetic interactions. From that point on, the basic physics of the universe was the same as it is today, but devoid of the complex macroscopic and microscopic structures that developed much later.

Implications for theology

There are no direct implications of GUT unification for religion and theology, but there are several important indirect influences. First, GUT unification, when it is finally achieved, will contribute to describing how everything in material reality is intimately interconnected in very basic ways. Those relationships constitute reality as it is and are an essential part of how God's continuing creative action is realized—through these "laws of nature." Second, a GUT characterizes a definite, very early stage in the evolution of the universe. A successful GUT will strengthen the already strong case for the evolution of the presently lumpy, cool, complex, and highly differentiated cosmos from a very hot, simple, homogeneous, and relatively undifferentiated primordial state, which was characterized by a much simpler physics. For the theistic thinker, a GUT represents one of the key ways in which God gradually brought into being the reality of which human beings are a part.

See also Field Theories

Bibliography

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william r. stoeger

grand unified theory (GUT) An attempt to describe the weak and strong nuclear forces and electromagnetism in a single mathematical theory. Unification of the weak force with electromagnetism has been achieved in the electroweak theory. Before about 10⁻¹² seconds after the Big Bang, by which time the Universe had cooled to about 10¹⁵ K, the electromagnetic and weak interactions acted as a single physical force; in the cooler temperatures since then, they have been distinct. Attempts to unify the electroweak force with the strong nuclear force have been only partially successful. It is thought that the temperature for their unification is of the order of 10²⁷ K, which occurs only 10⁻³⁶ s after the Big Bang. Particles surviving to the present day from this phase are possible candidates for non-baryonic dark matter. Unification of the GUT interaction with gravity may take place at higher energies still, but there is no satisfactory theory which unifies all four physical forces. Such a theory would be called a theory of everything (TOE).

grand unified theory (GUT) Theory that would demonstrate that three of the four fundamental forces are actually different aspects of the same fundamental force. The weak nuclear force and electromagnetic force have been incorporated as the electroweak force, as demonstrated by particle accelerator experiments. In order to prove the GUT, the electroweak force must be unified with the strong nuclear force. If the gravitational force could be incorporated, then a unified field theory would be produced.