Grand Unified Theories
Many of history's greatest physicists have devoted themselves to the search for a unified theory. A unified theory attempts to explain phenomena which seem different with a single set of postulates. These theories are expressed as single sets of equations that combine two or more fundamental forces with each other. For example, James Clerk Maxwell was able, between 1860 and 1865, to work out a theory that shows how electrical currents and the action of magnets are but two aspects of a single force, the electromagnetic force. Also, for a significant part of his career, Albert Einstein tried to find a way of combining the two fundamental forces known at his time, gravity and electromagnetism. He was, however, unsuccessful in that effort.
The most recent success in this type of effort occurred in the late 1960s with the development of the electroweak theory by Steven Weinberg, Abdus Salam, and Sheldon Glashow. The electroweak theory explains how electromagnetism and the weak force that affects certain subatomic particles can be viewed as two manifestations of a single force. The discovery in 1983 of the force particles that transmit the electroweak force, the W +, W-, and Z0 bosons, provided strong support for the Weinberg-Salam-Glashow theory.
The logical next step is to look for some system by which the electroweak and the strong forces can be integrated. The strong force is the force that holds the particles in the atomic nucleus together. The strong force is the strongest of all basic forces, 1045 times stronger than gravity and 107 times stronger than the weak force. But it operates over only very short distances, no more than about 10-15 meter. Theories that attempt to combine the electroweak and strong forces are known as "grand unified theories," or GUTs.
The form of mathematics used to develop GUTs is called gauge field theory. Gauge theories look for properties that remain constant under widely different conditions of scale. For example, at ordinary, everyday conditions, the electromagnetic force is very different from the weak force. But at higher energies, commonalities between the two forces can be identified.
GUTs assume that at even higher energies, similarities between the electroweak and strong forces can also be identified. The energies involved are very large indeed, of the order of 1014 Gev (gigaelectron volts), a trillion times greater than the energy available from the most powerful particle accelerators available today. These energies are available in cosmic ray showers, but the possibility of making the necessary observation using this source are very small indeed.
One of the most promising GUTs now available is that proposed by Howard Georgi and Sheldon Glashow in 1974. According to this theory, unification of the electroweak and strong forces can be described by a gauge theory known as SU(5). A number of predictions arise out of the Georgi-Glashow theory, as they do out of any GUT. One of the most dramatic of these is that three quarks are able to decay into a single lepton. If that were true, it means that a proton is not stable, but decays with a very long lifetime, of the order of 1031 years.
One test of the Georgi-Glashow theory, then, is to look for proton decay. No experiment could be devised that would wait 1031 years for a proton to decay, of course. However, if a very large number of protons can be observed over a period of time, there is a probability that one or two might decay during the experiment. this kind of observation is now being conducted, but no results confirming the theory have yet been reported.
There is no question that scientists will continue searching for unified theories. If and when a successful GUT is discovered, yet another horizon remains. The next effort would be to find a way of unifying the strong, weak, and electromagnetic forces with the fourth basic force, gravitation. If such a unification, called a supersymmetry theory, is successful, it will be able to account for the behavior of all known phenomena in the universe.
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