Grand unified theories, also called GUT’s, attempt to unite three of the four fundamental forces that are thought to govern the universe. These three forces, from the weakest to the strongest, are (1) the weak nuclear force, (2) electromagnetism, and (3) the strong nuclear force. The weak nuclear force is responsible for the radioactive decay of many kinds of atomic nuclei. Electromagnetism is the force that holds electrons to the nucleus. The strong nuclear force binds the nucleus together. Gravitation, the fourth fundamental force, is not included in GUT’s. The effect of gravity on a single atom or nucleus is so weak that scientists have never been able to detect it.
GUT’s were first proposed in the mid-1970’s. The theories consist of mathematical statements that describe electromagnetism and the strong and weak forces as a single unified force. According to GUT’s, the underlying unity becomes apparent only at extraordinarily high energies, like those that would have existed in the earliest moments in the history of the universe. At lower energies, such as those that exist today, the single superforce appears to act as three distinct forces. GUT’s have provided scientists with valuable insights about the interactions and relationships of the most elementary particles of matter. Cosmologists also use GUT’s to better understand how the universe developed into its present form.
GUT’s were conceived as an extension of the unified theory of the weak and electromagnetic forces, which was developed by three physicists—Sheldon L. Glashow and Steven Weinberg of the United States and Abdus Salam of Pakistan. This theory was completed by the early 1970’s, and in 1983, its key predictions were confirmed in high-energy experiments with collisions of subatomic particles using a device called a particle accelerator (see Particle accelerator).
An experiment to test GUT’s directly would require an energy about a trillion times higher than what is now available. But some predictions of GUT’s have been confirmed. GUT’s explain certain properties of the weak force and are the only known theories that explain why the charges of the electron and the proton have the same magnitude. GUT’s also predict the decay of the proton, a particle previously believed to be stable. Experiments are underway to detect proton decay and thus help verify the ideas behind GUT’s.
See also Force (Kinds of force).
