We have reached an extraordinary point in the history of science, for some physicists believe they are now on the verge of having a single theory that will unite all of their science under one mathematical umbrella. In particular this theory would unify the two great bastions of twentieth century physics - the general theory of relativity and quantum theory. Since general relativity describes the large scale, or cosmological, structure of the universe, and quantum theory describes the microscopic, or subatomic, structures, the unification of these theories would explain both the very big and the very small. This theory is often referred to as a "theory of everything".
In particular this theory would unify our understanding of all the fundamental physical forces in our universe. There are four such forces that physicists know of: gravity (which keeps planets revolving around their suns, and is responsible for the formation of stars and galaxies), the electromagnetic force (which is responsible for light, heat, electricity, and magnetism; and which is also responsible for holding atoms together), the weak nuclear force (which acts inside atomic nuclei, and is responsible for a certain kind of radioactive decay), and the strong nuclear force (which holds together the protons and neutrons in atomic nuclei, and is therefore crucial to the stability of matter). At the moment, physicists have separate theories for each of these forces, but they would like one unified theory of all four. That goal has partly been realized in that they now have a theory which unifies two of these forces - the electromagnetic and weak forces - but unifying all four is proving to be extremely difficult. Nonetheless, most TOE physicists are confident this goal will be realized in the next few decades.
Theoretical physicist, Steven Weinberg, who played a major role in unifying the electromagnetic and weak forces (for which he was awarded the Noble Prize, along with colleagues Abdus Salam and Sheldon Glashow), has called a theory of all four forces "a final theory." When physicists find this theory, he and others have suggested, then physics will have effectively achieved its end. Now the entire physical universe would be encompassed by a set of equations - or perhaps just one equation. But the question would still remain, what would that equation mean?
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