During the late 1960s, Steven Weinberg and Abdus Salam independently proposed a theory that linked two fundamental forces of nature, electromagnetic, and weak forces. The new electroweak theory was regarded as a major breakthrough in physicists' efforts to link all four of the fundamental forces of nature, an objective that had occupied the efforts of the greatest scientists from Isaac Newton to James Clerk Maxwell to Albert Einstein.

One problem with the original Weinberg-Salam theory, however, was that it applied to only one of the two basic types of particles, leptons. This problem was resolved by Sheldon Glashow. Glashow showed that the Weinberg-Salam theory could be extended to the second basic class of particles, quarks, by introducing a new physical concept, the charm quark.

In the early 1960s, Murray Gell-Mann had proposed that all forms of matter can be described as being composed of either leptons or quarks...