In 1869, Dmitri Mendeleev proposed the periodic law as a method for organizing the known chemical elements in a systematic way. The periodic law was important to chemists because it brought some order into the study of the 50-odd elements then known. But it was not until four decades later that an explanation was found--the Niels Bohr model of the atom--for Mendeleev's system of organization.

In 1961, the American physicist, Murray Gell-Mann, and the Israeli physicist, Yuval Ne'eman, independently suggested a method for organizing the known subatomic particles. The Gell-Mann-Ne'eman system became known as the eightfold way. The eightfold way was an important concept to physicists because it brought order to the study of more than a hundred subatomic particles discovered in the preceding two decades. Like the periodic law before it, however, the eightfold way was a purely theoretical concept. Neither Gell-Mann nor Ne'eman could explain why particles could...