Quarks and leptons come out to play

The past twenty years have seen much clarification in the understanding of the elementary constituents of matter and the fundamental forces among them. In 1934 the simple picture of the structure of matter consisted of four types of forces (gravitational, electric, strong, and weak), four types of charge, and four elementary entities (electron, neutrino, proton, and neutron) to carry the charges composing all matter and all energy. The crucial achievements if the last two decades in this field are the isolation of the four strong charges carried by four types of object - the so-called quarks, and the notion that there is a symmetry principle linking them, as well as the discovered link between electric and weak charge. It is here postulated that the basic principle which governs the number and nature of charges that may exist has still to be discovered. (U.K.)