ABSTRACT
Quantum mechanics had just about succeeded in bringing order and understanding to the atomic world — in principle at least—when chaos was again unleashed by the discovery of the neutron. Clearly, there in the nucleus lay a new force, an interaction whose coupling could be neither electric charge nor the gravitational constant. Some foundations for a new theory aiming at the strong interactions were laid by Yukawa. Yukawa conceived an exchange mechanism with a meson mediating the new interaction. Yet it was at this early stage that the complementary approach— symmetries—produced its first dividends in this new field of physics by requiring the mesons to form a triplet, including a neutral component. Hadrons participate in the nonstrong interactions too. Their electric charge coincides — except for the strength scale and an additive constant — with a particular component of the strong-interaction symmetry.
