In the first half of this course, Michael Green set out the historical background of string theory and the basic principles of string mechanics. He described the various consistent string theories and displayed the spectrum of particles which they produce. These always include candidates for gravitons and gauge bosons;
in the case of supersymmetric strings, one also finds candidates for approximately massless quarks and leptons. Thus, string theories provide a new basis for understanding the fundamental interactions, a generalized mechanics from which the dynamics of gauge-invariant fields can be derived as a low-energy limit.