ABSTRACT
This chapter describes the basic features of gauge theories and aims to prepare the way for our discussion of the Glashow–Weinberg–Salam theory of the weak interaction. The idea behind gauge theories is based on the fact that both in quantum mechanics and in classical physics there exist quantities which it is in principle impossible to measure. The chapter explains what is meant by internal symmetry. There are two types of internal symmetries, global and local. Symmetries of the latter type are identical with gauge symmetries. After the global symmetries, the chapter considers so-called local symmetries. It shows that the requirement for an invariance under local transformations forces the existence of an interaction whose form it also determines. The chapter describes a mechanism which may be used to save the gauge principle whilst at the same time resolving the differences from reality described.
