ABSTRACT
It is well known that the existence and interaction of certain fields, such as the electromagnetic field, can be closely related to the invariance properties of the theory. Thus, if the Lagrangian of matter fields is invariant under phase transformations with constant parameters a, the electromagnetic field can be introduced by demanding invariance under extended, local transformations, obtained by replacing a with a function of spacetime points, α → α(x). This idea was generalized by Yang and Mills (1954) to the case of SU(2) symmetry. Studying these internal local symmetries (appendix A) is particularly interesting from the point of view of the generalization to local spacetime symmetries.
