ABSTRACT
Many attempts to unify gravity with other fundamental interactions are based on the idea of gauge symmetry. The principle of gauge invariance was invoked for the first time in Weyl’s unified theory of gravitation and electromagnetism (Weyl 1918). The geometry of Weyl’s unified theory represents an extension of the Riemannian structure of spacetime in GR. Weyl introduced spacetime in which the principle of relativity applies not only to the choice of reference frames, but also to the choice of local standards of length.
