Covariant and locally Lorentz-invariant varying speed of light theories

We propose definitions for covariance and local Lorentz invariance applicable when the speed of light c is allowed to vary. They have the merit of retaining only those aspects of the usual definitions which are invariant under unit transformations and which can therefore legitimately represent the outcome of an experiment. We then discuss some possibilities for invariant actions governing the dynamics of such theories. We consider first the classical action for matter fields and the effects of a changing c upon quantization. We discover a peculiar form of quantum particle creation due to a varying c. We then study actions governing the dynamics of gravitation and the speed of light. We find the free, empty-space, no-gravity solution to be interpreted as the counterpart of Minkowksi space-time and highlight its similarities with Fock-Lorentz space-time. We also find flat-space string-type solutions, in which near the string core c is much higher. We label them fast tracks and compare them with gravitational wormholes. We finally discuss general features of cosmological and black hole solutions, and digress on the meaning of singularities in these theories.