  We develop a gauge theory of the combined gravitational-electromagnetic field
by expanding the Poincar\'e group to include clock synchronization
transformations. We show that the electromagnetic field can be interpreted as a
local gauge theory of the synchrony group. According to this interpretation,
the electromagnetic field equations possess nonlinear terms and electromagnetic
gauge transformations acquire a space-time interpretation as local synchrony
transformations. The free Lagrangian for the fields leads to the usual
Einstein-Maxwell field equations with additional gravitational-electromagnetic
coupling terms. The connection between the electromagnetic field and the
invariance properties of the Lagrangian under clock synchronization
transformations provides a strong theoretical argument in favor of the thesis
of the conventionality of simultaneity. This suggests that clock
synchronization invariance (or equivalently, invariance under transformations
of the one-way speed of light) is a fundamental invariance principle of
physics.
