A description of the gravitational red shift borrowed from the electrodynamics of continuous media

Abstract

The description of the effects of a gravitational field on the classical electromagnetic field equations in terms of generalized electric and magnetic permittivities is shown to define an “equivalent” medium with special properties. In particular it is remarked that such a medium can be chosen so as to include the behavior, in the presence of gravity, of the bodies that are used to produce and to detect the electromagnetic field. This is illustrated by the example of the gravitational red shift. The introduction of the proper time of the emitting and of the absorbing atoms, at rest in the gravitational field, is shown to define a medium interacting with the electromagnetic field through a drag velocity which increases linearly with time. For the sake of simplicity this example is described in the weak-field approximation.