  Geometry and gravity induced effective photon mass is known to arise in many
cases, such as various optical waveguides, Kaluza-Klein theories, and many
other optical and general relativity situations. Here we study the appearance
of effective photon mass in the Newtonian limit due to the presence of gravity
gradient in a locally inertial reference frame. The effective photon mass
squared appears to be proportional to the local gravity gradient, and it
becomes tachyonic around unstable equilibrium locations. A similar effect is
observed in the Kottler-Moller spacetime where the absolute value of the
gravity-induced effective photon mass appears to coincide with the Unruh
temperature. We demonstrate that similar to Unruh effect, a bath of thermal
radiation is observed near the unstable equilibrium, which temperature is
defined by the local gravity gradient, and which remains unchanged in the limit
of infinite light velocity.
