Effective photon mass in the presence of a gravity gradient emulated by an electromagnetic medium

Abstract

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 a gravity gradient emulated by an electromagnetic medium. The effective photon mass squared appears to be proportional to the local gravity gradient, and it becomes negative in an optical anti-waveguide around the unstable equilibrium location. A similar effect is observed in the emulated Kottler–Møller spacetime where the absolute value of the gravity-induced effective photon mass appears to coincide with the Unruh temperature. We demonstrate that similar to the Unruh effect, a bath of thermal radiation should be observed in an optical anti-waveguide near the unstable equilibrium, whose temperature is defined by the emulated local gravity gradient, and which remains unchanged in the c→∞ limit.