The not-so-effective mass of photons in a planar optical cavity

It is a well established and understood fact that photons propagating in free space interact with the gravitational field, leading to well-known effects such as gravitational redshift or gravitational lensing. While these phenomena might give an impression that photons in free space have a sort of mass, this impression falls short upon considering their dispersion relation. In this letter we show that unlike in free space, when photons are brought to a stop within a planar cavity, they acquire a mass that cannot not be distinguished from that of a solid-state body freely moving in a bidimensional space, both from the inertial and gravitational point of view.