Photon mass as an equivalent of the energy of field compression into a waveguide mode

The general case of a waveguide with an arbitrary cross section is used to show that a nonzero photon mass is equivalent to the energy of a quantum of the critical frequency of a waveguide mode. This energy is equal to the work done against the light-pressure forces of zero-point fluctuations of vacuum in the course of compression of unbounded free space into a waveguide.