Mensky's path integral and photon mass

Abstract

It is commonly assumed that zero and non-zero photon mass would lead to qualitatively different physics. For example, massless photon has two polarization degrees of freedom, while massive photon at least three. This feature seems counter-intuitive. In this paper we will show that if we change propagator by setting $i \epsilon$ (needed to avoid poles) to a finite value, and also introduce it in a way that breaks Lawrentz symmetry, then we would obtain the continuous transition we desire once the speed of the photons is "large enough" with respect to "preferred" frame. The two transverse polarization degrees of freedom will be long lived, while longitudinal will be short lived. Their lifetime will be near-zero if $m \ll \sqrt{\epsilon}$, which is where the properties of two circular polarizations arize. The $i \epsilon$ corresponds to the intensity of Mensky's "continuous measurement" and the short lifetime of the longitudinal photons can be understood as the "conversion" of quantum degrees of freedom (photons) into "classical" ones by the measurement device (thus getting rid of the former). While the "classical" trajectory of the longitudinal photons does arize, it plays no physical role due to quantum Zeno effect: intuitively, it is similar to an electron being kept at a ground state due to continuous measurement.