Physics of Mirror Photons

The physics of kinetic mixing between ordinary and mirror photons is elucidated. An important role is played by four linear combinations we dub the physical photon, the sterile photon, the physical mirror photon, and the sterile mirror photon. Because of the mass degeneracy between the two gauge bosons, quantum coherence effects are important. The physical photon becomes a certain coherent superposition of the bare ordinary photon and the bare mirror photon. Similarly, the physical mirror photon is another, but not orthogonal, coherent superposition. Using a Bloch equation analysis, we show that the physical mirror photon decoheres into a mixture of the physical (ordinary) photon and the sterile photon when it propagates through ordinary matter. Observational signatures for some hybrid ordinary/mirror binary as-trophysical systems are qualitatively discussed, and speculative connections with as yet mysterious observed objects are made. In particular, we speculate that the recently reported halo white dwarfs might actually be mirror halo stars.