Photon Transit-Time Asymmetry in Media with Molecular Mobility: A First-Harmonic Test of Preferred-Frame Effects

We identify a preferred-frame effect arising from photon transit-time asymmetry in dielectric media with translational molecular mobility. Thermally moving molecules contribute encounters proportional to the photon's orientation-dependent transit time, producing a first-harmonic phase shift of order (n−1) β (vₜh/c) per unit length. The signal is predicted to be strongly suppressed for solids, though this has never been tested in the required geometry. The model applies most cleanly when intermolecular distances exceed molecular dimensions (gases, supercritical fluids) ; in dense liquids the effect may be reduced by a factor of order unity, but the large refractive index still yields ~10⁻³ fringes/m. This first-order effect is invisible to all existing round-trip cavity experiments. A counterpropagating geometry with n passes enhances the signal by a factor of 2n and provides intrinsic common-mode rejection.