Directional Density of Gases Under FitzGerald-Lorentz Contraction: Predictions for Optical Interferometry

We show that the directional density of an ideal gas remains isotropic under FitzGerald-Lorentz contraction of its container, while that of a bound solid becomes anisotropic. Combined with the invariance of the optical phase delay per molecule, this yields parameter-free predictions for gas-filled interferometers under Lorentzian Relativity. For a single gas-filled arm measured against a direction-independent reference, the signal on rotation is ΔN = k(L/λ)(n−1)(v/c)², where k depends on the interferometric configuration. The signal arises from the orientation-dependent molecule count in the contracted gas path. We predict null results for solid-dielectric interferometers regardless of refractive index.