Isotropic Gauge as a Physical Representation of the Gravitational Vacuum

In general relativity, the choice between areal and isotropic coordinates is conventionally treated as a pure coordinate convention, without physical import. That is correct as far as it goes. But it does not exhaust the question. If the gravitational vacuum is hypothesized to possess an underlying locally isotropic optical or constitutive structure, then isotropic gauge acquires a different status. It is not privileged by general relativity alone. It is distinguished by the particular physical structure being investigated. Isotropic coordinates display the 3-metric as conformally Euclidean, isolating the spatial deformation into a single scalar factor B(ρ). This makes possible a clean decomposition of the effective optical refractive index into spatial dilation (n_spatial) and temporal lapse (n_lapse) components. The decomposition is exact. Whether the two factors correspond to physically distinct properties of the vacuum is a separate question — a hypothesis, not a consequence of the algebra.