The Refractive Vacuum Hypothesis: Deriving the Speed of Light from Informational Viscosity in a 5D Bulk Framework

We formulate a constrained 5D bulk–brane framework in which the observed 4D speed of light isnot treated as a primitive constant of nature, but as an effective limiting velocity induced by thelinear response of the quantum vacuum to propagating fields. The 4D vacuum is modeled as an informationallystructured, dispersive medium characterized by an effective refractive index η4D(ω, ρinfo),where ρinfo is a coarse-grained functional encoding microscopic vacuum complexity (e.g., condensates,strong-field polarization, or short-distance structure) in a way that can enter constitutiveresponse without being identified with ordinary rest-mass density. The operational propagationspeed of electromagnetic disturbances is…