Gravity as Hydrodynamics: Gradient-Index Refraction in a Variable-Density Ether Plenum

This paper develops the optical consequences of a compressible Ether Plenum whose density rises toward concentrations of mass-energy, and shows that three of the classical tests of gravitation follow from a single refractive index using Fermat's principle of least time rather than the geodesic equation. The index is derived rather than assumed. Taking the inward Radiative Undertow of magnitude GM per unit mass divided by the square of the radius, and imposing hydrostatic equilibrium with the equation of state pressure equals w times density times the square of light speed, integration yields a density profile that is exponential in the reciprocal radius. Applying the optical relation that the index equals the square root of the ratio of local to baseline density gives an index that is likewise exponential, and calibrating the single equation-of-state parameter against the measured solar light deflection fixes w at one quarter, so that the index becomes the exponential of twice GM divided by the product of the square of light speed and the radius, whose first-order expansion is one plus twice GM over that same product. This derivation matters for the standing of the series: earlier formulations imported this index from the established optical-mechanical analogy literature, and it is here obtained from the framework's own postulate. From the index, the paper obtains a solar-limb deflection of 1.7516 arcseconds against a measured 1.7500 plus or minus 0.0002, a round-trip Shapiro delay of 232.6 microseconds for the Earth-Venus geometry against a literature value of approximately 240, and a gravitational redshift of 2.459 times ten to the minus fifteen for the Pound-Rebka geometry against a measured 2.57 plus or minus 0.26. The paper states explicitly that the light-deflection agreement is not independent confirmation, since w was calibrated on that measurement, while the Shapiro delay and redshift are independent consequences of the calibrated index. It closes by identifying what the optical treatment does not address, principally the strong-field regime and the general-covariance question.