A Spacetime Geometric Interpretation of the Apparent Slowdown of Light in Media -- Based on the Hypothesis of Spacetime Path Decomposition

This paper expands the original Version 1.0 spacetime path decomposition hypothesis by introducing a controllable experimental system of thermostatted pure hydrogen gas. Quantitative relations among gas pressure, mass density and refractive index are fully integrated into the original four-dimensional spacetime component redistribution framework. The core premise remains unchanged: photons always travel at the vacuum speed of light c in four-dimensional spacetime. The apparent reduction of light speed inside media does not arise from a true change in the photon’s total propagation speed, but from weight redistribution between motion components along the temporal dimension and spatial dimensions. The spatial transverse shift Δ = S√n^2-1 serves as a measurable prediction of the model. The extended version incorporates the ideal gas law and the linear approximation from the Lorentz–Lorenz relation for dilute gases, forming a closed set of analytical formulas linking pressure P, medium density ρ, refractive index n, spacetime coupling angle θ, and equivalent temporal propagation speed v_T. Pure hydrogen enables continuous tuning of the refractive index via a single variable (gas pressure), offering a simple, repeatable experimental scheme to verify the spacetime geometric decomposition hypothesis. This model acts only as a complementary geometric description and does not replace microscopic electromagnetic polarization theories. It is fully consistent with classical optics and the special relativistic vacuum limit.