Paper I: The Substrate Field: A Physical Inference from the Finite Speed of Light

The speed of light in vacuum is finite and constant, yet the physical reason for this finiteness has never been explained. This paper adopts a first‑principle view: any finite propagation speed requires a physical medium. We therefore postulate the existence of a universal substrate field that fills all space, characterized by an effective density ρ₀ and a rigidity K, which determine the speed of light via c = √(K/ρ₀). Using only the refractive indices and mass densities of three common transparent materials (air, water, and glass), we derive numerical values for the substrate parameters: ρ₀ ≈ 4.4×10³ kg/m³ and K ≈ 4.0×10²⁰ Pa. The uniformity of the substrate ensures the isotropy and constancy of c. This work shows that the finite speed of light is not a brute fact but a quantitative consequence of a real physical medium, whose properties can be inferred from elementary optical data. The substrate field provides a new foundation for understanding light propagation, and its existence can be tested through the predictions made in subsequent papers of this series.Part I of the Substrate Field Theory series