Paper VI: Substrate Field Theory: Principles, Mathematical Formulation, and Responses to Common Criticisms

This is the sixth paper in the Substrate Field Theory series (preceding parts: 10. 5281/zenodo. 19297149, 10. 5281/zenodo. 19299418, 10. 5281/zenodo. 19343311, 10. 5281/zenodo. 19343466, 10. 5281/zenodo. 19343618). The speed of light is finite and constant, but why this is so has never been explained. Starting from the first principle that any finite speed requires a physical medium, we propose a universal substrate field that fills all space. The field has an effective density rho0 and rigidity K, related by c = sqrt (K/rho0). Using the refractive indices and densities of air, water, and glass, we derive rho0 ≈ 4. 4e3 kg/m³ and K ≈ 4. 0e20 Pa. Intrinsic dissipation in the substrate causes light to lose energy as it travels, providing a physical explanation for the Hubble redshift without dark energy. Gravity arises from density gradients of the substrate, offering a natural alternative to dark matter. The substrate's discrete granularity and dissipative nature also account for quantum superposition, entanglement, measurement collapse, and the second law of thermodynamics. This paper systematically derives the core equations: the wave equation with dissipation, dissipative redshift, kinetic energy loss for moving bodies, the gravitational field equation, cosmic scaling relations, and the decomposition of observed redshift into three physical contributions (Doppler, dissipative, and projection angle). Regarding the CMB dipole: its direction points toward the cosmic center, but the observed speed of 370 km/s is our velocity relative to the CMB radiation field, not necessarily our absolute velocity relative to the substrate. We also provide detailed responses to common criticisms, clarifying the nature of dissipation, blueshift in nearby objects, cosmic age, the missing mass problem, and the theory's testability. The theory predicts that the universe is currently in a collapsing phase and that the Moon's orbit exhibits a small additional inward drift beyond tidal recession. All predictions are consistent with existing astronomical and physical observations. Part VI of the Substrate Field Theory series