P23: Light, Photons, and the Universal Speed Limit: A First-Principles Derivation of c from Substrate Condensation Dynamics

Standard physics treats the speed of light c as a fundamental constant and photons as massless excitations of the electromagnetic field. The reason why c is universally limiting, why massless particles travel at exactly c, why gravitational waves share that same speed, and why massive particles cannot reach it, are treated as consequences of special relativity rather than as phenomena requiring physical explanation. This paper derives all of these results from the physical properties of the Spaticle substrate established in the BFUT programme. The universal speed limit c is the propagation speed of the Spaticle field itself, determined by the substrate stiffness-to-density ratio: c = √(K_s/ρ_s). A photon is a freely propagating organised excitation of the substrate that requires no stable localised condensation structure. All of its energy is available for propagation. It therefore travels at the maximum rate the substrate permits. Gravitational waves are propagating deformation disturbances of the same substrate. They travel at c for the same reason. The equivalence of light speed and gravitational wave speed, confirmed by GW170817 to one part in 10⁻¹⁶, is not a coincidence requiring a separate explanation. It follows directly from the common substrate origin of both phenomena. Massive particles travel below c because part of their energy budget is committed to maintaining their internal condensation structure rather than pure propagation. The velocity deficit from c is determined by the ratio of rest energy to total energy. Neutrinos, with their extremely small masses, travel within one part in 10⁻¹⁷ of c, consistent with the Supernova 1987A constraint. Photons travel at exactly c because they have no rest energy and therefore no condensation to maintain. Cosmic redshift is explained by Doppler motion of receding matter and substrate propagation dynamics. The universe is infinite and eternal; no expansion of space is involved. The Doppler effect, aberration, and time dilation all emerge from the finite substrate propagation speed. The paper also derives why the speed limit is absolute: no physical process can reorganise the substrate faster than the substrate propagates causal information. Light does not define the universal speed limit. The universal speed limit defines the behaviour of light.