The Unified Derivation of Universal Gravitation and Coulomb Force: A First-Principle Unified Field Theory of Light

Based on the First-Principle Unified Field Theory of Light, this paper uses identical axioms, mathematical structure, and logical steps to uniformly derive the physical essence and mathematical forms of universal gravitation and Coulomb force, realizing the fundamental unification of gravity and electromagnetic force. No action at a distance, no presupposed electric charge, no empirical fitting, and no additional assumptions are introduced. Both gravitation and Coulomb force originate from the density gradient effect of the light string field. They share exactly the same mathematical structure, constant structure, and physical origin, differing only in coupling strength. This paper completely eliminates "electric charge" as a fundamental physical entity, completes the key closed loop of the unified field theory, and lays a solid foundation for unifying the four fundamental interactions.