General relativity establishes a geometric theory of spacetime via the metric tensor, which accurately describes gravitational effects and spacetime curvature and is well consistent with various classical observations. Nevertheless, it merely provides a geometric mathematical description with-out endowing spacetime curvature with a clear physical ontology, and fails to fundamentally unify with electromagnetic theory. To address the fundamental separation between gravitational and electromagnetic systems, this paper proposes to physically reinterpret the metric tensor as the four-dimensional spacetime density tensor. While fully retaining the Riemannian geometry, Einstein field equations and the complete mathematical framework of general relativity, this work redefines the physical evolution of spacetime: gravity corresponds to the static gradient deformation of the four-dimensional spacetime density tensor, and electromagnetism corresponds to its dynamic fluctuation and shear perturbation. The two fundamental interactions are no longer independent entities, but two distinct evolutionary forms of the same four-dimensional spacetime substrate. Based on the weak-field approximation and linear perturbation theory, this paper conducts complete quantitative derivations, rigorously obtains the wave equation for electromagnetic potentials, and further deduces the classical Maxwell’s equations. The proposed model requires no extra dimensions or modifications to classical field equations, and complies with all existing experimental results. It provides a concise and self-consistent new theoretical framework for the fundamental unification of gravity and electromagnetism.
Lubo Hu (Mon,) studied this question.