Analysis of the Essence and Applicable Boundaries of Relativity from the Perspective of Light String First-Principle Unified Theory

Based on the First-Principle Unified Theory of Light Strings, this paper reinterprets the essence of special and general relativity while affirming their mathematical correctness and experimental validity. It clarifies that relativistic mass is a field resistance effect, the speed limit of light originates from the intrinsic propagation speed of the light string field, spacetime curvature is a mathematical equivalent of light string field density gradients, and gravity is the elastic rebound of the light string field. The gravitational constant G is rigorously derived as a composite parameter of the light string field. This work positions relativity as an accurate phenomenal-level tool and reveals its applicable boundaries, realizing a progressive leap from phenomenological description to essential physical interpretation.