On the Structural Nature of the Invariance of the Speed of Light Proper Time, Causal Boundaries, and Minimal Temporal Structure

This work is a foundational and conceptual contribution to the interpretation of relativistic spacetime. Rather than proposing new physical laws or modifying existing equations, the paper offers a structural clarification of the invariance of the speed of light by shifting the explanatory focus from kinematics to proper time. The analysis shows that the constancy of c can be understood as a consequence of the geometric definition of proper time and the role of null trajectories as causal boundaries in Lorentzian spacetime. The contribution is interpretive and structural in nature. It introduces no new postulates, entities, or dynamical assumptions, and remains fully consistent with standard special and general relativity. Its purpose is to clarify the foundational role of proper time in delimiting admissible physical regimes, rather than to advance a new theory or empirical model. This work is intended as a conceptual reference point for further discussions in the foundations of physics, causal structure, temporal interpretation, and related areas.