Null Propagation as the Causal-Limit Invariant: Why the Speed of Light Is a Metric Representative

This paper argues that the speed of light is not a fundamental physical quantity, despite its central role in modern physics and its exact numerical fixation in contemporary unit systems. The core claim is structural rather than empirical: the notion of speed presupposes metric structure and unit conventions, whereas the invariant physical content of relativity is carried by null causal structure, not by a numerical constant. The analysis introduces a clear criterion for fundamentality based on invariance under admissible re-descriptions (coordinate changes, unit rescalings, and other representational freedoms). Using this criterion, the paper shows that any numerical speed assigned to light propagation is representation-dependent, while the classification of relations into timelike, spacelike, and null—and the associated causal ordering—is invariant. In both special and general relativity, light propagation is fundamentally characterized by its null character, not by a primitive magnitude. The paper makes explicit the role of conformal invariance in general relativity, emphasizing that causal structure depends only on the conformal class of the spacetime metric, not on its overall scale. This clarifies why coordinate speeds of light may vary without physical contradiction and why fixing the numerical value of ccc by metrological convention does not confer physical fundamentality. Empirical implications are discussed carefully: experimental tests of relativity constrain Lorentz invariance, isotropy, dispersion, and causal signaling structure. These constraints attach directly to null structure and causal boundaries, even though they are often represented using a numerical speed parameter. The paper also clarifies common confusions surrounding so-called “varying-ccc” theories, distinguishing purely representational changes from substantive modifications of causal structure. Overall, the work reframes the status of the speed of light as a metric encoding of deeper invariants rather than a primitive constant, while preserving the full empirical content of special and general relativity. It is intended as a conceptual clarification for physicists and philosophers of physics interested in fundamentality, invariance, and the structure of relativistic theories.