Substantial Spacetime Dynamics (I) Beyond Einstein's Light Speed Postulate: The Mechanical Mechanism of the Light Speed Limit and the Cs² Framework of Substantial Spacetime

Substantial Spacetime Dynamics (I) Beyond Einstein’s Light Speed Postulate: The Mechanical Mechanism of the Light Speed Limit and the Cs² Framework of Substantial Spacetime Abstract: This paper provides the mechanical answer to why the speed of light is the universal limit, a fact that was previously only assumed as a postulate by Einstein. Recent gravitational wave detections demand a shift from geometric abstractions to a mechanical ontology of spacetime. While the constancy of the speed of light (c) has long been a foundational postulate, its physical origin and numerical relation to other constants remain unresolved. In this work, we propose that the observed speed c is not the ultimate limit, but a manifestation of a deeper structural bound determined by the intrinsic constitutive strength of spacetime, denoted as Cs. Based on the definitive natural verdict of gravitational wave experiments, this work identifies Cs as a newly discovered fundamental constant. Crucially, Cs is revealed as the "Meta-constant" of the universe, which serves as the primary sovereign governing the electromagnetic constants (epsilon₀, mu₀), the gravitational constant G, and the emergence of universal physical scales. We demonstrate that the true invariant limit resides in a quadratic structural threshold Cs², representing the maximal structural stress or response capacity of spacetime as a physical entity. Our CSSD framework proves that the structural response Cs is the true primary limit, while c is its secondary representative. This framework unifies disparate constants within a single structural ontology and establishes Cui’s Substantial Spacetime Dynamics (CSSD) as the foundational pathway toward resolving the architecture of the universe through its constitutive strength. Keywords: Spacetime Entity, Meta-constant (Cs), Cs² Structural Threshold, CSSD Framework, Gravitational Wave Verdict