Substantial Spacetime Dynamics (I) Beyond Einstein’s Light Speed Postulate: The Mechanical Mechanism of the Light Speed Limit and the Cs^2 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 (epsilon_0, mu_0), the gravitational constant G, and the emergence of universal physical scales. We demonstrate that the true invariant limit resides in a quadratic structural threshold Cs^2, 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), Cs^2 Structural Threshold, CSSD Framework, Gravitational Wave Verdict

实体时空动力学 (I):Cs^2 结构阈值与物理通用常数的力学起源 摘要: 本文为“光速为何是宇宙极限”提供了力学答案,而这一事实此前仅被爱因斯坦作为一个公设所承认。 近期的引力波探测要求物理学从几何抽象转向时空的力学本体论。虽然光速 (c) 的恒定性长期以来被视为现代物理学的基本假设,但其物理起源以及与其他基本常数的数值关系仍未得到解决。在本研究中,我们提出:观测到的光速 c 本身并非时空的最终速度极限,而是由时空内在力学本构强度(记为 Cs)所决定的深层结构边界的一种表现形式。 基于引力波实验的最终自然判决,本研究将 Cs 鉴定为一个新发现的基本物理常数。至关重要的是,Cs 被揭示为宇宙的“元常数”(Meta-constant),它是主导电磁常数 (epsilon_0, mu_0)、万有引力常数 G 以及通用物理尺度涌现的核心主权常数。我们证明,真正的对称性不变极限存在于“二阶结构阈值” Cs^2 中,它代表了时空作为物理实体的最大结构应力或响应能力。 我们的 CSSD 理论框架证明,结构响应速度 Cs 是真正的第一性极限,而 c 只是其二级表现。该框架在统一的时空本体论下整合了原本孤立的物理常数,确立了“崔氏实体时空动力学”(CSSD)作为通过力学本构强度解析宇宙架构的根本途径。 关键词: 时空实体、元常数 (Cs)、Cs^2 结构阈值、CSSD 框架、引力波判决