Action As a Fundamental Structure Dynamic Compatibility

The possibility of distinct physical systems remaining dynamically compatible over time is often implicitly assumed in various areas of physics, from interferometric descriptions to clock comparisons and the analysis of composite systems. In this work, we show that such continuous compatibility is not a generic property but encounters a structural limitation that stems directly from the variational formulation of dynamics. Starting from only minimal hypotheses—the existence of an accumulative action, monotonic proper parametrizations, and operational dependence on action differences via factors $e^{iS/\hbar}$—we demonstrate that dynamically independent systems cannot maintain functional coincidence of their actions over open intervals, except under fine-tuning. Consequently, moments of compatibility form a necessarily discrete set, and any coherent coupling is effective only within a time scale determined by the difference between the rates of action accumulation. The result does not depend on a specific microscopic model, nor on dissipative, statistical, or environmental mechanisms, being consistent with classical mechanics, relativity, and quantum field theory. It is shown, in particular, that effects such as the loss of interferometric visibility associated with time dilation can be understood as physical realizations of a more general constraint: the structural incompatibility between evolutions parameterized by distinct actions. Thus, the so-called ``compatibility window'' does not constitute a new dynamic principle, but an inevitable consequence of the variational architecture common to established physical theories.