We propose a conceptual reinterpretation of the fine-structure constant as a critical phase-coupling ratio that characterizes the boundary between wave-like energy states and structurally confined matter states. In this framework, electron–positron pair production is understood as a phase structure transition of high-density electromagnetic energy rather than a collision-induced production process, and the difficulty of stable propagation of ultra-high-frequency photons, as well as the reason why the speed of light constitutes the upper speed limit, are interpreted as consequences of common phase-structural boundary conditions. This work provides a unified and intuitive structural perspective on the continuity between radiation and matter, complementing the numerical success of existing theories without introducing new dynamics.