This paper presents a rigorous examination of the Separated Universes Theory (SUT), a novel ontological framework proposing that each fundamental particle and its associated spatial field constitute an independent, charge-neutral universe. Within this framework, we reinterpret the photon not as an elementary gauge boson, but as a composite oscillating structure carrying the "geometric template" of its source universe—a superposition of positive (particle center) and negative (deformed space) components. We propose that the atomic nucleus acts as a "Charge Prism" during pair production, decoupling the photon's composite structure into discrete matter (electron) and antimatter (positron) through differential electromagnetic interaction. Furthermore, we introduce the concept of Spatial Phase Transition to explain the generation of heavier leptons (muons, tauons) from the same spatial medium at higher energy densities, thereby resolving the "universality paradox" of photon-induced pair production. We demonstrate that SUT satisfies the three fundamental criteria of scientific validity: (1) Internal Mathematical Consistency across conservation laws; (2) Empirical Concordance with the zero rest mass of the photon and the 1.022 MeV pair-production threshold; and (3) Falsifiability via specific, quantitative experimental predictions proposed herein. This work extends the atomistic philosophy of Democritus from matter to reality itself, proposing that spacetime is not a passive stage but an active, quantized participant in physical phenomena.