Fractional Action, Refractive Vacuum, and Testable Memory Signatures in the Light Sector

This preprint introduces an original MetaTime-based framework for the light sector in which retardation, hysteresis, and effective damping arise from temporal memory encoded through a fractional action, rather than from phenomenological terms added by hand. The paper models the vacuum as an effective refractive medium with delayed response, leading to a history-dependent refractive index and to testable signatures such as phase lag, long-tail relaxation, cyclic hysteresis, and protocol-dependent attenuation. Its originality lies not in fractional calculus by itself, but in using it to build a falsifiable refractive-vacuum hierarchy connecting the local vacuum, an exponential-memory closure, and a fractional-memory extension. The work identifies two immediate observational pathways using existing datasets: Fast Radio Bursts (FRBs), where memory effects may appear as residual dispersion beyond the standard plasma law, and Gamma-Ray Bursts (GRBs), where cumulative non-instantaneous propagation effects may survive beyond standard local Lorentz-invariance-violation templates. Within the broader MetaTime program, this manuscript provides a minimal light-sector realization open to present-day observational falsification.