Photonic Vacuum Windows: A Casimir-Safe Operational Baseline

We propose a physical picture in which light propagation in vacuum is carried by a sea of fluctuating degrees of freedom. At the effective, linear-response scale, this collective medium determines the electromagnetic properties that govern how light moves, while preserving the characteristic impedance of the vacuum and avoiding artefacts arising from arbitrary parameter choices. This conceptual idea forms the primary aim of this work. To test whether this picture is realistic, we propose a platform-independent and testable approach. We introduce a simple band-averaged reporting measure that reduces any measurement to a single comparable quantity derived from the spectral weighting of the experiment. Using first-order, model-independent relations, we show how this quantity connects to observable shifts in cavity frequencies, interferometric phase, radiometric balance, and Casimir measurements. These links are presented solely as consequences that render the concept empirically testable, not as applications in themselves. The approach is constrained by the standard principles of linear response, causality, passivity, and a high-frequency limit consistent with Maxwell electrodynamics, so that the carried-light picture remains compatible with established physics and testable through reproducible, band-averaged limits on deviations from ideal vacuum propagation.