Selective Probing of Boundary-Modified Vacuum Observables in Fixed-Gap Casimir Cavities

Selective Probing of Boundary-Modified Vacuum Observables in Fixed-Gap Casimir Cavities — Preprint v1.0 This preprint proposes a methodological framework for using fixed-gap Casimir cavities as comparative platforms for selectively probing boundary-modified electromagnetic vacuum observables. The central idea is to hold the cavity gap fixed while systematically varying the boundary material, geometry, or physical state, and to evaluate differential observables between two configurations. The work introduces the normalized spectral asymmetry A_{A,B}(d) as an operational metric for comparing boundary-dependent vacuum responses under the same nominal geometry. Candidate observables include Casimir force, local electromagnetic density of states, two-point field correlators, quadrature noise, and quantum-emitter response. The proposed framework identifies three priority material comparisons: Au/doped Si, Au/graphene, and normal-metal/superconductor. A preliminary finite-temperature Lifshitz-theory evaluation is performed for the Au/doped-Si pair using parallel plates at T = 300 K. The calculation indicates a spectral asymmetry increasing from approximately 8% at a 50 nm gap to nearly 19% at a 200 nm gap, depending weakly on silicon carrier concentration. These results suggest that the limiting factor for a first experimental implementation is likely not signal magnitude or detector sensitivity, but the control of systematic effects such as patch potentials, surface roughness, and material-dependent preparation. The paper does not claim vacuum information storage, vacuum-energy extraction, spacetime manipulation, or new physics beyond standard Casimir and cavity-QED phenomena. Its contribution is methodological: it provides a controlled comparative protocol, supported by a preliminary Lifshitz calculation, for organizing future fixed-gap Casimir measurements as selective probes of boundary-modified vacuum observables. This is the third in a series of preprints on boundary-modified vacuum physics, following earlier work on boundary-modified density of states and optical squeezing, and on spacer-stabilized Casimir cavities.