Accidental and Intentional Constraints on an EM→ψ Back-Reaction Coupling: A Conservative Bound from Cavity Stability and a Practical Path to 10⁻¹⁴

Abstract (Theoretical Research; Preprint — experimental bounds and proposed test)

We investigate electromagnetic back-reaction on scalar background fields in extended gravity theories. We consider a minimal extension of Density Field Dynamics (DFD) in which the electromagnetic (EM) stress acts back on the scalar background ψ with a single dimensionless parameter λ. When λ=1, EM probes the optical metric n=e^ψ but does not source ψ; when |λ-1|≠0, EM can pump ψ. We show that the mere stability of existing high-Q cavities (no observed parametric instability near twice the drive frequency) provides an “accidental” constraint |λ-1|≲3×10⁻⁵ under deliberately conservative assumptions. The same equations, used intentionally with modest modulation depth and multi-cavity geometry, imply an immediately accessible search sensitivity approaching |λ-1|∼10⁻¹⁴. We state both a driven (2ω=Ω_ψ) and a parametric (2ω≃2Ω_ψ) route, derive compact design laws, and explain why such effects were not already seen in standard metrology workflows. Submitted to Annalen der Physik (2025).