Superlight: A Phase-State Framework for Scalar Field Dynamics and the Predicted 0.2 Hz Subharmonic Eigenmode

We present the Superlight framework, a scalar field model in which a single real-valued field Φ is governed by the inhomogeneous wave equation □Φ = J, where □ = ∂ 2 /∂t 2-c 2 ∇ 2 is the d'Alembertian operator and J(x, t) is a generalized source term encoding matter-energy coupling. The framework proposes that conventionally distinct physical phenomena-gravitational lensing, electromagnetic propagation, vacuum energy behavior-may be reinterpreted as phase-state behaviors of this single underlying scalar field. Depending on the local source term and boundary geometry, Φ exhibits qualitatively distinct dynamical regimes: freely propagating waves, standing-mode resonances, and condensate-like ground states. The principal quantitative prediction of the framework is a 0.2 Hz subharmonic eigenmode arising from the coupling of the Superlight field to matter within effective resonant cavities defined by large-scale cosmic structure. This paper outlines the theoretical basis of the framework, derives the eigenmode prediction, and proposes experimental pathways-including low-frequency magnetometry, superconducting gravimetry, and dedicated antenna arrays-through which the prediction may be tested or falsified.