Paper II UV Completion via Kaluza–Klein Compactification and String-Theoretic Embedding

This work develops a geometric framework for understanding quantum decoherence, entropy production, and the emergence of the arrow of time from higher-dimensional physics. The central object of the theory is the Entropion field, a scalar degree of freedom arising as the zero mode of a higher-dimensional field under Kaluza–Klein dimensional reduction. Starting from a (4+n) -dimensional action, we derive the effective four-dimensional dynamics of the Entropion field and show that its coupling to matter is naturally expressed through the trace of the energy-momentum tensor. This provides a well-defined microscopic interaction that links the field to local energy distributions. At macroscopic scales, this interaction admits an effective description in terms of entropy production, establishing a bridge between fundamental quantum dynamics and thermodynamic irreversibility. A key result of the framework is that the projection from higher-dimensional geometry into four-dimensional spacetime induces an effective coarse-graining over inaccessible degrees of freedom. This leads to non-unitary dynamics for the reduced system, which can be described by a Lindblad-type evolution equation. From this structure, a physically meaningful decoherence rate is derived, scaling with local mass-energy density and controlled by the compactification scale. Importantly, the theory preserves fundamental symmetries: the underlying higher-dimensional action remains unitary and time-reversal invariant. The apparent breaking of time symmetry emerges only at the effective level, as a consequence of projection and information loss. In this sense, the Entropion field provides a dynamical realization of the thermodynamic arrow of time without violating CPT symmetry. The framework also addresses consistency with known physics. The Entropion field is massless at the level of classical dimensional reduction but acquires a small, technically natural mass through symmetry-breaking effects such as fluxes and quantum corrections. Environmental screening mechanisms allow the field to remain light and dynamic in low-density or quantum-coherent regimes, while becoming effectively heavy in high-density environments, avoiding conflicts with experimental constraints. Overall, this work presents a unified picture in which quantum decoherence, entropy production, and temporal asymmetry arise from geometric projection in higher-dimensional spacetime. The results provide a concrete foundation for further phenomenological exploration, including experimental tests and cosmological implications developed in subsequent work.