Critical Switching Phenomena in the Quantum–Classical Crossover

Description This work introduces a structural framework for understanding the quantum–classical crossover using two internal quantities of the Thickness Structure Hypothesis (TSH): the spread deviation Δf and the shrink tension γT. These quantities characterize the competing tendencies of quantum spreading and classical or gravitational contraction. By organizing physical behaviors on the Δf–γT phase diagram, the framework provides a unified structural classification of quantum, classical, and gravitational regimes. A key result is that the sharp disappearance of quantum interference can be interpreted as a critical phenomenon. Near the boundary between the Stable (quantum) and Composite (classical) phases, the interference visibility follows a Landau‑type scaling law with critical exponent β = 1/2. This suggests that the abrupt switching observed in the quantum–classical transition arises from structural changes in Δf and γT rather than from external decoherence alone. Within this framework, wave‑packet collapse and the loss of interference visibility correspond to motion toward larger γT in the structural parameter space. The Δf–γT phase diagram thus provides a general, theory‑independent viewpoint for describing interference, localization, and the onset of classical behavior. The approach offers a structural mechanism for the sharpness of the quantum–classical crossover and connects quantum, classical, and gravitational behaviors within a single unified diagram.