Thermodynamic Geometry and the Emergence of Classicality through Decorrelation

Angelo Plastino

The quantum–classical transition is one of the most frequently invoked concepts in modern physics. Yet the notion of classicality itself is far from unique. Depending on the physical context, classical behavior may be associated with decoherence, the semiclassical limit, thermodynamic averaging, suppression of correlations, emergence of collective order, or geometric simplification of statistical state space. These viewpoints are often presented as if they described a single phenomenon, although they emphasize different physical mechanisms and different operational criteria. In this article we examine the principal notions of classicality that appear across quantum theory, statistical physics, condensed matter physics, and information geometry. We compare the corresponding mechanisms of classical emergence and analyze the physical quantities commonly used to characterize them, including coherence, entanglement, fluctuations, correlation length, Fisher information, statistical complexity, and information-geometric curvature. We argue that many apparently distinct routes toward classical behavior share a common structural feature: a reduction of effective fluctuation freedom. From this perspective, classicality may be interpreted as an emergent regime in which the accessible fluctuation manifold becomes progressively constrained, stabilized, or geometrically simplified. This viewpoint naturally unifies decoherence, semiclassical localization, thermodynamic averaging, decorrelation, and collective organization within a common conceptual framework. Rather than representing a unique physical process, the quantum–classical transition appears as a family of related mechanisms through which complex quantum fluctuation structure gives rise to effective macroscopic classical behavior.

John Goold

Dorje C. Brody, Lane P. Hughston

Hua Chen, Naoki Sasakura, Yūki Sato

It is shown that classical spaces with geometries emerge on boundaries of randomly connected tensor networks with appropriately chosen tensors in the thermodynamic limit. With variation of the tensors the dimensions of the spaces can be freely chosen, and the geometries---which are curved in general---can be varied. We give the explicit solvable examples of emergent flat tori in arbitrary dimensions, and the correspondence from the tensors to the geometries for general curved cases. The perturbative dynamics in the emergent space is shown to be described by an effective action which is invariant under the spatial diffeomorphism due to the underlying orthogonal group symmetry of the randomly connected tensor network. It is also shown that there are various phase transitions among spaces, including extended and point-like ones, under continuous change of the tensors.

Borros Arneth

We develop a pre-geometric reconstruction framework in which quantum mechanics, entanglement structure, gauge symmetry, locality, Lorentzian spacetime geometry, gravitation, and physical constants emerge from recursively stabilized coherent Hilbert-relational organization. The framework begins from a primitive coherent substrate consisting of compositional accessibility relations and complex transport amplitudes. No background spacetime manifold, metric geometry, or particle ontology is assumed fundamentally. The central proposal is that coherent Hilbert structure is more fundamental than spacetime geometry. Observable spacetime emerges through projection stabilization: a physical stability-selection mechanism dynamically selecting recursively stable coherent sectors from deeper relational structure. Quantum mechanics emerges from norm-preserving coherent transport. Entanglement emerges from nonfactorizable relational closure. Gauge symmetry emerges as redundancy of coherent transport representation. Classicality emerges through decoherence and stability selection. Lorentzian geometry emerges statistically from projected accessibility organization. Gravitation emerges thermodynamically as equilibrium relaxation of projected geometry. The framework is presented not as a completed unified theory, but as a reconstruction program aimed at explaining why physical reality organizes itself through Hilbert structure, gauge redundancy, locality, spacetime geometry, and thermodynamic gravitational dynamics.

H. A. Buchdahl