Emergence of Classical Time: Gravitational Temporal Decoherence and the Uniqueness of the Past

We propose a conservative theoretical framework addressing the emergence of classical time and the uniqueness of the past within quantum mechanics, without modifying its fundamental postulates or introducing objective collapse mechanisms. The central observation is that quantum superpositions of mass–energy distributions necessarily imply superpositions of gravitational potentials, and therefore superpositions of proper times. By treating time as an effective quantum degree of freedom and extending the system Hilbert space accordingly, we show that matter–time entanglement leads to an intrinsic dispersion of proper time. Gravitational fluctuations and irreversible memory formation dynamically suppress this dispersion, giving rise to an effective closure of temporal histories. The resulting dynamics admits stable fixed points corresponding to classical time and a unique past, while remaining fully compatible with global unitarity. We analyze the stability of this closure, its relation to decoherence and quantum Darwinism, and its implications for cosmology, including the interpretation of the Big Bang as a temporal closure process rather than a fundamental beginning of time. The framework clarifies the role of gravity in the quantum–classical transition and delineates the physical conditions under which a unique history can emerge.