On spontaneous irreversibility and objective thermalization in stochastic modifications of quantum theory

The deterministic and time-reversal symmetric dynamics of isolated quantum systems is at odds with irreversible equilibration observed in generic thermodynamic systems. Standard approaches at a reconciliation employ subjective restrictions on the space of observables or states and do not explain how a single macroscopic quantum system achieves equilibrium dynamically. We instead argue that quantum theory is an effective theory and requires corrections to accurately describe systems approaching the thermodynamic limit. We construct a stochastic extension of quantum theory which is practically identical to quantum mechanics for microscopic systems, yet allows single, isolated macroscopic systems to objectively thermalize, generically. A fluctuation-dissipation relation guarantees physical consistency including norm preservation, energy conservation, no superluminal signalling and the emergence of microcanonical equilibrium. We further discuss the inclusion of objective collapse, thereby realizing a falsifiable theory of spontaneous universal irreversibility which describes the quantum-to-classical crossover dynamics of macroscopic quantum systems. This model admits spontaneous symmetry breaking, quantum state reduction and objective quantum thermalization for individual systems while realizing an emergent hybrid, Born-Maxwell-Boltzmann-Gibbs-microcanonical distribution for ensembles.