Perspectives on modified quantum theory Measurements, irreversibility and stochasticity in the quantum-classical crossover

Although quantum theory in its present state is highly successful in prediciting ensemble averaged observations, it cannot describe the dynamics of a single experiment. Specifically, the fundamental dynamical laws of quantum theory are deterministic and reversible, yet what transpires in a single measurement is both random and irreversible. Thus, how irreversible phenomena in nature arise fundamentally is not understood within the quantum mechanical framework. This is the infamous measurement problem which has been connected to other irreversible phenomena in this thesis.
Modifications to quantum theory, contributing significantly at the quantum to classical crossover, have been proposed in this thesis and these modifications not only allow irreversible measurement like dynamics for the appropriate setting, but also describe spontaneous symmetry breaking in the dynamical regime, in the appropriate setting. An universal fluctuation dissipation relationship relating the deterministic and stochastic components of the modifications result in the emergence of Born's rules and guarantees that the theory does not lead to superluminal-signalling issues. The models and formalism established in this thesis complement previously constructed models, by allowing the use of arbitrary colored noise, one parameter white noise as well as two parameter space-time white noise. Such modifications may not only resolve the quantum measurement problem theoretically, but are also experimentally testable. Further, the models of modified quantum mechanics proposed in this thesis hint at an underlying stochastic universe which lead to the emergence of classical randomness in the macroscopic regime and may possibly allow an approach to a unified theory of irreversibility.