Gravitational Decoherence in Macroscopic Quantum Systems

Sammanfattning: The problem of how quantum mechanics gives rise to classicality has been debated for more than a century. A commonly proposed solution is decoherence, i.e. the gradual decay of superpositions in open quantum systems due to their inevitable interaction with their environment. However, the ability of decoherence to account for all aspects of the classical world is often questioned. A recently proposed model suggests that decoherence can occur even in isolated composite systems subject to gravitational time dilation, something which has sparked a debate. In this thesis we attempt to identify the precise role of decoherence in the quantum-to-classical transition (QTCT) and then use the result to analyze the validity of the newly proposed time dilation-induced decoherence mechanism. We find that the problem of the QTCT can be divided into two parts and that decoherence solves the first of these whereas the second is unsolvable without fundamental modifications to quantum theory. Moreover, we argue that the effect is fundamentally frame-dependent and we find a general formula for the rate of decoherence of macroscopic superpositions in the case where both the system and observer use Rindler coordinates. The result suggests that the frame-dependence may be utilized to increase the strength of the effect in experimental settings. Finally, the possibilities of experimental verification are discussed and we argue that recent advances in quantum measurement techniques in gravitational-wave observatories may enable tests of gravitational decoherence in the near future, finally providing an empirical glimpse into the resolution of one of the most critical debates in all of physics.