Abstract
In a quantum universe with a strong arrow of time, we postulate a low-entropy boundary condition (the past hypothesis, PH) to account for the temporal asymmetry. In this article, I show that the PH also contains enough information to simplify the quantum ontology and define a natural initial condition. First, I introduce ‘density-matrix realism’, the thesis that the quantum state of the universe is objective and impure. This stands in sharp contrast to wave-function realism, the thesis that the quantum state of the universe is objective and pure. Second, I suggest that the PH is sufficient to determine a natural density matrix, which is simple and unique. This is achieved by what I call the ‘initial projection hypothesis’: the initial density matrix of the universe is the (normalized) projection onto the PH-subspace (in the Hilbert space). Third, because the initial quantum state is unique and simple, we have a strong case for the nomological thesis: the initial quantum state of the universe is on a par with laws of nature. This new package of ideas has several interesting implications, including on the harmony between statistical mechanics and quantum mechanics, theoretical unity of the universe and the subsystems, and the alleged conflict between Humean supervenience and quantum entanglement. 1Introduction2Foundations of Quantum Mechanics and Statistical Mechanics2.1Quantum mechanics2.2Quantum statistical mechanics3Density-Matrix Realism3.1W-Bohmian mechanics3.2W-Everettian and W-Ghirardi–Rimini–Weber theories3.3Field intepretations of W4The Initial Projection Hypothesis4.1The past hypothesis4.2Introducing the initial projection hypothesis4.3Connections to the Weyl curvature hypothesis5Theoretical Pay-Offs5.1Harmony between statistical mechanics and quantum mechanics5.2Descriptions of the universe and the subsystems6The Nomological Thesis6.1The classical case6.2The quantum case6.3Humean supervenience7Conclusion