In Valia Allori (ed.), Statistical Mechanics and Scientific Explanation: Determinism, Indeterminism and Laws of Nature. World Scientific. pp. 479–515 (2020)

Authors
Eddy Keming Chen
University of California, San Diego
Abstract
In a quantum universe with a strong arrow of time, it is standard to postulate that the initial wave function started in a particular macrostate---the special low-entropy macrostate selected by the Past Hypothesis. Moreover, there is an additional postulate about statistical mechanical probabilities according to which the initial wave function is a ''typical'' choice in the macrostate. Together, they support a probabilistic version of the Second Law of Thermodynamics: typical initial wave functions will increase in entropy. Hence, there are two sources of randomness in such a universe: the quantum-mechanical probabilities of the Born rule and the statistical mechanical probabilities of the Statistical Postulate. I propose a new way to understand time's arrow in a quantum universe. It is based on what I call the Thermodynamic Theories of Quantum Mechanics. According to this perspective, there is a natural choice for the initial quantum state of the universe, which is given by not a wave function but by a density matrix. The density matrix plays a microscopic role: it appears in the fundamental dynamical equations of those theories. The density matrix also plays a macroscopic / thermodynamic role: it is exactly the projection operator onto the Past Hypothesis subspace. Thus, given an initial subspace, we obtain a unique choice of the initial density matrix. I call this property "the conditional uniqueness" of the initial quantum state. The conditional uniqueness provides a new and general strategy to eliminate statistical mechanical probabilities in the fundamental physical theories, by which we can reduce the two sources of randomness to only the quantum mechanical one. I also explore the idea of an absolutely unique initial quantum state, in a way that might realize Penrose's idea of a strongly deterministic universe.
Keywords time's arrow  Past Hypothesis  foundations of probability  quantum statistical mechanics  wave function realism  quantum ontology  density matrix  Weyl Curvature Hypothesis  Humean Supervenience
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References found in this work BETA

The Metaphysics Within Physics.Tim Maudlin - 2007 - Oxford University Press.
Time and Chance.David Z. Albert - 2000 - Harvard University Press.

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Citations of this work BETA

Quantum Mechanics in a Time-Asymmetric Universe: On the Nature of the Initial Quantum State.Eddy Keming Chen - 2021 - British Journal for the Philosophy of Science 72 (4):1155–1183.
The Past Hypothesis and the Nature of Physical Laws.Eddy Keming Chen - forthcoming - In Barry Loewer, Eric Winsberg & Brad Weslake (eds.), Time's Arrows and the Probability Structure of the World. Harvard University Press.
The Cosmic Void.Eddy Keming Chen - 2021 - In Sara Bernstein & Tyron Goldschmidt (eds.), Non-Being: New Essays on the Metaphysics of Nonexistence. Oxford University Press.

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