Time's Arrow in a Quantum Universe: On the Status of Statistical Mechanical Probabilities
Eddy Keming Chen
备注：
Forthcoming in Valia Allori (ed.), \emph{Statistical Mechanics and Scientific Explanation: Determinism, Indeterminism and Laws of Nature}, World Scientific
发表信息：
Valia Allori (ed.), Statistical Mechanics and Scientific Explanation: Determinism, Indeterminism and Laws of Nature. World Scientific, May 2020
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 (the Statistical Postulate). 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 initial density matrix of the universe is exactly the (normalized) projection operator onto the Past Hypothesis subspace (of the Hilbert space of the universe). 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 (1989) of a strongly deterministic universe.
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Non-coherent evolution of closed weakly interacting system leads to equidistribution of probabilities of microstates
Departamento de Fisica de Materia Condensada, GIYA, CAC-CNEA（凝聚态物理系，GIYA，CAC-CNEA）；Instituto de Nanociencia y Nanotecnologia, INN-CONICET-CNEA（纳米科学与技术研究所，INN-CONICET-CNEA）
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