München 2019 – scientific programme

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AGPhil: Arbeitsgruppe Philosophie der Physik

AGPhil 6: Philosophy of Cosmology VI

AGPhil 6.1: Talk

Wednesday, March 20, 2019, 12:30–13:00, HS 10

Time's Arrow in a Quantum Universe: On the Nature of the Initial Quantum State — •Eddy Keming Chen — Rutgers University, New Brunswick, NJ, USA

In a quantum universe with an arrow of time, we postulate a low-entropy boundary condition (the Past Hypothesis) to account for the temporal asymmetry. In this talk, I show that we can use the Past Hypothesis to determine a natural initial quantum state of the universe. First, I introduce the idea that the quantum state of the universe can be impure. This stands in sharp contrast to the standard view, according to which the quantum state of the universe is pure. Second, I suggest that the Past Hypothesis 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 Past Hypothesis subspace (in the Hilbert space). Third, because the initial quantum state is unique and simple, we no longer need to postulate fundamental statistical-mechanical probabilities to rule out anti-entropic quantum states (because there is only one possible initial state), and moreover we can interpret the quantum state to have the same status as laws of nature (because it is simple enough to be nomological). Hence, it offers a simple and unified answer to several open questions in philosophy of cosmology, including a natural choice for the universal quantum state (cf: Hartle and Hawking 1984), the status of the quantum state (cf: Durr et al. 1997), and the reduction of statistical mechanical probabilities (cf: Albert 2000 and Wallace 2012).