The Quantum Mechanical Arrows of Time

TL;DR

This paper explores how quantum mechanical and thermodynamic arrows of time can vary locally within the universe, challenging the assumption that they always align and are globally consistent.

Contribution

It introduces a time-neutral quantum framework applied to cosmological models, showing that local arrows of time can differ and coexist with global time symmetry.

Findings

Arrows of time may be locally defined and vary across spacetime.

Global time symmetry can coexist with local arrow variations.

Not all universes have well-defined or aligned arrows of time.

Abstract

The familiar textbook quantum mechanics of laboratory measurements incorporates a quantum mechanical arrow of time --- the direction in time in which state vector reduction operates. This arrow is usually assumed to coincide with the direction of the thermodynamic arrow of the quasiclassical realm of everyday experience. But in the more general context of cosmology we seek an explanation of all observed arrows, and the relations between them, in terms of the conditions that specify our particular universe. This paper investigates quantum mechanical and thermodynamic arrows in a time-neutral formulation of quantum mechanics for a number of model cosmologies in fixed background spacetimes. We find that a general universe may not have well defined arrows of either kind. When arrows are emergent they need not point in the same direction over the whole of spacetime. Rather they may be local, pointing in different directions in different spacetime regions. Local arrows can therefore be consistent with global time symmetry.