Constraints on Reversing the Thermodynamic Arrow of Time from Black Hole Thermodynamics, Wormholes, and Time-Symmetric Quantum Mechanics

TL;DR

This paper investigates whether the thermodynamic arrow of time can be reversed within a single universe using semiclassical gravity, black holes, wormholes, and time-symmetric quantum mechanics, concluding that such reversal is not possible under current constraints.

Contribution

It introduces a 'Global Entropy Transport' framework and derives inequalities showing entropy cannot be genuinely reversed in a single universe with current physical laws.

Findings

Black holes and wormholes can redistribute entropy but not reverse it.

The derived inequalities bound the net decrease of entropy, preventing true reversal.

Current constraints uphold the thermodynamic arrow of time in a single universe.

Abstract

Can the thermodynamic arrow of time in a single universe be reversed, even temporarily, within semiclassical gravity without invoking additional universes or branches? We address this question in a single, connected spacetime where quantum field theory is coupled to classical general relativity, and where black holes, traversable wormholes, and time-symmetric or retrocausal formulations of quantum mechanics might naively appear to open channels for entropy export or cancellation. After distinguishing fine-grained, coarse-grained, and generalized gravitational entropy, and formulating a cosmological coarse-grained entropy, we treat black hole evaporation, wormholes constrained by quantum energy inequalities, and two-time boundary-value frameworks (including absorber-type and two-state-vector formalisms) within a common information-theoretic language. We then introduce a "Global Entropy Transport" (GET) framework and derive a sectoral inequality that bounds the net decrease of matter-plus-radiation entropy in terms of changes in horizon area and correlation (mutual-information) terms, assuming the generalized second law and modern focusing and energy conditions. Within this framework, black holes, wormholes, and retrocausal protocols can at most redistribute entropy among matter, radiation, and gravitational sectors and reshape the local pattern of entropy production. They do not, under current semiclassical, holographic, and statistical-mechanical constraints, permit a genuine reversal of the universal thermodynamic arrow in a single connected universe.