From Time-symmetric Microscopic Dynamics to Time-asymmetric Macroscopic Behavior: An Overview

TL;DR

This overview explains how time-asymmetric macroscopic behavior, like the second law of thermodynamics, naturally emerges from time-symmetric microscopic laws considering scale disparity, initial conditions, and system observations.

Contribution

It clarifies the origins of the second law of thermodynamics from microscopic dynamics, contrasting it with other explanations and discussing quantum extensions.

Findings

Time asymmetry arises from initial conditions and scale differences.

Classical and quantum extensions of thermodynamic principles are discussed.

Features like diffusion depend on dynamical chaos.

Abstract

Time-asymmetric behavior as embodied in the second law of thermodynamics is observed in {\it individual macroscopic} systems. It can be understood as arising naturally from time-symmetric microscopic laws when account is taken of a) the great disparity between microscopic and macroscopic scales, b) a low entropy state of the early universe, and c) the fact that what we observe is the behavior of systems coming from such an initial state--not all possible systems. The explanation of the origin of the second law based on these ingredients goes back to Maxwell, Thomson and particularly Boltzmann. Common alternate explanations, such as those based on the ergodic or mixing properties of probability distributions (ensembles) already present for chaotic dynamical systems having only a few degrees of freedom or on the impossibility of having a truly isolated system, are either unnecessary, misguided or misleading. Specific features of macroscopic evolution, such as the diffusion equation, do however depend on the dynamical instability (deterministic chaos) of trajectories of isolated macroscopic systems. The extensions of these classical notions to the quantum world is in many ways fairly direct. It does however also bring in some new problems. These will be discussed but not resolved.