Asymmetry, abstraction and autonomy: justifying coarse-graining in statistical mechanics

Whilst the fundamental laws of physics are time-reversal invariant, most macroscopic processes are irreversible. Given that the fundamental laws are taken to underpin all other processes, how can the fundamental time-symmetry be reconciled with the asymmetry manifest elsewhere? In statistical mechanics, progress can be made with this question; what I dub the Zwanzig-Zeh-Wallace framework can be used to construct the irreversible equations of statistical mechanics from the underlying microdynamics. Yet this framework uses coarse-graining, a procedure that has faced much criticism. I focus on two objections in the literature: claims that coarse-graining makes time-asymmetry (i) ‘illusory’ and (ii) ‘anthropocentric’. I argue that these objections arise from an unsatisfactory justification of coarse-graining prevalent in the literature, rather than from coarse-graining itself. This justification relies on the idea of measurement imprecision. By considering the role that abstraction and autonomy play, I provide an alternative justification and offer replies to the illusory and anthropocentric objections. Finally I consider the broader consequences of this alternative justification: the connection to debates about inter-theoretic reduction and further, the implication that the time-asymmetry in statistical mechanics is weakly emergent.