Cage Escapes Cause Cage Escapes

A persistent question regarding the dynamics of supercooled liquids and glasses is whether the increasing strength and range of spatiotemporal correlations in these dynamics as temperature decreases is causal, with local plastic rearrangements perturbing their neighborhoods and causing new rearrangements to occur nearby, or merely correlational, resulting from correlations of hidden variables in the structure that might be predicted via a purely thermodynamic theory. We introduce a simple test which shows that while correlations of structure likely to influence the dynamics are present, these can only account for part of the dynamical correlations seen in particle simulations. In supercooled liquids, the question of causation is further complicated by time reversibility in equilibrium. We discuss how to reconcile causation with time reversibility, and discuss some of its consequences.