Eugene Y. S. Chua, Nanyang Technological University, Singapore
18/05/2026 14:30h, Sala Consiglio VII piano / 7th floor, DMAT, Politecnico di Milano,
Abstract: There is a tension in the foundations of thermodynamics. Philosophers argue that certain non-classical systems — black holes, relativistic systems, self-gravitating systems — do not ‘really’ behave thermodynamically. Nonetheless, physicists continue to extend thermodynamic — or thermodynamic-like — concepts into these and other exotic regimes. We situate and analyze this tension for the concepts of thermal equilibrium and equilibration in quantum statistical mechanics. First, we show how these concepts break down in three classes of quantum systems heavily discussed in quantum statistical mechanics: integrable systems, many-body localized systems, and periodically driven systems. Second, physicists nevertheless aim at — and succeed in — Â recovering other equilibrium-like concepts in and around these regimes, each with their own limited domains of applicability. While these concepts are not the classical ones, we argue that these concepts instantiate a broader mode of generalized equilibrium reasoning — a search for generalized equilibria with the following features: the scale-relative search for stationarity, stability under perturbation, and equilibration. We end by ruminating on whether it matters if such states are “really” thermodynamic, and on the role of generalized equilibrium reasoning in physics.