Subsystem evolution speed as indicator of relaxation

For dynamical quantum matter, many key features and properties address their steady states, such as the fundamental question of thermalization. However, it is a very concrete challenge to determine practically if a system has already relaxed and reached a steady state, as this requires an a priori understanding of steady-state properties, which must, in principle, be verified across a large set of observables individually. In this paper, we introduce a general method to quantify the relaxation of a quantum state by measuring the speed of subsystem evolution. This approach allows us to assess the relaxation of all conceivable observables, providing a general measure of relaxation without requiring any a priori knowledge.