I will describe ongoing work on the thermodynamics of quantum fields in far-from-equilibrium states. The key tool is modular flow, a nonstandard time-evolution map defined relative to a choice of state, which makes that state "look thermal." Famously, the modular flow for the Minkowski vacuum in the Rindler wedge is a geometric boost, which is one way of stating the Unruh effect. In this talk, I will outline a characterization of the settings in which modular flow is geometrically local, i.e., a complete list of "generalized Unruh effects" in arbitrary spacetimes and for arbitrary quantum field theories. The arguments involve analytic manipulations of position-space correlators, which may be of independent interest to those of you working on amplitudes.