The Unruh Effect in Relativistic Fluids

Abstract: We identify a relativistic-fluid counterpart of the Unruh effect, in which a comoving probe measures a \emph{Thermodynamic Unruh temperature}. Frame changes in first-order hydrodynamics can be recast as a local, time-dependent hyperbolic rotation in a Rindler-style state space; the instantaneous map between frames is the \emph{Thermodynamic Boost}, and its proper-time variation defines the \emph{Thermodynamic Acceleration} that imprints a thermal spectrum. Using the Israel--Stewart theory and extending it to modern relativistic fluid theories,we show that this temperature is frame-independent to leading order and universal for out-of-equilibrium relativistic fluids.