The Unruh effect is a prediction in quantum field theory that suggests an observer accelerating through a vacuum will perceive that vacuum as a warm bath of particles, or thermal radiation, while an inertial observer would see no particles at all. This phenomenon was first proposed by physicist William Unruh in 1976.
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An observer at constant proper acceleration in the Minkowski vacuum detects a thermal response at this temperature. The associated Rindler coordinates separate the observer's accessible wedge from a horizon. This is an observer-dependent particle/detector response, not an assertion that inertial observers measure a thermal bath.