According to the Unruh effect, an observer moving with uniform proper acceleration a, perceives the Minkowski vacuum as a heat bath at the Unruh temperature T=a/2pi. Unruh effect is usually demonstrated by means of an Unruh-DeWitt detector, which is a point-like quantum system interacting with a quantum field and moving along a path in Minkowski spacetime. The detector's excitation rate is calculated to leading order in perturbation theory and is found to follow a Planck distribution at the Unruh temperature.
In our talk, we argue that the above approach to the Unruh effect has a restricted domain of applicability. We motivate the treatment of Unruh-DeWitt detectors as open quantum systems, with the quantum field playing the role of the environment, and evaluate the response of a uniformly accelerated detector for different types of interaction between the detector and the field. Our analysis suggests that the relation between acceleration and temperature is best expressed in terms of the asymptotic state of the detector.