We review the properties of the cluttered Minkowski vacuum. In particular we discuss the example of a uniformly accelerated quantum oscillator in the Minkowski vacuum showing that it does not radiate. Equivalently, the presence of the oscillator does not lead to decoherence (i.e. the emergence of classical probabilities). Mach's Principle was related originally by Einstein to the non-existence of (classical) vacuum cosmological models. We speculate that Mach's Principle may acquire a quantum role as a condition for decoherence of the universe.

INTRODUCTION

Following Hawking's announcement (Hawking 1974,1975) of his result that black holes radiate a thermal flux, Davies (1975) applied an analogous technique to the spacetime of a uniformly accelerated observer in the Minkowski vacuum in the presence of a reflecting wall. He interpreted the result as a flux of radiation from the wall at a temperature ha/4π2ck, where a is the acceleration of the observer. Unruh (1976) independently showed that the Minkowski vacuum appears as a thermal state to any uniformly accelerated detector, the normal modes of which were defined with respect to its own proper time. There is no flux from the horizon but the detector is raised to an excited state with its levels populated according to a Boltzmann distribution at a temperature ha/4π2ck as it would be in an inertial radiation bath at this temperature.