 One important interpretive issue where unitarily inequivalent representations (UIRs) play a crucial role is the Unruh effect: a uniformly accelerated observer in a Minkowski vacuum should detect a thermal bath of particles, the so-called Rindler quanta (Unruh 1976, Unruh &amp; Wald 1984).  The Unruh effect constitutes a severe challenge to a particle interpretation of QFT, because it seems that the very existence of the basic entities of an ontology should not depend on the state of motion of the detectors. Teller (1995: 110–113) tries to dispel this problem by pointing out that while the Minkowski vacuum has the definite value zero for the Minkowski number operator, the particle number is indefinite for the Rindler number operator, since one has a superposition of Rindler quanta states. This means that there are only propensities for detecting different numbers of Rindler quanta but no actual quanta. However, this move is problematic since it seems to suggest that quantum physical propensities in general don’t need to be taken fully for real.