What do detectors detect?

TL;DR

This paper reviews the concept of quantum detectors, their responses in various scenarios, and explores phenomena like inversion of statistics and Planck-scale modifications, highlighting their nuanced interpretation beyond simple particle detection.

Contribution

It provides a comprehensive analysis of detector responses in different quantum field interactions, including nonlinear couplings, spacetime dimensions, and quantum gravitational effects.

Findings

Detector responses can differ from actual particle content.

Inversion of statistics occurs in odd dimensions with odd couplings.

Planck-scale modifications affect detector responses in flat spacetime.

Abstract

By a detector, one has in mind a point particle with internal energy levels, which when set in motion on a generic trajectory can get excited due to its interaction with a quantum field. Detectors have often been considered as a helpful tool to understand the concept of a particle in a curved spacetime. Specifically, they have been used extensively to investigate the thermal effects that arise in the presence of horizons. In this article, I review the concept of detectors and discuss their response when they are coupled linearly as well as non-linearly to a quantum scalar field in different situations. In particular, I discuss as to how the response of detectors does not necessarily reflect the particle content of the quantum field. I also describe an interesting `inversion of statistics' that occurs in odd spacetime dimensions for `odd couplings', i.e. the response of a uniformly accelerating detector is characterized by a Fermi-Dirac distribution even when it is interacting with a scalar field. Moreover, by coupling the detector to a quantum field that is governed by a modified dispersion relation arising supposedly due to quantum gravitational effects, I examine the possible Planck scale modifications to the response of a rotating detector in flat spacetime. Lastly, I discuss as to why detectors that are switched on for a finite period of time need to be turned on smoothly in order to have a meaningful response.