Abstract: There is no well-defined notion of a particle in quantum field theory in curved spacetime due to the lack of global symmetries. The standard procedure in quantum field theory is to treat fields rather than particles as the fundamental object of interest. Nevertheless, in a seminal 1976 paper by W. G. Unruh, an operational meaning was given to the particle concept by examining the absorption and emission of field quanta by a two-level atom. This is the so-called Unruh-DeWitt detector and, in this operational sense,we say a particle is what a particle detector detects!

Computing the response of an Unruh-DeWitt detector in black hole spacetimes is technically challenging – a fact which has largely limited the literature to stationary detectors. In this talk, we consider the response of an Unruh-DeWitt detector near an extremal charged black hole, modelling the near-horizon region of this extremal spacetime by the Bertotti-Robinson spacetime. The advantage of employing the Bertotti-Robinson limit is that the two-point functions for a massless scalar field are obtainable in closed form for a number of quantum states of interest, allowing us to explore the full gamut of the parameter space for a broader range of stationary and non-stationary trajectories than is typically possible. Particular attention will be paid to the thermalization of the detector as we look for evidence of the proposed anti-Unruh and anti-Hawking effects.