Nonlinear particle detectors across the Rindler firewall

Abstract

We investigate Unruh-DeWitt detectors coupled to composite observables of a quantum scalar field, including quadratic coupling to the field momentum and coupling to the local energy density. We develop a distributional framework for evaluating the corresponding detector response functions and apply it to detectors crossing the Rindler firewall. While we recover the finite response of the derivative-coupling model, we show that quadratic momentum coupling leads to ill-defined products of distributions and unavoidable formal $δ(0)$-type divergences. Since the local energy-density response is closely tied to the quadratic momentum response, our results provide strong evidence that the standard sharp firewall model is incompatible with nonlinear detector couplings to local observables. Our analysis further suggests that these pathologies originate from the discontinuous severing of correlations across the Rindler horizon, rather than from the detector model itself.