Zero mode, Zero influence: Spatial-derivative coupling in particle detectors

arXiv:2611.00309v1 Announce Type: new Abstract: It is common in particle detector models in cavity QED to often omit zero modes or assume their state, compromising causality or introducing ambiguity in their predictions. We show that spatial-derivative coupling eliminates both problems. For free massless scalar fields in static periodic or Neumann cavities of any spatial dimension, cavity-frame spatial derivatives annihilate the zero-mode operator while preserving microcausality. Detector dynamics and entanglement harvesting then depend only on the reduced detector-oscillator state, nonperturbatively and even with initial correlations with the zero mode. Time derivatives and moving-detector rest-frame spatial derivatives can retain zero-mode state dependence not compromising causality.