Würzburg Seminar on Quantum Field Theory and Gravity

Energy detectors are non-local operators on future null infinity measuring energy to a power Δ−2 in four-dimensional gauge theories and gravity. These operators transform as conformal primaries on the celestial sphere and provide a natural basis for describing energy-flux observables in scattering processes. I will derive the universal leading structure of the operator product expansion of this family of operators and discuss how they are related to the soft charges associated with BMS symmetry (or more generally asymptotic symmetries) of the S-matrix. The talk is based on a recent paper with Hernán González (2510.27348)