Christopher J. Fewster
University of YorkMeasurement in Quantum Field Theory: Problems and Solutions

Many presentations of quantum mechanics include a postulate that the state of a system undergoes an instantaneous change following a measurement. This is clearly incompatible with special and general relativity and raises questions concerning the description of measurement in quantum field theory (QFT). Attempts to extend measurement postulates to QFT by hand have produced pathologies, such as the “impossible measurements” described long ago by Sorkin. I will present a recent operational approach to these questions, which models measurement of one quantum field (the system) by coupling it to another (the probe). This is all accomplished in a model-independent way within algebraic quantum field theory (AQFT), and therefore applies to general QFTs. The resulting framework provides a description of measurement in QFT that is causal, covariant and consistent, includes state update rules for selective and non-selective measurements that are derived from the formalism, and works equally well in flat or curved spacetimes. As an application, It will be shown how the “impossible measurement” problem is resolved. No prior knowledge of AQFT will be assumed.