PhD Colloquium: Chris Fewster (U. of York)

Undergraduate courses in quantum mechanics pay a lot of attention to the rules for measurement, often presented in terms of an instantaneous collapse of the state vector. Meanwhile, undergraduate courses in special relativity make a point of the relativity of simultaneity. With this in mind, one might expect that courses in quantum field theory (QFT), which combines quantum theory and relativity, would address this apparent contradiction - but instead one encounters silence on the matter. In this lecture I will explain why the topic of measurement in quantum field theory presented more problems than solutions for many years, and describe a recently developed framework that clarifies how (and why) states should be updated in QFT following measurements, and which has among other things resolved the long-standing problem of "impossible measurements". The framework fits within an approach to QFT called algebraic quantum field theory, which I will explain. No prior knowledge of QFT will be assumed.