The Measurement Problem in QFT

A theoretical issue at the heart of the Measurement Problem in non-relativistic quantum mechanics (NRQM) is that the measurement theory (i.e., the Born rule and state update rules) cannot be derived from the application of the dynamical theory to a measurement scenario.

What form does the Measurement Problem take in relativistic quantum field theory (QFT), when both the measurement theory and the dynamical theory are relativistic? This question has recently become more tractable with the development of a number of measurement theories for QFT that, in contrast with the longstanding tradition in QFT, represent measurements as occurring in local spacetime regions. I will argue that the Measurement Problem takes a substantially different form in QFT than it does in NRQM. Two characteristic features of the Measurement Problem in NRQM do not carry over to the Measurement Problem in QFT. First, in QFT the Measurement Problem does not take the form of an apparent incompatibility between the post-measurement states prescribed by the dynamics and the state update rule. Second, the state update rules in the measurement theory for QFT do not describe an effective measurement collapse:  the state update rules do not admit a literal interpretation as representing a physical, measurement-induced change of state that occurs in some region of spacetime. These differences in the form that the Measurement Problem takes in QFT carry implications for the interpretation of QFT.