The Pragmatic QFT Measurement Problem and the need for a Heisenberg-like Cut in QFT

Abstract

Despite quantum theory's remarkable success, many philosophers worry that it nonetheless lacks some crucial connection between theory and experiment. One under-discussed aspect of the Quantum Measurement Problems is that it is sometimes unclear how to model our measurement processes in order to extract experimental predictions. Without a solution to these pragmatic worries, quantum theory would be at risk of losing both its evidential support and its physical salience. Avoiding these risks requires solving the Pragmatic Measurement Problem. For non-relativistic quantum theory, this problem has been solved as follows: One can model each of quantum theory's key experimental successes on a case-by-case in terms of measurement chains and Heisenberg cuts. From here, one can then strive for a wide-scoping measurement theory capable of modeling all (or nearly all) possible measurement processes. Indeed, for non-relativistic quantum theory this leads us to our usual projective measurement theory. But how does this story have to change when we move into the context of quantum field theory (QFT)? It is well known that in QFT almost all localized projective measurements violate causality, allowing for faster-than-light signaling. Despite this, I will argue that we can proceed largely as we did in the non-relativistic case. We first ought to build up a case-by-case measurement framework for QFT by using measurement chains and Heisdenberg-like cuts (where we switch from a QFT model to a non-QFT model). We can then strive for both a new measurement theory for QFT and an empirically meaningful characterization of its observables. It is at this point that significantly more theoretical work is needed. This paper ends by briefly reviewing the state of the art in the physics literature regarding the modeling of measurement processes involving quantum fields.