Haag as a How-To Theorem

Abstract

Haag's theorem is a classic no-go theorem. It rigorously demonstrates there is a logical problem with the interaction picture (IP), one of the most widely used modeling tools in quantum field theory (QFT). The significance of the theorem for the use of the IP in QFT has been the subject of long-running debate, focused around how ``worried'' we should be. In this paper, we argue for an alternative and opposite perspective on Haag's theorem, rejecting the `worry' framing in favor of emphasizing the no-go theorem's implications for model development.