Infrared Cancellation and Measurement

Quantum field theories containing massless particles are divergent not just in the ultraviolet but also in the infrared. Infrared divergences are typically regarded as less conceptually problematic than ultraviolet divergences because there is a cancellation mechanism that renders measurable physical observables such as decay rates and cross-sections infrared finite. In this article, I scrutinize the restriction to measurable physical observables that is required to arrive at infrared finite results. I argue that the restriction does not necessitate a retreat to operationalism about the meaning of the theory, but it does call attention to underappreciated features of the infrared regime.