Causality and Scattering Time delays

Since the 1950’s it has been understood that causality imposes restrictions on the time delays incurred in scattering events, and this in turn places useful constraints on the S-matrix. I will review the classic and modern incarnations of this idea, including its realization in spherically symmetric and pp-wave spacetimes, and how it can be used to put constraints on effective field theories in a complementary way to positivity bounds.