As I read “An electron–proton collider could bridge the gap between the LHC and its successor” (Physics Today, May 2017, page 29) and how it would serve in high-precision studies of Higgs decays as a portal to new physics, I was disappointed. I saw no mention of a long-standing problem in connection with the electron: What fraction of the electron’s mass is due to its interaction with the quantized electromagnetic field? Despite its enormous success in quantum electrodynamics, renormalization does not solve the problem, nor does it even tell us how to tackle it. Furthermore, the Higgs contribution to the electron’s mass is unknown. We also don’t know how to measure those respective contributions. Perhaps in thinking about the electron–proton collider, one should be thinking about opening portals to these long-neglected areas as well.

Physics Today