Scattering amplitudes in perturbative QED and gravity suffer from infrared divergences. These well understood divergences arise from our inability to distinguish between a “hard” particle with or without “soft hair”. (In non-abelian theories there are also collinear IR divergences. As these are absent in QED (and also in gravity) we will not discuss them here.) Hence, a physical asymptotic state entering any scattering process is a super-position of the hard particle with all possible soft hair arrangements. The way to construct the physical asymptotic states that yield an IR safe S-matrix was put forward by Kulish and Faddeev [10]. In their construction, the dressed incoming and outgoing states diagonalize the interacting asymptotic Hamiltonian. Once the S-matrix of these dressed electrons is expanded in perturbation theory, the cancellation of IR divergences is ensured by the existence of soft theorems.https://arxiv.org/pdf/1607.08599.pdf