2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/84846Radiation reaction is revisited, first in a new classical aproach, where the physical particle 4-momentum is redefined as the energy-momentum flux across the future light cone and is not parallel to the 4-velocity. Then in a semi-classical approach, it is shown that, when emitting a photon, the particle "side-slips" transversaly to its initial momentum, justifying the non-colinearity between momentum and mean velocity. Side-slipping is finally checked in a pure quantum mechanical treatment of synchrotron radiation.8 pages, 6 figures. presented at the Int. Conf. QEDSP 2001, dedicated to the 90th anniversary of A.I.Akhiezer, Kharkov (Ukraine) Oct. 30 - Nov. 3, 2001 To appear in the proceedingsClassical PhysicsHigh Energy Physics - PhenomenologyGeneral Physicstext