  Radiation 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.
