  Fast shocks that form in optically thick media are mediated by Compton
scattering and, if relativistic, pair creation. Since the radiation force acts
primarily on electrons and positrons, the question arises of how the force is
mediated to the ions which are the dominant carriers of the shock energy. It
has been widely thought that a small charge separation induced by the radiation
force generates electric field inside the shock that decelerates the ions. In
this paper we argue that, while this is true in sub-relativistic shocks which
are devoid of positrons, in relativistic radiation mediated shocks (RRMS),
which are dominated by newly created $e^+e^-$ pairs, additional coupling is
needed, owing to the opposite electric force acting on electrons and positrons.
Specifically, we show that dissipation of the ions energy must involve
collective plasma interactions. By constructing a multi-fluid model for RRMS
that incorporates friction forces, we estimate that momentum transfer between
electrons and positrons (and/or ions) via collective interactions on scales of
tens to thousands proton skin depths, depending on whether friction is
effective only between $e^+e^-$ pairs or also between pairs and ions, is
sufficient to coupe all particles and radiation inside the shock into a single
fluid. This leaves open the question whether in relativistic RMS particles can
effectively accelerate to high energies by scattering off plasma turbulence.
Such acceleration might have important consequences for relativistic shock
breakout signals.
