  We investigate the error induced by only considering binary collisions in the
momentum transport of hard-sphere granular materials, as is done in kinetic
theories. In this process, we first present a general microscopic derivation of
the momentum transport equation and compare it to the kinetic theory
derivation, which relies on the binary collision assumption. These two
derivations yield different microscopic expressions for the stress tensor,
which we compare using simulations. This provides a quantitative bound on the
regime where binary collisions dominate momentum transport and reveals that
most realistic granular flows occur in the region of phase space where the
binary collision assumption does not apply.
