  We construct a model of high energy heavy ion collisions as two
ultrarelativistic shock waves colliding in AdS_5. We point out that shock waves
corresponding to physical energy-momentum tensors of the nuclei completely stop
almost immediately after the collision in AdS_5, which, on the field theory
side, corresponds to complete nuclear stopping due to strong coupling effects,
likely leading to Landau hydrodynamics. Since in real-life heavy ion collisions
the large Bjorken x part of nuclear wave functions continues to move along the
light cone trajectories of the incoming nuclei leaving the small-x partons
behind, we conclude that a pure large coupling approach is not likely to
adequately model nuclear collisions. We show that to account for small-coupling
effects one can model the colliding nuclei by two (unphysical)
ultrarelativistic shock waves with zero net energy each (but with non-zero
energy density). We use this model to study the energy density of the
strongly-coupled matter created immediately after the collision. We argue that
expansion of the energy density in the powers of proper time squared
corresponds on the gravity side to a perturbative expansion of the metric in
graviton exchanges. Using such expansion we reproduce our earlier result that
the energy density of produced matter at mid-rapidity starts out as a constant
(of time) in heavy ion collisions at large coupling.
