  We compute and study the drag force acting on a heavy quark propagating
through the matter produced in the collision of two sheets of energy in a
strongly coupled gauge theory that can be analyzed holographically. Although
this matter is initially far from equilibrium, we find that the equilibrium
expression for heavy quark energy loss in a homogeneous strongly coupled plasma
with the same instantaneous energy density or pressure as that at the location
of the quark describes many qualitative features of our results. One
interesting exception is that there is a time delay after the initial collision
before the heavy quark energy loss becomes significant. At later times, once a
liquid plasma described by viscous hydrodynamics has formed, expressions based
upon assuming instantaneous homogeneity and equilibrium provide a
semi-quantitative description of our results - as long as the rapidity of the
heavy quark is not too large. For a heavy quark with large rapidity, the
gradients in the velocity of the hydrodynamic fluid result in qualitative
consequences for the 'drag' force acting on the quark. In certain
circumstances, the force required to drag the quark through the plasma can
point opposite to the velocity of the quark, meaning that the force that the
plasma exerts on a quark moving through it acts in the same direction as its
velocity. And, generically, the force includes a component perpendicular to the
direction of motion of the quark. Our results support a straightforward
approach to modeling the drag on, and energy loss of, heavy quarks with modest
rapidity in heavy ion collisions, both before and after the quark-gluon plasma
hydrodynamizes, and provide cautionary lessons at higher rapidity.
