  The redistribution in momentum space of heavy quarks via their interactions
in the quark-gluon plasma is an excellent probe of the heavy quark coupling
strength to the medium. We utilize a Monte Carlo Langevin calculation for
tracking heavy quark - antiquark pairs embedded in a viscous hydrodynamic
space-time evolution. We find that the nuclear modification factor ($R_{AA}$)
for charm quarks is relatively insensitive to the coupling to the quark-gluon
plasma at early times where the highest temperatures are acheived. In contrast
the azimuthal angular correlation of charm and anticharm quarks is extremely
sensitive to the early time evolution. For beauty quarks the situation is
reversed in terms of sensitivity. Future measurements of heavy quarks have the
potential to map out the temperature dependence of the shear viscosity to
entropy density ratio ({\eta}/s).
