  The QCD matter produced in nuclear collisions at the Relativistic Heavy Ion
Collider (RHIC) has been found to have a very low shear viscosity, which is
close to the lower bound allowed by unitarity. The matter has also been found
to strongly suppress the emission of energetic hadrons. This phenomenon, called
"jet quenching" is interpreted to be the result of a large energy loss by the
precursor parton on its path through the dense matter, primarily due to gluon
radiation. I discuss how the two phenomena are related. The RHIC data suggest,
in some scenarios of jet quenching, that the quark-gluon plasma created in
nuclear collisions is characterized by strong coupling, but still admits a
quasi-particle description.
