Parton Energy Loss and Jet Tomography: Probing the ``Perfect Liquid'' at RHIC

Experimental evidence from RHIC strongly suggests that matter having an energy density far in excess of the value required for the creation of a deconfined phase is produced in ultrarelativistic Au+Au collisions at $\sqrt{s_{_{\rm NN}}}$=200~GeV. This matter thermalizes rapidly, is strongly interacting, and displays hydrodynamic properties akin to a fluid with very low viscosity. Studies of the interaction of hard scattered partons with this matter provide an important probe of its properties.