  High energy heavy ion collisions at the Relativistic Heavy Ion Collider
(RHIC) produce a novel medium characterized by an initial energy density over
an order of magnitude above the expected phase transformation value and that
then evolves as a nearly inviscid liquid. Probing the medium with
auto-generated particles is a key methodology to quantitatively determine the
medium properties. Pairs of heavy quarks are an excellent probe since their
spatial separation to form various quarkonia states spans the relevant range of
color screening lengths in the medium. In this proceedings, we describe results
from the PHENIX experiment on $J/\psi$ production and discuss initial physics
implications of the measurements.
