  We argue that isotropization and, consequently, thermalization of the system
of gluons and quarks produced in an ultrarelativistic heavy ion collision does
not follow from Feynman diagram analysis to any order in the coupling constant.
We conclude that the apparent thermalization of quarks and gluons, leading to
success of perfect fluid hydrodynamics in describing heavy ion collisions at
RHIC, can only be attributed to the non-perturbative QCD effects not captured
by Feynman diagrams.
  We proceed by modeling these non-pertrubative thermalization effects using
viscous hydrodynamics. We point out that matching Color Glass Condensate inital
conditions with viscous hydrodynamics leads to continuous evolution of all
components of energy-momentum tensor and, unlike the case of ideal
hydrodynamics, does not give a discontinuity in the longitudinal pressure. An
important consequence of such a matching is a relationship between the
thermalization time and shear viscosity: we observe that small viscosity leads
to short thermalization time.
