  Anisotropic transverse flow is studied in Pb+Pb and Au+Au collisions at SPS
and RHIC energies. The centrality and transverse momentum dependence at
midrapidity of the elliptic flow coefficient v_2 is calculated in the
hydrodynamic and low density limits. Hydrodynamics is found to agree well with
the RHIC data for semicentral collisions up to transverse momenta of 1-1.5
GeV/c, but it considerably overestimates the measured elliptic flow at SPS
energies. The low density limit LDL is inconsistent with the measured magnitude
of v_2 at RHIC energies and with the shape of its p_t-dependence at both RHIC
and SPS energies. The success of the hydrodynamic model points to very rapid
thermalization in Au+Au collisions at RHIC and provides a serious challenge for
kinetic approaches based on classical scattering of on-shell particles.
