$v_2$) collectivity below 4 AGeV. Right: CERES[[@Agakichiev:2003gg]]{} data on elliptic flow at SPS is well below hydrodynamic predictions with freeze-out $T_f=120$ MeV required to reproduce the single inclusive radial flow. Early freeze-out with $T_f=160$ MeV, simulating effects of dissipation, is needed to reproduce the data.[]{data-label="spsv2"}](Danielv2.eps "fig:"){width="5cm" height="6cm"} ![Evidence for dissipative collective flow below RHIC energies. Left: Non equilibrium BUU nuclear transport theory [[@Danielewicz:2002pu; @Stoicea:2004kp]]{} can explain the observed elliptic squeeze-out (negative $v_2$) collectivity below 4 AGeV. Right: CERES[[@Agakichiev:2003gg]]{} data on elliptic flow at SPS is well below hydrodynamic predictions with freeze-out $T_f=120$ MeV required to reproduce the single inclusive radial flow. Early freeze-out with $T_f=160$ MeV, simulating effects of dissipation, is needed to reproduce the data.[]{data-label="spsv2"}](CERES_v2fig1.eps "fig:"){width="5cm"}

In order to account for the smallness of $v_2$ at SPS, hydrodynamics has to be frozen out at unphysically high $T_f\approx T_c=160$ MeV. However, the observed radial flow rules out this simple fix.

The discrepancy of $v_2$ and hydro at SPS energies can be traced to the important contribution of the dissipative final [*hadronic*]{} state interactions. The hadronic fluid is far from ideal. In approaches [@Bass:2000ib; @Teaney:2001av] that combine perfect fluid QGP hydrodynamics with non-equilibrium hadronic transport dynamics, the importance of dissipative hadron 