the pseudo rapidity dependence of elliptic from PHOBOS .[]{data-label="line1figb"}](PHOBOS_v2_eta.eps "fig:"){height="0.3\textheight" width="45.00000%"}

The long range nature of collective flow has also been conclusively established by STAR and PHOBOS seen in Fig.(\[line1figb\]). The sidewards flow is anti-correlated over 8 units of rapidity! In addition, its azimuthal orientation was shown to coincide with the azimuthal direction of the largest axis of elliptic deformation at $y=0$. This provides an important test of the overall consistency of the hydrodynamic origin of flow. The rapidity dependence of the elliptic flow in Fig.(\[line1figb\]) also shows the long range nature of bulk collectivity.

Why is $v_2$ more emphasized than $v_1$ or radial flow as a signature of QGP formation? The primary reason is that elliptic flow is generated mainly during the highest density phase of the evolution before the initial geometric spatial asymmetry of the plasma disappears. It comes from the azimuthal dependence of the pressure gradients, which can be studied by varying the centrality of the events [@Ollitrault:bk]. Detailed parton transport [@Molnar:2001ux] and hydrodynamic [@Teaney:2001av] calculations show that most of the $v_2$ at RHIC is produced before 3 fm/c and that elliptic flow is relatively insensitive to the late stage dissipative expansion of the hadronic phase. In 