Rhic ∗

Hydrodynamical models have certain advantages over transport model calculations in describing heavy ion collisions. One of the most important is that, once the equation of state and initial conditions of the matter are specified, the evolution of the system is determined. No knowledge of the underlying microscopic processes is required. This is especially important when studying the predicted phase transition from hadronic to partonic degrees of freedom (and vice versa) – a process for which details are still unknown. The hydrodynamical description is relatively simple and fulfills the conservation laws without additional constraints. The use of familiar concepts like temperature, pressure and flow velocity also provides an intuitive and transparent picture of the evolution. The price to be paid for these advantages is a set of bold assumptions: local kinetic and chemical equilibrium and lack of dissipation. This set of assumptions may or may not be valid in such a small system as that formed in a heavy ion collision. In a hydrodynamical description the evolution is assumed to proceed as follows: In the initial collision a large fraction of the kinetic energy of the colliding nuclei is used to create many secondary particles in a small volume. These particles will collide with each other sufficiently often to reach a state of local thermal equilibrium. When the system has reached local equilibrium