Learning Hydrodynamics with Yogiro Hama

A complete history of relativistic hydrodynamics is yet to be written. Some partial accounts can be found in [1, 2]. Here, I just want to remember some seminal works which started this field, which might be called hydrodynamics applied to multiparticle production in high energy hadronic collisions. In his pioneering work of 1953 [3] Landau developed several aspects of hydrodynamics and prepaired the formalism for future phenomenological applications. In the following year Khalatnikov [4] found the analytical solution of the one dimensional problem. These brilliant works went forgotten for many years. When remembered, they were regarded with suspition, if not with a frank opposition. About twenty years later, Carruthers used Landau hydrodynamics to study the, at that time, new data from the CERN ISR proton proton collisions [5]. The phenomenological success of this approach renewed the interest on hydrodynamics and, in particular, motivated the search for the three-dimensional solution of the Landau model. This first success of hydrodynamics found also opponents, for whom this type of collective treatment of a microscopic problem was, at best, a parametrization of data, which was successful only because it was an implementation of energy-momentum conservation applied to a system whose dynamics was dominated by phase space. Still according to these opponents, there was no chance that hydrodynamics would describe a real system because there was no fluid! The number of produced particles was too small and the lifetime of the system was so short that no thermal equilibrium would ever be achieved. It was in this period of phenomenological success but theoretical discredit, that Yogiro got interested in this subject.