Abstract. We discuss the roles of viscosity in relativistic fluid dynamics from the point of view of memory effects. Depending on the type of quantity to which the memory effect is applied, different terms appear in higher order corrections. We show that when the memory effect applies on the extensive quantities, the hydrodynamic equations of motion become non-singular. We further discuss the question of memory effect in the derivation of transport coefficients from a microscopic theory. We generalize the application of the Green-Kubo-Nakano (GKN) to calculate transport coefficients in the framework of projection operator formalism, and derive the general formula when the fluid is non-Newtonian.
Non-Newtonian Nature of a Dissipative Fluid in Relativistic RegimeThe effect of dissipation in relativistic fluids is one of current topics in the physics of relativistic heavy-ion collisions [1]. Here, we discuss this problem focusing on the memory effect on irreversible current. It is well-known that in a simple covariant extension of the Navier-Stokes theory there appears the problem of relativistic acausality and instability associated with it. First let us illustrate using the example of diffusion equation, the basic idea of memory effect as the solution for the problem of acausality in relativistic hydrodynamics.In the usual derivation of diffusion equation, we assume that the irreversible current J(t) is simply proportional to the corresponding thermodynamic force F (t),where D is a transport coefficient. The fluid whose irreversible current posseses this property is referred to as a Newtonian fluid, and the evolution is described by, for example, the Navier-Stokes equation. However, to be exact, there should exist some time retardation effect in generating the current in the medium, when a thermodynamic force is applied (remember the linear response theory). The expression (1) is justified only when there is clear separation of microscopic and macroscopic scales and the retardation effect is negligible. This assumption is usually satisfied in fluid around us, where the