A detailed study of entropy jump across shock waves in relativistic fluid dynamics

A detailed study of the function eta which characterizes entropy jump across shock waves is carried out for relativistic hydrodynamics at thermal equilibrium. It is shown that the function eta is defined only if the normal velocity of the shock waves does not exceed the speed of light in vacuo, consistently with the claims of relativity; moreover, the entropy jump goes to infinity as soon as the shock speed approaches the speed of light and γ is lower than 2,while, for γ 2, the lightlike shock vanishes