  In this paper we calculate the Rayleigh-Brillouin spectrum for a relativistic
simple fluid according to three different versions available for a relativistic
approach to non-equilibrium thermodynamics. An outcome of these calculations is
that Eckart's version predicts that such spectrum does not exist. This provides
an argument to question its validity. The remaining two results, which differ
one from another, do provide a finite form for such spectrum. This raises the
rather intriguing question as to which of the two theories is a better
candidate to be taken as a possible version of relativistic non-equilibrium
thermodynamics. The answer will clearly require deeper examination of this
problem.
