  This work applies a Rayleigh-Brillouin light spectrum analysis in order to
establish a causality test by means of a frequency cone. This technique allows
to identify forbidden and unforbidden regions in light scattering experiments
and establishes if a set of linearized transport equations admits causal
solutions. It is shown that, when studying a relativistic fluid with its
acoustic modes interacting with light, Eckart's formalism yields a non causal
behavior. In this case the solutions describing temperature, density and
pressure fluctuations are located outside the frequency cone. In contrast, the
set of equations that arises from modified Eckart's theory (based on
relativistic kinetic theory) yields solutions that lie within the cone, so that
they are causal.
