  Just as non relativistic fluids, oftentimes we find relativistic fluids in
situations where random fluctuations cannot be ignored, thermal and turbulent
fluctuations being the most relevant examples. Because of the theory's inherent
nonlinearity, fluctuations induce deep and complex changes in the dynamics of
the system. The Martin-Siggia-Rose technique is a powerful tool that allows us
to translate the original hydrodynamic problem into a quantum field theory one,
thus taking advantage of the progress in the treatment of quantum fields out of
equilibrium. To demonstrate this technique, we shall consider the thermal
fluctuations of the spin two modes of a relativistic fluid, in a theory where
hydrodynamics is derived by taking moments of the Boltzmann equation under the
relaxation time approximation.
