I read this wikipedia article https://en.wikipedia.org/wiki/Relativistic_heat_conduction It mentions the relativistic heat conduction equation: $$\frac{\partial \theta}{\partial t}=\Box\, \theta$$ Where $\theta$ is the Temperature and $\Box$ is the D'Alembert Operator How can one generalize this to situations in GR? As far as I know, it is clear that the D'Alembert Operator is invariant under Lorentz transformations and so one just has to replace the derivatives with the covariant derivatives. However what happens to the term on the LHS? Do I just replace it with the covariant derivative as well?