Taking the time derivative of the energy density of the energy momentum tensor should equal to the spatial derivatives or divergence of the momentum density components. How do the units work out though? shouldn't a time derivative of the energy density be in kg/xt^3 and the spatial derivative of the momentum density be kg/(x^2 t^2). I thought all components of the energy momentum tensor have the same units or am I missing a physical interpretation of what is going on? In the simplest form continuity of some flux ∇j=∂tρ the units make sense what am i missing?