Firstly, I am not a mathematician. So as I fumble through a likely ignorant question, please show pity. A part of my teaching and research frequently leads me to consult theories of astrophysics, in particular, EFE's. e.g. G mu nu = 8 pi T mu nu. The terms mu nu must be so fundamentally a part of the furniture that none of the discussions address these values within the elaboration. I would greatly appreciate any insight into the use and meaning of these values in the context of field equations. Sincerely, Oyate ps don't worry about being too technical in replying.