I have pretty much learned how to derive the left side of Einstein's field equations now (the Einstein tensor that is). Now I need to grasp that stress energy momentum tensor. Does anybody know of any good sources that will tell me how to derive the components of this tensor? I ask this because some of the meanings for the elements that I have heard confuse me. For example, T00 is said to be the energy density, but no source that I have seen has made any mention of volume. I'd think that energy density in this case would be something like relativistic energy divided by volume, but I have seen no mention of volume anywhere. The same problem holds true with the other elements in the top which are said to be the "momentum density". Furthermore, I have seen some sources refer to the top row (aside from T00) as energy flux, which confuses me because it does not seem like it would be the same thing as momentum density. The worst part of it all, is that I haven't seen a single formula to derive these elements. Can anyone link me to any good sources explaining how to properly derive the elements of this tensor or explain it themselves (preferably in a mathematical way)?