I'm fairly familiar with the elements of the stress-energy-momentum, but I have some questions. Is it right to say the stress-energy-momentum tensor is symmetric? And if so, why does the wikipedia article show energy flux across the top row vector, and momentum density down the first column vector? Or am I messing up the meaning of 'energy flux?' Maybe they're the same? As far as the 0,0 component goes.. I was watching this video lecture from MIT. You can find it by searching "Lec 3 | MIT 8.224 Exploring Black Holes" on youtube. And he uses the energy density as the 0,0 component. However, the wikipedia article states the 0,0 component is the relativistic mass density. I understand they only differ by a factor of c2 but which is correct to use? Also, the element of 'energy density,' is it simply the addition of the mechanical energy and the electromagnetic energy? And the same question goes for the momentum density, and stresses. Do you simply add the mechanical and electromagnetic contributions?