Hello everyone, I have some questions concerning the stress-energy-momentum tensor. I know it is a far more general object than the one used in GR, but I guess no one will disagree it plays a far more important role here. So, firstly, what is the proof of its existence and its tensor properties (I guess I have the same problem with the electromagnetic tensor). Everytime I read something on this it seems that the tensor just falls from the sky to accomplish our wishes. Secondly, the usual way of writing this tensor, e.g. the 00 component being the relativistic mass density, etc., is it purely conventional or not? Concerning GR, given that this tensor has different forms for different observers (but that's the whole point of the principle of general covariance), they will see differently the effects of the presence of massive bodies (for example an observer in uniform motion will measure a different momentum density). How does this difference is understood in GR?