I'd appreciate any explanations of the physical interpretations of Einstein's stress energy tensor....for general relativity....maybe a favorite explanatory/expository online source. Wikipedia is not very satisfying on this nor on the electromagnetic stress energy tensor. For example all involve momentum flux (flow) I think. It would be really interesting to see an explanatory description comparing special and general relativity tensors and also how those formulations differs from an analogous one for electromagnetic waves. Maybe even a comparison to the one(s) use in mechanical engineering. I get the concept that Einstein somehow included not only mass in his formalism but also stress and energy including negative pressure....Anybody have any historical information of where that all came from?? I've read a lot of books about physics but have seen only hints. Figuring our his "equivalence principle" is understandable but how did he ever formulate all the components in his GR equations? I've gathered some good information sources from online regarding the mathematical formalism of tensors (I posted those in the Learning section here) and the stress energy tensor for GR...for summer reading....but I'd really like to get some physical interpretations before attempting to study all that on my own... I'm actually less interested in the mathematical formalism than explanatory descriptions of the stress energy tensor for GR descriptions since I doubt that even if I get a solid understanding of the formalism on my own (somewhat doubtful!!!) I'd likely be able to even remotely approach the interpretations of experts over the last 85 years or so. I'm happy to study the insights of others. I do have BLACK HOLES AND TIME WARPS BY Kip Thorne for reading as that was a recommended reference somewhere here on physics forums...for GR. Thanks for any feedback.