My personal views, isolated because it is my view From reading Einsteins work I get the impression he thought of energy as a "quantity" and the universe as a bath full of it because of the laws of conservation. As he viewed it that way he arrived at modelling the pressure as a perfect fluid. I think his logic was energy had some property internally that makes it want to spread evenly like a something dissolved in liquid solution. As energy clump together it created an a pressure which in a normal liquid solution you would call osmotic pressure (
https://en.wikipedia.org/wiki/Osmotic_pressure)
The reason Einstein made the stress tensor always zero is because he made it like a surface tension of zero in a fluid. No such fluid exists by the way so why do it you ask.
Well surface tension is not a property of materials but of interfaces between two (or more) materials. It is implicit in its definition that the interface separates two kinds of materials that behave differently (otherwise the interface would be just some imaginary surface inside the one material with no physical meaning). In real liquids that surface is indeed subject to blurring because of forces which is why no zero surface tension fluids exist.
Worth a quick look it's short
https://en.wikipedia.org/wiki/Perfect_fluid I think Einstein wanted a clean division between the energy and its internal pressure and our physical world. It gets messy if you start modelling interface tension in GR and the maths blows up but some people have tried it. So Einstein wanted a clean divide between the energy domain and our physical world, I think the motivation was E=mc2 it's either one or other with c as a constant.
Do you understand now why you are never going to be able to measure the tensor directly as anything other than zero if Einstein is right?
However I also scratch my head if Einstein believed that of energy why did he have so much trouble with QM.
Anyhow there you have it why something called "Dark Energy" is actually needed in any non radiative version of gravity. GR is a non radiative version and has that back pressure inbuilt into it. If you make a field version of GR you must in build the back pressure of deforming the field. Finally the tensor is always zero in GR because of the clean symmetrical interface it defines.
Bonus now if anyone doing electronics asks you what the back-electromotive force or Lenz's law exists you can explain it
You know how I love cosmologists and consensus well apparently I am right to a science confidence of 99.996%
http://www.port.ac.uk/uopnews/2012/09/12/dark-energy-is-real-say-astronomers/ GR isn't my area and I can't go much deeper than this but I do know the basics but I could always be wrong !!!!!
If you understand all that you actually have got generalized field theory understood ... what a field is in physicality that is a whole other story
The other lesson is the one Bill G failed at... look at what is being said, not who said it. For example I always read Lubos Motl on GR even though he is a string theorist because he knows GR very very well, I have seen that on a number of physics forums. Don't take him as gospel but do read what he says very carefully.
