Here is Wikipedia's diagram of the stress-energy tensor in general relativity:
I notice that all of its elements are what would be termed "macroscopic" quantities in thermodynamics. That is, in statistical mechanics we would usuallt define these quantities in terms of an ensemble of systems, rather than in terms of the microscopic state of a single system. (This doesn't make much difference for large systems, but for small ones it does.) This observation leads me to a number of questions - I hope it's ok to post them all as a single Question, since they're so closely related:
- Am I correct in inferring that general relativity is actually a macroscopic, or phenomenological theory, rather than a theory about the microscopic level?
- Was Einstein explicit about this in deriving it? Or did he simply start by assuming that matter can be modelled as a continuously subdivisible fluid and take it from there?
- If general relativity is a macroscopic theory, what does the microscopic picture look like? (I'd expect that this is actually unknown, hence all the excitement about holography and whatnot, but perhaps I'm being naïve in thinking that.)
- Are there cases in which this continuous fluid approximation breaks down? For example, what if there are two weakly interacting fluids with different pressures?
- If general relativity is a macroscopic theory, does it imply that space and time themselves are macroscopic concepts?
- Is this related to the whole "is gravity an entropic force?" debate from a few years ago?