Thanks Tensor.
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Originally Posted by Tensor
Not quite sure what he's looking for here. GR treats spacetime as a Reimann manifold, so I guess you could say it's in the math. Although most relativists will say spacetime is the actual physical component of the mathematical manifold.
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Let's see what ngeo has to say ... I was merely introducing the topic (and letting any other folk who might be interested know - they can join as well, of course).
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In terms of the entire universe, the addition of stress-energy to the manifold causes either an expansion or contraction. The static solution resembles a pencil balanced on it's point. As soon as something changes, the manifold either contracts or expands. Within the math, it's the manifold that expands (as currently viewed). So, since spacetime is the physical componet of the manifold, it is spacetime that is expanding.
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I suspect that this is super-mysterious to a lot of people - is this just the way it is, from the math? And what is this 'stress-energy'?
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Not sure what you are looking for here Nereid, but here's an application of the Friedman equation. It doesn't start with the EFE, but the discussion starts with if EFE is correct, the first equation is correct. I'll see if I can scare up a page that shows how to get to the Friedman equation relates to the EFE.
EDIT Ok, I found this rather quickly.
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Let's wait for ngeo.
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Well, I don't think I have the math for this part of the question as I haven't studied QM, much less superstring theory, in mathematical depth. I think I could get concepts across. Although, since there are practicing particle phycisists on the board, they would be more qualified than I to describe these things.
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I suspect the first step is a very simple one - what, if anything, does GR itself have to say about the nature of 'particles' or 'bodies'? Or, putting this another way, is GR itself completely 'blind' to the
form that the things in the equations can take (mass, energy, ...)?
Not quite, because there's a distinction between massless entities (which must travel at c; but they don't have to be photons) and the rest (which cannot travel at c); and there's a special type of 'thing' - the black hole (whose mass can be anything, except zero!).
In other words, a purely GR universe is, potentially, a place with far more 'possibilities' than the real universe, if only because the interactions between real particles are constrained by the physics of electromagnetism, the weak force, the strong force, and so on.