Mass, Gravity, Spacetime and everything

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  • #26
JesseM
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So my current question about GR is: Does GR require a universe where mass creates gravity in order for it to work? What happens to GR when it's applied on a universe where gravity creates mass.. Or should I say "matter" instead of mass?
GR requires a universe where there is always a fixed relationship between the distribution of energy (including energy due to mass) and the curvature of spacetime at every point. It doesn't really tell you what "creates" what, it just gives you the mathematical relationship. But because this relationship is supposed to hold at every single point, you can't have a situation where there's first some curvature without the corresponding mass/energy distribution, and only later does it collect that mass/energy.
 
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So what has GR failed to predict untill now?
 
  • #28
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Also, what would guarantee that just the right amount of mass/energy would get trapped, not the slightest bit more or less than that which is predicted by the Einstein field equations?
The "less" part of this question is troubling me. Can it be that energy can pass through but if at any given time energy reaches this threshold you talk about which i'm calling mass, then and only then it gets trapped?
 
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  • #29
JesseM
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So what has GR failed to predict untill now?
GR is only meant to predict things relating to gravity, not relating to the other forces (electromagnetism and the strong and weak nuclear forces) or particle physics. In the domain of gravity it hasn't made any incorrect predictions, but theoretical arguments show it will conflict with quantum physics at very high energy scales (and very short distance and time scales), known as the Planck scale, so physicists expect it is just an approximation to a theory of "quantum gravity" which will make different predictions at the Planck scale but not be noticeably different at the scales we've observed so far. You can take a look at this thread for more on the theoretical conflict between GR and QM.
 

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