Somone in these forums linked to this and I finally got around to reading it and wanted to share the ideas:(adsbygoogle = window.adsbygoogle || []).push({});

What do you think of the overall description......and last paragraph?? Why??

http://blogs.discovermagazine.com/cosmicvariance/2010/02/22/energy-is-not-conserved/

[Mostly quotes but I changed some wording and omitted portions for brevity.]

Sean Carroll:I won't comment on the last paragraph yet, but would note this: We have discussed several related issues elsewhere, like 'do photons redshift or is that a frame related observation'; 'is spacetime a physical entity' or a mathematical artifact; 'how do we ascribe local energy density to the gravitational field' .........so one can see some potentially different perspectives and even theoretical issues. …..The reason... you thought energy was conserved was time-translation invariance: “the background on which particles and forces evolve, as well as the dynamical rules governing their motions, are fixed, not changing with time.” But in general relativity…. Einstein tells us that space and time are dynamical, and evolve with time. When the space through which particles move is changing, the total energy of those particles is not conserved....

The “energy-momentum conservation” equation looks like this:

∇_{u}T^{uv}= 0

.... the meaning of this equation is straightforward enough: Energy and momentum evolve in a precisely specified way in response to the behavior of spacetime around them. If that spacetime is standing completely still, the total energy is constant; if it’s evolving, the energy changes in a completely unambiguous way.

....Photons redshift losing energy as space expands..... If we keep track of a certain fixed number of photons, the number stays constant while the energy per photon decreases, so the total energy decreases. A decrease in energy is just as much a “violation of energy conservation” as an increase in energy, but it doesn’t seem to bother people as much.....

the fact that energy is not conserved in an expanding universe is absolutely central to getting the predictions of primordial nucleosynthesis correct.....

in my experience, saying “there’s energy in the gravitational field, but it’s negative, so it exactly cancels the energy you think is being gained in the matter fields” does not actually increase anyone’s understanding — it just quiets them down. Whereas if you say “in general relativity spacetime can give energy to matter, or absorb it from matter, so that the total energy simply isn’t conserved,” they might be surprised but I think most people do actually gain some understanding.....

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# Does spacetime emit and absorb energy?

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