> On Sep 30, 2014, at 2:05 PM, Benjamin Udell <bud...@nyc.rr.com> wrote: > > Regarding conservation of energy: My understanding is that, in general > relativity it's considered not to be conserved in an expanding or contracting > universe, although it's still regardable as conserved in normal situations > with some assumptions beyond those in special relativity.
Stack Overflow Physics does a good job on this http://physics.stackexchange.com/questions/296/is-energy-really-conserved <http://physics.stackexchange.com/questions/296/is-energy-really-conserved> Basically while Noether’s Theorem would imply non-conservation in classical (nonrelativistic) physics, within GR you can’t define a time direction. However in GR you have something called global hyperbolicity which leads to the same sort of result as Noether’s Theorem gives in classic physics. Sean Carroll over at Cosmic Variance dealt with this in a nice way a few years back. http://blogs.discovermagazine.com/cosmicvariance/2010/02/22/energy-is-not-conserved/ <http://blogs.discovermagazine.com/cosmicvariance/2010/02/22/energy-is-not-conserved/> I’m no cosmologist so this was new to me as I didn’t recall it from my GR text. I’d always just assume Noether’s Theorem held. Once it was explained it made complete sense though. No idea how this relates to Peirce though. I assume he just assumed classical physics.
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