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A comment on 100 Years After Einstein's Relativity, We're Still Just Figuring Gravity Out. Parent comment

Re:100 Years After Einstein's Relativity, We're Still Just F... by AnimalMuppet on Tuesday December 01, 2015 @01:13PM (#10657515)
We believe that certain symmetries apply to the laws of physics. For example, we believe that the laws of physics are independent of the observer's position. I mean, look (visualize me stepping sideways here). So we can demonstrate that one pretty easily. And they're independent of rotation. Again, look (visualize me turning 90 degrees here). And exchanging left and right. I mean, look... (visualize me pausing with a confused look on my face as I try to figure out how to exchange left and right).

I can't actually do that experiment. And it turns out, when we got deep enough, we found out that things actually behaved differently (parity non-conservation).

When it comes to time reversal, we can't actually do the experiment, either, so we don't really know whether physics behaves the same when time is reversed. We expect, philosophically, that it will, but we don't know. But it seems to me, given how relativity mixes time and space, if physics is not the same with left and right reversed, then it can't be the same with time reversed either.

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Re:100 Years After Einstein's Relativity, We're Still Just F... by danbruc on Wednesday December 02, 2015 @02:41AM (#10661543)
Saying we consider time reversibility true for more or less philosophical reasons seems a bit of an understatement to me. Conserved quantities prevent states differing in at least one conserved quantity from ever evolving into the same state, i.e. conserved quantities partition the phase space and states cannot cross those partition boundaries. This also has to hold for all isolated subsystems. And conserved quantities arise from symmetries like homogeneity and isotropy of space which are pretty solid empirical and experimental facts. It's all interlinked and I imagine it would not be to easy to eliminate time reversibility from the laws of physics without causing some odd consequences.
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