Time Reversal Spin: Why Does it Require Flipping?

aaaa202
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Does anyone know why time reversal requires spin to be flipped? It seems like my book assumes that, but I have no idea why.
 
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aaaa202 said:
Does anyone know why time reversal requires spin to be flipped? It seems like my book assumes that, but I have no idea why.

Well, it's obvious that orbital angular momentum reverses sign under time reversal, because it's \vec{L} = \vec{r} \times \vec{p}, and momentum reverses sign. Since angular momentum can move from spin to orbital, I don't think it would be consistent to reverse one and not the other. But I don't know of a proof.
 
Spin is angular momentum in the rest frame. Angular momentum has to change sign under time reversal by definition.
 

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