# Time reversal spin

1. Sep 28, 2015

### aaaa202

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

2. Sep 28, 2015

### stevendaryl

Staff Emeritus
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.

3. Sep 29, 2015

### DrDu

Spin is angular momentum in the rest frame. Angular momentum has to change sign under time reversal by definition.