# Photon Helicity

1. Jul 29, 2008

### Usaf Moji

(see http://en.wikipedia.org/wiki/Photon)

But for a photon, doesn't the spin vector always point in the same direction as the momentum vector - and therefore, shouldn't the magnitude of a photon's helicity equal it's spin magnitude, i.e. $$\sqrt{2}$$ $$\hbar$$?

2. Jul 30, 2008

### James R

The spin vector is always at an angle to the propagation vector, such that its component in the direction of propagation is $\pm \hbar$ and its magnitude is $\sqrt{s(s+1)}\hbar = \sqrt{2}\hbar$.

In theory, one might expect that the photon could also have a spin projection of zero. However, apparently this would require that the photon have non-zero rest mass (which it doesn't), so a zero helicity state is not observed.

If somebody can explain why a zero spin projection is ruled out by relativity in more detail, I would be grateful.

Last edited: Jul 30, 2008
3. Jul 30, 2008

### malawi_glenn

Usaf Moji: WHY must the spin of the photon be aligned in the same direction as its momentum-vector?