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Photon Helicity

  1. Jul 29, 2008 #1
    I'm perplexed about something that Wikipedia says about photon helicity:

    (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. [tex]\sqrt{2}[/tex] [tex]\hbar[/tex]?
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  3. Jul 30, 2008 #2

    James R

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

    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
  4. Jul 30, 2008 #3


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