Photon is a spin-1 particle. Were it massive, its spin projected along some direction would be either 1, -1, or 0. But photons can only be in an eigenstate of $S_z$ with eigenvalue $\pm 1$ (z as the momentum direction). I know this results from the transverse nature of EM waves, but how to derive this from the internal symmetry of photons? I read that the internal spacetime symmetry of massive particles are $O(3)$, and massless particles $E(2)$. But I can't find any references describing how $E(2)$ precludes the existence of photons with helicity 0.
$\begingroup$
$\endgroup$
2
-
1$\begingroup$ related: physics.stackexchange.com/q/46643 $\endgroup$– user4552Commented Aug 31, 2013 at 20:37
-
$\begingroup$ A discussion about the essence of photon’s spin and differences to the massive case: physics.stackexchange.com/q/19229 $\endgroup$– Incnis MrsiCommented Aug 18, 2014 at 12:34
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
11
It derives not from the internal symmetry itself but from the fact that it is a gauge symmetry.
Your symmetry group assignments are not those of the symmetry group but of the little group of the representation. If you assume in addition that the representation is irreducible, you end up in the massless case (with little group ISO(2)=E(2)) with a helicity representation, which picks up from a vector representation only the transversal part, corresponding to a gauge symmetry. Because of reflection symmetry (parity), there are two helicity degrees of freedom. Under the connected part of the Poincare group, this splits into two irreducible representations of fixed helicity, corresponding left and right circular polarization.
This is described in full detail in Section 5.9. of the quantum field theory book (Part I) by Weinberg. In particular, the 2-valuedness (rather than the 3-valuedness) of the helicity is discussed after (5.9.16).
answered Jun 8, 2012 at 17:17
-
$\begingroup$ That book has a chapter on massless particles, but does not mention E(2)-like little group. $\endgroup$ Commented Jun 9, 2012 at 13:25
-
1$\begingroup$ @KarsusRen: It mentions it on p.70 under the name ISO(2), which is just an alternative tradition for writing E(2). $\endgroup$ Commented Jun 10, 2012 at 10:23
-
1$\begingroup$ A freely available presentation by Nicolis that follow's Weinberg's is here: phys.columbia.edu/~nicolis/GR_from_LI.pdf $\endgroup$– user4552Commented Aug 31, 2013 at 20:38
-
$\begingroup$ @Arnold Neumaier: do you know a simple explanation how the Poincaré sphere structure appears directly from representations? $\endgroup$ Commented Aug 14, 2014 at 16:27
-
$\begingroup$ @IncnisMrsi:There are two helicity degrees of freedom, and any 2-level system has a fundamental SU(2) representastion, described by a poincare sphere = bloch sphere. $\endgroup$ Commented Aug 17, 2014 at 12:50