From: goldenfieldquaternions@gmail.com   
      
   On Saturday, February 18, 2017 at 9:32:22 AM UTC-6, matmzc%ho...   
   gtempaccount.com wrote:   
   > Hi all.  A massive spin one particle can have spin 1,0, or -1 in any   
   > spatial direction.  But for the massless photon spin in the direction of   
   > motion must be 1 or -1.  What is the easiest way to explain why 0 is   
   > excluded?  I am not asking for a proof, which can be found in texts on   
   > QFT.  But rather I would like a nice simple concept, if there is one,   
   > that makes it easy to understand.  So what is the simplest way to   
   > explain the gist of the matter?   
   >   
   > Mike   
      
   The spin m = -1 and +1 correspond to the projection of the spin along   
   the momentum of the photon. The m = 0 can only occur if there is a frame   
   where the spin can be perpendicular to the momentum. This requires the   
   spin particle to be timelike with a mass. A photon has no measured   
   mass. This is known to within 10^{-40} or so. So there is no m = 0 case   
   for massless particles on null geodesics.   
      
   The m = -1 and m = +1 classically correspond to helicities. The rotating   
   polarization is clockwise or counterclockwise. This defines the   
   oscillation of the electric field vector transverse (perpendicular) to   
   the momentum with time.   
      
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