åœ¨ 2017å¹´2月18日星期六 UTC+8下堠 
   ˆ11:32:22，matmzc%ho...@gtempaccount.com写é�“：   
   > 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   
      
   In classical field theory, massless EM field has gauge symmetry. It is   
   spin-1(tangent vector) and has 4 components in 3+1d spacetime. If one   
   adopts a particular gauge, one eliminates 2 degrees of freedom from it   
   (try a example). So we have 4-2=2 independent polarization of EM wave.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)