From: ram@zedat.fu-berlin.de   
      
   ram@zedat.fu-berlin.de (Stefan Ram) wrote or quoted:   
   >Photons lack a valid reference frame since Lorentz transformations   
   >are undefined at v = c.   
      
     However, some fields use "light-cone coordinates", introduced   
     by Paul Dirac and also known as "Dirac coordinates", where   
     the associated coordinate axes are the world-lines of beams   
     of light emitted from the origin along the x1 axis.   
      
     So, when x0 is the usual time coordinate and x1 the spatial   
     coordinate (for simplification, I assume just one spatial   
     dimension "x1"),   
      
   x+ :=( 1/sqrt(2) )( x0 + x1 ) and   
   x- :=( 1/sqrt(2) )( x0 - x1 ).   
      
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