# Photon with spin 1. But it only has two projection along z. Why?

1. Jan 23, 2009

### xfshi2000

Hi all:
I am confused about why photon only has two projection along the z-direction. This confusion came from what I read in shankar "printciple of quantum mechanics". In the chapter of field quantization, he explain that because photon must satisfy transverse condition. wave function component along propagation direction is zero. if it is non-zero, it must be remains invariant under rotation (Jz). that is to say, Sz=0 projection is not existed. I don't understand these words meaning. How did he get this conclusion? thanks.
xfshi