Quote from Introduction to High Energy Physics Edition 4 by Donald H. Perkins chapter 3.3.1 "It can be proved as a consequence of relativistic invariance that for any massless particle of spin $s$, there are only two possible spin substates, $s_1=\pm 1$, where $z$ is the direction of motion."
My questions were that:
How to prove the above statement?(with mathematics)
What happened to fermion.
Graviton was also massless, but had spin $2$. Does that mean gravition was not follow relativistic invariance?