It seems this theorem only applies to massive spin 1 particles, though: https://physics.stackexchange.com/questions/38427/proof-of-yangs-theorem https://physics

It seems this theorem only applies to massive spin 1 particles, though: https://physics.stackexchange.com/questions/38427/proof-of-yangs-theorem https://physics.stackexchange.com/questions/38427/proof-of-y...