It seems this theorem only applies to massive spin 1 particles, though: <a href="https:&#x2F;&#x2F;physics.stackexchange.com&#x2F;questions&#x2F;38427&#x2F;proof-of-yangs-theorem" rel="nofollow">https:&#x2F;&#x2F;physics.stackexchange.com&#x2F;questions&#x2F;38427&#x2F;proof-of-y...</a>