# Why does a photon have spin 1?

Are we taking the photon spin to be one to describe electromagnetic force or there is any equation (is it relativistic Schrodinger or Dirac equation?) with a solution that tells us that its value is one?

• For measurement of the photon's angular momentum, the classic paper is by Richard Beth, published in Physical Review in 1936, doi.org/10.1103/PhysRev.50.115
– rob
Nov 21, 2019 at 20:24
• The photons are the quanta of electromagnetic field so it's in those equations that you have to look..
– lcv
Nov 22, 2019 at 11:25

The question $$\textit{why it has spin 1}$$ is inappropriate. Particles, by definition, are embedded into irreducible representations of the Poincaré group, i.e., a field. Fields with distinct Lorentz representations have distinct phenomenology and so we must $$\textbf{choose}$$ the representation of the field in order to describe the correct phenomenology of the particle.
Photons are spin-1 particles, they can be polarized in two different ways, circular (left und right), like electromagnetic waves. They obey the Maxwell equation $$\partial_\mu F^{\mu \nu}=j^{\nu}$$. The question why is not so easy to answer. The photon is the U(1) gauge boson so it has to have integer spin.
• Photons obey $E=pc$. EM fields obey the Maxwell equations and its solutions describe where to expect a photon. Nov 23, 2019 at 18:03