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?
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3$\begingroup$ 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 $\endgroup$– rob ♦Nov 21, 2019 at 20:24
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$\begingroup$ The photons are the quanta of electromagnetic field so it's in those equations that you have to look.. $\endgroup$– lcvNov 22, 2019 at 11:25
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2 Answers
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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.
The photon is a particular case of this; it a boson with two degrees of freedom (two independent polarizations) which is its own antiparticle. In particular, the circular polarization of the photon is characteristic of massless spin 1 particles, since other spins like 0 or 2 have different polarization patterns. So, without going to deep into the theory, it is phenomenologically unavoidable to have a spin 1 photon.
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$\begingroup$ How do you sync what you said with this answer (e.g. "It is also what is meant by that photons have helicity, not spin")? $\endgroup$ Nov 23, 2019 at 14:57
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$\begingroup$ Oh, in my answer I am being sloppy by using the word "spin", but just like any other massless particle the photon has helicity, which accounts for two degrees of freedom. Is that what you meant? $\endgroup$– MassoNov 24, 2019 at 0:43
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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.
