Massless particles, as the representations of the Poincare group, are characterized by helicity $h$ but people always say that photon has spin 1.
What is the definition of spin for photon?
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1WP definition via Pauli-Lubanski. – Cosmas Zachos May 08 '20 at 00:09
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1Related. – Cosmas Zachos May 08 '20 at 00:12
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Photon spin is the internal angular momentum of photons as described by the electromagnetic field, much like electron spin is the internal AM of the Dirac spinor field. The EM field is a four component Lorentz vector potential, $A^\mu$, that can rotate. This rotation is EM spin. In a situation where the physics is independent of the orientation of the four vector, EM spin is conserved. The expression for EM spin is simply the Noether current associated with this symmetry.
But ... the generally accepted gauge invariant EM Lagrangian is not symmetric under EM potential rotation. So in the present EM theory EM spin is not conserved and no gauge invariant expression for photon spin exists. To remedy this a different Lagrangian can be used, the Fermi Lagrangian, which has the required symmetry. In a peer reviewed paper I propose and explore this approach. I show that it leads to a fully valid physically acceptable theory. I invite you and anybody interested in this fundamentally important issue to take a look at my work at https://arxiv.org/abs/physics/0106078.
my2cts
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1If spin is not conserved why people says that photon has spin 1? – amilton moreira May 07 '20 at 23:31
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It is conserved, just the theory does not conserve it. It is an experimental fact that a photon has spin 1. – my2cts May 07 '20 at 23:44
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1Famous experiment by Beth, 1935. Referency is in my paper or on Wikipedia. And a great multitude of atomic physics experiments. – my2cts May 07 '20 at 23:52
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1A modern version of Beth's experiment is described here https://phys.org/news/2016-09-photons-scientists.html . – my2cts May 08 '20 at 09:57
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1The freely accessible paper is here : https://advances.sciencemag.org/content/2/9/e1600485 – my2cts May 08 '20 at 10:12