Video 63. ""Quantum Mechanics 9a - Photon Spin and Schrödinger's Cat I" published by ViaScience on 2013-10-23." explains nicely how:
- incorporated into the Dirac equation as a natural consequence of special relativity corrections, but not naturally present in the Schrodinger equation, see also: the Dirac equation predicts spin
- photon spin can be either linear or circular
- the linear one can be made from a superposition of circular ones
- straight antennas produce linearly polarized photos, and Helical antennas circularly polarized ones
- a jump between 2s and 2p in an atom changes angular momentum. Therefore, the photon must carry angular momentum as well as energy.
- cannot be classically explained, because even for a very large estimate of the electron size, its surface would have to spin faster than light to achieve that magnetic momentum with the known electron charge
- as shown at Video 60. ""Quantum Mechanics 12b - Dirac Equation II" by ViaScience (2015)", observers in different frames of reference see different spin states