Why helicity of photon is 1 but not 3?

In summary: Weinberg's that covers also nonrenormalizable theories).In summary, the conversation discusses the reasons behind the spin values of various elementary particles and their relation to local quantum field theories. It is noted that the spin of a particle is related to the gauge principle and the representations of the Poincare group. It is also mentioned that there are attempts to create field theories for particles with higher spin, but they are not renormalizable and have issues with unphysical degrees of freedom. The book "Quantum Field Theory Vol 3" by Weinberg is recommended for further reading on the topic.
  • #36
WRT the 3 states of a spin 1 photon, I also like this visualization of orientation of the spin axis (or the polarization angle).

photon_ket3_small.jpg


It draws out the relationship between the Dirac Bra-ket notation (In Dirac's words, "I invented the bra"), the Jones vectors and one can visualize the action of Pauli matrices on the vectors.

Here the dot represents the highest probability of angle you would measure the spin axis of the photon at, and the arrow tells you the amount the spin axis itself is turning (precessing). If you were to measure one of these photons on the same angle as the dot, it would pass for sure. If you measure it 90 degrees off the dot, you will not pass for sure. If you measure any degree in between, the probability of passing relates the Cosine of the angle.

From this, I visualize the 3 kinds of photons. 1/ spin left photons, 2/ spin right photons, 3/ photons in a weird state where you only know the probabilities of their spin axis (ie. maybe vertical or maybe horizontal or maybe whatever depending on how you measure them).
 
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  • #37
If photon has tiny mass,then special relativity theory is not very precise?
 
  • #38
No, SRT stays the same. Only is the fundamental speed of SRT not the speed of electromagnetic waves in vacuo.
 

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