Why helicity of photon is 1 but not 3?

[edguy99]

The spin of the photon is 1; its helicity is +1 or -1 (if circularly polarized) or a superposition of these (otherwise).

Well said. I like to visualize photon spin 1 with 3 states as the photon axis turning to the left, turning to the right, or in some kind of stationary angle (vertical/horizontal). In any case, you never quite know for sure what you will measure since you only know the probability of getting such a measurement, a grey area if you like.

 

[vanhees71]

That's a totally misleading picture of a photon. A photon is not a little billard ball with an axis!

 

[edguy99]

That's a totally misleading picture of a photon. A photon is not a little billard ball with an axis!

I am not saying what a photon is or is not. I am expressing an opinion of a visual representation of a photon that I have (and like). Do you have a favorite visual representation of a photon?

 

[vanhees71]

Theoretically, an asymptotic free photon is represented by a one-photon Fock state. The only picture of photons we should use are momentum (or transverse-momentum) (energy) distributions or detection rates of photodetectors. Everything else is misleading.

 

[Demystifier]

The only picture of photons we should use are momentum (or transverse-momentum) (energy) distributions or detection rates of photodetectors.

I don't believe that this is the only picture of photons you have in your mind when you calculate Feynman diagrams or think about photons traveling from the source to the detector.

 

[edguy99]

WRT the 3 states of a spin 1 photon, I also like this visualization of orientation of the spin axis (or the polarization angle).

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).

 

[fxdung]

If photon has tiny mass,then special relativity theory is not very precise?

 

[vanhees71]

No, SRT stays the same. Only is the fundamental speed of SRT not the speed of electromagnetic waves in vacuo.