Anonymous at Tue, 26 Mar 2024 10:01:15 UTC No. 16097457

>>16097423
>chirality, helicity, polarization
same thing
>parity
a feature of anything with antisymmetry, such as pseudoscalers (helicity) and pseudovectors (such as torque)

Anonymous at Tue, 26 Mar 2024 14:59:15 UTC No. 16097736

>>16097423
I assume you are talking about particle physics in the following (all of these words have different meanings in other contexts)

>chirality
This is a property of spinor fields, rather than particles. It's not easy for me to give you a short explanation unless you've seen spinors before

>helicity
This is the component of the spin of a particle projected in the direction of momentum

>polarization
For circular polarization of light, this describes the sense in which the electric field rotates around the direction of propagation (clockwise or counter-clockwise). When light is described in terms of photons, the helicity of the photon is related to the circular polarization of the wave

>parity
If you are speaking of parity as a property, like the parity of a photon, it is the phase that a quantum state picks up upon flipping the signs of all the spatial directions.

Anonymous at Tue, 26 Mar 2024 18:15:44 UTC No. 16097936

>>16097736
>This is the component of the spin of a particle projected in the direction of momentum
can momentum and spin be measured separately, i.e their operators commute?
I read that because photons have spin 1 they can have three values of spin, -1, 0, 1 but in reality only two are realized, -1 or 1, is that related to helicity?

Anonymous at Tue, 26 Mar 2024 18:57:46 UTC No. 16098019

>>16097936
>can momentum and spin be measured separately, i.e their operators commute?
Yes. One way you can talk about spin when you have moving particles is using the "Pauli-Lubanski vector" which is something you can look up on Wikipedia. It commutes with momentum.

>but in reality only two are realized, -1 or 1, is that related to helicity?
Yes. For massive particles you can go to the rest frame and consider a large number of spin states (three in the spin 1 case, more if the total spin is higher), but since massless particles have no rest frame, angular momentum is treated differently, and you basically end up with only 2 distinct states of different helicity no matter the total spin. This isn't really obvious based on what I said, but it is shown for instance in chapter 2 of Weinberg's textbook on QFT.