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What is the direction of angular momentum vector of a photon?by jartsa
Tags: spin 
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#1
Feb1012, 08:50 AM

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Let's say momentum vector of a photon points to direction D. What are the possible directions that this photon's angular momentum vector can point to?
The spin angular momentum is the angular momentum that I'm interested of. 


#2
Feb1012, 09:13 AM

Mentor
P: 11,858

A photon's intrinsic angular momentum ("spin") is either parallel or antiparallel to its momentum. This is a consequence of its mass being zero.



#3
Feb1012, 02:17 PM

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#4
Feb1012, 03:25 PM

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Thanks
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What is the direction of angular momentum vector of a photon?
This is incorrect. The members of the Lorentz group that define a particle's angular momentum form the "little group", i.e. the subgroup that leaves its 4momentum invariant. For a particle with mass the little group is SO(3), and as you say the three generators of this group do not commute. For a massless particle the little group is the Euclidean group ISO(2), i.e. rotations and translations in two dimensions. One generator of this group is the rotation with axis along the direction of motion. The eigenvalues of this operator are S_{z}, the particle's helicity, namely ±1 for a photon. The other two generators are null rotations in the two transverse directions. They commute with each other, and for photons their effect is simply a gauge transformation.



#5
Feb1012, 10:04 PM

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#6
Feb1012, 10:33 PM

Sci Advisor
P: 1,594

Particles in 3space are points. The symmetries that leave a point invariant are SO(3).
Particles in 4dimensional spacetime are lines. Massive particles are represented by timelike lines. The symmetries that leave a timelike line invariant are SO(3). This is the "little group" of a massive particle. Massless particles are represented by null lines. The symmetries that leave a null line invariant are ISO(2). This is the "little group" of a massless particle. The reason we care about the 4momentum is because this vector is tangent to the particle's worldline. If it were possible to have a particle that appears for one instant and then disappears, so that it is pointlike in spacetime, then its angular momentum would be in a representation of the full SO(3,1). 


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