# Talk:Angular momentum of light

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## Requested move

The following discussion is an archived discussion of a requested move. Please do not modify it. Subsequent comments should be made in a new section on the talk page. No further edits should be made to this section.

The result of the move request was: moved by E-karimi (talk · contribs) (non-admin closure). Jenks24 (talk) 01:42, 27 September 2011 (UTC) --- Angular Momentum of LightAngular momentum of light – Grammatically correct version is without capital letters. 192.84.134.230 (talk) 14:52, 26 September 2011 (UTC)

The above discussion is preserved as an archive of a requested move. Please do not modify it. Subsequent comments should be made in a new section on this talk page. No further edits should be made to this section.

## Gauge Question

There seems to be a step missing in the logic of the article. The expressions for spin and orbital angular momentum are defined in terms of the components of the electromagnetic vector potential and it is correctly stated that these are not gauge invariant. The article goes on to say that the values of the vector potential are replaced by their components in the transverse gauge and it is claimed that this makes them gauge invariant. While this is true mathematically it would be just as true if the potentials of any other gauge were used. The expressions for the spin and orbital components would be gauge invariant but different for each gauge. What is so special about the transverse gauge that justifies its use here? For example, why not use the potentials of the Lorenz gauge? Xxanthippe (talk) 07:18, 14 October 2011 (UTC).

To expand the argument: any entity that describes a physical quantity is required to be gauge-covariant i.e. it has the same form in any gauge. One example is the total angular momentum of the electromagnetic field
${\displaystyle \mathbf {J} =\epsilon _{0}\int \mathbf {r} \times \left(\mathbf {E} \times \mathbf {B} \right)d^{3}\mathbf {r} .}$
This is gauge-covariant because both E and B are gauge-covariant. A similar property is required for the spin and orbital components if they are to be considered to be measurable entities. Although the third equation in the article is correct, each individual component on the right-hand side does not represent a physically measurable quantity because each depends on the gauge chosen; only their sum does not depend on gauge. I have inserted in the article "A justification for taking this step is yet to be provided." If editors are able to produce from the literature a good reason for the step, then they should revert my edit (and give the source). Note that the writer of the article has correctly described the steps in the argument given in the current literature; what is needed is an explanation of this one issue that will be accessible to readers. Xxanthippe (talk) 09:40, 13 December 2011 (UTC).