1
$\begingroup$

Gottfried and Yan's Quantum Mechanics has the following discussion of photon angular momentum:

$$ \boldsymbol{J}_\textbf{sp}= -i\hbar\sum_ka^\dagger_k\times a_k = \hbar \sum_{k\lambda} \lambda \hat{\boldsymbol{k}} \, n_{k\lambda}.$$

"Hence the one-photon state of of momentum $\hbar\boldsymbol{k}$ has angular momentum $\pm\hbar$ along $\boldsymbol{k}$, that is to say the photon has helicity $\pm1$. Note the striking difference from nonrelativistic angular momentum: the spin has no component along the direction of motion."

This seems to me to be directly contradicting itself. Does the photon have angular momentum along its direction of motion?

Improve this question
$\endgroup$
2
  • $\begingroup$ As an aside, the spacing on my last term seems bad. Is there a better way to do that? $\endgroup$ – NoethersOneRing Dec 25 '16 at 23:29
  • 1
    $\begingroup$ You can use \, in LaTeX to insert a small space, as I have demonstrated in an edit. $\endgroup$ – JamalS Dec 26 '16 at 0:26
3
$\begingroup$

Yes, it does. I think they meant to say "the spin has no component perpendicular to the direction of motion."

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.