Editing Angular momentum (section)

== Angular momentum in optics ==
In ''classical Maxwell electrodynamics'' the [[Poynting vector]]
is a linear momentum density of electromagnetic field.<ref name="Okulov2008">{{cite journal |doi=10.1088/0953-4075/41/10/101001|title=Angular momentum of photons and phase conjugation|journal=Journal of Physics B: Atomic, Molecular and Optical Physics|volume=41|issue=10|pages=101001|year=2008|last1=Okulov|first1=A Yu|arxiv = 0801.2675 |bibcode = 2008JPhB...41j1001O |s2cid=13307937 }}</ref>
<math display="block">\mathbf{S}(\mathbf{r}, t) = \epsilon_0 c^2 \mathbf{E}(\mathbf{r}, t) \times \mathbf{B}(\mathbf{r}, t).
</math>

The angular momentum density vector <math>\mathbf{L}(\mathbf{r}, t)</math> is given by a vector product
as in classical mechanics:<ref name="Okulov2008J">{{cite journal|url=http://www.jetpletters.ac.ru/ps/1852/article_28262.shtml|first=A.Y.|last=Okulov|title=Optical and Sound Helical structures in a Mandelstam – Brillouin mirror|journal=JETP Letters|volume=88|issue=8|pages=561–566|year=2008|language=ru|doi=10.1134/s0021364008200046|bibcode=2008JETPL..88..487O|s2cid=120371573|access-date=2015-10-31|archive-url=https://web.archive.org/web/20151222101259/http://www.jetpletters.ac.ru/ps/1852/article_28262.shtml|archive-date=2015-12-22|url-status=dead|url-access=subscription}}</ref>
<math display="block">\mathbf{L}(\mathbf{r}, t) = \epsilon_0 \mu_0 \mathbf{r} \times \mathbf{S}(\mathbf{r}, t).
</math>

The above identities are valid ''locally'', i.e. in each space point <math>\mathbf{r}</math> in a given moment <math>t</math>.