Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free.

In free space, the linear momentum density of an EM wave is given by the Poynting vector $\vec S$ over the speed of light squared, $\vec g=\frac{\vec S}{c^2}$.

In a medium, $S$ is generally not directed along the wave vector $\vec k$. If $v_g$ is the group velocity and $W$ is the density of EM wave energy, then $S = v_g W$. On the other hand, it is often assumed that the momentum of a photon is still $\hbar\vec k$. What is the macroscopic momentum density of EM wave in an anisotropic medium such as magnetized plasma ?

share|improve this question
Excellent link, Tobias. Here's a free version of the same: espace.library.uq.edu.au/eserv/UQ:134757/colloquium.pdf –  Andrew Mar 22 '11 at 12:47

1 Answer 1

up vote 1 down vote accepted

Best description of the problem of those I found is given by Nobel winner Vitaly Ginzburg. See, e.g.


or (even better)

VL Ginzburg, Theoretical Physics and Astrophysics (Pergamon, Oxford, 1979); other editions with somewhat different titles also available.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

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