  We calculate the vacuum polarization-induced ellipticity acquired by a
linearly polarized laser beam of angular frequency $\bar \omega$ on traversing
a region containing a transverse magnetic field rotating with a small angular
velocity $\Omega$ around the beam axis. The transmitted beam contains the
fundamental frequency $\bar \omega$ and weak sidebands of frequency $\bar
\omega \pm 2 \Omega$, but no other sidebands. To first order in small
quantities, the ellipticity acquired by the transmitted beam is independent of
$\Omega$, and is the same as would be calculated in the approximation of
regarding the magnetic field as fixed at its instantaneous angular orientation,
using the standard vacuum birefringence formulas for a static magnetic field.
Also to first order, there is no rotation of the polarization plane of the
transmitted beam. Analogous statements hold when the magnetic field strength is
slowly varying in time.

High Energy Physics - Phenomenology