  The photon magnetic moment for radiation propagating in magnetized vacuum is
defined as a pseudo-tensor quantity, proportional to the external
electromagnetic field tensor. After expanding the eigenvalues of the
polarization operator in powers of $k^2$, we obtain approximate dispersion
equations (cubic in $k^2$), and analytic solutions for the photon magnetic
moment, valid for low momentum and/or large magnetic field.
  The paramagnetic photon experiences a red shift, with opposite sign than the
gravitational one, which differs for parallel and perpendicular polarizations.
It is due to the drain of photon transverse momentum and energy by the external
field. By defining an effective transverse momentum, the constancy of the speed
of light orthogonal to the field is guaranteed. We conclude that the
propagation of the photon non-parallel to the magnetic behaves as if there is a
quantum compression of vacuum or warp of space-time in an amount depending on
its angle with regard to the field.
