Researchers at the Italian National Institute of Nuclear Physics have observed the rotation of the polarization of light in a vacuum due to a transverse magnetic field. Emilio Zavattini and colleagues in the PVLAS Collaboration sent a linearly polarized laser beam through a 5 T field produced by a 1-m long superconducting dipole magnet and observed the polarization. Using light at a wavelength of 1064 nm, they made 44,000 passes, measuring an average rotation of 3.9±0.5 × 10-12 rad for each pass.
Some rotation is predicted from quantum electrodynamics via an effective four-photon vertex. The result here, however, suggests additional, new physics, such as the presence of an axion coupling to the scalar product of electric and magnetic fields. Axions, which couple to two photons, are light, neutral bosons, originally hypothesized to explain the absence of CP violation in strong interactions. Further results are eagerly awaited.
Further reading
E Zavattini et al. 2006 Phys. Rev. Letts. 96 110406.