Text: Optical rotation sheds light on vacuum 27 March 2006 Physicists in Italy have demonstrated that empty space can cause light to rotate in the presence of a large magnetic field. Although the effect seen by Emilio Zavattini and co-workers on the PVLAS experiment at the INFN laboratory in Legnaro is extremely small, it could provide evidence for exotic new "dark matter" particles called axions (Phys. Rev. Lett. 96 110406). Classical physics tells us that space is empty, but the uncertainty principle allows particles and antiparticles to spontaneously appear and disappear, changing the structure of the vacuum in the process. In particular, a large magnetic field can cause the refractive index of space to vary with the polarization of light passing through it. In the PVLAS experiment a linearly polarized laser beam is sent through 5 Tesla magnetic field in a vacuum, and any changes in the polarization of the beam over a distance of 1m are measured. Based on 44,000 such measurements Zavattini and co-workers found that the beam emerges with a slight elliptical polarization, and that its polarization vector is rotated by 3.9±0.5x10-12 radians (less than half a billionth of a degree). http://physicsweb.org/articles/news/10/3/19/1

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