Experimental observation of vacuum birefringence

The PVLAS experiment operates an ellipsometer based on a Fabry-Perot optical cavity that embraces a superconducting rotating dipole magnet and can measure the ellipticity induced by the magnetic field onto linearly polarized laser light. With a residual pressure less than 10-7 mbar the apparatus gives ellipticity signals at the level of 10-11 rad per passage through 1 m of 5 Tesla transverse magnetic field of 532 nm wavelength laser light. These signals can be interpreted as being generated largely by vacuum birefringence. If this interpretation is valid, a tool has become available to characterize physical properties of vacuum as if it were an ordinary transparent medium. The main source of the induced ellipticity could be the existence of ultralight bosons with mass of the order of 10-3 eV that would couple to two photons and would be created in the experiment by interactions of photons of the laser beam with virtual photons of the magnetic field. The apparatus is calibrated in ampli...