  The PVLAS experiment operates an ellipsometer which embraces a
superconducting dipole magnet and can measure ellipticity and rotation induced
by the magnetic field onto linearly polarized light. The sensitivity of the
instrument is about 10-7rad Hz-1/2. With a residual pressure less than 10-7mbar
the apparatus gives both ellipticity and rotation signals at the 10-7rad level
with more than 8 sigma sob ratio in runs that last about 1000sec. These signals
can be interpreted as beeing generated largely by vacuum ellipticity and
dichroism induced by the transverse magnetic field.If this interpretation is
correct, a tool has become available to characterize physical properties of
vacuum as if it were an ordinary transparent medium. A microscopic effect
responsible for this induced dichroism could be the existence of ultralight
spin zero bosons with mass of the order of 10-3eV, that would couple to two
photons and would be created in the experiment by interaction of photons of the
laser beam with virtual photons of the magnetic field. The inverse of the
coupling constant to two photons would correspond to a mass M of the order of
10+6GeV.
