In 1936 Werner Heisenberg and Hans Euler predicted that as a consequence of quantum electrodynamics (QED), strong magnetic fields should affect the propagation of light through a vacuum. The effect, known as vacuum birefringence, is caused by virtual electron–positron pairs that get yanked along in the direction of the field. Light that is polarized in the same direction as the magnetic field should interact more strongly with the raging river of virtual particles, and thus propagate more slowly, than light polarized in other directions.
Because vacuum birefringence scales with the square of magnetic field strength, Nir Shaviv and Jeremy Heyl proposed analyzing the light emitted by neutron stars, which sport fields in excess of 1012 gauss. The signature would be a common direction of polarization, since the QED effect ensures that polarization follows the changing direction of the ultrastrong magnetic field.
Now a team led by Roberto Mignani from...
