It is well known that Maxwell's equations in vacuum are linear: the superposition principle is valid for electromagnetic fields. Heisenberg's principle, however, allows the vacuum to fluctuate, thereby producing virtual pairs of particles which, if charged, bring about non-linear effects in vacuum such as light-light scattering through a box diagram. Furthermore, hypothetical neutral particles coupling to photon pairs may also allow photon-photon interaction. In presence of an external field (magnetic or electric), these effects result in v≠c, birefringence, and dichroism.
The PVLAS experiment, with a new set-up currently being assembled at the Physics Department of the University of Ferrara, Italy, has this very aim of measuring both the real and imaginary part of the index of light refraction when traversing a strong magnetic field. The Trieste INFN is actively collaborating with this new set-up, which could lead to the very first detection of this minute but fundamental QED effect.
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