PVLAS Experiment

PVLAS: a search for the optical birefringence of vacuum

Nonlinear electrodynamic effects in vacuum have been predicted since the earliest days of quantum electrodynamics (QED), a few years after the discovery of positrons. One such effect is vacuum magnetic birefringence, closely connected to elastic light-by-light interaction. The effect is extremely small and has never yet been observed directly.
Although today QED is a very well-tested theory, the importance of detecting light-by-light interaction remains. First, QED has always been tested in the presence of charged particles either in the initial state or the final state. No tests exist in systems with only photons. More in general, no interaction has ever been observed directly between gauge bosons present in both the initial and final states. Second, to date, the evidence for zero-point quantum fluctuations relies entirely on the observation of the Casimir effect, which applies to photons only. Here we are dealing with the fluctuations of virtual charged particle-antiparticle pairs (of any nature, including hypothetical millicharged particles) and therefore the structure of fermionic quantum vacuum: to leading order, it would be a direct detection of loop diagrams. Finally, the observation of light-by-light interaction would be an evidence of the breakdown of the superposition principle and of Maxwell’s classical equations. One important consequence of a nonlinearity is that the velocity of light would depend on the presence or not of other electromagnetic fields.

The direct measurement of this effect is yet to be seen and has been the aim of the PVLAS experiment since its beginnings. The PVLAS experiment, receives grants from the Italian Istituto Nazionale di Fisica Nucleare (INFN) and MIUR, and is currently located in the Physics Dept. of the Ferrara University.

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