Photons Bouncing Off Each Other?

Light-light interactions in a vacuum will be possible soon.

Vacuum -- the very name suggests emptiness and nothingness -- is actually a realm rife with potentiality, courtesy of the laws of quantum electrodynamics (QED). According to QED, additional, albeit virtual, particles can be created in the vacuum, allowing light-light interactions.

Physicists from Umeå University, in Umeå, Sweden, and the Rutherford Appleton Lab, near Didcot, England, hope to explore the vacuum by aiming three powerful laser streams at each other. The laser light is not aimed at any material target and is not trying to initiate any nuclear fusion. Instead the three beams will merge to produce a fourth stream with a wavelength shorter than any of the input beams.

This idea of mixing beams has been broached before but the earlier proposals had the beams all in a single plane. The Swedish-British proposal (contact Mattias Marklund, +46-90-786-7717, mattias.marklund@physics.umu.se), by contrast, foresees a fully three-dimensional wave mixing process.

The actual experiment is planned to be carried out over the next year at the Rutherford Appleton Lab. By carefully polarizing the incoming light beams, the number of photons in the output beam can be controlled, providing valuable information about the interactions that took place in the vacuum.

What is this "four-wave mixing" good for? For studying QED itself, but also for testing theories that propose the existence of minor departures from Lorentz invariance, which is the proposition -- essential to special relativity -- that there is no preferred frame of reference. Light-light interactions might also be used to explore various hypotheses related to dark energy.

Lundström et al., Physical Review Letters, 3 March 2006
Contact Mattias Marklund, mattias.marklund@physics.umu.se, +46-90-786-7717