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The idea that particles of light can interact with one another – known as light-by-light scattering – has finally been observed some 80 years after it was first predicated. That's the claim of members of the ATLAS collaboration at CERN in Geneva, who have combed through data they took in 2015 when lead ions collided with each other in their detector. Some scientists, however, dispute the priority of the finding, arguing that light-by-light scattering was observed by an experiment at the SLAC National Accelerator Laboratory in California 20 years ago.

Classically, light cannot interact with light because photons – even though they mediate interactions between charged particles – do not themselves carry charge. However, Heisenberg's uncertainty principle, a cornerstone of quantum mechanics, says that photons can briefly transform into "virtual" pairs of particles and antiparticles, such as electrons and positrons. There is then a tiny chance that these virtual particles can recombine to create pairs of real photons.

The upshot is that two photons, each producing a virtual particle-antiparticle pair in the process, can scatter off one another. In doing so, they change direction but do not lose any energy. The interaction, in other words, is elastic.

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