Classically, light does not interact with itself because it is electrically neutral. But quantum-electrodynamic theory predicts that, in some circumstances, two photons can scatter off each other. In 2017, researchers at the Large Hadron Collider (LHC) published the first experimental evidence of this so-called light-by-light scattering, but with a certainty of 4.4 sigma, just below the commonly accepted threshold needed to claim a discovery. In an analysis of lead-ion collisions measured at the LHC in November 2018, researchers at the ATLAS experiment find evidence of photon-photon scattering with a certainty of 8.2 sigma.

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