Photonphoton

Photon-photon interactions, or light-by-light scattering, describe the scattering of two photons through higher-order quantum processes. In quantum electrodynamics (QED), photons do not interact at tree level, but can interact indirectly via virtual charged particles, most commonly an electron-positron loop that couples the photons.

Because the interaction proceeds through loop diagrams, the cross section is extremely small at ordinary energies, making the effect challenging to observe. The phenomenon is often described at low energies by the Euler-Heisenberg effective Lagrangian, which predicts nonlinear optical effects in vacuum such as four-photon scattering and vacuum birefringence.

Historically, light-by-light scattering was predicted in the 1930s as part of nonlinear QED by Euler and Heisenberg. Experimental evidence for the process in the laboratory emerged in the 2010s from ultraperipheral heavy-ion collisions at the Large Hadron Collider, with ATLAS reporting evidence in 2017 and subsequent analyses refining measurements.

Beyond confirming QED, photon-photon scattering provides a window into potential new physics that could modify the interaction, such as axion-like particles or other beyond-Standard-Model phenomena. It also motivates future experiments with higher luminosity colliders and high-intensity lasers that probe strong-field quantum electrodynamics.

In summary, photon-photon scattering is a rare but fundamental QED process arising from quantum loops, with ongoing experimental study in collider and laser facilities and potential implications for new physics.