Photons form the basis of light - packets of energy that travel at 299 792 458 m/s. Yet, when two light beams usually shine across each other, nothing interesting happens. Since photons have no electric charge, Quantum Electrodynamics (QED) predicts that there should be no direct photon-photon interactions.
Photo Credit: CERN/ATLAS Team
However, photons can fluctuate into fermion-antifermion pairs (such as electron-positron pairs) which are charged and thus interact through higher-order corrections.
Photo Credit: Wikipedia
A photon can temporarily disintegrate into a virtual electron and positron, before recombining back to form the photon. During this period, the 2 pairs of electrons and positrons from the 2 photons can bounce of each other, producing a scattering effect. This is only observed with photons of high energy. These photons are produced by smashing lead nuclei of together at near light speed and observing the resultant decay. When 2 energetic photons are lucky enough to be close to one another, these photon-photon interactions can possibly be observed.
Photo Credit: Mariola et al. (2016)
These observations form a further confirmation of the Perturbative Quantum Electrodynamics, and allow us to probe deeper into photon structure.
References
- Klusek-Gawenda, M., Lebiedowicz, P., & Szczurek, A. (2016). Light-by-light scattering in UPC at the LHC. arXiv preprint arXiv:1606.09038.
- Fichet, S., von Gersdorff, G., Lenzi, B., Royon, C., & Saimpert, M. (2015). Light-by-light scattering with intact protons at the LHC: from Standard Model to New Physics. Journal of High Energy Physics, 2015(2), 165.
- ATLAS Collaboration. (2017). Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC. arXiv preprint arXiv:1702.01625.
- Salgado, C. A., Alvarez-Muniz, J., Arleo, F., Armesto, N., Botje, M., Cacciari, M., ... & Gelis, F. (2011). Proton–nucleus collisions at the LHC: scientific opportunities and requirements. Journal of Physics G: Nuclear and Particle Physics, 39(1), 015010.