Light is pretty awesome. It’s made of subatomic particles called photons, which also behave like waves. It’s been demonstrated to act like both a particle and a wave simultaneously. Photons can be entangled at a distance. They reflect, refract and diffract. They have angular momentum, but no mass.
One thing they had never been observed doing was bouncing off each other and changing direction like snooker balls. But new research from the ATLAS experiment at CERN describes the first direct evidence of this actually happening.
The phenomenon is called light-by-light scattering, described by the Euler-Heisenberg Lagrangian published in 1936 by Hans Heinrich Euler and Werner Heisenberg (of uncertainty principle fame), and calculated by Robert Karplus and Maurice Neuman in 1951.
“According to classical electrodynamics, beams of light pass each other without being scattered,” explained Mateusz Dyndal, a researcher from DESY who performed a major role in data analysis, in a press release. “But if we take quantum physics into account, light can be scattered by light, even though this phenomenon seems very improbable.”
ATLAS researcher Jon Butterworth, a physics professor at University College London, likened it to two rubber balls bouncing off each other in an article he wrote for The Guardian.
The observation took place in the Large Hadron Collider, during a 2015 run in which it was smashing lead nuclei together. This is a much higher energy particle than the collider’s usual protons, which means there’s a dense cloud of photons involved. The heavy ions don’t usually collide with each other, but the photons can interact in what is called “ultra-peripheral collisions.”Out of four billion events analysed, the team found 13 candidate events for two photons interacting with each other and changing direction, rather than passing each other by.