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.
Visualising a detected light-by-light scattering event. Credit: ATLAS/CERN
In the visualisation above, we see data from a collision between two photons. The photons approach from behind and in front of the image, meeting in the centre of the circles. The green and yellow sections represent the energy deposited by the scattered photons, as detected by the ATLAS detector.
"This is a milestone result: the first direct evidence of light interacting with itself at high energy," said ATLAS physics coordinator Dan Tovey in a press release.
"This phenomenon is impossible in classical theories of electromagnetism; hence this result provides a sensitive test of our understanding of QED, the quantum theory of electromagnetism."
The next lead run of the Large Hadron Collider is scheduled to take place at the end of 2018. The research team hope to take more observations at that time to improve the precision of the result. There may also be the potential for a new field of study.
"Maybe we can find evidence for physics beyond the standard model of particle physics, for example axion-like particles that are a possible candidate for dark matter," Dyndal said. "Different theoretical concepts predict that light-by-light scattering can be sensitive to such particles."
The research has been published in the journal Nature Physics.