A proton or an atomic nucleus slams into Earth's atmosphere at tremendous speed, and a cascade of billions of particles is born. This extensive air shower races toward the surface, while sensitive telescopes catch its faint glow. From the shape of the glow, physicists determine how deep the shower penetrated the atmosphere and what it was made of.
But there's a catch. The theory of relativity calls the speed of light in a vacuum an absolute limit — nothing can exceed it. But what if, at unimaginably high energies, this rule is slightly violated? Then some processes would proceed differently. For example, the production of particle pairs (when energy creates an electron and its antiparticle, a positron) would become less likely. The electromagnetic component of the shower would decrease, and it would penetrate deeper.
The authors of the study took a simplified model of such a violation and ran it through a simulation of the detector response at the Pierre Auger Observatory in Argentina. They compared what showers with and without the violation would look like and matched them against real data. The method proved sensitive to the sought-after effect. The researchers obtained a preliminary lower limit on the energy at which the violation could manifest: more than a billion billion times higher than the energy achievable at the Large Hadron Collider.
So, is the speed of light still unbreakable? For now, yes. But physicists now have another tool to test this in the most extreme conditions nature has to offer — in showers produced by particles with energies millions of times higher than in any earthly accelerator.
An extensive air shower from a single ultra-high-energy particle can simultaneously cover an area of tens of square kilometers, like a small city.
In Neal Stephenson's novel Anathem, a violation of fundamental laws of physics in the distant past led to a catastrophe that changed the very fabric of civilization.