Cosmic rays do not travel in straight lines. Charged particles accelerated to enormous speeds loop through the Galaxy's magnetic field like dust motes in a gust of wind. It's usually assumed that they simply scatter along and across field lines. But the field has bends and eddies that also make particles drift — sliding sideways, like a skier cutting diagonally down a slope.
The authors built a computer model of the Galactic magnetic field, complete with a large-scale ordered component and small-scale turbulent ripples. They injected test particles and watched how they escaped the Galaxy. It turned out that at energies around one PeV (petaelectronvolt, or quadrillion electronvolts), drift can become the dominant escape mechanism. Particles start leaving faster, and the plot of their numbers develops a characteristic bend, the "knee."
However, for the knee to appear, a rare condition must hold: scattering along magnetic field lines must stop increasing with energy. Otherwise, particles would slip away too quickly, and drift wouldn't have a chance to show. The scientists considered two scenarios where this could happen, and in both, the bend indeed appears. This isn't the final answer, but it's an important step toward solving an old mystery — why the cosmic-ray spectrum breaks exactly where it does.
The knee in the cosmic-ray spectrum was discovered back in the 1950s, but its cause is still debated. One hypothesis is that particles simply can't be confined in the Galaxy at too high energies and start leaking into intergalactic space.