A pebble drops into a pond, and ripples spread across the water. But one wave suddenly doesn't spread out: it clings to an invisible fold on the surface and runs only along it. Something similar was discovered by physicists when they recalculated the encounter of light with an axion domain wall — a narrow region where the field changes sign, much like a mountain ridge separating two slopes.
Light is an oscillating electromagnetic field, and it has polarization: the wave 'rotates' either clockwise or counterclockwise. The authors of the work saw that the wall acts on these two rotations differently. One direction it attracts, like a groove collecting water, and around the wall, a light clump forms that cannot escape sideways. The other direction, the wall repels. This gives birth to a 'wall-hugging' beam that runs without attenuation and with zero rest energy.
Previously, it was thought that such walls are transparent to radiation. The new work shows that this is not the case. Trapped light is not just a mathematical curiosity; it could arise in the early Universe or inside hypothetical objects like axion stars. And if such a beam is ever detected, it would become a window into physics beyond the familiar particles.
So a simple fold of the field turns out to be a trap for light. And who knows how many more such 'streams' flow along the invisible boundaries of the cosmos, waiting to be noticed.
Axions, particle candidates for dark matter, were invented in the 1970s to solve a problem with symmetry violation in the strong interaction. Their name was borrowed from a laundry detergent because they 'clean up' the theory.