Two Echoes from One Particle: How a Telescope Will Hear Rare Showers ✛ ✎

Imagine a tiny particle—a proton or a helium nucleus—entering the atmosphere. It strikes air molecules, and a stream of particles rains down, like splashes from a stone thrown into water. Usually these splashes stay together. But occasionally one gets an especially strong kick and flies far ahead. Then the stream splits: you get two clumps instead of one.

Until now, such double showers have only been seen in computer simulations. Real instruments couldn't detect them because they are extremely rare. The new SKA-Low radio telescope is a vast field with thousands of antennas listening to radio waves from the atmosphere. It is so sensitive that it can pick up the signal from both clumps at once. Their waves combine, and at certain frequencies they cancel each other out. This creates dips in the spectrum, like two echoes arriving a fraction of a second apart.

The scientists built a simple model: they treated each clump as an independent source of radio waves. That was enough to calculate the altitude of both clumps from the dips. Then they tested the method on simulated events and achieved an accuracy of tens of grams per square centimeter. That's like measuring the thickness of a sheet of paper with a ruler.

Now, when the SKA-Low telescope operates at full capacity, it will be able to catch such double showers. The distance between the clumps will reveal the composition of the original particle. This matters because cosmic rays remain a mystery: we don't know exactly where they come from or what they are made of. Double showers will provide a new clue.