A dust grain hangs in the air, held by an invisible beam of light. Around it is ordinary air, but sometimes a tiny spark jumps across the grain. For the first time, scientists have observed such discharges up close and found that they are triggered not by the grain itself, but by invisible visitors from space and from the ground.
An optical tweezer (a beam that can hold microscopic objects) kept a dust grain the size of a bacterium in place. Every so often, the grain's charge would abruptly change by a few dozen elementary charges (as if a few dozen electrons had jumped onto or off the grain). These events were dubbed microdischarges.
Previously, it was thought that a discharge required some threshold, like an overflowing cup, where water spills over the rim. But here there was no threshold: discharges occurred at various charges on the grain. This means the cause is not the grain itself but the surrounding air. Natural radiation (from cosmic rays and traces of radioactive elements in the ground) constantly ionizes the air — knocking electrons off atoms. As the grain drifts through such an "ion cloud," it quickly captures ions and changes its charge.
In this way, the grain becomes a tiny radiation detector. Each spark on its surface is a trace of an invisible particle that just flew by. By studying such discharges, we can better understand how electricity behaves at the smallest scales — without wires or electrodes, simply in the air.
The grain's charge changed by only 40 elementary charges — billions of times less than the charge that builds up on a TV screen from friction.
In Charles Pellegrino's novel "Dust," microscopic particles serve as sensors of cosmic radiation, just as in this experiment.