It’s one of gentlest caresses imaginable: the touch of a single atom of gas. Now, scientists have measured its impact.

A levitating nanoparticle jiggled when hit by individual gas atoms or molecules in a nearly empty vacuum chamber, scientists report in the October 23 Physical Review Letters .

The force exerted by a gas is usually the result of many atoms or molecules bouncing around all at once: Think of the air inside a balloon keeping it inflated. But even an individual atom can exert a tiny force. To detect that force, researchers used a laser to levitate a glass sphere, just 100 nanometers across and weighing about a quadrillionth of a gram, inside a vacuum chamber. Then, the researchers added small amounts of a heavy gas, either krypton, xenon or sulfur hexafluoride. The first two are gases made up of atoms, while the third consists of molecules.

The nanosphere was about 10 million times as massive as an individual gas particle, making the impact extremely difficult to detect.…