A normal engine has cylinders, pistons and pumping parts. The smallest engine ever built has just one moving piece: a single calcium atom, floating in a beam of light. In 2016, physicists at the University of Mainz showed that even one atom can be made to do work - exactly like a microscopic motor.
The team trapped the atom inside an electric field shaped like a cone. They warmed and cooled the atom in cycles using lasers. As it got hotter it pushed outward, and as it cooled it slid back. That tiny push and pull is the same trick that makes engines in cars or trains run.
You couldn’t drive anywhere with this engine - its power output is only about a billion-billionth of a normal car’s. But the experiment proved something amazing. The rules that run engines as big as steam locomotives still hold when you shrink everything down to a single atom drifting in a trap.
