Steam engines run because of statistics — the particles in the machine have a predictable overall behavior, which drives the engine's piston up and down. But if you shrink the engine down to micrometer size, it'll contain far fewer particles, few enough that the behavior of each individual particle could potentially throw the entire apparatus out of whack.

So in order to construct the world's tiniest steam engine, researchers from the Max Planck Institute for Intelligent Systems had to tweak the traditional design.

Here's how it works: A Stirling engine converts heat to work through the compression and expansion of a gas-in the case of a steam engine, this gas would be water vapor. In its simplest, ideal form, the Stirling cycle is a four-step process. Imagine a cylinder filled with a gas and covered with a piston. The first two steps in the cycle decrease the pressure in the container. First, you raise the piston to increase the volume of the container while using an external heat reservoir to keep the temperature from changing. Next, the volume remains constant while the gas is cooled. The next two steps bring the pressure, temperature, and volume back to their original states. The piston is lowered to decrease the volume back to its original amount while a cold reservoir maintains the temperature. Finally, the temperature is heated back up to its original level.

If a Stirling cycle merely consisted of compression and expansion, then the work done by the piston and the work done on the piston would cancel out, and the engine would not have done any overall work. Because of the temperature changes, however, the net work done BY the piston will be positive.

Of course, not all engines are based on this exact single-cylinder structure. Another difference from the simple model is that instead of heat and cold reservoirs, they use "regenerators" to "move" heat into and out of the gas. But the basic principles of the Stirling cycle-the use of piston and gas, and the four-step process of expansion-cooling-compression-heating-remain the same in any Stirling engine.

In the tiny engine, the piston is replaced by a laser and the gas by a single "particle": a plastic bead, only three millionths of a meter wide, afloat in water. Instead of using a piston to compress and expand the volume of the cylinder in which the particle floats, researchers varied the intensity of the laser beam to limit the motion of the "particle," which in turn does work on the laser field. Like in a traditional Stirling engine, heat must be added into and subtracted from the system in order to make the piston do work. Specifically, the system must be heated during the expansion step, which the researchers did with a second laser beam that delivered a sudden burst of heat when switched on, but let the system cool when turned off.

In an average cycle, the tiny machine can run with as much efficiency as its much larger counterpart. There are issues, however, with the water molecules in which the bead floats. As the molecules of H2O collide randomly with the plastic bead, they exchange energy, which means that in each cycle, the amount of energy gained can vary widely.

Despite the erratic energy delivery, the fact that a micro-scale heat engine can actually do work proves that the statistical nature of thermodynamics is not an insurmountable obstacle to such an engine. And that, in turn, means that micro-engines based on this design could power efficient micromachines.