Andrea Blanco-Redondo

A laser pointer shines a neat spot: all its light waves march in step, like soldiers on parade. Such light is called coherent. But nature and technology also feature another kind of light—'messy' light, where waves are out of sync, like a crowd at a market. It's called partially coherent.

A wave packet is like a traveler who either loses steam and spreads out along the way, holds a steady pace, or gathers into a fist. In a 50-meter flume with water 5 meters deep, scientists tracked how a bell-shaped group of waves behaves when its height is changed but not its width.