dispersion

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.

If you push water in a narrow channel too fast, a steep wave forms and breaks into foam. In the world of ultracold atoms, cooled to near absolute zero, quantum laws prevent such a wave from breaking. Physicists took a cloud of rubidium atoms stretched into a thin thread and sharply compressed it from one end with a movable wall of light.

A disease outbreak is like a wildfire: the fire runs through dry grass, and its speed depends on wind and humidity. But an infection has its own 'wind'—how often people infect each other. When this rate changes along with the fraction of already infected, the epidemic wave can behave differently.

Physicists have achieved three-dimensional Anderson localization of electromagnetic waves — a phenomenon where waves become permanently entangled in a random labyrinth of metal particles and cease to propagate.

A pebble tossed into a pond sends ripples spreading in all directions. Now imagine the pond itself suddenly turning into jelly — instantly, faster than the wave can complete a single crest. Some ripples would keep spreading but change their spacing, while others would unexpectedly turn around and run back toward the point of impact, as if time were flowing backward for them.

Photographing the vicinity of a black hole is like shooting a fountain: a short exposure freezes the droplets, a long one blends everything into blurry streaks. Light from the superheated gas swirling around the hole reaches us along different paths. Some rays curve around it in an arc, others make multiple loops due to spacetime curvature.

A pebble drops into a pond, and neat circles spread across the water. But the pond's bottom is strewn with invisible glass shards: each one slightly delays the wave, scatters it here, or alters its pace. That's roughly how gravitational waves, the trembling of space itself, pass through distant galaxies and dark clumps of matter.

A swimmer cutting through water at the speed of the waves running on it finds themselves in a strange position: the waves they themselves generate can neither overtake them nor fall behind. They pile up at the swimmer's bow and freeze. A similar phenomenon occurs when a light source moves inside a medium at a speed matching the speed of energy transfer of the waves.

Fluid flows in the narrow gaps between brain cells. It doesn't stay still: each heartbeat pushes it forward, and breathing adds its own waves. This was seen in images ten years ago, but only now have researchers calculated how this pulsing accelerates molecules. If you drop ink into a stream, it drifts with the current. But if the stream speeds up and slows down, the ink spreads wider.

A stone drops into a pond — ripples spread across the water. Do short waves outrun long ones? No, in deep water it's the opposite: long waves overtake short ones. In metal, sound of one pitch travels at one speed, another pitch at another.

Shine a flashlight through a narrow slit, and the beam doesn't stay a thin stripe; it spreads out sideways. Light has a habit of bending around obstacles, and because of that, the smallest spot you can get with an ordinary lens is no smaller than half a wavelength.