Ehsan Roohi

Atoms in a gas behave like billiard balls: they fly, collide, and scatter in different directions. But instead of solid balls, they have a soft "coat" of attractive and repulsive forces. At close range, atoms repel sharply; at longer range, they attract slightly. Scientists need to calculate these collisions precisely to model gas flows around airplanes or inside microchips.

A cylinder slams into the air at enormous speed, and a shock wave is born around it — a sharp boundary where the air is instantly compressed and heated. Ernst Mach was the first to photograph these jumps, and since then we've thought of them as thin, almost invisible walls.