From Traffic Jams to Heart Attacks

A driver on the highway taps her brake. Just a tap. Maybe she was startled by a bird. Maybe she glanced at her phone and looked up. The car behind her brakes a little harder. The car behind that one, harder still. By the time the signal reaches the car half a mile back, it has become a wall of stopped traffic — and the woman who tapped her brake is already doing seventy miles an hour in clear road ahead. No accident. No lane closure. No reason visible from inside the jam. The jam is the reason. It made itself out of almost nothing — one small perturbation, and a wave that grew as it traveled backward through a system that was primed, by its own density, to amplify. Now forget the highway. Think about a heart. A healthy heart beats because electrical signals move through it in a coordinated wave — seventy times a minute, every chamber firing in sequence. The signal is elegant and self-sustaining. But introduce a small irregularity.