Water in a glass settles down once you stop stirring it. Oil in an engine dampens vibrations. It seems viscosity is always about damping. But in some flows, it's the opposite: add friction, and waves start to grow as if from nowhere.
Physicists have found a simple explanation. Such systems have a hidden energy reservoir, locked away by a special symmetry — called PT symmetry. As long as the symmetry is intact, the energy stays hidden and the flow remains calm. Viscosity breaks this symmetry, like a crack in a dam. The stored energy bursts out, and the flow loses stability: waves amplify on their own.
Previously, it was thought that viscosity only hinders waves. Now it's clear: it can be the very trigger that releases hidden energy. This matters for understanding plasma in fusion devices, ocean currents, and even the behavior of stellar gas.
Dam, spring, calm flow: it's all the same story. Symmetry stores energy, viscosity opens the lock, and calm gives way to a wave.
PT symmetry is when a system looks the same if you simultaneously mirror it and run time backward. You won't encounter that in everyday life, but in the physics of waves and light, such symmetries lurk in many equations.