Confine a chiral active fluid — particles that swim in circles — and currents appear along the edges. The standard intuition says this is a boundary effect: spinning particles collide with the wall and are deflected into a streaming motion. The explanation is local, mechanical, almost obvious.
The actual mechanism is global. Edge currents in chiral active liquids emerge from conservation of angular momentum, not from boundary interactions. The angular momentum injected by the active torques in the bulk must be balanced somewhere, and in a confined system the only available channel is a steady flow at the boundary. The current is dictated by a conservation law operating across the entire system, not by particles bouncing off walls.
The resulting edge current obeys an Ohmic-like conductance relation. Its magnitude depends on density, active torque, and drag — all intensive quantities, independent of system size. Double the container and the edge current stays the same. This is the signature of a bulk-determined phenomenon wearing a boundary disguise.
The distinction matters physically. If edge currents were a boundary effect, they would depend on boundary details — roughness, shape, material properties of the wall. Instead, they depend on bulk parameters that are insensitive to the container. Change the wall and the current doesn't care. Change the torque and the current responds proportionally.
A local phenomenon with a global cause. The edge is where the current flows, but the interior is where the current is decided. The boundary condition is the symptom; the conservation law is the disease.