Nonequilibrium steady states (NESS) sustain net probability currents J even at steady state — impossible at equilibrium (detailed balance: J=0).
Ring model: particle on a periodic potential with a nonconservative driving force F. Current J = e^{F/kT}·Z⁻¹ (Kramers).
2D landscape: probability density shown as heatmap; arrows show probability current J = P·v − D·∇P. Entropy production rate = ∮J·F/T·dx.
Increasing drive F breaks detailed balance and drives circulation — the hallmark of life, motors, and active matter.