N self-propelled particles move at constant speed, each aligning with neighbors within radius r — but with noise η. Below a critical noise level the system spontaneously orders: a nonequilibrium phase transition to collective flocking. The order parameter ⟨v⟩/v₀ jumps discontinuously, making it first-order in 2D (Vicsek et al., 1995; Toner-Tu theory).