The 1D contact process: infected sites recover (rate 1) and spread to neighbors (rate λ). At the critical rate λ_c ≈ 3.298, the system undergoes a second-order phase transition in the Directed Percolation universality class — one of the most fundamental non-equilibrium transitions.
Critical λ_c ≈ 3.298 (1D). Exponents: β≈0.277, ν_⊥≈1.097, ν_∥≈1.734 (DP universality class). Above λ_c: active phase persists. Below: absorbing state (all recovered) reached in finite time.