Contact Process — Absorbing State Transition

The contact process is the canonical model for absorbing-state phase transitions. Each site is either active (infected) or inactive. Active sites recover at rate μ and spread to neighbors at rate λ. Critical point λ_c ≈ 1.65μ (1D) separates the absorbing phase (extinction) from the active phase (survival). This transition belongs to the directed percolation universality class with exponents β≈0.277, ν_⊥≈1.097, ν_∥≈1.734. The space-time diagram below scrolls upward.