Kink-Antikink Collisions in the phi^4 Equation: The n-Bounce Resonance and the Separatrix Map

We provide a detailed mathematical explanation of a phenomenon known as the two-bounce resonance observed in collisions between kink and antikink traveling waves of the φ⁴ equations of mathematical physics. This behavior was discovered numerically in the 1980s by Campbell and his collaborators and subsequently discovered in several other equations supporting traveling waves. We first demonstrate the effect with new high-resolution numerical simulations. A pair of kink-like traveling waves may coalesce into a localized bound state or may reflect off each other. In the two-bounce resonance, they first coalesce, but later escape each other's embrace, with a very regular pattern governing the behaviors. Studying a finite-dimensional ``collective coordinates' model, we use geometric phase-plane based reasoning and matched asymptotics toexplain the mechanism underlying the phenomenon, including the origin of several mathematical assumptions needed by previous researchers. We derive a separatrix map for this problem---a simple algebraic recursion formula that explains the complex fractal-like dependence on initial velocity for kink-antikink interactions.