Korteweg–de Vries equation: u_t + 6u·u_x + u_xxx = 0. Two solitons of different heights travel at different speeds and collide — yet emerge perfectly unchanged. Elastic nonlinear wave collision, discovered 1965 (Zabusky & Kruskal).
t = 0.000
Pseudospectral integration with 4th-order Runge-Kutta. Periodic boundary conditions. Amplitude ratio controls speed ratio — taller soliton overtakes and passes through shorter one without deformation.