Effective dynamics for $N$-solitons of the Gross-Pitaevskii equation

We consider several solitons moving in a slowly varying external field. We show that the effective dynamics obtained by restricting the full Hamiltonian to the finite dimensional manifold of $N$-solitons (constructed when no external field is present) provides a remarkably good approximation to the actual soliton dynamics. That is quantified as an error of size $h^2$ where $h$ is the parameter describing the slowly varying nature of the potential. This also indicates that previous mathematical results of Holmer-Zworski for one soliton are optimal. For potentials with unstable equilibria the Ehrenrest time, $\log(1/h)/h$, appears to be the natural limiting time for these effective dynamics. We also show that the results of Holmer-Perelman-Zworski for two mKdV solitons apply numerically to a larger number of interacting solitons. We illustrate the results by applying the method with the external potentials used in Bose-Einstein soliton train experiments of Strecker et. al.