Metastability and dispersive shock waves in the Fermi-Pasta-Ulam system

We show the relevance of the dispersive analogue of the shock waves in the FPU dynamics. In particular we give strict numerical evidence that metastable states emerging from low frequency initial excitations are indeed constituted by dispersive shock waves travelling through the chain. Relevant characteristics of the metastable states, such as their frequency extension and their time scale of formation, are correctly obtained within this framework, using the underlying continuum model, the KdV equation.