Full derivation of the wave kinetic equation

TL;DR

This paper rigorously derives the wave kinetic equation from the cubic nonlinear Schrödinger equation in the limit of large system size and weak nonlinearity, confirming a key conjecture in wave turbulence theory.

Contribution

It provides the first rigorous derivation of the wave kinetic equation for a nonlinear dispersive system, analogous to Lanford's theorem for particle systems.

Findings

Derivation of wave kinetic equation from NLS in dimensions d≥3.

Validation of the wave turbulence conjecture under specific scaling laws.

Approximation holds up to times proportional to the inverse square of the nonlinearity strength.

Abstract

We provide the rigorous derivation of the wave kinetic equation from the cubic nonlinear Schr\"odinger (NLS) equation at the kinetic timescale, under a particular scaling law that describes the limiting process. This solves a main conjecture in the theory of wave turbulence, i.e. the kinetic theory of nonlinear wave systems. Our result is the wave analog of Lanford's theorem on the derivation of the Boltzmann kinetic equation from particle systems, where in both cases one takes the thermodynamic limit as the size of the system diverges to infinity, and as the interaction strength of waves or radius of particles vanishes to $0$, according to a particular scaling law (Boltzmann-Grad in the particle case). More precisely, in dimensions $d\geq 3$, we consider the (NLS) equation in a large box of size $L$ with a weak nonlinearity of strength $\alpha$. In the limit $L\to\infty$ and $\alpha\to 0$, under the scaling law $\alpha\sim L^{-1}$, we show that the long-time behavior of (NLS) is statistically described by the wave kinetic equation, with well justified approximation, up to times that are $O(1)$ (i.e independent of $L$ and $\alpha$) multiples of the kinetic timescale $T_{\text{kin}}\sim \alpha^{-2}$. This is the first result of its kind for any nonlinear dispersive system.