A fundamental question in wave turbulence theory is to understand how the wave kinetic equation describes the long-time dynamics of its associated nonlinear dispersive equation. Formal derivations in the physics literature, dating back to the work of Peierls in 1928, suggest that such a kinetic description should hold (for well-prepared random data) at a large kinetic time scale
$T_{\mathrm {kin}} \gg 1$
and in a limiting regime where the size L of the domain goes to infinity and the strength
$\alpha $
of the nonlinearity goes to
$0$
(weak nonlinearity). For the cubic nonlinear Schrödinger equation,
$T_{\mathrm {kin}}=O\left (\alpha ^{-2}\right )$
and
$\alpha $
is related to the conserved mass
$\lambda $
of the solution via
$\alpha =\lambda ^2 L^{-d}$
.
In this paper, we study the rigorous justification of this monumental statement and show that the answer seems to depend on the particular scaling law in which the
$(\alpha , L)$
limit is taken, in a spirit similar to how the Boltzmann–Grad scaling law is imposed in the derivation of Boltzmann’s equation. In particular, there appear to be two favourable scaling laws: when
$\alpha $
approaches
$0$
like
$L^{-\varepsilon +}$
or like
$L^{-1-\frac {\varepsilon }{2}+}$
(for arbitrary small
$\varepsilon $
), we exhibit the wave kinetic equation up to time scales
$O(T_{\mathrm {kin}}L^{-\varepsilon })$
, by showing that the relevant Feynman-diagram expansions converge absolutely (as a sum over paired trees). For the other scaling laws, we justify the onset of the kinetic description at time scales
$T_*\ll T_{\mathrm {kin}}$
and identify specific interactions that become very large for times beyond
$T_*$
. In particular, the relevant tree expansion diverges absolutely there. In light of those interactions, extending the kinetic description beyond
$T_*$
toward
$T_{\mathrm {kin}}$
for such scaling laws seems to require new methods and ideas.