Interaction renormalization and validity of kinetic equations for turbulent states

TL;DR

This paper investigates the limitations of kinetic equations in describing wave turbulence, revealing divergences at IR and UV scales and proposing a summation method to improve the theory's validity, especially for long cascades.

Contribution

It introduces a summation of UV-divergent terms in wave turbulence kinetic theory, showing the theory's breakdown at long cascades and linking strong turbulence to collective effects and bound states.

Findings

UV divergences can be summed to all orders, removing UV issues.

IR divergences imply increased effective coupling at large scales.

Strong turbulence may involve multi-wave bound states like solitons.

Abstract

We consider turbulence of waves that interact weakly via four-wave scattering (sea waves, plasma waves, spin waves, and many others). In the first non-vanishing order in the interaction, the occupation number of waves satisfy a closed kinetic equation which has stationary solutions describing turbulent cascades. We show that a straightforward perturbation theory beyond the kinetic equation gives terms that generally diverge both at small (IR) and large (UV) wavenumbers for a direct cascade. The analysis up to the third order identifies the most UV-divergent terms. In order to gain qualitative analytic control, we sum a subset of the most UV divergent term, to all orders, giving a perturbation theory which is generally free from UV divergence, showing that turbulence becomes independent of the dissipation scale when it goes to zero. On the contrary, the ever-present IR divergence means that the effective coupling is parametrically larger than the naive estimate and grows with the pumping scale (similar to anomalous scaling in fluid turbulence). This suggests that the kinetic equation does not describe wave turbulence even of arbitrarily small level if the cascade is sufficiently long. We show that the character of strong turbulence is determined by the sign of the coupling, that is, whether the effective four-wave interaction is enhanced or suppressed by collective effects. The enhancement possibly signals that strong turbulence is dominated by multi-wave bound states (solitons, shocks, cusps), similar to confinement in quantum chromodynamics.