Diffusive hydrodynamics from long-range correlations

TL;DR

This paper reveals that in integrable and linearly degenerate hydrodynamic systems, diffusive corrections are governed by ballistic transport of initial fluctuations, leading to reversible hydrodynamic equations that challenge the traditional Navier-Stokes framework.

Contribution

It introduces a non-local, ballistic fluctuation-based approach to describe diffusive effects, extending the understanding of hydrodynamics beyond linear response and gradient expansions.

Findings

Diffusive corrections are determined by ballistic transport of initial fluctuations.

Hydrodynamic equations become reversible, deviating from standard dissipative models.

The approach is validated within the framework of ballistic macroscopic fluctuation theory.

Abstract

In the hydrodynamic theory, the non-equilibrium dynamics of a many-body system is approximated, at large scales of space and time, by irreversible relaxation to local entropy maximisation. This results in a convective equation corrected by viscous or diffusive terms in a gradient expansion, such as the Navier-Stokes equations. Diffusive terms are evaluated using the Kubo formula, and possibly arising from an emergent noise due to discarded microscopic degrees of freedom. In one dimension of space, diffusive scaling is often broken as noise leads to super-diffusion. But in linearly degenerate hydrodynamics, such as that of integrable models, diffusive behaviors are observed, and it has long be thought that the standard diffusive picture remains valid. In this letter, we show that in such systems, the Navier-Stokes equation breaks down beyond linear response. We demonstrate that diffusive-order corrections do not take the form of a gradient expansion. Instead, they are completely determined by ballistic transport of initial-state fluctuations, and obtained from the non-local two-point correlations recently predicted by the ballistic macroscopic fluctuation theory (BMFT); the resulting hydrodynamic equations are reversible. To do so, we establish a regularised fluctuation theory, putting on a firm basis the recent idea that ballistic transport of initial-state fluctuations determines fluctuations and correlations beyond the Euler scale. This extends the idea of ``diffusion from convection'' previously developed to explain the Kubo formula in integrable systems, to generic non-equilibrium settings.