On the convergence of the gradient expansion in hydrodynamics

Abstract

Hydrodynamic excitations corresponding to sound and shear modes in fluids are characterised by gapless dispersion relations. In the hydrodynamic gradient expansion, their frequencies are represented by power series in spatial momenta. We investigate the analytic structure and convergence properties of the hydrodynamic series by studying the associated spectral curve in the space of complexified frequency and complexified spatial momentum. For the strongly coupled $\mathcal{N}=4$ supersymmetric Yang-Mills plasma, we use the holographic duality methods to demonstrate that the derivative expansions have finite non-zero radii of convergence. Obstruction to the convergence of hydrodynamic series arises from level-crossings in the quasinormal spectrum at complex momenta.

V3: 5 pages, 2 figures. Final version. Published in Physical Review Letters with the title "Convergence of the Gradient Expansion in Hydrodynamics"

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• Minkowski-space correlators in AdS/CFT correspondence: recipe and applications
• Relativistic viscous hydrodynamics, conformal invariance, and holography
• The stickiness of sound: An absolute lower limit on viscosity and the breakdown of second order relativistic hydrodynamics

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