Generalized global symmetries and dissipative magnetohydrodynamics

TL;DR

This paper develops a comprehensive relativistic hydrodynamic theory for magnetized plasmas based on generalized global symmetries, deriving transport coefficients, resistivity, and collective modes without assuming weak electromagnetic coupling.

Contribution

It systematically derives magnetohydrodynamics from symmetry principles, introduces a universal resistivity definition, and explores low-temperature regimes with emergent Lorentz invariance.

Findings

Seven dissipative transport coefficients identified.

Universal Kubo formula for resistivity derived.

New expressions for magnetosonic and Alfvén mode widths.

Abstract

The conserved magnetic flux of U(1) electrodynamics coupled to matter in four dimensions is associated with a generalized global symmetry. We study the realization of such a symmetry at finite temperature and develop the hydrodynamic theory describing fluctuations of a conserved 2-form current around thermal equilibrium. This can be thought of as a systematic derivation of relativistic magnetohydrodynamics, constrained only by symmetries and effective field theory. We construct the entropy current and show that at first order in derivatives, there are seven dissipative transport coefficients. We present a universal definition of resistivity in a theory of dynamical electromagnetism and derive a direct Kubo formula for the resistivity in terms of correlation functions of the electric field operator. We also study fluctuations and collective modes, deriving novel expressions for the dissipative widths of magnetosonic and Alfven modes. Finally, we demonstrate that a non-trivial truncation of the theory can be performed at low temperatures compared to the magnetic field: this theory has an emergent Lorentz invariance along magnetic field lines, and hydrodynamic fluctuations are now parametrized by a fluid tensor rather than a fluid velocity. Throughout, no assumption is made of weak electromagnetic coupling. Thus, our theory may have phenomenological relevance for dense electromagnetic plasmas.