Causality Bounds on Dissipative General-Relativistic Magnetohydrodynamics

We determine necessary and sufficient conditions under which a large class of relativistic generalizations of Braginskii's magnetohydrodynamics, described using Israel-Stewart theory, are causal and strongly hyperbolic in the fully nonlinear regime in curved spacetime. Our new nonlinear analysis provides stricter constraints on the dynamical variables that cannot be obtained via a standard linear expansion around equilibrium. Causality severely constrains the size of shear-viscous corrections, placing a bound on the far-from-equilibrium dynamics of magnetized weakly collisional relativistic plasmas, which rules out the onset of the firehose instability in such systems.