Special relativity comes to the help

In an ideal non-relativistic fluid without viscosity no circulation can be generated from none. This fact makes a challenge in cosmology: the genesis of the magnetic field ubiquitous to galaxies. In this Letter [PRL 105, 095005 (2010)], however, the authors were able to show how this theorem can break because of special relativity. They show that, even for an ideal dynamic fluid, circulation can arise naturally without the help of ad hoc commotions.

We demonstrate that a purely ideal mechanism, originating in the space-time distortion caused by the demands of special relativity, can break the topological constraint (leading to helicity conservation) that would forbid the emergence of a magnetic field (a generalized vorticity) in an ideal nonrelativistic dynamics. The new mechanism, arising from the interaction between the inhomogeneous flow fields and
inhomogeneous entropy, is universal and can provide a finite seed even for mildly relativistic flows.