Apparently superluminal superfluids

TL;DR

This paper demonstrates that within a certain relativistic superfluid theory, vortex solutions can have superluminal velocities without instabilities, challenging traditional interpretations of superfluid velocity.

Contribution

It provides the first proof of superluminal vortex solutions in a relativistic superfluid and analyzes their stability, contrasting with ordinary fluids.

Findings

Superluminal vortex solutions are consistent and stable within the theory.

Ordinary fluid vortices become unstable when superluminal velocities occur.

Superfluid velocity may not directly represent the physical velocity of a substance.

Abstract

We consider the superfluid phase of a specific renormalizable relativistic quantum field theory. We prove that, within the regime of validity of perturbation theory and of the superfluid effective theory, there are consistent and regular vortex solutions where the superfluid's velocity field as traditionally defined smoothly interpolates between zero and arbitrarily large superluminal values. We show that this solution is free of instabilities and of superluminal excitations. We show that, in contrast, a generic vortex solution for an ordinary fluid does develop an instability if the velocity field becomes superluminal. All this questions the characterization of a superfluid velocity field as the actual velocity of ``something".