# Dynamical Friction in Superfluids

## TL;DR

This paper investigates the dynamical friction experienced by a small object moving through a superfluid, revealing non-zero friction at subsonic speeds due to Jeans instability effects and highlighting limitations of standard effective theories.

## Contribution

It introduces a comprehensive analysis of dynamical friction in superfluids, accounting for Jeans instability and demonstrating the inadequacy of leading order effective theories for supersonic processes.

## Key findings

- Dynamical friction exists at subsonic velocities due to Jeans instability.
- Standard effective theories may fail to accurately describe supersonic processes.
- Subsonic perturbers can lose energy via phonon radiation in superfluids.

## Abstract

We compute the dynamical friction on a small perturber moving through an inviscid fluid, i.e., a superfluid. Crucially, we account for the tachyonic gravitational mass for sound waves, reminiscent of the Jeans instability of the fluid, which results in non-zero dynamical friction even for subsonic velocities. Moreover, we illustrate that the standard leading order effective theory in the derivative expansion is in general inadequate for analysing supersonic processes. We show this in two ways: (i) with a fluid treatment, where we solve the linearized hydrodynamical equations coupled to Newtonian gravity; and (ii) with a quasiparticle description, where we study the energy dissipation of a moving perturber due to phonon radiation. Ordinarily a subsonic perturber moving through a superfluid is kinematically prohibited from losing energy, however the Jeans instability modifies the dispersion relation of the fluid which can result in a small but non-vanishing dynamical friction force. We also analyse the soft phonon bremsstrahlung by a subsonic perturber scattered off an external field.