# Dynamical Friction in Interacting Relativistic Systems

## TL;DR

This paper introduces a new framework for analyzing dynamical friction in relativistic systems, smoothly interpolating between ideal gas and fluid limits, and accounting for finite system effects and corrections near the speed of sound.

## Contribution

It develops a novel theoretical approach that unifies different regimes of relativistic dynamical friction and includes finite system corrections using viscous fluid dynamics.

## Key findings

- Finite drag force at subsonic velocities.
- Corrections smooth the discontinuity at the speed of sound.
- Effective theory accurately approximates finite system effects.

## Abstract

We study dynamical friction in interacting relativistic systems with arbitrary mean free paths and medium constituent masses. Our novel framework recovers the known limits of ideal gas and ideal fluid when the mean free path goes to infinity or zero, respectively, and allows for a smooth interpolation between these limits. We find that in an infinite system the drag force can be expressed as a sum of ideal-gas-like and ideal-fluid-like contributions leading to a finite friction even at subsonic velocities. This simple picture receives corrections in any finite system and the corrections become especially significant for a projectile moving at a velocity $v$ close to the speed of sound $v\approx c_s$. These corrections smoothen the ideal fluid discontinuity around the speed of sound and render the drag force a continuous function of velocity. We show that these corrections can be computed to a good approximation within effective theory of viscous fluid dynamics.