Analytical Scaling of Relativistic Drag in the Interstellar Medium

TL;DR

This paper presents an analytical model for relativistic drag forces on probes in the interstellar medium, revealing a thermodynamic challenge due to extreme thermal loading at high speeds.

Contribution

It introduces scaling laws for relativistic drag forces, highlighting the thermal effects and the dominance of radiative versus baryonic regimes in interstellar travel.

Findings

Relativistic inertia keeps probe speed nearly constant over parsecs.

Thermal loading becomes extreme above 0.5c, surpassing passive material limits.

Radiative drag is negligible compared to baryonic drag in the galactic disk.

Abstract

This paper develops an analytical framework for the retarding forces on macroscopic spherical probes travelling through the interstellar medium (ISM) at relativistic speeds (0.1c to 0.99c). Integrating the aberrated momentum flux of both baryonic and radiative fields yields scaling laws that expose what this work calls the Magnitude Paradox: relativistic inertia (gamma^3) keeps a probe's speed nearly constant across parsec-scale distances, yet the same gamma^2 boost to the effective baryonic cross-section drives extreme thermal loading on the hull -- a relativistic correction that becomes significant only above beta > 0.5c and was not quantified in prior work focused on the Starshot regime (beta approx. 0.2c). The central conclusion is that ISM drag is not a kinematic problem -- a probe will not be slowed to a stop -- but a thermodynamic one: the forward surface faces energy deposition rates that no passive material can survive. A closed-form crossover condition is also derived separating the baryonic- and radiative-dominated regimes, showing that for any macroscopic probe in the galactic disk, total radiative drag is negligible by many orders of magnitude.