Rotating elastic string loops in flat and black hole spacetimes: stability, cosmic censorship and the Penrose process

TL;DR

This paper derives equations for elastic string loops in relativistic spacetimes, analyzes their stability, and explores their interactions with black holes, revealing constraints on overspinning and energy extraction efficiency.

Contribution

It provides a detailed stability analysis of rotating elastic string loops in flat and black hole spacetimes and examines their role in cosmic censorship and energy extraction processes.

Findings

String loops are stable below a certain rotation speed in Minkowski spacetime.

In Schwarzschild spacetime, equilibria are always linearly unstable.

Elastic string loops cannot overspin extremal black holes under the null energy condition.

Abstract

We rederive the equations of motion for relativistic strings, that is, one-dimensional elastic bodies whose internal energy depends only on their stretching, and use them to study circular string loops rotating in the equatorial plane of flat and black hole spacetimes. We start by obtaining the conditions for equilibrium, and find that: (i) if the string's longitudinal speed of sound does not exceed the speed of light then its radius when rotating in Minkowski's spacetime is always larger than its radius when at rest; (ii) in Minkowski's spacetime, equilibria are linearly stable for rotation speeds below a certain threshold, higher than the string's longitudinal speed of sound, and linearly unstable for some rotation speeds above it; (iii) equilibria are always linearly unstable in Schwarzschild's spacetime. Moreover, we study interactions of a rotating string loop with a Kerr black hole, namely in the context of the weak cosmic censorship conjecture and the Penrose process. We find that: (i) elastic string loops that satisfy the null energy condition cannot overspin extremal black holes; (ii) elastic string loops that satisfy the dominant energy condition cannot increase the maximum efficiency of the usual particle Penrose process; (iii) if the dominant energy condition (but not the weak energy condition) is violated then the efficiency can be increased. This last result hints at the interesting possibility that the dominant energy condition may underlie the well known upper bounds for the efficiencies of energy extraction processes (including, for example, superradiance).