The Harder They Fall, the Bigger They Become: Tidal Trapping of Strings by Microstate Geometries

TL;DR

This paper investigates how strings propagating in microstate geometries experience tidal forces that stretch and trap them, potentially causing gravitational echoes and shedding light on microstate dynamics.

Contribution

It introduces the concept of tidal trapping of strings in microstate geometries, showing how energy-dependent stretching leads to effective confinement and possible gravitational echoes.

Findings

Tidal forces cause energy-dependent string stretching.

Probes lose kinetic energy and become trapped in the throat.

Repeated passes lead to scrambling into microstates.

Abstract

We consider the fate of a massless (or ultra-relativistic massive) string probe propagating down the BTZ-like throat of a microstate geometry in the D1-D5 system. Far down the throat, the probe encounters large tidal forces that stretch and excite the string. The excitations are limited by the very short transit time through the region of large tidal force, leading to a controlled approximation to tidal stretching. We show that the amount of stretching is proportional to the incident energy, and that it robs the probe of the kinetic energy it would need to travel back up the throat. As a consequence, the probe is effectively trapped far down the throat and, through repeated return passes, scrambles into the ensemble of nearby microstates. We propose that this tidal trapping may lead to weak gravitational echoes.