Closest Safe Approach to an Accreting Black Hole

TL;DR

This paper investigates the complex causal and geometric structures of dynamical black holes in collapsing dust models, analyzing horizons, shockwaves, and the potential for near-horizon space exploration.

Contribution

It provides a detailed analysis of the horizons and causal boundaries in dynamical black holes, including shockwave effects and implications for near-horizon approaches.

Findings

Horizon kinks occur at shockwave intersections.

Discontinuities in separation functions relate to boundary geometry.

Insights into safe approaches to black hole horizons for space explorers.

Abstract

We examine the causal and geometric horizons of dynamical black holes in Lemaitre-Tolman-Bondi collapsing dust spacetimes. Marginally trapped tubes in these spacetimes may be spacelike, timelike or null and may also be sourced from or disappear into shell-crossing singularities which we resolve with (timelike) shockwaves. The event horizon kinks when it intersects a shockwave. We calculate the timelike separation between the crossable boundary (marginally trapped tubes plus connecting shockwaves) and event horizon. As measured along the crossable boundary this function can have discontinuities not only in its derivative but also in the function itself. These features are closely related to the geometry of the crossable boundary. Finally, we consider the application of this work for future space explorers seeking to make a closest (non-terminal) approach to a black hole horizon.