Tensile Strength and the Mining of Black Holes

TL;DR

This paper analyzes whether black holes can be mined to accelerate energy extraction, using thought experiments that place boxes, ropes, and other apparatus near horizons. It derives stringent constraints from the averaged null energy condition on tension, mass, and width of mining devices, showing that single boxes must be narrow (no wider than a local Hawking wavelength) and that both backreaction and melting limit how heavy and how long these devices can operate. Consequently, box-based mining offers no speed advantage over natural Hawking evaporation, and the author argues that threading the horizon with NEC-saturating strings provides a faster, more reliable destruction mechanism. In higher dimensions, mining can be even more efficient, altering the scaling of black hole lifetimes and informing discussions of information recovery and thermodynamics.

Abstract

There are a number of important thought experiments that involve raising and lowering boxes full of radiation in the vicinity of black hole horizons. This paper looks at the limitations placed on these thought experiments by the null energy condition, which imposes a fundamental bound on the tensile-strength-to-weight ratio of the materials involved, makes it impossible to build a box near the horizon that is wider than a single wavelength of the Hawking quanta and puts a severe constraint on the operation of 'space elevators' near black holes. In particular, it is shown that proposals for mining black holes by lowering boxes near the horizon, collecting some Hawking radiation and dragging it out to infinity cannot proceed nearly as rapidly as has previously been claimed and that as a consequence of this limitation the boxes and all the moving parts are superfluous and black holes can be destroyed equally rapidly by threading the horizon with strings.