Tensile Strength and the Mining of Black Holes

TL;DR

This paper examines the physical constraints imposed by the null energy condition on thought experiments involving black hole horizons, revealing limitations on black hole mining and the feasibility of space elevators near black holes.

Contribution

It demonstrates that the null energy condition severely restricts the design of boxes and devices near black holes, challenging previous claims about rapid black hole mining.

Findings

Mining black holes with boxes is slower than previously thought.

Physical constraints prevent building large boxes near the horizon.

Black holes can be destroyed rapidly using strings, bypassing box-based methods.

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.