The dangers of extremes

TL;DR

The paper argues that physical extremal black holes do not possess a smooth interior; perturbations induce a mass-inflation–driven singularity at the would-be inner horizon, eliminating the region of small curvature inside the horizon. This leads to the extreme limit being described by a horizon that becomes null-singular rather than smooth, with backreaction controlling when and where singularities form. The findings suggest possible observable quantum effects outside extreme or near-extreme black holes and imply that the microphysics of extremal black holes may differ significantly from Schwarzschild cases, complicating existing microscopic models such as string-theoretic or fuzzball descriptions. Overall, the work reframes the interior structure of extremal black holes and highlights the need for new quantum considerations in extreme regimes.

Abstract

While extreme black hole spacetimes with smooth horizons are known at the level of mathematics, we argue that the horizons of physical extreme black holes are effectively singular. Test particles encounter a singularity the moment they cross the horizon, and only objects with significant back-reaction can fall across a smooth (now non-extreme) horizon. As a result, classical interior solutions for extreme black holes are theoretical fictions that need not be reproduced by any quantum mechanical model. This observation suggests that significant quantum effects might be visible outside extreme or nearly extreme black holes. It also suggests that the microphysics of such black holes may be very different from that of their Schwarzschild cousins.