The dangers of extremes

TL;DR

This paper argues that the horizons of physical extreme black holes are effectively singular, implying that classical models are fictions and quantum effects could be observable outside such black holes.

Contribution

It challenges the assumption of smooth horizons in extreme black holes, highlighting the potential for observable quantum effects and different microphysics.

Findings

Test particles encounter singularity at the horizon

Objects with back-reaction cannot cross smooth horizons

Quantum effects may be visible outside extreme black holes

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.