The dangers of extremes

Understand

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.