“Physicists like to probe the extreme,” said Garrett Goon, a physicist at Carnegie Mellon University. “The fact that you can’t go further, that something is changing, something is blocking you — something interesting is happening there.”

For decades, black holes have played the headlining role in the thought experiments that physicists use to probe nature’s extremes. These invisible spheres form when matter becomes so concentrated that everything within a certain distance, even light, gets trapped by its gravity. Albert Einstein equated the force of gravity with curves in the space-time continuum, but the curvature grows so extreme near a black hole’s center that Einstein’s equations break. Thus generations of physicists have looked to black holes for clues about the true, quantum origin of gravity, which must fully reveal itself in their hearts and match Einstein’s approximate picture everywhere else.

Plumbing black holes for knowledge of quantum gravity originated with Stephen Hawking. In 1974, the British physicist calculated that quantum jitter at the surfaces of black holes cause them to evaporate, slowly shrinking as they radiate heat. Black hole evaporation has informed quantum gravity research ever since.

More recently, physicists have considered the extreme of the extreme — entities called extremal black holes — and found a fruitful new problem.

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