Yu-Peng Zhang

A black hole's shadow is a dark spot against the bright light bent by strong gravity. Usually, this spot is an almost perfect circle with a smooth edge. But if a black hole is not quite what Einstein's theory predicts, a sharp kink, like a corner on a crumpled piece of paper, can appear at the shadow's edge.

Gravitational wave detectors work like sensitive microphones, picking up the trembling of space from distant cosmic catastrophes. When two black holes merge into one, the newborn object rings like a bell, and that ringing is a unique 'voice' of gravity.