Edilberto O. Silva

A black hole is not just a void but a massive object that warps the space around it. If it finds itself in a cloud of invisible dark matter, matter that emits no light but makes itself known through gravity, its behavior changes.

In a water vortex swirling toward the drain, the flow speed near the center can exceed the speed of sound. Sound gets trapped: waves cannot escape back, forming a silent region — a sound shadow. Half a century ago, William Unruh realized that this is an exact copy of a black hole, but for sound instead of light.

The ocean bulges a full meter under the Moon: that's a tide. If a massive body sits near a black hole, the hole feels a tidal pull too. In ordinary Einsteinian gravity, it doesn't respond to such a static tide: its "tidal numbers" (measures of deformability) are zero.

Drop a stone into a calm lake, and ripples spread in even circles. Spacetime around a massive body behaves similarly: if the body is perfectly round, the disturbances it causes are symmetric. A black hole in general relativity resembles a perfectly smooth mirror of water.