Classic black holes face problems with singularities and the information paradox. An alternative is the black mirror model, where the event horizon reflects rather than absorbs. Gravitational waves can reveal a key difference: the spectrum of quasinormal modes (the "ringing" after a merger) is fundamentally different. The reflectivity of the mirror horizon is universal and depends only on the Hawking temperature. This changes the dynamics of objects spiraling in: at low spin, braking slows the process, while at high spin, superradiance vanishes and the inspiral accelerates. This mechanism explains the growth of supermassive black holes to high spin values.
Two drums: one swallows the beat silently, the other ricochets it back. That's how an ordinary black hole and its hypothetical twin — a black mirror — are built. The first has an event horizon, a point of no return. The second has a reflective surface: nothing falls in, everything is thrown back.
When a star spirals toward such an object, it punches spacetime, emitting gravitational waves. For a black hole, the sound quickly fades, swallowed by the horizon. For a black mirror, the waves bounce back and forth, creating a long echo with a distinct frequency spectrum — quasinormal modes. But the pace of infall also differs: a slow mirror decelerates the falling body by reflecting energy; a fast one accelerates it, because it suppresses the effect that, in ordinary holes, feeds orbital motion. Future observatories like LISA will notice this difference in rhythm.
🎯 The Hawking temperature of a solar-mass black hole is just 60 nanokelvins—billions of times colder than the cosmic microwave background.