If you briefly flash light near a black hole, it will respond with a long echo, like a bell that keeps ringing after being struck. The light gets trapped near the hole, bouncing repeatedly off an invisible boundary: the photon sphere, where rays circle around like sound in a metal.
Scientists replicated this on a tabletop using a curved surface that mimics curved spacetime. A light pulse striking this surface produced a long tail of bright bands — an echo, exactly like the one near a black hole.
Building on the work of Huygens and Schwarzschild, physicists decoded the echo using frequency analysis of light. It proved to be resonant: the light vibrates in tune with the gravitational “voice” of the black hole, like a string under a bow.
If you could hover inside a black hole’s photon sphere, you’d see the back of your own head: light emitted from there would loop around the hole and return to the observer.
In Interstellar, a black hole’s gravity enables communication through time; now physicists have discovered that real light signals can also create echoes, like messages from the past.