When photons hurtle toward a black hole, most are sucked into its depths, never to return, or gently deflected away. A rare few, however, skirt the hole, making a series of abrupt U-turns. Some of these photons keep circling the black hole practically forever.
Described by astrophysicists as a “cosmic movie camera” and an “infinite light trap,” the resulting ring of orbiting photons is among the weirdest phenomena in nature. If you detect the photons, “you’re going to see every object in the universe infinitely many times,” said Sam Gralla, a physicist at the University of Arizona.
But unlike the iconic event horizon of a black hole — the boundary within which gravity is so strong that nothing can escape — the photon ring, which orbits the hole farther away, has never received much attention from theorists. It makes sense that researchers have been preoccupied with the event horizon, since it marks the edge of their knowledge about the universe. Throughout most of the cosmos, gravity tracks with curves in space and time as described by Albert Einstein’s general theory of relativity. But space-time warps so much inside black holes that general relativity breaks down there. Quantum gravity theorists seeking a truer, quantum description of gravity have therefore looked to the horizon for answers.
To read more, click here.