A spinning top whirling so fast that it forces light around it into spirals: that's a fitting image for what happens near a black hole's horizon. Hot plasma falls in and emits radio waves. Their polarization (the plane in which the wave oscillates) becomes an arrow pointing in the direction of energy flow.
A team of theorists led by Andrew Chael has discovered that the polarization pattern born at the very edge of the abyss follows a strict law. It does not depend on how tangled the magnetic field lines are — only on the hole's spin rate and the tilt at which we see its disk. It's like a signature left by spacetime itself.
If future observations with the Event Horizon Telescope (EHT) or its more sensitive successors capture this pattern at the black hole in the center of galaxy M87, it would be direct evidence of the Blandford–Znajek process. That's the mechanism by which a spinning black hole gives up its energy, twisting magnetic fields and launching jets of matter outward. In essence, we would watch a giant cosmic top slow down.
The black hole at the center of M87 is so massive that one full rotation takes several minutes, even though its horizon is larger than our solar system.
In the movie Interstellar, the supermassive black hole Gargantua spins so fast that a ship can stay in a stable orbit right next to the horizon — exactly the region where the polarization pattern described in the paper is born.