A black hole doesn't just suck in matter — like a giant cosmic whirlpool, it warps the very fabric of reality and bends light rays around itself, focusing them. The effect is even stronger for black holes that spin at nearly the speed of light: their rotation twists space into a tight spiral.
Rays with left and right circular polarization — simply put, with different twists — are focused by such a hole into two points separated in space. The angle between them is microscopic, but if the lens is several light-years away, the distance between the points grows to trillions of kilometers.
On Earth, this means an observer in Brazil and an observer in Siberia will see the light from the same hole with different polarization. As you move, the polarization smoothly changes — as if the hole itself paints the planet with invisible patterns. This phenomenon turns every spinning black hole into a natural tool for brightness measurements and may help catch retrolensing — when light, circling the hole, returns back, creating rings in the sky.
A spinning black hole drags the very fabric of reality with it, like a giant whirlpool, and this splits light into left- and right-twisted rays.
In science fiction, such as in Star Trek, variable polarization is part of sensor systems. This same effect could inspire real methods for observing black holes.