Light usually behaves like a pedestrian on a two-way street: the path from a streetlamp to a camera matches the reverse path. But near a rotating black hole with broken spatial symmetry, this street becomes one-way. Swap the light source and observer, and the hole's shadow changes shape: from an oval resembling a rugby ball, it turns into a teardrop.
Now scientists can use observations of the black hole's shadow with existing telescopes to test the fundamental symmetries of the universe. Even tiny deviations from the usual laws will show up in the shadow's shape, warped by spacetime curvature.
If you fly around such a black hole, its shadow will 'wink'—turning from an oval to a teardrop and back.