A black hole is like a dark ball in space onto which luminous matter falls. A black hole has a shadow — a region from which rays cannot escape. But due to light bending, the shadow appears larger than the hole itself: just like a ball's shadow on a wall grows bigger when the lamp is far away.
It is proven that for a symmetric hairy black hole, the shadow radius is always at least 2.6 times the radius of the point of no return ($r_{\text{sh}}/r_{\text{H}} \ge 3\sqrt{3}/2$). The simplest 'bald' model gives this limit; any 'hair' enlarges the shadow. Remarkably, the shadow is not the hole itself but an illusion caused by the bent light of surrounding matter. Observations by telescopes like the Event Horizon Telescope turn shadows into a test for exotic physics. The shadow of M87 matches a 'bald' black hole, supporting Einstein's theory.
as written by the authors arXiv:2603.03421
The non-linearly coupled Einstein-matter field equations predict the existence of shadows with well-defined boundaries around black holes. We prove that, in spherically symmetric hairy black-hole spacetimes whose matter fields satisfy the weak energy condition, the radii of these shadows are bounded from below by the dimensionless relation $$r_{\text{sh}}/r_{\text{H}}\geq 3\sqrt{3}/2$$, where $$r_{\text{H}}$$ is the horizon radius of the central hairy black hole. The characteristic shadow of the (bald) Schwarzschild black-hole spacetime saturates the analytically derived lower bound.
The shadow of the M87 black hole, imaged in 2019, matched the predictions of general relativity: circular and with a radius 2.6 times larger than the point of no return.
The film 'Interstellar' used actual light-bending calculations to create the shadow of the Gargantua black hole.