A black hole is not just a dark spot in the sky. Its shadow falls on the glowing backdrop of infalling matter and carries the imprint of spacetime geometry itself. If gravity works even slightly differently than we think, the shadow will give it away.

In a new study, physicists traced light rays around a rotating black hole in so-called Rastall gravity. This theory adds two new numbers to Einstein's equations: the structure parameter γ and the Rastall parameter μ. Using computer simulations, they launched rays from a distant observer and watched what silhouette emerged against the accretion disk — the superheated matter swirling around the hole.

The result was clear: at a fixed spin rate, as γ increases, the shadow expands noticeably. The photon capture region grows, as if the black hole were 'swelling' in the eyes of a distant viewer. At the same time, the shadow becomes rounder — rotation deforms it less. The parameter μ also contributes, and the combined influence of both numbers leaves a distinct pattern on the shadow.

Thus, the shadow becomes a tool: by measuring its size and deviation from a perfect circle, we can probe the fundamental structure of gravity. Perhaps future observations with telescopes like the Event Horizon Telescope will help choose between the Einsteinian picture and its extensions.