A light ray passing by a massive body gets deflected: this is the effect of gravitational lensing, familiar from images of distant galaxies. The astrophysicists started with a rotating black hole (Kerr spacetime) and added a tiny correction to how light itself propagates. This correction comes from theories that attempt to unify gravity with quantum mechanics: it alters the usual "rule of motion" for particles of light.
The authors derived formulas for the deflection angle and the time delay when light bends around such a black hole. The correction turned out not to be a constant; it depends subtly on how fast the hole spins. Near the "photon sphere," where rays can travel in circles, the effect becomes most noticeable. This is like a slight change in traffic rules: on a straight road the difference is barely noticeable, but on a sharp curve the car starts to drift differently.
The bottom line is simple: if data from telescopes reveal a discrepancy with standard lensing calculations, it could be a "signature" of new laws of physics. The study provides a concrete mathematical tool for such a test, linking abstract theories to real observations.
The photon sphere around a black hole is an imaginary shell where a light ray could theoretically circle forever, neither falling in nor escaping. In practice, the slightest disturbance sends it either into the hole or out into space.
In the movie "Interstellar," the rotation of the black hole Gargantua distorts the accretion disk and creates striking light rings — precisely because of the strong bending of rays near the event horizon.