A black hole's shadow looks different depending on the spin of the light being used to see it.
Most models treat a black hole's shadow as a simple, dark silhouette caused by gravity. The gravitational spin Hall effect proves that light with different polarizations follows slightly different paths. This causes the edge of the shadow to shift or blur depending on the light's helicity. It reveals that black holes are not just geometric voids but interact with light in a way that depends on its quantum properties. This discovery will help astronomers more accurately interpret the high-resolution images taken by the Event Horizon Telescope.
Helicity-dependent corrections to black-hole shadows from the gravitational spin Hall effect
arXiv · 2605.02136
Black-hole shadows are purely geometric in the leading-order geometric-optics approximation: their boundary is set by null geodesics and carries no information about the polarization of the probing radiation. This changes at subleading order. We show that the gravitational spin Hall effect of light shifts the critical impact parameter governing photon capture by a helicity-dependent amount, causing polarized radiation with opposite helicities to trace slightly different shadow boundaries -- even