Macroscopic Optical Nonreciprocity: A Black Hole as an Optical Diode

TL;DR

This paper shows that a rotating black hole can act as an optical diode, breaking optical reciprocity due to Lorentz symmetry breaking, leading to asymmetric black hole shadows observable with horizon-scale imaging.

Contribution

It introduces a novel mechanism where Lorentz symmetry breaking causes macroscopic optical nonreciprocity in black hole shadows, enabling black holes to function as optical diodes.

Findings

Black hole shadow shape changes from rugby-ball to teardrop upon path reversal.

Lorentz symmetry breaking induces a high-contrast optical nonreciprocity.

Black holes can serve as cosmic-scale optical diodes for probing fundamental symmetries.

Abstract

Optical reciprocity--the principle that light retraces the same path when source and detector are interchanged--is a foundational concept in geometric optics. In this Letter, we demonstrate that this ``symmetry-protected'' behavior can be qualitatively overturned in a rotating black hole when spontaneous Lorentz symmetry breaking introduces a nonminimally coupled background structure with a preferred direction. Through numerical ray-tracing simulations, we reveal a striking macroscopic signature: upon optical-path reversal achieved by exchanging the source and the observer, the shadow of the same black hole morphs from a quasi-symmetric rugby-ball shape into a distinct teardrop profile. This high-contrast nonreciprocity effectively turns the black hole into a cosmic-scale optical diode, offering a novel pathway to probe fundamental symmetries using current and next-generation horizon-scale imaging.