In 2019, astronomers unveiled the first image of a black hole: M87*, a giant located 50 million light-years away. Two years later, they followed with Sagittarius A* (Sgr A*), the black hole at the heart of our Milky Way. These breakthroughs came from the Event Horizon Telescope (EHT), a global network of radio telescopes working together as a planet-sized observatory.
The next frontier is improving the sharpness of these images. A proposed mission called the Black Hole Explorer (BHEX) — whose project scientist Alex Lupsasca is a collaborator of the present work — would place a telescope in high Earth orbit and link it with the ground-based array. This would boost resolution fivefold, enabling astronomers not only to view M87* and Sgr A* in finer detail but also to image many more black holes. A key target is the photon ring—a thin circle of light predicted by Einstein’s theory but not yet directly observed. It forms from photons looping around a black hole before escaping and may carry uniquely precise information about the black hole’s mass, spin, and surrounding spacetime.
Meanwhile, gravitational-wave astronomy is probing black holes in a different way. When two black holes collide, the newborn black hole briefly vibrates in a “ringdown” phase, producing characteristic tones called quasinormal modes (QNMs). These vibrations act like a fingerprint of the black hole, revealing its size and spin. Interestingly, theory shows that photon rings and QNMs are deeply linked: in the high-frequency limit, the orbits of photons around a black hole determine the structure of these quasinormal vibrations. Thus, light from photon rings and ripples in spacetime from gravitational waves may be two sides of the same coin.
Some researchers think this link may also shed light on the holographic principle—the idea that everything happening inside a region of space, such as a black hole, can be fully described by information encoded on its boundary. If true, the photon ring might literally be part of the hologram, encoding aspects of the quantum description of black holes.
However, studying this in realistic black holes is extremely difficult. Their full QNM spectra are not known analytically and require heavy numerical work. To make progress, physicists often use simplified “toy models.” This paper advances the field by analyzing one such model: the Warped AdS₃ (WAdS₃) black hole, a mathematically simpler cousin of more realistic black holes. Unlike earlier models, WAdS₃ black holes possess a photon ring at a finite distance while still allowing exact calculations of the QNM spectrum.
By working out this connection in detail, the paper takes a step toward demonstrating – concretely, in a tractable setting – that the photon ring can indeed be part of a black hole’s holographic description. Future work on the dual quantum theory could complete this picture, providing a rare bridge between theoretical physics and observable black hole phenomena.
Stéphane Detournay, Sahaja Kanuri (ULB), Alexandru Lupsasca(Vanderbilt U.), Philippe Spindel(Brussels U., PTM and U. Mons), Quentin Vandermiers (ULB) and Raphaela Wutte (ULB and Arizona State U.) https://inspirehep.net/literature/2932043
