More study is needed before researchers can say for certain if the photon ring contains a black hole's inner contents. The new paper has a test for any quantum system that claims to be the black hole's dual. Juan Maldacena is one of the original architects of holograms.

Hiding in the Photon Ring

The excitement about the photon ring is that it is visible. Strominger changed his mind because of a picture of a black hole. He cried when he saw the event horizon telescope. It's amazing.

Elation began to spiral out of control. The black hole in the image had a thick ring of light around it, but physicists didn't know if it was the result of the hole's chaotic surrounding environment or if it included the black hole's photon ring. Strominger and his colleagues helped interpret the picture. They browsed the huge databank of computer simulations that were being used to disentangle the physical processes that produce light around black holes. The ring embedded in the larger, fuzzier orange doughnut of light could be seen in these simulations.

Shahar Hadar of the University of Haifa in Israel collaborated with Strominger and the EHT physicists on the research while at Harvard. Hadar said that the formation of the photon ring seems to be a universal effect.

The sharp line of the photon ring is different from the maelstrom of energetic particles colliding and fields surrounding black holes. Strominger said that it is the most beautiful and compelling way to view the black hole.

The picture of the black hole at the center of the galaxy is not sharp enough to resolve the photon ring. Future telescopes should be able to see photon rings. A new paper claims to have found the ring by removing layers from the original data but it has been met with skepticism.

Strominger and his colleagues wondered if the form of the photon rings suggested an even deeper meaning.

A Surprising Symmetry

It would appear to us as a single ring of light when a single U-turn around a black hole and then zip toward Earth was1-65561-65561-65561-65561-6556 on1-65561-65561-65561-6556 on1-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-6556 Two U-turns around the hole appear to be thinner than the first ring. There are nested rings, each fainter and thinner than the last, created by the photons that make three U-turns.

A series of snapshots of the universe were taken after the light from outer subrings was captured. The collaboration wrote in the 2020 paper that the set of subrings were similar to the frames of a movie.

Strominger wondered if there was an infinite number of copies of the universe at that screen. Couldn't that be the location of the dual?

Conformal symmetry is a group of symmetries that are suggestive of the ring. When you zoom in or out, the system looks the same. Each photon subring is a duplicate of the previous one. When all spatial coordinates are inverted, shifted and then inverted again, the system stays the same.

It's like waving red meat in front of a dog if you say conformal symmetry and black hole in the same sentence.

Andrew Strominger is a student at Harvard.

Strominger found conformal symmetry when he was studying a black hole. He and Cumrun Vafa were able to connect general relativity to the quantum world by understanding the details of this symmetry. They thought of cutting out the black hole and replacing it with a surface with a quantum system of particles. The properties of the system correspond to the properties of the black hole. They built a bridge between the descriptions of a black hole and general relativity.

Maldacena applied the same principle to a toy universe. A conformally symmetric quantum system living on the bottle's surface mapped onto properties of space-time and gravity in the bottle's interior. The interior was projected from its lower-dimensional surface into a hologram.

Many people think that the real universe is a hologram. Maldacena's universe is different from our own. The negatively curved space-time gives it a surface like outer boundary. Our universe is thought to be flat, and theorists don't know what a dual of flat space-time would look like. Strominger said that we need to get back to the real world while taking inspiration from hypothetical worlds.

The group decided to look at a spinning black hole like the one photographed by the event horizon telescope. The first question to ask is where the dual lives. Hadar asked, "What are the symmetries?"

Searching for the Holographic Dual

Conformal symmetry has been used in the search for quantum systems that map onto systems with gravity. Strominger said that saying conformal symmetry and black hole in the same sentence is like waving red meat in front of a dog.

The Kerr metric describes the spinning of black holes in general relativity. They thought of hitting the black hole with a hammer. When two black holes collide, they create ripples in the waves. The sound of a bell depends on the shape of space-time and the sound of a black hole depends on the shape of space-time.

The Kerr metric is so complex that it's not possible to figure out the exact pattern. The team approximated the pattern by taking into account the high frequencies that come from hitting the black hole very hard. The black hole's photon rings were seen to have a relationship with the pattern of waves. A new paper co-authored by Strominger, Hadar and Daniel Kapec of Harvard shows that the pattern is governed by the photon ring.

During the summer of 2020, there was a major event. The researchers were able to meet up in person after the benches and blackboards were set up on the grass. The sound of a ringing black hole is related to each other by conformal symmetry. Strominger said that the relationship between the photon rings and the black hole could be a troubling one.

The ring relates to the black hole's geometry in a strange way. Hadar said it was very strange. You are probing different depths into the black hole as you move along different points on the photon ring.

Strominger thinks that the photon ring is a natural candidate for a piece of the black hole's plate.

There is a mystery about what happens to information when objects fall into black holes. As a black hole slowly evaporates, recent calculations show that this information is still in the universe. Strominger thinks that the information could be in the plate. He said that information might not fall into the black hole, but it might stay in a cloud around the photon ring. We don't know how it's written in there or how it works.

A Call to Theorists

Strominger and company's hunch that the dual lives in or around the photon ring has been met with skepticism by some quantum gravity theorists. Daniel Harlow is a quantum gravity and black hole theorist at the Massachusetts Institute of Technology. Harlow stresses the importance of showing how the properties of the photon ring can be mapped into the quantum details of the black hole.

Several experts said that the new research offers a useful needle that a dual must thread if it is to be effective. Strominger said, "demanding the quantum system that describes the black hole reproduces all of that complexity is an incredibly powerful constraint." Eva Silverstein said that it seemed like a nice piece of theoretical data for people to try to reproduce.

Maldacena wanted to know how to incorporate this into a dual. It will likely cause some research to be done in that direction.

There will be more interest in the photon ring from theorists and watchers. It could start to detect photon rings within a few years if the upgrade to the event horizon telescope gets funded.

Extreme tests of general relativity near black holes will be possible thanks to the data from future measurement of these rings. It is up to theorists to figure out if the structure of the infinite light traps around black holes can be mathematical.