Most photons running towards a black hole are either softly directed or swallowed deep and never escape. However, a select handful avoid the crater by making a series of sharp U-turns. Some of these photons continue to orbit the black hole for what seems like an eternity.

The ring after orbiting photons, which astrophysicists refer to as the "cosmic film camera" and the "eternal light trap," is one of nature's most bizarre phenomena. If photons are detected, "you'll see every substance in the universe infinite times," says Sam Gralla, a physicist at the University of Arizona.

The ring of photons orbiting further away from the hole didn't draw much attention from theorists, unlike the black hole's iconic event horizon, the region where gravity is so intense that nothing can escape. Given that the event horizon represents the limit of what is known about the universe, it makes sense for scientists to be fascinated by it. According to Albert Einstein's general theory of relativity, gravity follows curves in space and time throughout most of the cosmos. But inside black holes, spacetime is bent so violently that general relativity no longer holds. Therefore, quantum gravity theorists have turned to the horizon for solutions in search of a more accurate, quantum description of gravity.

Andrew Strominger, black holes and quantum gravity theorist at Harvard University, said, “I had embraced the idea that the event horizon is what we need to understand. And I believed the photon ring was a technically challenging object of no profound significance.

Strominger is currently making a U-turn on his own and convincing other thinkers to follow suit. By “Kerr black holes,” he was referring to spinning black holes that form when stars die and gravitationally collapse. "We're excitedly exploring the idea that the photon ring is what you need to understand to unlock the secrets of Kerr black holes," he said.

In the Classic Quantum Gravity article online in May, Strominger and colleagues discovered that the ring of photons surrounding a spinning black hole has unexpected symmetry or can be changed and still remain the same. The symmetry implies that the ring may contain data about the quantum structure of the hole. The main issue for understanding the quantum dynamics of black holes is "it smells like something to do with symmetry," he said.

Researchers debate whether the photon ring is part of a black hole's "holographic twin"; this is a quantum system that is the same as a black hole and can be thought of as a black hole coming out of a hologram.

Alex Maloney, a theorist at McGill University in Canada who was not involved in the study, noted that "it opens up a very interesting way to understand the holography of [black hole] geometries." I find it very exciting that the new symmetry is governing the structure of black holes far from the event horizon.

Much more theoretical research is needed before researchers can definitively say whether or how the photon ring encodes the inner contents of a black hole. Theorists suggest that the new work provides a definitive test for at least any quantum system that claims to represent the black hole's holographic twin. "This is a target for a holographic representation," observed Juan Maldacena of the Institute for Advanced Study in Princeton, New Jersey, one of the founding fathers of holography.

Photon Ring: Concealment

The fact that the photon ring is actually visible, unlike the event horizon, adds to the excitement around it. Strominger has indeed made a U-turn and headed towards these rings as a result of the first image of a black hole. She admitted to crying when Event Horizon Telescope (EHT) revealed her in 2019.

Confusion quickly followed rejoicing. The black hole in the photo had a prominent ring of light surrounding it, but the EHT team's physicists were unsure whether this light was the result of the hole's chaotic environment or whether it contained the black hole's photon ring. They enlisted the help of Strominger and his theoretical colleagues to analyze the picture.

They examined the extensive database of computer simulations the EHT team used to decipher the physical mechanisms that cause light to be produced around black holes. In these computer-generated images, they were able to spot the small, bright ring inside the larger, more fuzzy orange ring of light.

According to Shahar Hadar of the University of Haifa in Israel, who worked on the work with Strominger and the EHT physicists while at Harvard, "you can't miss it when you look at all the simulations." According to Hadar, the creation of the photon ring appears to be a "universal effect" occurring around all black holes.

The theorists concluded that the sharp line of the photon ring provides direct information about the black hole's properties, including its mass and amount of spin, as opposed to the whirlwind of violently colliding particles and fields that surround black holes. According to Strominger, it's arguably the most elegant and fascinating way to actually observe the black hole.

According to a team of astronomers, simulators and theorists, the actual EHT image showing the black hole at the heart of the nearby galaxy Messier 87 wasn't precise enough to distinguish the photon ring, but it's not far off. In the 2020 publication, they proposed that the photon rings should be easily visible in future, higher resolution observatories.

Strominger and his colleagues also questioned whether the shape of the photon rings makes even deeper sense after staring at them for a long time in simulations.

If a single photon made a single U-turn around a black hole before accelerating towards Earth, we would see a single ring of light. The two U-rotating photons appear as a thinner, more ethereal subring within the main ring. In addition, photons that perform three U-turns appear as a subring within another subring, thus forming concentric rings, each gradually thinner and dimmer.

As a result of the fact that the light from the inner lower rings orbits more and is thus recorded before the light from the outer lower rings, time-delayed images of the surrounding cosmos are produced in tandem. In their 2020 article, the group stated that "together, the collection of subrings is like the stills of a movie, documenting the history of the visible universe as seen from the black hole."

According to Strominger and his colleagues, the EHT footage made us think there were an unlimited number of copies of the cosmos on that screen? Couldn't this be the place for the holographic duo?

The researchers concluded that the circular structure of the ring suggests a class of symmetry known as conformal symmetry. Scale invariance, or the ability to look the same whether you zoom in or out, is a feature of harmonious symmetric systems. Each photon subring in this example is a precise, scaled down copy of the one before it. A harmoniously symmetrical system retains its original shape when all spatial coordinates are inverted, shifted, and then reversed once more, as well as when time is moved forward or backward.

In the 1990s, Strominger encountered harmonious symmetry when it appeared in a particular type of five-dimensional black hole he was studying.

By carefully analyzing the properties of this symmetry, he and Cumrun Vafa discovered a new technique for connecting general relativity to the quantum world, at least within these extreme black hole types. They devised to remove the black hole and replace the event horizon with a surface they called the holographic plate, containing a system of quantum particles that adhere to conformal symmetry. They correlated the properties of the system with the properties of the black hole, as if it were a higher-dimensional hologram of the conformal quantum system of the black hole. In doing so, they created a link between the general relativity definition of a black hole and the quantum mechanical definition.

Maldacena applied the same holographic principle to a complete toy universe in 1997. A coherent symmetrical quantum system living on the surface of the bottle was perfectly reflected in the space-time and gravitational parameters inside the bottle, calling it a "universe within a universe." The interior appeared to be a “world” projected holographically from its lower-dimensional surface.

Many theorists have accepted the theory that the real cosmos is a hologram as a result of the discovery. The problem is that Maldacena's cosmos in a bottle is not the same as ours. Its outer boundary resembles a surface because it is filled with negatively curved spacetime.

Theorists believe our universe is flat, but aren't sure what the holographic duo of flat space-time would look like. “We need to return to the real world while being inspired by what we discover in these imaginary universes,” Strominger said.

So the team decided to investigate a plausible spinning black hole in flat space-time, similar to those seen in images taken by the Event Horizon Telescope. “The domicile of the holographic duo should be the first question to be determined. Then what are the symmetries?” said Hadar.

Trying to find the Holographic Binary

In the past, conformal symmetry has proven to be a reliable indicator when searching for quantum systems that map holographically to gravity-based systems.

According to Strominger, talking about conformal symmetry and black hole in the same sentence is equivalent to waving red meat in front of a dog.

The group began looking for signs of compatible symmetry from the definition of rotating black holes in general relativity, known as the Kerr metric. They imagined hammering the dark hole until it rang like a bell. For example, these slowly fading vibrations are waves in space, similar to the gravitational waves produced when two black holes collide. Similar to how the tones of a bell depend on the shape of the bell, the black hole will vibrate at some resonant frequency (i.e. the Kerr metric) dependent on the shape of space-time.

The Kerr metric is extremely complex and makes it impossible to determine the exact pattern of vibrations. Only by taking into account the high-frequency vibrations that result from very violent collisions with the black hole, the team was able to predict the pattern. They discovered a link between the structure of the black hole's photon rings and this high-energy wave pattern. The model is "entirely driven by the photon ring," according to Alex Lupsasca of the Vanderbilt Gravity, Waves, and Fluids Initiative in Tennessee, who, along with Harvard's Strominger, Hadar, and Daniel Kapec, co-authored the new study.

The Covid-2020 pandemic in the summer of 19 was a turning point. The researchers were finally able to meet face-to-face after blackboards and chairs were set up on the lawn in front of Harvard's Jefferson physics lab. They discovered that successive hues of ringed black holes are connected by harmonious symmetry, similar to the congruent symmetry connecting each photon ring to the lower ring below. According to Strominger, the connection between photon rings and black hole vibrations may represent a "precursor" of holography.

Another clue that the photon ring might be unusual comes from its unexpected relationship with the black hole's geometry. “It is extremely, very strange,” Hadar said.

As you walk through the various parts of the photon ring, you are actually exploring different radii or depths of the black hole.

These results show Strominger that the "natural candidate" for part of a spinning black hole's holographic plate is the photon ring, not the event horizon.

If so, it could be a new approach to thinking about the black hole information paradox, a longstanding puzzle involving what happens to data about objects falling into black holes. Recent estimates suggest that the universe somehow kept this information safe as a black hole eventually dissipated. Now, according to Strominger, the data can be held on the holographic plate.

Maybe the information doesn't actually enter the black hole, but rather hangs in a cloud surrounding it, which most likely extends to the photon ring, he suggested. “But we don't know exactly how it works or how it's coded out there,” he continued.

Some quantum gravity theorists disagree with Strominger and colleagues' hypothesis that the holographic twin exists in or near the photon ring, believing it to be an overly assertive extrapolation from the ring's concerted symmetry.

The question of where the holographic duo exists is much deeper than the issue of symmetry. Daniel Harlow, an expert on quantum gravity and black holes from the Massachusetts Institute of Technology, stated. Harlow emphasizes that in this case, a convincing holographic duality must show how the properties of the photon ring, such as the trajectories and frequencies of individual photons, match up mathematically.

However, the new finding provides a valuable needle that any proposed holographic binary should thread through, according to a number of experts: the duo should be able to encode the distinctive vibrational pattern of a black hole that spins after being struck like a bell.

According to Strominger, the requirement that the quantum system modeling the black hole must reproduce all this complexity is a very strong limitation that we never try to use. According to theoretical physicist Eva Silverstein of Stanford University, it looks like a pretty cool piece of theoretical data that people try to copy when trying to make a holographic binary definition.

It would be interesting to know how to incorporate that into a holographic duo, Maldacena agreed. Therefore, it will likely encourage some research in this area.

Maloney thinks the newly discovered symmetry of the photon ring will be of interest to theorists and audiences alike. In a few years, the Event Horizon Telescope could begin detecting photon rings if the hoped-for improvements are funded.

However, data from future observations of these rings will allow serious tests of general relativity near black holes rather than testing holography directly. The structure of the infinite light traps surrounding black holes, the ability to mathematically encode the secrets within, will need to be tested by theorists using pencil-and-paper calculations.

Source: Quanta Magazine / Thomas Lewton

## Be the first to comment