in this visualization of light flowing around a black hole,
contains a succession of images of the entire universe.
Source: NASA's Goddard Space Flight Center/Jeremy Schnittman
that the ring of photons orbiting a black hole
exhibits a special kind of symmetry,
hinting at a deeper meaning.
A rare few, however,
skirt the hole, making a series of abrupt U-turns. Some of these
photons keep circling the black hole practically forever.
Described by astrophysicists as a "cosmic movie camera" and an "infinite light trap," the resulting ring of orbiting photons is among the weirdest phenomena in nature. I
f you detect the photons,
But unlike the iconic event horizon of a black hole - the boundary within which gravity is so strong that nothing can escape - the photon ring, which orbits the hole farther away, has never received much attention from theorists.
It makes sense that researchers have been preoccupied with the event horizon, since it marks the edge of their knowledge about the universe.
Throughout most of the cosmos, gravity tracks with curves in space and time as described by Albert Einstein's general theory of relativity. But space-time warps so much inside black holes that general relativity breaks down there.
Quantum gravity theorists seeking a truer, quantum description of gravity have therefore looked to the horizon for answers.
Now Strominger is making his own U-turn and trying to convince other theorists to join him.
In a paper posted online in May and recently accepted for publication in Classical Quantum Gravity, Strominger and his collaborators revealed that the photon ring around a spinning black hole has an unexpected kind of symmetry:
The symmetry suggests that the ring may encode information about the hole's quantum structure.
The discovery has led researchers to debate whether the photon ring might even be part of a black hole's "holographic dual" - a quantum system that's exactly equivalent to the black hole itself, and which the black hole can be thought of as emerging out of like a hologram.
Photons that make a single U-turn around a black hole
before flying away from it create an image of a ring,
labeled n = 1 in the video.
Photons that redirect twice before flying away
from the hole form an image of a thinner ring
within the first ring, labeled n = 2 in the video, and so on.
Much more theoretical study is needed before researchers can say for sure whether, or in what way, the photon ring encodes a black hole's inner contents.
But at the very least, theorists say the new paper has detailed a precise test for any quantum system claiming to be the black hole's holographic dual.
Hiding in the Photon Ring
Part of the excitement about the photon ring is that, unlike the event horizon, it's actually visible.
In fact, Strominger's U-turn toward these rings happened because of a photograph:
When the Event Horizon Telescope (EHT) unveiled it in 2019,
Elation soon spiraled into confusion.
The black hole in the image had a thick ring of light around it, but physicists on the EHT team didn't know whether this light was the product of the hole's chaotic surrounding environment, or if it included the black hole's photon ring.
They went to Strominger and his theorist colleagues for help interpreting the image.
Together, they browsed the huge databank of computer simulations that the EHT team was using to disentangle the physical processes that produce light around black holes.
In these simulated images, they could see the thin, bright ring embedded in the larger, fuzzier orange doughnut of light.
The formation of the photon ring seems to be a "universal effect" that happens around all black holes, Hadar said.
Unlike the maelstrom of energetic colliding particles and fields that surrounds black holes, the theorists determined, the sharp line of the photon ring carries direct information about the black hole's properties, including its mass and amount of spin.
A global network of radio telescopes
known as the Event Horizon Telescope
released this first-ever photo of a black hole in 2019
— the supermassive one at the center
of the nearby galaxy Messier 87.
The collaboration of astronomers, simulators and theorists found that the EHT's actual photograph, which shows the black hole at the center of the nearby galaxy Messier 87, isn't sharp enough to resolve the photon ring, although it isn't far off.
They argued in a 2020 paper (Universal Interferometric Signatures of a Black Hole's Photon Ring) that future, higher-resolution telescopes should easily see photon rings.
(A new paper claims to have found the ring in the EHT's 2019 image by applying an algorithm to remove layers from the original data, but the claim has been met with skepticism.)
Still, having stared at photon rings for so long in the simulations, Strominger and his colleagues began to wonder if their form hinted at an even deeper meaning.
A Surprising Symmetry
Light from the inner subrings has made more orbits and was therefore captured before the light from outer subrings, resulting in a series of time-delayed snapshots of the surrounding universe.
Strominger said that when he and his collaborators looked at the EHT pictures,
The researchers realized that the ring's concentric structure is suggestive of a group of symmetries called conformal symmetry.
A system that has conformal symmetry exhibits "scale invariance," meaning it looks the same when you zoom in or out. In this case, each photon subring is an exact, demagnified copy of the previous subring.
Moreover, a conformally symmetric system stays the same when translated forward or backward in time and when all spatial coordinates are inverted, shifted and then inverted again.
Strominger encountered conformal symmetry in the 1990s when it turned up in a special kind of five-dimensional black hole he was studying.
By precisely understanding the details of this symmetry, he and Cumrun Vafa found a novel way to connect general relativity to the quantum world, at least inside these extreme kinds of black holes.
They imagined cutting out the black hole and replacing its event horizon with what they called a holographic plate,
They showed that the system's properties correspond to properties of the black hole, as if the black hole is a higher-dimensional hologram of the conformal quantum system.
In this way, they built a bridge between the description of a black hole according to general relativity and its quantum mechanical description.
In 1997, Maldacena extended this same holographic principle to an entire toy universe.
He discovered a "universe in a bottle," in which a conformally symmetric quantum system living on the bottle's surface exactly mapped onto properties of space-time and gravity in the bottle's interior.
The discovery led many theorists to believe that the real universe is a hologram.
The hitch is that Maldacena's universe in a bottle differs from our own.
Our universe is thought to be flat, and theorists have little idea what the holographic dual of flat space-time looks like.
And so the group decided to study a realistic spinning black hole sitting in flat space-time, like those photographed by the Event Horizon Telescope.
Searching for the Holographic Dual
Historically, conformal symmetry has proved a trustworthy guide in the search for quantum systems that holographically map onto systems with gravity.
Starting from the description of spinning black holes in general relativity, called the Kerr metric, the group began to look for hints of conformal symmetry.
Andrew Strominger and colleagues
recently discovered that a black hole's photon ring
has the kind of symmetry that often arises
when an object - in this case, the black hole - can be
described as a hologram.
Figuring out the exact pattern of vibrations is unfeasible because the Kerr metric is so complicated.
So the team approximated the pattern by only considering high-frequency vibrations, which result from hitting the black hole very hard. They noticed a relationship between the pattern of waves at these high energies and the structure of the black hole's photon rings.
A pivotal moment came in the summer of 2020 during the Covid-19 'pandemic'.
Blackboards and benches were set up on the grass outside Harvard's Jefferson physics lab, and the researchers could finally meet up in person.
They worked out that, like the conformal symmetry which relates each photon ring to the next subring, the successive tones of a ringing black hole are related to each other by conformal symmetry.
This relationship between the photon rings and the black hole vibrations could be a "harbinger" of holography, said Strominger.
Another clue that the photon ring may hold special significance comes from the counterintuitive way the ring relates to the black hole's geometry.
These findings imply to Strominger that the photon ring, rather than the event horizon, is a "natural candidate" for part of the holographic plate of a spinning black hole.
If so, there may be a new way to picture what happens to information about objects that fall into black holes - a long-standing mystery known as the black hole information paradox.
Recent calculations indicate that this information is somehow preserved by the universe as a black hole slowly evaporates.
Strominger now speculates that the information might be stored in the holographic plate.
A Call to Theorists
Strominger and company's hunch that the holographic dual lives in or around the photon ring has been met with skepticism by some quantum gravity theorists, who see it as too bold an extrapolation from the ring's conformal symmetry.
Although he is in favor of further research on the issue, Harlow stresses that a convincing holographic duality, in this case, must show how the properties of the photon ring, such as individual photons' orbits and frequencies, mathematically map onto the fine-grained quantum details of the black hole.
Nevertheless, several experts said that the new research offers a useful needle that any proposed holographic dual must thread:
Eva Silverstein, a theoretical physicist at Stanford University, said,
Maldacena agreed, saying,
Alex Maloney suspects that the newfound symmetry of the photon ring will spur interest among both theorists and observers. If hoped-for upgrades to the Event Horizon Telescope get funded, it could start to detect photon rings within a few years.
Future measurements of these rings won't directly test holography, though - rather, the data will allow extreme tests of general relativity near black holes.
It's up to theorists to determine with pen-and-paper calculations if the structure of the infinite light traps around black holes can mathematically encrypt the secrets within.