by Thomas Lewton September
08, 2022
from
QuantaMagazine Website

The photon ring, which glows orange

in this visualization of light flowing around a black hole,

contains a succession of images of the entire universe.
Olena Shmahalo for Quanta Magazine.

Source: NASA's Goddard Space Flight Center/Jeremy Schnittman

Physicists have discovered

that the ring of
photons orbiting a black hole

exhibits a
special kind of symmetry,

hinting at a
deeper meaning.

When photons hurtle toward
a black hole, most are sucked into
its depths, never to return, or gently deflected away.

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,

"you're going
to see every object in the universe infinitely many times," said
Sam Gralla, a physicist at the University of Arizona.

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.

"I had taken
the view that the event horizon was what we needed to
understand," said
Andrew Strominger, a leading black hole and quantum
gravity theorist at Harvard University.

"And I thought
of the photon ring as some sort of technical, complicated thing
which didn't have any deep significance."

Now Strominger is
making his own U-turn and trying to convince other theorists to join
him.

"We're exploring, excitedly, the possibility that the photon
ring is the thing that you have to understand to unlock the secrets
of Kerr black holes," he said, referring to the kind of spinning
black holes created when stars die and gravitationally collapse.

(The photon ring forms concurrently.)

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:

a way that it
can be transformed and still stay the same...

The symmetry
suggests that the ring may encode information about the hole's
quantum structure.

"This symmetry
smells like something to do with the central problem of
understanding the quantum dynamics of black holes," he said.

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.

"It opens up a
very interesting avenue for understanding the holography of
these [black hole] geometries," said
Alex Maloney, a theorist at McGill University in Canada
who was not involved in the research.

"The new
symmetry organizes the structure of black holes far from the
event horizon, and I think that's very exciting."

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.
Harvard-Smithsonian Center for Astrophysics

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.

"It's a target
for a holographic description," said Juan
Maldacena of the Institute for Advanced Study in
Princeton, New Jersey, one of the original architects of
holography.

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,

"I cried," he
said. "It's amazingly beautiful."

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.

"When you look
at all the simulations, you can't miss it," said
Shahar Hadar of the University of Haifa in Israel, who
collaborated with Strominger and the EHT physicists on the
research while at Harvard.

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.

"It's
definitely the most beautiful and compelling way to really see
the black hole," said Strominger.

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.
Event Horizon Telescope Collaboration

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.

(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

Photons that
make a single U-turn around
a black hole and then zip
toward Earth would appear to us as a single ring of light.

Photons that
make two U-turns around the hole appear as a fainter, thinner
subring within the first ring.

And photons
that make three U-turns appear as a subring within that subring,
and so on, creating nested rings, each fainter and thinner than
the last.

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.

"Together, the
set of subrings are akin to the frames of a movie, capturing the
history of the visible universe as seen from the black hole,"
the collaboration wrote in the 2020 paper.

Strominger said
that when he and his collaborators looked at the EHT pictures,

"we were like:
'Hey, there's an infinite number of copies of the universe right
there at that screen? Couldn't that be where the holographic
dual lives?'"

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,

a surface containing a quantum
system of particles that respect conformal symmetry.

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.

It was as if the
interior was a "universe" that projected from its lower-dimensional
surface like a hologram...

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.

It's filled
with a type of space-time that's negatively curved, which gives
it a surface-like outer boundary.

Our universe is
thought to be flat, and theorists have little idea what the
holographic dual of flat space-time looks like.

"We need to get
back to the real world, while taking inspiration from what we
learned from these hypothetical worlds," Strominger said.

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.

"The first
questions to ask are: Where does the holographic dual live? And
what are the symmetries?" said Hadar.

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.

"Saying
conformal symmetry and black hole in the same sentence to a
quantum gravity theorist is like waving red meat in front of a
dog," said Strominger.

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.

They imagined hitting the
black hole with a hammer to make it ring like a bell.

These slowly fading
vibrations are like the gravitational waves created when, say, two
black holes collide.

The black hole will
ring with some resonant frequencies that depend on the shape of
space-time (that is, on the Kerr metric) just as the ringing tones
of a bell depend on its shape.

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.
Allie Humenuk for Collapsar Films

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.

The pattern,

"turns out to
be completely governed by the photon ring," said
Alex Lupsasca of the Vanderbilt Initiative for
Gravity, Waves and Fluids in Tennessee, who co-authored the
new paper with Strominger, Hadar and Daniel Kapec of Harvard.

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.

"It's very,
very weird," Hadar said.

"As you move
along different points on the photon ring, you are actually
probing different radii" or depths into the black hole.

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.

"Perhaps
information doesn't really fall into the black hole, but it sort
of stays in a cloud around outside the black hole, which
probably extends to the photon ring," he said.

"But we don't
understand how it's coded in there, or exactly how that works."

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.

"Where the
holographic dual lives is a much deeper question than: What is
the symmetry?" said
Daniel Harlow, a quantum gravity and black hole theorist
at the Massachusetts Institute of Technology.

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:

The dual must
be able to encode the unusual vibration pattern of a spinning
black hole after it has been struck like a bell.

"Demanding
the quantum system that describes the black hole reproduces
all of that complexity is an incredibly powerful constraint
- and one that we've never tried to exploit before," said
Strominger.

Eva Silverstein, a theoretical physicist at Stanford
University, said,

"It seems like
a very nice piece of theoretical data for people to try to
reproduce when attempting a holographic dual description."

Maldacena agreed,
saying,

"One would like
to understand how to incorporate this into a holographic dual.
So it will probably stimulate some research in that direction."

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.