About a century ago, Albert Einstein amazed the world with his
groundbreaking theory of relativity, and ever since he shared this profound
understanding of gravity and spacetime, physicists everywhere have worked
hard to prove, refine and extend it. In the intervening decades, numerous
observations have borne Einstein out, with phenomena such as gravitational
lensing and redshift, shifts in planetary orbit and, more recently,
gravitational waves and observations of black holes.

However, for all the advances we’ve made in witnessing the more readily
observable, macro effects of gravity, there remains a gap — a chasm, really
— in our ability to understand gravity in the context of another profound
discovery: quantum mechanics, the physics of matter and energy at their
smallest scales.

“There is the longstanding problem, perhaps the greatest remaining from 20th
century physics, of reconciling quantum mechanics with gravity,” said UC
Santa Barbara theoretical physicist Steven Giddings(link is external). The
universe is quantum, and unlike the other fundamental forces — the
electromagnetic, the weak and the strong nuclear forces — which have been
described within quantum field theory, what we know of gravitation remains
solidly in the realm of classical physics.

“Associated with that problem is a gulf between theory and observation,”
said Giddings, who specializes in high energy and gravitational theory, as
well as quantum black holes, quantum cosmology and other quantum aspects of
gravity. Traditional thinking leads one to believe that quantum aspects of
gravity are only observable if we explore incredibly short distances, he
said, such as the Planck length (10-35 meter), thought to be the smallest
length in the universe and the length at which quantum gravity effects
become important. It’s also far beyond observational reach.

But what if it was possible to detect quantum gravity at longer, observable
length scales? Giddings, and fellow theorists Kathryn Zurek and Yanbei Chen
at Caltech, Cynthia Keeler and Maulik Parikh at Arizona State University,
and Ben Freivogel and Erik Verlinde at University of Amsterdam, think that
could be the case.

“Various theoretical developments have indicated that quantum gravity
effects may become important at much greater distances in certain contexts,
and that is truly exciting and worth exploring,” Giddings said. “We are
taking this seriously.”

And, thanks to support from the Heising-Simons Foundation, the team is
poised to bridge that chasm, by exploring ways in which quantum gravity may
be observed, via effects a longer length scales.

“We are thrilled that the Heising-Simons Foundation has chosen to support
this vision of exploring new effects, particularly at long distances, in
quantum gravity, and the possibility that they lead to observational
effects,” Giddings said of the $3.1 million in multi-institution grants to
help the team push the boundaries of our knowledge of quantum gravity.
“Their support should really move this research forward.”

### Quantum Effects at Longer Lengths

Reconciling relativity to quantum mechanics has challenged physicists for
the better part of a century, with puzzles such as the black hole
information paradox. That’s where relativity and quantum mechanics violently
conflict on the issue of what happens to information that falls into a black
hole, those extremely high-gravity voids in spacetime. A relativistic
picture indicates that the information gets destroyed as the black hole
slowly evaporates, while quantum mechanics states that that information
cannot be destroyed.

A suggested approach to that conundrum and other similarly complex issues
emerges with the proposed holographic principle, a fundamentally new idea
about the possible behavior of quantum gravity.

“There are different ways to explain it, but one is that the amount of
information you can put in a volume is not proportional to the volume but to
the surface area surrounding the volume,” Giddings explained. A consistent
theory incorporating this principle might explain how information is not
destroyed, resulting in a relativistic object, such as a black hole, obeying
quantum rules.

“When one tries to reconcile the existence of black holes with the
principles of quantum mechanics, one seems to be led to the conclusion that
new quantum gravity effects must become important not just at short
distances, but at distances comparable to the size of the black hole in
question — for the largest black holes we know, many times the size of our
solar system,” Giddings said.

The principle, which started out originally with black holes, has been
suggested to extend to the universe in general — what we perceive as our
three-dimensional reality may even, in a sense, have an underlying
two-dimensional description. This could make its mathematical description
more elegant and compelling.

“This is a big departure from the properties of quantum field theories that
describe other forces of nature — like electromagnetism and the strong force
— and is a feature of gravity that strongly suggests that a theory of
gravity has a very different underlying structure,” he added. This
fundamentally different structure might be part of a description with novel
properties, in which information is preserved.

A related argument for the observability of quantum gravity at greater
distances comes from the notion that very high energy collisions, though far
beyond what we have been able to accomplish, start producing quantum
gravitational effects at increasingly large distances.

“When one considers extremely high energy collisions of particles, one is
not probing shorter distances any longer — as has been true at accessible
energies — but instead one starts to see effects at longer distances, due to
basic properties of gravity,” Giddings said.

### Quantum Gravity at Work

Recent developments in experimental observations have made it possible to
detect and measure new effects of gravity, such as with Caltech’s Laser
Interferometer Gravitational Wave Observatory (LIGO), the Virgo
interferometer in Italy, and the Kamioka Gravitational Wave Detector (KAGRA)
in Japan. Each of those facilities is turned to space to sense gravitational
waves coming from major events, such as the mergers of massive celestial
bodies like black holes and neutron stars. These, as well as observations of
light from near black holes by the Event Horizon Telescope, may also be
sensitive to long-range quantum effects. In addition, ideas related to
holography suggest the possibility of new quantum effects in lab-based
settings, and newer experiments with interferometers may provide novel ways
to test them.

The task for the researchers as they resolve foundational issues and
understand aspects of the fundamental description of quantum gravity, is to
develop “effective descriptions” that can connect theory with observations
coming in from the interferometers and other instruments.

“In physics, we have often been in the situation where we don’t have the
complete theory, but we have an approximate description that captures
certain important properties of that theory,” Giddings explained. “Often,
such ‘effective descriptions’ can be surprisingly powerful, and lead to
deeper insight about the more fundamental theory.”

The group’s diverse mix of backgrounds is a strength of this collaboration,
with specializations ranging from quantum gravity to particle physics,
string theory to gravitational wave physics. Through a series of meetings to
be held over four years the collaboration will progress from foundational
issues, such as sharpening the description of holography and understanding
the mathematical structure of gravity, to studying models that may describe
behavior of quantum gravity, its interactions and potentially observable
effects, to developing specific observational tests with the interferometers
and observations of black holes.

Along the way, the collaboration will grow, starting with the seven core
members and adding postdoctoral fellows and graduate students, and finally
broadening activities to include additional physicists to discuss
collaboration results and related theoretical advances from the broader
community.

“If we are able to observe quantum effects of black holes, that will be
truly revolutionary,” Giddings said. “It would also likely help guide the
conceptual revolution of reconciling quantum mechanics with gravity, which
we expect to likely be as profound as the revolutionary discovery of quantum
mechanics.”

Super! Delicious! Quantum gravity has long been time to find and describe. However, why is all this eclectic sticking of insane ideas? Evaporation of black holes? Are you still seriously discussing this joke of Hawking? What two-dimensional holography? Our universe is a 3-dimensional hologram of a 4-dimensional ball, you have not enough of this? This is called a 3-sphere. You live within the boundaries of this film. Want information from 4 dimensions will be squeezed in 2? Do not make me laugh, it is a complete theoretical failure. But technical experiments warmly welcome.

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