# Heising-Simons Foundation (HSF) Collaboration

on Quantum Gravity

and Its Observational Signatures

(QuRIOS)

Seeking to bridge the divide between theory and observability

Explore## Mission

####
To catalyze recent theory progress

to explore the observability of Quantum Gravity.

## Background

Einstein's general theory of relativity is now over 100 years old. The beauty of the theory lies in its depiction of gravity as a manifestation of the curvature of spacetime; Einstein transformed the study of gravity into the study of space, time, and geometry. No less impressive, the theory remains in perfect agreement with all precision experiments to date. And yet, general relativity has a flaw: it is only a classical theory, obeying deterministic equations.

By contrast, quantum field theory, our regnant theory of matter and other forces, describes a reality in which the observed behavior of particles and fields is dictated by probability amplitudes. Quantum field theory has been validated for all forms of matter -- from electrons and quarks to photons and gluons.

It is not yet known how these two crowning achievements of twentieth-century physics are unified in a complete framework. Providing a thoroughgoing description of gravity consistent with quantum theory, and verified with observation, is among the deepest open problems in physics.

Although we still do not possess a complete understanding of the properties of quantum gravity, some fundamental aspects of the theory are already clear. The role of long distance scales in quantum gravity strongly motivates searching for novel observational signatures. A cultural chasm within the theory community has inhibited the development of this line of thought towards observational signatures.

We seek a paradigm shift that brings theory and observables together, simultaneously closing the chasm between quantum mechanics and gravity.

The Collaboration on Observational Signatures of Quantum Gravity brings together theorists from diverse backgrounds, from quantum gravity and string theory to astrophysics and phenomenology, to bridge the chasm between theory and observation, and develop theoretically well-founded proposals of quantum gravity that could be observed. We hope to change the culture in theory that has created this chasm, kick-starting a broader community effort on possible observational signatures of quantum gravity.

Read "Is Space Pixelated? Quantum Gravity: The Quest for the Pixelation of Space" on the *Caltech Magazine* website.

### Why now?

In the past few years, new observational tools have become available: Gravitational observatories (LIGO, Virgo and KaGRA) and the Event Horizon Telescope (EHT) have opened a new era of observations. And new experiments, such as the Gravity from Quantum Entanglement in Space-Time (GQuEST) interferometer, have been proposed.

Concurrently, multiple theoretical threads have combined to point to an important
role for *long* distances in Quantum Gravity. Theoretical tools such as holography
suggest correlations of unmeasurably short length scales of Quantum Gravity on longer
scales. Quantum consistency for black holes strongly indicates new physics at horizon
scales. Other developments make it possible to, in some cases, compute observables
in the infrared, utilizing well-established tools, such as Entanglement Entropy.

### Why this team?

A diverse group of scientists, each bringing their unique yet complementary expertise to the Collaboration, creates a team that is larger than the sum of its parts. Field theory, string theory, string theory, quantum gravity, cosmology, gravitational-wave science and astrophysics, particle physics and other related areas are the fields represented in the Collaboration. The team represents a robust theoretical background and is well-suited to build bridges to observable theory.

## Members

### Complementarity of Expertise

The Collaboration consists of four research hubs, Caltech, UC Santa Barbara, Arizona State University, and University of Amsterdam, and is directed by Kathryn Zurek at Caltech. We have brought together a team of scientists from disparate backgrounds, including quantum gravity, particle theory, string theory, and astrophysics (including gravitational waves), to make a whole that is greater than a sum of its parts:

*Particle theory, Effective field theory and models*

*String theory, emergent gravity*

*Particle theory, gravity*

*String theory, fluid gravity*

*Quantum gravity, black holes*

*String theory, cosmology, early universe*

*Astrophysics, gravitational waves, precision measurements*

## 2022 HSF Fellows in Observational Signatures of Quantum Gravity

## The Heising-Simons Foundation (HSF)

The Collaboration is supported by the The Heising-Simons Foundation. For more information, please visit the Foundation's website.