Quantum Gravity

Quantum gravity is concerned with unifying Einstein's general theory of relativity with quantum theory into a single theoretical framework. At Perimeter Institute, researchers are actively pursuing a number of approaches to this problem including loop quantum gravity, spin foam models, asymptotic safety, emergent gravity, string theory, and causal set theory. We are also particularly interested in experimental implications of these different proposals. As the aim is a unification of the laws of physics into a single theory, the search for quantum gravity overlaps with other areas such as cosmology, particle physics and the foundations of quantum theory.

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Glimpses into Loop Quantum Gravity

Abhay Ashtekar Pennsylvania State University
October 25, 2023

PIRSA:23100009
Quantum Gravity
Panel Discussion - QFT vs beyond-QFT approaches (Buoninfante, Rejzner, Smolin, Vieira)
- Luca Buoninfante Nordic Institute for Theoretical Physics
- Benjamin Knorr Nordic Institute for Theoretical Physics
- Katarzyna Rejzner University of York
- Lee Smolin Perimeter Institute for Theoretical Physics
- Pedro Vieira Perimeter Institute for Theoretical Physics
October 24, 2023

PIRSA:23100008
Quantum Gravity
Open Discussion with today's speakers (Buoninfante, Eichhorn, Holdom, Knorr, Loll)
- Luca Buoninfante Nordic Institute for Theoretical Physics
- Astrid Eichhorn Universität Heidelberg
- Bob Holdom University of Toronto
- Benjamin Knorr Nordic Institute for Theoretical Physics
- Renate Loll Radboud Universiteit Nijmegen
October 24, 2023

PIRSA:23100007
Quantum Gravity
Form Factors - a unifying language for Quantum Gravity

Benjamin Knorr Nordic Institute for Theoretical Physics
October 24, 2023

PIRSA:23100087
Quantum Gravity
The Case for Renormalizable Quantum Gravity: from local to nonlocal approaches (and back!)

Luca Buoninfante Nordic Institute for Theoretical Physics
October 24, 2023

PIRSA:23100064
Quantum Gravity
UV complete 4-derivative scalar field theory

Bob Holdom University of Toronto
October 24, 2023

PIRSA:23100006
Quantum Gravity
CDT as computational gateway to nonperturbative quantum gravity

Renate Loll Radboud Universiteit Nijmegen
October 24, 2023

PIRSA:23100062
Quantum Gravity
Status, perspective and three challenges in the asymptotic-safety paradigm for quantum gravity - VIRTUAL

Astrid Eichhorn Universität Heidelberg
October 24, 2023

PIRSA:23100005
Quantum Gravity
Panel Discussion - Strengths and limitations of EFT (Donoghue, Knorr, Montero, Quevedo, Tolley)
- John Donoghue University of Massachusetts Amherst
- Miguel Mlontero IFT Madrid
- Fernando Quevedo University of Cambridge
- Carlo Rovelli Aix-Marseille University
- Andrew Tolley Imperial College London
October 23, 2023

PIRSA:23100004
Quantum Gravity
Open Discussion with today's speakers (Donoghue, Erdmenger, Montero, Pasterski, Tolley)
- John Donoghue University of Massachusetts Amherst
- Johanna Erdmenger University of Würzburg
- Miguel Mlontero IFT Madrid
- Sabrina Pasterski Perimeter Institute for Theoretical Physics
- Andrew Tolley Imperial College London
October 23, 2023

PIRSA:23100003
Quantum Gravity
Piecing Together a Flat Hologram

Sabrina Pasterski Perimeter Institute for Theoretical Physics
October 23, 2023

PIRSA:23100061
Quantum Gravity
A hike through the Swamp

Miguel Mlontero IFT Madrid
October 23, 2023

PIRSA:23100002
Quantum Gravity

Glimpses into Loop Quantum Gravity

Abhay Ashtekar Pennsylvania State University
October 25, 2023

PIRSA:23100009
Quantum Gravity
The organizers requested an overview of Loop Quantum Gravity (LQG). However, this is an impossible task, especially for a 30 minute talk, given that the subject is over three decades old with tens of thousands of papers. Instead, I will outline some of the key distinguishing features of LQG and discuss an illustrative application. Hopefully these topics will complement those that will be covered by Bianca Dittrich and Lee Smolin, without too much of an overlap.
Panel Discussion - QFT vs beyond-QFT approaches (Buoninfante, Rejzner, Smolin, Vieira)
- Luca Buoninfante Nordic Institute for Theoretical Physics
- Benjamin Knorr Nordic Institute for Theoretical Physics
- Katarzyna Rejzner University of York
- Lee Smolin Perimeter Institute for Theoretical Physics
- Pedro Vieira Perimeter Institute for Theoretical Physics
October 24, 2023

PIRSA:23100008
Quantum Gravity
Open Discussion with today's speakers (Buoninfante, Eichhorn, Holdom, Knorr, Loll)
- Luca Buoninfante Nordic Institute for Theoretical Physics
- Astrid Eichhorn Universität Heidelberg
- Bob Holdom University of Toronto
- Benjamin Knorr Nordic Institute for Theoretical Physics
- Renate Loll Radboud Universiteit Nijmegen
October 24, 2023

PIRSA:23100007
Quantum Gravity
Form Factors - a unifying language for Quantum Gravity

Benjamin Knorr Nordic Institute for Theoretical Physics
October 24, 2023

PIRSA:23100087
Quantum Gravity
QFT's remarkable success lies in its ability to yield precise predictions through e.g. scattering amplitudes, capturing the essence of running couplings in a gauge-invariant manner. However, the introduction of gravity disrupts this straightforward notion of momentum dependence, posing a significant challenge to our theoretical framework. Enter form factors—an ingenious concept that extends the idea of momentum dependence to the curved spacetime of quantum gravity. They form the central ingredient when discussing quantum gravity in a QFT language. Furthermore, within an effective action, form factors offer a unifying language, allowing us to compare and contrast different quantum gravity approaches apples-to-apples. I will give an overview of the ideas underlying form factors, their application to scattering problems, and their potential to put different approaches on the same footing.
The Case for Renormalizable Quantum Gravity: from local to nonlocal approaches (and back!)

Luca Buoninfante Nordic Institute for Theoretical Physics
October 24, 2023

PIRSA:23100064
Quantum Gravity
In the context of perturbative quantum field theory (QFT), the addition of quadratic-curvature invariants to the Einstein-Hilbert action makes it possible to achieve strict renormalizability in four dimensions. This theory exhibits unusual features due to an additional massive spin-2 ghost which, in general, may cause instabilities. In the first part of this talk, we focus on the possibility of giving up locality as a way to avoid ghost-like degrees of freedom and provide a critical assessment on open questions in nonlocal theories of gravity, such as the uniqueness problem. In the second part of the talk, we take a step back and argue that, despite the presence of the ghost and actually thanks to it, Quadratic Gravity can still provide a consistent local perturbative QFT description of the gravitational interaction and explain new physics beyond Einstein's general relativity, e.g., it offers a natural explanation for the inflationary phase. Finally, we argue that a type of nonlocality in gravity can still occur non-perturbatively and show that a new lower bound on scattering amplitudes indicates that the gravitational interaction is intrinsically nonlocal if black holes form.
UV complete 4-derivative scalar field theory

Bob Holdom University of Toronto
October 24, 2023

PIRSA:23100006
Quantum Gravity
A scalar field theory with 4-derivative kinetic terms and 4-derivative cubic and quartic couplings is presented as a proxy for quadratic gravity. Unitarity and positivity appear as the key issues in the scalar field theory, just as they do in quadratic gravity. We have extended some calculations to show how these issues are resolved in the high energy limit of the theory. The results also show how it is that differential cross sections can have good high energy behaviour.
CDT as computational gateway to nonperturbative quantum gravity

Renate Loll Radboud Universiteit Nijmegen
October 24, 2023

PIRSA:23100062
Quantum Gravity
Based on quantitative results on quantum observables in a Planckian regime, I will argue that using dynamical lattices `a la CDT to construct and explore the nonperturbative path integral over Lorentzian geometries is a bona-fide quantum field-theoretic "method" rather than an "approach" to quantum gravity. Its most important features are universality, unitarity and the presence of powerful numerical tools to directly simulate and measure a chunk of quantum spacetime of about 20 Planck lengths across. This small observational window is a preferred place to look for clues to what quantum gravity is all about. Despite the diffeomorphism-invariant and nonlocal character of the observables, qualitatively new and surprising UV properties have already been discovered, and compatibility checks with semiclassical expectations for several cosmological observables ("classical limit") have been performed successfully. The underlying new mathematics is that of random geometry and beyond-Riemannian geometry.
Status, perspective and three challenges in the asymptotic-safety paradigm for quantum gravity - VIRTUAL

Astrid Eichhorn Universität Heidelberg
October 24, 2023

PIRSA:23100005
Quantum Gravity
In my talk, I'll review the key idea underlying asymptotically safe quantum gravity, before giving an overview of the current status and open questions. I'll then spotlight three challenges. The first challenge is how to test quantum gravity and I'll advocate that by linking quantum gravity to particle physics and cosmology, we can probe quantum gravity at all scales. The second challenge is how to connect to other ideas about quantum spacetime and I'll discuss effective asymptotic safety as a way to link to, e.g., string theory. The third challenge is the status of global symmetries and I'll discuss what we know about the preservation of global symmetries in asymptotic safety and what this implies for a broader perspective on the question. --- The presenter will be joining via Zoom for this talk.
Panel Discussion - Strengths and limitations of EFT (Donoghue, Knorr, Montero, Quevedo, Tolley)
- John Donoghue University of Massachusetts Amherst
- Miguel Mlontero IFT Madrid
- Fernando Quevedo University of Cambridge
- Carlo Rovelli Aix-Marseille University
- Andrew Tolley Imperial College London
October 23, 2023

PIRSA:23100004
Quantum Gravity
Open Discussion with today's speakers (Donoghue, Erdmenger, Montero, Pasterski, Tolley)
- John Donoghue University of Massachusetts Amherst
- Johanna Erdmenger University of Würzburg
- Miguel Mlontero IFT Madrid
- Sabrina Pasterski Perimeter Institute for Theoretical Physics
- Andrew Tolley Imperial College London
October 23, 2023

PIRSA:23100003
Quantum Gravity
Piecing Together a Flat Hologram

Sabrina Pasterski Perimeter Institute for Theoretical Physics
October 23, 2023

PIRSA:23100061
Quantum Gravity
The Celestial Holography program encompasses recent efforts to understand the flat space hologram in terms of a CFT living on the celestial sphere. A key development instigating these efforts came from understanding how soft limits of scattering encode infinite dimensional symmetry enhancements corresponding to the asymptotic symmetry group of the bulk spacetime. Historically, the construction of the bulk-boundary dual pair has followed bottom up approach matching symmetries on both sides. Recently, however, there has been exciting progress in formulating top down descriptions using insights from twisted holography. We review salient aspects of the celestial construction, the status of the dictionary, and active research directions.
A hike through the Swamp

Miguel Mlontero IFT Madrid
October 23, 2023

PIRSA:23100002
Quantum Gravity
At first sight, it seems that almost any QFT, seen as a low-energy effective field theory (EFT), can be coupled to gravity -- simply add an Einstein-Hilbert term, change all derivatives by covariant ones, and you're done. However, a growing body of evidence coming from String Theory compactifications, general properties of black hole evaporation, and rigorous results in holography, suggests that the opposite is true. We can work out the hidden constraints that quantum gravity consistency imposes in the matter sector systematically, an approach that receives the colourful name of "The Swampland Program". I will briefly review the program, its motivation, current results, and long-term goals.

Title	Speaker(s)	Date	Talk Type	Info link
Glimpses into Loop Quantum Gravity	Abhay Ashtekar	2023-10-25	Conference	View details
Panel Discussion - QFT vs beyond-QFT approaches (Buoninfante, Rejzner, Smolin, Vieira)	Luca Buoninfante, Benjamin Knorr, Katarzyna Rejzner, Lee Smolin, Pedro Vieira	2023-10-24	Conference	View details
Open Discussion with today's speakers (Buoninfante, Eichhorn, Holdom, Knorr, Loll)	Luca Buoninfante, Astrid Eichhorn, Bob Holdom, Benjamin Knorr, Renate Loll	2023-10-24	Conference	View details
Form Factors - a unifying language for Quantum Gravity	Benjamin Knorr	2023-10-24	Conference	View details
The Case for Renormalizable Quantum Gravity: from local to nonlocal approaches (and back!)	Luca Buoninfante	2023-10-24	Conference	View details
UV complete 4-derivative scalar field theory	Bob Holdom	2023-10-24	Conference	View details
CDT as computational gateway to nonperturbative quantum gravity	Renate Loll	2023-10-24	Conference	View details
Status, perspective and three challenges in the asymptotic-safety paradigm for quantum gravity - VIRTUAL	Astrid Eichhorn	2023-10-24	Conference	View details
Panel Discussion - Strengths and limitations of EFT (Donoghue, Knorr, Montero, Quevedo, Tolley)	John Donoghue, Miguel Mlontero, Fernando Quevedo, Carlo Rovelli, Andrew Tolley	2023-10-23	Conference	View details
Open Discussion with today's speakers (Donoghue, Erdmenger, Montero, Pasterski, Tolley)	John Donoghue, Johanna Erdmenger, Miguel Mlontero, Sabrina Pasterski, Andrew Tolley	2023-10-23	Conference	View details
Piecing Together a Flat Hologram	Sabrina Pasterski	2023-10-23	Conference	View details
A hike through the Swamp	Miguel Mlontero	2023-10-23	Conference	View details

Quantum Gravity

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