Name: Puzzles in the Quantum Gravity Landscape: viewpoints from different approaches
Start: 2023-10-23T08:30:00-04:00
End: 2023-10-27T17:00:00-04:00
Location: Perimeter Institute for Theoretical Physics

Puzzles in the Quantum Gravity Landscape: viewpoints from different approaches

from Monday, October 23, 2023 (8:30 AM) to Friday, October 27, 2023 (5:00 PM)

Monday, October 23, 2023

9:00 AM Opening Remarks
Opening Remarks
9:00 AM - 9:10 AM
Room: PI/1-100 - Theatre
9:10 AM Lessons of the Effective Field Theory Treatment of General Relativity - John Donoghue
Lessons of the Effective Field Theory Treatment of General Relativity
- John Donoghue
9:10 AM - 9:45 AM
Room: PI/1-100 - Theatre I will briefly review the key concepts underlying the Effective Field Theory of General Relativity, and give a couple of examples of how it works. Then I will describe seven lessons which can be extracted from the theory. Finally I discuss some of the limitations of the EFT framework.
9:45 AM Positivity Bounds and Effective Fields Theories (A Review) - Andrew Tolley
Positivity Bounds and Effective Fields Theories (A Review)
- Andrew Tolley
9:45 AM - 10:20 AM
Room: PI/1-100 - Theatre I will briefly review recent progress on how causality/analyticity and unitarity can put powerful constraints on both gravitational and non-gravitational EFTs that admit consistent UV completions.
10:20 AM Holography and its implications for quantum gravity - VIRTUAL - Johanna Erdmenger
Holography and its implications for quantum gravity - VIRTUAL
- Johanna Erdmenger
10:20 AM - 10:55 AM
Room: PI/1-100 - Theatre I will review implications of the AdS/CFT correspondence for quantum gravity, paying particular attention to holographic RG flows and the processing of information. Deformed hyperbolic geometries correspond to boundary field theory RG flows. Conversely, as I will exemplify using the recent approach of discrete holography, boundary RG flows may be used to reconstruct the bulk spacetime, for instance via tensor networks. I will discuss recent approaches towards including gravity dynamics into this reconstruction process. --- The presenter will be joining via Zoom for this talk.
11:00 AM Coffee Break
Coffee Break
11:00 AM - 11:30 AM
Room: PI/1-124 - Lower Bistro
11:30 AM A hike through the Swamp - Miguel Montero
A hike through the Swamp
- Miguel Montero
11:30 AM - 12:05 PM
Room: PI/1-100 - Theatre 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.
12:05 PM Piecing Together a Flat Hologram - Sabrina Pasterski
Piecing Together a Flat Hologram
- Sabrina Pasterski
12:05 PM - 12:40 PM
Room: PI/1-100 - Theatre 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.
12:45 PM Lunch
Lunch
12:45 PM - 1:45 PM
Room: PI/2-251 - Upper Bistro
2:00 PM Open Discussion with today's speakers (Donoghue, Erdmenger, Montero, Pasterski, Tolley) - Andrew Tolley Sabrina Pasterski Johanna Erdmenger John Donoghue Miguel Montero
Open Discussion with today's speakers (Donoghue, Erdmenger, Montero, Pasterski, Tolley)
- Andrew Tolley
- Sabrina Pasterski
- Johanna Erdmenger
- John Donoghue
- Miguel Montero
2:00 PM - 3:15 PM
Room: PI/1-100 - Theatre
3:15 PM Coffee Break
Coffee Break
3:15 PM - 3:45 PM
Room: PI/1-124 - Lower Bistro
3:45 PM Panel Discussion - Strengths and limitations of EFT (Donoghue, Knorr, Montero, Quevedo, Tolley) - Fernando Quevedo Carlo Rovelli Miguel Montero John Donoghue Andrew Tolley
Panel Discussion - Strengths and limitations of EFT (Donoghue, Knorr, Montero, Quevedo, Tolley)
- Fernando Quevedo
- Carlo Rovelli
- Miguel Montero
- John Donoghue
- Andrew Tolley
3:45 PM - 5:15 PM
Room: PI/1-100 - Theatre
5:15 PM Poster Session and Welcome Reception
Poster Session and Welcome Reception
5:15 PM - 7:00 PM
Tuesday, October 24, 2023
9:00 AM Status, perspective and three challenges in the asymptotic-safety paradigm for quantum gravity - VIRTUAL - Astrid Eichhorn
Status, perspective and three challenges in the asymptotic-safety paradigm for quantum gravity - VIRTUAL
- Astrid Eichhorn
9:00 AM - 9:35 AM
Room: PI/1-100 - Theatre 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.
9:35 AM CDT as computational gateway to nonperturbative quantum gravity - Renate Loll
CDT as computational gateway to nonperturbative quantum gravity
- Renate Loll
9:35 AM - 10:10 AM
Room: PI/1-100 - Theatre 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.
10:10 AM UV complete 4-derivative scalar field theory - Bob Holdom
UV complete 4-derivative scalar field theory
- Bob Holdom
10:10 AM - 10:45 AM
Room: PI/1-100 - Theatre 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.
10:45 AM Coffee Break
Coffee Break
10:45 AM - 11:15 AM
Room: PI/1-124 - Lower Bistro
11:15 AM The Case for Renormalizable Quantum Gravity: from local to nonlocal approaches (and back!) - Luca Buoninfante
The Case for Renormalizable Quantum Gravity: from local to nonlocal approaches (and back!)
- Luca Buoninfante
11:15 AM - 11:50 AM
Room: PI/1-100 - Theatre 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.
11:50 AM Form Factors - a unifying language for Quantum Gravity - Benjamin Knorr
Form Factors - a unifying language for Quantum Gravity
- Benjamin Knorr
11:50 AM - 12:25 PM
Room: PI/1-100 - Theatre 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.
12:30 PM Lunch
Lunch
12:30 PM - 1:30 PM
Room: PI/2-251 - Upper Bistro
2:00 PM Open Discussion with today's speakers (Buoninfante, Eichhorn, Holdom, Knorr, Loll) - Luca Buoninfante Benjamin Knorr Astrid Eichhorn Renate Loll Bob Holdom
Open Discussion with today's speakers (Buoninfante, Eichhorn, Holdom, Knorr, Loll)
- Luca Buoninfante
- Benjamin Knorr
- Astrid Eichhorn
- Renate Loll
- Bob Holdom
2:00 PM - 3:15 PM
Room: PI/1-100 - Theatre
3:15 PM Coffee Break
Coffee Break
3:15 PM - 3:45 PM
Room: PI/1-124 - Lower Bistro
3:45 PM Panel Discussion - QFT vs beyond-QFT approaches (Buoninfante, Rejzner, Smolin, Vieira) - Lee Smolin Luca Buoninfante Pedro Vieira Benjamin Knorr Kasia Rejzner
Panel Discussion - QFT vs beyond-QFT approaches (Buoninfante, Rejzner, Smolin, Vieira)
- Lee Smolin
- Luca Buoninfante
- Pedro Vieira
- Benjamin Knorr
- Kasia Rejzner
3:45 PM - 5:15 PM
Room: PI/1-100 - Theatre
5:15 PM Poster Session
Poster Session
5:15 PM - 6:15 PM

6:30 PM Pizza & Career: Panel for young researchers (Francesco di Filippo, Yasaman Yazdi, Miguel Montero, Kasia Rejzner)
Pizza & Career: Panel for young researchers (Francesco di Filippo, Yasaman Yazdi, Miguel Montero, Kasia Rejzner)
6:30 PM - 7:30 PM
Room: PI/2-251 - Upper Bistro
Wednesday, October 25, 2023
9:00 AM Glimpses into Loop Quantum Gravity - Abhay Ashtekar
Glimpses into Loop Quantum Gravity
- Abhay Ashtekar
9:00 AM - 9:35 AM
Room: PI/1-100 - Theatre 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.
9:35 AM Puzzles, Resolutions and Open Questions in Causal Set Theory - VIRTUAL - Sumati Surya
Puzzles, Resolutions and Open Questions in Causal Set Theory - VIRTUAL
- Sumati Surya
9:35 AM - 10:10 AM
Room: PI/1-100 - Theatre I will begin with a broad overview of the main ingredients of causal set theory, and then discuss the partial resolution of a long-standing "puzzle" in the causal set path sum. I will then discuss other causal set curiousities, ending with a brief overview of the challenges in constructing a realistic fully non-perturbative quantum dynamics. --- The presenter will be joining via Zoom for this talk.
10:10 AM Where is String Theory - Pedro Vieira
Where is String Theory
- Pedro Vieira
10:10 AM - 10:45 AM
Room: PI/1-100 - Theatre
10:45 AM Coffee Break
Coffee Break
10:45 AM - 11:15 AM
Room: PI/1-124 - Lower Bistro
11:15 AM Panel Discussion - Cosmological Puzzles (Ashtekar, Brandenberger, Geshnizjani, Sakellariadou, Yazdi) - Yasaman Yazdi Ghazal Geshnizjani Abhay Ashtekar Robert Brandenberger Mairi Sakellariadou
Panel Discussion - Cosmological Puzzles (Ashtekar, Brandenberger, Geshnizjani, Sakellariadou, Yazdi)
- Yasaman Yazdi
- Ghazal Geshnizjani
- Abhay Ashtekar
- Robert Brandenberger
- Mairi Sakellariadou
11:15 AM - 12:45 PM
Room: PI/1-100 - Theatre
12:45 PM Lunch
Lunch
12:45 PM - 1:45 PM
Room: PI/2-251 - Upper Bistro
1:45 PM Open Discussion with today's speakers (Ashtekar, Surya, Vieira) - Abhay Ashtekar Pedro Vieira Sumati Surya
Open Discussion with today's speakers (Ashtekar, Surya, Vieira)
- Abhay Ashtekar
- Pedro Vieira
- Sumati Surya
1:45 PM - 2:30 PM
Room: PI/1-100 - Theatre
2:30 PM Group Photo - meet in the Atrium
Group Photo - meet in the Atrium
2:30 PM - 2:35 PM
Room: PI/1-119 - Atrium

2:35 PM Coffee Break
Coffee Break
2:35 PM - 3:00 PM
Room: PI/1-124 - Lower Bistro

7:00 PM Livestream: Secrets of the Universe – Hiding in Plain Sight? Neil Turok Public Lecture
Livestream: Secrets of the Universe – Hiding in Plain Sight? Neil Turok Public Lecture
7:00 PM - 8:00 PM
Room: PI/1-119 - Atrium
Thursday, October 26, 2023
9:00 AM Quantum hair and information loss paradox - VIRTUAL - Xavier Calmet
Quantum hair and information loss paradox - VIRTUAL
- Xavier Calmet
9:00 AM - 9:35 AM
Room: PI/1-100 - Theatre In this talk I present our solution to the information paradox published in Phys.Rev.Lett. 128 (2022) 11, 111301 and Phys.Lett.B 827 (2022) 136995 (see EPL 139 (2022) 4, 49001 for a review). Long wavelength quantum gravitational effects allow the interior state of the black hole to influence Hawking radiation, leading to unitary evaporation. I explain why the Mathur theorem is evaded due to the complex nature of the Hawking radiation superposition state. --- The presenter will be joining via Zoom for this talk.
9:35 AM Hearts of Darkness: Nonsingular Black Holes Beyond General Relativity - Francesco di Filippo
Hearts of Darkness: Nonsingular Black Holes Beyond General Relativity
- Francesco di Filippo
9:35 AM - 10:10 AM
Room: PI/1-100 - Theatre Black holes contain, deep in their interior, theoretical evidence of the failure of general relativity. A series of fundamental results, starting from the 1965 Penrose singularity theorem, proved that physically realistic initial conditions will unavoidably produce a singular black hole spacetime. It is generally expected that a full theory of quantum gravity should remove the singularities that appear in general relativity. However, the lack of proper understanding of the dynamical laws dictating the evolution of spacetime and matter in these extreme situations hinders the extraction of predictions in specific models. I will discuss, in a model-independent manner, the different possibilities that singularity regularization may open. I will then focus on fundamental open issues that need to be addressed to obtain viable nonsingular black hole candidates.
10:10 AM Holographic Thermodynamics: C-ing a Black Hole - Ruth Gregory
Holographic Thermodynamics: C-ing a Black Hole
- Ruth Gregory
10:10 AM - 10:45 AM
Room: PI/1-100 - Theatre One of the fascinating things about Black Hole Thermodynamics is how we often approach it in a very classical fashion, yet - if real - it is an intrinsically quantum phenomenon. I'll use the illustration of thermodynamics of an accelerating black hole to highlight some of these issues, and I'd like to raise questions on how we derive the First Law, and how unique this derivation is.
10:45 AM Coffee Break
Coffee Break
10:45 AM - 11:15 AM
Room: PI/1-124 - Lower Bistro
11:15 AM Open Discussion with today's speakers (Calmet, di Filippo, Gregory) - Francesco di Filippo Xavier Calmet Ruth Gregory
Open Discussion with today's speakers (Calmet, di Filippo, Gregory)
- Francesco di Filippo
- Xavier Calmet
- Ruth Gregory
11:15 AM - 12:00 PM
Room: PI/1-100 - Theatre
12:00 PM Lunch
Lunch
12:00 PM - 1:00 PM
Room: PI/2-251 - Upper Bistro

1:10 PM Poster Prize Talk
Poster Prize Talk
1:10 PM - 1:30 PM
Room: PI/1-100 - Theatre
1:30 PM Panel Discussion - Black hole puzzles (Di Filippo, Gregory, Holdom, Myers, Stelle) - Francesco di Filippo Robert Myers Ruth Gregory Bob Holdom Kelly Stelle
Panel Discussion - Black hole puzzles (Di Filippo, Gregory, Holdom, Myers, Stelle)
- Francesco di Filippo
- Robert Myers
- Ruth Gregory
- Bob Holdom
- Kelly Stelle
1:30 PM - 3:00 PM
Room: PI/1-100 - Theatre
3:00 PM Coffee Break
Coffee Break
3:00 PM - 3:30 PM
Room: PI/1-124 - Lower Bistro
3:30 PM Colloquium - Insights from Warped Extra Dimensions - VIRTUAL - Lisa Randall
Colloquium - Insights from Warped Extra Dimensions - VIRTUAL
- Lisa Randall
3:30 PM - 5:00 PM
Room: PI/1-100 - Theatre Warped extra dimensions were originally introduced as a way of addressing the hierarchy problem. But insights from these scenarios have extended to black hole physics, cosmology, and even mechanisms for addressing issues in purely four dimensions. We investigate this scenario and some compelling lessons. The presenter will be joining via Zoom for this talk.
7:00 PM Fireside Chat & Career Q&A (Mairi Sakellariadou and Renate Loll)
Fireside Chat & Career Q&A (Mairi Sakellariadou and Renate Loll)
7:00 PM - 8:00 PM
Room: PI/3-394 - Skyroom
Friday, October 27, 2023
9:00 AM Balance Laws as Test of Gravity - VIRTUAL - Lavinia Heisenberg
Balance Laws as Test of Gravity - VIRTUAL
- Lavinia Heisenberg
9:00 AM - 9:35 AM
Room: PI/1-100 - Theatre I will discuss how one can use balance laws in full non-linear general relativity in order to test waveform models (arXiv:2309.12505) --- The presenter will be joining via Zoom for this talk.
9:35 AM The simplicial Lorentzian path integral and spin foams - Bianca Dittrich
The simplicial Lorentzian path integral and spin foams
- Bianca Dittrich
9:35 AM - 10:10 AM
Room: PI/1-100 - Theatre I will discuss two versions of the simplicial Lorentizian path integral, namely the (Lorentzian) quantum Regge and the spin foam version. I will do so in the simple context of de Sitter cosmology. This simple example will reveal the important role of light cone irregular configurations in the simplicial path integral — I will show that these can either lead to an exponentially enhanced or an exponentially suppressed amplitude. I will then highlight an important difference between the spin foams and quantum Regge path integral, which affects the probability for the creation of the (de Sitter) universe.
10:10 AM String Theory and the Cosmological Constant - Fernando Quevedo
String Theory and the Cosmological Constant
- Fernando Quevedo
10:10 AM - 10:45 AM
Room: PI/1-100 - Theatre An overview is given on moduli stabilization in string theory with emphasis in obtaining de Sitter space solutions. Recent work on vacuum transitions will be emphasised.
10:45 AM Coffee Break
Coffee Break
10:45 AM - 11:15 AM
Room: PI/1-124 - Lower Bistro
11:15 AM A minimal SM/LCDM cosmology - Neil Turok
A minimal SM/LCDM cosmology
- Neil Turok
11:15 AM - 11:50 AM
Room: PI/1-100 - Theatre Recent observations point to a surprisingly economical description of the universe on both very small and very large scales. Stimulated by these findings, Boyle and I have proposed a new, potentially more complete theoretical framework than currently popular paradigms. Our search has so far led to 1) the simplest-yet explanation for the cosmic dark matter, soon to be tested by galaxy surveys, 2) a thermodynamic explanation for the large scale geometry of the cosmos, based on the concept of gravitational entropy à la Hawking, 3) a new account of the big bang singularity as a “mirror” enforcing CPT-symmetric boundary conditions, realising Penrose's "Weyl curvature hypothesis" and 4) a new mechanism for cancelling the divergent vacuum energy and the trace anomalies in the Standard Model (SM). The new mechanism successfully predicts the primordial density perturbations in terms of the SM’s gauge couplings. It also explains why there are 3 generations of elementary particles, each including a RH neutrino, one of which is stable and comprises the dark matter. I’ll outline the challenges the new picture faces and the opportunities it presents, ranging from solving the gauge hierarchy problem to an improved description of quantum gravity along with prospective observational tests.
11:50 AM Open discussion with today's speakers (Dittrich, Heisenberg, Quevedo, Turok) - Neil Turok Lavinia Heisenberg Fernando Quevedo Bianca Dittrich
Open discussion with today's speakers (Dittrich, Heisenberg, Quevedo, Turok)
- Neil Turok
- Lavinia Heisenberg
- Fernando Quevedo
- Bianca Dittrich
11:50 AM - 12:50 PM
Room: PI/1-100 - Theatre
12:50 PM Lunch
Lunch
12:50 PM - 1:45 PM
Room: PI/2-251 - Upper Bistro
1:45 PM Panel Discussion - Future Directions in QG (Dittrich, Gregory, Loll, Sakellariadou, Surya) - Sumati Surya Renate Loll Mairi Sakellariadou Ruth Gregory Bianca Dittrich
Panel Discussion - Future Directions in QG (Dittrich, Gregory, Loll, Sakellariadou, Surya)
- Sumati Surya
- Renate Loll
- Mairi Sakellariadou
- Ruth Gregory
- Bianca Dittrich
1:45 PM - 3:00 PM
Room: PI/1-100 - Theatre
3:00 PM Break
Break
3:00 PM - 3:15 PM
Room: PI/1-100 - Theatre
3:15 PM Branes & Strings, Freedom & Safety and a bit of Cosmology -- Some current problems in Quantum Gravity - Kelly Stelle
Branes & Strings, Freedom & Safety and a bit of Cosmology -- Some current problems in Quantum Gravity
- Kelly Stelle
3:15 PM - 4:00 PM
Room: PI/1-100 - Theatre What relations are there between the various ways of wrangling with quantum gravity? String theory is now much more than just a theory of strings -- branes and braneworlds abound. Some kind of effective theory for the familiar world needs to emerge. Are there ways that one could glimpse underlying structure from aspects of an effective theory? That happens for pions -- is there anything like that for gravity? Effective theories also involve higher derivatives, and those can summon up spirits (i.e. ghosts) from the vasty deep. Do asymptotic freedom or asymptotic safety give ways to exorcise them? And what might the effective theory tell us about the earliest times?
4:00 PM Closing Remarks
Closing Remarks
4:00 PM - 4:10 PM
Room: PI/1-100 - Theatre