91TV

The path to quantum gravity with causal sets

07 - 08 September 2026 09:00 - 17:00 Holiday Inn, Manchester City Centre Free

Theo Murphy meeting organised by Dr Yasaman Yazdi and Dr Stav Zalel

This meeting will take a critical look at the past, present, and future of causal set theory – an approach to quantum gravity where spacetime is discrete. Progress in kinematics, phenomenology and dynamics will be discussed by researchers both in and outside the field, with emphasis on how these developments are transforming our understanding of the early universe and quantum fields programme.

The programme, including speaker biographies and abstracts, is available at the bottom of this page. Please note the programme may be subject to change.

Attending the event

This event is intended for researchers in relevant fields.

  • Free to attend and in-person only
  • When requesting an invitation, please briefly state your expertise and reasons for attending
  • Requests are reviewed by the meeting organisers on a rolling basis. You will receive a link to register if your request has been successful
  • Catering options will be available to purchase upon registering. Participants are responsible for booking their own accommodation
  • Please do not book accommodation until you have been invited to attend the meeting by the meeting organisers

Please note that scientific meetings hosted by the Royal Society do not necessarily represent a Royal Society position or signify an endorsement of the speakers or content presented.

Enquiries: Scientific Programmes team

Image credit © iStock.com / ktsimage

Organisers

  • Dr Stav Zalel

    Dr Stav Zalel

    Stav Zalel is a lecturer at Homerton College, University of Cambridge, and a member of the GR group at DAMTP. She obtained her PhD from Imperial College London in 2021. She was a member of the Theoretical Physics Group at Imperial College until moving to Cambridge in 2024. Her expertise is in Causal Set Theory and her research pioneers the use of combinatorics to connect physical phenomena on the largest cosmological scales to the nature of gravity on the smallest scales.

  • Dr Yasaman Yazdi

    Dr Yasaman Yazdi

    Yasaman Yazdi is a Research Ireland Pathway awardee and postdoctoral fellow at the Dublin Institute for Advanced Studies. She obtained her PhD from the Perimeter Institute for Theoretical Physics and the University of Waterloo in 2017. She has also held an Avadh Bhatia Fellowship at the University of Alberta, an Imperial College Research Fellowship, a Leverhulme Trust grant and an Emmy Noether Fellowship at the Perimeter Institute. Her research is on the fundamental nature of quantum gravity, as well as its applications. She works on the causal set theory approach to quantum gravity, and in particular, topics including quantum field theory, entanglement entropy, the Everpresent Λ dark energy model, and Lorentzian spectral geometry.

Schedule

Chair

Dr Yasaman Yazdi

Dr Yasaman Yazdi

Dublin Institute for Advanced Studies, Ireland

09:00-09:05 Welcome by the lead organiser
Dr Yasaman Yazdi

Dr Yasaman Yazdi

Dublin Institute for Advanced Studies, Ireland

09:05-09:35 Causal sets and continuum geometry: where are we?

This talk will present an overview of what is known about the relationship between causal sets and the continuum Lorentzian geometry model for spacetime. After a brief review of key elements in the structure of causal sets, the topics to be discussed address a sequence of questions with which we seek to understand the kinemtics of the theory, by asking: what it means for a causal set to be manifoldlike or associated with a Lorentzian manifold at large scales; how manifoldlike causal sets can be recognised; for those that are manifoldlike, the extent to which the associated continuum geometries are unique; and how we can estimate quantities characterising those geometries—such as dimensionality, timelike and spatial distances, and curvature. The talk will not address directly questions about how manifoldlike causal sets may arise dynamically or how matter may be included, but some of the topics covered are used in approaches to those aspects of the theory. In particular, considerations on the dynamics may inform our choice of important open questions on the kinematics and what we can consider to be a good definition of manifoldlike causal set.

Dr Luca Bombelli

Dr Luca Bombelli

University of Mississippi, US

09:35-10:05 Entanglement entropy in spacetime, and a signature of causal set discreteness

Entanglement entropy is one of the key candidates for the Bekenstein-Hawking entropy of black holes, although it is well known to be divergent without regularisation. In nature, this regularisation likely originates from a more fundamental theory, a theory of quantum gravity. Such a theory ought to be covariant, and as such, one requires a means of performing the calculation of entanglement entropy in spacetime, so as to properly incorporate the effects of quantum gravity. I will give such a spacetime formulation of entanglement entropy for quasifree fields, and show an example calculation on a causal set background. It will be seen that the resultant entanglement entropy carries within it a signature of the causal set discreteness. This talk will be based on: .

Mr Joshua Jones

Mr Joshua Jones

Dublin Institute for Advanced Studies, Ireland

10:05-10:35 Interacting QFT on causal sets

Causal set theory is an approach to quantum gravity in which spacetime is fundamentally discrete at the Planck scale and takes the form of a irregular Lorentzian lattice, or "causal set", from which continuum spacetime emerges in a large-scale (low-energy) approximation. Within this setting, we develop a quantum field theory formalism and derive a manifestly causal diagrammatic expansion for in-in correlators in local scalar field theories with finite polynomial interactions. The resulting expansion terminates at finite order in the interaction coupling, providing insight into how the underlying discreteness scale plays the role of an effective cut-off. In particular, we illustrate how this discreteness length can regularize expressions that diverge in the continuum limit.

Dr Emma Albertini

Dr Emma Albertini

International School for Advanced Studies, Italy

10:35-11:05 Coffee break
11:05-12:30 Panel discussion
Professor Marián Boguñá

Professor Marián Boguñá

Universitat de Barcelona, Spain

Dr Ren Yeats

Dr Ren Yeats

University of Waterloo, Canada

Dr Steven Johnston

Dr Steven Johnston

Independent researcher, US

Chair

Dr Yasaman Yazdi

Dr Yasaman Yazdi

Dublin Institute for Advanced Studies, Ireland

14:00-14:30 Talk title TBC
14:30-15:00 Bounds on swerves from dark matter and gravitational waves

Motivated by causal set quantum gravity, we formulate the covariant Brownian motion of massive and massless particles described by a stochastic geodesic equation. At the level of the Fokker-Planck equation, this approach provides the unique covariant diffusion equation in the absence of a preferred frame, which generalizes the earlier work by Dowker et al. to arbitrary curved spacetimes.

When applied to dark matter particles, it results in dynamical warming at late times, suppressing the matter power spectrum at small scales. Thus, we show that the model has potential for alleviating the S_8 tension.

When applied to gravitons, the model predicts spreading and drifting of the gravitational-wave power spectrum. Thus, LISA will be able to place bounds on the massless diffusion constants, assuming it detects a peaked spectrum of primordial gravitational waves. We show that a direct measurement and characterization of a gravitational wave (GW) background frequency spectrum can improve bounds on the diffusion and drift parameters by over 12 orders of magnitude compared to those from the CMB.

Dr Arad Nasiri

Dr Arad Nasiri

University of New Brunswick, Canada

15:00-15:30 Coffee break
15:30-17:00 Panel discussion
Dr Nosiphiwo Zwane

Dr Nosiphiwo Zwane

University of Swaziland, Eswatini

Dr Astrid Eichhorn

Dr Astrid Eichhorn

University of Heidelberg, Germany

Professor Dima Krioukov

Professor Dima Krioukov

Northeastern University, US

Chair

Dr Stav Zalel

Dr Stav Zalel

University of Cambridge, UK

09:00-09:30 Talk title TBC
09:30-10:00 Dynamics of discrete spacetimes with quantum‑enhanced Markov chain Monte Carlo

Quantum algorithms offer the potential for significant computational advantages; however, in many cases, it remains unclear how these advantages can be practically realized. Causal Set Theory is a discrete, Lorentz-invariant approach to quantum gravity which may be well positioned to benefit from quantum computing. In this work, we introduce a quantum algorithm that investigates the dynamics of causal sets by sampling the space of causal sets, improving on classical methods. Our approach builds on the quantum-enhanced Markov chain Monte Carlo technique developed by Layden et al. [Nature 619, 282 (2023)], adapting it to sample from the constrained spaces required for application. This is done by adding a constraint term to the Hamiltonian of the system. A qubit Hamiltonian representing the Benincasa-Dowker action (the causal set equivalent of the Einstein-Hilbert action) is also derived and used in the algorithm as the problem Hamiltonian. We achieve a super-quadratic quantum scaling advantage and, under some conditions, demonstrate a greater potential compared to classical approaches than previously observed in unconstrained QeMCMC implementations.

Dr Petros Wallden

Dr Petros Wallden

University of Edinburgh, UK

10:00-10:30 Causal sets and an emerging continuum

We understand reasonably well how to construct discrete causal sets that approximate a given spacetime manifold. The vast majority of causal sets, though, are not at all continuum-like, and if we take the discrete description to be fundamental, we must somehow suppress these "bad" sets. I will discuss some progress in showing that a very large class of non-manifoldlike sets is extremely strongly suppressed in the ordinary gravitational path integral. This is a first step, but it is by no means complete; I will end with suggestions for possible ways forward.

Professor Steve Carlip

Professor Steve Carlip

University of California, Davis, US

10:30-11:00 Coffee break
11:00-12:30 Panel discussion
Professor Denjoe O'Connor

Professor Denjoe O'Connor

Dublin Institute for Advanced Studies, Ireland

Dr David Rideout

Dr David Rideout

UC San Diego, US

Professor Neil Turok

Professor Neil Turok

University of Edinburgh, UK

Chair

Dr Stav Zalel

Dr Stav Zalel

University of Cambridge, UK

14:00-14:30 Causal brightness

Causal sets approximated by infinite volume Lorentzian manifolds, where we would resort to a Poisson process to obtain a sampling of points such that the expected number in any region is proportional to the volume of that region, must be countably infinite. In 1720, Edmund Halley reported to the Royal Society, “Another Argument I have heard urged, that if the number of Fixt Stars were more than finite, the whole superficies of their apparent Sphere would be luminous”, an apparently paradoxical consequence of the infinitude of fixed stars at varying distances postulated by Thomas Digges in 1576. In this talk I will describe some perhaps equally surprising consequences of the corresponding infinity in causal sets.

Professor David Meyer

Professor David Meyer

UC San Diego, US

14:30-15:00 From a spacetime to sprinkled causal sets, and back?

In causal set theory, we understand Lorentzian manifolds to be an effective description arising from (a collection of) causal sets as the underlying discrete structure. The fundamental conjecture ("Hauptvermutung") regards the correspondence of spacetimes to these discrete structures to be in some sense unique (up to scales below a given density). When such a statement holds, we may "reconstruct" a spacetime with all of its geometric properties from certain causal sets. In causal set investigations (for example, if we want to test such a reconstruction), we usually take the reverse route, starting with a given spacetime manifold and generated causal set models via a Poisson process (sprinkling).

I review the sprinkling process and use it to compute probabilities of certain causal set properties that may be taken as selection criteria for "manifold-like" causal sets. One of these properties is the number of layers, which is typically very small in generic finite partial orders but larger in sprinkled causal sets. Other properties are local symmetries, which I have shown to be absent in infinite causal sets, while finite causal sets (that arise in most numerical investigations) can admit local symmetries. Furthermore, I want to discuss the role of the sprinkling measure in the fundamental conjecture and the spacetime reconstruction.

Dr Christoph Minz

Dr Christoph Minz

Leibniz University Hannover, Germany

15:00-15:30 Coffee break
15:30-17:00 Panel discussion
Professor Fay Dowker

Professor Fay Dowker

Imperial College London, UK

Professor Sumati Surya

Professor Sumati Surya

Raman Research Institute, India