The path to quantum gravity with causal sets
Also in “ Scientific meeting”
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.
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Organisers
Schedule
Chair
Dr Yasaman Yazdi
Dublin Institute for Advanced Studies, Ireland
Dr Yasaman Yazdi
Dublin Institute for Advanced Studies, Ireland
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.
| 09:00-09:05 |
Welcome by the lead organiser
Dr Yasaman YazdiDublin Institute for Advanced Studies, Ireland
Dr Yasaman YazdiDublin Institute for Advanced Studies, Ireland 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. |
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| 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 BombelliUniversity of Mississippi, US
Dr Luca BombelliUniversity of Mississippi, US Dr Luca Bombelli is a theoretical physicist specialising in gravitational theory and mathematical physics. He earned his PhD from Syracuse University and held various temporary appointments including postdoctoral research positions at the University of Vienna, the University of Calgary and the Université Libre de Bruxelles before joining the faculty at the University of Mississippi, where he is currently Professor of Physics and Astronomy. Dr Bombelli has worked on the physics of black holes, particle and wave propagation in curved spacetimes including black holes and expanding universe models, and the canonical formulation of general relativity and its quantization in the loop quantum gravity approach. His current research centres on the causal set approach to spacetime structure and quantum gravity. |
| 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 JonesDublin Institute for Advanced Studies, Ireland
Mr Joshua JonesDublin Institute for Advanced Studies, Ireland Joshua Jones is a 3rd year PhD student at the Dublin Institute for Advanced Studies, supervised by Yasaman Yazdi. He did his undergraduate studies at Imperial College London, and a masters at the University of Cambridge. His interests are largely focused on entropy, black hole thermodynamics, and causal set theory. In particular, he studies entanglement entropy in the causal set as a potential source of the Bekenstein-Hawking entropy, championing a spacetime approach to the calculation such that one can regularise covariantly. |
| 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 AlbertiniInternational School for Advanced Studies, Italy
Dr Emma AlbertiniInternational School for Advanced Studies, Italy I am a postdoctoral researcher at the International School for Advanced Studies in Trieste, Italy. I began working on causal set theory during my master’s thesis and continued to develop this line of research throughout my PhD at Imperial College London, with a particular focus on quantum field theory on causal sets. My current research interests also include the effective field theory of gravity and environmental effects in compact binaries, exploring how surrounding media can influence gravitational dynamics and observables. |
| 10:35-11:05 |
Coffee break
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| 11:05-12:30 |
Panel discussion
Professor Marián BoguñáUniversitat de Barcelona, Spain
Professor Marián BoguñáUniversitat de Barcelona, Spain Marián Boguñá (Barcelona, 1967) is Full Professor at the Department of Condensed Matter Physics of the University of Barcelona. He earned his degree in Physics in 1994 and completed his PhD in 1998. After a postdoctoral appointment with Professor George H Weiss at the National Institutes of Health in Washington DC, he returned to Barcelona, where he was awarded a Ramón y Cajal fellowship in 2003 and obtained tenure in 2008. He has also been a visiting scientist at Indiana University. He has published in major peer-reviewed international scientific journals, including Nature, Nature Physics, Nature Reviews Physics, Nature Communications, Proceedings of the National Academy of Sciences of the United States of America, Physical Review Letters, and Physical Review X. He has co-chaired international conferences, served on program committees, received the Outstanding Referee Award of the American Physical Society, and was appointed ICREA Academia Researcher in 2010, 2015, 2020, and 2025. His research focuses on complex systems, especially the interplay between topology and function in networked systems such as social, biological, and communication networks. He is a leading contributor to the field of Network Geometry, which seeks to uncover the geometric principles underlying the discrete structures of complex systems.
Dr Ren YeatsUniversity of Waterloo, Canada
Dr Ren YeatsUniversity of Waterloo, Canada Ren Yeats works on combinatorial problems in fundamental physics including quantum field theory and causal set theory. They hold the Canada Research Chair in Combinatorics in Quantum Field theory in the Department of Combinatorics and Optimization at the University of Waterloo. They got their PhD from Boston University in 2008 under the supervision of Dirk Kreimer, then held a faculty position in the Department of Mathematics at Simon Fraser University before joining the University of Waterloo in 2016.
Dr Steven JohnstonIndependent researcher, US
Dr Steven JohnstonIndependent researcher, US Steven Johnston completed his PhD in 2010 at Imperial College London, with a thesis entitled "Quantum Fields on Causal Sets". His research focused on defining scalar field propagators on discrete causal set spacetime. He moved into industry after 2010 but has continued to publish causal set research, more recently focusing on general methods to embed causal sets into Minkowski spacetime. His current interests are on developing models for higher-spin particles on a causal set. |
Chair
Dr Yasaman Yazdi
Dublin Institute for Advanced Studies, Ireland
Dr Yasaman Yazdi
Dublin Institute for Advanced Studies, Ireland
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.
| 14:00-14:30 |
Talk title TBC
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| 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 NasiriUniversity of New Brunswick, Canada
Dr Arad NasiriUniversity of New Brunswick, Canada Arad Nasiri is a postdoctoral fellow at the University of New Brunswick where his research lies at the intersection of quantum gravity and cosmology. He completed his PhD at Imperial College London under the supervision of Professor Fay Dowker. His thesis, titled Stochastic Dark Sector, explored the interplay between ideas from quantum gravity and stochastic behaviour in dark matter and dark energy, as well as the phenomenological implications of causal sets in cosmology. As a postdoctoral fellow, his current interests span a range of topics, including the time-depended functional renormalisation group in the early universe, the quantum-to-classical transition via decoherence in cosmology, and the application of generative diffusion models in downscaling and weather forecasting in Atlantic Canada. |
| 15:00-15:30 |
Coffee break
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| 15:30-17:00 |
Panel discussion
Dr Nosiphiwo ZwaneUniversity of Swaziland, Eswatini
Dr Nosiphiwo ZwaneUniversity of Swaziland, Eswatini Nosiphiwo Zwane is a lecturer at the University of Eswatini and a founding member of the Eswatini Young Academy. With over eight years of academic experience, she bridges fundamental physics and complex systems. She is also coordinator for the University Physics Society, she is dedicated to fostering STEM engagement and mentoring the next generation of African physicists. Her research interest focuses on Causal Set Theory, a discrete approach to quantum gravity that replaces the space-time continuum with a partially ordered set of elementary events. She is interested in investigation involves identifying observational signatures of quantum gravity, such as the potential "swerving" or diffusion of high-energy cosmic rays as they travel through a discrete vacuum. She also interested in cosmological tests for alternative dark energy models, specifically the Ever-present Lambda (Λ), where the cosmological constant fluctuates based on the volume of the past.
Dr Astrid EichhornUniversity of Heidelberg, Germany
Dr Astrid EichhornUniversity of Heidelberg, Germany Astrid Eichhorn is a junior research group leader at the University of Heidelberg. She received her PhD from the University of Jena in Germany, in 2011. After that she became a postdoctoral researcher at the Perimeter Institute for Theoretical Physics in Canada, where she was promoted to senior postdoctoral researcher in 2014. She then joined the Theory Group at Imperial College as a junior research fellow, before moving to Heidelberg in 2016. Her research concentrates on physics beyond the Standard Model, with a particular focus on quantum gravity and its effects on matter.
Professor Dima KrioukovNortheastern University, US
Professor Dima KrioukovNortheastern University, US Professor Dima Krioukov graduated from Saint Petersburg State University with Diploma in Physics in 1993. In 1998 he received his PhD in Physics from Old Dominion University, and moved to the networking industry as a network architect with Dimension Enterprizes. Upon their acquisition by Nortel Networks in 2000, he accepted a research scientist position at Nortel. In 2004 he moved back to academia as a Senior Research Scientist at the Cooperative Association for Internet Data Analysis (CAIDA) at the University of California, San Diego (UCSD). In 2014 he moved to Northeastern University as an Associate Professor (Professor since 2024) at the Departments of Physics, Mathematics, and Electrical & Computer Engineering, and a core member of the , where he is the Director of the . DK-Lab research deals mostly with theory and fundamental aspects of network science. Research topics of particular interest to the lab are latent network geometry, maximum-entropy ensembles of random graphs and higher-order structures, random geometric graphs, causal sets, navigation in networks, and fundamental aspects of network dynamics. |
Chair
Dr Stav Zalel
University of Cambridge, UK
Dr Stav Zalel
University of Cambridge, UK
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.
| 09:00-09:30 |
Talk title TBC
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| 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 WalldenUniversity of Edinburgh, UK
Dr Petros WalldenUniversity of Edinburgh, UK Petros Wallden is Reader (Associate Professor) in the School of Informatics at the University of Edinburgh, and is the Deputy Director of the Quantum Software Lab (collaboration of Edinburgh with the UK National Quantum Computing Centre). His research focuses on quantum algorithms (theory and applications), quantum cryptography, and the foundations of quantum informatics. He was General Chair for the International Conference in Public Key Cryptography twice, and in the Editorial Board for the journals: Quantum, Cryptography and Theoretical Computer Science. |
| 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 CarlipUniversity of California, Davis, US
Professor Steve CarlipUniversity of California, Davis, US Steven Carlip received an undergraduate degree in physics from Harvard in 1975. After seven years as a printer, factory worker, and organiser, he returned to graduate school at the University of Texas, where he received his PhD under the supervision of Bryce DeWitt. Following a stint as a postdoc at the Institute for Advanced Study, he joined the faculty at the University of California at Davis in 1990, where he has just retired with the title of Distinguished Professor. His research covers a broad array of topics in quantum gravity, from lower dimensional models to black hole thermodynamics, across research programs ranging from string theory to path integrals to causal set theory; in the debate among approaches to quantum gravity, he considers himself, for now at least, "nonaligned." |
| 10:30-11:00 |
Coffee break
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| 11:00-12:30 |
Panel discussion
Professor Denjoe O'ConnorDublin Institute for Advanced Studies, Ireland
Professor Denjoe O'ConnorDublin Institute for Advanced Studies, Ireland
Dr David RideoutUC San Diego, US
Dr David RideoutUC San Diego, US David Rideout received his PhD in Physics at Syracuse University, studying under Rafael Sorkin. Together they formulated a discrete dynamical law for causal sets, based on sequential growth of the causal set. He went on to do a postdoc in the Astrophysical Relativity division at the Max Planck Institute for Gravitational Physics, developing the Cactus High Performance Computing Framework for numerical relativity. Since then he has taken positions at Hamilton College, Imperial College London, the Perimeter Institute, and University of California San Diego, bringing his expertise in High Performance Computing and the Cactus Framework to support large scale simulations in quantum gravity. Most recently he has been thinking about connections of his research to data science, to extend this understanding to serve humanity in a broader context. Professor Neil TurokUniversity of Edinburgh, UK Professor Neil TurokUniversity of Edinburgh, UK |
Chair
Dr Stav Zalel
University of Cambridge, UK
Dr Stav Zalel
University of Cambridge, UK
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.
| 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 MeyerUC San Diego, US
Professor David MeyerUC San Diego, US David Meyer wrote his PhD thesis on the dimension of causal sets under the guidance of Rafael Sorkin. He is currently professor of mathematics at the University of California, San Diego. After working on topics in quantum computation, including quantum walks, quantum error correcting codes, measures of entanglement, and quantum algorithms, for many years, he has begun thinking about causal set theory again. In the past couple of years he has studied dimensional reduction (or not) in causal sets, reconstruction of Minkowski spaces from timelike separations, and positive semidefiniteness of decoherence functionals on causal sets. He also taught a graduate course on causal set theory earlier this year, to a class of mathematicians, physicists, and philosophers. |
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| 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 MinzLeibniz University Hannover, Germany
Dr Christoph MinzLeibniz University Hannover, Germany Christoph Minz studied Physics at the Technical Universities of Ilmenau and Berlin, Germany, before he began his doctoral studies in Mathematical Physics at the University of York, UK. In his doctoral research, he worked on mathematical aspects of quantum field theory on discrete spacetime models (causal sets). In his post-doctoral projects at Leipzig University (Germany), SISSA (Italy) and currently at Leibniz University Hannover (Germany), he first used new numerical methods and then also analytic techniques to investigate modular Hamiltonians, which are objects that formalize entanglement measures in QFT. In side-projects, he continues research in topics of causal set theory and developed some tools for numerical investigations and visual representations of causal sets. |
| 15:00-15:30 |
Coffee break
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| 15:30-17:00 |
Panel discussion
Professor Fay DowkerImperial College London, UK
Professor Fay DowkerImperial College London, UK A Professor of Theoretical Physics at Imperial College London, specialising in quantum gravity and causal set theory. She did her PhD in at University of Cambridge. Before joining Imperial College, she was a lecturer at Queen Mary University of London. She completed her postdoctoral research at Fermilab, at the University of California, Santa Barbara, and the California Institute of Technology. She works on Quantum Gravity and Foundations of Quantum Theory.
Professor Sumati SuryaRaman Research Institute, India
Professor Sumati SuryaRaman Research Institute, India Professor Sumati Surya works at the Raman Research Institute, Bengaluru, India. Her primary focus of research is quantum gravity, with an allied interest in Lorentzian geometry. She obtained her PhD from Syracuse University, USA with Professor Rafael Sorkin. She held postdoctoral positions in IUCAA, Pune, India, TIFR, Mumbai, India, as well as at the University of British Columbia, Canada and the University of Alberta, Canada, during which time she worked on various aspects of Lorentzian geometry and quantum gravity. In the last couple of decades her work has been focused on the causal set approach to quantum gravity, which posits a fundamental spacetime discreteness. She has recently authored a Springer Lecture notes book on the causal set approach to quantum gravity. |