Celestial Holography Summer School 2024​

The Celestial Holography Summer School 2024 was held at the Perimeter Institute for Theoretical Physics, July 22-26th. The focus of the school was to provide pedagogical introductions to the broad array of topics under active investigation under the purview of celestial and flat space holography. In addition to 15 lecturers and PIs, the school hosted 36 Masters/PhD students, 32 postdocs and PI residents, and 178 virtual participants.

In addition to the main lecture series, there were TA sessions held every day where students could work through lecturer-assigned exercises, supervised and assisted by expert postdoctoral researchers in the field of celestial holography, including Collaboration-affiliated postdocs. There was also a poster session, and a series of 7 lightning talks, to give junior members of the collaboration a chance to present their work. To bolster collaborative investigations, and promote the growth of Collaboration endeavors, the school concluded with a panel discussion featuring the Collaboration PIs, who shared future visions, exciting prospects, and pressing confusions and challenges, in an informal discussion with the school’s students.

Lecture series were split into two hour-long sessions each. Ana Maria Raclariu (King’s College) introduced celestial holography from a bulk perspective, culminating in the construction of asymptotic charges at null infinity, while Andrea Puhm (Amsterdam) explored a boundary CFT point of view, and elucidated how conformal representation theory provides an organizing principle on universal boundary degrees of freedom. Prahar Mitra (Amsterdam) introduced the covariant phase space formalism, and applied it to a first principles construction of the BMS symmetry in all dimensions. Sebastian Mizera (IAS) explored the physical imprints of Shapiro time delay in a scattering amplitudes calculation, emphasizing a quantum mechanical toy model, while Hofie Hannesdottir (IAS) gave an in-depth survey of various low-energy properties of the S-matrix, including proposals for how to correct and understand its definition when scattering states are subject to long-range potentials (i.e. are not asymptotically free). Romain Ruzziconi (Oxford) introduced Carrollian physics as an ultralocal limit of various conventional QFTs, and its application to a putative 3d Carrollian CFT dual at null infinity. David Skinner (Cambridge) provided a rapid survey of the geometry of twistor theory, culminating in discussions of holomorphic QFTs on twistor space and their connection with celestial physics; Davide Gaiotto (PI) explained how 2d holomorphic QFTs can be studied in the framework of twisted holography more generally, explained the large-N combinatoric observables they compute in the context of the AdS/CFT correspondence, and described various generalizations.

Finally, the school featured several research-level survey and vision talks, aimed towards providing a review of past work, and a jumping off point to important open research directions in celestial holography: Lionel Mason (Oxford) explained how celestial symmetry algebras may be deformed to incorporate nonzero cosmological constants, with a broad hope of realizing these symmetry algebras on the complete S-matrix of gravitational and gauge theories as an expansion around the self-dual sector, which enjoys these celestial symmetries. Rob Myers (PI) provided an overview of holographic entanglement entropy in AdS/CFT and its successes, and then described nascent but intriguing proposals which generalize this concept to flat spacetimes. Andrew Strominger (Harvard) provided a broad survey of the bottom-up approach to the celestial holography program and pointed to a host of open questions and puzzles, including the unusual distributional nature of low-point celestial CFT correlators and how they might be ameliorated by his leaf amplitude formulation, the question of how to constrain 4-pt scattering in the gravitational S-matrix given the underspecification of the Wilsonian approach, and connections to observing new memory effects at upcoming LIGO/LISA experiments.