Abstracts
Lydia Bieri
Title: Asymptotics of Radiative Spacetimes
Abstract: In this talk, we shall discuss solutions of the Einstein equations for various physical scenarios. In particular, we will highlight structures at null infinity of asymptotically-flat spacetimes and extract information about gravitational radiation and memory. Questions around peeling, as well as antipodal symmetry and non-symmetry problems will be addressed along the way.
Silvia Nagy
Title: Sub^n-leading asymptotics in YM and gravity
Abstract: It is by now well understood how leading soft theorems follow as Ward identities of asymptotic symmetries defined at null infinity. For subleading infrared effects, the connection is more subtle, but it turns out that this can be formalised, to all orders in the energy expansion, by adapting the Stuckelberg procedure to construct an extended radiative phase space at null infinity. I will exemplify this with Yang-Mills theory and some work in progress in gravity, showing the construction of the extended phase space, as well as the charges corresponding to the subleading soft theorems at all orders. These turn out to satisfy simple recursion relations and organise themselves into infinite dimensional algebras. Time permitting, I will also review homotopy algebras, and show how they can, in the context of flat space holography, precisely encode the projection of fields to null infinity.
Evgeny Skvortsov
Title: M-theory of all self-dual ones: AdS/CFT, stringy and celestial holography
Abstract: I will give a brief overview of the quest to construct quantum gravity models via higher-spin gravities. The most promising candidate—chiral higher-spin gravity—can be understood as an 'M-theory' for all self-dual theories. In particular, it is expected to admit a natural formulation in twistor space, as is already the case for some of its truncations. Chiral higher-spin gravity is related via the AdS/CFT correspondence to (Chern-Simons) vector models and can explain the remarkable 3D bosonization duality. It also implies the existence of new 'self-dual' conformal field theories on the boundary. Along similar lines, there is a potential connection to tensionless strings in AdS(4) and the ABJ theory. Finally, I will cover recent developments in celestial holography that are connected to chiral higher-spin gravity.
Robert Wald
Title: Memory, Infrared Entanglement, and the Idealization of Scattering from Infinity
Abstract: The memory effect refers to the fact that in four dimensional asymptotically flat spacetimes, at order 1/r a massless field generically will not return to the same value at late retarded times as it had at early retarded times. In electromagnetism and gravity, when memory is present, the late retarded time field will differ from the early retarded time field by an asymptotic symmetry. There is a direct relationship between memory and the charges that generate the asymptotic symmetries. These charges must commute with any gauge invariant local observables in the bulk spacetime, thereby effectively decohering bulk states into superselection sectors of eigenstates of the large gauge charges. It can thereby be seen that in QED, states corresponding to "incoming bare electrons" from infinity (i.e., electron states with no incoming electromagnetic radiation) do not correspond to physical states in the bulk. The physical bulk states correspond at infinitely early and late times to Faddeev-Kulish states, in which the electrons are infinitely entangled with soft photons so as to produce eigenstates of the large gauge charges. However, for a physical bulk state, this entanglement will occur only logarithmically in time and should be completely negligible in the finite time required to do any realistic experiment in the bulk. In QED with massless charged particles, the Faddeev-Kulish construction yields singular states, and it does not appear that quantum scattering theory from infinity makes sense at all (even though classical scattering theory is well defined). In gravity, there are no eigenstates of the large gauge charges (apart from the vacuum), but there also are no gauge invariant local bulk observables, so it is not obvious what criteria should be imposed on scattering states so that they correspond to physically relevant bulk states. In all cases, the behavior of states at asymptotic infinity is very different from the behavior of states at the large but finite times relevant to experiments in the bulk spacetime. Although the idealization of scattering theory from infinity can be very usefully applied to various “practical calculations” (such as obtaining inclusive cross-sections), this work highlights that there are serious difficulties in elevating scattering from infinity to a fundamental status in the formulation of a theory.