Steve Hsu (Michigan State): Black Hole Information, Replica Wormholes, and Macroscopic Superposition States | 2/07/2025
We discuss recent applications of Euclidean path integrals to the black hole information problem. In calculations with replica wormholes as the next-to-leading order correction to the Gibbons-Hawking saddlepoint, the radiation density matrix approaches a pure state at late times, following the Page curve. We compare unitary evaporation of black holes (in real time), mediated by calculable quantum hair effects, with the replica wormhole results. Both replica wormhole and quantum hair approaches imply that radiation states are macroscopic superpositions of spacetime backgrounds, invalidating firewall and monogamy of entanglement constructions.
Martin Kruczenski (Purdue): The Gauge Theory Bootstrap: Computing Pion amplitudes and low energy parameters from QCD | 2/21/2025
Under the assumption of confinement and chiral symmetry breaking the low energy description of QCD is in terms of an effective theory of pions whose properties (couplings) should be derived from the high energy Lagrangian (QCD). In this talk I will describe recent work in collaboration with Yifei He where we propose a bootstrap method to compute pion scattering phase shifts and low energy effective action coefficients. The method looks for the most general S-matrix that matches at low energy the tree level amplitudes of the non-linear sigma model and at high energy, QCD sum rules and form factors. This is a theoretical/numerical calculation that uses as only data the pion mass, pion decay constant fpi and the QCD parameters Nc=3, Nf=2, mq and alpha_s. The results are in reasonable agreement with experiment. In particular, we find the rho(770), f2(1270) and rho(1450) resonances. The interplay between the UV gauge theory and low energy pion physics is an example of a general situation where we know the microscopic theory as well as the effective theory of long wavelength fluctuations but we want to solve the strongly coupled dynamics at intermediate energies. The bootstrap builds a bridge between the low and high energy by determining the consistent S-matrix that matches both and provides, in this case, a new direction to understand the strongly coupled physics of gauge theories.
Daniel Harlow (MIT): Quantum mechanics and observers for gravity in a closed universe | 3/28/2025
Recently there have been several arguments given for the seemingly absurd statement that the Hilbert space of quantum gravity in a closed universe is one-dimensional. How can this be consistent with the richness of our daily experience? In this talk I will review the arguments for a one-dimensional Hilbert space, and then present a possible resolution based on the idea that to do quantum mechanics in a closed system it is necessary to explicitly include a classical observer in the system.
Sabrina Pasterski (Perimeter): Lifting Swing Surfaces to AdS | 4/04/2025
The entanglement entropy for regions in a BMS field theory living at null infinity has been proposed to be holographically dual to certain ‘swing surfaces’ in flat space. We lift this construction to AdS/CFT and revisit both bulk and boundary aspects of this proposal.
Vladimir Rosenhaus (CUNY Graduate Center): Renormalization Group in far-from-equilibrium states | 4/11/2025
We study renormalization group flows in far-from-equilibrium states. The study is made tractable by focusing on states that are spatially homogeneous, time-independent, and scale invariant. Such states, in which mode k has occupation number $n_k = k^{-\gamma}$, are well-known in nonlinear physics (going under the name of wave turbulence). RG flow in such states is qualitatively different than in the vacuum -- a positive $\gamma$ decreases the dimension of an operator, turning marginal interactions into relevant interactions. We compute one loop beta functions. Depending on the sign of the beta function, backreaction may either cause a minor shift of the state in the IR, or completely change the nature of the state. Focusing on nearly marginal interactions, we construct an analog of the epsilon expansion and IR fixed points, with epsilon now set by the scaling of the interaction rather than the spacetime dimension. In the language of RG flow, critical balance scaling -- having applications in fields as varied as astrophysics and ocean waves -- corresponds to the state dynamically adjusting itself along the RG flow until the interaction becomes marginal.
Henry Lin (Stanford): Giant gravitons in Dp-brane holography | 4/18/2025
We consider half BPS operators in maximally supersymmetric Yang Mills (SYM) in p+1 dimensions. These operators satisfy trace relations that are identical to those discussed in the p=3 case (N = 4 SYM). Nevertheless, the bulk explanation of these trace relations must differ from the p = 3 case as their holographic duals are not AdS spacetimes. We identify giant graviton solutions in the dual holographic backgrounds for -1 <= p <= 4. In the ’t Hooft limit, these giants are D(6-p) branes that wrap the internal sphere. We also follow the giants into the strong coupling region where they become other branes. Despite propagating in a non-AdS geometry, we find that the branes “feel” like they are in AdS. This is closely related to the emergent scaling symmetry present in these boundary theories.
