Quantum Gravity

I'll discuss information retrieval from evaporating black holes, assuming that the internal dynamics of a black hole is unitary and rapidly mixing, and assuming that the retriever has unlimited control over the emitted Hawking radiation. If the evaporation of the black hole has already proceeded past the 'half-way' point, where half of the initial entropy has been radiated away, then additional quantum information deposited in the black hole is revealed in the Hawking radiation very rapidly. Information deposited prior to the half-way point remains concealed until the half-way point, and then emerges quickly. These conclusions hold because typical local quantum circuits are efficient encoders for quantum error-correcting codes that nearly achieve the capacity of the quantum erasure channel. The resulting estimate of a black hole's information retention time, based on speculative dynamical assumptions, is just barely compatible with the black hole complementary hypothesis. (Joint work with John Preskill).

We present new results from our Monte Carlo simulation of SUSY matrix quantum mechanics with 16 supercharges at finite temperature. The internal energy can be fitted nicely to the behavior predicted from the dual black hole thermodynamics including the alpha' corrections. The temporal Wilson loop can also be predicted from the gravity side, and it is directly related to the Schwarzschild radius of the dual black hole geometry. Our results for the Wilson loop indeed confirm this prediction up to subleading terms anticipated from the alpha' corrections on the gravity side. All these results give us strong support and a firm basis for the idea to use matrix model simulations to study quantum gravity.

I'll discuss some large N quantum mechanical theories that are toy models for eternal black holes in AdS via gauge/gravity duality. They can be used to study the classical limit and quantum corrections in gravity, and their roles in the information paradox. We demonstrate that such large N models can exhibit late time fall-off of a two-point function. By computing higher genus corrections explicitly, we argue that the fall-off, and thus information loss, persist even after perturbative gravity corrections are included.

I will classify the options to solve the black hole information loss problem by how radical a departure from semi-classical gravity they require outside the quantum gravitational regime. I will argue that the most plausible and conservative conclusion is that the problem of information loss originates in the presence of the singularity and that thus effort should be focused on understanding its avoidance. A consequence of accepting the accuracy of the semi-classical approximation is the surface interpretation of black hole entropy. I will summarize the arguments that have been raised for and against such scenarios. Reference: http://arxiv.org/abs/0901.3156

String theory gives a consistent theory of quantum gravity, so we can ask about the nature of black hole microstates in this theory. Studies of extremal and near-extremal microstates indicate that these microstates do not have a traditional horizon, which would have no data about the microstate in its vicinity. Instead, the information of the microstate is distributed throughout a horizon sized quantum `fuzzball'. If this picture holds for all microstates then it would resolve the information paradox. We review recent progress in the area, including some results on non-extremal states. We also discuss some conjectures about black hole dynamics suggested by the structure of fuzzballs.

Theories which have fundamental information destruction or decoherence are motivated by the black hole information paradox. However they have either violated conservation laws, or are highly non-local. Here, we show that the tension between conservation laws and locality can be circumvented by constructing a relational theory of information destruction. In terms of conservation laws, we derive a generalization of Noether's theorem for general theories, and show that symmetries imply a restriction on the type of evolution permissible. With respect to locality, we distinguish violations of causality from the creation or destruction of separated correlations. We show that violations of causality need not occur in a relational framework -- the only non-locality is that correlations decay faster than one might otherwise expect or can be created over spatial distances. The theories can be made time-symmetric, thus imposing no arrow of time.

Because the gravitational Hamiltonian is a pure boundary term on-shell, asymptotic gravitational fields store information in a manner not possible in local field theories. Two properties follow from this purely gravitational behavior. The first, 'Boundary Unitarity,' holds under AdS-like boundary conditions. This is the statement that the algebra of boundary observables is independent of time; i.e., that the algebra of boundary observables at any one time t_1 in fact coincides with the algebra of boundary observables at any other time t_2. As a result, any information available at the boundary at time t_1 remains available at any other time t_2. The second, 'Perturbative Holography,' holds under either AdS-like or asymptotically flat boundary conditions. In the AdS context, it is the statement that the algebra of boundary observables at any time t includes all perturbative observables anywhere in the spacetime. In the asymptotically flat context, Perturbative Holography is that statement that the algebra of observables on I^+ within any neighborhood of i^0 contains all perturbative observables. Perturbative Holography holds about any classical solution with a regular past infinity; i.e., spacetimes which collapse to form classical black holes are explicitly allowed. We derive the above properties and discuss their implications for information in black hole evaporation.

In this talk we introduce loop quantum gravity and we apply the theory to the black hole singularity problem. The Schwarzschild black hole solution inside the event horizon coincides with the Kantowski-Sachs space-time and we can study a simple spherically symmetric mini-superspace model. We show the classical black hole singularity is controlled by the quantum theory and the space-time can be dynamically extended beyond the classical singularity. We consider a semiclassical analysis of the black hole in LQG and we focus our attention on the space-time structure. The semiclassical solution is regular everywhere and similar to the Reissner-Nordstrom metric. The LQG black hole metric interpolates between two asymptotically flat regions, the 'r=infinity' region and the 'r=0' region. The metric is self-dual in the sense it is invariant under the symmetry (r ->1/r) which relates small and large distances. We study the thermodynamics of the semiclassical solution.

I present a viewpoint on black hole thermodynamics according to which the entropy: derives from horizon 'degrees of freedom''; is finite because the deep structure of spacetime is discrete; is ``objective'' thanks to the distinguished coarse graining provided by the horizon; and obeys the second law of thermodynamics precisely because the effective dynamics of the exterior region is not unitary.

In this talk I will describe a topos formulation of consistent histories obtained using the topos reformulation of standard quantum mechanics put forward by Doering and Isham. Such a reformulation leads to a novel type of logic with which to represent propositions. In the first part of the talk I will introduce the topos reformulation of quantum mechanics. I will then explain how such a reformulation can be extended so as to include temporally-ordered collection of propositions as opposed to single time propositions. Finally I will show how such an extension will lead to the possibility of assigning truth values to temporal propositions.

The handling of the constraints on initial data is a major issue in most canonical formulations of general relativity. Since the 1960s unconstrained initial data for GR that living on null hypersurfaces has been known, but no canoncial formulation based on these data was developed due to conceptual and technical difficulties. I will explain how these dificulties have been overcome and outline the resulting canonical framework. I will also explain how this might be the ideal setting to attempt a proof of the Bousso entropy bound, or to incorporate the associated holographic principle in a quantization of gravity.

New spin foam models for gravity have been recently proposed to deal with the shortcomings of the Barrett-Crane model. In particular, they draw a closer connection between the Loop Quantum Gravity and the Spin Foam approaches to non perturbative quantum gravity. In this talk, I will present the construction for the case of Lorentzian signature and finite Immirzi parameter. An area operator can be defined and its spectrum agrees with the one defined in LQG. Finally, the amplitude is shown to be finite after a suitable regularization.