Session J04: Quantum Aspects of Gravitation

Loop Quantum Gravity and the Quantization of Null Surfaces

It is arguably one of the main achievements of loop quantum gravity (LQG) to have demonstrated that space itself may have an atomic structure. One of the main open questions for LQG is how to reconcile its fundamental discreteness with general relativity in the continuum. In this talk, I will discuss recent progress regarding this question and I will show, in fact, that the loop gravity discreteness of space can be derived from a conventional Fock representation in the continuum. The derivation is based on certain boundary spinors that can be used as new canonical variables for general relativity on a null surface. I will discuss the geometric origin of these variables and explain their relevance for other approaches to quantum gravity, such as twistor theory.   

How low can the energy density go?

Quantum fields can sometimes have negative energy density. In gravitational contexts, this threatens to permit both causality violations (such as traversable wormholes, warp drives, and time machines) and violations of the Second Law for black holes. I will discuss the thermodynamic principles that rule out such pathological situations. These principles have led us to an interesting lower bound on the energy flux, even for field theories in flat spacetime! This Quantum Null Energy Condition has recently been proven for general relativistic field theories.   

Gravitational physics from quantum information constraints

In this talk, I will review some of the remarkable connections between gravitational physics and the physics of entanglement in conformal field theories. I will describe how the structure of entanglement in a conformal field theory can be captured by the geometry of an asymptotically AdS spacetime, and how constraints on entanglement imply (at least to second order in perturbation theory around AdS) that this spacetime must satisfy Einstein's equations. Further quantum information-theoretic constraints suggest new results in classical gravity, including a family of positive energy theorems for gravitational subsystems.