[HEG] The Unexpected Virtue of Ignorance

Who: Tom Yildirim (Oxford University)
Title: The Unexpected Virtue of Ignorance

Abstract: In this talk I will discuss some work in progress on the role of observers in quantum gravity. A realistic gravitational observer cannot access the entirety of spacetime and is limited to performing only a restricted class of sufficiently simple measurements. Subsequent predictions must therefore rely on a principle of maximal ignorance. An observer can represent their state of knowledge about the universe either by a maximum-entropy density matrix or by assigning a probability measure on the space of states. We show that the first approach generalises recently proposed “observer rules” for the gravitational path integral, while the second can reproduce the full sum over topologies, depending on a choice of prior on the global structure of the universe. Our results suggest that observers in closed universes may perceive conventional physics as long as they remain ignorant of the  global quantum state.