Dynamics and phenomenology of quantum gravity

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The aim of this proposal is to make definitive progress on the problem of quantum gravity, which is to bring together the two revolutionary developments initiated early in the 20th century: relativity and quantum theory. In the last 25 years a great deal of progress has been made, and indeed there are several promising directions under development. But so far none are completely satisfactory. In particular, none has made novel predictions which have been confirmed by experiment. In the proposed work we aspire to “make a push for the summit” and complete the remaining steps needed to generate experimental predictions from o quantum theory of gravity. We aim to accomplish this by joining together results and tools from several related approaches. In particular, we seek to link more theoretical approaches with more phenomenological approaches, and so be able to test basic principles by experiment. A basic and experimentally confirmed principle of gravitational theories which we implement in this proposal is background independence. This means that space and time, rather than being fixed stages on which the drama of nature play out, have a dynamical geometry which evolves in relation to matter. We build on recent progress in background independent approaches, especially loop quantum gravity. Recently several milestones have been achieved including explanations for black hole entropy and temperature, confirmations that general relativity is recovered as a semiclassical limit and increasing evidence that spacetime singularities are eliminated. These build confidence, so now is the time to raise the stakes and take on two key challenges: 1) Does the theory make experimental predictions by which it can be tested? 2) If correct, loop quantum gravity frames the language by which the other forces and particles are described. Are there then implications for the question of how gravity is to be unified with the rest of nature? Recent developments provide the basis for fresh attacks on these key questions. We start by joining two theoretical directions together, which are loop quantum gravity with causal set models of quantum spacetime. We plan to derive from these phenomenological models of the influence of quantum geometry on the propagation of photons and elementary particles. These describe effects such as an energy dependence of the speed of light which are consequences of the breaking or deforming of the symmetries of physics. These effects are in fact constrained by recent observations of photons from gamma ray bursts or cosmic rays from active galactic nuclei. We attack unification by building on two recent developments, one concerning the existence of chiral fermions, as required for the standard model, the other a natural extension of the theory that unifies general relativity with Yang-Mills theory. This offers a natural explanation for parity violation in the standard model.