Approaches to Quantum Gravity

21The causal set approach to Quantum GravityJ. HENSONHow can we reach a theory of Quantum Gravity? Many answers to this questionare proposed in the different chapters of this book. A more specific set ofquestions might be: what demands should we put on our framework, so that itis best able to meet all the challenges involved in creating a theory of QuantumGravity? What choices are most likely to give the correct theory, accordingto the clues we have from known physics? Are there any problems with our initialassumptions that may lead to trouble further down the road? The latter seemsto be one of the most important strategic questions when beginning to formulatea candidate theory. For example, can a canonical approach overcome the multifacetedproblem of time? And how far can a theory based on a fixed backgroundspacetime be pushed? On the one hand, these questions may only be answeredin the very attempt to formulate the theory. On the other, many such attemptshave been made, and now that Quantum Gravity research has built up some history,perhaps it is time to plough some of the experience gained back into anew approach, laying the groundwork for our theory in such a way as to avoidwell-known problems. The causal set program [1; 2; 3; 4; 5] represents such anattempt.In this review, some answers to the above questions, as embodied by the causalset program, are set out and explained, and some of their consequences are given.As part of this, the results and open problems in the program are discussed. Insection 21.1, reasons for hypothesising spacetime discreteness are reviewed. Thedefinition of a causal set is given, along with the proposed correspondence principlebetween this structure and the effective continuum description of spacetime. Thensome of the unique features of this discretisation scheme are discussed. In section21.2, ideas for causal set dynamics are given. Next, a review is made of some phenomenologicalmodels based on this Quantum Gravity program, and successes andchallenges in this line of work are summarised. Some results in this section touchon the issue of recovering locality for causal sets, something that is significant forApproaches to Quantum Gravity: Toward a New Understanding of Space, Time and Matter, ed. Daniele Oriti.Published by Cambridge University Press. c○ Cambridge University Press 2009.