The Case for Background Independence

This chapter explains the arguments behind the assertion that the correct quantum theory of gravity must be background independent. It begins by recounting how the debate over whether quantum gravity must be background independent is a continuation of a long-standing argument in the history of physics and philosophy over whether space and time are relational or absolute. This leads to a careful statement of what physicists mean when we speak of background independence. Given this we can characterize the precise sense in which general relativity is a background-independent theory. The leading background-independent approaches to quantum gravity are then discussed, including causal set models, loop quantum gravity, and dynamical triangulations, and their main achievements are summarized along with the problems that remain open. The relational/absolute debate has implications also for other issues such as unification and how the parameters of the standard models of physics and cosmology are to be explained. The chapter reviews recent issues concerning the string theory landscape and argues that they can only be resolved within the context of a background-independent formulation. Finally, it reviews some recent proposals to make quantum theory more relational.