Levels of Spacetime Emergence in Quantum Gravity

We explore the issue of spacetime emergence in quantum gravity by articulating several levels at which this can be intended. These levels correspond to the reconstruction moves that are needed to recover the classical and continuum notion of space and time, which are progressively lost in a progressively deeper sense in the more fundamental quantum gravity description. They can also be understood as successive steps in a process of widening of the perspective, revealing new details and new questions at each step. Each level carries indeed new technical issues and opportunities, and raises new conceptual issues. This deepens the scope of the debate on the nature of spacetime, both philosophically and physically.

Explanations of and in Time

Various approaches to quantum gravity render spacetime an emergent phenomenon, with the existence and properties of spacetime depending on a non-spatiotemporal underlying reality. This chapter investigates the mode of dependence that is involved. I explain and defend my recent proposal to classify different kinds of dependencies in terms of the principles mediating the dependency, and apply this proposal to the emergence of spacetime. While philosophers have typically interpreted spacetime emergence as involving metaphysical grounding, I show how premises that are widely endorsed lead us to classify the emergence of spacetime in loop quantum gravity as causal in nature. I recommend spacetime functionalism as a resolution of this puzzle that vindicates the natural view of spacetime emergence as non-causal. I then explore a different approach to spacetime emergence in quantum gravity, the ‘many-instant landscape’ scenario described by Gomes, and show how it fits into the proposed framework for spacetime emergence.