Effective spacetime geometry

Research Highlights•I examine the circumstances under which general relativity can be considered an effective theory are considered.•Consideration of the nature of the equivalence principle shows that it is relatively straightforward to interpret general relativity as an effective theory.•Poincaré gauge theories prove to provide classical examples of effective spacetime geometry.

I argue that the need to understand spacetime structure as emergent in quantum gravity is less radical and surprising it might appear. A clear understanding of the link between general relativity's geometrical structures and empirical geometry reveals that this empirical geometry is exactly the kind of thing that could be an effective and emergent matter. Furthermore, any theory with torsion will involve an effective geometry, even though these theories look, at first glance, like theories with straightforward spacetime geometry. As it's highly likely that there will be a role for torsion in quantum gravity, it's also highly likely that any theory of quantum gravity will require us to get to grips with emergent spacetime structure.