The dynamical approach to spin-2 gravity

This paper engages with the following closely related questions that have recently received some attention in the literature: (a) what is the status of the equivalence principle in general relativity (GR)?; (b) how does the metric field obtain its property of being able to act as a metric?; and (c) is the metric of GR derivative on the dynamics of the matter fields? The paper attempts to complement these debates by studying the spin-2 approach to (quantum) gravity. In particular, the paper argues that three lessons can be drawn from the spin-2 approach: (1) different from what is sometimes claimed in the literature, central aspects of the non-linear theory of GR are already derivable in classical spin-2 theory; in particular, ‘universal coupling’ can be considered a derived ‘theorem’ in both the classical and the quantum spin-2 approach; this provides new insights for the investigation of the equivalence principle; (2) the ‘second miracle’ that Read et al. argue characterises GR is explained in the classical as well as in the quantum version of the spin-2 approach; (3) the spin-2 approach allows for an ontological reduction of the metrical part of spacetime to the dynamics of matter fields.