The dynamical approach to spin-2 gravity

Abstract

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.

What represents spacetime? And what follows for substantivalism vs. relationalism and gravitational energy?

“…Previously I have found support in particle physics for at least some of Brown's project vis-a-vis space-time realist critiques (Freund et al, 1969;Pitts, 2011;Pitts, 2016c;Pitts, 2019). The result seems to be a coherent package of views that has not previously been seen clearly as a whole and in detail, and perhaps illustrates the fruitfulness of overcoming GR vs. particle physics divide-a divide that philosophers and historians have inherited, along with a GR exceptionalist bias, largely by default (but see (Menon, 2021;Salimkhani, 2020)).…”

“…Why seek a Lagrangian density of that sort given these ingredients? The particle physics spin 2 derivations of Einstein's equations (with citations in (Duff, 1975;Pitts, 2016a;Salimkhani, 2020)) provide rather compelling guidance from very weak premises about avoiding violent instability, locality, (at least) Poincaré invariance, and the empirical fact of the bending of light.…”

Miracles persist: a reply to Sus

“…As Sus himself concedes, this implies that their miraculous status continues to obtain ("... seeing the coincidence of all the symmetries of laws as a mere coincidence [is an option which] must be taken when there is no dynamically determined interaction that can be used to derive the constraints" (Sus, 2021, p. 27)). At least to us, it is not clear how the above-identified issues with Sus' argument can be overcome; in the absence of such a resolution, it remains the case that the best accounts of the miracles of general relativity (and ones which would be acceptable to proponents of the dynamical approach) lie in successor theories-such as spin-2 gravity (Salimkhani, 2020), or theories of quantum gravity (Read, 2019).…”

Why Not a Sound Postulate?

“…For more on the conceptual and ontological status of black holes, see[14] 25. For recent detailed philosophical discussion of the spin-2 approach, see e.g [48,55]…”

“… 25 For recent detailed philosophical discussion of the spin-2 approach, see e.g. [ 48 , 55 ]. These works include extensive references to the original physics literature on the subject.…”