A philosophical look at the particle physics (spin-2) approach to GR
I consider the “particle physics” (spin-2) approach to general relativity (GR) à la Kraichnan, Gupta, Feynman, Weinberg, Fang and Fronsdal, and Deser (among others), generally construed as an alternative to the standard “geometrical” textbook presentation. I begin with an overview of “the” particle physics approach by rehearsing its central “derivations” of GR (self-interaction, gauge deformation, and quantum variants). This sets the stage for addressing the big-picture question of what the particle physics approach offers us vis-à-vis the geometrical one.
This equipped, I thus first show that there is both ambiguity in what the decisive figures behind the approach have stated about its correct construal and a lack of consensus concerning the conceptual import of the approach in philosophical discussions (see, e.g., Pitts (2016, 2022), Salimkhani (2020, 2023), and NL, Smeenk and Baker (2023). I then charitably reconstruct possible takes systematically. This reconstruction features: (a) epistemic takes, on which the particle physics approach provides yet another means of arriving at the field equations à la MTW’s famous Six Routes, possibly even tied to more or less sensible alternative-history claims—think of Feynman’s Venusians discovering GR via some particle-physics approach; (b) intertheoretic takes, according to which the particle physics approach either shows the inevitability of GR under controlled generalisation (cf. Lovelock’s theorem) or demonstrates a theoretical reduction of GR (e.g. along the lines of GR reducing to SR in an appropriate limit); and (c) ontological takes, e.g. in the sense of a process ontology—on which the referents of spin-2 fields are genuinely interacting relative to what is represented through some background, presumably flat, metric, which can nevertheless also be effectively represented via the metric of GR—or in the sense of a design ontology, according to which it takes these and only these ingredients, including what spin-2 fields represent, to “make” the worlds we represent through GR (cf. creation metaphors from analytic metaphysics).
With all this on the table, I argue that the correct way to understand the approach is at a purely epistemic level. In particular, my position is that the ontological readings considered are either unattractive (as in the case of standard ontological reduction claims or that of the process ontology proposal) or are better collapsed into an effective epistemic reading after all (as in the case of the design ontology proposal).
Zoom
