Philosophy of Spin-2 Gravity Workshop

Quantum field theories and classical general relativity accurately model all observations to date. Although quantum gravity has been theoretically much studied, it still lacks empirical evidence. This makes “is spacetime/gravity quantum?” one of our most important open questions. I have pioneered an ambitious idea with my collaborators, “spin entanglement witness for quantum gravity,” to test the quantum nature of gravity in a lab. It exploits ideas from quantum information and combines a quantum spin with cooling and trapping technologies. It is based on entangling two neutral quantum masses solely by their gravitational interaction while all other interactions are mitigated, e.g. electromagnetic (EM) interactions between the masses. It proves the quantum nature of gravity, as classical gravity cannot mediate quantum correlations (entanglement). The potentially realisable protocol requires meeting a rich set of challenges: mitigating the EM interactions and background, creating spatial quantum superpositions for massive objects, and measuring spin correlations to witness the entanglement. We must also protect quantum superpositions from heating, recoil, blackbody radiation, acceleration, seismic and gravity-gradient noise.

A classical result in gravitational physics concerns a relation between the theory of general relativity and the theory of a self-coupling massless spin-2 particle. It was regarded as textbook knowledge that, when the self-coupling of a particle with these properties is considered, the structure of such a nonlinear theory is that of general relativity. However, recent works have shown how the reconciliation between the two theories is not so trivial, both at the mathematical level of carrying out the self-coupling interactions and at the level of interpreting the physical content of the two theories.
Then, I will revisit the recent approaches to the spin-2 field self-coupling derivation of general relativity and analyze what are the crucial steps in it. In particular, I will argue that different authors propose different and conflicting strategies to solve the self-coupling problem as a consequence of the different interpretations and commitments to the physical content of the theories. Starting from this analysis, I will reason on the level of equivalence between the two theories and conclude that the formalisms of spin-2 gravity and general relativity are not translatable into each other without committing in advance to some interpretation. Consequently, I will also conclude that spin-2 gravity and general relativity are theoretically inequivalent.

As a precursor to Niels Linnemann's talk, I will present my thoughts on how to make a philosophical case for the spin-2 approach by means of centering dependence relations and explanatory coherence. Finally, I will add a few preliminary remarks on recent replies.

I consider the particle physics approach (PPA) to general relativity (GR), generally construed as an alternative to the standard “geometrical” textbook presentation of GR. I begin with an overview of the PPA by both introducing a PPA re-interpretation of GR’s perturbative formulation and rehearsing the central “derivations” of GR from the PPA (self-interaction, gauge deformation, and quantum variants).

I then go on to show that there is both some fair degree of ambiguity in what the decisive figures behind the PPA have stated about its correct conceptualisation and a lack of consensus concerning the conceptual import of the approach in dedicated philosophical discussions (see, e.g., Pitts (2016, 2022), Salimkhani (2020, 2023) and Linnemann, Smeenk, and Baker (2023)). The range of conceptualisations include: (a) epistemological takes, on which the PPA provides yet another means of arriving at the field equations—à la Misner-Thorne-Wheeler’s famous Six Routes, partly tied to more or less alternative-history claims (think of Feynman’s Venusians discovering GR via some particle-physics approach); (b) intertheoretical takes, according to which the PPA either shows the inevitability of GR under controlled generalisation (in a similar spirit as Lovelock’s theorem; a form of anti-reduction) or, quite conversely, demonstrates a reduction of GR to a (typically flat-spacetime-based) particle physics theory (e.g. along the lines of GR reducing to SR in an appropriate limit); and (c) ontological takes, for instance 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 the spin-2 fields represent, to “make” the worlds we eventually represent through GR (cf. creation metaphors from analytic metaphysics).

In the third and last part, I argue 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 seen to collapse into epistemological readings after all (as in the case of the design ontology proposal).