Deflating the Spacetime-Matter Dichotomy
Antonio Ferreiro, Alex Fleuren, Niels C. M. Martens
TL;DR
This work interrogates the spacetime–matter dichotomy within scalar–tensor theories by examining Jordan and Einstein frames, arguing that conformal representations render the scalar field’s classification frame-dependent and thus undermine a black‑and‑white STM division. It shows that in the Jordan frame the scalar is neither matter nor spacetime, while in Einstein frames the scalar can be counted as either spacetime or matter depending on the representation, leading to a representational breakdown of the dichotomy. A key positive lesson is the frame-invariant status of gravity: gravitational fields, defined by how matter couples through the stress–energy tensor, remain robust across frames, suggesting a gravity/non-gravity separation as a more reliable organizing principle. Philosophical interpretations range from functionalist and symmetry-based stances to quietism and privilege-based views, but the authors argue there is no single frame that restores a strict STM dichotomy; instead, gravity provides a stable core for understanding these theories. The findings highlight the potential to decouple spacetime from gravity in scalar–tensor contexts and invite a shift toward a gravity-centered ontology in relativistic field theories.
Abstract
In this paper we analyse scalar-tensor theories-specific instances of which include mainstream inflation and dark energy models-in light of the spacetime-matter dichotomy. We argue that it is difficult to categorise the scalar fields as either a pure aspect of the spacetime structure or a pure form of matter, by focusing on the Jordan vs Einstein frames of these theories. We present and evaluate various interpretational options available, concluding that the spacetime-matter dichotomy becomes untenable in this context. At the same time, the ontological and conceptual category of spacetime can be decoupled from that of gravity, with the latter remaining viable in the context of scalar-tensor theories.