Emergent Spacetime and the Challenge of Empirical (In)coherence
In "Emergent Spacetime and Empirical (In)coherence," Huggett and Wüthrich address the complex challenge of reconciling theories of quantum gravity that lack a fundamental notion of spacetime with empirical science, which is inherently predicated on the ability to observe localized entities, termed "local beables." This paper scrutinizes the empirical viability of quantum gravity theories that radically deviate from the usual spacetime ontologies by either eliminating spacetime altogether or redefining its fundamental nature.
The paper examines multiple paradigms of quantum gravity and categorizes them according to their deviation from classical spatiotemporal concepts. It considers theories that manifest as simple discrete lattices to those involving non-commutative geometries, loop quantum gravity (LQG), and string theory dualities. Each of these theories challenges the traditional conception of spacetime and hence raises crucial questions of empirical coherence: if spacetime as a fundamental backdrop is absent, then how can the theories account for the observable universe and maintain empirical consistency?
Huggett and Wüthrich position various theories along a spectrum of "spacetimelessness." The simplest departure involves lattice spacetime, replacing the continuum with discrete structures akin to lattices that still preserve spatiotemporal relations albeit in a discretized form. More radically, the authors discuss non-metrical lattices, such as those in causal set theory, where events or entities are partially ordered causally but not embedded within any metric structure. This scenario poses profound challenges in recovering continuous spacetime and thus identifiable local beables.
The paper further analyzes LQG, which contemplates a quantum superposition of "spin networks," suggesting that spacetime should emerge from these by respecting certain semiclassical limits. LQG posits fundamental structures that lack intrinsic locality, hypothesizing that spacetime and its local beables could still emerge due to the dynamics of these interconnected quantum states.
In addressing string theoretic frameworks, the authors explore the implications of dualities: two theories can be mathematically equivalent yet imply distinct, even contradictory, spacetime structures. Here, dualities, like T-duality, imply that familiar spatial dimensions may not directly correlate with spatiotemporal structures intrinsic to the theories. The consideration of NCFTs pushes the boundaries further by employing algebraic structures where traditional concepts of spacetime are either undetermined or emerge through non-commutative relations.
In all these cases, an underlying theme persists: the empirical challenge of connecting the fundamental non-spatial theories to observable phenomena. While some theoretical constructs can map back to observable phenomena and save empirical coherence, whether through scattering event predictions or duality correspondences, achieving this without conceptual ambiguity remains pivotal.
While they recognize the theoretical prowess in potentially deriving spacetime emergently from non-spatiotemporal constructs, Huggett and Wüthrich raise philosophical concerns about the realist interpretation of these theories. They question whether the mathematical derivations that promise to connect fundamental descriptions to observable entities maintain their "physical salience" without a fundamental spacetime ontology.
In addressing such conceptual hurdles, the authors reflect on what it means for a theory to hold physical salience "from below" and how empirical coherence can be reconceptualized "from above." They engage both with Maudlin's philosophical challenge on empirical incoherence and with a broader scientific realism debate. For them, questioning the physical salience asks for careful scrutiny of what it means for a derivative structure to be physically valid and informative.
The paper presents a robust philosophical critique and insists on the significance of these theoretical developments. The research underlines an iterative scientific process where conceptual clarity and empirical evidence must evolve together to substantiate the reality implied by such theories. Huggett and Wüthrich ultimately observe that exploring quantum gravity theories and their implications for spacetime presents a rich ground for philosophers and physicists alike to interrogate fundamental assumptions about the universe, bridging profound theoretical inquiries with empirical grounding.
