Enrico Maresca

Thought experiments (TEs) are indispensable conceptual tools in scientific research, particularly in the study of quantum gravity. Many scholars argue that the epistemic significance of TEs hinges on the proper and ineliminable use of imagination. However, there is disagreement regarding the specific nature of the imagination involved. A valuable perspective on this debate is provided by a TE proposed by Matvei Bronstein in 1936 to support a quantum theory of gravity. His contribution serves as a notable example of destructive TE, aiming to highlight the internal inconsistency within a unified theory of both quantum mechanics and general relativity. In this paper, I reconstruct Bronstein’s TE in the context of recent discussions on the relationship between TEs and imagination. I argue that this case study challenges existing epistemological frameworks for understanding TEs. I contend that Bronstein’s TE introduces a new form of imagination, termed operational imagination, as indispensable for reaching its intended conclusion. I conclude that operational imagination can be integrated into simulative model-based accounts of TEs.

Noncommutative geometry is often taken to imply a radical departure from relativistic spacetime at the Planck scale. The structure it describes, noncommutaive spacetime (NCST), lacks spacetime points (pointlessness), thereby challenging its status as a genuine physical spatiotemporal structure, as opposed to a merely mathematical construct. This paper challenges that conclusion. First, I argue that claims of pointlessness presuppose an ambiguous criterion for distinguishing physical from merely mathematical structure: proposed specifications either generate conflicting interpretations or collapse into triviality in the noncommutative setting. Second, I contend that even granting pointlessness, non-spatiotemporality does not follow. The inference relies on an implicit constitution thesis (that spacetime is constituted by its points) which proves either insufficiently precise or too weak to sustain the conclusion. By analysing four functional roles of spacetime points in classical theories, I demonstrate that none requires their retention in NCST, thereby undermining claims that spacetime disappears in the noncommutative regime.

Noncommutative geometry (NCG) is a branch of pure mathematics with a wide range of applications to spacetime physics. Stemming from the divergence problem in QFT, modern contributions conjecture that the fundamental structure of spacetime is noncommutative. This seemingly homogeneous picture is the result of almost a century of discontinuous interest in noncommutative spacetime (NCST). In this paper, I present a three-phase division of the development of NCST approaches. The initial phase (1930s–1950s) introduced noncommutativity as a means of addressing the divergence problem while maintaining Lorentz-invariance. The second phase (1950s–early 1990s) initially dismissed noncominutativity and focused on the pressing problem of localisation at high energies. In this context, independent alternative approaches to QG identified the value of a fundamental length scale, which ultimately contributed to a reconsideration of the conjecture of spacetime noncommutativity. This was despite the fact that it undermined the original operationalist methodology. Finally, a third phase (1990s–today) has seen a renewed interest in NCST. I argue that this resurgence can be attributed to the discovery of new mathematics in the 1980s. Furthermore, I demonstrate how the third phase builds upon and continues instances from the previous attempts, but redirects the research question. In light of this, I finally argue that the history of NCG in physics can be understood as an example of “interlaced convergence.” This instance of theory convergence may prove usefill as a case study for the more general problem of theory building in QG.

Process jargon is widespread in the physical sciences. Beginning with the work of Wesley Salmon, several accounts in philosophy of science have attempted to provide a definition of "process" compatible with scientists' understanding of causation and explanation. The proposed characterisation links processes to the properties of the spacetime they inhabit as regards continuity and genuine causality. Recent developments in theories of quantum gravity challenge the validity of process ontologies at the fundamental scale. In particular, this paper examines how arguments based on minimal length in the literature question the traditional definition of process. Process realism does not favour the processualist against these arguments. I conclude that certain theories of quantum gravity prevent a processual representation of the intended phenomena at the fundamental scale because they predict a violation of either the spatiotemporal specification or the causality conditions. In the end, the processualist faces a dilemma: either weaken the accepted definition of process without falling into substance ontologies, or hope that problematic theories of quantum gravity will be disconfirmed.

Numerous theories of quantum gravity (QG) postulate non-spatiotemporal structures to describe physics at or beyond the Planck energy scale. This stands in stark contrast to the spatiotemporal framework provided by general relativity, which remains remarkably successful in low-energy regimes. The resulting tension gives rise to the so-called disappearance of spacetime (DST): the removal of spatiotemporal structures from the fundamental ontology of a theory and the corresponding challenge of reconciling this with the general relativistic picture. In this paper, I classify different instances of DST and highlight the necessary trade-off between theory-specific features and general patterns across QG approaches. I argue that a precise formulation of the DST requires prior clarification of the relevant conception of fundamentality. In particular, I distinguish two forms of disappearance, corresponding to intra-theoretic and inter-theoretic fundamentality relations. I argue that intra-theoretic analyses can yield meaningful results into the DST in QG only when supported by further justificatory arguments. To substantiate my claim, I examine the relationship between string theory, noncommutative geometry, and special relativity.