Baptiste Le Bihan

String theory is a minefield for philosophers in terms of conceptual difficulty and mathematical technicality. Presentations aimed at philosophers generally focus on providing precise and technical mathematical descriptions of toy models and inter-theoretical toy derivations to give an idea of what is at play in the field. This presentation takes the opposite approach, filtering out most mathematical considerations to provide an overview of the field, eyes on the ontology. The aim is to provide a compass for metaphysicians to find their way around string theory and the philosophy of string theory. The development of metaphysical arguments based on string theory against the fundamentality of spacetime is deferred to the next chapter.

Relativity supports a block-universe view on which past, present and future entities coexist within spacetime. Quantum gravity complicates this picture by suggesting that spacetime may be emergent. I distinguish spacetime emergence from block emergence: the former concerns the emergence of spatiotemporal structure, while the latter concerns the emergence of a unified domain of spatiotemporal existence simpliciter. This distinction clarifies three positions: standard eternalism, atemporal eternalism and atemporal fragmentalism. I argue that eternalism remains the default option once the existence of spacetime is accepted. Atemporal fragmentalism, however, remains a sound interpretation of certain approaches to quantum gravity.

Schneider and Bailey’s Superpsychism advances the Prototime Interpretation of quantum mechanics, on which entanglement unfolds in a non-spatiotemporal arena. By locating maximal consciousness at this fundamental level, they aim to gain leverage over both physicalism and cosmopsychism. I argue, however, that the view encounters two issues. First, if the fundamental level is non-spatiotemporal, our conception of the physical becomes unstable. At best one could redescribe what remains as a thin negative gap, but even that residual puzzle carries little dialectical force, reflecting ignorance rather than any principled contrast. Second, the contrast between superpsychism and cosmopsychism cannot rely on scale once spacetime is set aside; recast mereologically, it shows no clear advantage for superpsychism, and in fact risks collapsing into cosmopsychism unless further argument is supplied. The broader lesson is that the Prototime framework points more generally, and at least prima facie, toward non-spatiotemporal monism, while leaving open the task of showing whether and why superpsychism should be preferred within the family of monist views.

Can multiverse hypotheses ever receive empirical support? Critics argue that multiverse scenarios posit unobservable entities, face severe underdetermination, or fall outside the bounds of science. This chapter challenges that view by offering a naturalistic metaphysical counterpoint to Bayesian approaches, distinguishing fragmented from holistic multiverses. Scientific proposals are almost always holistic: they embed universes within a unifying physical or metaphysical structure that can, in principle, leave empirical signatures inside the universes. I develop a typology of such signatures and show how it applies to leading scenarios from quantum theory, cosmology, and string theory. This framework clarifies why objections such as the `this universe' objection and a newly articulated generalization, the epistemic isolation objection, fail against scientifically motivated multiverses. The upshot is a qualified defence: while fragmented multiverses remain empirically inaccessible, certain holistic multiverses could, in principle, be supported by the same epistemic standards used elsewhere in physics.

Causal Set Theory (CST) is a promising approach to fundamental physics that seems to treat causation as a basic posit. But in exactly what sense is CST causal? We argue that if the growth dynamics is interpreted as a physical process, then CST employs relations of actual causation between causal set elements, whereby elements bring one another into existence. This is important, as it provides a better sense of how CST works, highlights important differences from general relativity---where relations between spacetime points are typically seen as cases of mere causal connectibility rather than actual causation of the relevant type---and points toward a specific understanding of the emergence of spacetime within CST.

Physics not only describes past, present, and future events but also accounts for unrealized possibilities. These possibilities are represented through the solution spaces given by theories. These spaces are typically classified into two categories: kinematical and dynamical. The distinction raises important questions about the nature of physical possibility. How should we interpret the difference between kinematical and dynamical models? Do dynamical solutions represent genuine possibilities in the physical world? Should kinematical possibilities be viewed as mere logical or linguistic constructs, devoid of a deeper connection to the structure of physical reality? This chapter addresses these questions by analyzing some of the most significant theories in physics: classical mechanics, general relativity and quantum mechanics, with a final mention to quantum gravity. We argue that only dynamical models correspond to genuine physical possibilities.

A ‘‘duality’’ is a formal mapping between the spaces of solutions of two empirically equivalent theories. In recent times, dualities have been found to be pervasive in string theory and quantum field theory. Naïvely interpreted, duality-related theories appear to make very different ontological claims about the world—differing in, for example, spacetime structure, fundamental ontology, and mereological structure. In light of this, duality-related theories raise questions familiar from discussions of underdetermination in the philosophy of science: in the presence of dual theories, what is one to say about the ontology of the world? Int his paper, we undertake a comprehensive and nontechnical survey of the landscape of possible ontological interpretations of duality-related theories. We provide a significantly enriched and clarified taxonomy of options—several of which are novel to the literature.

The metaphysics of quantum gravity explores metaphysical issues related to research programs in theoretical physics clustered under the term quantum gravity. These research programs aim at the formulation of a theory that reconciles the theory of general relativity with quantum theory. The goal is not necessarily to come up with a unified single theory but, more pragmatically, to describe phenomena with a dual nature, embodying both quantum and relativistic features—such as black holes and the early universe. Approaches to quantum gravity are not yet fully worked-out theories. Nevertheless, they already provide a certain partial understanding of physical reality in different ways. Remarkably, they do so with a striking similarity: they virtually all deny the existence of some features usually regarded as essential to the existence of spacetime (or space and/or time) such as its four-dimensionality, the existence of distances and durations between events, or even the very partial ordering of events. This observation is particularly noteworthy, considering the pervasive influence of spatial and temporal organisation on the human mind across various facets of daily life and theoretical thinking, ranging from most ancient religions to contemporary scientific worldviews. The metaphysics of quantum gravity takes as its starting point the puzzling observation that physics could teach us that space and time are not fundamental. It draws on resources from traditional metaphysics to tackle a set of issues related to the possible non-fundamentality of spacetime, and it investigates its potential implications for venerable traditional issues in metaphysics. The metaphysics of quantum gravity is a relatively small and new research field, and thus as of now, its focus has been on explaining how spacetime could emerge from a more fundamental and non-spatiotemporal ontology. Consequently, this article is equally focused on questions regarding the status of spacetime and the emergence of spacetime. Section 1 situates the field within metaphysics of science more broadly. Sections 2 and 3 investigate, respectively, the status of spacetime in different approaches to quantum gravity and a number of potential issues with its lack of fundamentality. The article then covers the nature of the emerging spatiotemporal ontology (Section 4) and the building relation that relates it to the underlying non-spatiotemporal ontology (Section 5). Section 6 surveys various potential applications of spacetime emergence to a number of debates in metaphysics.

It is plausible that the models of scientific theories correspond to possibilities. But how do we know which models of which scientific theories so correspond? This paper provides a novel proposal for guiding belief about possibilities via scientific theories. The proposal draws on the notion of an effective theory: a theory that applies very well to a particular, restricted domain. We argue that it is the models of effective theories that we should believe correspond, at least in part, to possibilities. It is thus effective theories that should guide modal reasoning in science.

La vieille question du statut ontologique du présent refait aujourd’hui surface au travers du débat qui oppose présentisme et éternisme. Les présentistes défendent la thèse selon laquelle seul ce qui est présent existe. Les éternistes soutiennent quant à eux que le présent ne jouit d’aucun privilège ontologique, les choses passées et futures existant tout autant que les choses présentes. Dans cet article nous ne chercherons pas à départager les protagonistes mais à écarter les théories dites « hybrides » qui prétendent bénéficier des avantages de chacune des positions sans hériter de leurs inconvénients. Nous commencerons par retracer le débat entre présentisme et éternisme (section 1), puis nous présenterons les motivations des défenseurs des théories hybrides (section 2). Enfin, dans notre dernière section, nous avancerons un argument qui, s’il est juste, montre que les théories hybrides ne sont pas cohérentes.

Penrose's conformal cyclic cosmology describes the cosmos as a collection of successive universes, the so-called aeons. The beginning and ending of our universe are directly connected to two other, anterior and posterior, universes. Penrose considers but rules out a different interpretation of conformal cyclic cosmology: that the beginning of our universe is connected to its own end in a cosmic loop. The paper argues that the view, aeon monism, should be regarded as a natural interpretation of conformal cyclic cosmology and discusses its implications for the concept of eternal return in light of the most popular metaphysics of time.

The question of whether Everettian quantum mechanics (EQM) justifies the existence of metaphysical indeterminacy has recently come to the fore. Metaphysical indeterminacy has been argued to emerge from three sources: coherent superpositions, the indefinite number of branches in the quantum multiverse and the nature of these branches. This paper reviews the evidence and concludes that those arguments don’t rely on EQM alone and rest on metaphysical auxiliary assumptions that transcend the physics of EQM. We show how EQM can be ontologically interpreted without positing metaphysical indeterminacy by adopting a deflationary attitude towards branches. Two ways of developing the deflationary view are then proposed: one where branches are eliminated, and another where they are reduced to the universal quantum state.

La pertinence de la métaphysique analytique a fait l'objet de critiques : Ladyman et Ross, par exemple, ont suggéré d'abandonner ce domaine. French et McKenzie ont défendu la métaphysique analytique en affirmant qu'elle développe des outils qui pourraient s'avérer utiles pour la philosophie de la physique. Dans cet article, nous montrons dans un premier temps que cette défense heuristique de la métaphysique peut être étendue au domaine scientifique de l'ontologie appliquée, qui utilise des théories et outils issus de la métaphysique analytique. Dans un deuxième temps, nous développons le parallèle que font French et McKenzie entre les mathématiques et la métaphysique pour montrer que l'ensemble du domaine de la métaphysique analytique, étant donné son utilité non seulement pour la philosophie mais également pour la science, devrait continuer à exister en tant que domaine largement autonome.

Surface presentism is the combination of a general relativistic physics with a presentist metaphysics. In this paper, we provide an argument against this combination based on black holes. The problem focuses on the notion of an event horizon. We argue that the present locations of event horizons are ontologically dependent on future black hole regions, and that this dependence is incompatible with presentism. We consider five responses to the problem available to the surface presentist, and argue that none succeed. Surface presentism thus faces the prospect of refutation based on evidence that confirms the existence of black holes.

The existence and fundamentality of spacetime has been questioned in quantum gravity where spacetime is frequently described as emerging from a more fundamental non-spatiotemporal ontology. This is supposed to lead to various philosophical issues such as the problem of empirical coherence. Yet those issues assume beforehand that we actually understand and agree on the nature of spacetime. Reviewing popular conceptions of spacetime, we find that there is substantial disagreement on this matter, and little hope of resolving it. However, we argue that this should not trouble us as these issues, which seem to suggest the need for an account of spacetime in quantum gravity, can be addressed without one.

The present volume collects essays on the philosophical foundations of quantum theories of gravity, such as loop quantum gravity and string theory. Central for philosophical concerns is quantum gravity's suggestion that space and time, or spacetime, may not exist fundamentally, but instead be a derivative entity emerging from non-spatiotemporal degrees of freedom. In the spirit of naturalised metaphysics, contributions to this volume consider the philosophical implications of this suggestion. In turn, philosophical methods and insights are brought to bear on the foundations of quantum gravity itself. For instance, the idea of functionalism, borrowed from the philosophy of mind and discussed by several essays, exemplifies this mutual interaction the collection seeks to foster.

According to a number of approaches in theoretical physics, spacetime does not exist fundamentally. Rather, spacetime exists by depending on another, more fundamental, non-spatiotemporal structure. A prevalent opinion in the literature is that this dependence should not be analyzed in terms of composition. We should not say, that is, that spacetime depends on an ontology of non-spatiotemporal entities in virtue of having them as parts. But is that really right? On the contrary, we argue that a mereological approach to dependent spacetime is not only viable, but promises to enhance our understanding of the physical situation.

As a candidate theory of quantum gravity, the popularity of string theory has waxed and waned over the past four decades. One current source of scepticism is that the theory can be used to derive, depending upon the input geometrical assumptions that one makes, a vast range of different quantum field theories, giving rise to the so-called landscape problem. One apparent way to address the landscape problem is to posit the existence of a multiverse; this, however, has in turn drawn heightened attention to questions regarding the empirical testability and predictivity of string theory. We argue first that the landscape problem relies on dubious assumptions and does not motivate a multiverse hypothesis. Nevertheless, we then show that the multiverse hypothesis is scientifically legitimate and could be coupled to string theory for other empirical reasons. Looking at various cosmological approaches, we offer an empirical criterion to assess the scientific status of multiverse hypotheses.

A number of philosophers have argued in favour of extended simples on the grounds that they are needed by fundamental physics. The arguments typically appeal to theories of quantum gravity. To date, the argument in favour of extended simples has ignored the fact that the very existence of spacetime is put under pressure by quantum gravity. We thus consider the case for extended simples in the context of different views on the existence of spacetime. We show that the case for extended simples based on physics is far more complex than has been previously thought. We present and then map this complexity, in order to present a much more textured picture of the argument for extended simples.

The violation of Bell inequalities seems to establish an important fact about the world: that it is non-local. However, this result relies on the assumption of the statistical independence of the measurement settings with respect to potential past events that might have determined them. Superdeterminism refers to the view that a local, and determinist, account of Bell inequalities violations is possible, by rejecting this assumption of statistical independence. We examine and clarify various problems with superdeterminism, looking in particular at its consequences on the nature of scientific laws and scientific reasoning. We argue that the view requires a neo-Humean account of at least some laws, and creates a significant problem for the use of statistical independence in other parts of physics and science more generally.

L'éternalisme, la thèse selon laquelle les entités que nous catégorisons comme étant passées, présentes et futures existent tout autant, est la meilleure approche ontologique de l'existence temporelle qui soit en accord avec les théories de la relativité restreinte et de la relativité générale. Cependant, les théories de la relativité restreinte et générale ne sont pas fondamentales si bien que plusieurs programmes de recherche tentent de trouver une théorie plus fondamentale de la gravité quantique rassemblant tous les enseignements de la physique relativiste et de la physique quantique. Certaines de ces approches soutiennent que le temps n'est pas fondamental. Toutefois, si le temps n'est pas fondamental, quelles en sont les conséquences pour l'éternalisme et les débats sur l'existence dans le temps ? Premièrement, je soutiendrai que la non-fondamentalité du temps que l'on rencontre dans la théorie des cordes mène à l'éternalisme standard. Deuxièmement, je soutiendrai que la non-fondamentalité du temps rencontrée dans la gravité quantique à boucles implique l'éternalisme atemporel, à savoir une nouvelle position qui demeure fidèle à l'esprit de l'éternalisme standard.

Eternalism, the view that what we regard locally as being located in the past, the present and the future equally exists, is the best ontological account of temporal existence in line with special and general relativity. However, special and general relativity are not fundamental theories and several research programs aim at finding a more fundamental theory of quantum gravity weaving together all we know from relativistic physics and quantum physics. Interestingly, some of these approaches assert that time is not fundamental. If time is not fundamental, what does it entail for eternalism and the standard debate over existence in time? First, I will argue that the non-fundamentality of time to be found in string theory entails standard eternalism. Second, I will argue that the non-fundamentality of time to be found in loop quantum gravity entails atemporal eternalism, namely a novel position in the spirit of standard eternalism.

In a recent article, Ned Markosian gives an argument against four-dimensionalism understood as the view that time is one of four identical dimensions that constitute a single four-dimensional manifold. In this paper, I show that Markosian attacks a straw man as his argument targets a theory known to be false on empirical grounds. Four-dimensionalism rightly conceived in no way entails that time is identical to space. I then address two objections raised by Markosian against four-dimensionalism rightly conceived.

Spacetime functionalism is the view that spacetime is a functional structure implemented by a more fundamental ontology. Lam and Wüthrich have recently argued that spacetime functionalism helps to solve the epistemological problem of empirical coherence in quantum gravity and suggested that it also (dis)solves the hard problem of spacetime, namely the problem of offering a picture consistent with the emergence of spacetime from a non-spatio-temporal structure. First, I will deny that spacetime functionalism solves the hard problem by showing that it comes in various species, each entailing a different attitude towards, or answer to, the hard problem. Second, I will argue that the existence of an explanatory gap, which grounds the hard problem, has not been correctly taken into account in the literature.

La philosophie contemporaine du temps voit s’affronter deux conceptions du temps : celle du devenir qui identifie la réalité naturelle à un présent en constant renouvellement et celle de l’univers-bloc qui assimile la réalité naturelle à un espace-temps étendu dans quatre dimensions. Cette dernière approche implique notamment que les événements qui nous semblent passés et futurs sont tout aussi réels que les événements présents et que les êtres humains, bien que mortels, sont des êtres éternels. L’auteur défend cette théorie de l’univers-bloc en montrant que le raisonnement philosophique et les avancées les plus récentes de la physique contemporaine s’accordent à montrer que ce monde quadri-dimensionnel est bel et bien le nôtre.