# My talks

1 February 2019, Research Seminar Philosophy of Physics, Utrecht University:

**Spacetime is as spacetime does** (based on joint work with Vincent Lam)

**Abstract:**

15-16 January 2019, NYU Abu Dhabi:

**Black holes and membranes**

**Abstract:** The 'membrane paradigm' of black holes, developed in the 1980s by Kip Thorne and collaborators, 'stretches' the black hole's event horizon into a boundary between it and the universe. This boundary of the external physics considered in the paradigm--the 'membrane'--is then endowed with physical properties such as viscosity, charge, and conductivity in order to facilitate calculations of astrophysical phenomena in a 3+1 formalism. Recently, it has been claimed that the membrane paradigm strengthens significantly the case for thinking that black holes are thermodynamic objects. But does this paradigm really provide independent confirmation for black holes thermodynamics, or does it merely offer a check of its consistency? Or does it fail to even offer that, given that it only describes the physics away from the black hole itself?

20 October 2018, Foundational Problems of Black Holes and Gravitational Radiation, Ludwig-Maximilians-University Munich:

**Black holes and membranes**

**Abstract:** The 'membrane paradigm' of black holes, developed in the 1980s by Kip Thorne and collaborators, 'stretches' the black hole's event horizon into a boundary between it and the universe. This boundary of the external physics considered in the paradigm--the 'membrane'--is then endowed with physical properties such as viscosity, charge, and conductivity in order to facilitate calculations of astrophysical phenomena in a 3+1 formalism. Recently, it has been claimed that the membrane paradigm strengthens significantly the case for thinking that black holes are thermodynamic objects. But does this paradigm really provide independent confirmation for black holes thermodynamics, or does it merely offer a check of its consistency? Or does it fail to even offer that, given that it only describes the physics away from the black hole itself?

21 September 2018, Annual Meeting of the Swiss Society for Logic and Philosophy of Science, Università della Svizzera italiana, Lugano:

**The atemporal big bang** (partly based on joint work with Nick Huggett)

**Abstract:** Cosmological models based on quantum theories of gravity promise to offer a quantum treatment of the big bang. While they tend to erase what is classically a singularity, the quantum evolution that replaces it may not correspond to classical spacetime; instead, it seems to posit a non-spatiotemporal region, which somehow 'transitions' to a macroscopically spatiotemporal state. I will discuss the relation between the two different kinds of transitions and how they interact to yield a potential answer to the emergence of time and its direction. This will raise the bigger question of the emergence of classicality in a cosmological setting.

4 September 2018, Workshop 'Time in Physics', University of Salzburg:

**The atemporal big bang** (partly based on joint work with Nick Huggett)

29 August 2018, Time Time Time: Science, Art and Philosophy, Istituto Svizzero, Villa Maraini, Rome:

**Philosophy of physics and time, part II**

27 August 2018, Time Time Time: Science, Art and Philosophy, Istituto Svizzero, Villa Maraini, Rome:

**Philosophy of physics and time, part I**

18 July 2018, Split Summer School in Physics and Philosophy on 'The Chimera of Entropy', University of Split:

**Holography and black hole entropy**

**Abstract:** I will review the argument from entropy bounds to the holographic principle and assess the latter's status, implications, and promise to offer clues to the unification of quantum matter and gravity.

12 July 2018, The 19th UK and European Conference on Foundations of Physics, Utrecht:

**Laws beyond spacetime** (based on joint work with Vincent Lam)

**Abstract:** Are Humeanism and naturalism compatible? More specifically, can one be a Humean about laws of nature and a naturalist who takes fundamental physics seriously? The co-tenability of these two theses has of course been questioned before. We argue here that looking at physical theories beyond empirically established quantum physics and relativity suggests a novel, and much deeper naturalist challenge to Humeanism about laws of nature than those debated before. The principal problem that arises is how to even articulate a Humean account of laws in a world that is fundamentally non-spatiotemporal.

4 June 2018, Florence-Pisa Doctoral School in Philosophy Conference 'Philosophy, Knowledge, and the Sciences', Pistoia (Italy):

**Spacetime is as spacetime does** (based on joint work with Vincent Lam)

**Abstract:** In theories of quantum gravity, one often finds that the postulated fundamental structures lack significant aspects of relativistic spacetime. Yet any fundamentally non-spatiotemporal theory must face the challenge of its own empirical incoherence, as we could never believe a theory apparently precluding what appears to be necessary preconditions that must be in place for its own empirical confirmation, even if it were true. Thus, any quantum theory of gravity must establish how it is that relativistic spacetimes are, in many contexts, such excellent descriptions of our spatiotemporal reality. In this talk, it will be argued that in order to secure this emergence of spacetime, it suffices to recover those features of relativistic spacetimes functionally relevant in producing empirical evidence. In order to complete this task, an account must be given of how the more fundamental structures instantiate these functional roles. I will illustrate the general idea in the context of causal set theory and loop quantum gravity, two prominent approaches to quantum gravity.

15 May 2018, Vortragsverein Oberwallis and Naturforschende Gesellschaft Oberwallis, Brig (Switzerland):

**Ist unsere Wirklichkeit letztlich weder räumlich noch zeitlich?**

**Abstract:** Die Quantengravitation versucht, Einsichten der Quantentheorie, die so erfolgreich zu unserem Verständnis des Aufbaus der Materie beigetragen hat, und der allgemeinen Relativitätstheorie, unsere beste Theorie der Gravitation, zu einer einheitlichen Theorie zu verschmelzen. Dies ist nötig, um zum Beispiel schwarze Löcher und das ganz frühe Universum zu verstehen. Eine solche Theorie ist dabei auch von sehr grossem naturphilosophischen Interesse. Die Vorstellungen von Raum und Zeit, die aus der allgemeinen Relativitätstheorie erwachsen, sind bereits kontraintuitiv und es ist zu erwarten, dass sie durch die Quanteneffekte noch wesentlich verkompliziert werden. Es zeigt sich, dass die meisten Ansätze in der Quantengravitation zu verneinen scheinen, dass unsere Welt letztlich räumlich und zeitlich ist. Wie kann man sich eine solche Welt — wie sie vielleicht die unsere ist — überhaupt vorstellen? Sind in einer solchen Welt notwendige Bedingungen für die wissenschaftliche Forschung überhaupt erfüllbar? Wie können Raum und Zeit nicht fundamental, sondern bloss emergent, sein und also aus einer nicht raumzeitlichen Struktur hervorgehen? Diese Fragen sollen an konkreten Beispielen erläutert und beantwortet werden.

Coverage in the Swiss newspaper *Walliser Bote* on 11 May 2018.

17 April 2018, Annual Philosophy of Science Meeting, Dubrovnik:

**What we cannot learn from analogue experiments** (based on joint work with Karen Crowther and Niels Linnemann)

**Abstract:** Analogue experiments have attracted interest for their potential to shed light on inaccessible domains. For instance, 'dumb holes' in fluids and Bose-Einstein condensates, as analogues of black holes, have been promoted as means of confirming the existence of Hawking radiation in real black holes. We compare analogue experiments with other cases of material models and simulation, and we contrast these against *conventional* experiments and *semi-analogue* experiments (e.g., testing drugs for humans on mice). We argue---contra recent claims in the literature---that analogue experiments are not different in character from ordinary analogue reasoning. As they must assume the physical adequacy of the modelling framework used to describe the inaccessible target system, arguments to the conclusion that analogue experiments can yield confirmation for phenomena in those target systems, such as Hawking radiation in black holes, beg the question. Consequently, analogue experiments cannot provide confirmation for the existence of particular phenomena in inaccessible systems---or at least not more than any other form of analogue reasoning can do.

16 March 2018, DPG Spring Meeting, Berlin:

**Spacetime is as spacetime does** (based on joint work with Vincent Lam)

**Abstract:** In theories of quantum gravity, one often finds that the postulated fundamental structures lack significant aspects of relativistic spacetime. Yet any fundamentally non-spatiotemporal theory must face the challenge of its own empirical incoherence, as we could never believe a theory apparently precluding what appears to be necessary preconditions that must be in place for its own empirical confirmation, even if it were true. Thus, any quantum theory of gravity must establish how it is that relativistic spacetimes are, in many contexts, such excellent descriptions of our spatiotemporal reality. In this talk, it will be argued that in order to secure this emergence of spacetime, it suffices to recover those features of relativistic spacetimes functionally relevant in producing empirical evidence. In order to complete this task, an account must be given of how the more fundamental structures instantiate these functional roles. I will illustrate the general idea in the context of causal set theory and loop quantum gravity, two prominent approaches to quantum gravity.

27 October 2017, Workshop 'Spacetime: Fundamental or Emergent?', University of Bonn:

**Spacetime is as spacetime does** (based on joint work with Vincent Lam)

**Abstract:** In theories of quantum gravity, one often finds that the postulated fundamental structures lack significant aspects of relativistic spacetime. Yet any fundamentally non-spatiotemporal theory must face the challenge of its own empirical incoherence, as we could never believe a theory apparently precluding what appears to be necessary preconditions that must be in place for its own empirical confirmation, even if it were true. Thus, any quantum theory of gravity must establish how it is that relativistic spacetimes are, in many contexts, such excellent descriptions of our spatiotemporal reality. In this talk, it will be argued that in order to secure this emergence of spacetime, it suffices to recover those features of relativistic spacetimes functionally relevant in producing empirical evidence. In order to complete this task, an account must be given of how the more fundamental structures instantiate these functional roles. I will illustrate the general idea in the context of causal set theory and loop quantum gravity, two prominent approaches to quantum gravity.

25 August 2017, Annual Meeting of the Swiss Physical Society, Geneva:

**The disappearance of spacetime in quantum theories of gravity**

**Abstract:** Using loop quantum gravity as an example, I will present how essential aspects of relativistic spacetime disappear in quantum gravity. The absence of spacetime in a fundamental theory of physics seems to undermine the conditions necessary for its empirical confirmation and thereby threatens what could be called its 'empirical coherence'. I will sketch how relativistic spacetime is thought to be recovered in loop quantum gravity, and how this averts the threat of empirical incoherence.

17 July 2017, 5th Black Forest Summer School in Philosophy of Physics, Saig (Germany):

**The disappearance and re-emergence of spacetime in quantum theories of gravity based on general relativity**

**Abstract:** I will present how essential aspects of relativistic spacetime disappear in two approaches to quantum gravity based on general relativity—causal set theory and loop quantum gravity. The absence of spacetime in a fundamental theory of physics seems to undermine the conditions necessary for its empirical confirmation and thereby threatens its empirical coherence. I will sketch how relativistic spacetime is thought to be recovered in the two approaches, and how this averts the threat of empirical incoherence.

14 July 2017, British Society for the Philosophy of Science Annual Conference, Edinburgh:

**The temporal and atemporal emergence of (space-)time** (based on joint work with Nick Huggett)

**Abstract:** Space and time are conspicuous by their absence in many theories of quantum gravity, although the specific way in which and the extent to which the fundamental structure is non-spatiotemporal varies from approach to approach. The central question then arises as to how classical relativistic spacetime can be constituted by a fundamental, non-spatiotemporal structure, and whether it can be so constituted at all: how can spacetime emerge in a world not spatial, not temporal? This question concerns the atemporal relation between the more fundamental and the less fundamental. There is, however, also an issue of temporal emergence of spacetime, which becomes pressing in a cosmological setting: if the big bang must be described by a theory of quantum gravity, and this theory implies that the deep quantum regime of the very early universe is non-temporal, then physical time must emerge at some 'later time' 'after' the big bang. I will articulate these questions and sketch some answers in the context of loop quantum cosmology, the attempt to articulate a cosmological model based on loop quantum gravity, one of the predominant approaches to quantum gravity.

12 July 2017, XX International Summer School in Philosophy of Physics, Urbino:

**When the actual world is not even possible**

**Abstract:** Contemporary fundamental physics offers compelling reasons for the need of a quantum theory of gravity, i.e. a theory unifying classical general relativity with the quantum physics of the standard model of particle physics. Approaches to quantum gravity often involve the disappearance of space and time at the fundamental level. The metaphysical consequences of this disappearance are profound, as I will illustrate with David Lewis's analysis of modality. As Lewis's possible worlds are unified and isolated by the spatiotemporal relations among their parts, the non-fundamentality of spacetime---if borne out---suggests a serious problem for his analysis: his pluriverse, for all its ontological abundance, does not contain our world. Although the mere existence---as opposed to the fundamentality---of spacetime must be recovered from the fundamental structure in order to guarantee the empirical coherence of the non-spatiotemporal fundamental theory, it does not suffice to salvage Lewis's theory of modality from the charge of rendering our actual world impossible. Thus, non-spatiotemporal fundamental physics, naturalism, and Lewis's analysis of modality are incompatible, thereby exemplifying the profound metaphysical implications that quantum gravity may engender.

12 July 2017, XX International Summer School in Philosophy of Physics, Urbino:

**Spacetime is as spacetime does** (based on joint work with Vincent Lam)

**Abstract:** Space and time are conspicuous by their absence in fundamental theories of quantum gravity. In this talk, I illustrate this absence in the case of causal set theory, articulate why this poses a deep philosophical and scientific challenge to the theory--or indeed any candidate theory in quantum gravity--, and sketch ways in which this challenge can be met. In particular, I argue that a functionalist assumption constitutes a necessary part of a sufficient condition for the emergence of relativistic spacetimes from causal sets.

11 July 2017, XX International Summer School in Philosophy of Physics, Urbino:

**The structure of causal sets**

**Abstract:** In this talk, I will introduce a particularly simple approach to quantum gravity, causal set theory. This approach has been pioneered in the 1970s by Myrheim and 't Hooft and developed by a number of physicists around Rafael Sorkin at Syracuse since the 1980s. Even though the approach is still rather inchoate and requires substantial work in order to qualify as a genuine competitor for a full quantum theory of gravity, its conceptual simplicity offers a number of advantages of interest to physicists and philosophers alike. First, it provides a kind of conceptual laboratory for physicists to learn how various basic principles interact. Second, it promises to shed light on the important connection between a fundamentally discrete structure and relativistic spacetimes and on how the latter can emerge from the former. Third, it offers a paradigmatic case of a physical theory predestined to be interpreted in structuralist terms. I will explain the main motivations for structuralism in fundamental physics and show how philosophical interest in structuralism and technical issues in causal set theory interact.

10 July 2017, XX International Summer School in Philosophy of Physics, Urbino:

**Quantum gravity: motivations and implications**

**Abstract:** Do we need to quantize gravity, as it is tacitly assumed in much of fundamental physics? Even though the standard lore falls short of justifying an affirmative answer, the fact that matter is not classical requires our going beyond general relativity. Though conclusive in this respect, this fact offers no guidance as to the direction in which a quantum theory of gravity must be sought. Despite its being unobserved to date, Hawking radiation and black hole thermodynamics is widely considered our firmest hint, weak though it may be. As the theoretical derivation of Hawking radiation and black hole thermodynamics relies on a semi-classical mlange of physical principles, it raises the question of whether we need the unified blend of a full quantum theory of gravity at all, particularly given that the latter is generally expected to reject at least some of the principles of the former. Although this issue is not settled, it seems as if the difficulties in articulating a semi-classical theory can only be overcome in full quantum gravity.
This will lead us to the second part of my talk, in which I will sketch some of the philosophical and foundational problems that come up in quantum gravity and that will be addressed in various lectures and discussions throughout the week. The vantage and focal point for many of these issues will be the apparent disappearance of spacetime in full quantum theories of gravity.

7 July 2017, Summer School 'Physics and Philosophy', University of Split:

**What becomes of the future**

**Abstract:** Unlike the relativity theory it seeks to replace, causal set theory (CST) has been interpreted to leave space for a substantive, though perhaps ‘localized’, form of ‘becoming’. The possibility of fundamental becoming is nourished by the fact that the analogue of Stein’s theorem from special relativity does not hold in CST. Despite this, we find that in many ways, the debate concerning becoming parallels the well-rehearsed lines it follows in the domain of relativity. We present, however, some new twists and challenges. In particular, we show that a novel and exotic notion of becoming is compatible with causal sets. In contrast to the localized becoming considered compatible with the dynamics of CST by its advocates, our novel kind of becoming, while not answering to the typical A-theoretic demands, is global and objective.

6 July 2017, Summer School 'Physics and Philosophy', University of Split:

**The disappearance of space and time in quantum theories of gravity**

**Abstract:** There are compelling reasons to think that current physical theories will have to yield to a more fundamental quantum theory of gravity. As it turns out, space and time are conspicuous by their absence in many of these theories of quantum gravity, although the specific way in which and the extent to which the fundamental structure is non-spatiotemporal varies from approach to approach. My lecture will illustrate this non-spatiotemporality by example, articulating why this poses a deep philosophical and scientific challenge to any candidate theory in quantum theory, and sketch ways in which this challenge can be met.

13 June 2017, Fourth IAPT Meeting, Gargnano, Italy:

**Our world may be deterministic or indeterministic--or both**

**Abstract:** We show that our world may be, at the same time, deterministic and indeterministic. This apparent contradiction becomes possible on a Humean view in which (in)determinism is a property of theories, not worlds. This Humean view is actualist, rather than possibilist, but shares with other forms of Humeanism a commitment to Humean supervenience and something like a best systems account of laws of nature. On this view, a world may thus turn out to be described equally well by deterministic and indeterministic theories, and in this sense be both deterministic and indeterministic. Two empirically equivalent theories of non-relativistic quantum mechanics, Bohmian mechanics and Nelsonian mechanics serve as an illustration of this possibility. The conclusion is defended against two objections.

20 May 2017, Seven Pines Symposium XXI, Black Holes in the Spotlight, Outing Lodge, Stillwater, Minnesota:

**Considering information-theoretic and analogical reasoning**

**Abstract:** Same as Bad Honnef talk right below.

26 April 2017, Do Black Holes Exist? --- The Physics and Philosophy of Black Holes, 641st WE-Heraeus-Seminar, Bad Honnef (Germany):

**Considering information-theoretic and analogical reasoning in black-hole physics**

**Abstract:** Physicists working in quantum gravity radically diverge over the physical principles that they take as their starting point in articulating a quantum theory of gravity, over which direction to take from these points of departure, over what we should reasonably take as the goals of the enterprise and the criteria of success, and sometimes even over the legitimacy of different methods of evaluation and confirmation of the resulting theories. Yet, there is something that most of them agree upon: that black holes are thermodynamical objects, have entropy and radiate, and thus that the Bekenstein-Hawking formula for the entropy of a black hole must be recovered, more or less directly, from the microphysics of the fundamental degrees of freedom postulated and described by their theories.
The aim of this talk is twofold. First, I will investigate the foundation of this seeming, and curiously universal, agreement; in particular, I will analyze Jacob Bekenstein's original argument [1] that led him to propose his celebrated formula. Bekenstein relies on information-theoretic arguments to endow the formal similarity between thermodynamic entropy and the area of black hole’s event horizon with physical salience. Although the reasoning is suggestive, I will point out the many substantive and far from automatic steps it requires. Although this casts doubt on the original argument by Bekenstein there are other ways in which the thermodynamic character of black holes can be confirmed.
One such possibility of confirmation arises from the experimental detection of ‘Hawking radiation’ in analogue systems for black holes, for instance in a Bose-Einstein condensate [2]. Dardashti et al. [3] have claimed that the detection of analogue Hawking radiation could in principle confirm the existence of Hawking radiation in black holes. I will critically examine their argument and argue that the universality considerations which underlie the validity of analogical reasoning, and hence of confirmation of properties of black holes in terrestrial analogue models, are more problematic that they suggest.

7 April 2017, The Physics and Phenomenology of Time, University of Rome 3:

**The (a)temporal emergence of spacetime**(based on joint work with Nick Huggett)

15 March 2017, The Foundation of Reality: Fundamentality, Space and Time, Corpus Christi College, University of Oxford:

**Gounding (space-)time**

**Abstract:** Time might seem to be the sort of thing that is, by necessity, fundamental, primitive, and not grounded in anything non-temporal. However, leading research programs in quantum gravity suggest that (space-)time is not fundamental, but instead emerges from something non-spatiotemporal. How should we conceive of such a possibility metaphysically? I will try to illustrate just how this might work using the example of causal set theory, which asserts that time is grounded in causation, rather than the other way around.

10 February 2017, Center for Philosophy of Time colloquium, University of Milan:

**When the actual world is not even possible**

**Abstract:** Approaches to quantum gravity often involve the disappearance of space and time at the fundamental level. The metaphysical consequences of this disappearance are profound, as is illustrated with David Lewis’s analysis of modality. As Lewis’s possible worlds are unified by the spatiotemporal relations among their parts, the non-fundamentality of spacetime – if borne out – suggests a serious problem for his analysis: his pluriverse, for all its ontological abundance, does not contain our world. Although the mere existence – as opposed to the fundamentality – of spacetime must be recovered from the fundamental structure in order to guarantee the empirical coherence of the non-spatiotemporal fundamental theory, it does not suffice to salvage Lewis’s theory of modality from the charge of rendering our actual world impossible.

3 February 2017, eidos Graduate School in Metaphysics on 'Identity', Neuchâtel:

**Identity in physics**

20-21 January 2017, Wochenendseminare Studienkolleg, Forum Scientiarum, University of Tübingen:

**Blockseminar zur Ontologie von Raum und Zeit**

18 January 2017, The Metaphysics of Entanglement project, University of Oxford:

**When the actual world is not even possible**

**Abstract:** Contemporary fundamental physics offers compelling reasons for the need of a quantum theory of gravity, i.e. a theory unifying classical general relativity with the quantum physics of the standard model of particle physics. Approaches to quantum gravity often involve the disappearance of space and time at the fundamental level. The metaphysical consequences of this disappearance are profound, as I will illustrate with David Lewis's analysis of modality. As Lewis's possible worlds are unified and isolated by the spatiotemporal relations among their parts, the non-fundamentality of spacetime---if borne out---suggests a serious problem for his analysis: his pluriverse, for all its ontological abundance, does not contain our world. Although the mere existence---as opposed to the fundamentality---of spacetime must be recovered from the fundamental structure in order to guarantee the empirical coherence of the non-spatiotemporal fundamental theory, it does not suffice to salvage Lewis's theory of modality from the charge of rendering our actual world impossible. Thus, non-spatiotemporal fundamental physics, naturalism, and Lewis's analysis of modality are incompatible, thereby exemplifying the profound metaphysical implications that quantum gravity may engender.

6 January 2017, Cluster of Excellence 'Image Knowledge Gestaltung', Humboldt University Berlin:

**The philosophical depth of quantum gravity**

**Abstract:** The search for a quantum theory of gravity exemplifies the deep connections between fundamental physics and philosophy, as this talk will illustrate. Many approaches to quantum gravity such as loop quantum gravity, non-commutative geometry, but also string theory generically suggest that the fundamental reality we inhabit is not spatiotemporal. This immediately raises the philosophical concern that these theories may be empirical incoherent. Furthermore, some of these approaches, such as causal set theory, have emerged from genuinely philosophical traditions. Finally, the radical reconceptualization of fundamental reality that quantum gravity arguably engenders may have far-reaching implications for philosophy, including, for example, for distinctly metaphysical matters such as the nature of modality.

9 November 2016, EXRE Colloquium, Fribourg:

**The (a)temporal emergence of spacetime** (based on joint work with Nick Huggett)

**Abstract:** The question of the emergence of classical spacetime can be posed atemporally: 'how does some more fundamental theory explain the validity of GR?' But it can also be asked in a temporal sense: roughy, 'from what (if anything) did the big bang come?' Of course, the latter depends on the former, and whatever the answer, it is widely assumed that the initial singularity of GR signals its breakdown, a place where it will be extended by QG -- hopefully within its second century. Important philosophical issues are raised by this question: for example, what is asked is a causal form of 'why is there something rather than nothing?' Or again, one crucial possibility is that the answer will require us to go beyond the classical conception of time. This paper explains how three approaches to the temporal emergence of spacetime give different answers, and what conceptual issues are thus raised.

4 November 2016, 25th Biennial Meeting of the Philosophy of Science Association, Atlanta:

**The (a)temporal emergence of spacetime** (with Nick Huggett, Symposium 'General Relativity in Its Second Century)

**Abstract:** The question of the emergence of classical spacetime can be posed atemporally: 'how does some more fundamental theory explain the validity of GR?' But it can also be asked in a temporal sense: roughy, 'from what (if anything) did the big bang come?' Of course, the latter depends on the former, and whatever the answer, it is widely assumed that the initial singularity of GR signals its breakdown, a place where it will be extended by QG -- hopefully within its second century. Important philosophical issues are raised by this question: for example, what is asked is a causal form of 'why is there something rather than nothing?' Or again, one crucial possibility is that the answer will require us to go beyond the classical conception of time. This paper explains how three approaches to the temporal emergence of spacetime give different answers, and what conceptual issues are thus raised.

14 September 2016, Workshop 'Ground in Philosophy of Science', Geneva:

**Grounding time (and space)**

**Abstract:** Time seems to be the kind of aspect of our reality that ought to be primitive and hence fundamental in the metaphysical furniture of our world. However, as physicists have searched for a theory combining quantum physics with general relativity and replace them as fundamental physical theory, it has become increasingly clear that the ontology of such a fundamental theory will not contain anything resembling a (space-)time. Space and time, it appears, are absent at the fundamental level --they are grounded in something else -- and only 'emerge' as effective phenomena at a coarse-grained scale. If that is so, any such candidate theory must establish the emergence of spacetime and its dynamical content from the fundamental structure atemporally, i.e., without conceiving of this emergence as a dynamical process in time. The goal of this paper is to articulate what this means and to consider one approach to formulating such a theory, viz. loop quantum gravity, and to study how temporality may emerge atemporally in its cosmological models. This paper is part of a larger joint project with Nick Huggett addressing this issue in different approaches to quantum gravity.

21 July 2016, 4th International Summer School in Philosophy of Physics, Saig (Germany):

**Relativistic quantum physics**

**Abstract:** I will give an introduction to relativistic quantum physics. In particular, I will address how moving from non-relativistic quantum mechanics to relativistic quantum theory does not render the measurement problem any less pressing. Furthermore, I will address why we need a relativistic quantum theory in the first place, and what this implies for the issue of whether such a relativistic quantum theory is a theory of particles or of fields. This last discussion will help prepare the grounds for Adam Caulton?s lecture on particles and fields in QFT.

30 June 2016, Annual Meeting of the Société de philosophie des sciences, Lausanne:

**The atemporal emergence of temporality**

**Abstract:** Too long to post.

10 June 2016, International Association for the Philosophy of Time, Winston-Salem, North Carolina:

**Is time (or space-time) fundamental according to theories of quantum gravity?** (Symposium)

31 May 2016, Fourth International Conference on the Nature and Ontology of Spacetime, Varna (Bulgaria):

**Spacetime is as spacetime does** (with Vincent Lam)

**Abstract:** Too long to post.

27 May 2016, Lake Geneva Graduate Conference, Geneva:

**What becomes of the future**

21 April 2016, quodlibeta, University of Geneva:

**Our world may be deterministic or indeterministic--or both**

**Abstract:** Our world may be both deterministic and indeterministic. This apparent contradiction becomes possible on a Humean view in which (in)determinism is a property of theories, not worlds. A world may thus be described equally well by deterministic and indeterministic theories, and in this sense be both deterministic and indeterministic. Fleshing this out serves the bigger purpose of illustrating what I take to be the most attractive version of a Humean approach to understanding theories, laws of nature, and modality.

10 March 2016, Symposium `Methodological Challenges in Quantum Gravity', GWP, Düsseldorf:

**Considering the role of information theory in fundamental physics**

**Abstract:** Too long to post.

2 March 2016, Symposium 'Quantentheorie und Gravitation', Spring meeting of DPG, Hamburg:

**Quantum and gravity: blend or mélange?**

**Abstract:** Do we need to quantize gravity, as it is tacitly assumed in much of fundamental physics? The standard lore falls short of justifying an affirmative answer. Black hole thermodynamics is widely considered, faint though it may be, our firmest hint at a quantum theory of gravity--despite the failure to date to observe Hawking radiation or any other effect that would require going beyond a classical description of black holes. Hawking radiation hitherto merely enjoys a theoretical derivation in a semi-classical theory combining quantum matter with classical gravity. But how can a semi-classical mlange of physical principles possibly justify that the quantum and gravity are blended into a unified fundamental theory when the latter is generally expected to reject at least some of the principles in the former?

8 December 2015, Workshop 'Why trust a theory?', LMU Munich:

**Considering the role of information theory in fundamental physics**

**Abstract:** Information theory presupposes the notion of an epistemic agent, such as a scientist or an idealized human. Despite that, information theory is increasingly invoked by physicists concerned with fundamental physics, physics at very high energies, or generally with the physics of situations in which even idealized epistemic agents cannot exist. In this talk, I shall try to determine the extent to which the application of information theory in those contexts is legitimate. I will illustrate my considerations using the case of black hole thermodynamics and Bekenstein's celebrated argument for his formula for the entropy of black holes. This example is particularly pertinent to the current workshop because it is widely accepted as 'empirical data' in notoriously deprived quantum gravity, even though the laws of black hole thermodynamics have so far evaded direct empirical confirmation.

26 September 2015, EPSA15, Biennial Meeting of the European Philosophy of Science Association, Düsseldorf:

**Functional emergence of spacetime in quantum gravity** (contributed paper, with Vincent Lam)

**Abstract:** Relativistic spacetime, according to many quantum theories of gravity, does not exist, fundamentally. This threatens the very possibility of the empirical confirmation of these theories. Their empirical coherence can be restored by securing the emergence of spacetime from the fundamental non-spatio-temporal structures. Establishing this emergence requires not just mathematical limits and approximations, but also a successful argument that these technical procedures result in appropriately local 'beables'. We show in the context of two programs in quantum gravity--loop quantum gravity and causal set theory--that a recovery of the merely functionally relevant features of spacetime suffices to this end.

10 September 2015, Prospects for causal set quantum gravity, ICMS Edinburgh:

**What becomes of a causal set** (based on joint work with Craig Callender)

**Abstract:** Space and time are conspicuous by their absence in fundamental theories of quantum gravity. In this talk, I illustrate this absence in the case of causal set theory, articulate why this poses a deep philosophical and scientific challenge to the theory--or indeed any candidate theory in quantum gravity--, and sketch ways in which this challenge can be met. In particular, I argue that a functionalist assumption constitutes a necessary part of a sufficient condition for the emergence of relativistic spacetimes from causal sets.

21 July 2015, 3rd International Black Forest Summer School in the Philosophy of Physics, Lenzkirch-Saig, Germany:

**A philosophical Introduction to ontology**

**Abstract:** This talk gives a philosophical introduction to ontology, a central part of theoretical philosophy. It considers how to find a theory's ontological commitments and the Quinean method for doing so. It discusses relations of fundamentality and ontological dependence, and general points concerning the appropriate methodology of ontology.

25 April 2015, Southern California Group in Philosophy of Physics, Irvine:

**Spacetime from causal sets**

**Abstract:** I will illustrate how space and time vanish in causal set theory and address the central question of this research program, viz. how relativistic spacetimes re-emerge from the fundamental causal sets. Part of what I plan to talk about is covered in Section 3 of the attendant paper, which is a draft of Chapter 3 of my forthcoming book with Nick Huggett; the rest will be treated in what will be Chapter 4. Sections 1 and 2 of the paper can be read as a preparation for those who are not familiar with causal set theory, but I do not plan to discuss them in the meeting.

23 January 2015, Minnesota Center for Philosophy of Science:

**Space and time from causality**

**Abstract:** Space and time are conspicuous by their absence in fundamental theories of quantum gravity. Causal set theory is such a theory. It follows an eminent tradition of reducing spatiotemporal relations to causal ones. I will illustrate how the causal sets lack all spatial and most temporal structure. The absence of spacetime from the fundamental level of reality poses, however, a deep philosophical and scientific challenge. On the philosophical side, the threat of empirical incoherence looms. The scientific aspect arises from the need for any novel theory to explain the success, such as it was, of the theory it seeks to depose. Both sides of the challenge are resolved if we articulate a physically salient recovery of relativistic spacetime from the underlying fundamental causal sets. I will sketch ways in which this can be achieved.

16 December 2014, Workshop 'Emergent Time and Emergent Space in Quantum Gravity', Albert Einstein Institute, Golm (Germany):

**Functionalist causal set theory**

**Abstract:** Space and time are conspicuous by their absence in fundamental theories of quantum gravity. In this talk, I illustrate this absence in the case of causal set theory, articulate why this poses a deep philosophical and scientific challenge to the theory--or indeed any candidate theory in quantum gravity--, and sketch ways in which this challenge can be met. In particular, I argue that a functionalist assumption constitutes a necessary part of a sufficient condition for the emergence of relativistic spacetimes from causal sets.

7 November 2014, 24th Biennial Meeting of the Philosophy of Science Association, Chicago:

**What becomes of a causal set** (contributed paper, with Craig Callender)

**Abstract:** Unlike the relativity theory it seeks to replace, causal set theory has been interpreted to leave space for a substantive, though perhaps 'localized', form of 'becoming'. The possibility of fundamental becoming is nourished by the fact that the analogue of Stein's theorem from special relativity does not hold in causal set theory. Despite this, we find that in many ways, the debate concerning becoming parallels the well-rehearsed lines it follows in the domain of relativity. We present, however, some new twists and challenges. In particular, we show that a novel and exotic notion of becoming is compatible with causal sets. In contrast to the 'localized' becoming considered compatible with the dynamics of causal set theory by its advocates, our novel kind of becoming, while not answering to the typical A-theoretic demands, is 'global' and objective.

7 October 2014, Workshop on the History of Time Accuracy in Physics and Astronomy, Observatoire de Paris (cancelled):

**The conspicuous absence of space and time in quantum gravity**

**Abstract:** Space and time are conspicuous by their absence in fundamental theories of quantum gravity. My talk will illustrate this fact by example, articulate why this poses a deep philosophical and scientific challenge to any candidate theory in quantum theory, and sketch ways in which this challenge can be met.

26 July 2014, 2nd International Black Forest Summer School in the Philosophy of Physics, Lenzkirch-Saig, Germany:

**On the metaphysics and physics of determinism and indeterminism**

**Abstract:** In this presentation, I will first show how the conceptualization of (in)determinism differs in Humean and non-Humean approaches to the metaphysics of science. Second, I will illustrate how many of the standard 'verdicts' regarding (in)determinism allegedly issued by our best physical theories are quite limited in scope - much more so than is generally appreciated. To this end, I will extend some of John Earman's classic work on the topic. Third, I will argue that the recently announced 'Free Will Theorem' does not, as its authors John Conway and Simon Kochen claim, establish that our world must be indeterministic. Taken together, these points suggest that the determinism question remains wide open, and may in fact remain so forever.

10-11 July 2014, Annual Conference of the British Society for the Philosophy of Science, University of Cambridge:

**What becomes of a causal set** (with Craig Callender)

**Abstract:** Contemporary physics is notoriously hostile to an A-theoretic metaphysics of time. A recent approach to quantum gravity promises to reverse that verdict: advocates of causal set theory have argued that their framework is at least consistent with a fundamental notion of 'becoming'. In this paper, after presenting some new twists and challenges, we show that a novel and exotic notion of becoming is compatible with causal sets.

17 April 2014, Pacific APA, San Diego, Philosophy of Time Society:

**What becomes of a causal set** (with Craig Callender, presented by him)

**Abstract:** Contemporary physics is notoriously hostile to an A-theoretic metaphysics of time. A recent approach to quantum gravity promises to reverse that verdict: advocates of causal set theory have argued that their framework is at least consistent with a fundamental notion of 'becoming'. In this paper, after presenting some new twists and challenges, we show that a novel and exotic notion of becoming is compatible with causal sets.

28 September 2013, Seminar 'Philosophical Foundations of Quantum Gravity', University of Illinois, Chicago:

**Canonical quantum gravity for philosophers**

27 September 2013, Seminar 'Philosophical Foundations of Quantum Gravity', University of Illinois, Chicago:

**Why 'philosophical' foundations?** (with Nick Huggett)

14 September 2013, Workshop 'Metaphysics Meets the Philosophy of Physics', University of Rochester:

**Building bridges: potential lessons from quantum gravity**

5 July 2013, Workshop 'The Question "What if?" in the Sciences and Humanities', University of Geneva:

**What if the world was quantum gravitational?**

**Abstract:** There are compelling reasons to think that current physical theories will have to yield to a more fundamental quantum theory of gravity. Uncharacteristically for empirical science, these reasons do not include otherwise inexplicable data or some tension between the data and any available theories. If one accepts whatever these reasons may be, one is thus left with little empirical guidance for one's theorizing. It is exactly due to this speculative mode of theorizing that we find thought experiments assuming important roles in an exploration of physical possibility. Thought experiments thus beget theory in quantum gravity.

Conversely, theory stands, by necessity, at the origin of thought experiments. This is witnessed by their often undischarged assumptions, which span the modal spaces they are designed to reconnoitre. But it is *theory* which carves out these spaces. If this is right, we find a mutual dependence between thought experiment and theory. This interdependence will be illustrated in the case of quantum gravity, although it will be claimed to hold generally.

28 June 2013, Journée Philosophie de la physique au séminaire Philosophie et Physique du Laboratoire SPHERE, University of Paris 7, Denis Diderot:

**Time and space in causal set theory**

**Abstract:** Causal set theory offers an elegant and philosophically rich, though admittedly inchoate, approach to quantum gravity. After presenting its basic theoretical framework, I will show how space and time vanish from the fundamental picture it offers. The absence of space and time from the theory raises the serious question of whether such a theory can be empirically coherent at all, i.e, whether its truth would not undermine any justification we may have for believing it. If it can be shown that spacetime re-emerges from the fundamental structure in the appropriate limit, I will argue, then the threat of empirical incoherence is averted and it can be appreciated how space and time emerge from what there is, fundamentally, according to causal set theory. I shall close by sketching the prospects of the antecedent of this conditional claim.

1 June 2013, Quantum Gravity in Perspective, Munich Center for Mathematical Philosophy:

**Time and space in causal set theory**

**Abstract:** Causal set theory offers an elegant and philosophically rich, though admittedly inchoate, approach to quantum gravity. After presenting its basic theoretical framework, I will show how space and time vanish from the fundamental picture it offers. The absence of space and time from the theory raises the serious question of whether such a theory can be empirically coherent at all, i.e., whether its truth would not undermine any justification we may have for believing it. If it can be shown that spacetime re-emerges from the fundamental structure in the appropriate limit, I will argue, then the threat of empirical incoherence is averted and it can be appreciated how space and time emerge from what there is, fundamentally, according to causal set theory. I shall close by sketching the prospects of the antecedent of this conditional claim.

29 May 2013, University of Tübingen:

**Auf der Suche nach der verlorenen Raumzeit**

**Abstract:** Die Zeit erfährt in der modernen Physik wenig Gastfreundschaft: die Newtonsche Physik scheint ihr einen objektiven Fluss und eine objektiv ausgezeichnete Gegenwart zu verweigern, die Spezielle Relativitätstheorie verlangt, dass zwei Ereignisse nicht immer zeitlich geordnet sind, und die Allgemeine Relativitätstheorie lässt es sogar zu, dass ein Ereignis zeitlich sowohl früher als auch später als ein anderes Ereignis stattfindet. In zeitgenössischen Theorien der Quantengravitation wird allerdings ein neuer Tiefpunkt erreicht: viele dieser Theorien suggerieren, dass auf der fundamentalsten Ebene der physikalischen Beschreibung unserer Welt weder Raum noch Zeit vorkommen, sondern dies erst auf einer "höheren" Ebene tun. Raum und Zeit sind demnach bloss "emergente" Phänomene. Ich versuche zu klären, wie das zu verstehen ist. Insbesondere werde ich der Frage nachgehen, wie eine fundamentale Theorie ohne Raum und Zeit überhaupt empirisch kohärent sein kann. Dabei wird klar, was das grösste Problem dieser Theorien der Quantengravitation ist; nämlich aufzuzeigen, wie aus einer raumzeitlosen fundamentalen Struktur überhaupt etwas hervorgehen kann, das die Rolle einer relativistischen Raumzeit einnimmt. Wie philosophisch reich dieses Thema ist soll am den Beispielen der kausalen Mengentheorie und der Schleifenquantengravitation gezeigt werden.

17 May 2013, Annual Philosophy of Physics Conference, Western University (formerly University of Western Ontario):

**The problem of space in quantum gravity**

**Abstract:** There is, of course, the well-rehearsed problem of time (and change) in canonical theories of quantum gravity. Within the strictures of these approaches, the problem of time states that if we only accept a few seemingly innocuous assumptions, then we are forced into accepting that, fundamentally, there cannot be time, or at least that there cannot be genuine change. In this talk, I will suggest that in some approaches to quantum gravity, an analogous, though perhaps less severe, problem of space arises from similarly defensible assumptions. 'Space' in quantum gravity, it turns out, lacks quite a bit of the structure we normally attribute to it. In particular, I will consider two programs in quantum gravity--loop quantum gravity and causal set theory--in order to make this point evident. I do this by showing how what can reasonably be interpreted as that which corresponds to, or gives rise to, 'space' lacks several of what are naturally taken to be essential properties of physical space.

29 March 2013, Pacific APA, San Francisco:

**Comments on Oliver Pooley's 'Three types of gauge redundancy and the problem of time'**

2 February 2013, Law and Order conference, University of California, San Diego:

**Should the actual be possible?**

**Abstract:** Contemporary fundamental physics offers compelling reasons for the need of a quantum theory of gravity, i.e. a theory unifying classical general relativity with the quantum physics of the standard model of particle physics. Approaches to quantum gravity often involve the disappearance of space and time at the fundamental level. The metaphysical consequences of this disappearance are profound, as is illustrated with David Lewis's analysis of modality. As Lewis's possible worlds are unified by the spatiotemporal relations among their parts, the non-fundamentality of spacetime--if borne out--suggests a serious problem for his analysis: his pluriverse, for all its ontological abundance, does not contain our world.

If a Lewisian accepts, in a naturalistic vein, that physics may eliminate space and time from the fundamental furniture of the world, she can resist the argument by insisting that an empirical theory cannot coherently deny the existence of space and time, for all experience is experience of something being located somewhere at some time. I will argue that theories denying the fundamental existence of space and time are not incoherent in this sense, but only if they entail the non-fundamental, i.e., emergent existence of space and time. Unfortunately, this does not suffice to salvage the Lewisian account of modality. First, it is insufficient on principled grounds: a metaphysician will hardly be satisfied with being offered what is at best an approximately true account of modality which ignores the fundamental reality in favour of a description of emergent phenomena. Second, a Lewisian theory of modality trading in emergent spatiotemporal relations must remain inpotent in binding the basic constituents of actuality into one and the same possible world. The reason for this impotence is straightforward: there are no spatiotemporal (not even analogically spatiotemporal) relations exemplified in the fundamental structure, and the basal elements of this structure cannot be the relata of the emergent spacetime relations. Thus, non-spatiotemporal fundamental physics, naturalism, and Lewis's analysis of modality are incompatible.

15 October 2012, 40th Anniversary of the First Osgood Hill Conference on Quantum Gravity, Boston Center for Philosophy and History of Science, Boston University:

**The problem of space in quantum gravity**

**Abstract:** There is, of course, the well-rehearsed problem of time (and change) in canonical theories of quantum gravity. Within the strictures of these approaches, the problem of time states that if we only accept a few seemingly innocuous assumptions, then we are forced into accepting that, fundamentally, there cannot be time, or at least that there cannot be genuine change. In this talk, I will suggest that in some approaches to quantum gravity, an analogous, though perhaps less severe, problem of space arises from similarly defensible assumptions. 'Space' in quantum gravity, it turns out, lacks quite a bit of the structure we normally attribute to it. In particular, I will consider two programs in quantum gravity--loop quantum gravity and causal set theory--in order to make this point evident. I do this by showing how what can reasonably be interpreted as that which corresponds to, or gives rise to, 'space' lacks several of what are naturally taken to be essential properties of physical space.

22 September 2012, Workshop 'What is really possible 2 (WIRP-2): Logical and philosophical aspects of real possibility', Konstanz:

**Humean possibilities**

**Abstract:** This talk will contend that all modality is grounded in actuality, and in actuality only. 'Real' possibility can only ever arise a posteriori out of building blocks we collect in our interactions with the actual world. The theories we thus construct span ranges of possibilities, which do not physically exist but are 'real' to the extent to which we accept the theory on which they are based. Humans and other epistemically restricted agents do not have access to the entire 'Humean mosaic' of occurrent facts and are thus compelled to 'theorize' about the world they inhabit, i.e., they must hypothesize about its regularity structure. From this hypothesizing emerges the modality we ascribe to our world. Modality, on this view, is 'de theoria' rather than either 'de re' or 'de dicto'. The view sketched here thus rejects both modal realism, de re essentialism, and necessitarian combinatorialism, as well as the empiricist position which takes necessity to arise from stipulated semantic conventions.

18 September 2012, GAP8, Konstanz:

**When the actual world is not even possible**

14 September 2012, Institute for Theoretical Physics, University of Bern:

**The measurement problem in non-relativistic and relativistic quantum theories**

**Abstract:** The measurement problem (MP) in (non-relativistic) quantum mechanics can be understood as the inconsistency of the following three theses: (i) the wave function completely describes the quantum state, (ii) the evolution of the wave function is completely determined by its Schrödinger evolution, and (iii) measurements have determinate outcomes. This characterization lends itself naturally to categorizing solutions to the MP denying one of the theses as hidden-variable theories, collapse theories, and many-worlds theories, respectively. All of these solutions of the MP face their own difficulties. This talk will explore, in particular, the issues arising in attempts to transpose them into relativistic quantum (field) theories, where the MP still arises (as will be argued).

10 September 2012, First International Conference on Logic and Relativity: honoring István Németi's 70th birthday, Budapest:

**A prolegomenon to a quantum-information-theoretic complement to a general-relativistic implementation of a beyond-Turing computer**

**Abstract:** There exists a growing literature on the so-called physical Church-Turing thesis in a relativistic spacetime setting. The physical Church-Turing thesis is the conjecture that no computing device that is physically realizable (even in principle) can exceed the computational barriers of a Turing machine. By suggesting a concrete implementation of a beyond-Turing computer in a spacetime setting, István Németi and his collaborators Gábor Etesi and Gyula Dávid have shown how an appreciation of the physical Church-Turing thesis necessitates the confluence of mathematical, computational, physical, and indeed cosmological ideas. In this talk, I will honour István's seventieth birthday, as well as his longstanding interest in, and his seminal contributions to, this field going back to as early as 1987 by offering a prolegomenon to how the concrete implementation in proposed by Németi and collaborators may or may not be complemented by a quantum-information-theoretic communication protocol between the computing device and the logician who sets the beyond-Turing computer a task such as determining the consistency of Zermelo-Fraenkel set theory. This suggests that even the foundations of quantum theory and, ultimately, quantum gravity may play an important role in determining the validity of the physical Church-Turing thesis.

22 June 2012, 5th International Lauener Symposium on Analytical Philosophy in Honour of Professor Hilary Putnam, Bern:

**Putnam looks at quantum mechanics (again and again)**

**Abstract:** Hilary Putnam (1965, 2005) has argued that from a realist perspective, quantum mechanics stands in need of an interpretation. Ironically, this hypothesis may appear vulnerable against arguments drawing on Putnam's own work. Nancy Cartwright has recently urged that his 1962 essay on the meaning of theoretical terms suggests that quantum mechanics needs no interpretation and thus stands in tension with his claim of three years later. She furthermore contends that this conflict should be resolved in favour of the earlier work, as quantum mechanics, like all successful theories, does not need an interpretation. The first--longer--part of the talk deflates both of these objections. The second part addresses and evaluates Putnam's own assessments of the main interpretative options available in 1965 and 2005. Although we may disagree on some aspects, his pessimistic conclusion will come out largely unscathed. I will close by briefly stating the historical relevance of this work.

8 October 2011, 3rd Conference of the European Philosophy of Science Association, Athens:

**No categorical support for radical ontic structural realism** (contributed paper, with Vincent Lam)

**Abstract:** Radical ontic structural realism (ROSR) maintain that the world ultimately consists of 'free-standing' physical relations without relata. ROSRers have struggled to convert this idea into a functioning metaphysics adapted to fundamental physics because the theories of the latter make overt reference to objects. Jonathan Bain (2011) has recently argued that category theory offers a suitable framework for formulating these theories in a way which cleanses them of objects and thus realizes the ROSRer's vision. To make good on this claim, Bain considers the case of the category-theoretic extension of general relativity. The claim that spacetime points are eliminated relies on the fact that the algebraic counterparts of manifold points cannot be defined within the category-theoretic framework. The trouble is that while the reference to spacetime points is indeed eliminated, this is not the case for any physical objects whatsoever. Furthermore, it seems as if category-theoretic formulations prove to be impotent in determining or describing the structure of particular models of the theory, particularly in a way that connects with experimental practice. Category theory gets a beautifully general and unified handle on fundamental physical theories at the expense of being blinded as to the structure of the objects of a category--it can't see 'within' them. While we consider the questions raised by Bain and the solutions he offers to be of great foundational value, we submit that the radical shouldn't expect that invoking category theory will alleviate--let alone resolve--her ailings.

19-26 July 2011, 14th Congress of Logic, Methodology and Philosophy of Science, Nancy:

**How large is a structuralist universe?** (contributed paper)

**Abstract:** This paper evaluates how the symmetry problem that has recently been raised (Wüthrich 2009) for Friedmann-Lemaitre-Robertson-Walker spacetimes in general relativity (GR) and according to which spacetime structuralism seems committed to the claim that the universe has no spatial extension---it consists of one lonely point---is transposed into the causal sets approach to quantum gravity. It is shown that the introduction of an irreflexive relation does not, in general, resolve the difficulty. Even though the problem resurfaces in causal set theory, however, there is reason to believe that---unlike in GR---it does not affect physically relevant models of the theory.

29 June 2011, Universitat Autònoma de Barcelona:

**The physics and metaphysics of no time and space**

**Abstract:** Approaches to quantum gravity often involve the disappearance of space and time at the fundamental level. I will use this fact to construct a potentially fatal objection to David Lewis's pluriverse and thus to illustrate its profound consequences for philosophy. But before we abandon our cherished metaphysical theories, we should convince ourselves that a physical theory denying the existence of space and time does not suffer from empirical incoherence in that this denial undermines our justification for believing it to be true. Using the conceptually tractable example of causal set theory, I will show how the threat of empirical incoherence is blocked by giving an account of how space and time emerge from the fundamental structure.

23-24 June 2011, Workshop on Decoherence and No-Signalling, University of Bern:

**To what extent is relativistic QFT relativistic?**

28 March 2011, University of Geneva:

**Crushing the resurgence: presentism meets its physical fate**

18 March 2011, Utrecht University:

**The structure of causal sets** (Colloquium)

**Abstract:** In this talk, I will introduce a particularly simple approach to quantum gravity, causal set theory. This approach has been pioneered in the 1970s by Myrheim and 't Hooft and developed by a number of physicists around Rafael Sorkin at Syracuse since the 1980s. Even though the approach is still rather inchoate and requires substantial work in order to qualify as a genuine competitor for a full quantum theory of gravity, its conceptual simplicity offers a number of advantages of interest to physicists and philosophers alike. First, it provides a kind of conceptual laboratory for physicists to learn how various basic principles interact. Second, it promises to shed light on the important connection between a fundamentally discrete structure and relativistic spacetimes and on how the latter can emerge from the former. Third, it offers a paradigmatic case of a physical theory predestined to be interpreted in structuralist terms. I will explain the main motivations for structuralism in fundamental physics and show how philosophical interest in structuralism and technical issues in causal set theory interact.

16 March 2011, Annual Meeting of the *Deutsche Physikalische Gesellschaft*, Dresden:

**How the emergence of spacetime might save the structuralist** (invited talk)

**Abstract:** Spacetime structuralism maintains that spacetime is a relational complex consisting of spacetime points and the spatiotemporal relations they stand in. These points lack intrinsic properties and accrue their identity only by virtue of the position they inhabit in the relational complex. This view faces the difficulty that for highly symmetric spacetimes, such as the Friedman-Lemaitre-Robertson-Walker spacetimes, vast classes of points are identical as their relational positions are indiscernible (Wüthrich 2009). In one conceptually very clear approach to formulating a quantum theory of gravity, the so-called causal set theory, the fundamental structures--the causal sets--ought to be interpreted structurally, too. Analogously, structuralist readings of causal sets confront the challenge of distinguishing relationally indiscernible elements of highly symmetric causal sets. Compared to classical general relativity, however, the problem turns out to be much less severe. The reason for this has to do with the way relativistic spacetimes emerge from causal sets. My talk shall elucidate this emergence and how it acts against the symmetries requisite to challenge the structuralist.

10 March 2011, University of Oxford:

**The physical fate of presentism** (Philosophy of Physics Research Seminar)

**Abstract:** There has been a recent spate of essays defending presentism, the view in the metaphysics of time according to which all and only present events or entities exist. What is particularly striking about this resurgence is that it takes place on the background of the significant pressure exerted on the position by the relativity of simultaneity asserted in special relativity, and yet in several cases invokes modern physics for support. I classify the presentist arguments into a two by two matrix depending on whether they take a compatibilist or incompatibilist stance with respect to both special relativity in particular and modern physics in general. I then review and evaluate what I take to be some of the most forceful and intriguing presentist arguments turning on modern physics. Although nothing of what I will say eventuates its categorical demise, I hope to show that whatever presentism remains compatible with empirical facts and our best physics is metaphysically repugnant.

9 March 2011, University of Bristol:

**To the Planck scale and back: On the emergence of spacetime in quantum theories of gravity** (Colloquium Physics and Philosophy)

**Abstract:** Among the most striking features of most approaches to quantum gravity is their claim that spacetime as we know and love it disappears in one form or another from the fundamental furniture of the world. I shall explicate this disappearance, using the examples of loop quantum gravity and causal set theory. For any quantum theory of gravity to succeed, it must offer an explanation of how the smooth, classical spacetimes of general relativity re-emerge in the appropriate low-energy limit of the fundamental theory. I will show how similar philosophical and technical problems arise in the case of spacetime as they do for the emergence of classicality from ordinary quantum mechanics. The case at hand, however, poses the additional threat of empirical incoherence, as the absence of space and time deprives us of apparent prerequisites for the empirical testing of physical theories.

8 March 2011, University of Bristol:

**The fate of presentism in modern physics** (Workshop on the philosophy of time)

**Abstract:** There has recently been a remarkable resurgence of presentism in the philosophy of time. What is of particular interest in this renaissance is that a number of recent arguments supporting presentism are crafted in an untypically naturalistic vein, breathing new life into a metaphysics of time with a bad track record of co-habitation with modern physics. Against this trend, I will argue that the pressure on presentism exerted by special relativity and its core lesson of Lorentz symmetry cannot easily be shirked. A categorization of presentist responses to this pressure is offered. As a case in point, I analyze a recent argument by Monton (2006) presenting a case for the compatibility of presentism with quantum gravity. Monton claims that this compatibility arises because there are quantum theories of gravity that use fixed foliations of spacetime and that such fixed foliations provide a natural home for a metaphysically robust notion of the present. A careful analysis leaves Monton's argument wanting. In sum, the prospects of presentism to be alleviated from the stress applied by fundamental physics are faint.

7 March 2011, Sigma Club, London School of Economics:

**To the Planck scale and back: On the emergence of spacetime in quantum theories of gravity**

**Abstract:** Among the most striking features of most approaches to quantum gravity is their claim that spacetime as we know and love it disappears in one form or another from the fundamental furniture of the world. I shall explicate this disappearance, using the examples of loop quantum gravity and causal set theory. For any quantum theory of gravity to succeed, it must offer an explanation of how the smooth, classical spacetimes of general relativity re-emerge in the appropriate low-energy limit of the fundamental theory. I will show how similar philosophical and technical problems arise in the case of spacetime as they do for the emergence of classicality from ordinary quantum mechanics. The case at hand, however, poses the additional threat of empirical incoherence, as the absence of space and time deprives us of apparent prerequisites for the empirical testing of physical theories.

22 January 2011, Leibniz University Hannover:

**Die Nachrichten vom Ableben des Determinismus sind stark übertrieben**

6 November 2010, 22nd Meeting of the Philosophy of Science Association, Montreal:

**The emergence of spacetime in quantum theories of gravity** (symposium, co-organized with Nick Huggett; with Sean Carroll and Tim Maudlin)

**Abstract:** The search for a quantum theory of gravity has led many of the competing approaches to a revolutionary view according to which spacetime is no longer fundamental but instead "emerges" from a deeper, non-spatio-temporal structure. The proposal is that, at bottom, physical objects and processes are not "in" space and time; rather, inversely, space and time somehow arise only from underlying physical objects and processes. If borne out, this suggestion would shatter our conception of physical existence. Despite this obvious conceptual challenge, philosophy of science has so far remained almost entirely silent about what exactly the fundamental shift consists in, whether it is coherent, and, if so, about what it entails for our worldview. The proposed symposium addresses these issues and is intended to initiate a critical analysis of them.

5 November 2010, History of Science Society Annual Meeting, Montreal:

**A giants' singular struggle: Einstein, de Sitter, Weyl, and Klein's debate on an alleged singularity** (contributed paper)

**Abstract:** This paper explores an early episode in the history of Albert Einstein's general theory of relativity that turned on the interpretation of singularities. Einstein, in an attempt to restore stability to an otherwise imploding universe, introduced his infamous cosmological constant into his equations in 1917. Within a week, the Dutch astronomer Willem de Sitter produced a spacetime model of the modified equations. To Einstein's dismay, de Sitter's model violated his beloved "Mach's principle''. Consequently, Einstein set out to nullify de Sitter's model. The ensuing debate between Einstein and de Sitter, which also draws in, one by one, the mathematicians Hermann Weyl, Felix Klein and, from afar, David Hilbert quickly degenerates into what might uncharitably be depicted as a comedy of errors. For over a year, Einstein, de Sitter, and Weyl fail to recognize that the singularity--a mathematical pathology--they believe to have identified in de Sitter's model is a mere artefact of an unfortunate choice of coordinates in a perfectly regular geometrical space, akin to the perfectly regular origin of a polar coordinate system. This paper seeks to explain this failure and its persistence over a surprisingly long period by offering an analysis of their correspondence, the mathematical traditions that they have been trained and operate in, and--paradoxically--their precipitant desire to endow the mathematical structure with physical meaning. A case will be made that Einstein simply lacked, due to his own negligence, the requisite mathematical training, while Weyl was led astray by Hilbert's blatantly inadequate characterization of spacetime singularities.

4 November 2010, 22nd Meeting of the Philosophy of Science Association, Montreal:

**Determinism and general relativity** (contributed paper, with Chris Smeenk)

**Abstract:** The present article investigates the fate of determinism in the context of general relativity (GR) and undertakes an outlook beyond the classical theory into semi-classical and full quantum theories of gravity. Central to this investigation is an analysis of the status of global hyperbolicity, a necessary condition for determinism in GR. While global hyperbolicity may simply fail to be true of all physically reasonable models in GR, we are particularly interested in the appraisal of imposing global hyperbolicity by *fiat*, and justifying such a stipulation, vs. establishing it from the resources of the theory itself or from weaker assumptions, as is done in attempts to prove cosmic censorship theorems.

19 October 2010, Osher Lifelong Learning Institute, UCSD:

**Space, time, and spacetime, part II: emergent spacetime in quantum theories of gravity** [PDF]

**Abstract:** The next revolution in the physics of spacetime involves quantum theories of spacetime, or "quantum gravity". Amazingly, many approaches to quantum gravity suggest that space and time are not fundamental ingredients of the world. Instead, they "emerge" from deeper physics that does not rely on, or even permit, the (fundamental) existence of space and time. Surely, if true, this shatters our current conception of the universe and our place within it, along with many central philosophical ideas. I will try to convince you that this radical claim deserves to be taken seriously and urge that philosophers engage with this latest revolution in spacetime physics.

5 October 2010, Osher Lifelong Learning Institute, UCSD:

**Space, time, and spacetime, part I: from Newton's bucket to Einstein's hole** [PDF]

**Abstract:** The first part of this mini-series will discuss the traditionally central problems in the philosophical analysis of physical space and time. We will start with the famous debate between Leibniz and Newton which focused on the issue of whether or not space and time are substances that exist independently of any objects that they may contain. The advent of relativity in the early twentieth century challenges our most basic intuitions about space and time. Space and time, relativity tells us, are not independent from one another, they are curved, and they dynamically interact with the energy-matter content of the universe. After discussing some of these relativistic lessons, I will present Einstein's argument thought to have ruled out the view that spacetime is a substance. I will close by analyzing its consequences and by considering a spacetime "structuralism" as a potential resolution of the old debate.

22 July 2010, Workshop on Foundations of Spacetime Theories, University of Wuppertal:

**Raiders of the lost spacetime**

**Abstract:** Spacetime as we know and love it is lost in most approaches to quantum gravity. For many of these approaches, as inchoate and immature as they are, one of the main challenges is to relate what they take to be the fundamental non-spatiotemporal structure of the world back to the classical spacetime of general relativity. Time permitting, I will discuss this problem and sketch potential solutions for two approaches to quantum gravity, loop quantum gravity and causal set theory.

13 July 2010, Conference on Emergence in Physics, Institute of Philosophy, London:

**The emergence of spacetime: what it is not and what it might be**

1 July 2010, Department of Philosophy, University of Konstanz:

**Auf der Suche nach der verlorenen Raumzeit** (in German)

**Abstract:** Der erste Teil dieses Vortrags wird den Niedergang und Fall der Zeit in der modernen Physik in den letzten gut hundert Jahren nachzeichnen. Der Tiefpunkt wird dabei in zeitgenössischen Theorien der Quantengravitation erreicht: viele dieser Theorien suggerieren, dass auf der fundamentalsten Ebene der physikalischen Beschreibung unserer Welt weder Raum noch Zeit vorkommen, sondern dies erst auf einer "höheren" Ebene tun. Raum und Zeit sind demnach "emergente" Phänomene. Der zweite Teil des Vortrags soll klären, wie das zu verstehen ist. Schliesslich soll die Kohärenz einer Grundlagenphysik frei von Raum und Zeit diskutiert und die weit reichenden Konsequenzen der Emergenz der Raumzeit für die Philosophie aufgezeigt werden.

18 March 2010, Osher Lifelong Learning Institute, UCSD:

**Puzzling aspects of quantum mechanics, part II: Non-locality** [PDF]

4 March 2010, Osher Lifelong Learning Institute, UCSD:

**Puzzling aspects of quantum mechanics, part I: The measurement problem** [PDF]

20 November 2009, Department of Philosophy, UCSD:

**Space and time do not exist, fundamentally**

29 October 2009, Institute for History and Foundations of Science, University of Utrecht:

**An old and a new theorem in quantum mechanics and what they don't imply for indeterminism**

27 October 2009, Zentrale Einrichtung für Wissenschaftsheorie und Wissenschaftsethik (ZEWW), Leibniz University of Hannover:

**Raum und Zeit in der modernen Physik: Auf der Spur von Leibniz' Relationalismus in der Quantengravitation** (in German)

23 October 2009, 2nd Meeting of the European Philosophy of Science Association, Amsterdam:

**Demarcating presentism**

16 October 2009, LARSIM, Commissariat à l'Énergie Atomique, Orme des Merisiers (France):

**Let's go for a ride on a time machine**

15 October 2009, Institut d'Histoire and de Philosophie des Sciences et des Techniques (IHPST), Paris:

**An old and a new theorem in quantum mechanics and what they don't imply for indeterminism**

13 July 2009, British Society for the Philosophy of Science Annual Conference, University of East Anglia, Norwich (UK):

**The hole and the loop**

11 July 2009, Joint Session of the Aristotelian Society and the Mind Association, University of East Anglia, Norwich (UK)

**Demarcating presentism**

9 July 2009, Geneva Summer School in the Philosophy of Physics, Arolla (Switzerland):

**Gleason's theorem, Conway and Kochen's "free will theorem," and indeterminism**

7 July 2009, Geneva Summer School in the Philosophy of Physics, Arolla (Switzerland):

**Some technicalities of quantum mechanics**

6 July 2009, Geneva Summer School in the Philosophy of Physics, Arolla (Switzerland):

**Bell's theorem, the GHZ theorem, and non-locality**

21 June 2009, Osher Lifelong Learning Institute, UCSD:

**A philosopher looks at science**

27 March 2009, Department of Philosophy, University of Nevada, Las Vegas:

**Demarcating presentism**

19 December 2008, eidos workshop "Time and Time Experience", University of Geneva:

**Comment on E.J. Lowe's "Experience of change and change of experience"**

15 December 2008, eidos meeting, University of Fribourg (Switzerland):

**How properties and relations are had in a Lorentz-covariant way**

8 November 2008, 21st Biennial Meeting of the Philosophy of Science Association, Pittsburgh:

**Challenging the spacetime structuralist**

6 October 2008, Science Studies colloquium, UCSD:

**Who wants to take a ride on a time machine?**

3 September 2008, 8th eidos workshop, University of Geneva:

**Conceivability and possibility: a tutorial**

19 August 2008, 7th eidos workshop, University of Geneva:

**Demarcating a position: is presentism really distinct from eternalism?**

1 August 2008, Geneva Summer School in the Philosophy of Physics, Arolla (Switzerland):

**Einstein's nemesis conquered at last?**

31 July 2008, Geneva Summer School in the Philosophy of Physics, Arolla (Switzerland):

**Let's take a ride on a time machine**

15 July 2008, eidos Metaphysics Conference, Geneva:

**The many lives of presentism**

14 June 2008, Third International Conference on the Nature and Ontology of Spacetime, Concordia University, Montréal:

**No presentism in quantum gravity**

21 March 2008, Pacific APA, Pasadena:

**Comment on Bradford Skow's "Local and global relativity principles"**

21 December 2007, 3rd eidos workshop, University of Geneva:

**Challenging the spacetime structuralist**

12 September 2007, 2nd eidos workshop, University of Geneva:

**The end of time**

28 January 2007, Southern California Group in the Philosophy of Physics, UC Irvine:

**Loop quantum big bang**

30 June 2006, Department of Philosophy, University of Florence:

**Presentism, becoming, and the Rietdijk-Putnam argument reconsidered**

17 June 2006, Sigma Xi, Swiss Chapter, Bern:

**Albert Einstein's annus mirabilis of 1905 in Bern and its implications for the world**

24 March 2006, Philosophy of Physics Meeting, Centre Romand for Logic, History and Philosophy of Science, University of Lausanne:

**Challenging the spacetime structuralist**

27/30 January 2006, Department of History and Philosophy of Science, Indiana University; Department of Philosophy, Ohio State University:

**Approaching the Planck scale from a generally relativistic point of view**

26 January 2006, Department of History and Philosophy of Science, Indiana University:

**Singularities, time machines, and the future of gravity**

13 January 2006, Colloquium in History and Philosophy of Science, University of Bern:

**Quantum gravity from a generally relativistic point of view**

18 November 2005, Naturwissenschaftliche Gesellschaft (Society of Natural Sciences), Winterthur (Switzerland):

**Mit Einstein auf Zeitreisen** (in German)

10 November 2005, Cantonal Meeting of College Teachers of Information Science, Mathematics, and Physics, Thun (Switzerland):

**Das Schiffsmastexperiment--eines der wichtigsten Gedankenexperimente der Physik der frühen Neuzeit** (in German)

4 September 2005, Philosophical and Foundational Issues in Quantum Theory, European Science Foundation Scientific Network Conference, Budapest:

**General covariance in quantum general relativity**

8 August 2005, Logic in Hungary, Budapest:

**Misner spacetime as a counterexample against Krasnikov's no-go theorem?**

5 August 2005, Logic in Hungary, Budapest:

**Let's build a time machine**

30 July 2005, New PhDs' Seminar in Physics and Philosophy, Department of Philosophy, University of Illinois at Chicago:

**Foundational issues in loop quantum gravity**

15 July 2005, Philosophy, Probability and Physics Lecture Series, University of Konstanz:

**Costs and gains of a Hamiltonian formulation of general relativity**

30 June 2005, Chambre of Commerce and of Industry of the Canton of Bern:

**Die Bedeutung von Albert Einsteins Annus mirabilis 1905 für Bern--und für die Welt** (in German)

5 April 2005, Astronomical Society Bern:

**Einstein und de Sitter: Grenzen der Mathematik oder viel Lärm um nichts?** (in German)

12 March 2005, Seventh International Conference on the History of General Relativity, La Orotava, Tenerife:

**Einstein's nemesis conquered at last? The fate of singularities in loop quantum cosmology**

1 December 2004, Albert Einstein Colloquium for college teachers, University of Bern:

**Mit Einstein auf Zeitreisen** (in German)

19 November 2004, 19th Biennial Meeting of the Philosophy of Science Association, Austin:

**To quantize or not to quantize: fact and folklore in quantum gravity**

14 June 2004, Rendez-vous des Thésards, Centre de Physique Théorique, Marseille:

**Quantum gravity and the 3D vs. 4D controversy**

12 May 2004, International Conference on the Ontology of Spacetime, Concordia University, Montréal:

**Quantum gravity and the 3D vs. 4D controversy**

30 January 2004, Séminaire de Travail en Physique Mathématique, Centre de Physique Théorique, Marseille:

**Conceptualizing time machines**

16 January 2004, Colloquium for History and Philosophy of Science, University of Bern:

**Vorwärts im Lichtkegel und zurück in die Vergangenheit: Zeitreisen und Zeitmaschinen in der modernen Physik** (in German)

21 November 2003, Philosophy Colloquium, University of Waterloo (Canada):

**Philosophical problems at the Planck scale**

12 April 2003, UCSD Graduate Student Philosophy Conference, University of California, San Diego:

**Does modern physics permit the operation of time machines?**

5 April 2003, Midwest Junto for the History of Science, University of Minnesota, Minneapolis:

**An early episode in the perennial struggle with singularities**

18 March 2003, Grad Expo, University of Pittsburgh:

**Does modern physics permit the operation of time machines?**

16 March 2002, Fourth Annual Carnegie Mellon University/University of Pittsburgh Graduate Philosophy Conference:

**No-go theorems for time machines in classical general relativity**