The Colloquium’s goal is to provide a window into the state-of-the-art in the foundations of spacetime physics. It brings together top-level philosophers and physicists willing to share their current research with an international audience.

**Winter Semester Program:**

**2 October **(16:00-18:00 CEST)** – Carlo Rovelli** (Aix-Marseille University) – “Why can we decide what we shall do tomorrow, but we cannot decide what we did yesterday? Time reversibility and the physics of an agent”. (Watch on YouTube)

Abstract:

*Much of the confusion in the philosophy of spacetime stems from the failure to recognize that ‘space’ and ‘time’ (and a fortiori ‘spacetime’) are layered concepts used to denote a variety of different notions. I disentangle the different uses of these words, and discuss what we understand about the different layers in contemporary physics. In the second part of the talk I discuss in particular the arrow of time: the reason why the past is fixed and the future open. I show that we remember the past (not the future) and we can affect the future (not the past) because of the entropy gradient. Hence the fact that the past is fixed and the future is open is a macroscopic, statistical, phenomenon.*

**16 October **(16:00-18:00 CEST)** – J. Brian Pitts**
(University of Lincoln, University of Cambridge, University of South
Carolina) – “Change in observables in Hamiltonian general relativity”.

Abstract:

*Since the 1950s it has been claimed that change is missing in the formulation of General Relativity most straightforwardly quantized, the Hamiltonian (“canonical”) formulation. In particular, “observables” are said to be constants of the motion and to require integration over the whole universe. This talk gives a technical evaluation of that claim and a sketch of the trajectory of canonical GR co-founder Peter Bergmann’s thoughts on the topic. Technically one finds that the typical notion of observables (as having 0 Poisson bracket with each first-class constraint) contains 2 suspect ingredients. One is the use of first-class constraints separately rather than as a team, the Rosenfeld-Anderson-Bergmann-Castellani “gauge generator” G, which preserves Hamilton’s equations. Use of separate constraints violates Hamiltonian-Lagrangian equivalence, a principle that Bergmann claimed to uphold. The second suspect ingredient, having 0 Poisson bracket (as opposed to a suitable nonzero Lie derivative) under coordinate gauge transformations, in its usual form contradicts daily experience and the principle that equivalent theories have equivalent observables. A reformed definition of observables uses the gauge generator G and takes them to be invariant under internal gauge transformations but only covariant under coordinate transformations. This definition makes the metric and the electromagnetic field strength observables for Einstein-Maxwell: observables are local fields that vary spatio-temporally. Change is essential time dependence, cashed out technically as the lack of a time-like Killing vector field or (with matter) an analogous condition. Change in the presence of spinors and Yang-Mills (weak & strong forces) fields is sketched. Classically, change in Hamiltonian GR is just where it should have been, at least for solutions of Einstein’s equations. Quantum imposition of the constraints is another matter.*

**30 October **(16:00-18:00 CET)** – James Read** (University of Oxford) – “Shifts and reference”. (Watch on YouTube)

Abstract:

*Maudlin’s ‘metric essentialist’ response to the hole argument of general relativity is well-known, but differs strikingly from his response to what is often regarded as being the analogous problem in the context of Newtonian gravity (namely, the possibility of a Leibnizian static shift), which centres around a certain epistemological argument. In this talk, I explicate the reasons underlying this divergence of responses. I then apply recent work from the philosophy of language in order to assess Dasgupta’s arguments that Maudlin’s epistemological argument given in response to the static shift is unsuccessful, which are based upon the notion of ‘inexpressible ignorance’. I argue that Dasgupta’s reading of Maudlin is not quite correct; rather, Maudlin should be read as endorsing both (a) Hawthorne and Manley’s ‘liberalism about reference and singular thought’, and (b) Kaplan’s conception of indexicals. That said, Dasgupta’s point can still be made by rejecting either (a) or (b). Finally, I analyse how the epistemological argument plays out in the context of the gauge redundancy in electromagnetism, finding that the situation is interestingly different from the spacetime case. (Based upon joint work with Bryan Cheng.)*

**13 November **(16:00-18:00 CET)** – Karen Crowther** (University of Oslo) and **Sebastian De Haro** (University of Amsterdam) – “The role of singularities in the search for quantum gravity”. (Watch on YouTube)

Abstract:

*Singularities in general relativity and quantum field theory
(i.e., the standard model) not only motivate the search for a
more-fundamental theory (quantum gravity), but also serve to
characterise this new theory and shape expectations of what it is to
achieve. But what can singularities in current theories tell us about
new physics? We explore the different possible interpretations of these
singularities in regards to their significance for quantum gravity. In
doing so, we aim not only to gain insight into the theory being sought,
and to articulate some of the constraints upon it, but also to better
understand the nature of fundamental physical theories more generally,
and the relationships between singularities and scientific explanation.*

**27 November** (16:00-18:00 CET) **– Claus Kiefer** (University of Cologne) – “Time in quantum gravity”.

Abstract:

*Time is absolute in standard quantum theory and dynamical in
general relativity. The combination of the two theories into a theory of
quantum gravity thus leads to a `problem of time’. In my talk, I shall
investigate those consequences for the concept of time that can be drawn
without a detailed knowledge of quantum gravity. The only assumptions
are the experimentally supported universality of the superposition
principle and the recovery of general relativity in the classical limit.
Among the consequences are the fundamental timelessness of quantum
gravity, the approximate nature of a semiclassical time, and the
correlation of entropy with the size of the Universe. The last
consequence gives rise to the irreversibility of our world.*

**11 December **(16:00-18:00 CET) **– Radin Dardashti** (University of Wuppertal) – “The rise and fall of scientific problems”. (Watch on YouTube)

Abstract:

*The everyday practice of scientists is to a large extent
determined by the scientific problems they are confronted with. The
conceptual analysis of scientific problems and how they change,
therefore, may allow for a more fine-grained investigation of the
development of a scientific discipline. In this talk I discuss what
constitutes a scientific problem, what its elements are and how they
change, following and building on the work of Thomas Nickles. I will
illustrate the advantages of a more problem-focused approach in
understanding the development of modern particle physics and shed some
light on recent debates about the naturalness problem and whether it
constitutes a “genuine” problem.*

**8 January **(16:00-18:00 CET)** – Karim Thébault**
(University of Bristol) – “Poincaré, dark energy, and the deadly robots
of Krikkit: Solving the problem of time via superpositions of the
cosmological constant”. (Watch on YouTube)

Abstract:

*Henri Poincaré, in a strangely neglected passage towards the end
of his monumental essay on `Relative and Absolute Motion’ (Science and
Hypothesis, 1905 Chapter 7), appeals to the idea of a planet entirely
secluded from the rest of the universe by clouds to argue that ‘as long
as nature has secrets’ the distinction between constants of nature and
constants of motion will remain ‘highly arbitrary and always precarious’
(p.87). To what extent do such arguments support a relationship between
a scientist’s epistemic access to different scales and the
categorisation of constants? What are the implications of this view for
modern cosmology, in particular the interpretation of the cosmological
constant, the nature of time, and the quantization of gravity? And what
does any of this have to do with Robots? In this talk I will attempt to
answer these and other related questions. (Based on work with Sean Gryb
(University of Groningen))*

P.S.: After the talk, Professor Lévy-Leblond (University of Nice) wanted to share his past work on this subject. You can find it here and here.

**22 January **(16:00-18:00 CET)** – Vera Matarese** (University of Bern) – “Space the many substances”. (Watch on YouTube)

Abstract:

*The view that loop quantum gravity’s
spin-networks represent concrete atoms of space is proposed in Vassallo
& Esfeld (2014), which adopts a primitive ontology approach to
spacetime. Rovelli (2015), on the contrary, warns against this literal
interpretation, and regards the ‘chunks’ of space represented by
spin-networks as ‘modes of interactions’. In my talk, I will not engage
in the legitimacy of the interpretation of spin-networks as concrete
atoms of space from a physical perspective. I will rather spell out the
metaphysical advantages and disadvantages of endorsing such a view and
discuss conceptual issues by drawing on the metaphysical debate on the
atomistic view of spacetime. My conclusion will be that the loop quantum
gravity’s discrete model of space, according to which space is a
collection of many substances—‘the atoms of space’—has certain metaphysically advantageous consequences that have been hitherto overlooked.*

**Summer Semester Program:**

**26 February **(17:00-19:00 CET)** – Sean Carroll **(Caltech) – “From Quantum Mechanics to Spacetime”. (Watch on YouTube)

Abstract:

*Nine decades in, the foundations of quantum mechanics remain
mysterious. Meanwhile, modern physicists puzzle over how to reconcile
quantum mechanics with gravity. I will suggest that these problems are
related, and that a promising strategy suggests itself: rather than
“quantizing gravity,” we should look for gravity within quantum
mechanics. This approach has interesting consequences for how we think
about the nature of space and time.*

**12 March **(17:00-19:00 CET)** – Daniele Oriti **(LMU Munich) – “Which spacetime emergence in quantum gravity?”. (Slides – Watch on YouTube)

Abstract:

*We discuss several issues in quantum gravity, related to the notion of spacetime being emergent and not fundamental, focusing on their conceptual aspects more than their (possible) technical solution:*

*– what it means to have spacetime as merely emergent and how this goes beyond what classical GR tells us already;*

*– which different types of spacetime emergence we can envisage in quantum gravity, among which the picture of spacetime emerging from suitable coarse graining, and the suggestion that this involves a phase transition of the underlying quantum gravity system;*

*– whether and how such phase transition (geometrogenesis) can be understood as a proper physical process, and how it can enter our picture of the evolution of the universe.*

*We illustrate these issues, the related proposals, and their possible realizations, with examples taken from quantum gravity formalisms like tensorial group field theory and the related loop quantum gravity, and with a focus on emergent spacetime physics in a cosmological context.*

**26 March **(17:00-19:00 CET)** – Sabine Hossenfelder **(Frankfurt Institute for Advanced Studies) – “Superdeterminism”. (Watch on YouTube)

Abstract:

*In this talk I will explain what superdeterminism is, why the objections that have so far been raised to it are unjustified, and why it is the most promising way to solve the measurement problem.*

**09 April **(17:00-19:00 CEST)** – Erik Curiel **(LMU Munich, Harvard University) – “Interaction and Evolution in Classical Mechanics and in Quantum Mechanics”. (Watch on YouTube)

Abstract:

*I describe a few theorems that show that, in classical mechanics
(Newtonian and Lagrangian mechanics), the intrinsic structure of the
dynamics naturally distinguishes the concept of interaction from that of
evolution, and, correlatively, position from momentum. One gets for
free, moreover, a characterization of “free” evolution (or “isolation”).
This is not the case in Hamiltonian and quantum mechanics. I show that
the theorems allow one to entirely reconstruct the full 4-dimensional
spacetime structure of Newtonian physics from classical dynamics, again
in a way not possible in quantum theory. Thus, the dynamics of quantum
theory needs to be hooked up to background spacetime structure “by hand”
in a way not required in classical mechanics. I conclude by discussing
what I take to be the lessons for all this with regard to the
Measurement Problem: not only is the idea of “measurement” problematic
in quantum theory, but the entire idea of “interaction” per se in
quantum theory is more deeply problematic than has been recognized.
There may also be some lessons for quantum gravity somewhere in here,
but who really knows?*

**23 April **(16:00-18:00 CEST)** – Marco Giovanelli **(University of Turin) – “Special Relativity as a Theory of Principles. Reflections On Einstein’s Distinction between Constructive and Principle Theories”. (Watch on YouTube)

Abstract:

*In a 1919 article for the Times of London, Einstein declared relativity theory to be a ‘principle theory,’ like thermodynamics, rather than a ‘constructive theory,’ like the kinetic theory of gases. Over the last decades, Einstein’s distinction has attracted considerable attention. Philosophers have often considered it as Einstein’s fundamental insight into the nature of spacetime, historians as an unoriginal variation on the 19th-century theme. The paper argues that both stances grasp only part of the truth. To understand Einstein’s “theories of theories” properly, one has to disentangle the two threads of its fabric. Einstein introduced at the same time (a) classification of existing theories (b) classification of strategies for finding new theories. Unlike the usual physical theories, special relativity, like thermodynamics, does not directly attempt to construct models of specific physical systems; it provides empirically motivated and mathematically formulated criteria for such theories’ acceptability. After his early success, Einstein became convinced that, in general, instead of directly searching for new theories, it is often more convenient to search for the formal conditions that constraint the number of possible theories. It is indeed using this strategy that Einstein achieved most of his successes. The paper concludes that these two aspects of Einstein’s principles/constructive theories distinction are best framed by resorting to the opposition between ‘byproducts’ and ‘constraints’ (Lange). For Lorentz and Poincaré, the Lorentz-transformations were a by-product of the actual laws governing field and matter, as a feature that they happen to satisfy. Einstein elevated such coincidence into a constraint, a requirement that all possible laws of nature must satisfy. From this perspective, the relativity principle is not a categorical statement about the real but a modal statement about the possible. In this sense, the paper will defend the characterization as special relativity as a “principle theory”—providing general constraints on laws or theories of whatever nature—rather than as constructive theory—either about the material structure of rods and clocks (Brown) or about the geometrical structure of spacetime (Janssen). *

**07 May **(17:00-19:00 CEST)** – Tim Maudlin **(New York University) – “S = k ln(B(W)): Boltzmann entropy, the Second Law, and the Architecture of Hell”. (Slides –Watch on YouTube)

Abstract:

*The concept of entropy was introduced in the setting of classical thermodynamics, which was agnostic about the microscopic structure of matter. The advent of the atomic theory and the kinetic theory of heat allowed for the same phenomena to be approached from the standpoint of statistical mechanics, a plan originated by Maxwell and Boltzmann. But in order to “translate” or “reduce” classical thermodynamics into the language of statistical mechanics and the kinetic theory, one needs definitions of terms like “temperature” and “pressure” and “entropy” in the language of the microphysics. One famous such definition—commonly ascribe to Boltzmann but actually due to Planck—is S = k ln(W). I will propose that in a certain explanatory sense this is the wrong definition. And further, that the proper definition sheds light on the reversibility paradoxes and on the role—or lack of one—of the Past Hypothesis in the reduction of classical thermodynamics to statistical mechanics.*

**21 May **(17:00-19:00 CEST)** – Baptiste Le Bihan **(University of Geneva) – “Quantum Gravity and Mereology: Not So Simple”. (Watch on YouTube)

Abstract:

*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. (Joint work with Sam Baron)*

**28 May **(17:00-19:00 CEST)** – JB Manchak **(University of California, Irvine) – “On the (In?)Stability of Spacetime Inextendibility”. (Watch on YouTube)

Abstract:

*Within the context of general relativity, the “stability” of various spacetime properties has been one important focus of study. It has been argued that “in order to be physically significant, a property of space-time ought to have some form of stability, that is to say, it should be a property of ‘nearby’ space-times” (Hawking and Ellis 1973, p. 197). Questions concerning the stability of spacetime properties are often made precise using the so-called “C^k fine” topologies on any collection of spacetimes with the same underlying manifold. (The property of “stable causality” is often defined using the C^0 fine topology.) Here we review what is known concerning the (in)stability of spacetime properties within this framework. After considering some foundational results concerning causal properties (Hawking 1969; Geroch 1970) and a fascinating drama concerning geodesic (in)completeness (Beem et al. 1996), we focus on the property of spacetime inextendibility about which very little is known. Because inextendibility is defined relative to a background “possibility space” in the form of a standard collection of spacetimes, one can naturally consider variant definitions relative to other collections. (Some formulations of the “cosmic censorship” conjecture rely on such variant definitions of inextendibility.) We find that the stability of “inextendibility” can be highly sensitive to the choice of definition — even when attention is limited to definitions that are relative to “physically reasonable” collections of spacetimes. Indeed, it is not yet clear that there is a physically significant sense in which “inextendibility” is a stable property. We close by drawing attention to some precise open questions which could be explored to clarify the situation.*