Abstracts

What is a reference frame in General Relativity?

Nicola Bamonti (University of Geneva/SNS)

I argue that in General Relativity, the notions of reference frame and coordinate system must be distinguished. The former refers to physical systems which interact necessarily with the gravitational field, except for possible approximations, while the latter refers to a set of physically empty mathematical variables. This distinction is lost in pre-General-Relativistic physics, where we can choose as a reference frame a system of objects unaffected by and unable to affect the physical system under consideration. Therefore, we can make it coincide with a coordinate system, without requiring approximations. In contrast, within general relativistic physics we are not allowed to consider reference frames as “external”, non-dynamical objects, as we do in pre-General-Relativistic physics. Essentially, this is because no existing physical system is gravitationally neutral. There is no way to disregard the interaction between the gravitational field and the reference objects. The only way this could be done is through approximations on the material system constituting the reference frame itself. Consequently, in General Relativity a reference frame must have a distinct nature from that of a coordinate system, which is a set of mathematical variables that naturally have no dynamics whatsoever. Departing from the orthodox definitions given to separate a reference frame from a coordinate system, I propose a new criterion to distinguish between the two concepts in General Relativity, as well as a distinction between three ways to understand the notion of a reference frame, seen as a material system. In particular, I will distinguish between Idealized Reference Frames (IRFs), Physical Reference Frames (PRFs) and Real Reference Frames (RRFs), according to their increasing physical role into the total dynamic. The proposed classification will allow me to identify a possible reason why the notions of reference frame and coordinate systems are conflated in General Relativity. I claim that such confusion arises from the practical, but not conceptual, equivalence between IRFs and coordinate systems. In a nutshell, IRFs hide an approximation procedure, whereas coordinates do not, since in General Relativity they are naturally non-dynamic and “external” objects.

Introduction to shape dynamics

Tim Koslowski (University of Würzburg)

There exist no physical reference structures or processes outside the universe, so the description of the universe as a whole can not rely on external structure, i.e. no frame, no unit, no scale, no duration other than those that can be defined relationally among the physical structures within the universe itself. Pure shape dynamics implements these ideas by insisting that the dynamics of a classical universe has to be described as an equation of state of the geometry of the curve in relational configuration space (shape space). For pedagogical reasons, I will derive this equation of state explicitly for a Newtonian universe and explain how the universe spontaneously generates these reference structures and processes and how these structures generate the gravitational arrow of time, which is obtained as the direction in which increasing amounts of information are stored within local structures. Using the Newtonian equations as a foundation, I will present the pure shape dynamics description of general relativity and discuss the consequences.

Super-substantivalist Becoming in Physics

Tannaz Najafi (University of Geneva/University of Lisbon)

Recently in the literature the super-substantivalist view on spacetime and entities is gaining more attention, both from a philosophical but also physical perspective. However, what this new interpretation exactly is and how we may conceive of ‘becoming’ or ‘the passage of time’ in such a framework remains insufficiently specified. In this paper I will analyze in depth what is meant by super-substantivalism and how many variants of it we may have. 

Special Relativity's Lessons for Quantum Mechanics: Two Case Studies

Tim Riedel (University of Geneva) and Ryan Miller (University of Geneva)

Special relativity and quantum mechanics are often either treated as totally independent or as a 'solved problem' in combination after Dirac. We investigate the plausibility of these hypotheses by taking as case studies the most and least relativity-inspired versions of quantum mechanics (relational quantum mechanics and Bohmian mechanics) and seeing how they fare.

Relational primitivism about the direction of time

Michael Esfeld (University of Lausanne)

Primitivism about the direction of time is the thesis that the direction of time does not call for an explanation because it is a primitive posit in one’s ontology. In the literature, primitivism has in general come along with a substantival view of time according to which time is an independent substance. In this paper, we defend a new primitivist approach to the direction of time –relational primitivism. According to it, time is primitively directed because change is primitive. By relying on Leibnizian relationalism, we argue that a relational ontology of time must be able to distinguish between spatial relations and temporal relations to make sense of the distinction between variation and change. This distinction, however, requires the assumption of a primitive directionality of change, which is metaphysically equivalent to the direction of time. Relational primitivism is an attractive view for those that want to avoid substantivalism about time but retain a primitive direction of time in a more parsimonious ontology.

Rethinking geometry in physics

Amine Rusi (University of Lausanne)

The role of geometry in physics underwent a significant shift from Newton to Einstein. While Newton considered geometry as an a priori and fixed framework to describe the physical world, Einstein challenged this view and emphasized the importance of linking geometry to physical experiments and operational definitions. This led to the development of Einstein's "practical geometry" which played a crucial role in the development of his theory of general relativity. However, this practical geometry suffered from an ontological problem related to the status of rods and clocks, which are ad hoc theoretical constructs used to formulate empirical equivalent statements of geometrical concepts.

As we move towards the quantum world, the questions of how to think about geometry and its ontological status become even more crucial. Can we go beyond Einstein's practical geometry? What is the relationship between geometry and quantum theory of matter? These are the questions that I am currently exploring. By examining alternative theories of geometry, such as Alain Connes' spectral geometry, we can gain new insights into the ontological status of geometry and its relationship with physical concepts. In this talk, I aim to provide a critical evaluation of Einstein's practical geometry and explore new avenues for thinking about geometry in the quantum world.