Recordings

Julian Barbour [Slides, Video]

Karen Crowther [Slides]

Juliusz Doboszewski [Slides, Video]

George Ellis [Slides, Video]

Astrid Eichhorn [not available]

Simon Friederich [Slides, Video]

Steffen Gielen [Slides, Video]

Henrique Gomes [Slides]

Sean Gryb [Slides, Video]

Tim Koslowski [Slides, Video]

Mairi Sakellariadou [Slides, Video]

David Sloan [Slides, Video]

Timetable

Click titles to see abstracts (PDF of timetable and abstracts).

Follow links to access slides and video of talks (where available).

Tuesday 22 May 2018

Wednesday 23 May 2018

Abstracts

Julian Barbour

Shape Dynamics: Architectonic Structure and Unexpected Perspectives

Shape Dynamics is a relational theory of the universe in which rods and clocks emerge as substructures within the universe and all physical degrees of freedom are dimensionless ratios. History is an unparametrized undirected curve in shape space, the quotient of the extended configuration space with respect to translations, rotations and dilatations in particle dynamics and diffeomorphisms and conformal transformations in the dynamics of three-dimensional Riemannian geometry. In the latter case a small subset of the closed-space solutions of general relativity is obtained together with all of its hitherto experimentally confirmed predictions. The realization that the ideal case of a shape-dynamic universe is one in which all possible solutions divide in two at a unique ‘Janus’ point of minimum (possibly zero) size leads unexpectedly to a possible resolution of the problem of time’s arrows, including the origin of the second law of thermodynamics. In accordance with it, the growth of entropy is not due to a very special condition in the past but to the fundamental structure of the law of the universe, which dictates bidirectional arrows of time pointing away from the Janus point.

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Karen Crowther

Scale invariance and fundamentality

What does it mean for a theory to be fundamental, and what drives physics to seek ever more fundamental theories? By first examining the reasons that current theories are perceived as non-fundamental, I articulate a list of conditions that a theory of physics should satisfy if it is to be counted as fundamental, including: unification, uniqueness, UV-completeness, non-reliance upon approximations, internal consistency, level-comprehensiveness, and background independence. I argue that there are two general principles that underlie these conditions: full, non-overlapping coverage of description, and comprehensiveness of explanation. I explore how the idea of scale-invariance relates to these conditions and the general principles underlying the attribution of fundamentality. Finally, I consider how these conditions feature in the search for quantum gravity.

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Juliusz Doboszewski

Do we have a black hole singularity resolution?

I will critically discuss black to white hole quantum tunneling scenarios, focusing on:

(a) conditions for a “singularity resolution” to be successful,

(b) the extent to which white hole instability is well-established (in particular: issues related to choice of observables), and potential roles it could play in constraining black-to-white hole tunneling,

(c) conceptual differences between cosmological singularity resolution and black hole singularity resolution,

(d) compatibility with a Generalized Second Law of thermodynamics.

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George Ellis

The passage of time and the arrow of time

Whatever theory one may have about observers and the relativity of measurement, at the emergent levels of thermodynamics and biology there must be an experience of the passage of time and of the arrow of time. I will discuss how this can fit into a suitable spacetime view (an Emerging Block Universe) where the future is not uniquely fixed by the past (because of quantum uncertainty), and make some comments on how this relates to the emergence of brains that experience the passage of time. I remain puzzled as to how scale invariance fits into this picture.

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Astrid Eichhorn

The role of observations for quantum gravity

To make progress in quantum gravity, establishing a link to observations is crucial. I will discuss how the measured mass of the Higgs and the absence of new physics at the LHC point to a scenario where a “desert” lies between the electroweak scale and the Planck scale. This allows to forge a direct link between observations at the electroweak scale, and Planck-scale physics. I will explain how this can be used to confront quantum gravity models, such as asymptotically safe quantum gravity, with data in particle physics. Specifically, I will review how asymptotic safety leads to a prediction of the Higgs mass, and “retrodictions” of top mass and Abelian gauge coupling as well as the Higgs-portal coupling to scalar dark matter. I will also comment on the question of naturalness in this context.

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Simon Friederich

The observer reference class problem and researchers’ degrees of freedom in multiverse cosmology

The assumption that we are typical observers plays a core role in attempts to make multiverse theories empirically testable. A widely shared worry about this assumption is that it suffers from systematic ambiguity concerning the reference class of observers with respect to which typicality is assumed. As a way out, Srednicki and Hartle recommend that we empirically test typicality with respect to different candidate reference classes in analogy to how we test physical theories. Unfortunately, this idea fails because typicality is not the kind of assumption that can be subjected to empirical tests. As an alternative, a background information constraint on observer reference class choice is suggested according to which the observer reference class should be chosen such that it includes precisely those observers who one could possibly be, given one’s assumed background Information.

Based on this – formal – solution to the observer reference class problem, I discuss the prospects for subjecting multiverse theories to rigorous empirical tests and come to a pessimistic conclusion. The most severe problem that I see is that researchers using the background information constraint must in practice make pragmatic choices concerning an “observer proxy” and, in the context of eternal inflation, a “cosmic measure” to make multiverse theories testable. We can expect researchers to – consciously or unconsciously – become victims of confirmation bias and exploit thoses choices to arrive at findings compatible with their preferred multiverse frameworks, thus undermining the credibility of any claimed successful tests of concrete multiverse theories.

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Steffen Gielen

Hamiltonian general relativity with and without Weyl invariance

Shape dynamics incorporates the idea that physics should be invariant under a local choice of units, as emphasized by Hermann Weyl, by giving a locally scale invariant Hamiltonian formulation of GR.

In the Lagrangian, spacetime covariant setting, a Weyl invariant formulation of GR was discussed by Dirac already in the 1970’s. In the talk I will discuss the Hamiltonian structure of Dirac’s theory, and its relations to shape dynamics, usual (Einstein) gravity as well as unimodular gravity. Dirac’s theory can in some respects play the role of a “linking theory”, from which all the other formalisms arise under additional assumptions.

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Henrique Gomes

Why go Spatial?

Shape dynamics is usually introduced from relational demands. Common complaints are: i) that this demand relies on spatial, not spacetime relationalism, ii) that it rings a bit like empiricism and iii) that it still relies on knowing GR, which comes from a spacetime view. Here I will argue for spatial relationalism from a different perspective — that of quantum gravity. This approach evades counter-arguments i and ii. I will discuss a host of issues arising in the quantization of the principle of relativity of simultaneity, which go beyond what is usually termed “The problem of time”. I will then argue for spatial relationalism as the solution to these issues. Relativity of simultaneity need only be an emergent symmetry — it emerges only when the gravitational degrees of freedom approximately satisfy the equations of motion. I will then try to present a toy model built strictly from these insights, and see where it gets us. This part goes towards answering point iii.

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Sean Gryb

Time, scale and symmetry in gravity

In a groundbreaking paper in 1973 York claimed that: “the true dynamical degrees of freedom of the gravitational field can be identified directly with the conformally invariant geometry of three–dimensional spacelike hypersurfaces”. But how could this be possible when the symmetries and fundamental ontology of General Relativity are based on spacetime concepts that don’t have any obvious connection with spatial conformal invariance? We will investigate York’s claim in detail and assess its motivations and validity. Our considerations will provide the basic framework for a new starting point for quantum gravity called ‘Shape Dynamics’ that raises fascinating questions regarding the nature of time, scale and symmetry. I will outline some philosophical points regarding the foundations for this theory and discuss recent advances that will be explored throughout the conference.

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Tim Koslowski

A geometric approach to Shape Dynamics

Shape dynamics is a framework that builds up fundamental physics from simple relational first principles, which do not allow for any non-dynamical reference structures. In particular there are no non-dynamical rods and clocks, there are only evolving ratios of physical quantities. This can be formalized as the statement that shape dynamics describes the dynamics of the universe as a pure curve (without parametrization) in relational configuration space (shape space). I formalize this by the mathematical statement: “The dynamics of the universe is described as an equation of state of the local differential geometry of a pure curve on shape space.” The local geometric properties are a point in the unit tangent bundle over shape space and curvature degrees of freedom. The space of these geometric properties is the shape phase space and the equations of motion are a section in the unit tangent bundle over this shape phase space.

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Mairi Sakellariadou

Science does not get along with make-dos, like models: it needs theories

I will discuss the current concordance cosmological model, emphasising its phenomenological origin, and examining its consistency with current data. I will thus question the validity of the current widely used approach, which consists in extrapolating the validity of our knowledge in local (both space and time) scales in order to construct models that describe the laws of the universe in large scales and early times. I will then highlight two quantum gravity proposals, one from a top-down approach and the other from a bottom-up one. I will argue that while we accumulate more and more data, we lack a satisfactory theory to accommodate them and we are playing with models rather than theories.

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David Sloan

Dynamical Similarity

If dynamics is purely relational, why does Hamiltonian mechanics work with scales? I will show how a system with scale can have a shape system hidden inside, and how to translate between the two. In doing so I will establish some of the mathematics underpinning Shape Dynamics: The phase-space symmetry that represents scale, the autonomy of shape equations of motion and the contact Hamiltonian system that gives rise to Shape Dynamics. From this I will give general forms of Janus points, measures on shape spaces and their relation to their Hamiltonian counterparts.

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