Theories of quantum gravity suggest that classical spacetime may not be fundamental, but only arise as higher order phenomena. What implications does such a possibility have for cosmology, which generally assumes the classical framework of general relativity? Such implications are in part physical, as one expects new physics to play a role in Planckian scales, for instance in the early universe. But they are also conceptual and methodological. How does the physics of a whole universe relate to the physics of its parts? What reasoning is involved in drawing inferences for the early universe and the physics of the very small, from observation of the current universe? In regimes in which classical spacetime breaks down, how are observables, laws, or indeed dynamics to be understood — and what of the universe ‘before’ the Big Bang?

In the seminar, leading philosophers and physicists will address these questions from the point of view of a range of approaches to quantum gravity.

The seminar is a collaboration between The Center for Philosophy of Science, and the Cosmology Beyond Spacetime project funded by the John Templeton Foundation.

WEDNESDAYS 10:15 AM – 12:00 PM EST

Talks in this series will be held in Zoom webinar format and pre-registration is required.  You may register for all talks, or just a few.

REGISTER HERE:  https://pitt.zoom.us/webinar/register/WN_BYAmu_RxTeCCSU7JXYk_Ig

Titles, abstracts, coming soon.

Speakers

Feb 3:  “Laws For Nowhere”  Nick Huggett, University of Illinois at Chicago, Senior Visiting Fellow at The Center

Abstract: The standard concept of law is, I suggest, significantly spatiotemporal, posing the question of how there can be laws in non-spatiotemporal theories, and most pointedly how laws could hold in non-spatiotemporal regions of spacetime. I describe a couple of quantum gravity models of the Big Bang (in string theory and group field theory), in a provisional attempt to demonstrate how such questions might arise.

Feb 10:  “Laws Beyond Spacetime”  Christian Wüthrich, University of Geneva

Abstract: Quantum gravity’s suggestion that spacetime may be emergent and so only exist contingently would force a radical reconception of extant analyses of laws of nature. Humeanism presupposes a spatiotemporal mosaic of particular matters of fact on which laws supervene. I will show how the Humean supervenience basis of non-modal facts can be reconceived, avoiding a reliance on fundamental spacetime. However, it is unclear that naturalistic forms of Humeanism can maintain their commitment to there being no necessary connections among distinct entities. This talk is based on a joint project with Vincent Lam.

Feb 17:  “Quantum Gravity in Practice”  Francesca Vidotto, Western University

Abstract: I present a recent concrete calculation in Spinfoam Cosmology -the application of the covariant LQG techniques to the cosmos- as an example to discuss a number of conceptual issues that are at the core of quantum gravity and cosmology. These include: What are the observables when localization does not rely on background space and time? What are the degrees of freedom? What is the role of quantum fluctuations of spacetime? What’s the interplay between the Planck scale and the cosmological scale? How should we think about time in this picture?

Feb 24:  “Temporal Naturalism”  Lee Smolin, Perimeter Institute for Theoretical Physics

I discuss the progress of a research program called temporal naturalism, whose aim is to reframe naturalism and relationalism based on the hypotheses that time is fundamental, while space is emergent.  By the fundamentality of time we mean that all that is real are causal processes that continually make definite facts out of previously indefinite possibilities, thereby producing novel events out of predecessor events.  Good be-ables to construct such a theory from are the views of events, which describe what properties an event was endowed with by its predecessor events, such as energy and momentum. Thus there is a single universe made up of partial views of itself.

In a relational setting and with no distance, coordinates, fields, or trajectories to draw from, the dynamics must be formulated in terms of views, and in particular in terms of differences amongst views. The change between the view of an event and its immediate predecessors provides a notion of kinetic energy while potential energy is related to the variety, which is the total diversity of present causally unrelated views. This is enough to derive a version of many body quantum theory; which lives in a space that we show emerges from the solutions of the theory.

Thus the program shows promise of reconciling both the problems of quantum foundations and quantum gravity, within a single completion.

The part of the program just described has been developed under the names of energetic causal sets and the causal theory of views.  Another key aspect is the view that the laws of physics cannot be fixed, but must evolve, in a way as to explain how the choices the universe has made of the fundamental forces and particles have come about through an evolutionary, dynamical process.  If there is time I will discuss three realizations of this idea: cosmological natural selection, the principle of precedence, and the hypothesis that the vacua of quantum fields can learn to navigate a landscape of possible laws, using the same mechanisms that allow a deep neural network to learn.

This work has appeared in six books and a number of papers; key collaborators have included Julian Barbour, Fotini Markopoulou, Stuart Kauffman, Joao Magueijo, Stephon Alexander, Roberto Mangabeira Unger, Jaron Lanier, Marina Cortes, Andrew Liddle and Clelia Verde.

March 3:  “Decoupling from the Initial State?”  Christopher Smeenk, Western University

According to inflationary cosmology, the universe passed through a transient phase of exponential expansion that leaves several characteristic imprints in the universe’s post-inflationary state. This paradigm has enjoyed considerable phenomenological success, as a wide range of inflationary models are compatible with observations. The extent to which this success lends credibility to inflation has been a subject of ongoing debate. Here I will focus on whether the predictions of inflation are robust to changes in high-energy physics, or to features of the pre-inflationary initial state. The prospect of describing the early universe successfully without resolving the mysteries of quantum gravity has always been one of inflation’s appealing features. I will review arguments that inflation does not decouple from high energy physics in the same sense as other effective field theories in physics. Establishing how inflation can be implemented in a theory of quantum gravity is an ongoing challenge, and doing so is needed to address several long-standing foundational questions.

March 10:  “If Time Had No Beginning”  Fay Dowker, Imperial College London

Could the universe have had no beginning? I don’t mean, in raising this question, to deny or throw doubt on Big Bang cosmology and the existence in the past of a hot, dense state of Planckian curvature and temperature. But, in that case, how is the question to be interpreted given that—if we accept the standard cosmology— the Lorentzian manifold structure of spacetime breaks down at the Big Bang? Certainly,  the continuum concept of time in our cosmological epoch “begins” at the Big Bang. Can we even ask what happened “before” that?  The causal set approach to the problem of quantum gravity provides an arena in which to address the question of origins, in which it makes sense to ask what happened before the Big Bang and in which a clear distinction can be made between models of the universe which are “past infinite” and “past finite”. I will describe work with Stav Zalel and Bruno Bento in which we construct a framework for dynamics for causal sets which can result in past infinite universes.