Sean Gryb: Hidden symmetries and the role of scale in physics

While measurement outcomes are necessarily recorded in terms of dimensionless numbers (e.g., the length ratio between an object and some probe), scientific theories are almost exclusively formulated in terms of dimension-full quantities (e.g., meters). Such usage of fixed scales seems justifiable for theories that make reference to external reference structures. However, for cosmological theories such as general relativity, a more careful scrutiny of the role played by scale is required.

We will study various pitfalls associated with the demarkation of the otiose and genuine physical structures of a dynamical theory. To illustrate a rather peculiar case, we will study a particular attempt, first advocated by Barbour, for formulating fundamental physics in terms of dimensionless quantities. We show how an implementation of Barbour’s ontology leads to surprising insights about the nature of symmetries. In particular, we will see that symmetries can be ‘hidden’ and even ‘traded’ for one another in a framework known as 'Shape Dynamics'. We will then comment on the broader implications of such observations towards the meaning of symmetries and the role of scale in physics.

When & where?

Monday, 5 February 2018, 3-5pm
Faculty of Philosophy, room Aquarium