Relational clocks, decoherence, and the arrow of time

Julian Barbour

Complexity as time

Elimination from the N-body gravitational problem of all traces of the absolute elements that Newton introduced in dynamics together with adoption of the Newton gravitational potential made scale-invariant as both time and a measure of complexity reveals remarkable properties hidden in standard presentations. Time and structure become one and the same thing and Newton's theory of universal gravity, properly interpreted, is not one of a clockwork universe but a theory of creation. An arrow of time is a dynamical necessity, not something that needs to be explained by imposition of a special condition low-entropy condition in the past.

Sean Gryb

Counting what counts: how what's possible under symmetry may point time's arrow

In this talk, I will propose two things. The first will be a principle that can be used to define conditions for when the symmetries of a theory are well matched with empirical facts. I will then use this principle to argue for a new kind of gauge symmetry in cosmology. Applying normative prescriptions suggested by the principle will lead to my second proposal: a new way of solving the problem of the arrow of time. A key insight of this proposal is the role played by its privileged clocks. These are no longer clocks in which the laws appear in a time-symmetric fashion. Instead, they highlight the time-asymmetric features of the solution space of the theory.

Katie Robertson

Discovering simplicity: the past hypothesis and the foundations of statistical mechanics

In this talk, I give a unified account of how probabilities and time-asymmetry emerge from the underlying unitary dynamics. But doing so requires an initial condition - that the initial state is 'simple' in some way - and in some guises this is called a 'Past Hypothesis'. Much debate and controversy surrounds the Past Hypothesis. I will suggest one way of making a 'Gibbsian' Past Hypothesis more precise, and one way to dodge some of the controversies suggested inter alia by Gryb (2021).

Eddy Chen

Time's Arrow and Decoherence in a Mixed-State Universe

Standard discussions of decoherence implicitly assume that the quantum state of the Universe is a pure state, represented by a wave function. I suggest that decoherence applies even when the Universe is in a fundamental mixed state. I discuss its implications for justifying the Born rule and understanding the arrow of time. (This talk is based on joint work with Eugene Chua.)

Simon Saunders

Decoherence and the Probability Arrow of Time

It appears that there is a probability arrow of time that in principle is independent of the entropic arrow of time. In fact I shall argue that - -at least in Everettian quantum theory -- the two go together, and that there is a strong case for the view that there is no place for an independent (e.g. ‘primitive’) understanding of time’s arrow (or for that matter ‘time-flow’). Whether there is more to time itself is a more difficult question; as is the connection, if any, between the arrow of time thus identified and the concept of clock time. But notably, both involve the selection of in some sense ‘distinguished’ degrees of freedom.