**Collection Number**C21004

**Collection Date**

**Collection Type**Conference/School

## Relational dynamics in an emergent spacetime context

Luca Marchetti
Università di Pisa

I discuss the new dimension that the relational approach to the problem of time takes in quantum gravity contexts in which spacetime and geometry are understood as emergent. I argue that, in this case, the relational strategy is best realized at an approximate and effective level, after suitable coarse graining and only in terms of special quantum states. I then show a concrete realization of such effective relational dynamics in the context of a cosmological application of the tensorial group field theory formalism for quantum gravity.

## Relational observables and quantum diffeomorphisms on the worldline

Leonardo Chataignier
Universität zu Köln

"Candidate theories of quantum gravity must answer the questions: how can the dynamics of quantum states of matter and geometry be defined in a diffeomorphism-invariant way? How are the quantum states related to probabilities in the absence of a preferred time? To address these issues, we discuss the construction and interpretation of relational observables in quantum theories with worldline diffeomorphism invariance, which serve as toy models of quantum gravity.

## Discussion Session 4

## Measuring Quantum Discreteness of Time in the Lab with Gravity Entanglement Interference

Carlo Rovelli
Centre de Physique Théorique (CPT)

The concrete perspective of using interference to measure Gravity Induced Entanglement in the lab is a very exciting development for quantum gravity. While the measurements considered so far only test the nonrelativistic regime, the same technique might allow access to genuine relativistic quantum effects. Among these, there might be the possibility of direct detection of time quantum discreteness.

## Measuring time with stationary quantum clocks

Mischa Woods
ETH Zurich

Time plays a fundamental role in our ability to make sense of the physical laws in the world around us. The nature of time has puzzled people –- from the ancient Greeks to the present day -– resulting in a long running debate between philosophers and physicists alike to whether time needs change to exist (the so-called relatival theory), or whether time flows regardless of change (the so-called substantival theory).

## Time in Physics and Intuitionistic Mathematics

Nicolas Gisin
Université de Genève

"Physics is formulated in terms of timeless axiomatic mathematics. However, time is essential in all our stories, in particular in physics. For example, to think of an event is to think of something in time. A formulation of physics based of intuitionism, a constructive form of mathematics built on time-evolving processes, would offer a perspective that is closer to our experience of physical reality and may help bridging the gap between static relativity and quantum indeterminacy.
Historically, intuitionistic mathematics was introduced by L.E.J.

## God does not play dice (He plays sudoku)

Emily Adlam
Basic Research Community for Physics

I argue that modern physics gives us good reason to take seriously the possibility of laws which are non-local, global, or in some other way non-dynamical. I set out a general framework for lawhood which does not presuppose the standard kinematical/dynamical split, and I apply it to the problem of giving a generalized definition of determinism for the non-dynamical context. Finally I make some suggestions about how to draw conclusions about the global structure of the laws of nature from the local observations we are able to make.

## Time and Noether's (first) theorem

Harvey Brown
University of Oxford

"It is widely believed that the homogeneity of time is the symmetry related by Noether's (first) theorem to the conservation of energy, and indeed that it explains energy conservation. Both claims are questionable, and in particular seemingly hard to reconcile with the modern version of Noether's first theorem due independently to Martínes Alonso (1979) and Olver (1986).
The talk is based on: 'Do symmetries ""explain"" conservation laws? ...'
arXiv:2010.10909v1"

## Discussion Session 3

## The Issue of Time in Generally Covariant Theories

Rodolfo Gambini
Universidad de Montevideo

A possible solution of the problem of time in quantum gravitational systems is presented based on a relational description between the parameterized Dirac observables of the system under consideration and the clocks. The use of physical clocks required by a quantum gravitational description of time is shown to induce a loss of unitarity. The evolution is described by a Lindblad-type master equation unless it is possible to construct a perfect clock.