**OBSERVERS IN QUANTUM GRAVITY**

**Maximilian Lock**

**Imperial College London/University of Vienna**

**Title**

A quantum version of the relativistic light-clock

**Abstract**

The conflict between quantum theory and the theory of relativity is exemplified in their treatment of time. We examine the ways in which their conceptions differ, and consider a quantum version of the light-clock thought experiment commonly used to illustrate relativistic time dilation. This ``semiclassical'' clock model combines elements of both theories, and exhibits both quantitative and qualitative differences from the corresponding proper time of an ideal pointlike observer. Einstein begins his original formulation of relativity with a kind of operationalism, defining simultaneity by the ability of two separated clocks to synchronise. Our results show that a similarly operational definition of time, now accounting for quantum mechanics, must include extra considerations.