**OBSERVERS IN QUANTUM GRAVITY**

**Caslav Brukner**

**University of Vienna**

**Monday 22 talk**

**Title**

Bell’s theorem for temporal order

**Abstract**

In general relativity causal relations between any pair of events is uniquely determined by locally predefined variables – the distribution of matter-energy degrees of freedom in the events’ past light-cone. Under the assumption of locally predefined causal order, agents performing freely chosen local operations on an initially local quantum state cannot violate Bell inequalities. However, superposition of massive objects can effectively lead to “entanglement” in the temporal order between groups of local operations, enabling the violation of the inequalities. This shows that temporal orders between events can be “indefinite” in non-classical space-times.

**Tuesday 23 talk**

**Title**

A no-go theorem for ``facts of the world''

**Abstract**

Wigner's friend is a thought experiment in which, from the point of view of Wigner, the friend, who measures a quantum system, is entangled with the state of the system. Both Wigner and the friend can in principle confirm their state assignment by performing an appropriate measurement. The result of each of these measurements – as manifested trough a click in a detector or a specific position of the pointer -- can be seen as a “fact”. I will show that the view that holds the coexistence of the "facts of the world" common both for Wigner and the friend runs into the problem of the hidden variable program, for which I will derive a Bell-type theorem. A possible implication of theorem would be that “facts” are defined only relative to an observer. Still considering Wigner's friend thought experiment I will show how the friend can prove herself that she is in a superposition, and how in such a situation the application of “standard quantum formalism” seems to fail.

Reference: arXiv:1507.05255