Observers and Causality in Quantum Gravity

The conference Observers and Causality in Quantum Gravity took place from 21 to 24 April 2026 in Bratislava, Slovakia. It was jointly organised by working groups of the COST Actions Relativistic Quantum Information (RQI) and Bridging High and Low Energies in Search of Quantum Gravity (BridgeQG), bringing together researchers from a range of backgrounds including quantum foundations, quantum information and quantum gravity.

The central aim of the meeting was to discuss the notion of observers and causality, in particular related to quantum gravity. Some key questions that were discussed include the following:

• What structures are required to define observers in quantum gravity?

• What is the relation among the notions of observers, observables, and measurements?

• Which features of causal structure are genuinely new in quantum gravity compared to classical spacetime?

• Which other fundamental concepts should be examined to help build a shared conceptual language?

The scientific programme consisted of invited and contributed plenary talks, including several members of our society. A distinctive feature of the meeting was the emphasis on collective reflection and exchange: several sessions were explicitly devoted to moderated discussions, during which participants were invited to articulate and compare their perspectives on key conceptual issues. In some cases, informal voting procedures were used to gauge the distribution of opinions within the community on specific questions. These discussions will be continued in future similar conferences and other events of the COST Actions. In addition to the plenary programme, a poster session provided further opportunities for scientific exchange. The posters remained on display in the conference room throughout the meeting, facilitating continued discussion during breaks.

The schedule also included a conference dinner in a restaurant on the Danube riverbank and a social excursion to Devín Castle, located at the confluence of the Danube and Morava rivers, allowing beautiful views over the border region between Slovakia and Austria.