Observer-dependent locality of quantum events

Understand

In general relativity, the causal structure between events is dynamical, but it is definite and observer-independent; events are point-like and the membership of an event A in the future or past light-cone of an event B is an observer-independent statement.

• When events are defined with respect to quantum systems however, nothing guarantees that the causal relationship between A and B is definite.
• We propose to associate a causal reference frame corresponding to each event, which can be interpreted as an observer-dependent time according to which an observer describes the evolution of quantum systems.
• In the causal reference frame of one event, this particular event is always localised, but other events can be "smeared out" in the future and in the past.