Researchers at the University of Vienna and the Austrian Academy of Sciences develop a new theoretical framework to describe how causal structures in quantum mechanics transform. They analyse under which conditions quantum mechanics allows the causal structure of the world to become "fuzzy." In this case, a fixed order of events is not possible. The results are published in the journal Physical Review X.

The idea that events occur one after the other in a fixed causal order is part of our intuitive picture of the physical world. Imagine that Alice can send a message to Bob via a wire that connects them. Alice decides to have a barbecue and can invite Bob via the wire connection. If he gets invited, Bob decides to prepare some Ćevapčići to bring along. This is an example where the event in which Alice decides to invite Bob to the barbecue influences the event in which Bob decides to prepare food. Such an order of events characterizes a definite causal structure. However, research in the foundations of quantum mechanics suggests that, at the quantum level, causal structures may be "indefinite." In an indefinite causal structure there might not be a fixed order in which events happen, i.e. whether Alice influences Bob or Bob influences Alice might not be defined.

If causality is indeed indefinite, where do indefinite causal structures come from? Can they be obtained dynamically so that definite causal structures become indefinite? And, if so, under which conditions can this happen? Answers to these questions would be remarkable, because they would shed light on the nature of causality in the quantum world.

To read more, click here.