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 analyze 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 were 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.

“Our results demonstrate that under physically reasonable assumptions of continuity and reversibility a world with definite causal order will never become a world with an indefinite causal order and vice versa”, says Esteban Castro, one of the authors of the paper. This insight may lead to a more complete understanding of what the role of causality is in the quantum world. (1)

In the beginning there was chaos.

And then the cosmos was born.

We like to look into patterns.

We like to indulge into our hallucinations.

But every night, when we fall asleep, we remember.

It is not the Sun we celebrate.

What exists cannot change.

We are not scientists.

We are poets.

Admiring the Moon…