Causality

 

Touching on the Quantum end of it

Causality is a concept in everyday life and so deeply rooted that you may take it with as little consideration as the air around you. This is the idea that events that you experience in the present are caused by events in the past and, in turn, act as causes for what happens in the future, but can a cause be its own affect? It seems so in the quantum world.

A caused B and B caused A

This of course is hypothetical, but many well constructed hypotheses often find themselves in the real-proven world and this could have far-reaching implications for the foundations of quantum mechanics, quantum gravity and quantum computing.

In everyday life and in classical physics, events are ordered in time: a cause can only influence an effect in its future not in its past. Einstein said it, so it must be true! As a simple example, imagine a person walking into a room and finding something on their cell phone in a text note. After reading what is written in the text you erase the note and leave your own message there. Another person walks into the same room at some other time and does the same: they read, erase and rewrite some message on the phone’s notepad. If they enter the room after you, they will be able to read what you wrote; however, you will not have a chance to know their message. In this case, your writing is the "cause" and what they read is the "effect". Each time the two repeat the procedure, only one will be able to read what the other wrote. Even if they don't have watches and don't know who enters the room first, they can deduce it by what they write and read on the phone’s notepad. For example, you might write "I was here today", such that if they read the message, they will know that they came to the room after you.

Now we get to quantum violation of causal order

If only the laws of classical physics are allowed, the order of events is fixed: either them or you are first to enter the room and leave a message for the other person. When quantum mechanics enters into play, however, the picture may change drastically. According to quantum mechanics, objects can lose their well-defined classical properties, such as, for example, a particle that can be at two different locations at the same time. In quantum physics this is called a "superposition". Now an international team of physicists led by Caslav Brukner from the University of Vienna have shown that even the causal order of events could be in such a superposition. If, in our example, you and the other have a quantum system instead of an ordinary phone to write your messages on, then you can end up in a situation where each can read a part of the message written by the other. Effectively, one has a superposition of two situations: "You enter the room first and leave a message before they do" and "they enter the room first and leaves a message before you".

"Such a superposition, however, has not been considered in the standard formulation of quantum mechanics since the theory always assumes a definite causal order between events", says Ognyan Oreshkov from the Université Libre de Bruxelles (formerly University of Vienna). "But if we believe that quantum mechanics governs all phenomena, it is natural to expect that the order of events could also be indefinite, similarly to the location of a particle or its velocity", adds Fabio Costa from the University of Vienna.

The work provides an important step towards understanding that definite causal order might not be a mandatory property of nature. "The real challenge is finding out where in nature we should look for superpositions of causal orders", explains Caslav Brukner from the Quantum Optics, Quantum Nanophysics, Quantum Information group of the University of Vienna.

[Information here is from https://phys.org/news/2012-10-quantum-causal.html]