Quantum Causal Relations: A Causes B Causes A; Single Event Can Be Both, A Cause And An Effect

Although it is still not known if such situations can be actually found in nature, the sheer possibility that they could exist may have far-reaching implications for the foundations of quantum mechanics, quantum gravity and quantum computing.

Causal relations: who influences whom

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. As a simple example, imagine a person, Alice, walking into a room and finding there a piece of paper. After reading what is written on the paper Alice erases the message and leaves her own message on the piece of paper. Another person, Bob, walks into the same room at some other time and does the same: he reads, erases and re-writes some message on the paper. If Bob enters the room after Alice, he will be able to read what she wrote; however Alice will not have a chance to know Bob’s message. In this case, Alice’s writing is the “cause” and what Bob reads the “effect”. 

Quantum violation of causal order

As long as only the laws of classical physics are allowed, the order of events is fixed: either Bob or Alice is 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 e.g. a particle that can be at two different locations at the same time. In quantum physics this is called a “superposition”.