What you do today could affect what happened yesterday – that is the bizarre conclusion of a thought experiment in quantum physics described in a preprint article by Yakir Aharonov of Tel-Aviv University in Israel and colleagues.

It sounds impossible, indeed as though it is violating one of science’s most cherished principles – causality – but the researchers say that the rules of the quantum world conspire to preserve causality by ‘hiding’ the influence of future choices until those choices have actually been made.

At the heart of the idea is the quantum phenomenon of ‘nonlocality’, in which two or more particles exist in interrelated or ‘entangled’ states that remain undetermined until a measurement is made on one of them. Once the measurement takes place, the state of the other particle is instantly fixed too, no matter how far away it is. Albert Einstein first pointed out this instantaneous ‘action at a distance’ in 1935, when he argued that it meant quantum theory must be incomplete. Modern experiments have confirmed that this instantaneous action is, in fact, real, and it now holds the key to practical quantum technologies such as quantum computing and cryptography.

Aharonov and his co-workers describe an experiment for a […]