Two particles affecting one another faster than light seemed unimaginable but there is no denying the facts. This mystery has inspired much skepticism from lay folk and Nobel prize winners alike.  But how do we solve this? Emily Adlam argues, that retrocausality, the idea the future can affect the past, is the key to solving this quantum enigma.

One of the most famous and puzzling features of quantum mechanics is the fact that it exhibits what Einstein called ‘spooky action at a distance, - correlations between measurements performed on particles very far apart in space. Now, of course distant correlations in and of themselves are nothing surprising. For example, suppose I take a pair of socks out of my drawer, separate them, and then send one off to my friend Alice in London and the other to my friend Bob in Auckland. If my friends observe the socks and then compare their results and discover that they both have socks of the same colour, there’s no mystery about that - the socks are the same colour because of a common cause in their shared past, i.e. the fact that they both came from the same pair. But the correlations appearing in quantum mechanics are special because they cannot be explained in this way. For example, distant quantum correlations are typically demonstrated in a ‘Bell experiment,’ in which I prepare a pair of quantum particles and send one to Alice and another to Bob, and then Alice and Bob both choose a measurement and perform that measurement on their particle. A famous theorem due to the physicist John Bell tells us that in this scenario, correlations explained by a common cause in the past must obey a certain inequality - but it turns out that we can choose preparations and measurements in quantum mechanics which lead to a violation of Bell’s inequality. This seems to indicate that Alice’s decision to make one measurement rather than another has an instantaneous influence on Bob’s particle and thereby affects the results that Bob obtains in his own measurement, regardless of how far apart they are.