In the first instalment of our series on the Foundations of Quantum Mechanics, Tim Maudlin revisits Einstein’s famous disquiet with a prediction of quantum mechanics: the possibility of "spooky action at a distance".  As John Bell later proved, quantum mechanics is indeed a non-local theory, allowing for the outcomes of experiments separated by enormous distances to causally influence each other.

It seems to be a manifest fact about the physical world that in order for one action or event to have an effect on another there must be some continuous process—an exchange of particles, flow of electricity, flash of light, etc—that connects them. If such continuous processes are required and if those processes are in turn limited by the speed of light, then there would be a strict limitation on what can have a causal effect on what. Indeed, denying this sort of limitation on causation is what Albert Einstein famously referred to as “spooky action-at-a-distance” or “telepathy”, and he roundly rejected the possibility. But quantum mechanics shows that, despite  Einstein's protestations, the physical world is non-local.