In the 1920s, Albert Einstein thought he had found a fundamental flaw in quantum physics. This set off a chain of investigations that, over several decades, showed he had instead discovered a crucial feature of quantum theory – and one of its oddest. This property, now called Bell non-locality, which involves quantum objects maintaining coordinated behaviours even across cosmically large distances, has been unkind to our intuition. Yet embracing it in the 21st century has turned out to be a fantastic idea. The issue can be set out with the help of two hypothetical experimenters, Alice and Bob, who each have one of a pair of “entangled” particles. Entanglement allows the particles to exhibit correlations even if they are so far apart that no signal could ever pass between them quickly enough to make a difference. Yet, for those correlations to become obvious, each experimenter must interact with their particle. Do the particles “know” they are correlated before Alice or Bob interacts with them, or is there something spooky going on between them? Einstein, working with Nathan Rosen …