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?