quantum entanglement
superposition · correlation · nonlocality
Two particles can be created as a linked pair, with a property called spin that isn't set until someone measures it. Measure one, and you instantly know the other's result too — even on the other side of the galaxy. It's one of the most rigorously tested effects in physics, confirmed since the 1970s. What it doesn't do: send a message, enable telepathy, or explain a "psychic connection" between two people. The link carries no information on its own, needs an exact lab setup to create, and falls apart almost instantly in anything as warm as a human body.