Quantum entanglement is real, faster than light — and cannot be used to send information

When two particles are quantum-entangled, measuring one instantly determines the state of the other, no matter how far apart they are. Einstein called it 'spooky action at a distance' and believed it proved quantum mechanics was incomplete. John Bell's 1964 theorem and subsequent experiments confirmed entanglement is real. But there's a catch: it cannot be used to transmit information faster than light, because the outcomes are random. You cannot control what result you get, only that the results will correlate. Entanglement enables quantum encryption and quantum computing but not faster-than-light communication.

Source: Bell, J.S., 'On the Einstein Podolsky Rosen Paradox', Physics (1964); Aspect et al., Physical Review Letters (1982)