What phenomenon allows two particles to be correlated across space?

Quantum entanglement is the phenomenon that allows two particles to be correlated across space. Their quantum states can become linked so that measuring one helps determine the corresponding outcome for the other, even when the particles are far apart.

Entanglement was central to the 1935 Einstein–Podolsky–Rosen argument, which questioned whether quantum mechanics gave a complete description of reality. John Bell later showed that experiments could distinguish quantum predictions from local hidden-variable theories. Bell-test experiments have repeatedly supported quantum mechanics.

Entanglement does not let people send usable messages faster than light. Each measurement result is individually random, and the correlation becomes evident only when observers later compare their results. It is also different from quantum teleportation: teleportation transfers an unknown quantum state using entanglement plus classical communication, while entanglement itself is the shared relationship between systems.