Quantum entanglement happens when two or more particles are created or interact in a way that ties their properties together. Once entangled, measuring a property of one particle, like its spin, immediately fixes the corresponding property of its partner, even if that partner is on the other side of the galaxy.
This is not a signal traveling between them. Nothing is sent, and no information can be transmitted faster than light this way. What is strange is that before the measurement, neither particle has a definite value at all. The pair is described by a single shared wave function, and measuring one collapses the whole system at once.
Einstein famously called this “spooky action at a distance” because it seemed to break the rule that nothing travels faster than light. Decades of experiments, including ones that closed every loophole skeptics raised, have confirmed entanglement is real. It now underpins quantum computing and quantum cryptography.
The debate over entanglement traces back to a 1935 paper by Einstein, Boris Podolsky, and Nathan Rosen, now known as the EPR paradox, which argued that quantum mechanics must be an incomplete theory since it seemed to allow this kind of instantaneous correlation. For nearly three decades the disagreement remained mostly philosophical, until physicist John Stewart Bell devised a mathematical test in 1964 that could actually distinguish entanglement’s predictions from any theory based on hidden, predetermined variables. Experiments testing Bell’s inequality, refined over the following decades to close every loophole critics raised, consistently sided with quantum mechanics over Einstein’s intuition, work that was recognized with the 2022 Nobel Prize in Physics, awarded to Alain Aspect, John Clauser, and Anton Zeilinger.
Entanglement also underlies quantum teleportation, a real experimental technique, despite its science fiction sounding name, that lets physicists transfer the exact quantum state of one particle onto another distant particle, without physically moving the original particle itself. No matter or energy is teleported in the process, only information, and the technique still requires a conventional, slower than light communication channel to complete, so it does not allow faster than light signaling.