Physics
Non-Locality
Quick fact
In 1935, Einstein called non-locality 'spooky action at a distance', but experiments have repeatedly confirmed that entangled particles behave as a single system, even across the universe.
Why this is interesting
Imagine two dice that always show the same number when rolled, no matter how far apart they are. Could that really happen?
Read the full explanation
Understanding Non-Locality
Non-locality is a property of certain quantum systems called entangled particles. When two particles are entangled, measuring one particle's property (like spin or polarization) instantly determines the same property of the other particle, even if they are light-years apart. This seems to violate the classical idea that nothing can travel faster than light. However, it does not allow sending information faster than light—the outcome at each particle is random, but the correlation between them is perfect. Think of it like a pair of pre-agreed magic coins: when flipped, one always shows heads and the other tails, but you can't control which is which. In quantum mechanics, this correlation is stronger than any classical system can achieve, as proven by Bell's theorem.
A deeper explanation
The mechanism behind non-locality lies in the nature of quantum superposition and entanglement. When two particles become entangled, their combined state cannot be described as independent states—they form a single, holistic quantum state. For example, an entangled pair of photons can be in a superposition of 'both vertical polarization' and 'both horizontal polarization'. When you measure one photon, the superposition 'collapses' into a definite state, instantly forcing the other photon into the corresponding state. This instantaneous correlation does not involve any signal or influence traveling between them; it's a fundamental property of the quantum description. Bell's theorem mathematically shows that no local hidden variable theory can reproduce all predictions of quantum mechanics. Experiments, starting with Alain Aspect in the 1980s, have confirmed that nature is indeed non-local. Non-locality matters because it reveals that the universe at its most fundamental level is interconnected in a way classical physics cannot explain. It is also the resource behind quantum cryptography, teleportation, and certain computations.