Albert Einstein spent his final decades insisting that quantum entanglement was an incomplete illusion masking hidden common-sense variables; John Stewart Bell devised a single mathematical inequality that allowed laboratory experiments to test whether Einstein or quantum mechanics was right. Published in an obscure, short-lived journal in 1964, Bell’s Theorem proved that the physical universe is fundamentally non-local—founding modern quantum computing, quantum cryptography, and quantum teleportation.

In 1935, Albert Einstein mocked quantum mechanics for predicting "spooky action at a distance"—the idea that measuring one particle instantly dictates the state of another entangled particle across the universe. Einstein argued that particles must carry secret internal instructions (local hidden variables), like a pair of matching shoes packed into two boxes.
Northern Irish physicist John Stewart Bell realized this philosophical debate could be settled with pure mathematics. He proved that if Einstein was right and particles had hidden pre-set properties, the statistical correlation between measurements could never exceed a strict mathematical limit: Bell's Inequality.
Laboratory experiments smashed Bell's limit, proving Einstein was wrong and quantum entanglement is real. By earning the 2022 Nobel Prize in Physics for Aspect, Clauser, and Zeilinger, by enabling unhackable quantum satellite encryption, and by launching the quantum computer race, Bell's theorem created quantum technology.
On the Einstein Podolsky Rosen paradox
Received 4 November 1964DOI:https://doi.org/10.1103/PhysicsPhysiqueFizika.1.195Copyright © 1964 Physics Publishing Co.
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