An experimentally testable inequality that any local hidden-variable theory must satisfy — and that quantum mechanics, and reality, provably violate.
Why it matters
It is the proof that entanglement is not just correlated randomness set up in advance. It also underpins device-independent cryptography, and it won the 2022 Nobel Prize in Physics.
After this you will be able to
Hand-picked and ordered. If you only have time for one, take the first.
The prize to Aspect, Clauser and Zeilinger for experimentally proving Bell inequality violations. The popular and scientific background documents are excellent free explanations of nonlocality.
Free, self-contained and rigorous, with exercises. Widely used as the standard course text in Europe and the best single free replacement for a paid textbook.
A drag-and-drop optical table in the browser — lasers, beam splitters, polarisers — that lets you build interference and Bell-test experiments and see the physics rather than the algebra.