In 1991, GHZ fundamentally updated Bell's result, essentially by
investigating Bell-like relationships in correlated systems of more than
two particles. What they showed surpassed Bell's result by eliminating
the statistical nature of the proof.
They show a situation involving three particles where after measuring two
of the three, the third becomes an actual test contrasting between
locality and the quantum picture: a local theory predicts one value is
inevitable for the third particle, while quantum mechanics absolutely
predicts a diffent value.
(So, we only have to run the experiment once.)
This is really equivalent to older proofs about the modeling of the
quantum state by underlying variables, as was pointed out by David Mermin
- Kochen and Specher required over a hundred particles in their original
proof, while GHZ have it down to three.
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