PART 11 · QUANTUM EXPERIMENTS
Bell’s Inequality & Quantum Entanglement
One source, two distant particles, and correlations no local hidden-variable model can reproduce.
1A pair from one source
An entangled pair is produced together and sent to distant measurement stations.
Each side chooses a measurement setting and records a local outcome.
Each local outcome is individually random; no controllable message is sent to the other side.
Memory image: one center point giving birth to two distant points.
2Angles and correlations
Repeat the experiment many times with different settings.
The crucial object is not a single result but the statistical correlation between both sides.
Bell showed that a broad class of local hidden-variable theories must obey a statistical bound.
Quantum mechanics — and experiments — violate that bound.
3What Bell really tells us
Bell violations rule out local hidden-variable explanations for all observed correlations.
They do not provide faster-than-light signalling.
Loophole-free experiments from 2015 onward greatly strengthened the evidence.
Sources: Bell (1964); Aspect and colleagues; Hensen et al. (2015); Nobel Prize 2022.
◦Three things to remember
The image that should stay with you
From distant correlations we move to a wave that crosses a barrier classical physics would reject.