John Bell

John Stewart Bell was a physicist from Belfast who spent most of his career at CERN in Geneva. In 1964 he proved that no local hidden-variable theory can reproduce all the predictions of quantum mechanics, deriving an inequality that such theories must satisfy and quantum mechanics violates. This turned the Einstein–Podolsky–Rosen question about completeness and locality into one that experiments could test. He was also a sharp critic of the vague role of 'measurement' in standard formulations of quantum theory.

Key ideas

• Any theory in which outcomes are fixed by local hidden variables must satisfy certain statistical inequalities.
• Quantum mechanics predicts violations of these inequalities, so the question became experimental.
• Experiments have since found the violations, putting local hidden-variable theories under severe pressure.
• A fundamental physical theory should not rely on an undefined notion of 'measurement'.

Major works

• "On the Einstein Podolsky Rosen Paradox" (1964, Physics)
• "On the Problem of Hidden Variables in Quantum Mechanics" (1966, Reviews of Modern Physics)
• Speakable and Unspeakable in Quantum Mechanics (1987)
• "Against 'Measurement'" (1990)

In the conversation

• Question What did Bell’s theorem actually prove? I keep hearing three different answers Ethan Mercer · 4 replies