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Bell's Theorem
Direct answer
Bell's Theorem: Bell's theorem is John Bell's proof of 1964 that no theory in which particles carry preset properties and influence one another only locally can reproduce all the predictions of quantum mechanics.
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At a glance
- Also called
- bell's inequality, bell test, bells theorem explained, local hidden variables
- Found in
- Consciousness & Spiritual Psychology
- Related
- Quantum Entanglement, Copenhagen Interpretation, Wave Function, Observer Effect
Bell's theorem is John Bell's proof of 1964 that no theory in which particles carry preset properties and influence one another only locally can reproduce all the predictions of quantum mechanics.
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What does Bell's theorem prove?
Einstein, Podolsky and Rosen argued in 1935 that the linked results seen in entangled particles must come from properties fixed in advance, which quantum theory fails to describe. Bell showed that this idea can be tested. Any theory of preset local properties limits how strongly the results at two distant detectors can be correlated; that limit is Bell's inequality. Quantum mechanics predicts correlations that exceed it.
Experiments have sided with quantum mechanics, the strictest of them without exception. The usual conclusion is that nature is not locally real in Einstein's sense: either the properties are not there before measurement, or distant events are connected in a way no signal carries. The correlations cannot be used to send a message, since each side sees only random results until the two records are compared. They are no evidence for telepathy.
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What sets Bell's Theorem apart
The theorem's force comes from its generality. Bell did not refute one hidden-variable model; he bounded all of them at once, by assuming only that results are fixed by local factors and that experimenters choose their settings freely. The version tested in laboratories, from Clauser, Horne, Shimony and Holt in 1969, sets that bound at 2 for a certain sum of correlations, while quantum mechanics allows up to about 2.83. Experiments return values above 2, and that numerical gap is the whole argument.
- Published: John S. Bell, On the Einstein Podolsky Rosen Paradox, 1964
- Testable form: the CHSH inequality, 1969: local theories give at most 2
- Quantum maximum: two times the square root of two, about 2.83
- First test: Freedman and Clauser, Berkeley, 1972
- Loophole-free tests: Delft, Vienna and Boulder, 2015
- Nobel Prize: Aspect, Clauser and Zeilinger, 2022
Often mistaken for something else: The EPR argument of 1935 claimed quantum mechanics must be incomplete; Bell's theorem showed that completing it in the way Einstein hoped would contradict its own tested predictions.
The story behind Bell's Theorem
John Stewart Bell, a physicist from Belfast working at CERN, published the theorem in 1964 in the short-lived journal Physics. John Clauser and Stuart Freedman ran the first test in 1972, Alain Aspect's group in Paris closed a major gap in 1982, and three teams reported tests free of the main loopholes in 2015. Clauser, Aspect and Anton Zeilinger shared the Nobel Prize in Physics in 2022 for this work.
Why Bell's Theorem matters
Bell's Theorem turned a philosophical quarrel between Einstein and Bohr into a question experiments could settle, and they did.
How to read Bell's Theorem
Bell tests now certify entanglement in quantum cryptography and computing, where breaking the inequality shows a device is behaving quantum mechanically.
Bell's Theorem in practice: an example
A reader who hears that entanglement proves all minds are linked can note that Bell correlations carry no message at all.
Where readings of Bell's Theorem go wrong
Bell's Theorem rules out one class of physical theory; it does not show instant communication, shared consciousness or action by intent.
Sources and editorial method
This entry summarizes the tradition in the Almanac's own words; where no source is listed, it is an editorial summary rather than a primary text, and symbolic readings are named as the tradition's, not as fact.
- Source
- The Great Almanac Dictionary
- Category
- Consciousness & Spiritual Psychology
- Aliases
- bell's inequality, bell test, bells theorem explained, local hidden variables, epr paradox
- Last updated
- 2026-09-26
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Common questions
What does Bell's theorem prove?
That the predictions of quantum mechanics cannot all be matched by any theory in which particles have definite properties beforehand and are affected only by their immediate surroundings.
What is Bell's inequality?
It is a numerical limit on how strongly measurements on two separated particles can agree if their results were fixed locally in advance. Entangled particles are measured to break the limit.
Does Bell's theorem allow faster-than-light communication?
No. Each observer's results look random by themselves. The pattern appears only when the two lists are brought together by ordinary means, so nothing usable travels faster than light.
