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The EPR paradox: "spooky action at a distance"
1935· Discovery / experiment
ProvenTested many times; part of working physics.
A 1935 paper by Einstein, Podolsky and Rosen arguing quantum mechanics must be incomplete because of the strange correlations it predicts.
See it on the Science map →How it works
EPR imagined two particles that interact, then fly apart. Measuring one's position reveals the other's position; measuring one's momentum reveals the other's. Since either choice can be made without touching the distant particle, they argued, it must have had both values all along, which quantum theory denies. So the theory misses something. Einstein later called the alternative 'spooky action at a distance' (1947 letter to Max Born).
Why it matters
It identified entanglement as quantum theory's strangest feature and framed the question that Bell later made testable.
How we know
The correlations EPR described are real and observed. But Bell's 1964 theorem and later experiments showed their preferred explanation, hidden pre-set local values, cannot work.
Still a mystery
What the correlations mean for reality: whether nature is non-local, particles lack definite values before measurement, or something else.
Connections (3)
▸Albert Einstein challenged quantum theory with The EPR paradox: "spooky action at a distance"Proven
The relationship
Einstein accepted that quantum mechanics predicted correctly but believed it was incomplete. With Podolsky and Rosen he designed a thought experiment: two linked particles fly apart, and measuring one seems to fix the other's properties instantly. Either reality holds hidden extra details, or there is instant influence at a distance. He judged the second absurd.
How the link was found
Einstein, Boris Podolsky and Nathan Rosen at Princeton, in a May 1935 Physical Review paper. Podolsky wrote the text; Einstein felt it buried his main point.
Why it's proven
The historical link is fully documented, and Bohr replied within months. Later Bell tests confirmed the correlations EPR described are real, but showed Einstein's preferred local hidden-detail explanation cannot work.
Open questions and mysteries
Einstein's real concern, that physics should describe a reality independent of observation, is still debated among interpretations of quantum theory.
▸The EPR paradox: "spooky action at a distance" first described Quantum entanglementProven
The relationship
EPR's thought experiment used two particles whose properties were linked after interacting, so measuring one fixed the other. That linkage is entanglement. EPR meant it as evidence the theory was incomplete; instead it identified the theory's most distinctive and useful feature.
How the link was found
Einstein, Podolsky and Rosen in May 1935; Schrödinger coined the word 'entanglement' months later in response.
Why it's proven
The correlations EPR described have been produced and measured directly since the 1970s, exactly as quantum mechanics predicts.
Open questions and mysteries
Whether entanglement reveals non-local connections or the absence of definite properties before measurement is still debated.
▸The EPR paradox: "spooky action at a distance" was made testable by Bell's theoremProven
The relationship
For thirty years EPR's question, whether hidden pre-set values exist, seemed untestable. Bell showed that any local hidden-value theory limits how strongly results at two distant detectors can agree as their settings vary. Quantum mechanics predicts more agreement than that limit. The philosophical debate became a measurable prediction.
How the link was found
John Stewart Bell, a Northern Irish physicist at CERN, in a 1964 paper written on leave in the United States, inspired partly by David Bohm's hidden-variable theory.
Why it's proven
Bell's theorem is a mathematical proof. Clauser, Horne, Shimony and Holt turned it into a practical experimental test in 1969.
Open questions and mysteries
Which assumption fails, locality or definite pre-existing values, is not settled by Bell's theorem alone.