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Why did Albert Einstein refer to quantum entanglement as "spooky action at a distance"?

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photoelectric effect: Einstein's Nobel Prize-winning discovery — quantum mechanics Brian Greene discusses the key formula in the photoelectric effect, an insight that helped launch the quantum revolution. This video is an episode in his *Daily Equation* series.

Albert Einstein referred to quantum entanglement as "spooky action at a distance" because it appeared to violate the principle of locality, a fundamental concept in physics. Quantum entanglement describes a phenomenon where two or more subatomic particles become linked in such a way that they share a common quantum mechanical state, regardless of the distance separating them.1

In 1935, Einstein, along with Boris Podolsky and Nathan Rosen, proposed a thought experiment (the EPR paradox) that highlighted this issue. Their reasoning suggested that if quantum mechanics were correct, measuring a property of one entangled particle would instantaneously determine the corresponding property of the other, even if they were light-years apart. This apparent faster-than-light influence contradicted Einstein's theory of special relativity, which states that no information can travel faster than the speed of light. Einstein believed that the universe operated deterministically and that such instantaneous correlations were impossible, leading him to question the completeness of quantum theory. However, subsequent experiments have confirmed that quantum mechanics is indeed non-local, meaning these correlations exist despite the vast distances.2345