Author
John S. Bell
1 reading card from 1 book · 1964.
1 card
On the Einstein Podolsky Rosen Paradox · 1964
Entangled pairs agree better than any hidden ticket would allow, but each end sees only chance, so you cannot send messages through them.
Start with an analogy. You put a left glove and a right glove into two boxes without looking, and send one box to Cluj and the other to Tokyo. Whoever opens the box in Cluj and finds the left glove knows at once that the right one is in Tokyo. Nothing mysterious: it was all settled at departure. Einstein believed entangled particles work the same way — each would carry a kind of hidden ticket with the answers, and quantum mechanics simply could not read it. In 1964 John Bell found a way to test this. His idea: if each particle carries answers written in advance for every question, and nothing travels between them, then the agreement between results, when you ask different questions at the two ends (for example different filter angles), has a ceiling. Quantum mechanics predicts that entangled pairs go past that ceiling. So both ideas cannot be true, and an experiment can decide. Experiments have decided: the ceiling is broken, again and again, from the 1970s up to the 2015 tests that closed the last loopholes. This is where the glove analogy breaks: the answers were not written in the box. For this work Alain Aspect, John Clauser and Anton Zeilinger received the 2022 Nobel Prize in Physics. What entanglement does NOT mean: it is not a telephone. At each end, the results look like perfect coin tosses, half one way, half the other. The link appears only when the two lists are laid side by side, and to do that they must be brought together through ordinary channels, at most at the speed of light. No message can be sent faster than light.
“In a theory in which parameters are added to quantum mechanics to determine the results of individual measurements, without changing the statistical predictions, there must be a mechanism whereby the setting of one measuring device can influence the reading of another instrument, however remote.”