Action at A Distance (physics) - Quantum Mechanics

Quantum Mechanics

See also: Quantum nonlocality

Since the early 20th century, quantum mechanics has posed new challenges for the view that physical processes should obey locality. Whether quantum entanglement counts as action-at-a-distance hinges on the nature of the wave function and decoherence, issues over which there is still considerable debate among scientists and philosophers. One important line of debate originated with Einstein, who challenged the idea that quantum mechanics offers a complete description of reality, along with Boris Podolsky and Nathan Rosen. They proposed a thought experiment to involving an entangled pair of observables with non-commuting operators (e.g. position and momentum).

This thought experiment, which came to be known as the EPR paradox, hinges on the principle of locality. A common presentation of the paradox is as such: two particles interact and fly off in opposite directions. Even when the particles are so far apart that any classical interaction would be impossible (see principle of locality), a measurement of one particle nonetheless determines the corresponding measurement of the other.

After the EPR paper, several scientists such as de Broglie took up interest in local hidden variables theories. In the 1960s John Bell derived an inequality that showed a testable difference between the predictions of quantum mechanics and local hidden variables theories. Experiments testing Bell-type inequalities in situations analogous to the EPR thought experiment have been consistent with the predictions of quantum mechanics, suggesting that local hidden variables theories can be ruled out. Whether or not this is interpreted as evidence for nonlocality depends on one's interpretation of quantum mechanics.

Non-standard interpretations of quantum mechanics also vary in their response to the EPR-type experiments. Bohm interpretation gives an explanation based on nonlocal hidden variables for the correlations seen in entanglement. Many advocates of the many-worlds interpretation argue that it can explain these correlations in a way that does not require a violation of locality, by allowing measurements to have non-unique outcomes.

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