In 1935 Einstein et al.  challenged the ► orthodox approach to the quantum formalism by asking whether the formalism was complete or not. The specific point that led them to this conclusion was based on a puzzle that arose when two particles were in an entangled state (► entanglement). These states are characterised by the fact that the ► wave function of the individual particles are not well defined, being ambiguous until the state of one of them was measured. The difficulty arose when the two particles were separated by a large distance and were not interacting with each other through any known classical potential. If a measurement was made on one of the particles, the state of the other became immediately well defined, even though it was removed far from the apparatus measuring the state of the first particle. How does this come about?
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