Quantum entanglement stands as one of the strangest and hardest concepts to understand in physics. Two or more particles can interact in a specific ways that leave them entangled, such that a later measurement on one system identifies what the outcome of a similar measurement on the second system—no matter how far they are separated in space.
Repeated experiments have verified that this works even when the measurements are performed more quickly than light could travel between the sites of measurement: there's no slower-than-light influence that can pass between the entangled particles. However, one possible explanation for entanglement would allow for a faster-than-light exchange from one particle to the other. Odd as it might seem, this still doesn't violate relativity, since the only thing exchanged is the internal quantum state—no external information is passed.
But a new analysis by J-D. Bancal, S. Pironio, A. Acín, Y-C. Liang, V. Scarani, and N. Gisin shows that any such explanation would inevitably open the door to faster-than-light communication. In other words, quantum entanglement cannot involve the passage of information—even hidden, internal information, inaccessible to experiment—at any velocity, without also allowing for other types of interactions that violate relativity.
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