Quantum entanglement
lastmod 2019-10-28created 2019-10-281 edits
In quantum mechanics, quantum entanglement, or simply entanglement, is a set of non-classical correlations that can exist between two subsystems. Entanglement can exist even when the two subsystems are spatially far apart from each other.
For example, put two particles in a certain quantum state so that their spins are always opposite. (For example, the singlet state of two spins.) According to quantum mechanics, the state of the two particles cannot be known until it is measured. But once a measurement is made, the state of one system is determined at that moment, and this immediately determines the state of the other system entangled with it as well. It looks as if information moved instantly from one system to the other.
After this theory of quantum entanglement appeared, experiments in quantum cryptography, quantum computers, quantum teleportation and so on were carried out steadily, and through them the predictions of the theory of quantum entanglement could be verified. While these experimental results kept piling up, philosophical discussion also went on steadily. One of the points was that quantum entanglement violates the principle of locality. The principle of locality says that information about the state of a system can always be mediated only through the system's surroundings. If information were transmitted by quantum entanglement, information could be transmitted without going through the surroundings, which contradicts the principle of locality. In the end, the discussion of how information is actually transmitted in quantum entanglement continued, and later new interpretations of quantum mechanics that can remove this contradiction emerged.
Quantum entanglement violates locality, but it does not violate the theory of relativity, because it cannot transmit information faster than light. However, quantum mechanical information can be sent along with classical information through quantum entanglement; this is called quantum teleportation. Even in this case, information cannot be transmitted faster than light.