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Quantum entanglement is a possible property of a quantum mechanical state of a system of two or more objects in which the quantum states of the constituting objects are linked together so that one object can no longer be adequately described without full mention of its counterpart — even though the individual objects may be spatially separated. This interconnection leads to non-classical correlations between observable physical properties of remote systems, often referred to as nonlocal correlations. For example, quantum mechanics holds that states such as spin are indeterminate until such time as some physical intervention is made to measure the spin of the object in question. It is equally likely that any given particle will be observed to be spin-up as that it will be spin-down. Measuring any number of particles will result in an unpredictable series of measures that will tend more and more closely to half up and half down. However, if this experiment is done with entangled particles the results are quite different. When two members of an entangled pair are measured, one will always be spin-up and the other will be spin-down. The distance between the two particles is irrelevant. Theories involving 'hidden variables' have been proposed in order to explain this result; these hidden variables account for the spin of each particle, and are determined when the entangled pair is created. It may appear then that the hidden variables must be in communication no matter how far apart the particles are, that the hidden variable describing one particle must be able to change instantly when the other is measured. If the hidden variables stop interacting when they are far apart, the statistics of multiple measurements must obey an inequality (called Bell's inequality), which is, however, violated — both by quantum mechanical theory and in experiments.
but its probably safe to say that in practice it's going to be waaaaaaaaaaaay too complicated.
Mr Scientist (and SC) made me wonder about entanglement. Lets say we have two entangled particles, one is made to be placed in a gravity well. We do not observe them while doing so, but we do it afterwards. Now those particles will have not only traveled in distance but also in time differently when we finally compare them. Will the entanglement be there? And we need to do it with something 'ticking' not photons I think, otherwise it would maybe be possible to test with a optical black hole.