Uploaded on Feb 13, 2010
http://www.facebook.com/ScienceReason ... Quantum Mechanics (Chapter 6): Quantum Entanglement - The Weirdness Of Quantum Mechanics.
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Quantum entanglement, also called the quantum non-local connection, is a 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 if the individual objects are spatially separated in a spacelike manner.
The property of entanglement was understood in the early days of quantum theory, although not by that name. Quantum entanglement is at the heart of the EPR paradox developed by Albert Einstein, Boris Podolsky, and Nathan Rosen in 1935. This interconnection leads to non-classical correlations between observable physical properties of remote systems, often referred to as nonlocal correlations.
Quantum mechanics holds that observables, for example spin, are indeterminate until some physical intervention is made to measure an observable of the object in question. In the singlet state of two spin, it is equally likely that any given particle will be observed to be spin-up or spin-down.
Measuring any number of particles will result in an unpredictable series of measurements that will tend to a 50% probability of the spin being up or down. However, the results are quite different if this experiment is done with entangled particles. For example, when two members of an entangled pair are measured, their spin measurement results will be correlated.
Two (out of infinitely many) possibilities are that the spins will be found to always have opposite spins (in the spin-anti-correlated case), or that they will always have the same spin (in the spin-correlated case). Measuring one member of the pair therefore tells you what spin the other member would have if it were also measured. The distance between the two particles is irrelevant.
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