Quantum Mechanics - Quantum Entanglement Since the early 20th century, quantum mechanics has posed new challenges for the view that physical processes should obey locality. The collapse of the wave function of an electron being measured, for instance, is presumed to be instantaneous. Whether this counts as action-at-a-distance hinges on the nature of the wave function and its collapse, issues over which there is still considerable debate amongst scientists and philosophers. One important line of debate originated with Einstein, who challenged the idea that the wave function offers a complete description of the physical reality of a particle by showing that such a view leads to a paradox. Einstein, along with Boris Podolsky and Nathan Rosen, proposed a thought experiment to demonstrate how two physical quantities with non-commuting operators (eg position and momentum) can have simultaneous reality. Since the wave function does not ascribe simultaneous reality to both quantities and yet they can be shown to exist simultaneously, Einstein, Podolsky and Rosen (EPR) argued that the quantum mechanical description of reality must not be complete.[2] This thought experiment, which came to be known as the EPR paradox, hinges on the principle of locality. A common presentation of the paradox is as such: two particles interact briefly and then are sent off in opposite directions. One could imagine an atomic transition that releases two photons A and B (spin-1 particles) with no ...


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