A thought experiment suggested by Einstein as an objection to the Copenhagen interpretation of quantum mechanics. In a simplified form of this experiment one considers a particle with zero spin converted into two particles with spin. The particles fly apart and it is known that they must have opposite spins because spin is conserved. At a certain point one measures the spin direction of one of the particles, and immediately knows that the other particle's spin direction is opposite. However, according to Bohr, the spin is not defined until the measurement is made, at which point the wave function collapses. How does the second particle ‘know’ the result of the measurement on the first particle? According to Einstein, this implies that the spins of the particles were set at the time the particles were formed and that hidden variables are involved in quantum mechanics. In 1964 John Bell showed a way of testing this experimentally (see Bell's theorem) and experiments were eventually done (see Aspect experiment). These seem to show that the Copenhagen interpretation is correct and that the particles are part of a single entangled state, even though they are far apart. See also quantum entanglement (Feature).
http://www.drchinese.com/David/EPR.pdf The original 1935 paper in Physical Review