EPR paradox is a seeming paradox conceived as a thought experiment by Albert Einstein (1879–1955), Boris Podolsky, and Nathan Rosen in 1935 as a challenge to the Copenhagen Interpretation of quantum mechanics. The title of the published paper was "Can Quantum-Mechanical Description of Physical Reality be Considered Complete?" Einstein did not believe that objects only had properties when they were observed. He tried to conceive of a situation in which the observation of one thing would lead to a situation in which it was one hundred percent certain that another state would be completely certain irrespective of whether it had been observed.

Atomic states can be created so that their total spin is zero. If the state is allowed to decay into two spinning particles that move off rapidly in opposite direction close to the speed of light then the particles must have spin of +1 and -1 units respectively, so that they add to zero, as...