EPR experiment

The EPR paradox, was a 1935 thought experiment by Albert Einstein, Boris Podolsky and Nathan Rosen. This is a famous example of entanglement phenomenon. It involves two particles created from a common source. These correlated particles were sent off in opposing directions. The idea was that this thought experiment would prove that the wave function does not accurately describe the natural world and that the Copenhagen interpretation of quantum mechanics could not be a complete description of reality. The paradox assumed two properties:

• They assumed that local realism was valid. Local realism is a combination of the principle of locality (that an object is only directly influenced by it's immediate surroundings) with the assumption that a particle must have a pre-existing value before a measurement is made. They also thought that there would have to be hidden parameters to explain how the measurement of one particle could affect the other. The combination of these two proposals led to what is known as the "principle of local action". 

• They proposed that unless momentum and position were real properties that can be understood at the same time (unlike in the Heisenberg uncertainty principle, which is central to the Copenhagen interpretation of quantum mechanics) than quantum mechanics would predict non-locality. 

That was the paradox. However, physicists today have shown that it is theoretically consistent for quantum particles to remain entangled despite their vast distance from one another.