The EPR-paradox, as it is commonly known as, is a thought experiment targeted at the philosophy of quantum mechanics, at that time represented by the Copenhagen Interpretation

It was published as an article in 1935 written by Albert EinsteinBoris Podolsky and Nathan Rosen

The thought experiment showed how two electrons, first joined closely and then separated by a great distance, can affect each other instantaneously. More in detail, it had to do with how a two-electron system with spin angular momentum. Measuring this on one electron, you immediately know what the other electron has, since the sum has to be conserved. The paradox is that when you measure the angular momentum along one of the (x,y,z) axis, you also define the state of the electron. What state depends on what axis you measure relative to. Therefore, one electron can get a defined state, when another electron light-years away has its angular momentum measured. The conclusion of E-P-R was that there must be something incomplete about the quantum mechanics and that there had to exist what they called "hidden variables". "God doesn't play dice" was what Einstein said about this, meaning that there had to be something that we couldn't measure.

This thought experiment was further examined and refined by John Bell in 1964, leading to Bell's Inequality. In short, he showed that if there were any hidden variables, then the correlation for the values of measurements as mentioned above had to follow a certain quantitative relationship.

Experiments performed later broke Bell's Inequality and complied with quantum mechanics, proving that there are no hidden variables.

In 1982 French scientist Alain Aspect and his team formed a experiment that actually simulated the one suggested by E-P-R. They actually measured two particles in such a short time interval that no information could be transferred between them - according to Einstein's Special Theory of Relativity. The result was that the measurement of one particle clearly defined the state of the other. Since it was already proven that there are no hidden variables, and no information was exchanged, the conclusion was that the original Copenhagen Interpretation is correct. The wave/particle duality is real in the physical world.

Source:, my own head after reading too many physics books, for English terminology