According to quantum mechanics, the spin of one electron cannot be known in advance of a measurement yet will be perfectly correlated with the other, even if it is in a distant location. Einstein didn't like this because it seemed to imply that the information can be sent from one electron to the other instantaneously – breaking a rule that says nothing can travel faster than the speed of light. He instead thought that there were "hidden variables" encoded in each electron that could determine the result if only we could access them.