 In 1935, Einstein, along with Boris Podolski and Nathan Rosen, argued that quantum mechanics was not complete as a theory. They wrote that, to be correct, the theory must match what we observe through experiment and measurement. To be complete, every element of physical reality must have a corresponding element in the theory. Einstein used the following thought experiment to illustrate this point. Consider two identical entangled particles starting from the same place and moving at the same speed in opposite directions from a common starting point. Letting x represent the distance traveled, x2 would have the opposite sign of x1. Meaning p represent particle momentum and given that the initial momentum was zero, p2 would have the opposite momentum of p1. So their sum would be zero. That each particle has a location and a momentum means that these quantities are elements of a physical reality. Heisenberg's uncertainty principle rules out the ability to measure these two quantities at the same time for any one particle. Because interacting with one of these properties impacts the ability to measure the other. But according to Einstein, measuring x2 allows us to predict x1 and measuring p1 allows us to predict p2. With this we can know both the position and momentum of both particles at the same time. According to Einstein, this is how a complete theory would work. But in quantum mechanics, given that these two particles are under a single wave function, measuring x2 impacts x1 in such a way as to make it impossible to measure p1. From Einstein's point of view, this was spooky action at a distance and made quantum mechanics incomplete. Einstein proposed that there are hidden variables at play that determine the state of particles like these in advance. One of his examples went like this. Suppose we have a pair of gloves, one is right-handed and one is left-handed. We place them in two identical boxes and mix up the boxes to the point where we do not know which glove is in which box. Now send these two boxes down to channels A and B. As soon as you open one and find out which handedness it was, you immediately know the other. He thought that someday a new physics theory will uncover these currently hidden variables. Niels Bohr responded with support for quantum mechanics. In his view, reality follows the wave nature of matter without any need for hidden variables. At the time, there was no way to prove whether hidden variables did or could not exist. In fact, how can you even go about proving that a hidden variable doesn't exist?