The EPR paradox is an early and strong criticism of quantum mechanics. Albert Einstein and his co-workers, Boris Podolsky and Nathan Rosen, said that Niels Bohr, Werner Heisenberg, and the other scientists in Copenhagen were wrong about uncertainty. Heisenberg claimed that you could never know, for any one time, both the position and momentum (or velocity, or trajectory) of any atom-sized or smaller particle. The idea was that the two could not be measured at the same time, and that as soon as one was measured a change would occur and you would not get the same answer for the other one that you would have gotten if you had measured it first. Einstein and his group said that Heisenberg should think again. Suppose you had two very small particles. You measured the weight of each of them and then stuck them together. You gave them a little push. Then something broke them apart. They should have both positions and velocities that were related. So if you measured the position of one of them, then even if you assumed that Heisenberg was right and you could not involve messing up its velocity in the process of measuring it, surely that did not mean that it never had a definite velocity. For proof, Einstein said that you could next measure the velocity of the second particle and, since everything was mathematically related, you would then know the velocity of the first particle. 