There was one way that Heisenberg could be right, a reason that Einstein thought was nonsense: What if measuring the position of the first particle would mess up the velocity of the second particle. That would be like magic. How could anybody explain such an influence? Suppose that the two particles were moving away very fast and a long time had gone by. If something that happened to the first particle somehow influenced the second particle, its influence would have to travel faster than the speed of light, which is impossible. Physicists like Erwin Schrödinger suggested that maybe the relationship in position and velocity would just gradually go away somehow. Schrödinger called the connection between the two particles (and anything like it that happened to other things) "entanglement." 