**1925**
Werner Heisenberg, a German theoretical physicist, proposed in 1925 in his famous Uncertainty Principle that we can know either the position or the momentum of a subatomic particle, but not both. Further, Heisenberg said, the more precisely we know the particle’s momentum, the less we can know about its position. At the atomic scale, Newton’s laws of classical mechanics give way to mathematical functions, developed by Erwin Schrödinger in 1926, that describe particle behavior in terms of probabilities. The work of Heisenberg and Schrödinger is the foundation of quantum mechanics, the theory that has proved eminently successful in describing and predicting the behavior of subatomic particles.