The Uncertainty principle is also called the Heisenberg uncertainty principle. Werner Heisenberg stumbled on a secret of the universe: Nothing has a definite position, a definite trajectory, or a definite momentum. Trying to pin a thing down to one definite position will make its momentum less well pinned down, and vice-versa. In everyday life we can successfully measure the position of an automobile at a definite time and then measure its direction and speed (assuming it is coasting along at a steady rate) in the next few moments. That is because the uncertainties in position and velocity are so small that we could not detect them. We assume, quite correctly, that the trajectory of the automobile will not be noticeably changed when we drop a marker on the ground and click a stopwatch at the same time to note the car's position in time and space.