Next we will discover that the more closely we try to pin down some location for the electron on its way toward the detection screen, the more it and all others like it will be likely to miss that target. Moreover, we may justifiably assume that a photon produced by a laser aimed at a detection screen will hit very near to its target on that screen, and confirm this prediction by any number of experiments. We need to learn that the electron did not have a definite position before we located it, and that it also did not have a definite momentum before we measured the trajectory. We may bring that experience to the world of atomic-sized phenomena and incorrectly assume that if we measure the position of something like an electron as it moves along its trajectory it will continue to move along that same trajectory, which we imagine we can then accurately detect in the next few moments. 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. That is because the uncertainties in position and velocity are so small that we could not detect them.

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. Trying to pin a thing down to one definite position will make its momentum less well pinned down, and vice-versa. Werner Heisenberg stumbled on a secret of the universe: Nothing has a definite position, a definite trajectory, or a definite momentum. The Uncertainty principle is also called the Heisenberg uncertainty principle. You can help Wikipedia by reading Wikipedia:How to write Simple English pages, then simplifying the article. The English used in this article or section may not be easy for everybody to understand.