Uncertainty principle
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 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. 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. Next we will discover that the more closely we try to measure some location of the electron on its way toward the detection screen, the more it and all others like it will be likely to miss that target. So the mere act of measuring the location of an electron makes the trajectory more indefinite, indeterminate, or uncertain. If the trajectory were made more clear and then we were to try to locate that electron along an extension of the trajectory we just staked out, then we would find that the more precise we made our knowledge of the trajectory, the less likely we would be to find the electron where ordinary expectations would lead us to believe it to be. If pitchers threw electrons instead of baseballs, and an overhead camera and side-facing camera were placed somewhere between the pitcher's mound and home plate so that the exact position of the electron could be determined in mid flight, then without the cameras being turned on, the pitcher would throw straight balls, and with the cameras turned on his pitches would start out straight but gyrate wildly after their pictures were taken. The more clearly we know where the ball was halfway toward home place, the more trouble the batter will have in getting ready to hit it with his bat.