uncertainty principle

n. In quantum physics, the idea, introduced by Werner Heisenberg in 1927, that it is impossible to measure both the position and the momentum of a subatomic particle at the same time because of the fact that all known methods of measurement of one of these quantities will change the other in unpredictable ways. The implied suggestion that thinking in terms of cause and effect may not be the best way of understanding the workings of the world has also been of influence in other sciences, including psychology.