uncertainty principle
the principle, introduced by German physicist Werner Heisenberg (1901–1976), that it is impossible to measure accurately both the position and the momentum of a particle at a specific moment in time. It arises from the fact that the act of measurement will itself interfere with the system being measured in unpredictable ways. The same principle applies to other paired variables, such as energy and time. The uncertainty principle has implications for philosophy, in that it appears to undermine the laws of cause and effect. Also called Heisenberg principle; indeterminacy principle.