The principle in quantum theory stated by Werner Heisenberg in 1927 which says that it is not possible to simultaneously determine the position and momentum of a particle. Mathematically, this means that the momentum observable P and the position observable X do not commute or, more precisely Δψ(P)Δψ(X)≥ℏ/2, where Δψ denotes dispersion of a wave function ψ, and ℏ is the reduced Planck’s constant.