What is the name of the principle that states it is impossible to simultaneously know the exact position and momentum of a particle?

The principle stating that a particle’s exact position and exact momentum cannot both be known simultaneously is the Heisenberg uncertainty principle. It is a fundamental feature of quantum mechanics, not merely a limitation caused by imperfect instruments.

For position and momentum, the principle is commonly written as ΔxΔp ≥ ħ/2, where ħ is the reduced Planck constant. A state tightly localized in position must contain a wider range of possible momenta; a state with sharply defined momentum is spread out in position. This relationship follows from the mathematical structure of quantum states and the non-commuting position and momentum operators.

Werner Heisenberg introduced the principle in 1927 during the early development of quantum mechanics. Later work by Earle Kennard, Hermann Weyl, Howard Robertson, and Erwin Schrödinger gave it more precise mathematical forms.

The principle is often described as measurement disturbance, but that is incomplete. Disturbing a particle during measurement can matter, yet the deeper statement concerns the uncertainties inherent in the quantum state itself. The complementarity principle is related, but it is broader and is not the canonical answer to this question.