It is impossible to determine both momentum and position by means of the same measurement.

Heisenberg argues that every measurement destroys part of our knowledge of the system that was obtained by previous measurements. The uncertainty principle states a fundamental property of quantum systems. In quantum mechanics, there can be no states that describe a particle with both a definite position and a definite momentum. The more precise the position, the less precise is the momentum and vice versa.

The uncertainty principle can be restated in terms of other measurement processes, which involves collapse of the wave function. When the position is initially localized by preparation, the wave function collapses to a narrow bump in an interval Δx > 0, and the momentum wave function becomes spread out. The particle's momentum is left uncertain by an amount inversely proportional to the accuracy of the position measurement. For example even though an atom is in a state |Ψ> to start with, when an observation is made on it in order to determine the energy, the atom jumps to one of the base states. This jump is referred to as collapse of the wave function.