Uncertainty Principle:

The position and the velocity of the bodies which we come across in our daily life can be determined accurately at a particular instant of time. Hence the path or trajectories of such bodies can be predicted. However, Werner Heisenberg in 1927 pointed out that we can never measure simultaneously and accurately both the position and velocity (or momentum) of a microscopic particle as small as an electron. Thus, it is not possible to talk of trajectory of an electron. This principle, which is a direct consequence of the dual nature of matter and radiation, states that, "it is impossible to measure simultaneously both the position and velocity (or momentum) of a microscopic particle with absolute accuracy or certainty."

The only kind of wave with a definite position is concentrated at one point, and such a wave has an indefinite wavelength (and therefore an indefinite momentum). Conversely, the only kind of wave with a definite wavelength is an infinite regular periodic oscillation over all space, which has no definite position. So 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 the momentum.

The uncertainty principle can be restated in terms of measurements, which involves collapse of the wavefunction. When the position is measured, the wavefunction collapses to a narrow bump near the measured value, and the momentum wavefunction becomes spread out. The particle's momentum is left uncertain by an amount inversely proportional to the accuracy of the position measurement. The amount of left-over uncertainty can never be reduced below the limit set by the uncertainty principle, no matter what the measurement process.

This means that the uncertainty principle is related to the observer effect, with which it is often conflated. The uncertainty principle sets a lower limit to how small the momentum disturbance in an accurate position experiment can be, and vice versa for momentum experiments.