The uncertainty principle is one of the main differences between the study of classical mechanics and quantum mechanics. In classical mechanics, a physical quantity can be simultaneously assigned to any particle. In quantum mechanics, the uncertainty principle states the more closely you measure the momentum of a particle, the less certain you can measure the position.
An example of this would be if a microscope was used to measure the position of a particle, the measurement would have an accuracy dependent on the wavelength of the light used to take the measurement. When the light strikes the particle and is reflected back to you to record, it imparts energy to the particle and changes its momentum. The shorter the wavelength used by the microscope, the better the resolving power to pinpoint the position. Unfortunately, the shorter wavelength means more energy is involved in the collision with the particle and the less can be known of its momentum.
Quantum mechanics deals mainly with probabilities. A particle has a quantum state with probabilities for physical properties such as position, momentum, energy and angular momentum. The uncertainty principle outlines the theoretical limit of knowing how close you can measure any of these properties. Find out more about Heisenberg and what else occurred on this day in science history.