Heisenberg Uncertainty Principle

The Heisenberg uncertainty principle states that the position and the velocity of an object cannot both be measured exactly, at the same time. The rule says that the product of the uncertainties in position and velocity is equal to or greater than Planck’s constant, so only for exceedingly small masses of atoms and subatomic particles does the product of the uncertainties become significant.

Any attempt to measure the velocity of an electron will knock it about in an unpredictable way so that a simultaneous measurement of its position has no validity. The uncertainty principle arises from the wave-particle duality. Every particle has a wave associated with it; a measurement of one observable involves a relatively large uncertainty in the measurement of the other.

The double-slit experiment is a demonstration that light and matter can display characteristics of both waves and particles. A laser beam illuminates a plate pierced by two parallel slits, and the light passing through the slits is observed on a screen behind the plate. The wave nature of light causes the light waves passing through the two slits to interfere, producing bright and dark bands on the screen – a result that would not be expected if light consisted of classical particles. However, the light is always found to be absorbed at the screen at discrete points, as individual particles. These results demonstrate the principle of wave–particle duality. Other atomic-scale entities, such as electrons, are found to exhibit the same behavior when fired towards a double slit.