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. Initially the light projected on the screen is a dot, and as the slit is made smaller, the dot becomes smaller, as expected. However, beyond a point of uncertainty, as the slit becomes very small, the dot widens. The light has actually taken two directions and spread out left and right.

In the diagram, a laser shines through a glass slab and as expected, its light is refracted at its angle of incidence through the slab and out the bottom. However, some of the light reflects off the top surface as well. Why do some photons choose to refract and others to reflect? This aptly demonstrates the uncertainty principle as well as duality.