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. A deep thought that arises from this, is that if photons and electrons can do it, then why not people, and why not universes? This makes the multiverse and beyond feasible.

The uncertainty principle is a notable consequence of quantum indeterminism. The Copenhagen interpretation of quantum mechanics is probabilistic because measuring an eigenvalue of a quantum system causes the associated wavefunction of that quantum system to collapse. However, according to the Many-Worlds interpretation of quantum mechanics, the wavefunction never actually collapses. Even though the total number of all possible measurable eigenvalues of such a system is about 10^{500}, it is still regarded to be deterministic.