In quantum mechanics, the uncertainty principle, also known as Heisenberg's uncertainty principle, is any of a variety of mathematical inequalities asserting a fundamental limit to the precision with which certain pairs of physical properties of a particle, known as complementary variables, such as position x and momentum p, can be known simultaneously.

Introduced first in 1927, by the German physicist Werner Heisenberg, it states that the more precisely the position of some particle is determined, the less precisely its momentum can be known, and vice versa. The formal inequality relating the standard deviation of position σ_x and the standard deviation of momentum σ_p was derived by Earle Hesse Kennard later that year and by Hermann Weyl in 1928:

(ħ is the reduced Planck constant, h / 2π).

Historically, the uncertainty principle has been confused with a somewhat similar effect in physics, called the observer effect, which notes that measurements of certain systems cannot be made without affecting the systems. Heisenberg offered such an observer effect at the quantum level (see below) as a physical "explanation" of quantum uncertainty. It has since become clear, however, that the uncertainty principle is inherent in the properties of all wave-like systems, and that it arises in quantum mechanics simply due to the matter wave nature of all quantum objects. Thus, the uncertainty principle actually states a fundamental property of quantum systems, and is not a statement about the observational success of current technology. It must be emphasized that measurement does not mean only a process in which a physicist-observer takes part, but rather any interaction between classical and quantum objects regardless of any observer.

The uncertainty principle is a fundamental concept in quantum physics.

Uncertainty principle may also refer to:

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"Uncertainty Principle" is the second episode of the first season of the American television series Numb3rs. Based on a real bank robbery case, the episode features a Federal Bureau of Investigation (FBI) math consultant's prediction being incomplete after FBI agents find themselves in an unexpected shootout with suspected bank robbers. Series writers Cheryl Heuton and Nicolas Falacci wanted to explore the emotional effects of the case on Dr. Charlie Eppes (David Krumholtz). For the mathematics used in the case, they included several mathematical and physics concepts, such as the Heisenberg uncertainty principle, P vs. NP, and Minesweeper.

The episode was directed by Davis Guggenheim and filmed in Los Angeles, California. Due to the type of scenes in the episode, filming took over nine days. During production, network executives objected both to the level of violence depicted in the episode and to the episodes plot. Guggenheim refused to tone down the violence, and Heuton and Falacci felt as though the points to which the executives objected served a purpose in the episode.