# Uncertainty principle

## Foundational principle in quantum physics / From Wikipedia, the free encyclopedia

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The **uncertainty principle**, also known as **Heisenberg's indeterminacy principle**, is a fundamental concept in quantum mechanics. It states that there is a limit to the precision with which certain pairs of physical properties, such as position and momentum, can be simultaneously known. In other words, the more accurately one property is measured, the less accurately the other property can be known.

More formally, the uncertainty principle is any of a variety of mathematical inequalities asserting a fundamental limit to the product of the accuracy of certain related pairs of measurements on a quantum system, such as position, *x*, and momentum, *p*.^{[1]} Such paired-variables are known as complementary variables or canonically conjugate variables.

First introduced in 1927 by German physicist Werner Heisenberg,^{[2]}^{[3]}^{[4]}^{[5]} the formal inequality relating the standard deviation of position *σ _{x}* and the standard deviation of momentum

*σ*was derived by Earle Hesse Kennard

_{p}^{[6]}later that year and by Hermann Weyl

^{[7]}in 1928:

$\sigma _{x}\sigma _{p}\geq {\frac {\hbar }{2}}$

where $\hbar ={\frac {h}{2\pi }}$ is the reduced Planck constant.

The quintessentially quantum mechanical uncertainty principle comes in many forms other than position–momentum. The energy–time relationship is widely used to relate quantum state lifetime to measured energy widths but its formal derivation is fraught with confusing issues about the nature of time. The basic principle has been extended in numerous directions; it must be considered in many kinds of fundamental physical measurements.