Physics Concepts and Definitions

What is Heisenberg's uncertainty principle?
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  1. Proposed by German theoretical physicist Werner Heisenberg in 1927, the uncertainty principle dictates that, at the miniscule level of quantum mechanics, it becomes impossible to measure a particle's exact location with any degree of precision. In fact, you can't measure the particle's position without affecting its momentum in an unpredictable way. The concepts of exact position and exact velocity together have no meaning in the natural world. In other words, uncertainty is inherent in the quantum realm.

    Momentum is mass times velocity, and although Heisenberg's principle does not seem Earth-shattering of itself -- after all, it has the word uncertainty in it -- the relationship of momentum and position are critical to the study of concepts such as causality, or determination of future events.

    Also called the principle of indeterminacy, Heisenberg's theory began as a debate he had with a close colleague in 1926. The debates centered mostly on whether light and matter could only behave in continuous waves or make quantum jumps. Some of his colleagues were able to produce equations that formed the basis of what is now considered quantum mechanics.  As Heisenberg studied these quantum equations, he noticed that imprecisions, or uncertainties, occurred whenever the observer tried to measure a particle's position and momentum simultaneously. He presented his findings -- which he said were inherent to quantum mechanics and not the fault of the observer -- in a letter to a colleague. The letter eventually was modified to become a published paper on the uncertainty principle [source: American Institute of Physics].

    Heisenberg's uncertainty principle also applies to other observable properties of matter and light: Energy and time are one example. An unstable atom or nucleus radiates an uncertain quantity of energy and an uncertain lifetime as it transitions to a more stable state [source: University of Oregon]. One of the most profound concepts in the uncertainty principle is that concepts such as orbiting electrons did not exist unless they were observed.

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