In physics, particularly in quantum physics, a system **observable** is a measurable operator, or gauge, where the property of the system state can be determined by some sequence of physical operations. For example, these operations might involve submitting the system to various electromagnetic fields and eventually reading a value off some gauge. In systems governed by classical mechanics, any experimentally observable value can be shown to be given by a real-valued function on the set of all possible system states.

Physically meaningful observables must also satisfy transformation laws which relate observations performed by different observers in different frames of reference. These transformation laws are automorphisms of the state space, that is bijective transformations which preserve some mathematical property.

Read more about Observable: Quantum Mechanics, Incompatibility of Observables in Quantum Mechanics

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### Famous quotes containing the word observable:

“To develop an empiricist account of science is to depict it as involving a search for truth only about the empirical world, about what is actual and *observable*.... It must involve throughout a resolute rejection of the demand for an explanation of the regularities in the *observable* course of nature, by means of truths concerning a reality beyond what is actual and *observable*, as a demand which plays no role in the scientific enterprise.”

—Bas Van Fraassen (b. 1941)

“Every living language, like the perspiring bodies of living creatures, is in perpetual motion and alteration; some words go off, and become obsolete; others are taken in, and by degrees grow into common use; or the same word is inverted to a new sense or notion, which in tract of time makes an *observable* change in the air and features of a language, as age makes in the lines and mien of a face.”

—Richard Bentley (1662–1742)