Editing Observable (section)

{{Short description|Any entity that can be measured}}
{{About|the use in physics|the use in statistics|Observable variable|the use in control theory|Observability|the use in software engineering|Observer pattern}}

{{More footnotes|date=May 2009}}

In [[physics]], an '''observable''' is a [[physical property]] or [[physical quantity]] that can be [[Measurement|measured]]. In [[classical mechanics]], an observable is a [[real number|real]]-valued "function" on the set of all possible system states, e.g., [[Position (vector)|position]] and [[momentum]]. In [[quantum mechanics]], an observable is an [[quantum operator|operator]], or [[gauge theory|gauge]], where the property of the [[quantum state]]  can be determined by some sequence of [[operational definition|operations]]. For example, these operations might involve submitting the system to various [[electromagnetic field]]s and eventually reading a value.

Physically meaningful observables must also satisfy [[linear map|transformation]] laws that relate observations performed by different [[observation|observer]]s in different [[frames of reference]]. These transformation laws are [[automorphism]]s of the [[state space (physics)|state space]], that is [[bijection|bijective]] [[transformation (mathematics)|transformation]]s that preserve certain mathematical properties of the space in question.