Editing Wave function collapse (section)

{{Short description|Process by which a quantum system takes on a definitive state}}
{{for|the constraint-solving algorithm| Wave function collapse (algorithm)}}
{{Use American English|date=January 2019}}
[[File:Wave-particle duality.gif|thumb|Particle impacts during a [[double-slit experiment]]. The total [[interference pattern]] represents the original [[wave function]], while each particle impact represents an individual wave function collapse.]]
{{Quantum mechanics}}

In various [[Interpretations of quantum mechanics|interpretations]] of [[quantum mechanics]], '''wave function collapse''', also called '''reduction of the state vector''',<ref>{{Cite journal |last=Penrose |first=Roger |date=May 1996 |title=On Gravity's role in Quantum State Reduction |url=http://link.springer.com/10.1007/BF02105068 |journal=General Relativity and Gravitation |language=en |volume=28 |issue=5 |pages=581–600 |doi=10.1007/BF02105068 |issn=0001-7701|url-access=subscription }}</ref> occurs when a [[wave function]]—initially in a [[quantum superposition|superposition]] of several [[eigenstates]]—reduces to a single eigenstate due to [[Fundamental interaction|interaction]] with the external world. This interaction is called an [[Observation (physics)|''observation'']] and is the essence of a [[measurement in quantum mechanics]], which connects the wave function with classical [[observable]]s such as [[position (vector)|position]] and [[momentum]]. Collapse is one of the two processes by which [[quantum system]]s evolve in time; the other is the continuous evolution governed by the [[Schrödinger equation]].<ref name="Grundlagen">
{{cite book
 |author=J. von Neumann
 |year=1932
 |title=Mathematische Grundlagen der Quantenmechanik
 |publisher=[[Springer (publisher)|Springer]]
 |location=Berlin
|language=de}}<br/>
:{{cite book
 |author=J. von Neumann
 |year=1955
 |title=Mathematical Foundations of Quantum Mechanics
 |url=https://archive.org/details/mathematicalfoun0613vonn
 |url-access=registration
 |publisher=[[Princeton University Press]]
}}</ref>

In the [[Copenhagen interpretation]], wave function collapse connects quantum to classical models, with a special [[Copenhagen interpretation#Role of the observer|role for the observer]]. By contrast, [[Objective-collapse theory|objective-collapse]] proposes an origin in physical processes. In the [[many-worlds interpretation]], collapse does not exist; all wave function outcomes occur while [[quantum decoherence]] accounts for the appearance of collapse.

Historically, [[Werner Heisenberg]] was the first to use the idea of wave function reduction to explain quantum measurement.<ref>[[Werner Heisenberg|Heisenberg, W.]] (1927). Über den anschaulichen Inhalt der quantentheoretischen Kinematik und Mechanik, ''Z. Phys.'' '''43''': 172–198. Translation as [https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19840008978.pdf "The actual content of quantum theoretical kinematics and mechanics"].</ref><ref name="C. Kiefer-2002" />