In [[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}}</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">
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Calculations of [[quantum decoherence]] show that when a quantum system interacts with the environment, the superpositions ''apparently'' reduce to mixtures of classical alternatives. Significantly, the combined wave function of the system and environment continue to obey the Schrödinger equation throughout this ''apparent'' collapse.<ref name=Zurek>{{cite journal |last=Zurek |first=Wojciech Hubert |title=Quantum Darwinism |journal=Nature Physics |year=2009 |volume=5 |issue=3 |pages=181–188 |doi=10.1038/nphys1202 |arxiv = 0903.5082 |bibcode = 2009NatPh...5..181Z|issue=3 |s2cid=119205282}}</ref> More importantly, this is not enough to explain ''actual'' wave function collapse, as decoherence does not reduce it to a single eigenstate.<ref name=Schlosshauer>{{cite journal |last=Schlosshauer |first=Maximilian |title=Decoherence, the measurement problem, and interpretations of quantum mechanics |journal=Rev. Mod. Phys. |year=2005 |volume=76 |issue=4 |pages=1267–1305 |doi=10.1103/RevModPhys.76.1267 |arxiv = quant-ph/0312059 |bibcode = 2004RvMP...76.1267S |s2cid=7295619}}</ref><ref name="Stanford1">{{cite encyclopedia
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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 'The actual content of quantum theoretical kinematics and mechanics' [https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19840008978.pdf here"The actual content of quantum theoretical kinematics and mechanics"].</ref>{{Citation needed|date=March 2024|reason=need a secondary ref to support this claim}}
