Editing Measurement problem (section)

{{Short description|Theoretical problem in quantum physics}}
{{distinguish|Measure problem (disambiguation)}}
{{quantum mechanics}}
In [[quantum mechanics]], the '''measurement problem''' is the ''problem of definite outcomes:'' quantum systems have superpositions but quantum measurements only give one definite result.<ref>{{Cite journal |last=Schlosshauer |first=Maximilian |date=2005-02-23 |title=Decoherence, the measurement problem, and interpretations of quantum mechanics |url=https://link.aps.org/doi/10.1103/RevModPhys.76.1267 |journal=Reviews of Modern Physics |language=en |volume=76 |issue=4 |pages=1267–1305 |doi=10.1103/RevModPhys.76.1267 |issn=0034-6861|arxiv=quant-ph/0312059 }}</ref><ref>{{Cite journal |last=Leggett |first=A. J. |date=2005-02-11 |title=The Quantum Measurement Problem |url=https://www.science.org/doi/10.1126/science.1109541 |journal=Science |language=en |volume=307 |issue=5711 |pages=871–872 |doi=10.1126/science.1109541 |issn=0036-8075|url-access=subscription }}</ref> 

The [[wave function]] in [[quantum mechanics]] evolves [[deterministic system|deterministically]] according to the [[Schrödinger equation]] as a linear [[Quantum superposition|superposition]] of different states. However, actual measurements always find the physical system in a definite state. Any future evolution of the wave function is based on the state the system was discovered to be in when the measurement was made, meaning that the measurement "did something" to the system that is not obviously a consequence of [[Schrödinger evolution]]. The measurement problem is describing what that "something" is, how a superposition of many possible values becomes a single measured value.

To express matters differently (paraphrasing [[Steven Weinberg]]),<ref name=Weinberg>{{cite book |title=The Oxford History of the Twentieth Century |first=Steven |last=Weinberg |chapter=The Great Reduction: Physics in the Twentieth Century |page=[https://archive.org/details/oxfordhistoryoft00howa/page/26 26] |chapter-url=https://books.google.com/books?id=WGvbAApi2roC&pg=PA22 |isbn=0-19-820428-0 |year=1998 |publisher=Oxford University Press |editor1=Michael Howard |name-list-style=amp |editor2=William Roger Louis |url-access=registration |url=https://archive.org/details/oxfordhistoryoft00howa/page/26 }}</ref><ref name=Weinberg2>{{cite journal |last=Weinberg |first=Steven |title=Einstein's Mistakes |journal=[[Physics Today]] |date=November 2005 |volume=58 |issue=11 |pages=31–35 |bibcode=2005PhT....58k..31W |doi=10.1063/1.2155755 |s2cid=120594692 |doi-access=free }}</ref> the Schrödinger equation determines the wave function at any later time. If observers and their measuring apparatus are themselves described by a deterministic wave function, why can we not predict precise results for measurements, but only probabilities? As a general question: How can one establish a correspondence between quantum reality and classical reality?<ref name=Zurek>{{cite journal |last1=Zurek |first1=Wojciech Hubert |title=Decoherence, einselection, and the quantum origins of the classical |journal=[[Reviews of Modern Physics]] |date=22 May 2003 |volume=75 |issue=3 |pages=715–775 |arxiv=quant-ph/0105127 |bibcode=2003RvMP...75..715Z |doi=10.1103/RevModPhys.75.715|s2cid=14759237 }}</ref>