Editing Measurement problem (section)

==Interpretations==
{{main|Interpretations of quantum mechanics}}

The views often grouped together as the [[Copenhagen interpretation]] are the oldest and, collectively, probably still the most widely held attitude about quantum mechanics.<ref>{{cite journal |last1=Schlosshauer |first1=Maximilian |last2=Kofler |first2=Johannes |last3=Zeilinger |first3=Anton |title=A snapshot of foundational attitudes toward quantum mechanics |journal=[[Studies in History and Philosophy of Science Part B]] |date=August 2013 |volume=44 |issue=3 |pages=222–230 |arxiv=1301.1069 |bibcode=2013SHPMP..44..222S |doi=10.1016/j.shpsb.2013.04.004|s2cid=55537196 }}</ref><ref>{{Cite journal | url=https://www.nature.com/news/experts-still-split-about-what-quantum-theory-means-1.12198 | doi=10.1038/nature.2013.12198| title=Experts still split about what quantum theory means| journal=[[Nature (journal)|Nature]]| year=2013| last1=Ball| first1=Philip| author-link=Philip Ball |s2cid=124012568| url-access=subscription}}</ref> [[N. David Mermin]] coined the phrase "Shut up and calculate!" to summarize Copenhagen-type views, a saying often misattributed to [[Richard Feynman]] and which Mermin later found insufficiently nuanced.<ref>{{cite journal|journal=[[Physics Today]] |volume=42 |number=4 |year=1989 |page=9 |doi=10.1063/1.2810963 |first=N. David |last=Mermin |author-link=N. David Mermin |title=What's Wrong with this Pillow?|bibcode=1989PhT....42d...9D }}</ref><ref>{{cite journal|doi=10.1063/1.1768652|title=Could Feynman have said this?|journal=[[Physics Today]]|volume=57|issue=5|pages=10–11|year=2004|last1=Mermin|first1=N. David|author-link=N. David Mermin |bibcode=2004PhT....57e..10M|doi-access=free}}</ref>

Generally, views in the Copenhagen tradition posit something in the act of observation which results in the [[Wave function collapse|collapse of the wave function]]. This concept, though often attributed to [[Niels Bohr]], was due to [[Werner Heisenberg]], whose later writings obscured many disagreements he and Bohr had during their collaboration and that the two never resolved.<ref>{{Cite journal|last=Howard|first=Don|date=December 2004|title=Who Invented the "Copenhagen Interpretation"? A Study in Mythology|url=https://www.journals.uchicago.edu/doi/10.1086/425941|journal=[[Philosophy of Science (journal)|Philosophy of Science]] |language=en|volume=71|issue=5|pages=669–682|doi=10.1086/425941|s2cid=9454552 |issn=0031-8248}}</ref><ref>{{Cite journal|last=Camilleri|first=Kristian|date=May 2009|title=Constructing the Myth of the Copenhagen Interpretation|url=http://www.mitpressjournals.org/doi/10.1162/posc.2009.17.1.26|journal=Perspectives on Science|language=en|volume=17|issue=1|pages=26–57|doi=10.1162/posc.2009.17.1.26|s2cid=57559199 |issn=1063-6145|url-access=subscription}}</ref> In these schools of thought, wave functions may be regarded as statistical information about a quantum system, and wave function collapse is the updating of that information in response to new data.<ref>{{Cite journal|last=Englert|first=Berthold-Georg|author-link=Berthold-Georg Englert|date=2013-11-22|title=On quantum theory|journal=[[The European Physical Journal D]]|language=en|volume=67|issue=11|pages=238|arxiv=1308.5290|doi=10.1140/epjd/e2013-40486-5|bibcode=2013EPJD...67..238E |s2cid=119293245 |issn=1434-6079}}</ref><ref name="Peierls">{{Cite journal|last=Peierls|first=Rudolf|author-link=Rudolf Peierls |date=1991|title=In defence of "measurement"|journal=[[Physics World]] |language=en|volume=4|issue=1|pages=19–21|doi=10.1088/2058-7058/4/1/19|issn=2058-7058}}</ref> Exactly how to understand this process remains a topic of dispute.<ref name="stanford2">{{Cite book|chapter-url=https://plato.stanford.edu/entries/quantum-bayesian/|title=[[Stanford Encyclopedia of Philosophy]]|last=Healey|first=Richard|publisher=Metaphysics Research Lab, Stanford University|year=2016|editor-last=Zalta|editor-first=Edward N.|chapter=Quantum-Bayesian and Pragmatist Views of Quantum Theory}}</ref>

Bohr discussed his views in a 1947 letter to Pauli.<ref>{{citation |author=Bohr |first=Niels |title=Niels Bohr: Collected Works |date=1985 |volume=6: Foundations of Quantum Physics I (1926–1932) |pages=451–454 |editor=Jørgen Kalckar |orig-date=May 16, 1947 |url=https://www.nbarchive.dk/publications/bcw/ |author-link=Niels Bohr}}</ref> Bohr points out that the measurement processes such as cloud chambers or photographic plates involve enormous amplification requiring energies far in excess of the quantum effects being studied and he notes that these processes are irreversible.<ref>{{citation |author=Stenholm |first=Stig |title=Quantum Optics, Experimental Gravitation, and Measurement Theory |pages=121 |year=1983 |editor=Meystre |editor-first=Pierre |chapter=To fathom space and time |publisher=Plenum Press |quote=The role of irreversibility in the theory of measurement has been emphasized by many. Only this way can a permanent record be obtained. The fact that separate pointer positions must be of the asymptotic nature usually associated with irreversibility has been utilized in the measurement theory of Daneri, Loinger and Prosperi (1962). It has been accepted as a formal representation of Bohr's ideas by Rosenfeld (1966). |author-link=Stig Stenholm |editor1-link=Pierre Meystre}}</ref> He considered a consistent account of this issue to be an unsolved problem.

[[Hugh Everett]]'s [[many-worlds interpretation]] attempts to solve the problem by suggesting that there is only one wave function, the superposition of the entire universe, and it never collapses—so there is no measurement problem. Instead, the act of measurement is simply an interaction between quantum entities, e.g. observer, measuring instrument, electron/positron etc., which entangle to form a single larger entity, for instance ''living cat/happy scientist''. Everett also attempted to demonstrate how the probabilistic nature of [[quantum mechanics]] would appear in measurements, a work later extended by [[Bryce DeWitt]]. However, proponents of the Everettian program have not yet reached a consensus regarding the correct way to justify the use of the [[Born rule]] to calculate probabilities.<ref>{{cite encyclopedia|first=Adrian |last=Kent |author-link=Adrian Kent |title=One world versus many: the inadequacy of Everettian accounts of evolution, probability, and scientific confirmation |arxiv=0905.0624 |encyclopedia=Many Worlds? |year=2010 |pages=307–354 |publisher=[[Oxford University Press]] |isbn=9780199560561 |oclc=696602007}}</ref><ref name="stanford1">{{Cite book|chapter-url=https://plato.stanford.edu/entries/qm-everett/|title=[[Stanford Encyclopedia of Philosophy]]|last=Barrett|first=Jeffrey|publisher=Metaphysics Research Lab, Stanford University|year=2018|editor-last=Zalta|editor-first=Edward N.|chapter=Everett's Relative-State Formulation of Quantum Mechanics}}</ref>

The [[de Broglie–Bohm theory]] tries to solve the measurement problem very differently: the information describing the system contains not only the wave function, but also supplementary data (a trajectory) giving the position of the particle(s). The role of the wave function is to generate the velocity field for the particles. These velocities are such that the probability distribution for the particle remains consistent with the predictions of the orthodox quantum mechanics. According to the de Broglie–Bohm theory, interaction with the environment during a measurement procedure separates the wave packets in configuration space, which is where apparent wave function collapse comes from, even though there is no actual collapse.<ref>{{cite book|chapter-url=https://plato.stanford.edu/entries/qm-bohm/ |first=Goldstein |last=Sheldon |author-link=Sheldon Goldstein |chapter=Bohmian Mechanics |title=[[Stanford Encyclopedia of Philosophy]] |year=2017 |publisher=Metaphysics Research Lab, Stanford University |editor-last=Zalta|editor-first=Edward N.}}</ref>

A fourth approach is given by [[Objective-collapse theory|objective-collapse models]]. In such models, the [[Schrödinger equation]] is modified and obtains nonlinear terms. These nonlinear modifications are of [[stochastic]] nature and lead to behaviour that for microscopic quantum objects, e.g. electrons or atoms, is unmeasurably close to that given by the usual Schrödinger equation. For macroscopic objects, however, the nonlinear modification becomes important and induces the collapse of the wave function. Objective-collapse models are [[Effective theory|effective theories]]. The stochastic modification is thought to stem from some external non-quantum field, but the nature of this field is unknown. One possible candidate is the gravitational interaction as in the models of Diósi and [[Penrose interpretation|Penrose]]. The main difference of objective-collapse models compared to the other approaches is that they make [[Falsifiability|falsifiable]] predictions that differ from standard quantum mechanics. Experiments are already getting close to the parameter regime where these predictions can be tested.<ref>{{cite journal |author=Bassi |first1=Angelo |last2=Lochan |first2=Kinjalk |last3=Satin |first3=Seema |last4=Singh |first4=Tejinder P. |last5=Ulbricht |first5=Hendrik |year=2013 |title=Models of wave-function collapse, underlying theories, and experimental tests |journal=[[Reviews of Modern Physics]] |volume=85 |issue=2 |pages=471–527 |arxiv=1204.4325 |bibcode=2013RvMP...85..471B |doi=10.1103/RevModPhys.85.471 |s2cid=119261020}}</ref> 

The [[Ghirardi–Rimini–Weber theory|Ghirardi–Rimini–Weber (GRW) theory]] proposes that wave function collapse happens spontaneously as part of the dynamics. Particles have a non-zero probability of undergoing a "hit", or spontaneous collapse of the wave function, on the order of once every hundred million years.<ref>Bell, J. S. (2004). "Are there quantum jumps?". Speakable and Unspeakable in Quantum Mechanics. pp. 201–212.</ref> Though collapse is extremely rare, the sheer number of particles in a measurement system means that the probability of a collapse occurring somewhere in the system is high. Since the entire measurement system is entangled (by quantum entanglement), the collapse of a single particle initiates the collapse of the entire measurement apparatus. Because the GRW theory makes different predictions from orthodox quantum mechanics in some conditions, it is not an interpretation of quantum mechanics in a strict sense.