Editing Schrödinger's cat (section)

== Interpretations ==
Since Schrödinger's time, other interpretations of quantum mechanics have been proposed that give different answers to the questions posed by Schrödinger's cat of how long superpositions last and when (or ''whether'') they collapse.

=== Copenhagen interpretation ===
{{Main|Copenhagen interpretation}}

A commonly held interpretation of quantum mechanics is the Copenhagen interpretation.<ref name="Wimmel1992">{{cite book|first=Hermann|last=Wimmel|title=Quantum physics & observed reality: a critical interpretation of quantum mechanics|url=https://books.google.com/books?id=-4sJ_fgyZJEC&pg=PA2|access-date=9 May 2011|year=1992|publisher=World Scientific|isbn=978-981-02-1010-6|page=2|url-status=live|archive-url=https://web.archive.org/web/20130520185205/http://books.google.com/books?id=-4sJ_fgyZJEC&pg=PA2|archive-date=20 May 2013}}</ref>  In the Copenhagen interpretation, a measurement results in only one state of a superposition. This thought experiment makes apparent the fact that this interpretation simply provides no explanation for the state of the cat while the box is closed. The wavefunction description of the system consists of a superposition of the states "decayed nucleus/dead cat" and "undecayed nucleus/living cat". Only when the box is opened and observed can we make a statement about the cat.<ref name=BaggottStory/>{{rp|157|q=As far as the Copenhagen interpretation is concerned...it is meaningless to speculate on whether it is alive or dead until the box is opened.}}

===Role of consciousness===
{{Main|Consciousness causes collapse}}
In 1932, [[John von Neumann]] described in his book ''[[Mathematical Foundations of Quantum Mechanics]]'' a pattern where the radioactive source is observed by a device, which itself is observed by another device and so on. It makes no difference in the predictions of quantum theory where along this chain of causal effects the superposition collapses.<ref name="Tales of the Quantum">{{cite book |last1=Hobson |first1=Art |title=Tales of the Quantum: Understanding Physics' Most Fundamental Theory |date=2017 |publisher=Oxford University Press |location=New York, NY |isbn=9780190679637 |pages=200–202 |url=https://books.google.com/books?id=mGduDQAAQBAJ |access-date=April 8, 2022}}</ref> This potentially infinite chain could be broken if the last device is replaced by a conscious observer. This solved the problem because it was claimed that an individual's consciousness cannot be multiple.<ref>{{cite book |last1=Omnès |first1=Roland |title=Understanding Quantum Mechanics |date=1999 |publisher=Princeton University Press |location=Princeton, New Jersey |isbn=0-691-00435-8 |pages=60–62 |url=https://books.google.com/books?id=XET_DwAAQBAJ |access-date=April 8, 2022}}</ref> Eugene Wigner asserted that an observer is necessary for a collapse to one or the other (e.g., either a live cat or a dead cat) of the terms on the right-hand side of a [[wave function]]. Wigner discussed the interpretation in a thought experiment known as [[Wigner's friend]].<ref name="Surfing the Quantum World">{{cite book |last1=Levin |first1=Frank S. |title=Surfing the Quantum World |date=2017 |publisher=Oxford University Press |location=New York, NY |isbn=978-0-19-880827-5 |pages=229–232 |url=https://books.google.com/books?id=Y1w-DwAAQBAJ |access-date=April 8, 2022}}</ref>

Wigner supposed that a friend opened the box and observed the cat without telling anyone. From Wigner's conscious perspective, the friend is now part of the wave function and has seen a live cat and seen a dead cat. To a third person's conscious perspective, Wigner himself becomes part of the wave function once Wigner learns the outcome from the friend. This could be extended indefinitely.<ref name="Surfing the Quantum World"/>

A resolution of the paradox is that the triggering of the Geiger counter counts as a measurement of the state of the radioactive substance. Because a measurement has already occurred deciding the state of the cat, the subsequent observation by a human records only what has already occurred.<ref>{{cite book |last1=Puri |first1=Ravinder R. |title=Non-Relativistic Quantum Mechanics |date=2017 |publisher=Cambridge University Press |location=Cambridge, United Kingdom |isbn=978-1-107-16436-9 |page=146 |url=https://books.google.com/books?id=qDbSDgAAQBAJ |access-date=April 8, 2022}}</ref> Analysis of an actual experiment by [[Roger Carpenter]] and A. J. Anderson found that measurement alone (for example by a Geiger counter) is sufficient to collapse a quantum wave function before any human knows of the result.<ref name="Carpenter2006">{{cite journal | title = The death of Schrödinger's cat and of consciousness-based wave-function collapse | journal = [[Annales de la Fondation Louis de Broglie]] | year = 2006 | author = Carpenter RHS, Anderson AJ | volume = 31 | issue = 1 | pages = 45–52| url = http://www.ensmp.fr/aflb/AFLB-311/aflb311m387.pdf  | access-date = 2010-09-10 |archive-url = https://web.archive.org/web/20061130173850/http://www.ensmp.fr/aflb/AFLB-311/aflb311m387.pdf |archive-date = 2006-11-30}}</ref> The apparatus indicates one of two colors depending on the outcome. The human observer sees which color is indicated, but they don't consciously know which outcome the color represents. A second human, the one who set up the apparatus, is told of the color and becomes conscious of the outcome, and the box is opened to check if the outcome matches.<ref name="Tales of the Quantum"/> However, it is disputed whether merely observing the color counts as a conscious observation of the outcome.<ref name="Okon2006">{{cite journal | title = How to Back up or Refute Quantum Theories of Consciousness | journal = Mind and Matter | year = 2016 | author = Okón E, Sebastián MA | volume = 14 | issue = 1 | pages = 25–49}}</ref>

===Bohr's interpretation===
Analysis of the work of  [[Niels Bohr]], one of the main scientists associated with the Copenhagen interpretation, suggests he viewed the state of the cat before the box is opened as indeterminate. The superposition itself had no physical meaning to Bohr: Schrödinger's cat would be either dead or alive long before the box is opened but the cat and box form a inseparable combination.<ref name="Faye2008">{{cite encyclopedia | url = http://plato.stanford.edu/entries/qm-copenhagen/ | title = Copenhagen Interpretation of Quantum Mechanics | access-date = 2010-09-19 | last = Faye | first = J | date = 2008-01-24 | encyclopedia = [[Stanford Encyclopedia of Philosophy]] | publisher = The Metaphysics Research Lab Center for the Study of Language and Information, [[Stanford University]]}}</ref> 
Bohr saw no role for a human observer.<ref>{{cite journal |title=Against 'measurement' |author=[[John Stewart Bell|John Bell]] |journal=Physics World |volume=3 |number=8 |year=1990 |pages=33–41 |doi=10.1088/2058-7058/3/8/26}}</ref>{{rp|35}}
Bohr emphasized the classical nature of measurement results.
An "irreversible" or effectively irreversible process imparts the classical behavior of "observation" or "measurement".<ref>{{cite book |author=[[Niels Bohr]] |orig-date=May 16, 1947 |date=1985 |series=Niels Bohr: Collected Works |volume=6 |title=Foundations of Quantum Physics I (1926-1932) |editor=Jørgen Kalckar |pages=451–454 |url=https://www.nbarchive.dk/publications/bcw/}}</ref><ref>{{cite book |chapter=To fathom space and time |pages=121 |author=Stig Stenholm | title=Quantum Optics, Experimental Gravitation, and Measurement Theory|editor1-link=Pierre Meystre |editor=Pierre Meystre |publisher=Plenum Press |year=1983 |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).}}</ref><ref>{{cite journal |title=Classical motion of meter variables in the quantum theory of measurement |author=Fritz Haake |date= April 1, 1993 |journal=[[Physical Review A]] |doi=10.1103/PhysRevA.47.2506 |volume=47 |issue=4 |pages=2506–2517 |pmid=9909217 |bibcode=1993PhRvA..47.2506H }}</ref>

=== Many-worlds interpretation ===
{{Main|Many-worlds interpretation}}
[[File:Schroedingers cat film.svg|thumb|The quantum-mechanical "Schrödinger's cat" paradox according to the many-worlds interpretation. In this interpretation, every event is a branch point. The cat is both alive and dead—regardless of whether the box is opened—but the "alive" and "dead" cats are in different branches of the universe that are equally real but cannot interact with each other.]]

In 1957, [[Hugh Everett]] formulated the many-worlds interpretation of quantum mechanics, which does not single out observation as a special process. In the many-worlds interpretation, both alive and dead states of the cat persist after the box is opened, but are [[quantum decoherence|decoherent]] from each other. In other words, when the box is opened, the observer and the possibly-dead cat split into an observer looking at a box with a dead cat and an observer looking at a box with a live cat. But since the dead and alive states are decoherent, there is no communication or interaction between them.

When opening the box, the observer becomes entangled with the cat, so "observer states" corresponding to the cat's being alive and dead are formed; each observer state is [[quantum entanglement|entangled]], or linked, with the cat so that the observation of the cat's state and the cat's state correspond with each other.  Quantum decoherence ensures that the different outcomes have no interaction with each other. Decoherence is generally considered to prevent simultaneous observation of multiple states.<ref name="zurek03">{{cite journal | last1 = Zurek | first1 = Wojciech H. | author-link = Wojciech H. Zurek | year = 2003 | title = Decoherence, einselection, and the quantum origins of the classical | arxiv = quant-ph/0105127 | journal = Reviews of Modern Physics | volume = 75 | issue = 3| page = 715 | doi=10.1103/revmodphys.75.715| bibcode = 2003RvMP...75..715Z | s2cid = 14759237 }}</ref><ref name="zurek91">[[Wojciech H. Zurek]], "Decoherence and the transition from quantum to classical", ''Physics Today'', 44, pp. 36–44 (1991)</ref>

A variant of the Schrödinger's cat experiment, known as the [[Quantum suicide and immortality|quantum suicide]] machine, has been proposed by cosmologist [[Max Tegmark]]. It examines the Schrödinger's cat experiment from the point of view of the cat, and argues that by using this approach, one may be able to distinguish between the Copenhagen interpretation and many-worlds.

=== Ensemble interpretation ===

The [[Ensemble Interpretation|ensemble interpretation]] states that superpositions are nothing but subensembles of a larger statistical ensemble. The state vector would not apply to individual cat experiments, but only to the statistics of many similarly prepared cat experiments. Proponents of this interpretation state that this makes the Schrödinger's cat paradox a trivial matter, or a non-issue.

This interpretation serves to ''discard'' the idea that a single physical system in quantum mechanics has a mathematical description that corresponds to it in any way.<ref>{{Cite journal|last=Smolin|first=Lee|date=October 2012|title=A real ensemble interpretation of quantum mechanics|journal=Foundations of Physics|volume=42|issue=10|pages=1239–1261|doi=10.1007/s10701-012-9666-4|issn=0015-9018|arxiv=1104.2822|bibcode=2012FoPh...42.1239S|s2cid=118505566}}</ref>

=== Relational interpretation ===
{{main|Relational quantum mechanics}}
The [[Relational quantum mechanics|relational interpretation]] makes no fundamental distinction between the human experimenter, the cat, and the apparatus or between animate and inanimate systems; all are quantum systems governed by the same rules of wavefunction [[Time evolution|evolution]], and all may be considered "observers". But the relational interpretation allows that different observers can give different accounts of the same series of events, depending on the information they have about the system.<ref>{{Cite journal|last = Rovelli|first = Carlo|author-link = Carlo Rovelli|title = Relational Quantum Mechanics|journal = International Journal of Theoretical Physics|volume = 35|pages = 1637–1678|year = 1996|arxiv = quant-ph/9609002 |doi = 10.1007/BF02302261|bibcode = 1996IJTP...35.1637R|issue = 8 |s2cid = 16325959}}</ref> The cat can be considered an observer of the apparatus; meanwhile, the experimenter can be considered another observer of the system in the box (the cat plus the apparatus). Before the box is opened, the cat, by nature of its being alive or dead, has information about the state of the apparatus (the atom has either decayed or not decayed); but the experimenter does not have information about the state of the box contents. In this way, the two observers simultaneously have different accounts of the situation: To the cat, the wavefunction of the apparatus has appeared to "collapse"; to the experimenter, the contents of the box appear to be in superposition. Not until the box is opened, and both observers have the same information about what happened, do both system states appear to "collapse" into the same definite result, a cat that is either alive or dead.

=== Transactional interpretation ===

In the [[transactional interpretation]] the apparatus emits an advanced wave backward in time, which combined with the wave that the source emits forward in time, forms a standing wave. The waves are seen as physically real, and the apparatus is considered an "observer". In the transactional interpretation, the collapse of the wavefunction is "atemporal" and occurs along the whole transaction between the source and the apparatus.  The cat is never in superposition.  Rather the cat is only in one state at any particular time, regardless of when the human experimenter looks in the box. The transactional interpretation resolves this quantum paradox.<ref>{{Cite book|last=Cramer|first=John G.|url=https://www.researchgate.net/publication/280926546|title=The transactional interpretation of quantum mechanics|publisher=Reviews of Modern Physics|date=July 1986|volume=58|pages=647–685}}</ref>

=== Objective collapse theories ===

According to [[objective-collapse theory|objective collapse theories]], superpositions are destroyed spontaneously (irrespective of external observation) when some objective physical threshold (of time, mass, temperature, [[irreversibility]], etc.) is reached. Thus, the cat would be expected to have settled into a definite state long before the box is opened. This could loosely be phrased as "the cat observes itself" or "the environment observes the cat".

Objective collapse theories require a modification of standard quantum mechanics to allow superpositions to be destroyed by the process of time evolution.<ref>{{Cite journal|last1=Okon|first1=Elias|last2=Sudarsky|first2=Daniel|date=2014-02-01|title=Benefits of Objective Collapse Models for Cosmology and Quantum Gravity|journal=Foundations of Physics|language=en|volume=44|issue=2|pages=114–143|doi=10.1007/s10701-014-9772-6|issn=1572-9516|arxiv=1309.1730|bibcode=2014FoPh...44..114O|s2cid=67831520}}</ref> These theories could ideally be tested by creating mesoscopic superposition states in the experiment. For instance, energy cat states has been proposed as a precise detector of the quantum gravity related energy decoherence models.<ref>{{Cite journal|last1=Khazali|first1=Mohammadsadegh|last2=Lau|first2=Hon Wai|last3=Humeniuk|first3=Adam|last4=Simon|first4=Christoph|date=2016-08-11|title=Large energy superpositions via Rydberg dressing|url=http://dx.doi.org/10.1103/physreva.94.023408|journal=Physical Review A|volume=94|issue=2|page=023408|doi=10.1103/physreva.94.023408|issn=2469-9926|arxiv=1509.01303|bibcode=2016PhRvA..94b3408K|s2cid=118364289}}</ref>