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{{Short description|Thought experiment in quantum mechanics}}
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[[File:Schrodingers cat.svg|thumb|upright=1.5|Schrödinger's cat: a cat, a flask of poison, and a [[radioactive]] source connected to a [[Geiger counter]] are placed in a sealed box. As illustrated, the quantum description uses a superposition of an alive cat and one that has died.]]
In [[quantum mechanics]], '''Schrödinger's cat''' is a [[thought experiment]] concerning [[quantum superposition]]. In the thought experiment, a hypothetical [[cat]] may be considered simultaneously both alive and dead, while it is unobserved in a closed box, as a result of its fate being linked to a random [[Subatomic particle|subatomic]] event that may or may not occur. This experiment viewed this way is described as a [[paradox]]. This thought experiment was devised by physicist [[Erwin Schrödinger]] in 1935<ref name="Schrodinger1935">{{cite journal | author-link = Erwin Schrödinger | first = Erwin| last=Schrödinger | title = Die gegenwärtige Situation in der Quantenmechanik (The Present Situation in Quantum Mechanics) | journal = [[Naturwissenschaften]] | doi = 10.1007/BF01491891 | volume = 23 | issue = 48 | pages = 807–812 | quote = Man kann auch ganz burleske Fälle konstruieren. Eine Katze wird in eine Stahlkammer gesperrt, zusammen mit folgender Höllenmaschine (die man gegen den direkten Zugriff der Katze sichern muß): in einem Geigerschen Zählrohr befindet sich eine winzige Menge radioaktiver Substanz, so wenig, daß im Laufe einer Stunde vielleicht eines von den Atomen zerfällt, ebenso wahrscheinlich aber auch keines; geschieht es, so spricht das Zählrohr an und betätigt über ein Relais ein Hämmerchen, das ein Kölbchen mit Blausäure zertrümmert. Hat man dieses ganze System eine Stunde lang sich selbst überlassen, so wird man sich sagen, daß die Katze noch lebt, wenn inzwischen kein Atom zerfallen ist. Der erste Atomzerfall würde sie vergiftet haben. Die Psi-Funktion des ganzen Systems würde das so zum Ausdruck bringen, daß in ihr die lebende und die tote Katze (s.v.v.) [sit venia verbo] zu gleichen Teilen gemischt oder verschmiert sind. Das Typische an solchen Fällen ist, daß eine ursprünglich auf den Atombereich beschränkte Unbestimmtheit sich in grobsinnliche Unbestimmtheit umsetzt, die sich dann durch direkte Beobachtung entscheiden läßt. Das hindert uns, in so naiver Weise ein „verwaschenes Modell“ als Abbild der Wirklichkeit gelten zu lassen. An sich enthielte es nichts Unklares oder Widerspruchsvolles. Es ist ein Unterschied zwischen einer verwackelten oder unscharf eingestellten Photographie und einer Aufnahme von Wolken und Nebelschwaden.| bibcode=1935NW.....23..807S | date=November 1935| s2cid=206795705}}</ref> in a discussion with [[Albert Einstein]]<ref name="Fine">{{cite web |last1=Fine |first1=Arthur |title=The Einstein-Podolsky-Rosen Argument in Quantum Theory |url=https://plato.stanford.edu/entries/qt-epr/ |website=Stanford Encyclopedia of Philosophy |access-date=11 June 2020}}</ref> to illustrate what Schrödinger saw as the problems of the [[Copenhagen interpretation]] of quantum mechanics.

In Schrödinger's original formulation, a cat, a flask of poison, and a [[radioactive]] source are placed in a sealed box. If an internal radiation monitor (e.g. a [[Geiger counter]]) detects radioactivity (i.e. a single atom decaying), the flask is shattered, releasing the poison, which kills the cat. The Copenhagen interpretation implies that, after a while, the cat is ''simultaneously'' alive ''and'' dead. Yet, when one looks in the box, one sees the cat ''either'' alive ''or'' dead, not both alive ''and'' dead. This poses the question of when exactly [[quantum superposition]] ends and reality resolves into one possibility or the other.

Although originally a critique on the Copenhagen interpretation, Schrödinger's seemingly paradoxical thought experiment became part of the foundation of quantum mechanics. The scenario is often featured in theoretical discussions of the [[interpretations of quantum mechanics]], particularly in situations involving the [[measurement problem]]. As a result, Schrödinger's cat has had enduring [[Schrödinger's cat in popular culture|appeal in popular culture]]. The experiment is not intended to be actually performed on a cat, but rather as an easily understandable illustration of the behavior of atoms. Experiments at the atomic scale have been carried out, showing that very small objects may exist as superpositions; but superposing an object as large as a cat would pose considerable technical difficulties.<ref>{{cite web |last1=Ball |first1=Philip |title= Real-Life Schrödinger’s Cats Probe the Boundary of the Quantum World |url= https://www.quantamagazine.org/real-life-schrodingers-cats-probe-the-boundary-of-the-quantum-world-20180625/ |website=Quanta Magazine |access-date=24 February 2025}}</ref>

Fundamentally, the Schrödinger's cat experiment asks how long quantum superpositions last and when (or ''whether'') they collapse. Different [[interpretations of quantum mechanics|interpretations of the mathematics of quantum mechanics]] have been proposed that give different explanations for this process.

== Origin and motivation ==
{{unsolved|physics|How does the quantum description of reality, which includes elements such as the superposition of states and wavefunction collapse or quantum decoherence, give rise to the reality we perceive? Another way of stating this question regards the measurement problem: What constitutes a "measurement" that apparently causes the wave function to collapse into a definite state?}}

Schrödinger intended his thought experiment as a discussion of the [[EPR paradox|EPR article]]—named after its authors [[Albert Einstein|Einstein]], [[Boris Podolsky|Podolsky]], and [[Nathan Rosen|Rosen]]—in 1935.<ref>[http://prola.aps.org/abstract/PR/v47/i10/p777_1 Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?] {{webarchive|url=https://web.archive.org/web/20060208145129/http://prola.aps.org/abstract/PR/v47/i10/p777_1 |date=2006-02-08 }} A. Einstein, B. Podolsky, and N. Rosen, Phys. Rev. 47, 777 (1935)</ref><ref name="Stanford1">{{cite encyclopedia | last = Fine | first = Arthur | title = The Einstein-Podolsky-Rosen Argument in Quantum Theory | encyclopedia = Stanford Encyclopedia of Philosophy | publisher = Stanford University | date = 2017 | url = https://plato.stanford.edu/entries/qt-epr/ | access-date = 11 April 2021}}</ref> The EPR article highlighted the counterintuitive nature of [[quantum superposition]]s, in which a quantum system for two particles does not separate<ref name=BaggottStory/>{{rp|150}} even when the particles are detected far from their last point of contact. The EPR paper concludes with a claim that this lack of separability meant that quantum mechanics as a theory of reality was incomplete. 

Schrödinger and [[Albert Einstein|Einstein]] exchanged letters about [[EPR paradox|Einstein's EPR article]], in the course of which Einstein pointed out that the  state of an [[Instability|unstable]] keg of [[gunpowder]] will, after a while, contain a superposition of both exploded and unexploded states.<ref name="Stanford1" />

To further illustrate, Schrödinger described how one could, in principle, create a superposition in a large-scale system by making it dependent on a quantum particle that was in a superposition.  He proposed a scenario with a cat in a closed steel chamber, wherein the cat's life or death depended on the state of a [[Radioactive decay|radioactive]] atom, whether it had decayed and emitted radiation or not. According to Schrödinger, the Copenhagen interpretation implies that ''the cat remains both alive and dead'' until the state has been observed. Schrödinger did not wish to promote the idea of dead-and-live cats as a serious possibility; on the contrary, he intended the example to illustrate the absurdity of the existing view of quantum mechanics,<ref name="Schrodinger1935" /> thus employing [[reductio ad absurdum]].

Since Schrödinger's time, various [[interpretations of quantum mechanics|interpretations of the mathematics of quantum mechanics]] have been advanced by physicists, some of which regard the "alive and dead" cat superposition as quite real, others do not.<ref name="Polkinghorne">{{cite book | last1       = Polkinghorne | first1      = J. C. | title       = The Quantum World | publisher   = [[Princeton University Press]] | date        = 1985 | pages       = 67 | url         = https://books.google.com/books?id=lp4JPYnLrtEC&q=%22schrodinger's+cat%22+%22alive+dead&pg=PA67 | isbn        = 0691023883 | url-status     = live | archive-url  = https://web.archive.org/web/20150519001623/https://books.google.com/books?id=lp4JPYnLrtEC&pg=PA67&dq=%22schrodinger's+cat%22+%22alive+dead | archive-date = 2015-05-19 }}</ref><ref name="Tetlow">{{cite book | last1       = Tetlow | first1      = Philip | title       = Understanding Information and Computation: From Einstein to Web Science | publisher   = Gower Publishing, Ltd. | date        = 2012 | pages       = 321 | url         = https://books.google.com/books?id=Rk7O3EG0Xn4C&q=%22alive+and+dead%22&pg=PA321 | isbn        = 978-1409440406 | url-status     = live | archive-url  = https://web.archive.org/web/20150519001741/https://books.google.com/books?id=Rk7O3EG0Xn4C&pg=PA321&dq=%22alive+and+dead%22 | archive-date = 2015-05-19 }}</ref> Intended as a critique of the Copenhagen interpretation (the prevailing orthodoxy in 1935), the Schrödinger's cat thought experiment remains a [[Touchstone (metaphor)|touchstone]] for modern interpretations of quantum mechanics and can be used to illustrate and compare their strengths and weaknesses.<ref>{{cite arXiv |last1=Lazarou |first1=Dimitris |title=Interpretation of quantum theory - An overview |date=2007 |class=quant-ph |eprint=0712.3466 }}</ref>
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== Thought experiment ==
[[File:Schroedinger cat.jpg|thumb|upright=1.2|A life-size cat figure in the garden of Huttenstrasse 9, Zurich, where [[Erwin Schrödinger]] lived from 1921 to 1926. Depending on the light conditions, the figure appears to be either a live cat or a dead one.]]

Schrödinger wrote:<ref name="Schrodinger1935" /><ref>{{cite journal|last1=Trimmer|first1=John D.|title=The Present Situation in Quantum Mechanics: A Translation of Schrödinger's "Cat Paradox" Paper|journal=Proceedings of the American Philosophical Society|date=1980|volume=124|issue=5|pages=323–338|jstor=986572}} The English translation here is based on the German original, not on the inaccurate version in this source's translation of the entire article:
[https://archive.today/20121204184041/http://www.tuhh.de/rzt/rzt/it/QM/cat.html#sect5 Schrödinger: "The Present Situation in Quantum Mechanics." 5. Are the Variables Really Blurred?]</ref>

{{Blockquote|
One can contrive even completely burlesque [farcical] cases. A cat is put in a steel chamber along with the following infernal device (which must be secured against direct interference by the cat): in a [[Geiger counter]], there is a tiny amount of radioactive substance, so tiny that in the course of an hour one of the atoms will perhaps decay, but also, with equal probability, that none of them will; if it does happen, the counter tube will discharge and through a relay release a hammer that will shatter a small flask of [[hydrocyanic acid]]. If one has left this entire system to itself for an hour, one would tell oneself that the cat is still alive if no atom has [[radioactive decay|decayed]] in the meantime. Even a single atomic decay would have poisoned it. The [[wave function|psi-function]] of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or spread out in equal parts.

It is typical of these cases that an indeterminacy originally restricted to the atomic domain turns into a sensually observable [macroscopic] indeterminacy, which can then be resolved by direct observation. This prevents us from so naïvely accepting a "blurred model" as representative of reality. Per se, it would not embody anything unclear or contradictory. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks.}}

Schrödinger developed his famous [[thought experiment]] in correspondence with Einstein. He suggested this 'quite ridiculous case' to illustrate his conclusion that the wave function cannot represent reality.<ref name=BaggottStory>{{Cite book |last=Baggott |first=J. E. |title=The quantum story: a history in 40 moments |date=2013 |publisher=Oxford Univ. Press |isbn=978-0-19-965597-7 |edition=Impression: 3 |location=Oxford}}</ref>{{rp|p=153|q='I am long past the stage where I thought that one can consider the <math>\psi</math>-function as somehow a direct description of reality'}} 
The wave function description of the complete cat system implies that the reality of the cat mixes the living and dead cat.<ref name=BaggottStory/>{{rp|154}} Einstein was impressed by the ability of the thought experiment to highlight these issues. In a letter to Schrödinger dated 1950, he wrote:<ref name=BaggottStory/>{{rp|157}}
{{Blockquote|You are the only contemporary physicist, besides [[Max von Laue|Laue]], who sees that one cannot get around the assumption of reality, if only one is honest. Most of them simply do not see what sort of risky game they are playing with reality—reality as something independent of what is experimentally established. Their interpretation is, however, refuted most elegantly by your system of radioactive atom + amplifier + charge of gun powder + cat in a box, in which the psi-function of the system contains both the cat alive and blown to bits. Nobody really doubts that the presence or absence of the cat is something independent of the act of observation.<ref>{{cite journal|title=Induction and Scientific Realism: Einstein versus van Fraassen Part Three: Einstein, Aim-Oriented Empiricism and the Discovery of Special and General Relativity|first=Nicholas|last=Maxwell|date=1 January 1993|volume=44|issue=2|pages=275–305|doi=10.1093/bjps/44.2.275|jstor=687649|journal=The British Journal for the Philosophy of Science}}</ref>}}
Note that the charge of gunpowder is not mentioned in Schrödinger's setup, which uses a Geiger counter as an amplifier and hydrocyanic poison instead of gunpowder. The gunpowder had been mentioned in Einstein's original suggestion to Schrödinger 15 years before, and Einstein carried it forward to the present discussion.<ref name="Stanford1" />

== Analysis ==
In modern terms Schrodinger's hypothetical cat experiment describes the [[measurement problem]]: quantum theory describes the cat system as a combination of two possible outcomes but only one outcome is ever observed.<ref name="PeresCat">{{Cite journal |last=Peres |first=Asher |date=January 1988 |title=Schrödinger's immortal cat |url=http://link.springer.com/10.1007/BF01882873 |journal=Foundations of Physics |language=en |volume=18 |issue=1 |pages=57–76 |doi=10.1007/BF01882873 |bibcode=1988FoPh...18...57P |issn=0015-9018}}</ref>{{rp|57|q=Nobody has ever seen a cat in such a strange
situation.}}<ref name="SchlosshauerDecoherenceReview">{{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 |volume=76 |issue=4 |pages=1267–1305 |doi=10.1103/RevModPhys.76.1267|arxiv=quant-ph/0312059 }}</ref>{{rp|p=1269|q=A book has never been observed to be in a state of being both “here” and “there” si.e., to be in a superposition of macroscopically distinguishable positionsd, nor does a Schrödinger cat that is a superposition of being alive and dead bear much resemblence to reality as we perceive it.}} 
The experiment poses the question, "''when'' does a quantum system stop existing as a superposition of states and become one or the other?"  (More technically, when does the actual quantum state stop being a non-trivial [[linear combination]] of states, each of which resembles different classical states, and instead begin to have a unique classical description?) Standard microscopic quantum mechanics describes multiple possible outcomes of experiments but only one outcome is observed. The thought experiment illustrates this apparent paradox. Our intuition says that the cat cannot be in more than one state simultaneously—yet the quantum mechanical description of the thought experiment requires such a condition.

== 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.}}

===Von Neumann interpretation===
{{Main|Von Neumann–Wigner interpretation}}
In 1932, [[John von Neumann]] described in his book ''[[Mathematical Foundations of Quantum Mechanics|Mathematical Foundations]]'' 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> Neumann asserted that a conscious 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]]. This interpretation was later adopted by [[Eugene Wigner]], who then rejected 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>

== Applications and tests ==
[[File:Quantum superposition of states and decoherence.ogv|thumb|upright=1.5|Schrödinger's cat quantum superposition of states and effect of the environment through decoherence]]

The experiment as described is a purely theoretical one, and the machine proposed is not known to have been constructed. However, successful experiments involving similar principles, e.g. superpositions of [[mesoscopic|relatively large]] (by the standards of quantum physics) objects have been performed.<ref>{{cite web|url=http://physics.stackexchange.com/questions/3309/what-is-the-worlds-biggest-schrodinger-cat|title=What is the world's biggest Schrodinger cat?|website=stackexchange.com|url-status=live|archive-url=https://web.archive.org/web/20120108000629/http://physics.stackexchange.com/questions/3309/what-is-the-worlds-biggest-schrodinger-cat|archive-date=2012-01-08}}</ref>{{better source needed|date=December 2022|reason=StackExchange is a web forum}} These experiments do not show that a cat-sized object can be superposed, but the known upper limit on "[[cat state]]s" has been pushed upwards by them. In many cases the state is short-lived, even when cooled to near [[absolute zero]].
* A "cat state" has been achieved with photons.<ref>{{cite web|url=http://www.science20.com/news_articles/schr%C3%B6dingers_cat_now_made_light|title=Schrödinger's Cat Now Made Of Light|date=27 August 2014|website=www.science20.com|url-status=live|archive-url=https://web.archive.org/web/20120318091956/http://www.science20.com/news_articles/schr%C3%B6dingers_cat_now_made_light|archive-date=18 March 2012}}</ref>
* A beryllium ion has been trapped in a superposed state.<ref>{{cite journal|first1=C. |last1=Monroe |first2=D. M. |last2=Meekhof |first3=B. E. |last3=King |first4=D. J. |last4=Wineland |title=A "Schrödinger's cat" Superposition State of an Atom |journal=Science |volume=272 |number=5265 |date=1996-05-24 |pages=1131–1136 |doi=10.1126/science.272.5265.1131|pmid=8662445 |bibcode=1996Sci...272.1131M |s2cid=2311821 }}</ref>
* An experiment involving a [[superconducting quantum interference device]] ("SQUID") has been linked to the theme of the thought experiment: "The superposition state does not correspond to a billion electrons flowing one way and a billion others flowing the other way. Superconducting electrons move en masse. All the superconducting electrons in the SQUID flow both ways around the loop at once when they are in the Schrödinger's cat state."<ref>{{cite web| url = https://physicsworld.com/a/schrodingers-cat-comes-into-view/| title = Physics World: ''Schrödinger's cat comes into view''| date = 5 July 2000}}</ref>
* A [[piezoelectric]] "tuning fork" has been constructed, which can be placed into a superposition of vibrating and non vibrating states. The resonator comprises about 10 trillion atoms.<ref>[http://www.scientificamerican.com/article.cfm?id=quantum-microphone Scientific American :'' Macro-Weirdness: "Quantum Microphone" Puts Naked-Eye Object in 2 Places at Once: A new device tests the limits of Schrödinger's cat''] {{webarchive|url=https://web.archive.org/web/20120319021316/http://www.scientificamerican.com/article.cfm?id=quantum-microphone |date=2012-03-19 }}</ref>
*  An experiment involving a flu virus has been proposed.<ref>{{cite journal|first1=O. |last1=Romero-Isart |first2=M. L. |last2=Juan |first3=R. |last3=Quidant |first4=J. I. |last4=Cirac |title=Toward Quantum Superposition of Living Organisms |journal=New Journal of Physics |year=2010 |volume=12 |number=3 |pages=033015 |doi=10.1088/1367-2630/12/3/033015 |arxiv=0909.1469 |bibcode=2010NJPh...12c3015R|s2cid=59151724 }}</ref>
* An experiment involving a bacterium and an electromechanical oscillator has been proposed.<ref>{{cite web|url=http://physicsworld.com/cws/article/news/2015/sep/21/could-schrodingers-bacterium-be-placed-in-a-quantum-superposition|title=Could 'Schrödinger's bacterium' be placed in a quantum superposition?|website=physicsworld.com|url-status=live|archive-url=https://web.archive.org/web/20160730174613/http://physicsworld.com/cws/article/news/2015/sep/21/could-schrodingers-bacterium-be-placed-in-a-quantum-superposition|archive-date=2016-07-30}}</ref>

In [[quantum computing]] the phrase "cat state" sometimes refers to the [[Greenberger–Horne–Zeilinger state|GHZ state]], wherein several qubits are in an equal superposition of all being 0 and all being 1; e.g.,

:<math> | \psi \rangle = \frac{1}{\sqrt{2}} \bigg( | 00\ldots0 \rangle + |11\ldots1 \rangle \bigg). </math>

According to at least one proposal, it may be possible to determine the state of the cat ''before'' observing it.<ref name="LS-20191107">{{cite news |last=Najjar |first=Dana |title=Physicists Can Finally Peek at Schrödinger's Cat Without Killing It Forever |url=https://www.livescience.com/schrodingers-cat-can-be-peeked-at.html |date=7 November 2019 |work=[[Live Science]] |access-date=7 November 2019 }}</ref><ref name="NJP-20191001">{{cite journal |last1=Patekar |first1=Kartik |last2=Hofmann |first2=Holger F. |title=The role of system–meter entanglement in controlling the resolution and decoherence of quantum measurements |journal=[[New Journal of Physics]] |volume=21 |issue=10 |pages=103006 |doi=10.1088/1367-2630/ab4451|arxiv=1905.09978  |year=2019 |bibcode=2019NJPh...21j3006P |doi-access=free }}</ref>

== Extensions ==

In August 2020, physicists presented studies involving interpretations of [[quantum mechanics]] that are related to the Schrödinger's cat and [[Wigner's friend]] paradoxes, resulting in conclusions that challenge seemingly established assumptions about [[reality]].<ref name="SA-20200817">{{cite news |last=Merali |first=Zeeya |title=This Twist on Schrödinger's Cat Paradox Has Major Implications for Quantum Theory - A laboratory demonstration of the classic "Wigner's friend" thought experiment could overturn cherished assumptions about reality |url=https://www.scientificamerican.com/article/this-twist-on-schroedingers-cat-paradox-has-major-implications-for-quantum-theory/ |date=17 August 2020 |work=[[Scientific American]] |access-date=17 August 2020 }}</ref><ref name="SM-20200817">{{cite news |last=Musser |first=George |title=Quantum paradox points to shaky foundations of reality |url=https://www.science.org/content/article/quantum-paradox-points-shaky-foundations-reality |date=17 August 2020 |work=[[Science Magazine]] |access-date=17 August 2020 }}</ref><ref name="NAT-20200817">{{cite journal |author=Bong, Kok-Wei |display-authors=et al. |title=A strong no-go theorem on the Wigner's friend paradox |date=17 August 2020 |journal=[[Nature Physics]] |volume=27 |issue=12 |pages=1199–1205 |doi=10.1038/s41567-020-0990-x |arxiv=1907.05607 |bibcode=2020NatPh..16.1199B |doi-access=free }}</ref>

== See also ==
{{Portal|Physics}}
{{cols|colwidth=26em}}
* [[Basis function]]
* [[Cat state]]
* [[Complementarity (physics)]]
* [[Double-slit experiment]]
* [[Elitzur–Vaidman bomb tester]]
* [[Heisenberg cut]]
* [[Modal realism]]
* [[Observer effect (physics)]]
* [[Ray cat]]
* [[Schroedinbug]]
* [[Schrödinger's cat in popular culture]]
{{colend}}

== References ==

{{reflist}}

== Further reading ==

* {{cite journal|last1=Einstein |first1=Albert |last2=Podolsky|first2=Boris |last3=Rosen|first3=Nathan|author-link1=Albert Einstein |author-link2=Boris Podolsky |author-link3=Nathan Rosen |title=Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?|journal=Physical Review|date=15 May 1935|volume=47|issue=10|pages=777–780|doi=10.1103/PhysRev.47.777 |bibcode=1935PhRv...47..777E|doi-access=free}}
* {{cite web |last1=Leggett |first1=Tony |title=New Life for Schrödinger's Cat |url=https://jrfriedman.people.amherst.edu/Leggett%20Physics%20World%20article/PW%20article.pdf |pages=23–24 |date= August 2000 |publisher=Physics World |access-date=28 February 2020}} An article on experiments with "cat state" superpositions in superconducting rings, in which the electrons go around the ring in two directions simultaneously.
* {{cite journal | first=John D.| last=Trimmer | title=The Present Situation in Quantum Mechanics: A Translation of Schrödinger's "Cat Paradox" Paper | journal=[[Proceedings of the American Philosophical Society]] | volume =124| issue= 5 | pages=323–338 | date=1980 | jstor=986572 }}{{registration required}}
* {{cite journal |last1=Yam |first1=Phillip |title=Bringing Schrödinger's Cat to Life |journal=Scientific American |date=October 9, 2012 |url=https://www.scientificamerican.com/article/bringing-schrodingers-quantum-cat-to-life/ |access-date=28 February 2020}} A description of investigations of quantum "cat states" and wave function collapse by [[Serge Haroche]] and [[David J. Wineland]], for which they won the 2012 [[List of Nobel laureates in Physics|Nobel Prize in Physics]].
* {{cite conference |last1=Violaris |first1=M. |title=A Physics Lab Inside Your Head: Quantum Thought Experiments as an Educational Tool |date=2023 |conference=2023 IEEE International Conference on Quantum Computing and Engineering (QCE) |location=Bellevue, WA, USA |pages=58–67 |doi=10.1109/QCE57702.2023.20325 |arxiv=2312.07840}} Reduction of the Schrödinger's cat to a simple [[quantum circuit]].

== External links ==
{{commons category}}

* [[:File:Schrodingers-cat.ogg|A spoken word version]] of this article (created from a revision of the article dated 2013-08-12).
* ''[http://www.informationphilosopher.com/solutions/experiments/schrodingerscat/ Schrödinger's Cat]'' from the Information Philosopher.
* [http://www.sixtysymbols.com/videos/schrodinger.htm Schrödinger's Cat - Sixty Symbols] - a video published by  the [[University of Nottingham]].
* [http://soundcloud.com/siftpodcast/schr-dingers-cat Schrödinger's Cat] - a podcast produced by Sift.

{{Quantum mechanics topics}}
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