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{{Cosmology}} | {{Cosmology}} | ||
The '''multiverse''' is the [[Hypothesis|hypothetical]] set of all [[universe]]s.<ref>{{cite news |title=We are closer than ever to finally proving the multiverse exists |url=https://www.newscientist.com/article/mg26234971-300-we-are-closer-than-ever-to-finally-proving-the-multiverse-exists/ |access-date=18 July 2024 |work=New Scientist|url-access=subscription}}</ref>{{efn|In some models, such as those of [[brane cosmology]], many parallel structures may exist within the same universe.}} Together, these universes are presumed to comprise everything that exists: the entirety of [[space]], [[time]], [[matter]], [[energy]], [[information]], and the [[physical law]]s and [[Physical constant|constants]] that describe them. The different universes within the multiverse are called "parallel universes", "flat universes", "other universes", "alternate universes", "multiple universes", "plane universes", "parent and child universes", "many universes", or "many worlds". One common assumption is that the multiverse is a "patchwork quilt of separate universes all bound by the same laws of physics."<ref>{{cite book |last1=Swain |first1=Frank |title=The Universe Next Door: A Journey Through 55 Alternative Realities, Parallel Worlds and Possible Futures |date=2017 |publisher=New Scientist |location=London |isbn= | The '''multiverse''' is the [[Hypothesis|hypothetical]] set of all [[universe]]s.<ref>{{cite news |title=We are closer than ever to finally proving the multiverse exists |url=https://www.newscientist.com/article/mg26234971-300-we-are-closer-than-ever-to-finally-proving-the-multiverse-exists/ |access-date=18 July 2024 |work=New Scientist|url-access=subscription}}</ref>{{efn|In some models, such as those of [[brane cosmology]], many parallel structures may exist within the same universe.}} Together, these universes are presumed to comprise everything that exists: the entirety of [[space]], [[time]], [[matter]], [[energy]], [[information]], and the [[physical law]]s and [[Physical constant|constants]] that describe them. The different universes within the multiverse are called "parallel universes", "flat universes", "other universes", "alternate universes", "multiple universes", "plane universes", "parent and child universes", "many universes", or "many worlds". One common assumption is that the multiverse is a "patchwork quilt of separate universes all bound by the same laws of physics."<ref>{{cite book |last1=Swain |first1=Frank |title=The Universe Next Door: A Journey Through 55 Alternative Realities, Parallel Worlds and Possible Futures |date=2017 |publisher=New Scientist |location=London |isbn=978-1-4736-5867-7 |page=12}}</ref> | ||
The concept of multiple universes, or a multiverse, has been discussed throughout history. It has evolved and has been debated in various fields, including cosmology, physics, and philosophy. Some physicists have argued that the multiverse is a philosophical notion rather than a scientific hypothesis, as it cannot be empirically falsified. In recent years, there have been proponents and skeptics of multiverse theories within the physics community. Although some scientists have analyzed data in search of evidence for other universes, no statistically significant evidence has been found. Critics argue that the multiverse concept lacks testability and falsifiability, which are essential for scientific inquiry, and that it raises unresolved metaphysical issues. | The concept of multiple universes, or a multiverse, has been discussed throughout history. It has evolved and has been debated in various fields, including cosmology, physics, and philosophy. Some physicists have argued that the multiverse is a philosophical notion rather than a scientific hypothesis, as it cannot be empirically falsified. In recent years, there have been proponents and skeptics of multiverse theories within the physics community. Although some scientists have analyzed data in search of evidence for other universes, no statistically significant evidence has been found. Critics argue that the multiverse concept lacks testability and falsifiability, which are essential for scientific inquiry, and that it raises unresolved metaphysical issues. | ||
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== History of the concept == | == History of the concept == | ||
According to some, the idea of infinite worlds was first suggested by the pre-Socratic Greek philosopher [[Anaximander]] in the sixth century BCE.<ref>{{ | According to some, the idea of infinite worlds was first suggested by the pre-Socratic Greek philosopher [[Anaximander]] in the sixth century BCE.<ref>{{cite book |last=Tarán |first =Leonardo |chapter=The Text of Simplicius' Commentary on Aristotle's Physics |title=Simplicius. Sa vie, son oeuvre, sa survie |year=1987 |location=Berlin, Germany |publisher=DE GRUYTER |doi=10.1515/9783110862041.246 |isbn =978-3-11-086204-1 }}</ref> However, there is debate as to whether he believed in multiple worlds, and if he did, whether those worlds were co-existent or successive.<ref> | ||
{{Cite journal |last=Kočandrle |first=Radim |title=Infinite Worlds in the Thought of Anaximander |date=December 2019 |url=https://www.cambridge.org/core/product/identifier/S000983882000004X/type/journal_article |journal=The Classical Quarterly |language=en |volume=69 |issue=2 |pages=483–500 |doi=10.1017/S000983882000004X |s2cid=216169543 |issn=0009-8388}} | {{Cite journal |last=Kočandrle |first=Radim |title=Infinite Worlds in the Thought of Anaximander |date=December 2019 |url =https://www.cambridge.org/core/product/identifier/S000983882000004X/type/journal_article |journal=The Classical Quarterly |language=en |volume=69 |issue=2 |pages=483–500 |doi =10.1017/S000983882000004X |s2cid=216169543 |issn=0009-8388|url-access=subscription }} | ||
</ref><ref> | </ref><ref> | ||
{{cite book |last1=Gregory |first1=Andrew |title=Anaximander: A Re-assessment |date=25 February 2016 |publisher=Bloomsbury Publishing |isbn=978-1-4725-0625-2 |page=121 |url=https://books.google.com/books?id=7TE0CwAAQBAJ&pg=PA121}} | {{cite book |last1 =Gregory |first1 =Andrew |title =Anaximander: A Re-assessment |date =25 February 2016 |publisher=Bloomsbury Publishing |isbn=978-1-4725-0625-2 |page=121 |url =https://books.google.com/books?id=7TE0CwAAQBAJ&pg=PA121}} | ||
</ref><ref> | </ref><ref> | ||
{{cite book |last1=Curd |first1=Patricia |last2=Graham |first2=Daniel W. |title=The Oxford Handbook of Presocratic Philosophy |date=27 October 2008 |publisher=Oxford University Press |isbn=978-0-19-972244-0 |pages=239–241 |url=https://books.google.com/books?id=lDvRCwAAQBAJ&pg=PA239}} | {{cite book |last1 =Curd |first1 =Patricia |last2 =Graham |first2 =Daniel W. |title=The Oxford Handbook of Presocratic Philosophy |date =27 October 2008 |publisher =Oxford University Press |isbn =978-0-19-972244-0 |pages =239–241 |url =https://books.google.com/books?id=lDvRCwAAQBAJ&pg=PA239}} | ||
</ref><ref name="Hatleback2014"> | </ref><ref name="Hatleback2014"> | ||
{{cite thesis |last=Hatleback |first=Eric Nelson |date=2014 |title=Chimera of the Cosmos |type=PhD |chapter= |publisher=University of Pittsburgh |url=https://d-scholarship.pitt.edu/22668/1/Chimera_of_the_Cosmos_2.pdf |place=Pittsburgh, Pennsylvania}}</ref> | {{cite thesis |last =Hatleback |first =Eric Nelson |date =2014 |title =Chimera of the Cosmos |type =PhD |chapter= |publisher=University of Pittsburgh |url=https://d-scholarship.pitt.edu/22668/1/Chimera_of_the_Cosmos_2.pdf |place=Pittsburgh, Pennsylvania}}</ref> | ||
The first to whom | The first figures to whom historians can definitively attribute the concept of innumerable worlds are the Ancient Greek [[Atomists]], beginning with [[Leucippus]] and [[Democritus]] in the 5th century BCE, followed by [[Epicurus]] (341–270 BCE) and the Roman Epicurean [[Lucretius]] (1st century BCE).<ref name="Siegfried2019"> | ||
{{cite book |last1=Siegfried |first1=Tom |title=The Number of the Heavens: A History of the Multiverse and the Quest to Understand the Cosmos |date=17 September 2019 |publisher=Harvard University Press |isbn=978-0-674-97588-0 |pages=51–61 |url=https://books.google.com/books?id=L36mDwAAQBAJ&pg=PA51 |quote="In some worlds there is no sun and moon, in others they are larger than in our world, and in others more numerous. The intervals between the worlds are unequal; in some parts there are more worlds, in others fewer; some are increasing, some at their height, some decreasing; in some parts they are arising, in others falling. They are destroyed by collision one with another. There are some worlds devoid of living creatures or plants or any moisture." ... Only an infinite number of atoms could have created the complexity of the known world by their random motions... In this sense, the atomist-multiverse theory of antiquity presents a striking parallel to the situation in science today. The Greek atomists' theory of the ultimate nature of matter on the smallest scales implied the existence of multiple universes on cosmic scales. Modern science's most popular attempt to describe the fundamental nature of matter—superstring theory—also turns out (much to the theorists' surprise) to imply a vast multiplicity of vacuum states, essentially the same thing as predicting the existence of a multiverse.}} | {{cite book |last1 =Siegfried |first1 =Tom |title =The Number of the Heavens: A History of the Multiverse and the Quest to Understand the Cosmos |date=17 September 2019 |publisher =Harvard University Press |isbn=978-0-674-97588-0 |pages=51–61 |url =https://books.google.com/books?id=L36mDwAAQBAJ&pg=PA51 |quote ="In some worlds there is no sun and moon, in others they are larger than in our world, and in others more numerous. The intervals between the worlds are unequal; in some parts there are more worlds, in others fewer; some are increasing, some at their height, some decreasing; in some parts they are arising, in others falling. They are destroyed by collision one with another. There are some worlds devoid of living creatures or plants or any moisture." ... Only an infinite number of atoms could have created the complexity of the known world by their random motions... In this sense, the atomist-multiverse theory of antiquity presents a striking parallel to the situation in science today. The Greek atomists' theory of the ultimate nature of matter on the smallest scales implied the existence of multiple universes on cosmic scales. Modern science's most popular attempt to describe the fundamental nature of matter—superstring theory—also turns out (much to the theorists' surprise) to imply a vast multiplicity of vacuum states, essentially the same thing as predicting the existence of a multiverse.}} | ||
</ref><ref> | </ref><ref> | ||
{{cite book |last1=Dick |first1=Steven J. |title=Plurality of Words: The Extraterrestrial Life Debate from Democritus to Kant |date=29 June 1984 |publisher=Cambridge University Press |isbn=978-0-521-31985-0 |pages=6–10 |url=https://books.google.com/books?id=Uak5AAAAIAAJ&pg=PA6 |quote=Why should other worlds have become the subject of scientific discourse, when they were neither among the phenomena demanding explanation?... it derived from the cosmogonic assumption of ancient atomism: the belief that the constituent bodies of the cosmos are formed by the chance coalescence of moving atoms, the same type of indivisible particles of which matter on Earth was composed... Given the occurrence of these natural processes, and the obvious example of potential stability revealed in our own finite world, it was not unreasonable to suppose the existence of other stable conglomerations. The atomists further employed the principle that when causes were present, effects must occur.6 Atoms were the agents of causality and their number was infinite. The effect was innumerable worlds in formation, in collision, and in decay."}} | {{cite book |last1 =Dick |first1 =Steven J. |title =Plurality of Words: The Extraterrestrial Life Debate from Democritus to Kant |date =29 June 1984 |publisher =Cambridge University Press |isbn=978-0-521-31985-0 |pages=6–10 |url=https://books.google.com/books?id=Uak5AAAAIAAJ&pg=PA6 |quote =Why should other worlds have become the subject of scientific discourse, when they were neither among the phenomena demanding explanation?... it derived from the cosmogonic assumption of ancient atomism: the belief that the constituent bodies of the cosmos are formed by the chance coalescence of moving atoms, the same type of indivisible particles of which matter on Earth was composed... Given the occurrence of these natural processes, and the obvious example of potential stability revealed in our own finite world, it was not unreasonable to suppose the existence of other stable conglomerations. The atomists further employed the principle that when causes were present, effects must occur.6 Atoms were the agents of causality and their number was infinite. The effect was innumerable worlds in formation, in collision, and in decay."}} | ||
</ref><ref name="Hatleback2014"/><ref name="Rubenstein2014"> | </ref><ref name="Hatleback2014"/><ref name="Rubenstein2014"> | ||
{{cite book |last1=Rubenstein |first1=Mary-Jane |title=Worlds Without End: The Many Lives of the Multiverse |date=11 February 2014 |publisher=Columbia University Press |isbn=978-0-231-15662-2 |pages=40–69 |chapter-url=https://books.google.com/books?id=fV-sAgAAQBAJ&pg=PA40 |chapter=Ancient Openings of Multiplicity}} | {{cite book |last1 =Rubenstein |first1 =Mary-Jane |title =Worlds Without End: The Many Lives of the Multiverse |date =11 February 2014 |publisher =Columbia University Press |isbn =978-0-231-15662-2 |pages =40–69 |chapter-url =https://books.google.com/books?id=fV-sAgAAQBAJ&pg=PA40 |chapter=Ancient Openings of Multiplicity}} | ||
</ref><ref name="Sedacca2017"> | </ref><ref name="Sedacca2017"> | ||
{{cite web |last1=Sedacca |first1=Matthew |title=The Multiverse Is an Ancient Idea |url=https://nautil.us/the-multiverse-is-an-ancient-idea-236401/ |website=Nautilus |access-date=4 December 2022 |date=30 January 2017 |quote=The earliest hints of the multiverse are found in two ancient Greek schools of thought, the Atomists and the Stoics. The Atomists, whose philosophy dates to the fifth century B.C., argued that that the order and beauty of our world was the accidental product of atoms colliding in an infinite void. The atomic collisions also give rise to an endless number of other, parallel worlds less perfect than our own.}} | {{cite web |last1 =Sedacca |first1=Matthew |title=The Multiverse Is an Ancient Idea |url =https://nautil.us/the-multiverse-is-an-ancient-idea-236401/ |website =Nautilus |access-date =4 December 2022 |date =30 January 2017 |quote=The earliest hints of the multiverse are found in two ancient Greek schools of thought, the Atomists and the Stoics. The Atomists, whose philosophy dates to the fifth century B.C., argued that that the order and beauty of our world was the accidental product of atoms colliding in an infinite void. The atomic collisions also give rise to an endless number of other, parallel worlds less perfect than our own.}} | ||
</ref><ref> | </ref><ref> | ||
{{cite web |last1=Siegfried |first1=Tom |title=Long Live the Multiverse! |url=https://blogs.scientificamerican.com/observations/long-live-the-multiverse/ |website=Scientific American Blog Network |date=2019 |quote=Leucippus and Democritus believed that their atomic theory required an infinity of worlds... Their later follower, Epicurus of Samos, also professed the reality of multiple worlds. "There are infinite worlds both like and unlike this world of ours"...}} | {{cite web |last1=Siegfried |first1=Tom |title=Long Live the Multiverse! |url =https://blogs.scientificamerican.com/observations/long-live-the-multiverse/ |website =Scientific American Blog Network |date =2019 |quote =Leucippus and Democritus believed that their atomic theory required an infinity of worlds... Their later follower, Epicurus of Samos, also professed the reality of multiple worlds. "There are infinite worlds both like and unlike this world of ours"...}} | ||
</ref> In the third century BCE, the philosopher [[Chrysippus]] suggested that the world eternally expired and regenerated, effectively suggesting the existence of multiple universes across time.<ref name=Sedacca2017/> The concept of multiple universes became more defined in the [[Middle Ages]].<ref>{{Cite news |last=Laskow |first=Sarah |date=20 June 2017 |title=Medieval Scholars Believed in the Possibility of Parallel Universes |url=https://www.atlasobscura.com/articles/medieval-multiverse-theory |access-date=20 June 2025 |work=Atlas Obscura}}</ref> | </ref> In the third century BCE, the philosopher [[Chrysippus]] suggested that the world eternally expired and regenerated, effectively suggesting the existence of multiple universes across time.<ref name=Sedacca2017/> The concept of multiple universes became more defined in the [[Middle Ages]].<ref>{{Cite news |last =Laskow |first =Sarah |date =20 June 2017 |title =Medieval Scholars Believed in the Possibility of Parallel Universes |url =https://www.atlasobscura.com/articles/medieval-multiverse-theory |access-date =20 June 2025 |work =Atlas Obscura}}</ref> In the Renaissance, [[Giordano Bruno]] (1548-1600) expressed the concept of infinite worlds.<ref> | ||
{{cite book | |||
|editor-last1 = Sgarbi | |||
|editor-first1 = Marco | |||
|editor-link1 = Marco Sgarbi | |||
|date = 27 October 2022 | |||
|title = Encyclopedia of Renaissance Philosophy | |||
|url = https://books.google.com/books?id=81yYEAAAQBAJ | |||
|publication-place = New York | |||
|publisher = Springer Nature | |||
|page = 255 | |||
|isbn = 978-3-319-14169-5 | |||
|access-date = 22 August 2025 | |||
|quote = [...] we [...] encounter an acknowledged debt to Democritus (who, like Bruno, had postulated infinite worlds) [...]. | |||
}} | |||
</ref> | |||
The American philosopher and psychologist [[William James]] used the term "multiverse" in 1895, but in a different context.<ref>James, William, ''The Will to Believe'', 1895; and earlier in 1895, as cited in [[OED]]'s new 2003 entry for "multiverse": {{citation|url=https://books.google.com/books?id=HA0MAAAAIAAJ&pg=PA10|date=October 1895|last=James|first=William|journal=Int. J. Ethics|volume=6|page=10|title=Is Life Worth Living?|quote=Visible nature is all plasticity and indifference, a multiverse, as one might call it, and not a universe.|doi=10.1086/205378|issue=1}}</ref> | The American philosopher and psychologist [[William James]] used the term "multiverse" in 1895, but in a different context.<ref>James, William, ''The Will to Believe'', 1895; and earlier in 1895, as cited in [[OED]]'s new 2003 entry for "multiverse": {{citation |url =https://books.google.com/books?id=HA0MAAAAIAAJ&pg=PA10 |date=October 1895 |last=James |first=William |journal=Int. J. Ethics |volume=6 |page=10 |title=Is Life Worth Living? |quote=Visible nature is all plasticity and indifference, a multiverse, as one might call it, and not a universe. |doi=10.1086/205378 |issue=1}}</ref> | ||
The concept first appeared in the modern scientific context in the course of the debate between [[Ludwig Boltzmann|Boltzmann]] and [[Ernst Zermelo|Zermelo]] in 1895.<ref>{{cite book |last1=Ćirković |first1=Milan M. |editor1-last=Kragh |editor1-first=Helge |editor2-last=Longair |editor2-first=Malcolm |title=The Oxford Handbook of the History of Modern Cosmology |date=6 March 2019 |publisher=Oxford University Press |isbn=978-0-19-254997-6 |chapter-url=https://books.google.com/books?id=OsKKDwAAQBAJ&pg=PA467 |chapter=Stranger things: multiverse, string cosmology, physical eschatology}}</ref> | The concept first appeared in the modern scientific context in the course of the debate between [[Ludwig Boltzmann|Boltzmann]] and [[Ernst Zermelo|Zermelo]] in 1895.<ref>{{cite book |last1 =Ćirković |first1 =Milan M. |editor1-last =Kragh |editor1-first =Helge |editor2-last =Longair |editor2-first =Malcolm |title =The Oxford Handbook of the History of Modern Cosmology |date =6 March 2019 |publisher =Oxford University Press |isbn =978-0-19-254997-6 |chapter-url =https://books.google.com/books?id=OsKKDwAAQBAJ&pg=PA467 |chapter =Stranger things: multiverse, string cosmology, physical eschatology}}</ref> | ||
In [[Dublin]] in 1952, [[Erwin Schrödinger]] gave a lecture in which he jocularly warned his audience that what he was about to say might "seem lunatic". He said that when his equations seemed to describe several different histories, these were "not alternatives, but all really happen simultaneously".<ref>{{Cite web |date=2012-04-05 |title=Erwin Schrödinger and the Quantum Revolution by John Gribbin: review |url=https://www.telegraph.co.uk/culture/books/bookreviews/9188438/Erwin-Schrodinger-and-the-Quantum-Revolution-by-John-Gribbin-review.html |access-date=2023-09-24 |website=The Telegraph |language=en}}</ref> This sort of duality is called | In [[Dublin]] in 1952, [[Erwin Schrödinger]] gave a lecture in which he jocularly warned his audience that what he was about to say might "seem lunatic". He said that when his equations seemed to describe several different histories, these were "not alternatives, but all really happen simultaneously".<ref>{{Cite web |date =2012-04-05 |title =Erwin Schrödinger and the Quantum Revolution by John Gribbin: review |url =https://www.telegraph.co.uk/culture/books/bookreviews/9188438/Erwin-Schrodinger-and-the-Quantum-Revolution-by-John-Gribbin-review.html |access-date =2023-09-24 |website =The Telegraph |language=en}}</ref> This sort of duality is called [[Quantum superposition | "superposition"]]. | ||
== Search for evidence == | == Search for evidence == | ||
In the 1990s, after recent works of fiction about the concept gained popularity, scientific discussions about the multiverse and [[Scientific journal|journal]] articles about it gained prominence.<ref>{{Cite web |last=Romeo |first=Jess |date=2022-01-07 |title=The Real Science of the Multiverse |url=https://daily.jstor.org/the-real-science-of-the-multiverse/ |access-date=2023-07-15 |website=JSTOR Daily |publisher=[[JSTOR]] |language=en-US}}</ref> | In the 1990s, after recent works of fiction about the concept gained popularity, scientific discussions about the multiverse and [[Scientific journal|journal]] articles about it gained prominence.<ref>{{Cite web |last=Romeo |first=Jess |date=2022-01-07 |title=The Real Science of the Multiverse |url=https://daily.jstor.org/the-real-science-of-the-multiverse/ |access-date=2023-07-15 |website=JSTOR Daily |publisher=[[JSTOR]] |language=en-US}}</ref> | ||
Around 2010, scientists such as Stephen M. Feeney analyzed [[Wilkinson Microwave Anisotropy Probe]] (WMAP) data and claimed to find evidence suggesting that this universe collided with other (parallel) universes in the distant past.<ref>{{cite web|title=Astronomers Find First Evidence Of Other Universe|url=https://www.technologyreview.com/s/421999/astronomers-find-first-evidence-of-other-universes/|publisher=technologyreview.com|access-date=12 October 2013|date=13 December 2010}}</ref><ref>{{cite web |author=Tegmark |first1=Max |last2=Vilenkin |first2=Alexander |date=19 July 2011 |title=The Case for Parallel Universes |url=https://www.scientificamerican.com/article/multiverse-the-case-for-parallel-universe/ |access-date=12 October 2013 |website=[[Scientific American]]}}</ref><ref>{{cite web|title=Is Our Universe Inside a Bubble? First Observational Test of the 'Multiverse'|url=https://www.sciencedaily.com/releases/2011/08/110803102844.htm|work=Science Daily|publisher=sciencedaily.com|access-date=12 October 2013|date=3 August 2011}}</ref> However, a more thorough analysis of data from the WMAP and from the [[Planck (spacecraft)|Planck satellite]], which has a resolution three times higher than WMAP, did not reveal any statistically significant evidence of such a [[bubble universe]] collision.<ref name="Feeney">{{cite journal |title=First observational tests of eternal inflation: Analysis methods and WMAP 7-year results |journal=Physical Review D |volume=84 |issue=4 | | Around 2010, scientists such as Stephen M. Feeney analyzed [[Wilkinson Microwave Anisotropy Probe]] (WMAP) data and claimed to find evidence suggesting that this universe collided with other (parallel) universes in the distant past.<ref>{{cite web|title=Astronomers Find First Evidence Of Other Universe|url=https://www.technologyreview.com/s/421999/astronomers-find-first-evidence-of-other-universes/|publisher=technologyreview.com|access-date=12 October 2013|date=13 December 2010}}</ref><ref>{{cite web |author=Tegmark |first1=Max |last2=Vilenkin |first2=Alexander |date=19 July 2011 |title=The Case for Parallel Universes |url=https://www.scientificamerican.com/article/multiverse-the-case-for-parallel-universe/ |access-date=12 October 2013 |website=[[Scientific American]]}}</ref><ref>{{cite web|title=Is Our Universe Inside a Bubble? First Observational Test of the 'Multiverse'|url=https://www.sciencedaily.com/releases/2011/08/110803102844.htm|work=Science Daily|publisher=sciencedaily.com|access-date=12 October 2013|date=3 August 2011}}</ref> However, a more thorough analysis of data from the WMAP and from the [[Planck (spacecraft)|Planck satellite]], which has a resolution three times higher than WMAP, did not reveal any statistically significant evidence of such a [[bubble universe]] collision.<ref name="Feeney">{{cite journal |title=First observational tests of eternal inflation: Analysis methods and WMAP 7-year results |journal=Physical Review D |volume=84 |issue=4 |article-number=43507 |date=2011 |arxiv=1012.3667 |last1=Feeney|first1=Stephen M. |display-authors=1 |bibcode=2011PhRvD..84d3507F |last2=Johnson |first2=Matthew C.|last3=Mortlock |first3=Daniel J. |last4=Peiris |first4=Hiranya V. |doi=10.1103/PhysRevD.84.043507 |s2cid=43793857 }}</ref><ref name="Feeney2">{{cite journal|title=First observational tests of eternal inflation |journal=Physical Review Letters |volume=107 |issue=7 |article-number=071301 |date=2011 |arxiv=1012.1995 |author=Feeney |display-authors=etal |bibcode = 2011PhRvL.107g1301F |doi = 10.1103/PhysRevLett.107.071301 |pmid=21902380 |s2cid=23560957 }}. {{Cite journal|arxiv=1309.4060|last1=Bousso|first1=Raphael|title=Inflation after False Vacuum Decay: Observational Prospects after Planck|journal=Physical Review D|volume=91|issue=8|article-number=083527|last2=Harlow|first2=Daniel|last3=Senatore|first3=Leonardo|year=2015|doi=10.1103/PhysRevD.91.083527|bibcode = 2015PhRvD..91h3527B |s2cid=118488797}}</ref> In addition, there was no evidence of any gravitational pull of other universes on ours.<ref>{{Cite journal|arxiv=1303.5090 |title= Planck intermediate results. XIII. Constraints on peculiar velocities |journal=Astronomy & Astrophysics |volume=561 |pages=A97 |date=2013-03-20 |last1=Collaboration |first1=Planck |last2= Ade |first2=P. A. R. |last3= Aghanim |first3=N.|author3-link=Nabila Aghanim |last4= Arnaud |first4=M. |last5= Ashdown |first5=M. |last6= Aumont |first6=J. |last7= Baccigalupi |first7=C. |last8= Balbi |first8=A. |last9= Banday |first9=A. J. |last10= Barreiro |first10=R. B. |last11= Battaner |first11=E. |last12= Benabed |first12=K. |last13= Benoit-Levy |first13=A. |last14= Bernard |first14=J. -P. |last15= Bersanelli |first15=M. |last16= Bielewicz |first16=P. |last17= Bikmaev |first17=I. |last18= Bobin |first18=J. |last19= Bock |first19=J. J. |last20= Bonaldi |first20=A. |last21= Bond |first21=J. R. |last22= Borrill |first22=J. |last23= Bouchet |first23=F. R. |last24= Burigana |first24=C. |last25= Butler |first25=R. C. |last26= Cabella |first26=P. |last27= Cardoso |first27=J. -F. |last28= Catalano |first28=A. |last29= Chamballu |first29=A. |last30= -Y Chiang |first30=L. |display-authors=29 |doi=10.1051/0004-6361/201321299 |bibcode = 2014A&A...561A..97P |s2cid= 2745526 }}</ref><ref>{{cite news|title=Blow for 'dark flow' in Planck's new view of the cosmos|url=https://www.newscientist.com/article/dn23340-blow-for-dark-flow-in-plancks-new-view-of-the-cosmos|access-date=10 March 2014|newspaper=[[New Scientist]]|date=3 April 2013}}</ref> | ||
In 2015, an astrophysicist may have found evidence of alternate or parallel universes by looking back in time to a time immediately after the [[Big Bang]], although it is still a matter of debate among physicists.<ref name="www.usatoday.com">{{cite web |title=Study may have found evidence of alternate, parallel universes |url=http://www.usatoday.com/story/tech/2015/11/03/alternate-universes-discovered/75102502/ |publisher=www.usatoday.com}}<!-- auto-translated by Module:CS1 translator --> por Doyle Rice, ''[[USA Today]]'' (2015).</ref> Dr. Ranga-Ram Chary, after analyzing the [[Cosmic microwave background|cosmic radiation spectrum]], found a signal 4,500 times brighter than it should have been, based on the number of [[Proton|protons]] and [[Electron|electrons]] scientists believe existed in the very early universe. This signal—an emission line that arose from the formation of atoms during the era of recombination—is more consistent with a universe whose ratio of matter particles to photons is about 65 times greater than our own. There is a 30% chance that this signal is noise, and not really a signal at all; however, it is also possible that it exists because a parallel universe dumped some of its matter particles into our universe. If additional protons and electrons had been added to our universe during recombination, more atoms would have formed, more photons would have been emitted during their formation, and the signature line that arose from all of these emissions would be greatly enhanced. Chary | In 2015, an astrophysicist may have found evidence of alternate or parallel universes by looking back in time to a time immediately after the [[Big Bang]], although it is still a matter of debate among physicists.<ref name="www.usatoday.com">{{cite web |title=Study may have found evidence of alternate, parallel universes |url=http://www.usatoday.com/story/tech/2015/11/03/alternate-universes-discovered/75102502/ |publisher=www.usatoday.com}}<!-- auto-translated by Module:CS1 translator --> por Doyle Rice, ''[[USA Today]]'' (2015).</ref> Dr. Ranga-Ram Chary, after analyzing the [[Cosmic microwave background|cosmic radiation spectrum]], found a signal 4,500 times brighter than it should have been, based on the number of [[Proton|protons]] and [[Electron|electrons]] scientists believe existed in the very early universe. This signal—an emission line that arose from the formation of atoms during the era of recombination—is more consistent with a universe whose ratio of matter particles to photons is about 65 times greater than our own. There is a 30% chance that this signal is noise, and not really a signal at all; however, it is also possible that it exists because a parallel universe dumped some of its matter particles into our universe. If additional protons and electrons had been added to our universe during recombination, more atoms would have formed, more photons would have been emitted during their formation, and the signature line that arose from all of these emissions would be greatly enhanced. Chary said:{{quotation|text=Many other regions beyond our observable universe would exist with each such region governed by a different set of physical parameters than the ones we have measured for our universe.<ref name="www.usatoday.com"/>|author=Ranga-Ram Chary|title=''USA Today''|source=}} | ||
Chary also noted:<ref name="phys.org">{{cite web |title=Cosmologist thinks a strange signal may be evidence of a parallel universe |url=http://phys.org/news/2015-11-cosmologist-strange-evidence-parallel-universe.html#jCp |publisher=phys.org}}<!-- auto-translated by Module:CS1 translator --> por Vanessa Janek, "Universe Today" (2015).</ref>{{quotation|text=Unusual claims like evidence for alternate universes require a very high burden of proof.<ref name="phys.org"/>|author=Ranga-Ram Chary|title="Universe Today"|source=}} | Chary also noted:<ref name="phys.org">{{cite web |title=Cosmologist thinks a strange signal may be evidence of a parallel universe |url=http://phys.org/news/2015-11-cosmologist-strange-evidence-parallel-universe.html#jCp |publisher=phys.org}}<!-- auto-translated by Module:CS1 translator --> por Vanessa Janek, "Universe Today" (2015).</ref>{{quotation|text=Unusual claims like evidence for alternate universes require a very high burden of proof.<ref name="phys.org"/>|author=Ranga-Ram Chary|title="Universe Today"|source=}} | ||
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== Proponents and skeptics == | == Proponents and skeptics == | ||
Modern proponents of one or more of the multiverse hypotheses include <!-- To add anyone to this list we MUST have a citation that shows that they belong on the list. -->[[Lee Smolin]],<ref>Smolin, Lee. The Life of the Cosmos. Oxford University Press. ISBN 978-0195126648.</ref> [[Don Page (physicist)|Don Page]],<ref>{{Cite web |last=Page |first=Don |date=8 March 2018 |title=Does God exist in the multiverse? |work=Grandin Media |url=https://grandinmedia.ca/god-exist-multiverse/}}</ref> [[Brian Greene]],<ref name="C4WDefault-4326764">{{cite interview |url= https://www.npr.org/2011/01/24/132932268/a-physicist-explains-why-parallel-universes-may-exist |title= A Physicist Explains Why Parallel Universes May Exist |date= 24 January 2011 |access-date= 12 September 2014 |work= npr.org |last= Greene |first= Brian |interviewer= Terry Gross |archive-url= https://web.archive.org/web/20140913065530/http://www.npr.org/2011/01/24/132932268/a-physicist-explains-why-parallel-universes-may-exist |archive-date= 13 September 2014 |url-status= live | Modern proponents of one or more of the multiverse hypotheses include <!-- To add anyone to this list we MUST have a citation that shows that they belong on the list. -->[[Lee Smolin]],<ref>Smolin, Lee. The Life of the Cosmos. Oxford University Press. ISBN 978-0195126648.</ref> [[Don Page (physicist)|Don Page]],<ref>{{Cite web |last=Page |first=Don |date=8 March 2018 |title=Does God exist in the multiverse? |work=Grandin Media |url=https://grandinmedia.ca/god-exist-multiverse/}}</ref> [[Brian Greene]],<ref name="C4WDefault-4326764">{{cite interview |url= https://www.npr.org/2011/01/24/132932268/a-physicist-explains-why-parallel-universes-may-exist |title= A Physicist Explains Why Parallel Universes May Exist |date= 24 January 2011 |access-date= 12 September 2014 |work= npr.org |last= Greene |first= Brian |interviewer= Terry Gross |archive-url= https://web.archive.org/web/20140913065530/http://www.npr.org/2011/01/24/132932268/a-physicist-explains-why-parallel-universes-may-exist |archive-date= 13 September 2014 |url-status= live }}</ref><ref name="C4WDefault-379179">{{cite interview |url=https://www.npr.org/templates/transcript/transcript.php?storyId=132932268 |title=Transcript:A Physicist Explains Why Parallel Universes May Exist |date=24 January 2011 |access-date=12 September 2014 |work=npr.org |last=Greene |first=Brian |interviewer=Terry Gross |archive-url=https://web.archive.org/web/20140913065827/http://www.npr.org/templates/transcript/transcript.php?storyId=132932268 |archive-date=13 September 2014 |url-status=live }}</ref> [[Max Tegmark]],<ref name="X0302131" /> [[Alan Guth]],<ref>{{cite web |last=Guth |first=Alan |title=Inflationary Cosmology: Is Our Universe Part of a Multiverse? |url=https://www.youtube.com/watch?v=d1bImPhPw9c |url-status=live |archive-url=https://ghostarchive.org/varchive/youtube/20211211/d1bImPhPw9c |archive-date=2021-12-11 |access-date=6 October 2014 |work=YouTube|date=May 2014 }}{{cbignore}}</ref> [[Andrei Linde]],<ref name="C4WDefault-6178546">{{cite web |url=http://www.ctc.cam.ac.uk/stephen70/talks/swh70_linde.pdf |title=Inflation in Supergravity and String Theory: Brief History of the Multiverse |date=27 January 2012 <!-- No date available; last modification date used. --> |access-date=13 September 2014 |work=ctc.cam.ac.uk |last=Linde |first=Andrei |archive-url=https://web.archive.org/web/20140714173722/http://www.ctc.cam.ac.uk/stephen70/talks/swh70_linde.pdf |archive-date=14 July 2014 |url-status= live}}</ref> [[Michio Kaku]],<ref>{{Cite web |last=Kaku |first=Michio |title=e-reading.ws |url=http://www.e-reading.ws/bookreader.php/136469/Parallel_Worlds:_A_Journey_Through_Creation,_Higher_Dimensions,_and_the_Future_of_the_Cosmos.pdf |website=www.e-reading.ws}}</ref> [[David Deutsch]],<ref>David Deutsch (1997). "The Ends of the Universe". The Fabric of Reality: The Science of Parallel Universes—and Its Implications. London, England: Penguin Press. {{ISBN|0-7139-9061-9}}.</ref> [[Leonard Susskind]],<ref name="BoussoSusskind">{{Cite journal |last1=Bousso |first1=Raphael |last2=Susskind |first2=Leonard |year=2012 |title=Multiverse interpretation of quantum mechanics |journal=Physical Review D |volume=85 |issue=4 |article-number=045007 |arxiv=1105.3796 |bibcode=2012PhRvD..85d5007B |doi=10.1103/PhysRevD.85.045007 |s2cid=118507872}}</ref> [[Alexander Vilenkin]],<ref>{{Cite book|url= https://books.google.com/books?id=9nRGwQnvGx0C|isbn= 978-0-374-70714-9|date= 2007|title= Many Worlds in One: The Search for Other Universes|first= Alex |last= Vilenkin|publisher= Farrar, Straus and Giroux}}</ref> [[Yasunori Nomura]],<ref name="Nomura">{{Cite journal |last1=Nomura |first1=Yasunori |year=2011 |title=Physical theories, eternal inflation, and the quantum universe |journal=Journal of High Energy Physics |volume=2011 |issue=11 |article-number=63 |arxiv=1104.2324 |bibcode=2011JHEP...11..063N |doi=10.1007/JHEP11(2011)063 |s2cid=119283262}}</ref> [[Raj Pathria]],<ref>{{Cite journal | doi = 10.1038/240298a0| title = The Universe as a Black Hole| journal = Nature| volume = 240| issue = 5379| pages = 298–299| year = 1972| last1 = Pathria | first1 = R. K.| bibcode=1972Natur.240..298P| s2cid = 4282253}}</ref> [[Laura Mersini-Houghton]],<ref name="TG-20220827">{{cite news |last=Fox |first=Killian |date=27 August 2022 |title=Cosmologist Laura Mersini-Houghton: 'Our universe is one tiny grain of dust in a beautiful cosmos' – Interview |url=https://www.theguardian.com/science/2022/aug/27/cosmologist-laura-mersini-houghton-before-the-big-bang-interview |access-date=28 August 2022 |work=[[The Guardian]]}}</ref> [[Neil deGrasse Tyson]],<ref name="C4WDefault-2084173">{{cite news |url=http://www.huffingtonpost.com/2014/03/04/neil-degrasse-tyson-cosmos-god-alien-life-multiverses-interview_n_4790408.html |title=Why Revive 'Cosmos?' Neil DeGrasse Tyson Says Just About Everything We Know Has Changed |date=4 March 2014 |access-date=12 September 2014 |work=huffingtonpost.com |last=Freeman |first=David |archive-url=https://web.archive.org/web/20140913063109/http://www.huffingtonpost.com/2014/03/04/neil-degrasse-tyson-cosmos-god-alien-life-multiverses-interview_n_4790408.html |archive-date=13 September 2014 |url-status=live }}</ref> [[Sean M. Carroll|Sean Carroll]]<ref>{{cite magazine |author=Carroll |first=Sean |date=18 October 2011 |title=Welcome to the Multiverse |url=http://discovermagazine.com/2011/oct/18-out-there-welcome-to-the-multiverse |magazine=[[Discover (magazine)|Discover]] |access-date=5 May 2015}}</ref> and [[Stephen Hawking]].<ref>{{cite book|title= Universe or Multiverse|isbn= 978-0-521-84841-1|page= 19|quote= Some physicists would prefer to believe that string theory, or M-theory, will answer these questions and uniquely predict the features of the Universe. Others adopt the view that the initial state of the Universe is prescribed by an outside agency, code-named God, or that there are many universes, with ours being picked out by the anthropic principle. Hawking argued that string theory is unlikely to predict the distinctive features of the Universe. But neither is he is an advocate of God. He therefore opts for the last approach, favoring the type of multiverse which arises naturally within the context of his own work in quantum cosmology.|last1= Carr|first1= Bernard|date= 2007-06-21|publisher= Cambridge University Press}}</ref> | ||
Scientists who are generally skeptical of the concept of a multiverse or popular multiverse hypotheses include <!-- To add anyone to this list we MUST have a citation that shows that they belong on the list. -->[[Sabine Hossenfelder]],<ref name="Sabine">{{cite magazine |last1=Geek's Guide to the Galaxy |title=Have Some Scientists Gotten Too Excited About the Multiverse? |url=https://www.wired.com/2022/09/geeks-guide-sabine-hossenfelder/ |access-date=16 February 2024 |magazine=Wired |publisher=Wired |date=9 September 2022}}</ref> [[David Gross]],<ref name="Davies2008">{{cite book |first=Paul |last=Davies |date=2008 |title=The Goldilocks Enigma: Why Is the Universe Just Right for Life? |chapter=Many Scientists Hate the Multiverse Idea |publisher=Houghton Mifflin Harcourt |isbn= | Scientists who are generally skeptical of the concept of a multiverse or popular multiverse hypotheses include <!-- To add anyone to this list we MUST have a citation that shows that they belong on the list. -->[[Sabine Hossenfelder]],<ref name="Sabine">{{cite magazine |last1=Geek's Guide to the Galaxy |title=Have Some Scientists Gotten Too Excited About the Multiverse? |url=https://www.wired.com/2022/09/geeks-guide-sabine-hossenfelder/ |access-date=16 February 2024 |magazine=Wired |publisher=Wired |date=9 September 2022}}</ref> [[David Gross]],<ref name="Davies2008">{{cite book |first=Paul |last=Davies |date=2008 |title=The Goldilocks Enigma: Why Is the Universe Just Right for Life? |chapter=Many Scientists Hate the Multiverse Idea |publisher=Houghton Mifflin Harcourt |isbn=978-0-547-34846-9 |chapter-url=https://books.google.com/books?id=ktkvIoOanJsC&pg=PT207 |page=207}}</ref> [[Paul Steinhardt]],<ref name="edge-steinhardt-2014retire">{{cite web |url=http://www.edge.org/response-detail/25405 |title=Theories of Anything |work=edge.org |date=9 March 2014 <!-- No date available; last modification date used. --> |access-date=9 March 2014 |first=Paul |last=Steinhardt |author-link=Paul Steinhardt |archive-url=https://web.archive.org/web/20140310063831/http://www.edge.org/response-detail/25405 |archive-date=10 March 2014 |url-status=live |at=2014 : WHAT SCIENTIFIC IDEA IS READY FOR RETIREMENT? |quote=Theories of Anything<br />A pervasive idea in fundamental physics and cosmology that should be retired: the notion that we live in a multiverse in which the laws of physics and the properties of the cosmos vary randomly from one patch of space to another }}</ref><ref name="Multimess"/> Anna Ijjas,<ref name="Multimess">{{citation |last1=Ijjas |first1=Anna |title=Cosmic Inflation Theory Faces Challenges |date=February 2017 |journal=Scientific American |volume=316 |issue=2 |pages=32–39 |doi=10.1038/scientificamerican0217-32 |pmid=28118351 |last2=Loeb |first2=Abraham |last3=Steinhardt |first3=Paul}}.</ref> [[Abraham Loeb]],<ref name="Multimess"/> [[David Spergel]],<ref>{{Cite news| url = https://www.youtube.com/watch?v=hgCaBxAIcGE| title = Is Nature Simple? 2018 Breakthrough Prize Symposium Panel | work = YouTube | access-date = 14 January 2018}}</ref> [[Neil Turok]],<ref>{{cite journal |last1=Gibbons |first1=G. W. |last2=Turok |first2=Neil |year=2008 |title=The Measure Problem in Cosmology |journal=Physical Review D |volume=77 |issue=6 |article-number=063516 |arxiv=hep-th/0609095 |bibcode=2008PhRvD..77f3516G |doi=10.1103/PhysRevD.77.063516 |s2cid=16394385}}</ref> [[Viatcheslav Mukhanov]],<ref>{{cite journal | last1 = Mukhanov | first1 = Viatcheslav| year = 2014 | title = Inflation without Selfreproduction | journal = Fortschritte der Physik | volume = 63 | issue = 1 | pages = 36–41 | doi = 10.1002/prop.201400074 | bibcode = 2015ForPh..63...36M|arxiv = 1409.2335 | s2cid = 117514254}}</ref> [[Michael S. Turner]],<ref>{{cite web|url=http://www.math.columbia.edu/~woit/wordpress/?p=7793 | title= A Crisis at the (Western) Edge of Physics | last1=Woit|first1= Peter| work= [[Not Even Wrong]] |date= 9 June 2015}}</ref> [[Roger Penrose]],<ref>{{cite web|url= http://www.math.columbia.edu/~woit/wordpress/?p=7812 | title= CMB @ 50 | last1=Woit|first1= Peter| work= [[Not Even Wrong]] |date= 14 June 2015}}</ref> [[George Francis Rayner Ellis|George Ellis]],<ref name="SciAmer-731548">{{cite journal |url= http://www.scientificamerican.com/article/does-the-multiverse-really-exist/ |title= Does the Multiverse Really Exist? |last=Ellis |first=George F. R. |date=1 August 2011 |journal= [[Scientific American]] |volume=305 |issue=2 |pages=38–43 |access-date= 12 September 2014 |doi= 10.1038/scientificamerican0811-38 |pmid= 21827123 |url-access=subscription|bibcode = 2011SciAm.305a..38E }}</ref><ref name="C4WDefault-1835434">{{cite web |url=http://www.aei.mpg.de/~axkl/HermannFestProceedings/Ellis.pdf |title=The Multiverse: Conjecture, Proof, and Science |date=2012 |access-date=12 September 2014 |work=Slides for a talk at Nicolai Fest Golm 2012 |last=Ellis |first=George |archive-url=https://web.archive.org/web/20140913064557/http://www.aei.mpg.de/~axkl/HermannFestProceedings/Ellis.pdf |archive-date=13 September 2014 }}</ref> [[Joe Silk]],<ref>{{citation |title= Scientific Method: Defend the Integrity of Physics |last1= Ellis |first1= George |last2=Silk |first2=Joe |date=16 December 2014 |journal= Nature |doi=10.1038/516321a |volume=516 |issue= 7531 |pages=321–323|bibcode = 2014Natur.516..321E |pmid=25519115|doi-access= free }}</ref> [[Carlo Rovelli]],<ref>{{citation |last1=Scoles |first1=Sarah |title=Can Physics Ever Prove the Multiverse is Real |date=19 April 2016 |work=Smithsonian.com |url=http://www.smithsonianmag.com/science-nature/can-physicists-ever-prove-multiverse-real-180958813/?no-ist }}.</ref> [[Adam Frank]],<ref name="Crisis" >{{cite news |url= https://www.nytimes.com/2015/06/07/opinion/a-crisis-at-the-edge-of-physics.html?_r=0 |title= A Crisis at the Edge of Physics |last1= Frank|first1= Adam |last2=Gleiser |first2=Marcelo |work = [[The New York Times]] |date= 5 June 2015}}</ref> [[Marcelo Gleiser]],<ref name = "Crisis" /> [[Jim Baggott]]<ref name="Amazon-1605984728">{{cite book|title= Farewell to Reality: How Modern Physics Has Betrayed the Search for Scientific Truth|last= Baggott|first= Jim|author-link= Jim Baggott|date= 1 August 2013|publisher= Pegasus|isbn= 978-1-60598-472-8|url-access= registration|url= https://archive.org/details/isbn_9781605984728}} | ||
</ref><!-- The person who added the original ref did not specify which version of the book they used. --> and [[Paul Davies]].<ref>{{cite news |url= https://www.nytimes.com/2003/04/12/opinion/a-brief-history-of-the-multiverse.html |title= A Brief History of the Multiverse |last1= Davies |first1= Paul | work = [[The New York Times]] |date= 12 April 2003}}</ref> | </ref><!-- The person who added the original ref did not specify which version of the book they used. --> and [[Paul Davies]].<ref>{{cite news |url= https://www.nytimes.com/2003/04/12/opinion/a-brief-history-of-the-multiverse.html |title= A Brief History of the Multiverse |last1= Davies |first1= Paul | work = [[The New York Times]] |date= 12 April 2003}}</ref> | ||
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[[Hugh Everett III]]'s [[many-worlds interpretation]] (MWI) is one of several mainstream [[interpretations of quantum mechanics]]. | [[Hugh Everett III]]'s [[many-worlds interpretation]] (MWI) is one of several mainstream [[interpretations of quantum mechanics]]. | ||
In brief, one aspect of quantum mechanics is that certain observations cannot be predicted absolutely. Instead, there is a range of possible observations, each with a different [[probability]]. According to the MWI, each of these possible observations corresponds to a different "world" within the [[Universal wavefunction]], with each world as real as ours. Suppose a six-sided | In brief, one aspect of quantum mechanics is that certain observations cannot be predicted absolutely. Instead, there is a range of possible observations, each with a different [[probability]]. According to the MWI, each of these possible observations corresponds to a different "world" within the [[Universal wavefunction]], with each world as real as ours. Suppose a six-sided die is thrown and that the result of the throw corresponds to [[observable]] quantum mechanics. All six possible ways the die can fall correspond to six different worlds. In the case of the Schrödinger's cat thought experiment, [[Schrödinger's cat#Many-worlds interpretation and consistent histories|both outcomes would be "real" in at least one "world"]]. | ||
Tegmark argues that a Level III multiverse does not contain more possibilities in the Hubble volume than a Level I or Level II multiverse. In effect, all the different worlds created by "splits" in a Level III multiverse with the same physical constants can be found in some Hubble volume in a Level I multiverse. Tegmark writes that, "The only difference between Level I and Level III is where your [[doppelgänger]]s reside. In Level I they live elsewhere in good old three-dimensional space. In Level III they live on another quantum branch in infinite-dimensional [[Hilbert space]]." | Tegmark argues that a Level III multiverse does not contain more possibilities in the Hubble volume than a Level I or Level II multiverse. In effect, all the different worlds created by "splits" in a Level III multiverse with the same physical constants can be found in some Hubble volume in a Level I multiverse. Tegmark writes that, "The only difference between Level I and Level III is where your [[doppelgänger]]s reside. In Level I they live elsewhere in good old three-dimensional space. In Level III they live on another quantum branch in infinite-dimensional [[Hilbert space]]." | ||
Similarly, all Level II bubble universes with different physical constants can, in effect, be found as "worlds" created by "splits" at the moment of spontaneous symmetry breaking in a Level III multiverse.<ref name="TegmarkPUstaple"/> According to [[Yasunori Nomura]],<ref name="Nomura"/> [[Raphael Bousso]], and [[Leonard Susskind]],<ref name="BoussoSusskind"/> this is because global spacetime appearing in the (eternally) inflating multiverse is a redundant concept. This implies that the multiverses of Levels I, II, and III are, in fact, the same thing. This hypothesis is referred to as "Multiverse = Quantum Many Worlds". According to [[Yasunori Nomura]], this quantum multiverse is static, and time is a simple illusion.<ref>{{Cite journal |arxiv = 1205.5550|doi = 10.1103/PhysRevD.86.083505|title = Static quantum multiverse|journal = Physical Review D|volume = 86|issue = 8| | Similarly, all Level II bubble universes with different physical constants can, in effect, be found as "worlds" created by "splits" at the moment of spontaneous symmetry breaking in a Level III multiverse.<ref name="TegmarkPUstaple"/> According to [[Yasunori Nomura]],<ref name="Nomura"/> [[Raphael Bousso]], and [[Leonard Susskind]],<ref name="BoussoSusskind"/> this is because global spacetime appearing in the (eternally) inflating multiverse is a redundant concept. This implies that the multiverses of Levels I, II, and III are, in fact, the same thing. This hypothesis is referred to as "Multiverse = Quantum Many Worlds". According to [[Yasunori Nomura]], this quantum multiverse is static, and time is a simple illusion.<ref>{{Cite journal |arxiv = 1205.5550|doi = 10.1103/PhysRevD.86.083505|title = Static quantum multiverse|journal = Physical Review D|volume = 86|issue = 8|article-number = 083505|year = 2012|last1 = Nomura|first1 = Yasunori|last2 = Johnson|first2 = Matthew C.|last3 = Mortlock|first3 = Daniel J.|last4 = Peiris|first4 = Hiranya V.|bibcode = 2012PhRvD..86h3505N|s2cid = 119207079}}</ref> | ||
Another version of the many-worlds idea is [[H. Dieter Zeh]]'s [[many-minds interpretation|''many-minds'' interpretation]]. | Another version of the many-worlds idea is [[H. Dieter Zeh]]'s [[many-minds interpretation|''many-minds'' interpretation]]. | ||
==== Level IV: Ultimate ensemble ==== | ==== Level IV: Ultimate ensemble ==== | ||
The ultimate [[mathematical universe hypothesis]] is Tegmark's own hypothesis.<ref name="Tegmark2014">{{cite book |first=Max |last=Tegmark |date=2014 |title=Our Mathematical Universe: My Quest for the Ultimate Nature of Reality |publisher=Knopf Doubleday Publishing Group |isbn= | The ultimate [[mathematical universe hypothesis]] is Tegmark's own hypothesis.<ref name="Tegmark2014">{{cite book |first=Max |last=Tegmark |date=2014 |title=Our Mathematical Universe: My Quest for the Ultimate Nature of Reality |publisher=Knopf Doubleday Publishing Group |isbn=978-0-307-59980-3|title-link=Our Mathematical Universe: My Quest for the Ultimate Nature of Reality }}</ref> | ||
This level considers all universes to be equally real which can be described by different mathematical structures. | This level considers all universes to be equally real which can be described by different mathematical structures. | ||
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[[Jürgen Schmidhuber]], however, says that the set of mathematical structures is not even [[well-defined]] and that it admits only universe representations describable by [[constructive mathematics]]—that is, [[computer programs]]. | [[Jürgen Schmidhuber]], however, says that the set of mathematical structures is not even [[well-defined]] and that it admits only universe representations describable by [[constructive mathematics]]—that is, [[computer programs]]. | ||
Schmidhuber explicitly includes universe representations describable by non-halting programs whose output bits converge after a finite time, although the convergence time itself may not be predictable by a halting program, due to the [[Undecidable problem|undecidability]] of the [[halting problem]].<ref>[[Jürgen Schmidhuber|J. Schmidhuber]] (1997): A Computer Scientist's View of Life, the Universe, and Everything. Lecture Notes in Computer Science, pp. 201–208, Springer: [http://www.idsia.ch/~juergen/everything/ IDSIA – Dalle Molle Institute for Artificial Intelligence].</ref><ref>{{Cite arXiv|eprint=quant-ph/0011122|last1=Schmidhuber|first1=Juergen|title=Algorithmic Theories of Everything|year=2000}}</ref><ref>[[Jürgen Schmidhuber|J. Schmidhuber]] (2002): Hierarchies of generalized Kolmogorov complexities and nonenumerable universal measures computable in the limit. ''International Journal of Foundations of Computer Science'' 13 (4): 587–612. [http://www.idsia.ch/~juergen/kolmogorov.html IDSIA – Dalle Molle Institute for Artificial Intelligence].</ref> He also explicitly discusses the more restricted ensemble of quickly computable universes.<ref>[[Jürgen Schmidhuber|J. Schmidhuber]] (2002): The Speed Prior: A New Simplicity Measure Yielding Near-Optimal Computable Predictions. Proc. 15th Annual Conference on Computational Learning Theory (COLT 2002), Sydney, Australia, Lecture Notes in Artificial Intelligence, pp. 216–228. Springer: [http://www.idsia.ch/~juergen/speedprior.html IDSIA – Dalle Molle Institute for Artificial Intelligence].</ref> | Schmidhuber explicitly includes universe representations describable by non-halting programs whose output bits converge after a finite time, although the convergence time itself may not be predictable by a halting program, due to the [[Undecidable problem|undecidability]] of the [[halting problem]].<ref>[[Jürgen Schmidhuber|J. Schmidhuber]] (1997): A Computer Scientist's View of Life, the Universe, and Everything. Lecture Notes in Computer Science, pp. 201–208, Springer: [http://www.idsia.ch/~juergen/everything/ IDSIA – Dalle Molle Institute for Artificial Intelligence].</ref><ref>{{Cite arXiv|eprint=quant-ph/0011122|last1=Schmidhuber|first1=Juergen|title=Algorithmic Theories of Everything|year=2000 }}</ref><ref>[[Jürgen Schmidhuber|J. Schmidhuber]] (2002): Hierarchies of generalized Kolmogorov complexities and nonenumerable universal measures computable in the limit. ''International Journal of Foundations of Computer Science'' 13 (4): 587–612. [http://www.idsia.ch/~juergen/kolmogorov.html IDSIA – Dalle Molle Institute for Artificial Intelligence].</ref> He also explicitly discusses the more restricted ensemble of quickly computable universes.<ref>[[Jürgen Schmidhuber|J. Schmidhuber]] (2002): The Speed Prior: A New Simplicity Measure Yielding Near-Optimal Computable Predictions. Proc. 15th Annual Conference on Computational Learning Theory (COLT 2002), Sydney, Australia, Lecture Notes in Artificial Intelligence, pp. 216–228. Springer: [http://www.idsia.ch/~juergen/speedprior.html IDSIA – Dalle Molle Institute for Artificial Intelligence].</ref> | ||
=== Brian Greene's nine types === | === Brian Greene's nine types === | ||
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=== Twin-world models === | === Twin-world models === | ||
[[File:Universe Antiuniverse model.png|thumb|upright=1.4|<!--{{better image needed}}-->Concept of a twin universe, with the [[Cosmogony|beginning of time]] in the middle]] | [[File:Universe Antiuniverse model.png|thumb|upright=1.4|<!--{{better image needed}}-->Concept of a twin universe, with the [[Cosmogony|beginning of time]] in the middle]] | ||
There are models of two related universes that e.g. attempt to explain the [[baryon asymmetry]] – why there was more matter than [[antimatter]] at the beginning – [[Baryon asymmetry#Mirror anti-universe|with a mirror anti-universe]].<ref>{{cite news |title=Our universe has antimatter partner on the other side of the Big Bang, say physicists |url=https://physicsworld.com/a/our-universe-has-antimatter-partner-on-the-other-side-of-the-big-bang-say-physicists/ |access-date=22 June 2022 |work=Physics World |date=3 January 2019}}</ref><ref>{{cite news |last1=Letzter |first1=Rafi |title=Why some physicists really think there's a 'mirror universe' hiding in space-time |url=https://www.space.com/truth-behind-nasa-mirror-parallel-universe.html |access-date=22 June 2022 |work=Space.com |date=23 June 2020 |language=en}}</ref><ref>{{cite journal |last1=Boyle |first1=Latham |last2=Finn |first2=Kieran |last3=Turok |first3=Neil |title=CPT-Symmetric Universe |journal=Physical Review Letters |date=20 December 2018 |volume=121 |issue=25 | | There are models of two related universes that e.g. attempt to explain the [[baryon asymmetry]] – why there was more matter than [[antimatter]] at the beginning – [[Baryon asymmetry#Mirror anti-universe|with a mirror anti-universe]].<ref>{{cite news |title=Our universe has antimatter partner on the other side of the Big Bang, say physicists |url=https://physicsworld.com/a/our-universe-has-antimatter-partner-on-the-other-side-of-the-big-bang-say-physicists/ |access-date=22 June 2022 |work=Physics World |date=3 January 2019}}</ref><ref>{{cite news |last1=Letzter |first1=Rafi |title=Why some physicists really think there's a 'mirror universe' hiding in space-time |url=https://www.space.com/truth-behind-nasa-mirror-parallel-universe.html |access-date=22 June 2022 |work=Space.com |date=23 June 2020 |language=en}}</ref><ref>{{cite journal |last1=Boyle |first1=Latham |last2=Finn |first2=Kieran |last3=Turok |first3=Neil |title=CPT-Symmetric Universe |journal=Physical Review Letters |date=20 December 2018 |volume=121 |issue=25 |article-number=251301 |doi=10.1103/PhysRevLett.121.251301|pmid=30608856 |arxiv=1803.08928 |bibcode=2018PhRvL.121y1301B |s2cid=58638592 }}</ref> One two-universe cosmological model<!-- – already extensively studied to find out why gravity appears much weaker than other known forces –--> could explain the [[Hubble's law#Hubble tension|Hubble constant (H<sub>0</sub>) tension]] via interactions between the two worlds. The "mirror world" would contain copies of all existing fundamental particles.<ref>{{cite news |title=Mirror world of dark particles could explain cosmic anomaly |url=https://physicsworld.com/a/mirror-world-of-dark-particles-could-explain-cosmic-anomaly/ |access-date=22 June 2022 |work=Physics World |date=31 May 2022}}</ref><ref>{{cite journal |last1=Cyr-Racine |first1=Francis-Yan |last2=Ge |first2=Fei |last3=Knox |first3=Lloyd |title=Symmetry of Cosmological Observables, a Mirror World Dark Sector, and the Hubble Constant |journal=Physical Review Letters |date=18 May 2022 |volume=128 |issue=20 |article-number=201301 |doi=10.1103/PhysRevLett.128.201301|pmid=35657861 |arxiv=2107.13000 |bibcode=2022PhRvL.128t1301C |s2cid=248904936 }}</ref> Another twin/pair-world or "bi-world" cosmology is shown to theoretically be able to solve the [[cosmological constant problem|cosmological constant (Λ) problem]], closely related to [[dark energy]]<!--while describing other important parts of physics-->: two interacting worlds with a large Λ each could result in a small shared effective Λ.<ref>{{cite news |last1=Bedford |first1=Bailey |title=Bilayer graphene inspires two-universe cosmological model |url=https://phys.org/news/2022-05-bilayer-graphene-two-universe-cosmological.html |access-date=22 June 2022 |work=Joint Quantum Institute |language=en}}</ref><ref>{{cite journal |last1=Parhizkar |first1=Alireza |last2=Galitski |first2=Victor |title=Strained bilayer graphene, emergent energy scales, and moir\'e gravity |journal=Physical Review Research |date=2 May 2022 |volume=4 |issue=2 |article-number=L022027 |doi=10.1103/PhysRevResearch.4.L022027|arxiv=2108.04252 |bibcode=2022PhRvR...4b2027P |s2cid=236965490 }}</ref><ref>{{cite arXiv |last1=Parhizkar |first1=Alireza |last2=Galitski |first2=Victor |date=2022 |title=Moiré Gravity and Cosmology |class=hep-th |eprint=2204.06574}}</ref> | ||
=== Cyclic theories === | === Cyclic theories === | ||
| Line 164: | Line 179: | ||
== M-theory == | == M-theory == | ||
{{see also|Introduction to M-theory|M-theory|Brane cosmology|String theory landscape}} | {{see also|Introduction to M-theory|M-theory|Brane cosmology|String theory landscape}} | ||
A multiverse of a somewhat different kind has been envisaged within [[string theory]] and its higher-dimensional extension, [[Introduction to M-theory|M-theory.]]<ref>{{cite arXiv|eprint=hep-th/0511037v1|last1=Weinberg|first1=Steven|title=Living in the Multiverse|date=2005}}</ref> | A multiverse of a somewhat different kind has been envisaged within [[string theory]] and its higher-dimensional extension, [[Introduction to M-theory|M-theory.]]<ref>{{cite arXiv|eprint=hep-th/0511037v1|last1=Weinberg|first1=Steven|title=Living in the Multiverse|date=2005 }}</ref> | ||
These theories require the presence of 10 or 11 spacetime dimensions respectively. The extra six or seven dimensions may either be compactified on a very small scale, or our universe may simply be localized on a dynamical (3+1)-dimensional object, a [[D3-brane]]. This opens up the possibility that there are other [[brane]]s which could support other universes.<ref name="Richard J Szabo 2004">Richard J. Szabo, ''An introduction to string theory and D-brane dynamics'' (2004).</ref><ref name="Maurizio Gasperini 2007">Maurizio Gasperini, ''Elements of String Cosmology'' (2007).</ref> | These theories require the presence of 10 or 11 spacetime dimensions respectively. The extra six or seven dimensions may either be compactified on a very small scale, or our universe may simply be localized on a dynamical (3+1)-dimensional object, a [[D3-brane]]. This opens up the possibility that there are other [[brane]]s which could support other universes.<ref name="Richard J Szabo 2004">Richard J. Szabo, ''An introduction to string theory and D-brane dynamics'' (2004).</ref><ref name="Maurizio Gasperini 2007">Maurizio Gasperini, ''Elements of String Cosmology'' (2007).</ref> | ||
| Line 188: | Line 203: | ||
== Possible worlds and real worlds == | == Possible worlds and real worlds == | ||
In any given set of possible universes – e.g. in terms of histories or variables of nature – not all may be ever realized, and some may be realized many times.<ref>{{cite journal |last1=Ellis |first1=G. F. R. |last2=Kirchner |first2=U. |last3=Stoeger |first3=W. R. |title=Multiverses and physical cosmology |journal=Monthly Notices of the Royal Astronomical Society |date=21 January 2004 |volume=347 |issue=3 |pages=921–936 |doi=10.1111/j.1365-2966.2004.07261.x|doi-access=free |arxiv=astro-ph/0305292 |bibcode=2004MNRAS.347..921E |s2cid=119028830 }}</ref> For example, over infinite time there could, in some potential theories, be infinite universes, but only a small or relatively small real number of universes where humanity could exist and only one where it ever does exist (with a unique history).{{citation needed|date=July 2022}} It has been suggested that a universe that "contains life, in the form it has on Earth, is in a certain sense radically non-[[Ergodicity|ergodic]], in that the vast majority of possible organisms will never be realized".<ref>{{cite arXiv |last1=Cortês |first1=Marina |last2=Kauffman |first2=Stuart A. |last3=Liddle |first3=Andrew R. |last4=Smolin |first4=Lee | | In any given set of possible universes – e.g. in terms of histories or variables of nature – not all may be ever realized, and some may be realized many times.<ref>{{cite journal |last1=Ellis |first1=G. F. R. |last2=Kirchner |first2=U. |last3=Stoeger |first3=W. R. |title=Multiverses and physical cosmology |journal=Monthly Notices of the Royal Astronomical Society |date=21 January 2004 |volume=347 |issue=3 |pages=921–936 |doi=10.1111/j.1365-2966.2004.07261.x|doi-access=free |arxiv=astro-ph/0305292 |bibcode=2004MNRAS.347..921E |s2cid=119028830 }}</ref> For example, over infinite time there could, in some potential theories, be infinite universes, but only a small or relatively small real number of universes where humanity could exist and only one where it ever does exist (with a unique history).{{citation needed|date=July 2022}} It has been suggested that a universe that "contains life, in the form it has on Earth, is in a certain sense radically non-[[Ergodicity|ergodic]], in that the vast majority of possible organisms will never be realized".<ref>{{cite arXiv |last1=Cortês |first1=Marina |last2=Kauffman |first2=Stuart A. |last3=Liddle |first3=Andrew R. |last4=Smolin |first4=Lee |author-link4=Lee Smolin |title=Biocosmology: Biology from a cosmological perspective |eprint=2204.09379 |date=28 April 2022|class=physics.hist-ph }}</ref> On the other hand, some scientists, theories and popular works conceive of a multiverse in which the universes are so similar that humanity exists in many equally real separate universes but with varying histories.<ref>{{cite web |title=What is the multiverse—and is there any evidence it really exists? |url=https://www.nationalgeographic.com/science/article/what-is-the-multiverse |archive-url=https://web.archive.org/web/20220504150431/https://www.nationalgeographic.com/science/article/what-is-the-multiverse |archive-date=4 May 2022 |website=Science |access-date=12 July 2022 |language=en |date=4 May 2022}}</ref> | ||
There is a [[Many-worlds interpretation#Debate whether the other worlds are real|debate about whether the other worlds are real]] in the many-worlds interpretation (MWI) of quantum [[mechanics]]. In [[Quantum Darwinism]] one does not need to adopt a MWI in which all of the branches are equally real.<ref>{{cite journal | | There is a [[Many-worlds interpretation#Debate whether the other worlds are real|debate about whether the other worlds are real]] in the many-worlds interpretation (MWI) of quantum [[mechanics]]. In [[Quantum Darwinism]] one does not need to adopt a MWI in which all of the branches are equally real.<ref>{{cite journal |author-link1=Wojciech H. Zurek |last1=Zurek |first1=Wojciech Hubert |title=Quantum theory of the classical: quantum jumps, Born's Rule and objective classical reality via quantum Darwinism |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |date=13 July 2018 |volume=376 |issue=2123 |article-number=20180107 |doi=10.1098/rsta.2018.0107|pmid=29807905 |pmc=5990654 |arxiv=1807.02092 |bibcode=2018RSPTA.37680107Z }}</ref> | ||
=== Modal realism === | === Modal realism === | ||
| Line 220: | Line 235: | ||
{{refbegin}} | {{refbegin}} | ||
* {{cite book |last=Carr |first=Bernard |edition=2007 |author-link=Bernard Carr |title=Universe or Multiverse? |publisher=Cambridge University Press}} | * {{cite book |last=Carr |first=Bernard |edition=2007 |author-link=Bernard Carr |title=Universe or Multiverse? |publisher=Cambridge University Press}} | ||
* {{cite journal |first=David |last=Deutsch |author-link=David Deutsch |date=1985 |title=Quantum theory, the Church–Turing principle and the universal quantum computer |journal=Proceedings of the Royal Society of London A |issue=1818 |pages=97–117 |url=http://www.cs.berkeley.edu/~christos/classics/Deutsch_quantum_theory.pdf |doi=10.1098/rspa.1985.0070 |volume=400 |bibcode=1985RSPSA.400...97D |citeseerx=10.1.1.41.2382 |s2cid=1438116 |access-date=15 September 2014 |archive-url=https://web.archive.org/web/20160309202304/http://www.cs.berkeley.edu/~christos/classics/Deutsch_quantum_theory.pdf |archive-date=9 March 2016 | * {{cite journal |first=David |last=Deutsch |author-link=David Deutsch |date=1985 |title=Quantum theory, the Church–Turing principle and the universal quantum computer |journal=Proceedings of the Royal Society of London A |issue=1818 |pages=97–117 |url=http://www.cs.berkeley.edu/~christos/classics/Deutsch_quantum_theory.pdf |doi=10.1098/rspa.1985.0070 |volume=400 |bibcode=1985RSPSA.400...97D |citeseerx=10.1.1.41.2382 |s2cid=1438116 |access-date=15 September 2014 |archive-url=https://web.archive.org/web/20160309202304/http://www.cs.berkeley.edu/~christos/classics/Deutsch_quantum_theory.pdf |archive-date=9 March 2016 }} | ||
* {{cite journal |last1=Ellis |first1=George F.R. |author-link=George Francis Rayner Ellis |last2=Stoeger |title=Multiverses and physical cosmology |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=347 |issue=3 |pages=921–936 |date=2004 |arxiv=astro-ph/0305292 |doi=10.1111/j.1365-2966.2004.07261.x |bibcode=2004MNRAS.347..921E |first2=William R. |doi-access=free |author-link2=William R. Stoeger |s2cid=119028830}} | * {{cite journal |last1=Ellis |first1=George F.R. |author-link=George Francis Rayner Ellis |last2=Stoeger |title=Multiverses and physical cosmology |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=347 |issue=3 |pages=921–936 |date=2004 |arxiv=astro-ph/0305292 |doi=10.1111/j.1365-2966.2004.07261.x |bibcode=2004MNRAS.347..921E |first2=William R. |doi-access=free |author-link2=William R. Stoeger |s2cid=119028830}} | ||
* Andrei Linde, [https://web.stanford.edu/~alinde/1032226.pdf The Self-Reproducing Inflationary Universe], [[Scientific American]], November 1994 – Touches on multiverse concepts at the end of the article. | * Andrei Linde, [https://web.stanford.edu/~alinde/1032226.pdf The Self-Reproducing Inflationary Universe], [[Scientific American]], November 1994 – Touches on multiverse concepts at the end of the article. | ||
| Line 230: | Line 245: | ||
{{commons category|Multiverse}} | {{commons category|Multiverse}} | ||
* [http://thoughtcast.org/casts/the-end-of-our-universe-among-other-timely-topics Interview with Tufts cosmologist Alex Vilenkin] on his book, "Many Worlds in One: The Search for Other Universes" on the podcast and public radio interview program [http://www.thoughtcast.org/ ThoughtCast.] {{Webarchive|url=https://web.archive.org/web/20200818134713/https://thoughtcast.org/ |date=18 August 2020 }}. | * [http://thoughtcast.org/casts/the-end-of-our-universe-among-other-timely-topics Interview with Tufts cosmologist Alex Vilenkin] on his book, "Many Worlds in One: The Search for Other Universes" on the podcast and public radio interview program [http://www.thoughtcast.org/ ThoughtCast.] {{Webarchive|url=https://web.archive.org/web/20200818134713/https://thoughtcast.org/ |date=18 August 2020 }}. | ||
* [ | * [https://www.bbc.co.uk/programmes/b008z744 Multiverse] – an episode of the series [[In Our Time (radio series)|In Our Time]] with [[Melvyn Bragg]], on [[BBC Radio 4]]. | ||
* [http://www.bbc.com/earth/story/20160318-why-there-might-be-many-more-universes-besides-our-own Why There Might be Many More Universes Besides Our Own], by Phillip Ball, March 21, 2016, bbc.com. | * [http://www.bbc.com/earth/story/20160318-why-there-might-be-many-more-universes-besides-our-own Why There Might be Many More Universes Besides Our Own], by Phillip Ball, March 21, 2016, bbc.com. | ||
Latest revision as of 15:57, 7 November 2025
Template:Short description Script error: No such module "Distinguish". Template:Redirect-distinguish Script error: No such module "other uses". Template:Pp-pc Template:Use dmy dates Template:Cosmology
The multiverse is the hypothetical set of all universes.[1]Template:Efn Together, these universes are presumed to comprise everything that exists: the entirety of space, time, matter, energy, information, and the physical laws and constants that describe them. The different universes within the multiverse are called "parallel universes", "flat universes", "other universes", "alternate universes", "multiple universes", "plane universes", "parent and child universes", "many universes", or "many worlds". One common assumption is that the multiverse is a "patchwork quilt of separate universes all bound by the same laws of physics."[2]
The concept of multiple universes, or a multiverse, has been discussed throughout history. It has evolved and has been debated in various fields, including cosmology, physics, and philosophy. Some physicists have argued that the multiverse is a philosophical notion rather than a scientific hypothesis, as it cannot be empirically falsified. In recent years, there have been proponents and skeptics of multiverse theories within the physics community. Although some scientists have analyzed data in search of evidence for other universes, no statistically significant evidence has been found. Critics argue that the multiverse concept lacks testability and falsifiability, which are essential for scientific inquiry, and that it raises unresolved metaphysical issues.
Max Tegmark and Brian Greene have proposed different classification schemes for multiverses and universes. Tegmark's four-level classification consists of Level I: an extension of our universe, Level II: universes with different physical constants, Level III: many-worlds interpretation of quantum mechanics, and Level IV: ultimate ensemble. Brian Greene's nine types of multiverses include quilted, inflationary, brane, cyclic, landscape, quantum, holographic, simulated, and ultimate. The ideas explore various dimensions of space, physical laws, and mathematical structures to explain the existence and interactions of multiple universes. Some other multiverse concepts include twin-world models, cyclic theories, M-theory, and black-hole cosmology.
The anthropic principle suggests that the existence of a multitude of universes, each with different physical laws, could explain the asserted appearance of fine-tuning of our own universe for conscious life. The weak anthropic principle posits that we exist in one of the few universes that support life. Debates around Occam's razor and the simplicity of the multiverse versus a single universe arise, with proponents like Max Tegmark arguing that the multiverse is simpler and more elegant. The many-worlds interpretation of quantum mechanics and modal realism, the belief that all possible worlds exist and are as real as our world, are also subjects of debate in the context of the anthropic principle.
History of the concept
According to some, the idea of infinite worlds was first suggested by the pre-Socratic Greek philosopher Anaximander in the sixth century BCE.[3] However, there is debate as to whether he believed in multiple worlds, and if he did, whether those worlds were co-existent or successive.[4][5][6][7]
The first figures to whom historians can definitively attribute the concept of innumerable worlds are the Ancient Greek Atomists, beginning with Leucippus and Democritus in the 5th century BCE, followed by Epicurus (341–270 BCE) and the Roman Epicurean Lucretius (1st century BCE).[8][9][7][10][11][12] In the third century BCE, the philosopher Chrysippus suggested that the world eternally expired and regenerated, effectively suggesting the existence of multiple universes across time.[11] The concept of multiple universes became more defined in the Middle Ages.[13] In the Renaissance, Giordano Bruno (1548-1600) expressed the concept of infinite worlds.[14]
The American philosopher and psychologist William James used the term "multiverse" in 1895, but in a different context.[15]
The concept first appeared in the modern scientific context in the course of the debate between Boltzmann and Zermelo in 1895.[16]
In Dublin in 1952, Erwin Schrödinger gave a lecture in which he jocularly warned his audience that what he was about to say might "seem lunatic". He said that when his equations seemed to describe several different histories, these were "not alternatives, but all really happen simultaneously".[17] This sort of duality is called "superposition".
Search for evidence
In the 1990s, after recent works of fiction about the concept gained popularity, scientific discussions about the multiverse and journal articles about it gained prominence.[18]
Around 2010, scientists such as Stephen M. Feeney analyzed Wilkinson Microwave Anisotropy Probe (WMAP) data and claimed to find evidence suggesting that this universe collided with other (parallel) universes in the distant past.[19][20][21] However, a more thorough analysis of data from the WMAP and from the Planck satellite, which has a resolution three times higher than WMAP, did not reveal any statistically significant evidence of such a bubble universe collision.[22][23] In addition, there was no evidence of any gravitational pull of other universes on ours.[24][25]
In 2015, an astrophysicist may have found evidence of alternate or parallel universes by looking back in time to a time immediately after the Big Bang, although it is still a matter of debate among physicists.[26] Dr. Ranga-Ram Chary, after analyzing the cosmic radiation spectrum, found a signal 4,500 times brighter than it should have been, based on the number of protons and electrons scientists believe existed in the very early universe. This signal—an emission line that arose from the formation of atoms during the era of recombination—is more consistent with a universe whose ratio of matter particles to photons is about 65 times greater than our own. There is a 30% chance that this signal is noise, and not really a signal at all; however, it is also possible that it exists because a parallel universe dumped some of its matter particles into our universe. If additional protons and electrons had been added to our universe during recombination, more atoms would have formed, more photons would have been emitted during their formation, and the signature line that arose from all of these emissions would be greatly enhanced. Chary said:Template:Quotation
Chary also noted:[27]Template:Quotation
The signature that Chary has isolated may be a consequence of incoming light from distant galaxies, or even from clouds of dust surrounding our own galaxy.[27]
Proponents and skeptics
Modern proponents of one or more of the multiverse hypotheses include Lee Smolin,[28] Don Page,[29] Brian Greene,[30][31] Max Tegmark,[32] Alan Guth,[33] Andrei Linde,[34] Michio Kaku,[35] David Deutsch,[36] Leonard Susskind,[37] Alexander Vilenkin,[38] Yasunori Nomura,[39] Raj Pathria,[40] Laura Mersini-Houghton,[41] Neil deGrasse Tyson,[42] Sean Carroll[43] and Stephen Hawking.[44]
Scientists who are generally skeptical of the concept of a multiverse or popular multiverse hypotheses include Sabine Hossenfelder,[45] David Gross,[46] Paul Steinhardt,[47][48] Anna Ijjas,[48] Abraham Loeb,[48] David Spergel,[49] Neil Turok,[50] Viatcheslav Mukhanov,[51] Michael S. Turner,[52] Roger Penrose,[53] George Ellis,[54][55] Joe Silk,[56] Carlo Rovelli,[57] Adam Frank,[58] Marcelo Gleiser,[58] Jim Baggott[59] and Paul Davies.[60]
Arguments against multiverse hypotheses
In his 2003 New York Times opinion piece, "A Brief History of the Multiverse", author and cosmologist Paul Davies offered a variety of arguments that multiverse hypotheses are non-scientific:[61] Template:Quotation
George Ellis, writing in August 2011, provided a criticism of the multiverse, and pointed out that it is not a traditional scientific theory. He accepts that the multiverse is thought to exist far beyond the cosmological horizon. He emphasized that it is theorized to be so far away that it is unlikely any evidence will ever be found. Ellis also explained that some theorists do not believe the lack of empirical testability and falsifiability is a major concern, but he is opposed to that line of thinking: Template:Quotation
Ellis says that scientists have proposed the idea of the multiverse as a way of explaining the nature of existence. He points out that it ultimately leaves those questions unresolved because it is a metaphysical issue that cannot be resolved by empirical science. He argues that observational testing is at the core of science and should not be abandoned:[62] Template:Quotation
Philosopher Philip Goff argues that the inference of a multiverse to explain the apparent fine-tuning of the universe is an example of Inverse Gambler's Fallacy.[63]
Stoeger, Ellis, and Kircher[64]Template:Rp note that in a true multiverse theory, "the universes are then completely disjoint and nothing that happens in any one of them is causally linked to what happens in any other one. This lack of any causal connection in such multiverses really places them beyond any scientific support".
In May 2020, astrophysicist Ethan Siegel expressed criticism in a Forbes blog post that parallel universes would have to remain a science fiction dream for the time being, based on the scientific evidence available to us.[65]
Scientific American contributor John Horgan also argues against the idea of a multiverse, claiming that they are "bad for science."[66]
Types
Max Tegmark and Brian Greene have devised classification schemes for the various theoretical types of multiverses and universes that they might comprise.
Max Tegmark's four levels
Script error: No such module "anchor".Cosmologist Max Tegmark has provided a taxonomy of universes beyond the familiar observable universe. The four levels of Tegmark's classification are arranged such that subsequent levels can be understood to encompass and expand upon previous levels. They are briefly described below.[67][68]
Level I: An extension of our universe
A prediction of cosmic inflation is the existence of an infinite ergodic universe, which, being infinite, must contain Hubble volumes realizing all initial conditions.
Accordingly, an infinite universe will contain an infinite number of Hubble volumes, all having the same physical laws and physical constants. In regard to configurations such as the distribution of matter, almost all will differ from our Hubble volume. However, because there are infinitely many, far beyond the cosmological horizon, there will eventually be Hubble volumes with similar, and even identical, configurations. Tegmark estimates that an identical volume to ours should be about 1010115 meters away from us.[32]
Given infinite space, there would be an infinite number of Hubble volumes identical to ours in the universe.[69] This follows directly from the cosmological principle, wherein it is assumed that our Hubble volume is not special or unique.
Level II: Universes with different physical constants
In the eternal inflation theory, which is a variant of the cosmic inflation theory, the multiverse or space as a whole is stretching and will continue doing so forever,[70] but some regions of space stop stretching and form distinct bubbles (like gas pockets in a loaf of rising bread). Such bubbles are embryonic level I multiverses.
Different bubbles may experience different spontaneous symmetry breaking, which results in different properties, such as different physical constants.[69]
Level II also includes John Archibald Wheeler's oscillatory universe theory and Lee Smolin's fecund universes theory.
Level III: Many-worlds interpretation of quantum mechanics
Hugh Everett III's many-worlds interpretation (MWI) is one of several mainstream interpretations of quantum mechanics.
In brief, one aspect of quantum mechanics is that certain observations cannot be predicted absolutely. Instead, there is a range of possible observations, each with a different probability. According to the MWI, each of these possible observations corresponds to a different "world" within the Universal wavefunction, with each world as real as ours. Suppose a six-sided die is thrown and that the result of the throw corresponds to observable quantum mechanics. All six possible ways the die can fall correspond to six different worlds. In the case of the Schrödinger's cat thought experiment, both outcomes would be "real" in at least one "world".
Tegmark argues that a Level III multiverse does not contain more possibilities in the Hubble volume than a Level I or Level II multiverse. In effect, all the different worlds created by "splits" in a Level III multiverse with the same physical constants can be found in some Hubble volume in a Level I multiverse. Tegmark writes that, "The only difference between Level I and Level III is where your doppelgängers reside. In Level I they live elsewhere in good old three-dimensional space. In Level III they live on another quantum branch in infinite-dimensional Hilbert space."
Similarly, all Level II bubble universes with different physical constants can, in effect, be found as "worlds" created by "splits" at the moment of spontaneous symmetry breaking in a Level III multiverse.[69] According to Yasunori Nomura,[39] Raphael Bousso, and Leonard Susskind,[37] this is because global spacetime appearing in the (eternally) inflating multiverse is a redundant concept. This implies that the multiverses of Levels I, II, and III are, in fact, the same thing. This hypothesis is referred to as "Multiverse = Quantum Many Worlds". According to Yasunori Nomura, this quantum multiverse is static, and time is a simple illusion.[71]
Another version of the many-worlds idea is H. Dieter Zeh's many-minds interpretation.
Level IV: Ultimate ensemble
The ultimate mathematical universe hypothesis is Tegmark's own hypothesis.[72]
This level considers all universes to be equally real which can be described by different mathematical structures.
Tegmark writes: Template:Quotation
He argues that this "implies that any conceivable parallel universe theory can be described at Level IV" and "subsumes all other ensembles, therefore brings closure to the hierarchy of multiverses, and there cannot be, say, a Level V."[32]
Jürgen Schmidhuber, however, says that the set of mathematical structures is not even well-defined and that it admits only universe representations describable by constructive mathematics—that is, computer programs.
Schmidhuber explicitly includes universe representations describable by non-halting programs whose output bits converge after a finite time, although the convergence time itself may not be predictable by a halting program, due to the undecidability of the halting problem.[73][74][75] He also explicitly discusses the more restricted ensemble of quickly computable universes.[76]
Brian Greene's nine types
Script error: No such module "anchor".The American theoretical physicist and string theorist Brian Greene discussed nine types of multiverses:[77]
- Quilted
- The quilted multiverse works only in an infinite universe. With an infinite amount of space, every possible event will occur an infinite number of times. However, the speed of light prevents us from being aware of these other identical areas.
- Inflationary
- The inflationary multiverse is composed of various pockets in which inflation fields collapse and form new universes.
- Brane
- The brane multiverse version postulates that our entire universe exists on a membrane (brane) which floats in a higher dimension or "bulk". In this bulk, there are other membranes with their own universes. These universes can interact with one another, and when they collide, the violence and energy produced is more than enough to give rise to a Big Bang. The branes float or drift near each other in the bulk, and every few trillion years, attracted by gravity or some other force we do not understand, collide and bang into each other. This repeated contact gives rise to multiple or "cyclic" Big Bangs. This particular hypothesis falls under the string theory umbrella as it requires extra spatial dimensions.
- Cyclic
- The cyclic multiverse has multiple branes that have collided, causing Big Bangs. The universes bounce back and pass through time until they are pulled back together and again collide, destroying the old contents and creating them anew.
- Landscape
- The landscape multiverse relies on string theory's Calabi–Yau spaces. Quantum fluctuations drop the shapes to a lower energy level, creating a pocket with a set of laws different from that of the surrounding space.
- Quantum
- The quantum multiverse creates a new universe when a diversion in events occurs, as in the real-worlds variant of the many-worlds interpretation of quantum mechanics.
- Holographic
- The holographic multiverse is derived from the theory that the surface area of a space can encode the contents of the volume of the region.
- Simulated
- The simulated multiverse exists on complex computer systems that simulate entire universes. A related hypothesis, as put forward as a possibility by astronomer Avi Loeb, is that universes may be creatable in laboratories of advanced technological civilizations who have a theory of everything.[78] Other related hypotheses include brain in a vat[79]-type scenarios where the perceived universe is either simulated in a low-resource way or not perceived directly by the virtual/simulated inhabitant species.Template:Additional citation needed
- Ultimate
- The ultimate multiverse contains every mathematically possible universe under different laws of physics.
Twin-world models
There are models of two related universes that e.g. attempt to explain the baryon asymmetry – why there was more matter than antimatter at the beginning – with a mirror anti-universe.[80][81][82] One two-universe cosmological model could explain the Hubble constant (H0) tension via interactions between the two worlds. The "mirror world" would contain copies of all existing fundamental particles.[83][84] Another twin/pair-world or "bi-world" cosmology is shown to theoretically be able to solve the cosmological constant (Λ) problem, closely related to dark energy: two interacting worlds with a large Λ each could result in a small shared effective Λ.[85][86][87]
Cyclic theories
Script error: No such module "Labelled list hatnote". In several theories, there is a series of, in some cases infinite, self-sustaining cycles – typically a series of Big Crunches (or Big Bounces). However, the respective universes do not exist at once but are forming or following in a logical order or sequence, with key natural constituents potentially varying between universes (see § Anthropic principle).
M-theory
Script error: No such module "Labelled list hatnote". A multiverse of a somewhat different kind has been envisaged within string theory and its higher-dimensional extension, M-theory.[88]
These theories require the presence of 10 or 11 spacetime dimensions respectively. The extra six or seven dimensions may either be compactified on a very small scale, or our universe may simply be localized on a dynamical (3+1)-dimensional object, a D3-brane. This opens up the possibility that there are other branes which could support other universes.[89][90]
Black-hole cosmology
Script error: No such module "Labelled list hatnote". Black-hole cosmology is a cosmological model in which the observable universe is the interior of a black hole existing as one of possibly many universes inside a larger universe.[91] This includes the theory of white holes, which are on the opposite side of space-time.
Anthropic principle
Script error: No such module "Labelled list hatnote". The concept of other universes has been proposed to explain how our own universe appears to be fine-tuned for conscious life as we experience it.
If there were a large (possibly infinite) number of universes, each with possibly different physical laws (or different fundamental physical constants), then some of these universes (even if very few) would have the combination of laws and fundamental parameters that are suitable for the development of matter, astronomical structures, elemental diversity, stars, and planets that can exist long enough for life to emerge and evolve.
The weak anthropic principle could then be applied to conclude that we (as conscious beings) would only exist in one of those few universes that happened to be finely tuned, permitting the existence of life with developed consciousness. Thus, while the probability might be extremely small that any particular universe would have the requisite conditions for life (as we understand life), those conditions do not require intelligent design as an explanation for the conditions in the Universe that promote our existence in it.
An early form of this reasoning is evident in Arthur Schopenhauer's 1844 work "Von der Nichtigkeit und dem Leiden des Lebens", where he argues that our world must be the worst of all possible worlds, because if it were significantly worse in any respect it could not continue to exist.[92]
Occam's razor
Proponents and critics disagree about how to apply Occam's razor. Critics argue that to postulate an almost infinite number of unobservable universes, just to explain our own universe, is contrary to Occam's razor.[93] However, proponents argue that in terms of Kolmogorov complexity the proposed multiverse is simpler than a single idiosyncratic universe.[69]
For example, multiverse proponent Max Tegmark argues: Template:Quotation
Possible worlds and real worlds
In any given set of possible universes – e.g. in terms of histories or variables of nature – not all may be ever realized, and some may be realized many times.[94] For example, over infinite time there could, in some potential theories, be infinite universes, but only a small or relatively small real number of universes where humanity could exist and only one where it ever does exist (with a unique history).Script error: No such module "Unsubst". It has been suggested that a universe that "contains life, in the form it has on Earth, is in a certain sense radically non-ergodic, in that the vast majority of possible organisms will never be realized".[95] On the other hand, some scientists, theories and popular works conceive of a multiverse in which the universes are so similar that humanity exists in many equally real separate universes but with varying histories.[96]
There is a debate about whether the other worlds are real in the many-worlds interpretation (MWI) of quantum mechanics. In Quantum Darwinism one does not need to adopt a MWI in which all of the branches are equally real.[97]
Modal realism
Possible worlds are a way of explaining probability and hypothetical statements. Some philosophers, such as David Lewis, posit that all possible worlds exist and that they are just as real as the world we live in. This position is known as modal realism.[98]
See also
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References
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Citations Template:Reflist
Further reading
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- Andrei Linde, The Self-Reproducing Inflationary Universe, Scientific American, November 1994 – Touches on multiverse concepts at the end of the article.
External links
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- Interview with Tufts cosmologist Alex Vilenkin on his book, "Many Worlds in One: The Search for Other Universes" on the podcast and public radio interview program ThoughtCast. Template:Webarchive.
- Multiverse – an episode of the series In Our Time with Melvyn Bragg, on BBC Radio 4.
- Why There Might be Many More Universes Besides Our Own, by Phillip Ball, March 21, 2016, bbc.com.
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