Editing Theory of everything (section)

===String theory and M-theory===
{{unsolved|physics|Is [[string theory]], [[superstring theory]], or [[M-theory]], or some other variant on this theme, a step on the road to a "theory of everything", or just a blind alley?}}

Since the 1990s, some physicists such as [[Edward Witten]] believe that 11-dimensional [[M-theory]], which is described in some limits by one of the five [[perturbation theory|perturbative]] [[superstring theory|superstring theories]], and in another by the maximally-[[supersymmetry|supersymmetric]] [[eleven-dimensional supergravity]], is the theory of everything. There is no widespread consensus on this issue.

One remarkable property of [[string theory|string]]/[[M-theory]] is that seven extra dimensions are required for the theory's consistency, on top of the four dimensions in our universe. In this regard, string theory can be seen as building on the insights of the [[Kaluza–Klein theory]], in which it was realized that applying general relativity to a 5-dimensional universe, with one space dimension small and curled up, looks from the 4-dimensional perspective like the usual general relativity together with [[Maxwell's equations|Maxwell's electrodynamics]]. This lent credence to the idea of unifying [[gauge theory|gauge]] and [[gravity]] interactions, and to extra dimensions, but did not address the detailed experimental requirements. Another important property of string theory is its [[supersymmetry]], which together with extra dimensions are the two main proposals for resolving the [[hierarchy problem]] of the [[standard model]], which is (roughly) the question of why gravity is so much weaker than any other force. The extra-dimensional solution involves allowing gravity to propagate into the other dimensions while keeping other forces confined to a 4-dimensional spacetime, an idea that has been realized with explicit stringy mechanisms.<ref>{{cite journal |pmid=16196251 |date=2005 |title=The Beauty of Branes |journal=Scientific American |pages=38–40 |doi=10.1038/scientificamerican1005-38 |url=http://randall.physics.harvard.edu/RandallCV/ScientificAm10-05.pdf |access-date=August 13, 2012 |last1=Holloway |first1=M |volume=293 |issue=4 |bibcode=2005SciAm.293d..38H |archive-date=November 22, 2014 |archive-url=https://web.archive.org/web/20141122023615/http://randall.physics.harvard.edu/RandallCV/ScientificAm10-05.pdf  }}</ref>

Research into string theory has been encouraged by a variety of theoretical and experimental factors. On the experimental side, the particle content of the standard model supplemented with [[Seesaw mechanism|neutrino masses]] fits into a [[spinor]] representation of [[SO(10)]], a subgroup of [[E8 (mathematics)|E8]] that routinely emerges in string theory, such as in [[heterotic string theory]]<ref>{{cite journal |arxiv=0806.3905 |doi=10.1140/epjc/s10052-008-0740-1 |title=From strings to the MSSM |year=2009 |last1=Nilles |first1=Hans Peter |last2=Ramos-Sánchez |first2=Saúl |last3=Ratz |first3=Michael |last4=Vaudrevange |first4=Patrick K. S. |journal=The European Physical Journal C |volume=59 |issue=2 |pages=249–267 |bibcode=2009EPJC...59..249N |s2cid=17452924 }}</ref> or (sometimes equivalently) in [[F-theory]].<ref>{{cite journal |doi=10.1088/1126-6708/2009/01/058 |arxiv=0802.3391 |title=GUTs and exceptional branes in F-theory — I |date=2009 |last1=Beasley |first1=Chris |last2=Heckman |first2=Jonathan J |last3=Vafa |first3=Cumrun |journal=Journal of High Energy Physics |volume=2009 |issue=1 |page=058 |bibcode=2009JHEP...01..058B |s2cid=119309173 }}</ref><ref>{{cite arXiv |eprint=0802.2969v3 |last1=Donagi |first1=Ron |title=Model Building with F-Theory |last2=Wijnholt |first2=Martijn |class=hep-th |year=2008}}</ref> String theory has mechanisms that may explain why fermions come in three hierarchical generations, and explain the [[CKM matrix|mixing rates]] between quark generations.<ref>{{Cite journal |arxiv=0811.2417 |last1=Heckman |first1=Jonathan J. |title=Flavor Hierarchy from F-theory |journal=Nuclear Physics B |volume=837 |issue=1 |pages=137–151 |last2=Vafa |first2=Cumrun |year=2010 |doi=10.1016/j.nuclphysb.2010.05.009 |bibcode=2010NuPhB.837..137H |s2cid=119244083 }}</ref> On the theoretical side, it has begun to address some of the key questions in [[quantum gravity]], such as resolving the [[black hole information paradox]], counting the correct [[black hole thermodynamics|entropy of black holes]]<ref>{{cite journal |doi=10.1016/0370-2693(96)00345-0 |arxiv=hep-th/9601029 |title=Microscopic origin of the Bekenstein-Hawking entropy |date=1996 |last1=Strominger |first1=Andrew |last2=Vafa |first2=Cumrun |journal=Physics Letters B |volume=379 |issue=1–4 |pages=99–104 |bibcode=1996PhLB..379...99S |s2cid=1041890 }}</ref><ref>{{cite arXiv<!--Citation bot deny, the arxiv metadata is wrong-->|arxiv=gr-qc/9604051 |last1=Horowitz |first1=Gary |title=The Origin of Black Hole Entropy in String Theory}}</ref> and allowing for [[topology]]-changing processes.<ref>{{cite journal |doi=10.1016/0550-3213(95)00371-X |arxiv=hep-th/9504145 |title=Black hole condensation and the unification of string vacua |date=1995 |last1=Greene |first1=Brian R. |last2=Morrison |first2=David R. |last3=Strominger |first3=Andrew |journal=Nuclear Physics B |volume=451 |issue=1–2 |pages=109–120 |bibcode=1995NuPhB.451..109G |s2cid=11145691 }}</ref><ref>{{cite journal |doi=10.1016/0550-3213(94)90321-2 |arxiv=hep-th/9309097 |title=Calabi-Yau moduli space, mirror manifolds and spacetime topology change in string theory |date=1994 |last1=Aspinwall |first1=Paul S. |last2=Greene |first2=Brian R. |last3=Morrison |first3=David R. |journal=Nuclear Physics B |volume=416 |issue=2 |page=414 |bibcode=1994NuPhB.416..414A |s2cid=10927539 }}</ref><ref>{{cite journal |doi=10.1088/1126-6708/2005/10/033 |arxiv=hep-th/0502021 |title=Things fall apart: Topology change from winding tachyons |date=2005|author-link1=Allan Adams |last1=Adams |first1=Allan |last2=Liu |first2=Xiao |last3=McGreevy |first3=John |last4=Saltman |first4=Alex |last5=Silverstein |first5=Eva |journal=Journal of High Energy Physics |volume=2005 |issue=10 |page=033 |bibcode=2005JHEP...10..033A |s2cid=14320855 }}</ref> It has also led to many insights in [[pure mathematics]] and in ordinary, strongly-coupled [[gauge theory]] due to the [[AdS/CFT|Gauge/String duality]].

In the late 1990s, it was noted that one major hurdle in this endeavor is that the number of possible 4-dimensional universes is incredibly large. The small, "curled up" extra dimensions can be [[compact dimension|compactified]] in an enormous number of different ways (one estimate is 10<sup>500</sup>&nbsp;) each of which leads to different properties for the low-energy particles and forces. This array of models is known as the [[string theory landscape]].<ref name="Impey2012" />{{rp|347}}

One proposed solution is that many or all of these possibilities are realized in one or another of a huge number of universes, but that only a small number of them are habitable. Hence what we normally conceive as the [[fundamental constants]] of the universe are ultimately the result of the [[anthropic principle]] rather than dictated by theory. This has led to criticism of string theory,<ref>{{cite book |last=Smolin |first=Lee |title=The Trouble With Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next |date=2006 |publisher=Houghton Mifflin |isbn=978-0-618-55105-7|title-link=The Trouble With Physics }}</ref> arguing that it cannot make useful (i.e., original, [[falsifiable]], and verifiable) predictions and regarding it as a [[pseudoscience]]/[[philosophy]]. Others disagree,<ref>{{cite journal |author=Duff, M. J. |arxiv=1112.0788 |doi=10.1007/s10701-011-9618-4 |title=String and M-Theory: Answering the Critics |date=2011 |journal=Foundations of Physics |volume=43 |issue=1 |pages=182–200 |bibcode=2013FoPh...43..182D |s2cid=55066230 }}</ref> and string theory remains an active topic of investigation in [[theoretical physics]].<ref>{{Cite news|url=https://www.symmetrymagazine.org/article/may-2007/the-great-string-debate|title=The Great String Debate|last=Chui|first=Glennda|date=May 1, 2007|work=Symmetry Magazine|access-date=2018-10-17|language=en|archive-date=2018-10-17|archive-url=https://web.archive.org/web/20181017123651/https://www.symmetrymagazine.org/article/may-2007/the-great-string-debate|url-status=live}}</ref>