Editing Theory of everything (section)

==Modern physics==
{{multiple image
|image1  =cube_of_theoretical_physics.svg
|caption1=A depiction of the [[cGh physics|''cGh'' cube]]
|image2  =Venn_diagram_of_theoretical_physics.svg
|caption2=Depicted as a Venn diagram
}}
===Conventional sequence of theories===
A theory of everything would unify all the [[fundamental interaction]]s of nature: [[gravitation]], the [[strong interaction]], the [[weak interaction]], and [[electromagnetism]]. Because the weak interaction can transform [[elementary particles]] from one kind into another, the theory of everything should also predict all the different kinds of particles possible. The usual assumed path of theories is given in the following graph, where each unification step leads one level up on the graph.

{{tree chart/start}}
{{tree chart| | | | |TOE| | TOE=Theory of everything }}
{{tree chart| | | | | |!| | }}
{{tree chart| | | | |QG| | QG=[[Quantum gravity]] }}
{{tree chart| | | | | |!| | }}
{{tree chart| |REL|-|^|-|GUT| | REL=[[General Relativity|Space Curvature]] | GUT=Electronuclear force ([[Grand Unified Theory]]) }}
{{tree chart| | |!| | | | | |!| | }}
{{tree chart| |SMC| | | |SMP| SMC=[[Lambda-CDM model|Standard model of cosmology]] | SMP=[[Standard model of particle physics]]}}
{{tree chart| | | | | | | | |!| | }}
{{tree chart| | | | |QCD|-|^|-|-|EWT| | QCD=[[Strong interaction]]<br />[[Special unitary group|SU(3)]] | EWT=[[Electroweak interaction]]<br />[[Special unitary group|SU(2)]] x [[Unitary group|U(1)]]<sub>[[hypercharge|Y]]</sub> }}
{{tree chart| | | | | | | | | | | | |!| | }}
{{tree chart| | | | | | | | |WNF|-|^|-|EMF||WNF=[[Weak interaction]]<br />[[Special unitary group|SU(2)]]|EMF=[[Electromagnetism]]<br />[[Unitary group|U(1)]]<sub>EM</sub>}}
{{tree chart| | | | | | | | | | | | | | | |!| | | | | | }}
{{tree chart| | | | | | | | | | | |EF|-|^|-|MF| |EF=[[Electricity]]|MF=[[Magnetism]] }}
{{tree chart/end}}

In this graph, electroweak unification occurs at around 100 GeV, grand unification is predicted to occur at 10<sup>16</sup> GeV, and unification of the GUT force with gravity is expected at the [[Planck energy]], roughly 10<sup>19</sup> GeV.

Several [[Grand Unified Theory|Grand Unified Theories]] (GUTs) have been proposed to unify electromagnetism and the weak and strong forces. Grand unification would imply the existence of an electronuclear force; it is expected to set in at energies of the order of 10<sup>16</sup> GeV, far greater than could be reached by any currently feasible [[particle accelerator]]. Although the simplest grand unified theories have been experimentally ruled out, the idea of a grand unified theory, especially when linked with [[supersymmetry]], remains a favorite candidate in the theoretical physics community. Supersymmetric grand unified theories seem plausible not only for their theoretical "beauty", but because they naturally produce large quantities of dark matter, and because the inflationary force may be related to grand unified theory physics (although it does not seem to form an inevitable part of the theory). Yet grand unified theories are clearly not the final answer; both the current standard model and all proposed GUTs are [[quantum field theory|quantum field theories]] which require the problematic technique of [[renormalization]] to yield sensible answers. This is usually regarded as a sign that these are only [[effective field theory|effective field theories]], omitting crucial phenomena relevant only at very high energies.<ref name="Carlip" />

The final step in the graph requires resolving the separation between quantum mechanics and gravitation, often equated with general relativity. Numerous researchers concentrate their efforts on this specific step; nevertheless, no accepted theory of [[quantum gravity]], and thus no accepted theory of everything, has emerged with observational evidence. It is usually assumed that the theory of everything will also solve the remaining problems of grand unified theories.

In addition to explaining the forces listed in the graph, a theory of everything may also explain the status of at least two candidate forces suggested by modern [[physical cosmology|cosmology]]: an [[inflation (cosmology)|inflationary force]] and [[dark energy]]. Furthermore, cosmological experiments also suggest the existence of [[dark matter]], supposedly composed of fundamental particles outside the scheme of the standard model. However, the existence of these forces and particles has not been proven.

===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>

===Loop quantum gravity===
Current research on [[loop quantum gravity]] may eventually play a fundamental role in a theory of everything, but that is not its primary aim.<ref>{{cite web
 |last=Potter
 |first=Franklin
 |date=15 February 2005
 |url=http://www.sciencegems.com/discretespace.pdf
 |title=Leptons And Quarks In A Discrete Spacetime
 |work=Frank Potter's Science Gems
 |access-date=2009-12-01
 |archive-date=2021-03-09
 |archive-url=https://web.archive.org/web/20210309121814/http://www.sciencegems.com/discretespace.pdf
 |url-status=live
 }}</ref> Loop quantum gravity also introduces a lower bound on the possible length scales.

There have been recent claims that loop quantum gravity may be able to reproduce features resembling the [[Standard Model]]. So far only the first generation of [[fermion]]s ([[lepton]]s and [[quark]]s) with correct parity properties have been modelled by [[Sundance Bilson-Thompson]] using [[preon]]s constituted of braids of spacetime as the building blocks.<ref>{{cite journal |title=Quantum gravity and the standard model |last=Bilson-Thompson |first=Sundance O. |author2=Markopoulou, Fotini |author3=Smolin, Lee |doi=10.1088/0264-9381/24/16/002 |date=2007 |journal=Classical and Quantum Gravity |volume=24 |issue=16 |pages=3975–3994 |arxiv=hep-th/0603022 |bibcode=2007CQGra..24.3975B|s2cid=37406474 }}</ref> However, there is no derivation of the [[Lagrangian (field theory)|Lagrangian]] that would describe the interactions of such particles, nor is it possible to show that such particles are fermions, nor that the gauge groups or interactions of the Standard Model are realised. Use of [[quantum computing]] concepts made it possible to demonstrate that the particles are able to survive [[quantum fluctuation]]s.<ref>{{cite journal |url=https://www.newscientist.com/channel/fundamentals/mg19125645.800 |journal=New Scientist |title=You are made of space-time |first=Davide |last=Castelvecchi |author2=Valerie Jamieson |issue=2564 |date=August 12, 2006 |access-date=September 16, 2017 |archive-date=February 9, 2008 |archive-url=https://web.archive.org/web/20080209025208/http://www.newscientist.com/channel/fundamentals/mg19125645.800 |url-status=live }}</ref>

This model leads to an interpretation of electric and color charge as topological quantities (electric as number and chirality of twists carried on the individual ribbons and colour as variants of such twisting for fixed electric charge).

Bilson-Thompson's original paper suggested that the higher-generation fermions could be represented by more complicated braidings, although explicit constructions of these structures were not given. The electric charge, color, and parity properties of such fermions would arise in the same way as for the first generation. The model was expressly generalized for an infinite number of generations and for the weak force bosons (but not for photons or gluons) in a 2008 paper by Bilson-Thompson, Hackett, Kauffman and Smolin.<ref>{{cite arXiv |eprint=0804.0037 |class=hep-th |first1=Sundance |last1=Bilson-Thompson |first2=Jonathan |last2=Hackett |title=Particle Identifications from Symmetries of Braided Ribbon Network Invariants |date=2008 |author3=Kauffman, Lou |author4=Smolin, Lee}}</ref>

===Other attempts===
Among other attempts to develop a theory of everything is the theory of [[causal fermion system]]s,<ref name="CFSIntro">{{Cite journal |author=Finster |first1=F. |last2=Kleiner |first2=J. |date=2015 |title=Causal fermion systems as a candidate for a unified physical theory |journal=Journal of Physics: Conference Series |volume=626 |issue=2015 |page=012020 |arxiv=1502.03587 |bibcode=2015JPhCS.626a2020F |doi=10.1088/1742-6596/626/1/012020 |s2cid=33471826}}</ref> giving the two current physical theories ([[general relativity]] and [[quantum field theory]]) as limiting cases.

Another theory is called [[Causal Sets]]. As some of the approaches mentioned above, its direct goal isn't necessarily to achieve a theory of everything but primarily a working theory of quantum gravity, which might eventually include the standard model and become a candidate for a theory of everything. Its founding principle is that spacetime is fundamentally discrete and that the spacetime events are related by a [[partial order]]. This partial order has the physical meaning of the [[causality relation]]s between relative [[past and future distinguishing]] spacetime events.<!--Please do _not_ insert "Time Cube" and "Heim Theory" here without first gaining a consensus on the talk page for including these theories. Changes without such a consensus will be promptly reverted. Thanks!-->

[[Causal dynamical triangulation]] does not assume any pre-existing arena (dimensional space), but rather attempts to show how the spacetime fabric itself evolves.

Another attempt may be related to [[ER=EPR]], a conjecture in physics stating that [[Quantum entanglement|entangled]] particles are connected by a [[wormhole]] (or Einstein–Rosen bridge).<ref name=Cowen>{{cite journal|last1=Cowen|first1=Ron|title=The quantum source of space-time|journal=Nature|date=16 November 2015|volume=527|issue=7578|pages=290–293|bibcode=2015Natur.527..290C|doi=10.1038/527290a|pmid=26581274|s2cid=4447880}}</ref>

===Present status===
At present, there is no candidate theory of everything that includes the standard model of particle physics and general relativity and that, at the same time, is able to calculate the [[fine-structure constant]] or the [[mass of the electron]].<ref name="NYT-20201123" /> Most particle physicists expect that the outcome of ongoing experiments – the search for new particles at the large [[particle accelerator]]s and for [[dark matter]] – are needed in order to provide further input for a theory of everything.