View source for Theory of everything

{{Short description|Hypothetical physical concept}}
{{About|the hypothetical physical concept}}
{{Beyond the Standard Model|expanded=Theories}}

A '''theory of everything''' ('''TOE'''), '''final theory''', '''ultimate theory''', '''unified field theory''', or '''master theory''' is a hypothetical, singular, all-encompassing, coherent [[theoretical physics|theoretical framework of physics]] that fully explains and links together all aspects of the [[universe]].<ref name="Weinberg2011">{{cite book |author=Weinberg |first=Steven |title=Dreams of a Final Theory: The Scientist's Search for the Ultimate Laws of Nature |date=2011-04-20 |publisher=Knopf Doubleday Publishing Group |isbn=978-0-307-78786-6 |language=en-us}}</ref>{{rp|6}} Finding a theory of everything is one of the major [[unsolved problems in physics]].<ref name="NYT-20201123" /><ref name="NYT-20230911">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |date=11 September 2023 |title=Don't Expect a 'Theory of Everything' to Explain It All – Not even the most advanced physics can reveal everything we want to know about the history and future of the cosmos, or about ourselves. |work=[[The New York Times]] |url=https://www.nytimes.com/2023/09/11/science/space/astronomy-universe-simulations.html |url-status=live |access-date=11 September 2023 |archive-url=https://archive.today/20230911043212/https://www.nytimes.com/2023/09/11/science/space/astronomy-universe-simulations.html |archive-date=11 September 2023}}</ref>

Over the past few centuries, two theoretical frameworks have been developed that, together, most closely resemble a theory of everything. These two theories upon which all modern physics rests are [[general relativity]] and [[quantum mechanics]]. General relativity is a theoretical framework that only focuses on [[gravity]] for understanding the universe in regions of both large scale and high mass: [[planets]], [[stars]], [[galaxies]], [[Galaxy cluster|clusters of galaxies]], etc. On the other hand, quantum mechanics is a theoretical framework that focuses primarily on three non-gravitational forces for understanding the universe in regions of both very small scale and low mass: [[subatomic particles]], [[atoms]], and [[molecules]]. Quantum mechanics successfully implemented the [[Standard Model]] that describes the three non-gravitational forces: [[strong force|strong nuclear]], [[weak force|weak nuclear]], and [[electromagnetism|electromagnetic]] force – as well as all observed elementary particles.<ref name="Hawking2006">{{cite book |author=Hawking |first=Stephen W. |title=The Theory of Everything: The Origin and Fate of the Universe |date=28 February 2006 |publisher=Phoenix Books; Special Anniversary |isbn=978-1-59777-508-3}}</ref>{{rp|122}}

General relativity and quantum mechanics have been repeatedly validated in their separate fields of relevance. Since the usual domains of applicability of general relativity and quantum mechanics are so different, most situations require that only one of the two theories be used.<ref>{{cite journal |last1=SMOLIN |first1=L. |title=An Invitation to Loop Quantum Gravity |date=2004 |journal=Quantum Theory and Symmetries |volume=[] |pages=655–682 | doi=10.1142/9789812702340_0078|arxiv=hep-th/0408048 |bibcode=2004qts..conf..655S |isbn=978-981-256-068-1 |s2cid=16195175}}</ref><ref name="Carlip">{{cite journal |last=Carlip |first=Steven |title=Quantum Gravity: a Progress Report |journal=Reports on Progress in Physics |volume=64 |issue=8 |pages=885–942 |date=2001 |doi=10.1088/0034-4885/64/8/301 |arxiv=gr-qc/0108040 |bibcode=2001RPPh...64..885C |s2cid=118923209 }}</ref><ref name="Priest2010">{{cite book |author=Priest |first=Susanna Hornig |title=Encyclopedia of Science and Technology Communication |date=14 July 2010 |publisher=SAGE Publications |isbn=978-1-4522-6578-0}}</ref>{{rp|842–844}} The two theories are considered incompatible in regions of extremely small scale – the [[Planck scale]] – such as those that exist within a black hole or during the beginning stages of the universe (i.e., the moment immediately following the [[Big Bang]]). To resolve the incompatibility, a theoretical framework revealing a deeper underlying reality, unifying gravity with the other three interactions, must be discovered to harmoniously integrate the realms of general relativity and quantum mechanics into a seamless whole: a theory of everything may be defined as a comprehensive theory that, in principle, would be capable of describing all physical phenomena in the universe. 

In pursuit of this goal, [[quantum gravity]] has become one area of active research.<ref name="NYT-20221010">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |date=10 October 2022 |title=Black Holes May Hide a Mind-Bending Secret About Our Universe – Take gravity, add quantum mechanics, stir. What do you get? Just maybe, a holographic cosmos. |work=[[The New York Times]] |url=https://www.nytimes.com/2022/10/10/science/black-holes-cosmology-hologram.html |url-status=live |access-date=22 October 2022 |archive-url=https://web.archive.org/web/20221116151210/https://www.nytimes.com/2022/10/10/science/black-holes-cosmology-hologram.html |archive-date=16 November 2022}}</ref><ref name="SA-20221116">{{cite news |last=Starr |first=Michelle |title=Scientists Created a Black Hole in The Lab, And Then It Started to Glow |url=https://www.sciencealert.com/scientists-created-a-black-hole-in-the-lab-and-then-it-started-to-glow |date=16 November 2022 |work=[[ScienceAlert]] |access-date=16 November 2022 |archive-date=15 November 2022 |archive-url=https://web.archive.org/web/20221115234327/https://www.sciencealert.com/scientists-created-a-black-hole-in-the-lab-and-then-it-started-to-glow |url-status=live }}</ref> One example is [[string theory]], which evolved into a candidate for the theory of everything, but not without drawbacks (most notably, its apparent lack of currently [[testable]] [[prediction]]s) and controversy. String theory posits that at the [[Planck epoch|beginning of the universe]] (up to 10<sup>−43</sup> seconds after the Big Bang), the [[four fundamental forces]] were once a single fundamental force. According to string theory, every particle in the universe, at its most ultramicroscopic level ([[Planck length]]), consists of varying combinations of vibrating strings (or strands) with preferred patterns of vibration. String theory further claims that it is through these specific oscillatory patterns of strings that a particle of unique mass and force charge is created (that is to say, the [[electron]] is a type of string that vibrates one way, while the [[up quark]] is a type of string vibrating another way, and so forth). String theory/[[M-theory]] proposes six or seven [[dimensions]] of [[spacetime]] in addition to the four common dimensions for a ten- or eleven-dimensional spacetime.

==Name==
Initially, the term ''theory of everything'' was used with an ironic reference to various overgeneralized theories. For example, a grandfather of [[Ijon Tichy]] – a character from a cycle of [[Stanisław Lem]]'s [[science fiction]] stories of the 1960s – was known to work on the "[[General Theory of Everything]]". Physicist [[Harald Fritzsch]] used the term in his 1977 lectures in [[Varenna]].<ref>{{Cite journal
 |first=Harald |last=Fritzsch
 |date=1977
 |journal=CERN Report
 |title=THE WORLD OF FLAVOUR AND COLOUR
 |volume=Ref.TH.2359-CERN}} (download at https://cds.cern.ch/record/875256/files/CM-P00061728.pdf {{Webarchive|url=https://web.archive.org/web/20200212051758/https://cds.cern.ch/record/875256/files/CM-P00061728.pdf |date=2020-02-12 }} )</ref> Physicist [[John Ellis (physicist, born 1946)|John Ellis]] claims<ref>
{{cite journal
 |first=John |last=Ellis
 |date=2002
 |journal=[[Nature (journal)|Nature]]
 |title=Physics gets physical (correspondence)
 |volume=415 |page=957
 |doi=10.1038/415957b
 |bibcode=2002Natur.415..957E
 |issue=6875
 |pmid=11875539|doi-access=free
 }}</ref> to have introduced the acronym "TOE" into the technical literature in an article in ''[[Nature (magazine)|Nature]]'' in 1986.<ref>
{{Cite journal
 |first=John |last=Ellis
 |date=1986
 |journal=Nature
 |title=The Superstring: Theory of Everything, or of Nothing?
 |volume=323 |pages=595–598
 |doi=10.1038/323595a0
 |bibcode=1986Natur.323..595E
 |issue=6089|s2cid=4344940
 }}</ref> Over time, the term stuck in popularizations of [[theoretical physics]] research.

==Historical antecedents==

===Antiquity to 19th century===
Many ancient cultures such as [[Babylonian astronomers]] and [[Indian astronomy]] studied the pattern of the ''Seven Sacred Luminaires''/[[Classical Planets]] against the background of [[stars]], with their interest being to relate celestial movement to human events ([[astrology]]), and the goal being to predict events by recording events against a time measure and then look for recurrent patterns. The debate between the universe having either [[Temporal finitism|a beginning]] or [[Cyclic model|eternal cycles]] can be traced to ancient [[Babylonia]].<ref name="Hodge">{{cite book |last1=Hodge |first1=John C. |title=Theory of Everything: Scalar Potential Model of the Big and the Small |date=2012 |isbn=978-1-4699-8736-1 |pages=1–13, 99 |publisher=CreateSpace Independent Publishing Platform }}</ref> [[Hindu cosmology]] posits that time is infinite with a ''cyclic universe'', where the current universe was preceded and will be followed by an infinite number of universes.<ref>{{cite book |author=Sushil Mittal |author2=Gene Thursby |page=284 |title=Hindu World |publisher=Routledge |year=2012 |isbn=978-1-134-60875-1}}</ref><ref>{{cite book |author=Jones |first=Andrew Zimmerman |title=String Theory For Dummies |publisher=John Wiley & Sons |year=2009 |isbn=978-0-470-59584-8 |page=262}}</ref> Time scales mentioned in [[Hindu cosmology]] correspond to those of modern scientific cosmology. Its cycles run from our ordinary day and night to a day and night of Brahma, 8.64 billion years long.<ref>{{cite book |author=Sagan, Carl |title=Cosmos |year=2006 }}</ref>

The [[natural philosophy]] of [[atomism]] appeared in several ancient traditions. In ancient [[Greek philosophy]], the [[Pre-Socratic philosophy|pre-Socratic philosophers]] speculated that the apparent diversity of observed phenomena was due to a single type of interaction, namely the motions and collisions of atoms. The concept of 'atom' proposed by [[Democritus]] was an early philosophical attempt to unify phenomena observed in nature. The concept of 'atom' also appeared in the [[Nyaya]]-[[Vaisheshika]] school of ancient [[Indian philosophy]].

[[Archimedes]] was possibly the first philosopher to have described nature with axioms (or principles) and then deduce new results from them. Any "theory of everything" is similarly expected to be based on axioms and to deduce all observable phenomena from them.<ref name="Impey2012" />{{rp|340}}

Following earlier atomistic thought, the [[mechanical philosophy]] of the 17th century posited that all forces could be ultimately reduced to [[contact force]]s between the atoms, then imagined as tiny solid particles.<ref name="Burns2001">{{cite book |author=Burns |first=William E. |title=The Scientific Revolution: An Encyclopedia |date=1 January 2001 |publisher=ABC-CLIO |isbn=978-0-87436-875-8}}</ref>{{rp|184}}<ref>
{{cite book
 |first=Steven |last=Shapin
 |date=1996
 |title=The Scientific Revolution
 |url=https://archive.org/details/scientificrevolu00shap_0 |url-access=registration |publisher=[[University of Chicago Press]]
 |isbn=978-0-226-75021-7
}}</ref>

In the late 17th century, [[Isaac Newton]]'s description of the long-distance force of gravity implied that not all forces in nature result from things coming into contact. Newton's work in his ''[[Philosophiæ Naturalis Principia Mathematica|Mathematical Principles of Natural Philosophy]]'' dealt with this in a further example of [[Unification (physics)|unification]], in this case unifying [[Galileo]]'s work on terrestrial gravity, [[Kepler]]'s laws of planetary motion and the phenomenon of [[tide]]s by explaining these apparent actions at a distance under one single law: the law of [[universal gravitation]].<ref>{{cite book |page=255 |url=https://books.google.com/books?id=6EqxPav3vIsC&pg=PA255 |title=The Mathematical Principles of Natural Philosophy |volume=II |last1=Newton |first1=Sir Isaac |date=1729}}</ref>

In 1814, building on these results, [[Laplace]] famously suggested that a [[Laplace's demon|sufficiently powerful intellect]] could, if it knew the position and velocity of every particle at a given time, along with the laws of nature, calculate the position of any particle at any other time:<ref name="Carroll2010">{{cite book <!-- Citation bot deny--> |author=Carroll |first=Sean |title=[[From Eternity to Here: The Quest for the Ultimate Theory of Time]] |publisher=Penguin Group US |year=2010 |isbn=978-1-101-15215-7 |language=en-us}}</ref>{{rp |ch 7}}

{{quote|An intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature is composed, if this intellect were also vast enough to submit these data to analysis, it would embrace in a single formula the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be present before its eyes.|''Essai philosophique sur les probabilités'', Introduction. 1814}}

Laplace thus envisaged a combination of gravitation and mechanics as a theory of everything. Modern [[quantum mechanics]] implies that [[Heisenberg uncertainty|uncertainty is inescapable]], and thus that Laplace's vision has to be amended: a theory of everything must include gravitation and quantum mechanics. Even ignoring quantum mechanics, [[chaos theory]] is sufficient to guarantee that the future of any sufficiently complex mechanical or astronomical system is unpredictable.

In 1820, [[Hans Christian Ørsted]] discovered a connection between electricity and magnetism, triggering decades of work that culminated in 1865, in [[James Clerk Maxwell]]'s theory of [[electromagnetism]]. During the 19th and early 20th centuries, it gradually became apparent that many common examples of forces – contact forces, [[elasticity (physics)|elasticity]], [[viscosity]], [[friction]], and [[pressure]] – result from electrical interactions between the smallest particles of matter.

In his experiments of 1849–1850, [[Michael Faraday]] was the first to search for a unification of [[gravity]] with electricity and magnetism.<ref>
{{Cite journal
 |first=M.|last=Faraday
 |date=1850
 |title=Experimental Researches in Electricity. Twenty-Fourth Series. On the Possible Relation of Gravity to Electricity
 |journal=Abstracts of the Papers Communicated to the Royal Society of London
 |volume=5 |pages=994–995
 |doi=10.1098/rspl.1843.0267
|doi-access=free
 }}</ref> However, he found no connection.

In 1900, [[David Hilbert]] published a famous list of mathematical problems. In [[Hilbert's sixth problem]], he challenged researchers to find an axiomatic basis to all of physics. In this problem he thus asked for what today would be called a theory of everything.<ref>{{Cite journal |doi=10.1090/S0273-0979-2013-01439-3 |title=Hilbert's 6th Problem: Exact and approximate hydrodynamic manifolds for kinetic equations |journal=Bulletin of the American Mathematical Society |volume=51 |issue=2 |page=187 |year=2013 |last1=Gorban |first1=Alexander N. |last2=Karlin |first2=Ilya|arxiv=1310.0406 |bibcode=2013arXiv1310.0406G |s2cid=7228220 }}</ref>

===Early 20th century===
In the late 1920s, the then new quantum mechanics showed that the [[chemical bond]]s between [[atom]]s were examples of (quantum) electrical forces, justifying [[Paul Dirac|Dirac]]'s boast that "the underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known".<ref>
{{cite journal
 |last=Dirac |first=P.A.M.
 |date=1929
 |title=Quantum mechanics of many-electron systems
 |journal=[[Proceedings of the Royal Society of London A]]
 |volume=123 |pages=714–733
 |doi=10.1098/rspa.1929.0094
 |bibcode=1929RSPSA.123..714D
 |issue=792|doi-access=free
 }}</ref>

After 1915, when [[Albert Einstein]] published the theory of gravity ([[general relativity]]), the search for a [[unified field theory]] combining gravity with electromagnetism began with a renewed interest. In Einstein's day, the strong and the weak forces had not yet been discovered, yet he found the potential existence of two other distinct forces, gravity and electromagnetism, far more alluring. This launched his 40-year voyage in search of the so-called ''"unified field theory"'' that he hoped would show that these two forces are really manifestations of one grand, underlying principle. During the last few decades of his life, this ambition alienated Einstein from the rest of mainstream of physics, as the mainstream was instead far more excited about the emerging framework of quantum mechanics. Einstein wrote to a friend in the early 1940s, "I have become a lonely old chap who is mainly known because he doesn't wear socks and who is exhibited as a curiosity on special occasions." Prominent contributors were [[Gunnar Nordström]], [[Hermann Weyl]], [[Arthur Eddington]], [[David Hilbert]],<ref>{{cite book |arxiv=physics/0405110 |doi=10.1007/0-8176-4454-7_14 |isbn=978-0-8176-4454-3 | title=Hilbert's "World Equations" and His Vision of a Unified Science |series=Einstein Studies |volume=11 |pages=259–276 |year=2005 |last1=Majer |first1=U. |last2=Sauer |first2=T. |bibcode=2005ugr..book..259M |journal=<!-- Citation bot--> |s2cid=985751 }}</ref> [[Theodor Kaluza]], [[Oskar Klein]] (see [[Kaluza–Klein theory]]), and most notably, Albert Einstein and his collaborators. Einstein searched in earnest for, but ultimately failed to find, a unifying theory<ref name="Pais1982">{{cite book |author=Abraham Pais |title=Subtle is the Lord: The Science and the Life of Albert Einstein: The Science and the Life of Albert Einstein |url=https://archive.org/details/subtleislordscie00pais |url-access=registration |date=23 September 1982 |publisher=Oxford University Press |isbn=978-0-19-152402-8|author-link=Abraham Pais }}</ref>{{rp|ch 17}} (see Einstein–Maxwell–Dirac equations).

===Late 20th century and the nuclear interactions===
In the 20th century, the search for a unifying theory was interrupted by the discovery of the [[strong force|strong]] and [[weak force|weak]] nuclear forces, which differ both from gravity and from electromagnetism. A further hurdle was the acceptance that in a theory of everything, quantum mechanics had to be incorporated from the outset, rather than emerging as a consequence of a deterministic unified theory, as Einstein had hoped.

Gravity and electromagnetism are able to coexist as entries in a list of classical forces, but for many years it seemed that gravity could not be incorporated into the quantum framework, let alone unified with the other fundamental forces. For this reason, work on unification, for much of the 20th century, focused on understanding the three forces described by quantum mechanics: electromagnetism and the weak and strong forces. The first two were [[electroweak interaction|combined]] in 1967–1968 by [[Sheldon Glashow]], [[Steven Weinberg]], and [[Abdus Salam]] into the electroweak force.<ref>Weinberg (1993), Ch. 5</ref>
Electroweak unification is a [[broken symmetry]]: the electromagnetic and weak forces appear distinct at low energies because the particles carrying the weak force, the [[W and Z bosons]], have non-zero masses ({{val|80.4|u=GeV/c2}} and {{val|91.2|u=GeV/c2}}, respectively), whereas the [[photon]], which carries the electromagnetic force, is massless. At higher energies W bosons and Z bosons can be [[matter creation|created]] easily and the unified nature of the force becomes apparent.

While the strong and electroweak forces coexist under the [[Standard Model]] of particle physics, they remain distinct. Thus, the pursuit of a theory of everything remained unsuccessful: neither a unification of the strong and electroweak forces&nbsp;– which Laplace would have called 'contact forces'&nbsp;– nor a unification of these forces with gravitation had been achieved.

==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.

==Arguments against==
In parallel to the intense search for a theory of everything, various scholars have debated the possibility of its discovery.

===Gödel's incompleteness theorem===
A number of scholars claim that [[Gödel's incompleteness theorem]] suggests that attempts to construct a theory of everything are bound to fail. Gödel's theorem, informally stated, asserts that any formal theory sufficient to express elementary arithmetical facts and strong enough for them to be proved is either inconsistent (both a statement and its denial can be derived from its axioms) or incomplete, in the sense that there is a true statement that can't be derived in the formal theory.

[[Stanley Jaki]], in his 1966 book ''The Relevance of Physics'', pointed out that, because a "theory of everything" will certainly be a consistent non-trivial mathematical theory, it must be incomplete. He claims that this dooms searches for a deterministic theory of everything.<ref>
{{cite book
 |last=Jaki |first=S.L.
 |date=1966
 |pages=127–130
 |title=The Relevance of Physics
 |publisher=Chicago Press
 }}</ref>

[[Freeman Dyson]] has stated that "Gödel's theorem implies that pure mathematics is inexhaustible. No matter how many problems we solve, there will always be other problems that cannot be solved within the existing rules. […] Because of Gödel's theorem, physics is inexhaustible too. The laws of physics are a finite set of rules, and include the rules for doing mathematics, so that Gödel's theorem applies to them."<ref>Freeman Dyson, NYRB, May 13, 2004</ref>

[[Stephen Hawking]] was originally a believer in the Theory of Everything, but after considering Gödel's Theorem, he concluded that one was not obtainable. "Some people will be very disappointed if there is not an ultimate theory that can be formulated as a finite number of principles. I used to belong to that camp, but I have changed my mind."<ref>Stephen Hawking, [http://www.hawking.org.uk/godel-and-the-end-of-physics.html Gödel and the end of physics] {{Webarchive|url=https://web.archive.org/web/20200529232552/http://www.hawking.org.uk/godel-and-the-end-of-physics.html |date=2020-05-29 }}, July 20, 2002</ref>

[[Jürgen Schmidhuber]] (1997) has argued against this view; he asserts that Gödel's theorems are irrelevant for [[computable]] physics.<ref>{{cite book
 |last=Schmidhuber
 |first=Jürgen
 |date=1997
 |title=A Computer Scientist's View of Life, the Universe, and Everything. Lecture Notes in Computer Science
 |volume=1337
 |url=http://www.idsia.ch/~juergen/everything/
 |pages=201–208
 |publisher=[[Springer (publisher)|Springer]]
 |isbn=978-3-540-63746-2
 |doi=10.1007/BFb0052071
 |citeseerx=10.1.1.580.1970
 |s2cid=21317070
 |access-date=2008-03-26
 |archive-date=2014-02-27
 |archive-url=https://web.archive.org/web/20140227162350/http://www.idsia.ch/~juergen/everything/
 |url-status=live
 }}</ref> In 2000, Schmidhuber explicitly constructed limit-computable, deterministic universes whose [[pseudo-randomness]] based on [[undecidable problem|undecidable]], Gödel-like [[halting problem]]s is extremely hard to detect but does not prevent formal theories of everything describable by very few bits of information.<ref>
{{Cite journal
 |author=Schmidhuber, Jürgen
 |date=2002
 |title=Hierarchies of generalized Kolmogorov complexities and nonenumerable universal measures computable in the limit
 |journal=International Journal of Foundations of Computer Science
 |volume=13
 |issue=4
 |pages=587–612
 |arxiv=quant-ph/0011122
 |bibcode=2000quant.ph.11122S
 |doi=10.1142/s0129054102001291
}}</ref>

Related critique was offered by [[Solomon Feferman]]<ref>{{cite web
 |last=Feferman
 |first=Solomon
 |url=http://math.stanford.edu/~feferman/papers/Godel-IAS.pdf
 |title=The nature and significance of Gödel's incompleteness theorems
 |publisher=[[Institute for Advanced Study]]
 |date=17 November 2006
 |access-date=2009-01-12
 |archive-date=2008-12-17
 |archive-url=https://web.archive.org/web/20081217035530/http://math.stanford.edu/~feferman/papers/Godel-IAS.pdf
 |url-status=live
 }}</ref> and others. Douglas S. Robertson offers [[Conway's game of life]] as an example:<ref>
{{cite journal
 |last=Robertson |first=Douglas S.
 |date=2007
 |title=Goedel's Theorem, the Theory of Everything, and the Future of Science and Mathematics
 |journal=[[Complexity (journal)|Complexity]]
 |volume=5 |pages=22–27
 |doi=10.1002/1099-0526(200005/06)5:5<22::AID-CPLX4>3.0.CO;2-0
 |issue=5
|bibcode=2000Cmplx...5e..22R}}</ref> The underlying rules are simple and complete, but there are formally undecidable questions about the game's behaviors. Analogously, it may (or may not) be possible to completely state the underlying rules of physics with a finite number of well-defined laws, but there is little doubt that there are questions about the behavior of physical systems which are formally undecidable on the basis of those underlying laws.

Since most physicists would consider the statement of the underlying rules to suffice as the definition of a "theory of everything", most physicists argue that Gödel's Theorem does ''not'' mean that a theory of everything cannot exist.{{Citation needed|reason=Precarious wording, unclear significance, and any relevance of the purported affidavits necessitate scrutiny|date=August 2021}} On the other hand, the scholars invoking Gödel's Theorem appear, at least in some cases, to be referring not to the underlying rules, but to the understandability of the behavior of all physical systems, as when Hawking mentions arranging blocks into rectangles, turning the computation of [[prime number]]s into a physical question.<ref>{{cite web
 |last=Hawking
 |first=Stephen
 |date=20 July 2002
 |title=Gödel and the end of physics
 |url=http://www.damtp.cam.ac.uk/strings02/dirac/hawking/
 |access-date=2009-12-01
 |archive-date=2011-05-21
 |archive-url=https://web.archive.org/web/20110521123113/http://www.damtp.cam.ac.uk/strings02/dirac/hawking/
 
 }}</ref> This definitional discrepancy may explain some of the disagreement among researchers.

===Fundamental limits in accuracy===
No physical theory to date is believed to be precisely accurate. Instead, physics has proceeded by a series of "successive approximations" allowing more and more accurate predictions over a wider and wider range of phenomena. Some physicists believe that it is therefore a mistake to confuse theoretical models with the true nature of reality, and hold that the series of approximations will never terminate in the "truth".<ref>{{cite book|title=The New Cosmic Onion: Quarks and the Nature of the Universe|first=search|last=results|date=17 December 2006|publisher=CRC Press|isbn = 978-1-58488-798-0}}</ref> Einstein himself expressed this view on occasions.<ref>Einstein, letter to Felix Klein, 1917. (On determinism and approximations.) Quoted in Pais (1982), Ch. 17.</ref>

===Definition of fundamental laws===
There is a philosophical debate within the physics community as to whether a theory of everything deserves to be called ''the'' fundamental law of the universe.<ref>Weinberg (1993), Ch 2.</ref> One view is the hard [[reductionist]] position that the theory of everything is the fundamental law and that all other theories that apply within the universe are a consequence of the theory of everything. Another view is that [[emergence|emergent]] laws, which govern the behavior of [[complex system]]s, should be seen as equally fundamental. Examples of emergent laws are the [[second law of thermodynamics]] and the theory of [[natural selection]]. The advocates of emergence argue that emergent laws, especially those describing complex or living systems are independent of the low-level, microscopic laws. In this view, emergent laws are as fundamental as a theory of everything.

The debates do not make the point at issue clear. Possibly the only issue at stake is the right to apply the high-status term "fundamental" to the respective subjects of research. A well-known debate over this took place between Steven Weinberg and [[Philip Warren Anderson|Philip Anderson]].<ref>{{Cite book|title=Superstrings, P-branes and M-theory|page=7}}</ref>

====Impossibility of calculation====
Weinberg<ref>Weinberg (1993) p. 5</ref> points out that calculating the precise motion of an actual projectile in the Earth's atmosphere is impossible. So how can we know we have an adequate theory for describing the motion of projectiles? Weinberg suggests that we know ''principles'' (Newton's laws of motion and gravitation) that work "well enough" for simple examples, like the motion of planets in empty space. These principles have worked so well on simple examples that we can be reasonably confident they will work for more complex examples. For example, although [[general relativity]] includes equations that do not have exact solutions, it is widely accepted as a valid theory because all of its equations with exact solutions have been experimentally verified. Likewise, a theory of everything must work for a wide range of simple examples in such a way that we can be reasonably confident it will work for every situation in physics. Difficulties in creating a theory of everything often begin to appear when combining [[quantum mechanics]] with the theory of [[general relativity]], as the equations of quantum mechanics begin to falter when the force of gravity is applied to them. 

==See also==
{{Portal|Physics}}
{{div col|colwidth=30}}
* {{Annotated link|Absolute (philosophy)}}
* {{Annotated link|Argument from beauty}}
* {{Annotated link|Attractor}}
* {{Annotated link|Black hole thermodynamics#Beyond black holes|Beyond black holes}}
* {{Annotated link|Beyond the standard model}}
* {{Annotated link|cGh physics}}
* {{Annotated link|Chronology of the universe}}
* {{Annotated link|ER{{=}}EPR}}
* {{Annotated link|Grand Unified Theory}}
* {{Annotated link|Holographic principle}}
* {{Annotated link|List of unsolved problems in mathematics}}
* {{Annotated link|List of unsolved problems in neuroscience}}
* {{Annotated link|List of unsolved problems in physics}}
* {{Annotated link|Mathematical beauty}}
* {{Annotated link|Mathematical universe hypothesis}}
* {{Annotated link|Multiverse}}
* {{Annotated link|Penrose interpretation}}
* {{Annotated link|Standard Model (mathematical formulation)}}
* {{Annotated link|Superfluid vacuum theory}} (SVT)
* {{Annotated link|The Theory of Everything (2014 film)|''The Theory of Everything''}}
* {{Annotated link|Timeline of the Big Bang}}
* {{Annotated link|Unification (physics)|Unification}}
* {{Annotated link|Unity of science}}
* {{Annotated link|Zero-energy universe}}
{{div col end}}

==References==
{{reflist|35em|refs=

<ref name="Impey2012">{{cite book |author=Impey |first=Chris |url=https://archive.org/details/howitbegantimetr0000impe |title=How It Began: A Time-Traveler's Guide to the Universe |date=26 March 2012 |publisher=W. W. Norton |isbn=978-0-393-08002-5 |url-access=registration}}</ref>

<ref name="NYT-20201123">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |date=23 November 2020 |title=Can a Computer Devise a Theory of Everything? - It might be possible, physicists say, but not anytime soon. And there's no guarantee that we humans will understand the result. |work=[[The New York Times]] |url=https://www.nytimes.com/2020/11/23/science/artificial-intelligence-ai-physics-theory.html |archive-url=https://web.archive.org/web/20201123100612/https://www.nytimes.com/2020/11/23/science/artificial-intelligence-ai-physics-theory.html |archive-date=23 November 2020 |url-status=live |access-date=23 November 2020}}</ref>

}}

===Bibliography===
* [[Abraham Pais|Pais, Abraham]] (1982) ''[[Subtle is the Lord: The Science and the Life of Albert Einstein]]'' (Oxford University Press, Oxford. Ch. 17, {{ISBN|0-19-853907-X}}
* [[Steven Weinberg|Weinberg, Steven]] (1993) ''Dreams of a Final Theory: The Search for the Fundamental Laws of Nature'', Hutchinson Radius, London, {{ISBN|0-09-177395-4}}
* [[Corey S. Powell]] ''Relativity versus quantum mechanics: the battle for the universe'', The Guardian (2015) [https://www.theguardian.com/news/2015/nov/04/relativity-quantum-mechanics-universe-physicists Relativity versus quantum mechanics: the battle for the universe]

==External links==
{{commons category}}
{{wikiquote}}
* [https://www.pbs.org/wgbh/nova/elegant/program.html The Elegant Universe], ''[[Nova (American TV series)|Nova]]'' episode about the search for the theory of everything and string theory.
* [http://www.vega.org.uk/video/programme/7 Theory of Everything], freeview video by the [[Vega Science Trust]], BBC and [[Open University]].
* [http://iai.tv/video/the-theory-of-everything The Theory of Everything]: Are we getting closer, or is a final theory of matter and the universe impossible? Debate between [[John Ellis (physicist, born 1946)|John Ellis (physicist)]], [[Frank Close]] and [[Nicholas Maxwell]].
* [http://iai.tv/video/why-the-world-exists Why The World Exists], a discussion between physicist [[Laura Mersini-Houghton]], cosmologist [[George Francis Rayner Ellis]] and philosopher David Wallace about dark matter, parallel universes and explaining why these and the present Universe exist.
* [https://www.bbc.co.uk/programmes/p004y24b Theories of Everything], BBC Radio 4 discussion with Brian Greene, John Barrow & Val Gibson (''In Our Time'', March 25, 2004).

{{Theories of gravitation|selected=GR alternatives}}
{{World view}}
{{Standard model of physics}}

{{Authority control}}

[[Category:Physics beyond the Standard Model]]
[[Category:Theories of gravity]]