A **theory of everything** (**ToE**), **final theory**, **ultimate theory**, or **master theory** is a hypothetical single, all-encompassing, coherent theoretical framework of physics that fully explains and links together all physical aspects of the universe.^{[1]}^{:6} Finding a ToE is one of the major unsolved problems in physics. Over the past few centuries, two theoretical frameworks have been developed that, as a whole, most closely resemble a ToE. These two theories upon which all modern physics rests are general relativity (GR) and quantum field theory (QFT). GR is a theoretical framework that only focuses on gravity for understanding the universe in regions of both large-scale and high-mass: stars, galaxies, clusters of galaxies, etc. On the other hand, QFT is a theoretical framework that only focuses on three non-gravitational forces for understanding the universe in regions of both small scale and low mass: sub-atomic particles, atoms, molecules, etc. QFT successfully implemented the Standard Model and unified the interactions (so-called Grand Unified Theory) between the three non-gravitational forces: weak, strong, and electromagnetic force.^{[2]}^{:122}

## Quantum theoryEdit

Through years of research, physicists have experimentally confirmed with tremendous accuracy virtually every prediction made by these two theories when in their appropriate domains of applicability. In accordance with their findings, scientists also learned that general relativity (GR) and quantum field theory (QFT) are mutually incompatible – they cannot both be right. Since the usual domains of applicability of GR and QFT are so different, most situations require that only one of the two theories be used.^{[3]}^{[4]}^{:842–844} As it turns out, this incompatibility between GR and QFT is apparently only an issue in regions of extremely small-scale and high-mass, 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 (*See also* Universe expansion).) To resolve this conflict, a theoretical framework revealing a deeper underlying reality, unifying gravity with the other three interactions, must be discovered to harmoniously integrate the realms of GR and QFT into a seamless whole: a single theory that, in principle, is capable of describing all phenomena. In pursuit of this goal, quantum gravity has become an area of active research.

## String theoryEdit

Eventually a single explanatory framework, called "string theory", emerged that intends to be the ultimate theory of the universe. String theory posits that at the beginning of the universe (up to 10^{−43} 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 microscopic 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).

### M-theoryEdit

Since the 1990s, some physicistsTemplate:Who believe that 11-dimensional M-theory, which is described in some limits by one of the five perturbative superstring theories, and in another by the maximally-supersymmetric 11-dimensional supergravity, is the theory of everything. However, there is no widespread consensus on this issue.

A surprising property of string/M-theory is that extra dimensions are required for the theory's consistency. 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 five-dimensional universe (with one of them small and curled up) looks from the four-dimensional perspective like the usual general relativity together with Maxwell's electrodynamics. This lent credence to the idea of unifying 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 four-dimensional spacetime, an idea that has been realized with explicit stringy mechanisms.^{[5]}

### F-TheoryEdit

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 neutrino masses fits into a spinor representation of SO(10), a subgroup of E8 that routinely emerges in string theory, such as in heterotic string theory^{[6]} or (sometimes equivalently) in F-theory.^{[7]}^{[8]} String theory has mechanisms that may explain why fermions come in three hierarchical generations, and explain the mixing rates between quark generations.^{[9]} 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 entropy of black holes^{[10]}^{[11]} and allowing for topology-changing processes.^{[12]}^{[13]}^{[14]} It has also led to many insights in pure mathematics and in ordinary, strongly-coupled gauge theory due to the Gauge/String duality.

### CriticismEdit

In the late 1990s, it was noted that one major hurdle in this endeavor is that the number of possible four-dimensional universes is incredibly large. The small, "curled up" extra dimensions can be compactified in an enormous number of different ways (one estimate is 10^{500} ) 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.^{[15]}^{:347} One proposed solution is that many or all of these possibilities are realised 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,^{[16]} arguing that it cannot make useful (i.e., original, falsifiable, and verifiable) predictions and regarding it as a pseudoscience. Others disagree,^{[17]} and string theory remains an extremely active topic of investigation in theoretical physics.Template:Citation needed

## See alsoEdit

## ReferencesEdit

- ↑ Steven Weinberg.
*Dreams of a Final Theory: The Scientist's Search for the Ultimate Laws of Nature*. Knopf Doubleday Publishing Group. ISBN 978-0-307-78786-6. - ↑ Stephen W. Hawking (28 February 2006).
*The Theory of Everything: The Origin and Fate of the Universe*. Phoenix Books; Special Anniv. ISBN 978-1-59777-508-3. - ↑ Carlip, Steven (2001). "Quantum Gravity: a Progress Report".
*Reports on Progress in Physics***64**(8): 885. doi: . Bibcode: 2001RPPh...64..885C. - ↑ Susanna Hornig Priest (14 July 2010).
*Encyclopedia of Science and Technology Communication*. SAGE Publications. ISBN 978-1-4522-6578-0. - ↑ Holloway, M (2005). "The Beauty of Branes".
*Scientific American*(Scientific American)**293**(4): 38. doi: . PMID 16196251. Bibcode: 2005SciAm.293d..38H. http://randall.physics.harvard.edu/RandallCV/ScientificAm10-05.pdf. Retrieved on 13 August 2012. - ↑ Nilles, Hans Peter; Ramos-Sánchez, Saúl; Ratz, Michael; Vaudrevange, Patrick K. S. (2008). "From strings to the MSSM".
*The European Physical Journal C***59**(2): 249. doi: . Bibcode: 2009EPJC...59..249N. - ↑ Beasley, Chris; Heckman, Jonathan J; Vafa, Cumrun (2009). "GUTs and exceptional branes in F-theory — I".
*Journal of High Energy Physics***2009**: 058. doi: . Bibcode: 2009JHEP...01..058B. - ↑ Template:Cite arXiv
- ↑ Heckman, Jonathan J.; Vafa, Cumrun (2008). "Flavor Hierarchy from F-theory".
*Nuclear Physics B***837**: 137–151. doi: . Bibcode: 2010NuPhB.837..137H. - ↑ Strominger, Andrew; Vafa, Cumrun (1996). "Microscopic origin of the Bekenstein-Hawking entropy".
*Physics Letters B***379**: 99. doi: . Bibcode: 1996PhLB..379...99S. - ↑ Horowitz, Gary (1996). "Gravitational Wave Astronomy".
*The Origin of Black Hole Entropy in String Theory*. Astrophysics and Space Science Library.**211**. pp. 95. doi: . ISBN 978-94-010-6455-2. - ↑ Greene, Brian R.; Morrison, David R.; Strominger, Andrew (1995). "Black hole condensation and the unification of string vacua".
*Nuclear Physics B***451**: 109. doi: . Bibcode: 1995NuPhB.451..109G. - ↑ Aspinwall, Paul S.; Greene, Brian R.; Morrison, David R. (1994). "Calabi-Yau moduli space, mirror manifolds and spacetime topology change in string theory".
*Nuclear Physics B***416**(2): 414. doi: . Bibcode: 1994NuPhB.416..414A. - ↑ Adams, Allan; Liu, Xiao; McGreevy, John; Saltman, Alex; Silverstein, Eva (2005). "Things fall apart: Topology change from winding tachyons".
*Journal of High Energy Physics***2005**(10): 033. doi: . Bibcode: 2005JHEP...10..033A. - ↑ Chris Impey (26 March 2012).
*How It Began: A Time-Traveler's Guide to the Universe*. W. W. Norton. ISBN 978-0-393-08002-5. https://books.google.com/books?id=IjEz8Tq-EdIC. - ↑ Smolin, Lee (2006).
*The Trouble With Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next*. Houghton Mifflin. ISBN 978-0-618-55105-7. - ↑ Duff, M. J. (2011). "String and M-Theory: Answering the Critics".
*Foundations of Physics***43**: 182. doi: . Bibcode: 2013FoPh...43..182D.