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In physics, unified field theory is an attempt to unify all the fundamental forces and the interactions between elementary particles into a single theoretical framework. The term was coined by Einstein who attempted to reconcile the general theory of relativity with electromagnetism in a single field theory. His quest proved elusive and a unified field theory, sometimes grandiosely referred to as the Theory of Everything (TOE, for short), has remained the holy grail for physicists, the long-sought theory which would explain the nature and behavior of all matter.

In physics, the forces between objects can be described as mediated by fields. Current theory says that at subatomic distances, these fields are replaced by quantum fields interacting according to the laws of quantum mechanics. Alternatively, using the particle-wave duality of quantum mechanics, fields can be described in terms of exchange particles that transfer momentum and energy between objects. Crudely speaking, objects interact as they emit and absorb exchanged particles, in effect playing a subatomic game of “catch”. The essential belief of a unified field theory is that the four fundamental forces (see below) as well as all matter are simply different manifestations of a single fundamental field.

A unified field theory aims to reconcile the four fundamental forces (or fields) of nature, namely:
• Strong force: Force responsible for holding quarks together to form neutrons and protons, and holding neutrons and protons together to form nuclei. The exchange particles that mediate this force are gluons.
• Electromagnetic force: It is the familiar force that acts on electrically charged particle. The photon is the exchange particle for this force.
• Weak force: Responsible for radioactivity, it is a repulsive short-range interaction that acts on electrons, neutrinos and quarks. It is governed by the W boson.
• Gravitational force: A long-range attractive force that acts on all particles. The exchange particles have been postulated and named gravitons.

History
Historically, the first unified field theory was developed by James Clerk Maxwell. In 1831, Michael Faraday made the observation that time-varying magnetic fields could induce electric currents. Until then, electricity and magnetism had been thought as unrelated phenomena. In 1864, Maxwell published his famous paper on a dynamical theory of the electromagnetic field. This was the first example of a theory that was able to encompass previous theories (namely electricity and magnetism) to provide a unifying theory of electromagnetism. However, today we know that the classical electrodynamics developed by Maxwell eventually breaks down near the quantum limit (for large momentum and energy transfer). A complete quantum description of the electromagnetic force was achieved in the 1940s, a theory known as quantum electrodynamics (QED). This theory represents the interactions of charged particles mediated by force carriers named photons. The theory is based on a space-time symmetry of the field called gauge (really phase) symmetry. The theory was so successful that the principle of continuous gauge symmetry was soon adopted for all forces.

In 1967, two Americans Sheldon Glashow and Steven Weinberg and a Pakistani Abdus Salam proposed independently a theory unifying electromagnetism and the weak nuclear forces. They found that in seeking a quantum gauge field theory of the weak forces they were forced to introduce an additional force. They demonstrated that the gauge field from the weak interaction was structurally identical to the electromagnetic field. Quantum electrodynamics is then a consequence of a spontaneous symmetry breaking in a theory in which initially the weak and electromagnetic interactions are unified. This unified theory was governed by the exchange of four particles: the photon for electromagnetic interactions, and a neutral Z particle and two charged W particles for weak interaction. As a result of the spontaneous symmetry breaking the weak force becomes short range and the Z and W bosons acquire masses of the order of 90 GeV / c2. Their theory was given experimental support by the discovery, in 1983, of the Z and W bosons at CERN by Carlo Rubbia’s team. For their insights, Glashow, Weinberg and Salam were awarded the Nobel Prize in Physics in 1979. Carlo Rubbia and Simon van der Meer received the Prize in 1984.

The next logical step towards the unification of the fundamental forces of nature was to include the strong interaction with the electroweak forces in a theory called the Grand Unified Theory (GUT). A quantum theory of the strong force had been developed in the 1970s under the name of Quantum Chromodynamics. The strong interaction acts between quarks via the exchange of particles called gluons. There are eight types of gluons, each carrying a color charge and an anti-color charge. Based on this theory, Sheldon Glashow and Howard Georgi proposed the first grand unified theory in 1974, which applied to energies above 1000 GeV. Since then there have been several proposals for GUTs, although none is currently universally accepted. A major problem for experimental tests of such theories is the energy scale involved, which is well beyond the reach of current accelerators. However, there are some falsifiable predictions that have been made for low energy processes that do not involve accelerators. One of these predictions is that the proton is unstable and can decay. It is at present unknown if the proton can decay although experiments have determined a lower bound of 1035 years for its lifetime. It is therefore uncertain, at the present time, whether any GUT can provide an accurate description of matter.

Gravity has yet to be included in a theory of everything. Theoretical physicists have been so far incapable of formulating a consistent theory that combines general relativity and quantum mechanics. The two theories have proved to be incompatible and the quantization of gravity remains an outstanding problem in the field of physics. In recent years the quest for a unified field theory has largely focused on string theory. Much hope has been put on one of its offshoots known as M-theory (M. Kaku, B. Greene). Others theories that attempt to explain the quantization of gravity are twistor theory (R. Penrose and W. Rindler), Noncommutative geometry (A. Connes, J. Madore) and loop quantum gravity (L. Smolin, R. Gambini and J. Pullin).
See also dynamic theory of gravity, generalized theory of gravitation.

Unifications in physics
• electricity + magnetism = electromagnetism (due to Maxwell (1860s))
• electromagnetism + weak interaction = electroweak interaction (due to Glashow, Salam and Weinberg (1960s))
• electroweak interaction + strong interaction = grand unified theory (none yet verified)
• grand unified theory + general relativity = unified field theory (none yet known)

Reductionism
There is much debate about the intrinsic value of searching for a possibly successful unified field theory. Besides the argument that such a theory may not exist, some have argued that finding the final theory, that is the ultimate foundation of nature, will not unlock the mystery of the universe. This is the view that the understanding of the ultimate particles will not yield a complete knowledge of the behaviour of atoms and molecules or some higher level structure. Some physicists (e.g P.W. Anderson) have argued that large structures undergo collective behaviors which are not most usefully described in terms of the behavior of their constituents and therefore there is no reason to label the lower-level behaviors as more fundamental.

Amateur theories
Many amateur theories have been proposed. These are often couched in cryptic language with numerous neologisms apparently intended to impress or obscure meaning. These attempts are for the most part ill-conceived and devoid of merit. Such theories typically contain little in the way of falsifiable results or predictions; and, for the most part, have not been through a process equivalent to peer-review.
The unified field theory must be consistent, explain all previously known aspects of gravity on a large scale, and of quantum mechanics on the subatomic level, in a single framework while making new and falsifiable predictions.

A theory of everything (TOE) is a theory of theoretical physics and mathematics that fully explains and links together all known physical phenomena. Initially, the term was used with an ironic connotation to refer to various overgeneralized theories. For example, a great-grandfather of Ijon Tichy — a character from a cycle of Stanisław Lem’s science fiction stories of 1960s — was known to work on “General Theory of Everything” (Polish: “Ogólna Teoria Wszystkiego”). Over time, the term stuck in popularizations of quantum physics to describe a theory that would unify the theories of the four fundamental interactions of nature.

There have been numerous theories of everything proposed by theoretical physicists over the last century, but as yet none has been able to stand up to experimental scrutiny or there is tremendous difficulty in getting the theories to produce even experimentally testable results. The primary problem in producing a TOE is that the accepted theories of quantum mechanics and general relativity propose radically different descriptions of the universe, and straightforward ways of combining the two lead quickly to the renormalization problem in which the theory does not give finite results for experimentally testable quantities.

Mainstream physics
Albert Einstein was the first well-known scientist who spent most of his life trying to find a TOE; he believed that the only task was to unify general relativity and electromagnetism.

Current mainstream physics concepts require that a TOE unify the four fundamental interactions of nature: gravity, the strong nuclear force, the weak nuclear force, and the electromagnetic force; it should also explain the spectrum of elementary particles. There has been progress toward a TOE in unifying electromagnetism and the weak nuclear force in an electroweak unified field theory and in unifying all of the forces except for gravity (which in the present theory of general relativity is not a force) in the grand unification theory. One missing piece in a theory of everything involves combining quantum mechanics and general relativity into a theory of quantum gravity.

The only mainstream candidate for a theory of everything at the moment is superstring theory / M-theory; current research on loop quantum gravity may eventually play a fundamental role in a TOE, but that is not its primary aim. These theories attempt to deal with the renormalization problem by setting up some lower bound on the length scales possible. Also, early 21st century theories of everything tend to suppose that the universe actually has more dimensions than the easily observed three of space and one of time. The motivation behind this approach began with the Kaluza-Klein theory in which it was noted that adding one dimension to general relativity would produce the electromagnetic Maxwell’s equations. This has led to efforts to work with theories with large number of dimensions in the hopes that this would produce equations which are similar to known laws of physics. The notion of extra dimensions also helps to resolve the hierarchy problem which is the question of why gravity is so much weaker than any other force. The common answer involves gravity leaking into the extra dimensions in ways that the other forces do not.

In the late 1990s, it was noted that one problem with several of the candidates for theories of everything was that they did not constrain the characteristics of the predicted universe. For example, many theories of quantum gravity can create universes with arbitrary numbers of dimensions or with arbitrary cosmological constants. One bit of speculation is that there may indeed be a huge number of universes, but that only a small number of them are habitable, and hence the fundamental constants of the universe are ultimately the result of the anthropic principle rather than a consequence of the theory of everything. Max Tegmark has taken this principle to its logical conclusion with his “Ultimate Ensemble”, whose only postulate is that “all structures that exist mathematically exist also physically”. In this theory, certain mathematical structures are complex enough to contain self aware substructures, who subjectively perceive themselves as existing in a physically real world.

Unsolved problems in 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?

There is also a philosophical debate within the physics community as to whether or not a “theory of everything” should be seen as the fundamental law of the universe. One view is the hard reductionist view that the TOE is the fundamental law of the universe and that all other theories of the universe are a consequence of the TOE. Another view is that there are laws which Steven Weinberg calls free floating laws which govern the behavior of complex systems, and while these laws are related to the theory of everything, they cannot be seen as less fundamental than the TOE. Some argue that this explanation would violate Occam’s Razor if a completely valid TOE were formulated.

Other possibilities which may frustrate the explanatory capacity of a TOE may include sensitivity to the boundary conditions of the universe, or the existence of mathematical chaos in its solutions, making its predictions precise, but useless.

There have been several attempts to advance the general theory of relativity as a theory of everything. As mentioned above, Einstein was responsible for one of these: in collaboration with with Rosen he attempted to model particles as tiny wormholes, hence the term Einstein-Rosen Bridge. Wormholes have also been proposed at various times (for instance, by Shimony and by Durand [1]) to explain Bell violations not as superluminal influences but influences that take a shortcut through a wormhole. Such theories face a number of hurdles: the creation of wormholes changes the topology of spacetime by creating a new “handle” which implies violations of causality (see Hadley [2]), and the general theory of relativity predicts its own breakdown at a Gravitational singularity by theorems of Stephen Hawking and Roger Penrose. A recent effort to surmount this hurdle notes that the equivalence principle can be applied along curves rather than at a single point (Iliev [3]), which would imply that time dilation of (1 − v2) − 1 / 2 is indistinguishable locally (along the curve) from a relative velocity v and the unbounded time dilation observed as an event horizon emerges at the center of a collapsing star implies that the center is in reality as well as appearance receding at a velocity approaching the speed of light, producing a bubble-like local inflation of the star’s interior (Monroe [4]). This approach skirts the trapped surface assumption of the theorems of Hawking and Penrose.

Where the Standard Model comes up short
The Standard Model of physics is among the most successful theories in history, but it fails to explain everything. It doesn’t explain the origins of the universe before the big bang or the cause of that event. There are 18 arbitrary constants and several dozen elementary particles in the Standard Model. Why are there so many? The Standard Model also fails to explain over 90% of the apparent mass-energy of the universe. The existence of dark matter and dark energy, although never observed directly, is all but guaranteed if current theory is correct.

Why is so much of the universe invisible? What is the state of matter within a black hole? Is spacetime curved, or is it flat? How many dimensions of space and time are there? What is the origin of matter and energy? What is the reason for them at all? Are some particles the “most fundamental”? What happens beyond Planck scales? Why is momentum quantized? Is the speed of light the fastest speed in the universe? These are among the many questions left unanswered by the most modern theories in physics. A successful TOE would explain each of these questions and provide solutions to every situation which could exist in the universe.

Other efforts
Attempts to create theories of everything are common among people outside the professional physics community. Some are created by amateurs, and their theories are often criticised on the basis of inability to make quantifiable and/or falsifiable predictions. For example, a theory of everything would provide some insight into the relative strength of forces, and predictions of particle lifetimes and cross sections. It would need to be shown to explain all known universal phenomena. Unlike professional physicists, who are generally aware that their proposed theory is incomplete, untested, and likely to be wrong and who are aware of the huge difficulties and challenges involved in creating a TOE, amateurs who create TOE’s tend to be unaware of what work has already been done, the mechanisms for testing scientific theories and the fact that most proposed theories are wrong.

Burkhard Heim and quantised general relativity
Burkhard Heim’s theory of quantised general relativity purports to be a TOE but this theory, begun in the 1950s and still under development, had until recently sunk into obscurity. A sign that it is undergoing a renewal of interest is that a paper by Droescher and Haeuser on aerospace applications of Heim Theory was published by the AIAA in 2005 and was awarded the prize for best paper of the year by the Nuclear and Future Flight Propulsion Technical Committee. Supporters claim that Heim’s six dimensional theory can predict the masses of some fundamental particles with considerable accuracy, which no established theory has yet been able to do. Heim’s theory bears a resemblance to loop quantum gravity in that Heim’s network of metrons is similar to the spin networks of LQG, and predates them by several decades.

Eino Kaila
The prolific Finnish philosopher Eino Kaila attempted to construct a theory of everything based on the philosophical implications of quantum mechanics in the 1950s. His attempt did not get much attention outside Finland, and he only managed to write the first part of what he planned on making an extensive study on the subject. “Terminalkausalität als die Grundlage eines unitarischen Naturbegriffs” (”terminal causality as the foundation of a unitarian notion of nature”), published in 1956, formulated a new type of causality and was meant to be followed by similar works on psychology and biology.

Time Cube
Gene Ray’s Time Cube concept is an example of an amateur TOE that is quite well-known, although this is mainly due to entertainment value rather than its scientific merit. Mr. Ray claims to explain all known universal phenomena through the postulate that “Time is cubic, not linear”. See list of alternative, speculative and disputed theories. Like many similar theories, it is regarded as pseudoscience.

Expansion Theory
Expansion Theory purports to offer a theory of everything in which all physical phenomena are explained by universal accelerating expansion. Author Mark McCutcheon described the theory in the book The Final Theory: Rethinking our Scientific Legacy, in 2002, although the theory itself is much older. The theory argues that current scientific theory is inconsistent and incomplete in that it predicts yet doesn’t explain Action at a distance, violates its own conservation laws, and fails to live up to experimental data or concur with the laws of common sense. Under expansion theory, Classical Mechanics, General Relativity, Special Relativity, Quantum Mechanics are discarded and replaced with an atomic expansion that, according to the author, accounts for phenomena like magnetism, light, gravity, and atomic forces. [5]

Expansion Theory holds little to no acceptance within the scientific community. Many of the predictions of the theory don’t hold empirically, and the theory doesn’t explain any anomalous data. Like other purported theories of everything, many regard the theory to be a form of pseudoscience.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article “Unified Field Theory and The Theory of Everything” .

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