Several models for unifying the electroweak and strong nuclear force have also been proposed, and physicists refer to them as grand unified theories GUTs because they would bring together three out of the four forces. The GUT candidates remain untested and unconfirmed, however, because the energies required to investigate them far exceed the capacities of even the most powerful particle colliders.

See also: Electroweak interaction ; Grand unification theories ; Quantum electrodynamics. Gravity remains the hardest of the forces to unify because a convincing model for quantum gravity has yet to emerge.

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In technical terms, the obstacle is that the quantum field theories for the electroweak and strong nuclear forces allow the use of mathematical techniques called renormalization to avoid problematic infinities arising in the calculations. The field equations for gravity are not renormalizable. See also: Renormalization. Currently, many physicists are developing ambitious concepts often called theories of everything, which would not only unify gravity with the other forces but would also, ideally, explain why fundamental physical constants such as the gravitational constant, G have the values they do.

Among the approaches considered promising by many physicists is a class of ideas often labeled string theory, in which the ultimate building blocks of matter are not particles but rather extremely tiny one-dimensional strings highly coiled through more than four dimensions. Some well-developed concepts related to string theory go by the name of superstring theory and M-theory.

A criticism of the string theory approach, however, is that experimental observations may never be able to distinguish which of a nearly infinite number of variations on it is correct. Loop quantum gravity is a rival to string theory with many advocates: It proposes that space itself has a discrete granularity on the order of the Planck scale about 10 —35 meters.

Formulations of loop quantum gravity still have difficulty predicting how smooth space emerges at larger scales, however. Moreover, even if loop quantum gravity does reconcile relativity and quantum mechanics, it would not directly help with the unification of gravity, electromagnetism, and the two nuclear interactions as manifestations of a single underlying force. See also: The quest for loop quantum gravity.

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Contributors include more than highly qualified scientists and 43 Nobel Prize winners. Physics Unification theories and a theory of everything. BR An ongoing endeavor in physics for at least the past three centuries has been the attempt to unify the disparate phenomena of nature within a single theoretical framework and a single set of descriptive equations. See also: Quantum field theory ; Universe Starting with Einstein, physicists sought for decades to devise a unified field theory that would present all four fundamental forces as specialized cases of a single underlying field equation.

See also: Electroweak interaction ; Grand unification theories ; Quantum electrodynamics Gravity remains the hardest of the forces to unify because a convincing model for quantum gravity has yet to emerge. See also: Renormalization Currently, many physicists are developing ambitious concepts often called theories of everything, which would not only unify gravity with the other forces but would also, ideally, explain why fundamental physical constants such as the gravitational constant, G have the values they do. You may already have access to this content.

Robertson offers Conway's game of life as an example: [46] 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.

## Maybe there is no grand unified theory? | Physics Forums

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". Einstein himself expressed this view on occasions. On the other hand, it is often claimed that, despite the apparently ever-increasing complexity of the mathematics of each new theory, in a deep sense associated with their underlying gauge symmetry and the number of dimensionless physical constants , the theories are becoming simpler.

If this is the case, the process of simplification cannot continue indefinitely. 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. Another view is that emergent laws, which govern the behavior of complex systems , 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 TOE. 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 Anderson [ citation needed ]. Although the name "theory of everything" suggests the determinism of Laplace's quotation, this gives a very misleading impression.

### Grand Unified Theory

Determinism is frustrated by the probabilistic nature of quantum mechanical predictions, by the extreme sensitivity to initial conditions that leads to mathematical chaos , by the limitations due to event horizons, and by the extreme mathematical difficulty of applying the theory. Thus, although the current standard model of particle physics "in principle" predicts almost all known non-gravitational phenomena, in practice only a few quantitative results have been derived from the full theory e. The TOE would almost certainly be even harder to apply for the prediction of experimental results, and thus might be of limited use.

A motive for seeking a TOE, [ citation needed ] apart from the pure intellectual satisfaction of completing a centuries-long quest, is that prior examples of unification have predicted new phenomena, some of which e. And like in these prior examples of unification, the TOE would probably allow us to confidently define the domain of validity and residual error of low-energy approximations to the full theory.

Frank Close regularly argues that the layers of nature may be like the layers of an onion, and that the number of layers might be infinite. Weinberg [51] 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 TOE 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.

## Grand Unified Theory

From Wikipedia, the free encyclopedia. This article is about the physical concept. For other uses, see Theory of everything disambiguation.

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Simulated Large Hadron Collider CMS particle detector data depicting a Higgs boson produced by colliding protons decaying into hadron jets and electrons.

Quantum gravity. String theory Loop quantum gravity Loop quantum cosmology Causal dynamical triangulation Causal fermion systems Causal sets Event symmetry Canonical quantum gravity Superfluid vacuum theory. 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?

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### Aeon for Friends

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Theory of everything.