Beyond the Standard Model

Standard Model Quantum gravity String theory Loop quantum gravity Causal dynamical triangulation Canonical general relativity

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Quantum gravity is the field of theoretical physics attempting to unify quantum mechanics, which describes three of the fundamental forces of nature (electromagnetism, weak interaction, and strong interaction), with general relativity, the theory of the fourth fundamental force: gravity. In Physics, the Standard Model of Particle physics is currently the best description of all experimental data The Standard Model of Particle physics is a theory that describes three of the four known Fundamental interactions together with the Elementary particles String theory is a still-developing scientific approach to Theoretical physics, whose original building blocks are one-dimensional extended objects called strings Loop quantum gravity (LQG, also known as loop gravity and Quantum geometry, is a proposed quantum theory of Spacetime which attempts to reconcile the theories Causal dynamical triangulation (abbreviated as "CDT" invented by Renate Loll, Jan Ambjørn and Jerzy Jurkiewicz is an approach to Quantum In Physics, canonical quantum gravity is an attempt to quantize the canonical formulation of general relativity (or canonical gravity) Theoretical physics employs Mathematical models and Abstractions of Physics in an attempt to explain experimental data taken of the natural world Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons In Physics, a fundamental interaction or fundamental force is a mechanism by which particles interact with each other and which cannot be explained in terms General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 Gravitation is a natural Phenomenon by which objects with Mass attract one another One ultimate goal hoped to emerge as a result of this is a unified framework for all fundamental forces— called a "theory of everything" (TOE). A theory of everything ( TOE) is a putative Theory of Theoretical physics that fully explains and links together all known physical phenomena

Overview

Unsolved problems in physics: How can the theory of quantum mechanics be merged with the theory of general relativity/gravitational force and remain correct at microscopic length scales? What verifiable predictions does any theory of quantum gravity make?

Much of the difficulty in merging these theories at all energy scales comes from the different assumptions that these theories make on how the universe works. This is a list of some of the major unsolved problems in Physics. Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 Gravitation is a natural Phenomenon by which objects with Mass attract one another Quantum field theory depends on particle fields embedded in the flat space-time of special relativity. In quantum field theory (QFT the forces between particles are mediated by other particles Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial General relativity models gravity as a curvature within space-time that changes as a gravitational mass moves. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 SpaceTime is a patent-pending three dimensional graphical user interface that allows end users to search their content such as Google Google Images Yahoo! YouTube eBay Amazon and RSS Historically, the most obvious way of combining the two (such as treating gravity as simply another particle field) ran quickly into what is known as the renormalization problem. In Quantum field theory, the Statistical mechanics of fields and the theory of self-similar geometric structures renormalization refers to a collection In the old-fashioned understanding of renormalization, gravity particles would attract each other and adding together all of the interactions results in many infinite values which cannot easily be cancelled out mathematically to yield sensible, finite results. This is in contrast with quantum electrodynamics where, while the series still do not converge, the interactions sometimes evaluate to infinite results, but those are few enough in number to be removable via renormalization. Quantum electrodynamics ( QED) is a relativistic Quantum field theory of Electrodynamics.

Effective field theories

In recent decades, however, this antiquated understanding of renormalization has given way to the modern idea of effective field theory. In Quantum field theory, the Statistical mechanics of fields and the theory of self-similar geometric structures renormalization refers to a collection In Physics, an effective field theory is an approximate theory (usually a Quantum field theory) that includes appropriate degrees of freedom to describe All quantum field theories come with some high-energy cutoff, beyond which we do not expect that the theory provides a good description of nature. The "infinities" then become large but finite quantities proportional to this finite cutoff scale, and correspond to processes that involve very high energies near the fundamental cutoff. These quantities can then be absorbed into an infinite collection of coupling constants, and at energies well below the fundamental cutoff of the theory, to any desired precision only a finite number of these coupling constants need to be measured in order to make legitimate quantum-mechanical predictions. This same logic works just as well for the highly successful theory of low-energy pions as for quantum gravity. In Particle physics, pion (short for pi meson) is the collective name for three Subatomic particles, and. Indeed, the first quantum-mechanical corrections to graviton-graviton scattering and Newton's law of gravitation have been explicitly computed (although they are so astronomically small that we may never be able to measure them), and any more fundamental theory of nature would need to replicate these results in order to be taken seriously. In Physics, the graviton is a hypothetical Elementary particle, a Boson to be exact that mediates the force of Gravity in the framework Scattering is a general physical process whereby some forms of Radiation, such as Light, Sound or moving particles for example are forced to deviate from Newton 's law of universal Gravitation is a physical law describing the gravitational attraction between bodies with mass In fact, gravity is in many ways a much better quantum field theory than the Standard Model, since it appears to be valid all the way up to its cutoff at the Planck scale. The Standard Model of Particle physics is a theory that describes three of the four known Fundamental interactions together with the Elementary particles In Particle physics and Physical cosmology, the Planck scale is an Energy scale around 1 (By comparison, the Standard Model is expected to start to break down above its cutoff at the much smaller scale of around 1000 GeV. )

While confirming that quantum mechanics and gravity are indeed consistent at reasonable energies (in fact, the complete structure of gravity can be shown to arise automatically from the quantum mechanics of spin-2 massless particles), this way of thinking makes clear that near or above the fundamental cutoff of our effective quantum theory of gravity (the cutoff is generally assumed to be of order the Planck scale), a new model of nature will be needed. In Quantum mechanics, spin is a fundamental property of atomic nuclei, Hadrons and Elementary particles For particles with non-zero spin That is, in the modern way of thinking, the problem of combining quantum mechanics and gravity becomes an issue only at very high energies, and may well require a totally new kind of model.

Quantum gravity theory for the highest energy scales

The general approach taken in deriving a theory of quantum gravity that is valid at even the highest energy scales is to assume that the underlying theory will be simple and elegant and then to look at current theories for symmetries and hints for how to combine them elegantly into an overarching theory. One problem with this approach is that it is not known if quantum gravity will be a simple and elegant theory (that resolves the conundrum of special and general relativity with regard to the uniformity of acceleration and gravity, in the former case and spacetime curvature in the latter case). Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 This page is about the scientific concept of relativity for philosophical or sociological theories about relativity see Relativism. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916

Such a theory is required in order to understand those problems involving the combination of very large mass or energy and very small dimensions of space, such as the behavior of black holes, and the origin of the universe. A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e The Big Bang is the cosmological model of the Universe that is best supported by all lines of scientific evidence and Observation.

Quantum Mechanics and General Relativity

Gravity Probe B (GP-B) will measure spacetime curvature near Earth to test related models in application of Einstein's general theory of relativity. Gravity Probe B ( GP-B) is a Satellite -based mission which launched in 2004

The graviton

Main article: Graviton

At present, one of the deepest problems in theoretical physics is harmonizing the theory of general relativity, which describes gravitation, and applies to large-scale structures (stars, planets, galaxies), with quantum mechanics, which describes the other three fundamental forces acting on the atomic scale. In Physics, the graviton is a hypothetical Elementary particle, a Boson to be exact that mediates the force of Gravity in the framework General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons In Physics, a fundamental interaction or fundamental force is a mechanism by which particles interact with each other and which cannot be explained in terms History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny This problem must be put in the proper context, however. In particular, contrary to the popular but erroneous claim that quantum mechanics and general relativity are fundamentally incompatible, one can in fact demonstrate that the structure of general relativity essentially follows inevitably from the quantum mechanics of interacting theoretical spin-2 massless particles (called gravitons). The word theory has many distinct meanings in different fields of Knowledge, depending on their methodologies and the context of discussion. In Physics, the graviton is a hypothetical Elementary particle, a Boson to be exact that mediates the force of Gravity in the framework

While there is no concrete proof of the existence of gravitons, all quantized theories of matter necessitate their existence. Supporting this theory is the observation that all other fundamental forces have one or more messenger particles, except gravity, leading researchers to believe that at least one most likely does exist; they have dubbed these hypothetical particles gravitons. The word theory has many distinct meanings in different fields of Knowledge, depending on their methodologies and the context of discussion. In Physics, a fundamental interaction or fundamental force is a mechanism by which particles interact with each other and which cannot be explained in terms In Particle physics, the Quantum field theory called the Standard Model describes the strong, weak and electromagnetic Fundamental Many of the accepted notions of a unified theory of physics since the 1970s, including string theory, superstring theory, M-theory, loop quantum gravity, all assume, and to some degree depend upon the existence of the graviton. String theory is a still-developing scientific approach to Theoretical physics, whose original building blocks are one-dimensional extended objects called strings See also String theory Superstring theory is an attempt to explain all of the particles and Fundamental forces of nature in one theory by modelling In Theoretical physics, M-theory is a new limit of String theory in which 11 dimensions of Spacetime may be identified Loop quantum gravity (LQG, also known as loop gravity and Quantum geometry, is a proposed quantum theory of Spacetime which attempts to reconcile the theories Many researchers view the detection of the graviton as vital to validating their work. CERN plans to dedicate a large timeshare to search for the graviton using the Large Hadron Collider. The European Organization for Nuclear Research (Organisation Européenne pour la Recherche Nucléaire known as CERN

Nonrenormalizability of gravity

Further information: Renormalization

Historically, many believed that general relativity was in fact fundamentally inconsistent with quantum mechanics. In Quantum field theory, the Statistical mechanics of fields and the theory of self-similar geometric structures renormalization refers to a collection General relativity, like electromagnetism, is a classical field theory. Electromagnetism is the Physics of the Electromagnetic field: a field which exerts a Force on particles that possess the property of A classical field theory is a Physical theory that describes the study of how one or more physical fields interact with matter One might expect that, as with electromagnetism, there should be a corresponding quantum field theory. In quantum field theory (QFT the forces between particles are mediated by other particles

However, gravity is nonrenormalizable. In Quantum field theory, the Statistical mechanics of fields and the theory of self-similar geometric structures renormalization refers to a collection For a quantum field theory to be well-defined according to this understanding of the subject, it must be asymptotically free or asymptotically safe. In Physics, asymptotic freedom is the property of some gauge theories in which the interaction between the particles such as Quarks, becomes arbitrarily A theory has a UV fixed point if its Renormalization group flow runs towards a Fixed point in the ultraviolet (i The theory must be characterized by a choice of finitely many parameters, which could, in principle, be set by experiment. For example, in quantum electrodynamics, these parameters are the charge and mass of the electron, as measured at a particular energy scale. Quantum electrodynamics ( QED) is a relativistic Quantum field theory of Electrodynamics.

On the other hand, in quantizing gravity, there are infinitely many independent parameters needed to define the theory. For a given choice of those parameters, one could make sense of the theory, but since we can never do infinitely many experiments to fix the values of every parameter, we do not have a meaningful physical theory:

• At low energies, the logic of the renormalization group tells us that, despite the unknown choices of these infinitely many parameters, quantum gravity will reduce to the usual Einstein theory of general relativity. In Theoretical physics, renormalization group (RG refers to a mathematical apparatus that allows one to investigate the changes of a physical system as one views General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916
• On the other hand, if we could probe very high energies where quantum effects take over, then every one of the infinitely many unknown parameters would begin to matter, and we could make no predictions at all.

As explained below, there is a way around this problem by treating QG as an effective field theory. In Physics, an effective field theory is an approximate theory (usually a Quantum field theory) that includes appropriate degrees of freedom to describe

Any meaningful theory of quantum gravity that makes sense and is predictive at all energy scales must have some deep principle that reduces the infinitely many unknown parameters to a finite number that can then be measured.

• One possibility is that normal perturbation theory is not a reliable guide to the renormalizability of the theory, and that there really is a UV fixed point for gravity. This article describes perturbation theory as a general mathematical method A theory has a UV fixed point if its Renormalization group flow runs towards a Fixed point in the ultraviolet (i Since this is a question of non-perturbative quantum field theory, it is difficult to find a reliable answer, but some people still pursue this option. In Mathematics and Physics, a non-perturbative function or process is one that cannot be accurately described by Perturbation theory.
• Another possibility is that there are new symmetry principles that constrain the parameters and reduce them to a finite set. This is the route taken by string theory, where all of the excitations of the string essentially manifest themselves as new symmetries. String theory is a still-developing scientific approach to Theoretical physics, whose original building blocks are one-dimensional extended objects called strings

QG as an effective field theory

Main article: Effective field theory

In an effective field theory, all but the first few of the infinite set of parameters in a nonrenormalizable theory are suppressed by huge energy scales and hence can be neglected when computing low-energy effects. In Physics, an effective field theory is an approximate theory (usually a Quantum field theory) that includes appropriate degrees of freedom to describe In Physics, an effective field theory is an approximate theory (usually a Quantum field theory) that includes appropriate degrees of freedom to describe Thus, at least in the low-energy regime, the model is indeed a predictive quantum field theory^[1]. (A very similar situation occurs for the very similar effective field theory of low-energy pions. ) Furthermore, many theorists agree that even the Standard Model should really be regarded as an effective field theory as well, with "nonrenormalizable" interactions suppressed by large energy scales and whose effects have consequently not been observed experimentally.

Recent work^[1] has shown that by treating general relativity as an effective field theory, one can actually make legitimate predictions for quantum gravity, at least for low-energy phenomena. In Physics, an effective field theory is an approximate theory (usually a Quantum field theory) that includes appropriate degrees of freedom to describe An example is the well-known calculation of the tiny first-order quantum-mechanical correction to the classical Newtonian gravitational potential between two masses. Such predictions would need to be replicated by any candidate theory of high-energy quantum gravity.

Spacetime background dependence

Main article: Background independence

A fundamental lesson of general relativity is that there is no fixed spacetime background, as found in Newtonian mechanics and special relativity; the spacetime geometry is dynamic. Background independence is a condition in theoretical physics especially in Quantum gravity (QG that requires the defining equations of a theory to be independent of the actual Classical mechanics is used for describing the motion of Macroscopic objects from Projectiles to parts of Machinery, as well as Astronomical objects Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial While easy to grasp in principle, this is the hardest idea to understand about general relativity, and its consequences are profound and not fully explored, even at the classical level. To a certain extent, general relativity can be seen to be a relational theory,^[2] in which the only physically relevant information is the relationship between different events in space-time. … This article is about relational theory in physics and philosophy

On the other hand, quantum mechanics has depended since its inception on a fixed background (non-dynamical) structure. In the case of quantum mechanics, it is time that is given and not dynamic, just as in Newtonian classical mechanics. For other uses see Time (disambiguation Time is a component of a measuring system used to sequence events to compare the durations of In relativistic quantum field theory, just as in classical field theory, Minkowski spacetime is the fixed background of the theory. In Physics and Mathematics, Minkowski space (or Minkowski spacetime) is the mathematical setting in which Einstein's theory of Special relativity

String theory

Interaction in the subatomic world: world lines of point-like particles in the Standard Model or a world sheet swept up by closed strings in string theory

String theory started out as a generalization of quantum field theory where instead of point particles, string-like objects propagate in a fixed spacetime background. In physics the world line of an object is the unique path of that object as it travels through 4- Dimensional Spacetime. A subatomic particle is an elementary or composite Particle smaller than an Atom. The Standard Model of Particle physics is a theory that describes three of the four known Fundamental interactions together with the Elementary particles A string is one of the main objects of study in String theory, a branch of Theoretical physics. A string is one of the main objects of study in String theory, a branch of Theoretical physics. String theory is a still-developing scientific approach to Theoretical physics, whose original building blocks are one-dimensional extended objects called strings Although string theory had its origins in the study of quark confinement and not of quantum gravity, it was soon discovered that the string spectrum contains the graviton, and that "condensation" of certain vibration modes of strings is equivalent to a modification of the original background. Color confinement, often called just confinement, is the Physics phenomenon that Color charged particles (such as Quarks cannot be isolated singularly In Physics, the graviton is a hypothetical Elementary particle, a Boson to be exact that mediates the force of Gravity in the framework In this sense, string perturbation theory exhibits exactly the features one would expect of a perturbation theory that may exhibit a strong dependence on asymptotics (as seen, for example, in the AdS/CFT correspondence) which is a weak form of background dependence. For the relation of the AdS/CFT correspondence to the general context of string theory see String theory. Background independence is a condition in theoretical physics especially in Quantum gravity (QG that requires the defining equations of a theory to be independent of the actual

Background independent theories

Loop quantum gravity is the fruit of an effort to formulate a background-independent quantum theory. Loop quantum gravity (LQG, also known as loop gravity and Quantum geometry, is a proposed quantum theory of Spacetime which attempts to reconcile the theories Background independence is a condition in theoretical physics especially in Quantum gravity (QG that requires the defining equations of a theory to be independent of the actual

Topological quantum field theory provided an example of background-independent quantum theory, but with no local degrees of freedom, and only finitely many degrees of freedom globally. A topological quantum field theory (or topological field theory or TQFT) is a Quantum field theory which computes Topological invariants This is inadequate to describe gravity in 3+1 dimensions which has local degrees of freedom according to general relativity. In 2+1 dimensions, however, gravity is a topological field theory, and it has been successfully quantized in several different ways, including spin networks. In Physics, a spin network is a type of diagram which can be used to represent states and interactions between particles and fields in Quantum physics

Fields vs particles

Quantum field theory on curved (non-Minkowskian) backgrounds, while not a quantum theory of gravity, has shown that some of the assumptions of quantum field theory cannot be carried over to curved spacetime, let alone to full-blown quantum gravity. In quantum field theory (QFT the forces between particles are mediated by other particles In particular, the vacuum, when it exists, is shown to depend on the path of the observer through space-time (see Unruh effect). The Unruh effect, discovered in 1976 by Bill Unruh of the University of British Columbia, is the prediction that an accelerating observer will observe

Also, some argue that in curved spacetime, the field concept is seen to be fundamental over the particle concept (which arises as a convenient way to describe localized interactions). However, since it appears possible to regard curved spacetime as consisting of a condensate of gravitons, there is still some debate over which concept is truly the more fundamental.

Points of tension

There are two other points of tension between quantum mechanics and general relativity.

• First, classical general relativity breaks down at singularities, and quantum mechanics becomes inconsistent with general relativity in a neighborhood of singularities (however, no one is certain that classical general relativity applies near singularities in the first place).
• Second, it is not clear how to determine the gravitational field of a particle, since under the Heisenberg uncertainty principle of quantum mechanics its location and velocity cannot be known with certainty. In Quantum physics, the Heisenberg uncertainty principle states that locating a particle in a small region of space makes the Momentum of the particle uncertain The resolution of these points may come from a better understanding of general relativity^[3].

Candidate theories

There are a number of proposed quantum gravity theories. ^[4] Currently, there is still no complete and consistent quantum theory of gravity, and the candidate models still need to overcome major formal and conceptual problems. They also face the common problem that, as yet, there is no way to put quantum gravity predictions to experimental tests, although there is hope for this to change as future data from cosmological observations and particle physics experiments becomes available. ^[5]

String theory

Main article: String theory

Projection of a Calabi-Yau manifold, one of the ways of compactifying the extra dimensions posited by string theory

One suggestive starting point are ordinary quantum field theories which, after all, are successful in describing the other three basic fundamental forces in the context of the standard model of elementary particle physics. String theory is a still-developing scientific approach to Theoretical physics, whose original building blocks are one-dimensional extended objects called strings In mathematics Calabi&ndashYau manifolds are compact Kähler manifolds whose Canonical bundle is trivial The Standard Model of Particle physics is a theory that describes three of the four known Fundamental interactions together with the Elementary particles Particle physics is a branch of Physics that studies the elementary constituents of Matter and Radiation, and the interactions between them However, while this leads to an acceptable effective (quantum) field theory of gravity at low energies,^[6] gravity turns out to be much more problematic at higher energies. In Physics, an effective field theory is an approximate theory (usually a Quantum field theory) that includes appropriate degrees of freedom to describe Where, for ordinary field theories such as quantum electrodynamics, a technique known as renormalization is an integral part of deriving predictions which take into account higher-energy contributions,^[7] gravity turns out to be nonrenormalizable: at high energies, applying the recipes of ordinary quantum field theory yields models that are devoid of all predictive power. Quantum electrodynamics ( QED) is a relativistic Quantum field theory of Electrodynamics. In Quantum field theory, the Statistical mechanics of fields and the theory of self-similar geometric structures renormalization refers to a collection In Quantum field theory, the Statistical mechanics of fields and the theory of self-similar geometric structures renormalization refers to a collection ^[8]

One attempt to overcome these limitations is to replace ordinary quantum field theory, which is based on the classical concept of a point particle, with a quantum theory of one-dimensional extended objects: string theory. A point particle (or point-like, often spelled pointlike) is an idealized object heavily used in Physics. String theory is a still-developing scientific approach to Theoretical physics, whose original building blocks are one-dimensional extended objects called strings ^[9] At the energies reached in current experiments, these strings are indistinguishable from point-like particles, but, crucially, different modes of oscillation of one and the same type of fundamental string appear as particles with different (electric and other) charges. In this way, string theory promises to be a unified description of all particles and interactions. A theory of everything ( TOE) is a putative Theory of Theoretical physics that fully explains and links together all known physical phenomena ^[10] The theory is successful in that one mode will always correspond to a graviton, the messenger particle of gravity; however, the price to pay are unusual features such as six extra dimensions of space in addition to the usual three. In Physics, the graviton is a hypothetical Elementary particle, a Boson to be exact that mediates the force of Gravity in the framework In Particle physics, the Quantum field theory called the Standard Model describes the strong, weak and electromagnetic Fundamental In Physics, Kaluza–Klein theory (or KK theory, for short is a model that seeks to unify the two fundamental forces of Gravitation and ^[11] In what is called the second superstring revolution, it was conjectured that both string theory and a unification of general relativity and supersymmetry known as supergravity^[12] form part of a hypothesized eleven-dimensional model known as M-theory, which would constitute a uniquely defined and consistent theory of quantum gravity. The second superstring revolution was the intense wave of breakthroughs in String theory that took place approximately between 1994 and 1997. In Particle physics, supersymmetry (often abbreviated SUSY) is a Symmetry that relates elementary particles of one spin to another particle that In Theoretical physics, supergravity ( supergravity theory) is a field theory that combines the principles of Supersymmetry and General relativity In Theoretical physics, M-theory is a new limit of String theory in which 11 dimensions of Spacetime may be identified ^[13]

Loop quantum gravity

Main article: loop quantum gravity

Simple spin network of the type used in loop quantum gravity

Another approach to quantum gravity starts with the canonical quantization procedures of quantum theory. Loop quantum gravity (LQG, also known as loop gravity and Quantum geometry, is a proposed quantum theory of Spacetime which attempts to reconcile the theories In Physics, a spin network is a type of diagram which can be used to represent states and interactions between particles and fields in Quantum physics In Physics, canonical quantization is one of many procedures for quantizing a Classical theory. Starting with the initial-value-formulation of general relativity (cf. the section on evolution equations, above), the result is an analogue of the Schrödinger equation: the Wheeler-deWitt equation which, regrettably, turns out to be ill-defined. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 In Physics, especially Quantum mechanics, the Schrödinger equation is an equation that describes how the Quantum state of a Physical system ^[14] A major break-through came with the introduction of what are now known as Ashtekar variables, which represent geometric gravity using mathematical analogues of electric and magnetic fields. In Theoretical physics, Ashtekar (new variables (named after Abhay Ashtekar who invented them represent an unusual way to rewrite the metric on the three-dimensional In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges ^[15] The resulting candidate for a theory of quantum gravity is Loop quantum gravity, in which space is represented by a network structure called a spin network, evolving over time in discrete steps. Loop quantum gravity (LQG, also known as loop gravity and Quantum geometry, is a proposed quantum theory of Spacetime which attempts to reconcile the theories In Physics, a spin network is a type of diagram which can be used to represent states and interactions between particles and fields in Quantum physics ^[16]

Other candidates

There are a number of other approaches to quantum gravity. The approaches differ depending on which features of general relativity and quantum theory are accepted unchanged, and which features are modified^[17]. Examples include:

• Acoustic metric and other analog models of gravity
• An Exceptionally Simple Theory of Everything
• Causal Dynamical Triangulation
• Dynamical triangulations,^[18]
• Causal sets,^[19]
• The Omega Point and the quantum gravity Theory of Everything
• path-integral based models of quantum cosmology. In Mathematical physics, a metric describes the arrangement of relative distances within a surface or volume usually measured by signals passing through the region – essentially An Exceptionally Simple Theory of Everything is a Preprint proposing a basis for a Unified field theory, named E8 Theory, and has Causal dynamical triangulation (abbreviated as "CDT" invented by Renate Loll, Jan Ambjørn and Jerzy Jurkiewicz is an approach to Quantum The causal sets programme is an approach to Quantum gravity. Its founding principle is that Spacetime is fundamentally discrete and that the spacetime points are related The Omega Point is a term used by Tulane University professor of mathematics and physics Frank J In Theoretical physics, Quantum cosmology is a field attempting to study the effect of Quantum mechanics on the creation of the universe or ^[20]
• Process physics
• Regge calculus
• Supergravity
• Twistor models^[21]

Weinberg-Witten theorem

There is a theorem in quantum field theory called the Weinberg-Witten theorem which places some constraints on theories of composite gravity/emergent gravity. In General relativity, Regge calculus is a formalism for producing simplicial approximations of spacetimes which are solutions to the Einstein field equation In Theoretical physics, supergravity ( supergravity theory) is a field theory that combines the principles of Supersymmetry and General relativity The twistor theory, originally developed by Roger Penrose in 1967, is the mathematical theory which maps the Geometric objects of the four dimensional space-time In quantum field theory (QFT the forces between particles are mediated by other particles Steven Weinberg and Edward Witten consider the so-called emergent theories to be misguided In Theoretical physics, composite gravity refers to models that attempted to derive General relativity in a framework where the Graviton is constructed Induced gravity (or Emergent gravity) is an idea in Quantum gravity that space-time background emerges asa Mean field approximation of underlying microscopic

In popular culture

• The famous spoof of postmodernism by Alan Sokal (see Sokal Affair) was entitled Transgressing the Boundaries: Toward a Transformative Hermeneutics of Quantum Gravity. Postmodernism literally means 'after the modernist movement' While " Modern " itself refers to something "related to the present" the movement of modernism Alan David Sokal (born 1955) is a professor of Physics and faculty member of the physics department at New York University. The Sokal affair (also Sokal's hoax) was a Hoax by physicist Alan Sokal perpetrated on the editorial staff and readership of the Postmodern
• Quantum gravity is said to be the reason behind the dual realities in the anime film The Place Promised in Our Early Days. (anime in Japanese, is a 90 minute Japanese Anime film created and directed by Makoto Shinkai, following his previous work Voices of a Distant Star.
• Hard science fiction author Greg Egan proposed in his short story The Planck Dive a concept of quantum gravity in which spacetime itself is a function of networks of all possible world lines and gravity is merely the alteration of probabilities associated with individual worldlines according the increased or decreased incidences of quantum phase shift due to time dilation which is a result of interactions with virtual particles. Hard science fiction is a category of Science fiction characterized by an emphasis on scientific or technical detail or on scientific accuracy or on both Greg Egan (born 20 August 1961) is an Australian science fiction author. SpaceTime is a patent-pending three dimensional graphical user interface that allows end users to search their content such as Google Google Images Yahoo! YouTube eBay Amazon and RSS In physics the world line of an object is the unique path of that object as it travels through 4- Dimensional Spacetime. The phase of an oscillation or wave is the fraction of a complete cycle corresponding to an offset in the displacement from a specified reference point at time t = 0 This article discusses a concept in physics For the concept in sociology see Time displacement. In Physics, a virtual particle is a particle that exists for a limited time and space introducing uncertainty in their energy and momentum due to the Heisenberg Uncertainty

See also

Notes

References

• Ashtekar, Abhay (2007), Loop Quantum Gravity: Four Recent Advances and a Dozen Frequently Asked Questions, arXiv:0705.2222 
• Ashtekar, Abhay & Lewandowski, Jerzy (2004), “Background Independent Quantum Gravity: A Status Report”, Class. The arXiv ( pronounced " Archive " as if the "X" were the Greek letter Chi, χ is an Archive for electronic Quant. Grav. 21: R53-R152, arXiv:gr-qc/0404018 
• Ashtekar, Abhay (1986), “New variables for classical and quantum gravity”, Phys. The arXiv ( pronounced " Archive " as if the "X" were the Greek letter Chi, χ is an Archive for electronic Rev. Lett. 57: 2244–2247, DOI 10. 1103/PhysRevLett. 57. 2244 
• Ashtekar, Abhay (1987), “New Hamiltonian formulation of general relativity”, Phys. Rev. D36: 1587–1602, DOI 10. 1103/PhysRevD. 36. 1587 
• Carlip, Steven (2001), “Quantum Gravity: a Progress Report”, Rept. Prog. Phys. 64: 885, arXiv:gr-qc/0108040 
• Donoghue, John F. The arXiv ( pronounced " Archive " as if the "X" were the Greek letter Chi, χ is an Archive for electronic (1995), “Introduction to the Effective Field Theory Description of Gravity”, in Cornet, Fernando, Effective Theories: Proceedings of the Advanced School, Almunecar, Spain, 26 June–1 July 1995, arXiv:gr-qc/9512024, ISBN 9810229089 
• Duff, Michael (1996), “M-Theory (the Theory Formerly Known as Strings)”, Int. The arXiv ( pronounced " Archive " as if the "X" were the Greek letter Chi, χ is an Archive for electronic J. Mod. Phys. A11: 5623–5642, arXiv:hep-th/9608117 
• Goroff, Marc H. The arXiv ( pronounced " Archive " as if the "X" were the Greek letter Chi, χ is an Archive for electronic & Sagnotti, Augusto (1985), “Quantum gravity at two loops”, Phys. Lett. 160B: 81–86 
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External links

• Quantum Gravity Perimeter Institute for Theoretical Physics
• Prima facie questions in quantum gravity Chris J. Steven Weinberg (born May 3, 1933) is an American Physicist, and Nobel laureate in Physics for his contributions with Abdus Salam Isham
• Quantum Gravity in Stanford Encyclopedia of Philosophy
• The shape of things to come New Scientist, July 30 2005
• Quantum gravity articles
• Quantum Gravity pages by Lee Smolin (no longer active, archived at http://web.archive.org/web/20060428001322/http://www.qgravity.org/)