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Encyclopedia > Quantum gravity

Quantum gravity is the field of theoretical physics attempting to unify quantum mechanics, which describes three of the fundamental forces of nature, with general relativity, the theory of the fourth fundamental force: gravity. One ultimate goal hoped to emerge as a result of this is a unified framework for all fundamental forces—a "theory of everything", or "Grand Unified Theory" (GUT). Theoretical physics employs mathematical models and abstractions of physics, as opposed to experimental processes, in an attempt to understand nature. ... For a less technical and generally accessible introduction to the topic, see Introduction to quantum mechanics. ... A fundamental interaction or fundamental force is a mechanism by which particles interact with each other, and which cannot be explained in terms of another interaction. ... For a less technical and generally accessible introduction to the topic, see Introduction to general relativity. ... “Gravity” redirects here. ... This article or section is in need of attention from an expert on the subject. ... Grand unification, grand unified theory, or GUT is a theory in physics that unifies the strong interaction and electroweak interaction. ...

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?

Contents

Image File history File links No higher resolution available. ... This is a list of some of the unsolved problems in physics. ... For a less technical and generally accessible introduction to the topic, see Introduction to quantum mechanics. ... For a less technical and generally accessible introduction to the topic, see Introduction to general relativity. ... “Gravity” redirects here. ...

Overview

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. Quantum field theory depends on particle fields embedded in the flat space-time of special relativity. General relativity models gravity as a curvature within space-time that changes as a gravitational mass moves. 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 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 don't converge, the interactions sometimes evaluate to infinite results, but those are few enough in number to be removable via renormalization. Quantum field theory (QFT) is the quantum theory of fields. ... For a less technical and generally accessible introduction to the topic, see Introduction to special relativity. ... For a less technical and generally accessible introduction to the topic, see Introduction to general relativity. ... For other uses of this term, see Spacetime (disambiguation). ... Figure 1. ... Quantum electrodynamics (QED) is a relativistic quantum field theory of electrodynamics. ...


In recent decades, however, this antiquated understanding of renormalization has given way to the modern idea of effective field theory. 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. 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 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. (By comparison, the Standard Model is expected to break down above its cutoff at the much smaller TeV scale.) Figure 1. ... In physics, an effective field theory is an approximate theory (usually a quantum field theory) that contains the appropriate degrees of freedom to describe physical phenomena occurring at a chosen length scale, but ignores the substructure and the degrees of freedom at shorter distances (or, equivalently, higher energies). ... In particle physics, pion (short for pi meson) is the collective name for three subatomic particles: π0, π+ and π−. Pions are the lightest mesons and play an important role in explaining low-energy properties of the strong nuclear force. ... The Standard Model of Fundamental Particles and Interactions For the Standard Model in Cryptography, see Standard Model (cryptography). ... In physics, Planck units are physical units of measurement originally proposed by Max Planck. ... The electronvolt (symbol eV) is a unit of energy. ...


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. 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. In physics, spin refers to the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is the motion of its center of mass about an external point. ...


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 space time curvature in the latter case).


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. For other uses, see Black hole (disambiguation). ... For other uses, see Big Bang (disambiguation). ...


The "incompatibility" of quantum mechanics and general relativity

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. 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). 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 particle, EXCEPT gravity, leading researchers to believe that at least one most likely does exist, which they have named, the graviton. For a less technical and generally accessible introduction to the topic, see Introduction to general relativity. ... STARS can mean: Shock Trauma Air Rescue Society Special Tactics And Rescue Service, a fictional task force that appears in Capcoms Resident Evil video game franchise. ... A planet (from the Greek πλανήτης, planetes or wanderers) is a body of considerable mass that orbits a star and that produces very little or no energy through nuclear fusion. ... This article is about a celestial body. ... A fundamental interaction is a mechanism by which particles interact with each other, and which cannot be explained by another more fundamental interaction. ... The general meaning of atomic is irreducible. That is, reduced to the smallest possible part. ... In mathematics, theory is used informally to refer to a body of knowledge about mathematics. ... In physics, the graviton is a hypothetical elementary particle that transmits the force of gravity in most quantum gravity systems. ... The word theory has a number of distinct meanings in different fields of knowledge, depending on their methodologies and the context of discussion. ... A fundamental interaction is a mechanism by which particles interact with each other, and which cannot be explained by another more fundamental interaction. ... Messenger particles are sub-atomic particles that are exchanged between matter and are responsible for force, (i. ... In physics, the graviton is mainly still considered to be a hypothetical elementary particle that mediates the force of gravity in the framework of quantum field theory. ...


If the graviton turns out not to exist, it will render all work based on quantized macroscopic physics flawed, and destroy virtually all the accepted notions of a unified theory of physics since the 1970's, including String theory, Superstring theory, M-theory, Loop Quantum Gravity, and Quantum Gravity, among others. In an attempt to prove that modern physics is on the right track, CERN has promised to dedicate a large timeshare to search for the graviton using the Large Hadron Collider, the world's largest particle accelerator and collider, which is scheduled to complete construction in May 2008. Interaction in the subatomic world: world lines of pointlike particles in the Standard Model or a world sheet swept up by closed strings in string theory String theory is a model of fundamental physics, whose building blocks are one-dimensional extended objects called strings, rather than the zero-dimensional point... Superstring theory is an attempt to explain all of the particles and fundamental forces of nature in one theory by modeling them as vibrations of tiny supersymmetric strings. ... M-theory is a solution proposed for the unknown theory of everything which would combine all five superstring theories and 11-dimensional supergravity together. ... Loop quantum gravity (LQG), also known as loop gravity and quantum geometry, is a proposed quantum theory of spacetime which attempts to reconcile the seemingly incompatible theories of quantum mechanics and general relativity. ... This article does not cite any references or sources. ... CERN logo The European Organization for Nuclear Research (French: ), commonly known as CERN (see Naming), pronounced (or in French), is the worlds largest particle physics laboratory, situated just northwest of Geneva on the border between France and Switzerland. ... The Large Hadron Collider (LHC) is a particle accelerator and collider located at CERN, near Geneva, Switzerland (). Currently under construction, the LHC is scheduled to begin operation in May 2008. ...


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. 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. In physics, an effective field theory is an approximate theory (usually a quantum field theory) that contains the appropriate degrees of freedom to describe physical phenomena occurring at a chosen length scale, but ignores the substructure and the degrees of freedom at shorter distances (or, equivalently, higher energies). ...


Historically, many believed that general relativity was in fact fundamentally inconsistent with quantum mechanics. General relativity, like electromagnetism, is a classical field theory. One might expect that, as with electromagnetism, there should be a corresponding quantum field theory. However, gravity is nonrenormalizable. For a quantum field theory to be well-defined according to this now-outdated understanding of the subject, it must be asymptotically free or asymptotically safe. 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. 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. 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. A classical field theory is a physical theory that describes the study of how one or more physical fields interact with matter. ... Quantum field theory (QFT) is the quantum theory of fields. ... In physics, the adjective renormalizable refers to a theory (usually a quantum field theory) in which all ultraviolet divergences, infinities and other seemingly meaningless results can be cured by the process of renormalization. ... In physics, asymptotic freedom is the property of some gauge theories in which the interaction between the particles, such as quarks, becomes arbitrarily weak at ever shorter distances, i. ... A theory has a UV fixed point if its renormalization group flow runs towards a fixed point in the ultraviolet limit. ... Quantum electrodynamics (QED) is a relativistic quantum field theory of electrodynamics. ... In theoretical physics, renormalization group (RG) refers to a set of techniques and concepts related to the change of physics with the observation scale. ... For a less technical and generally accessible introduction to the topic, see Introduction to general relativity. ...


However, from the perspective of effective field theory, one sees that all but the first few such parameters are suppressed by huge energy scales and hence can be neglected when computing low-energy effects. Thus, at least in the low-energy regime, the model is indeed a predictive quantum field theory[2]. (A very similar situation occurs for the very similar effective field theory of low-energy pions.) Furthermore, most 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. In physics, an effective field theory is an approximate theory (usually a quantum field theory) that contains the appropriate degrees of freedom to describe physical phenomena occurring at a chosen length scale, but ignores the substructure and the degrees of freedom at shorter distances (or, equivalently, higher energies). ...


However, 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. 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. 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. Perturbation theory comprises mathematical methods that are used to find an approximate solution to a problem which cannot be solved exactly, by starting from the exact solution of a related problem. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... In mathematics and physics, a non-perturbative function or process is one that cannot be described by perturbation theory. ... Interaction in the subatomic world: world lines of pointlike particles in the Standard Model or a world sheet swept up by closed strings in string theory String theory is a model of fundamental physics, whose building blocks are one-dimensional extended objects called strings, rather than the zero-dimensional point...


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. 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, in which the only physically relevant information is the relationship between different events in space-time. It has been suggested that this article or section be merged with Classical mechanics. ... For a less technical and generally accessible introduction to the topic, see Introduction to special relativity. ... This article is about relational theory in physics. ...


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. In relativistic quantum field theory, just as in classical field theory, Minkowski spacetime is the fixed background of the theory. Finally, 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. 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. 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. Look up time in Wiktionary, the free dictionary. ... In physics and mathematics, Minkowski space (or Minkowski spacetime) is the mathematical setting in which Einsteins theory of special relativity is most conveniently formulated. ... Interaction in the subatomic world: world lines of pointlike particles in the Standard Model or a world sheet swept up by closed strings in string theory String theory is a model of fundamental physics, whose building blocks are one-dimensional extended objects called strings, rather than the zero-dimensional point... This article refers to a particle physics phenomenon. ... In physics, the graviton is mainly still considered to be a hypothetical elementary particle that mediates the force of gravity in the framework of quantum field theory. ... In physics, the AdS/CFT correspondence is the equivalence between a string theory or supergravity defined on some sort of Anti de Sitter space and a conformal field theory defined on its boundary whose dimension is lower by one. ...


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 particular, the vacuum, when it exists, is shown to depend on the path of the observer through space-time (see Unruh effect). 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. Quantum field theory (QFT) is the quantum theory of fields. ... The Unruh effect, discovered in 1976 by Bill Unruh of the University of British Columbia, is the prediction that an accelerating observer will observe black-body radiation where an inertial observer would observe none, that is, the accelerating observer will find themselves in a warm background. ...


Loop quantum gravity is the fruit of an effort to formulate a background-independent quantum theory. 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. 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. Loop quantum gravity (LQG), also known as loop gravity and quantum geometry, is a proposed quantum theory of spacetime which attempts to reconcile the seemingly incompatible theories of quantum mechanics and general relativity. ... Background independence is a condition in theoretical physics, especially in quantum gravity, that requires the defining equations of a theory to be independent of the actual shape of the spacetime and the value of various fields within the spacetime. ... A topological quantum field theory (or topological field theory or TQFT) is a quantum field theory which computes topological invariants. ... A spin network is a graph whose edges are associated with representations of a Lie group, G and vertices are associated with intertwiners of the edge reps adjacent to it. ...


There are two other points of tension between quantum mechanics and general relativity. First, general relativity predicts its own breakdown at singularities, and quantum mechanics becomes inconsistent with general relativity in a neighborhood of singularities (however, no one is certain that classical general relativity should necessarily be trusted near singularities in the first place). Second, it is not clear how to determine the gravitational field of a particle, if under the Heisenberg uncertainty principle of quantum mechanics its location and velocity cannot be known with certainty. The resolution of these points may come from a better understanding of general relativity [3]. In quantum physics, the Heisenberg uncertainty principle, sometimes called the Heisenberg indeterminacy principle, expresses a limitation on accuracy of (nearly) simultaneous measurement of observables such as the position and the momentum of a particle. ...


Theories

There are a number of proposed quantum gravity theories:

Interaction in the subatomic world: world lines of pointlike particles in the Standard Model or a world sheet swept up by closed strings in string theory String theory is a model of fundamental physics, whose building blocks are one-dimensional extended objects called strings, rather than the zero-dimensional point... Superstring theory is an attempt to explain all of the particles and fundamental forces of nature in one theory by modeling them as vibrations of tiny supersymmetric strings. ... M-theory is a solution proposed for the unknown theory of everything which would combine all five superstring theories and 11-dimensional supergravity together. ... In theoretical physics, supergravity (supergravity theory) refers to a field theory which combines the two theories of supersymmetry and general relativity. ... In physics, the AdS/CFT correspondence (anti-de-Sitter space/conformal field theory correspondence), sometimes called the Maldacena duality, is the conjectured equivalence between a string theory defined on one space, and a quantum field theory without gravity defined on the conformal boundary of this space, whose dimension is lower... In theoretical physics, the Wheeler-deWitt equation is an equation that a wave function of the Universe should satisfy in a theory of quantum gravity. ... Loop quantum gravity (LQG), also known as loop gravity and quantum geometry, is a proposed quantum theory of spacetime which attempts to reconcile the seemingly incompatible theories of quantum mechanics and general relativity. ... Euclidean quantum gravity refers to a Wick rotated version of quantum gravity, formulated as a quantum field theory. ... The causal sets programme is an approach to quantum gravity. ... 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 (Minkowski space) into the geometric objects in the 4-dimensional complex space with the metric signature (2,2). ... Sakharov proposed the idea of induced gravity as an alternative theory of quantum gravity. ... In general relativity, Regge calculus is a formalism for producing simplicial approximations of spacetimes which are solutions to the Einstein field equation. ... In mathematical physics, a metric (mathematics) describes the arrangement of relative distances within a surface or volume, usually measured by signals passing through the region – essentially describing the intrinsic geometry of the region. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Causal dynamical triangulation (abbreviated as CDT) invented by Renate Loll, Jan Ambjørn and Jerzy Jurkiewicz is an approach to quantum gravity that like loop quantum gravity is background independent. ...

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. Quantum field theory (QFT) is the quantum theory of fields. ... 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 as a composite bound state of more elementary particles, usually fermions. ... It has been suggested that this article or section be merged with induced gravity. ...


Quantum gravity theorists

See list of quantum gravity researchers This is a list of researchers in quantum gravity. ...


See also

A Centauro event is a kind of anomalous event observed in cosmic-ray detectors since 1972. ... In physics, Hawking radiation (also known as Bekenstein-Hawking radiation) is a thermal radiation thought to be emitted by black holes due to quantum effects. ... M-theory is a solution proposed for the unknown theory of everything which would combine all five superstring theories and 11-dimensional supergravity together. ... In physics, the AdS/CFT correspondence (anti-de-Sitter space/conformal field theory correspondence), sometimes called the Maldacena duality, is the conjectured equivalence between a string theory defined on one space, and a quantum field theory without gravity defined on the conformal boundary of this space, whose dimension is lower... To meet Wikipedias quality standards, this article or section may require cleanup. ... Quantum field theory in curved spacetimes is an extension of the standard quantum field theory to curved spacetimes. ... Semiclassical gravity is the approximation to the theory of quantum gravity in which one treats matter fields as being quantum and the gravitational field as being classical. ... Interaction in the subatomic world: world lines of pointlike particles in the Standard Model or a world sheet swept up by closed strings in string theory String theory is a model of fundamental physics, whose building blocks are one-dimensional extended objects called strings, rather than the zero-dimensional point... The Abraham-Lorentz force is the average force on an accelerating charged particle caused by the particle emitting electromagnetic radiation. ... Invariance mechanics, in its simplest form, is the rewriting of the laws of quantum field theory in terms of invariant quantities only. ... To meet Wikipedias quality standards, this article or section may require cleanup. ...

In popular culture

Postmodernism is a term applied to a wide-ranging set of developments in critical theory, philosophy, architecture, art, literature, and culture, which are generally characterized as either emerging from, in reaction to, or superseding, modernism. ... Alan David Sokal (born 1955) is a physicist at New York University. ... The Sokal Affair was a hoax by physicist Alan Sokal on the editorial staff and readership of a leading journal in the academic humanities. ... “Animé” redirects here. ... The Place Promised in Our Early Days , lit. ... 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 (August 20, 1961, Perth, Western Australia) is an Australian computer programmer and science fiction author. ... For other uses of this term, see Spacetime (disambiguation). ... World line of the orbit of the Earth depicted in two spatial dimensions X and Y (the plane of the Earth orbit) and a time dimension —usually put as the vertical axis. ... Waves with the same phase Waves with different phases The phase of a wave relates the position of a feature, typically a peak or a trough of the waveform, to that same feature in another part of the waveform (or, which amounts to the same, on a second waveform). ... Time dilation is the phenomenon whereby an observer finds that anothers clock which is physically identical to their own is ticking at a slower rate as measured by their own clock. ... In physics, a virtual particle is a particle which exists for such a short time and space that its energy and momentum do not have to obey the usual relationship. ...

References

External links

  • 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/)

  Results from FactBites:
 
Quantum gravity - definition of Quantum gravity in Encyclopedia (1357 words)
Quantum gravity is the field of theoretical physics attempting to unify the theory of quantum mechanics, which describes three of the fundamental forces of nature, with general relativity, the theory of the fourth fundamental force: gravity.
Quantum field theory on curved (non-Minkowskian) backgrounds, while not a quantum theory of gravity, has shown that some of the core assumptions of quantum field theory cannot be carried over to curved spacetime, let alone to full-blown quantum gravity.
Loop quantum gravity of Ashtekar, Smolin and Rovelli
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