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66-587: [REDACTED] Look up q.g. in Wiktionary, the free dictionary. QG may refer to: Science and technology [ edit ] Quantum gravity , a theory in physics that aims to unify general relativity and quantum mechanics Quasigeostrophic, an atmospheric dynamics theory; see Geostrophic wind Quadrature Generator; see quadrature amplitude modulation Nissan QG engine , an automotive engine series ATCvet code QG , Genito-urinary system and sex hormones ,

132-449: A gravitational force. Essentially, a non-zero vacuum energy is expected to contribute to the cosmological constant , which affects the expansion of the universe . The field strength of vacuum energy is a concept proposed in a theoretical study that explores the nature of the vacuum and its relationship to gravitational interactions. The study derived a mathematical framework that uses the field strength of vacuum energy as an indicator of

198-468: A curved background (the Unruh effect occurs even in flat Minkowskian backgrounds). The vacuum state is the state with the least energy (and may or may not contain particles). A conceptual difficulty in combining quantum mechanics with general relativity arises from the contrasting role of time within these two frameworks. In quantum theories, time acts as an independent background through which states evolve, with

264-454: A definitive conclusion regarding the value of G. This lack of consensus has puzzled scientists and called for alternative explanations. To test the theoretical predictions regarding the field strength of vacuum energy, specific experimental conditions involving the position of the moon are recommended in the theoretical study. These conditions aim to achieve consistent outcomes in precision measurements of G. The ultimate goal of such experiments

330-422: A field in spacetime: they represent directly quantum states of spacetime. The theory is based on the reformulation of general relativity known as Ashtekar variables , which represent geometric gravity using mathematical analogues of electric and magnetic fields . In the quantum theory, space is represented by a network structure called a spin network, evolving over time in discrete steps. The dynamics of

396-477: A fixed spacetime background, although the interactions among closed strings give rise to space-time in a dynamic way. 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

462-465: 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. It

528-439: A number of other approaches to quantum gravity. The theories differ depending on which features of general relativity and quantum theory are accepted unchanged, and which features are modified. Examples include: As was emphasized above, quantum gravitational effects are extremely weak and therefore difficult to test. For this reason, the possibility of experimentally testing quantum gravity had not received much attention prior to

594-400: A reliable answer is difficult, pursued in the asymptotic safety program . Another possibility is that there are new, undiscovered 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. In an effective field theory , not all but

660-451: A scale far smaller, and hence only accessible with far higher energies, than those currently available in high energy particle accelerators . Therefore, physicists lack experimental data which could distinguish between the competing theories which have been proposed. Thought experiment approaches have been suggested as a testing tool for quantum gravity theories. In the field of quantum gravity there are several open questions – e.g., it

726-603: A section of the Anatomical Therapeutic Chemical Classification System for veterinary medicinal products quod google (q.g.) , an internet variation on Latin reference quod vide to indicate further information available Other uses [ edit ] Quetta Gladiators , a cricket team franchise in Pakistan Super League Queen's Gambit , a chess opening move Citilink , an IATA airline code Topics referred to by

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792-462: A simple harmonic oscillator requires the lowest possible energy, or zero-point energy of such an oscillator to be E = 1 2 ℏ ω   {\displaystyle {E}={\tfrac {1}{2}}\hbar \omega \ } Summing over all possible oscillators at all points in space gives an infinite quantity. To remove this infinity, one may argue that only differences in energy are physically measurable, much as

858-588: A verification of the existence of vacuum energy; in 1955, however, Evgeny Lifshitz offered a different origin for the Casimir effect. In 1957, Lee and Yang proved the concepts of broken symmetry and parity violation , for which they won the Nobel prize. In 1973, Edward Tryon proposed the zero-energy universe hypothesis: that the Universe may be a large-scale quantum-mechanical vacuum fluctuation where positive mass –energy

924-459: A very large number (10 by some estimates) of consistent vacua, comprising the so-called " string landscape ". Sorting through this large family of solutions remains a major challenge. Loop quantum gravity seriously considers general relativity's insight that spacetime is a dynamical field and is therefore a quantum object. Its second idea is that the quantum discreteness that determines the particle-like behavior of other field theories (for instance,

990-458: Is balanced by negative gravitational potential energy . During the 1980s, there were many attempts to relate the fields that generate the vacuum energy to specific fields that were predicted by attempts at a Grand Unified Theory and to use observations of the Universe to confirm one or another version. However, the exact nature of the particles (or fields) that generate vacuum energy, with a density such as that required by inflation theory, remains

1056-426: Is based on Albert Einstein 's general theory of relativity , which incorporates his theory of special relativity and deeply modifies the understanding of concepts like time and space. Although general relativity is highly regarded for its elegance and accuracy, it has limitations: the gravitational singularities inside black holes , the ad hoc postulation of dark matter , as well as dark energy and its relation to

1122-428: Is called phenomenological quantum gravity . Much of the difficulty in meshing these theories at all energy scales comes from the different assumptions that these theories make on how the universe works. General relativity models gravity as curvature of spacetime : in the slogan of John Archibald Wheeler , "Spacetime tells matter how to move; matter tells spacetime how to curve." On the other hand, quantum field theory

1188-412: Is conceivable that, in the correct theory of quantum gravity, the infinitely many unknown parameters will reduce 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, finding

1254-421: Is dependent on the mass of the black hole (the equations indicate that the smaller the black hole, the more rapidly it evaporates) but could be on the order of 10 years for large solar-mass black holes. The vacuum energy also has important consequences for physical cosmology . General relativity predicts that energy is equivalent to mass, and therefore, if the vacuum energy is "really there", it should exert

1320-459: Is different from Wikidata All article disambiguation pages All disambiguation pages Quantum gravity Three of the four fundamental forces of nature are described within the framework of quantum mechanics and quantum field theory : the electromagnetic interaction , the strong force , and the weak force ; this leaves gravity as the only interaction that has not been fully accommodated. The current understanding of gravity

1386-435: Is fixed a priori , developing quantum field theory becomes more mathematically challenging, and many ideas physicists use in quantum field theory on flat spacetime are no longer applicable. It is widely hoped that a theory of quantum gravity would allow us to understand problems of very high energy and very small dimensions of space, such as the behavior of black holes , and the origin of the universe . One major obstacle

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1452-489: Is hope for this to change as future data from cosmological observations and particle physics experiments become available. The central idea of string theory is to replace the classical concept of a point particle in quantum field theory with a quantum theory of one-dimensional extended objects: string theory. At the energies reached in current experiments, these strings are indistinguishable from point-like particles, but, crucially, different modes of oscillation of one and

1518-452: 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 . Quantum field theory on curved (non-Minkowskian) backgrounds, while not a full quantum theory of gravity, has shown many promising early results. In an analogous way to

1584-423: Is known as the graviton . These particles act as a force particle similar to the photon of the electromagnetic interaction. Under mild assumptions, the structure of general relativity requires them to follow the quantum mechanical description of interacting theoretical spin-2 massless particles. Many of the accepted notions of a unified theory of physics since the 1970s assume, and to some degree depend upon,

1650-442: Is not known how spin of elementary particles sources gravity, and thought experiments could provide a pathway to explore possible resolutions to these questions, even in the absence of lab experiments or physical observations. In the early 21st century, new experiment designs and technologies have arisen which suggest that indirect approaches to testing quantum gravity may be feasible over the next few decades. This field of study

1716-461: Is quantized, and the quanta are the photons. In the case of gravity, the operators representing the area and the volume of each surface or space region likewise have discrete spectra. Thus area and volume of any portion of space are also quantized, where the quanta are elementary quanta of space. It follows, then, that spacetime has an elementary quantum granular structure at the Planck scale, which cuts off

1782-649: Is that for quantum field theory in curved spacetime with a fixed metric, bosonic / fermionic operator fields supercommute for spacelike separated points . (This is a way of imposing a principle of locality .) However, in quantum gravity, the metric is dynamical, so that whether two points are spacelike separated depends on the state. In fact, they can be in a quantum superposition of being spacelike and not spacelike separated. The observation that all fundamental forces except gravity have one or more known messenger particles leads researchers to believe that at least one must exist for gravity. This hypothetical particle

1848-520: Is the well-known calculation of the tiny first-order quantum-mechanical correction to the classical Newtonian gravitational potential between two masses. An other example is the calculation of the corrections to the Bekenstein-Hawking entropy formula. A fundamental lesson of general relativity is that there is no fixed spacetime background, as found in Newtonian mechanics and special relativity ;

1914-501: Is to either falsify or provide confirmations to the proposed theoretical framework. The significance of exploring the field strength of vacuum energy lies in its potential to revolutionize our understanding of gravity and its interactions. In 1934, Georges Lemaître used an unusual perfect-fluid equation of state to interpret the cosmological constant as due to vacuum energy. In 1948, the Casimir effect provided an experimental method for

1980-417: Is typically formulated in the flat spacetime used in special relativity . No theory has yet proven successful in describing the general situation where the dynamics of matter, modeled with quantum mechanics, affect the curvature of spacetime. If one attempts to treat gravity as simply another quantum field, the resulting theory is not renormalizable . Even in the simpler case where the curvature of spacetime

2046-461: The Casimir effect , and the Lamb shift , and are thought to influence the behavior of the Universe on cosmological scales . Using the upper limit of the cosmological constant , the vacuum energy of free space has been estimated to be 10 joules (10 ergs ), or ~5 GeV per cubic meter. However, in quantum electrodynamics , consistency with the principle of Lorentz covariance and with the magnitude of

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2112-533: The Hamiltonian operator acting as the generator of infinitesimal translations of quantum states through time. In contrast, general relativity treats time as a dynamical variable which relates directly with matter and moreover requires the Hamiltonian constraint to vanish. Because this variability of time has been observed macroscopically , it removes any possibility of employing a fixed notion of time, similar to

2178-503: The Planck constant suggests a much larger value of 10 joules per cubic meter. This huge discrepancy is known as the cosmological constant problem or, colloquially, the "vacuum catastrophe." Quantum field theory states that all fundamental fields , such as the electromagnetic field , must be quantized at every point in space. A field in physics may be envisioned as if space were filled with interconnected vibrating balls and springs, and

2244-423: The cosmic microwave background (in particular its polarization), and decoherence induced by fluctuations in the space-time foam . The latter scenario has been searched for in light from gamma-ray bursts and both astrophysical and atmospheric neutrinos , placing limits on phenomenological quantum gravity parameters. ESA 's INTEGRAL satellite measured polarization of photons of different wavelengths and

2310-434: The cosmological constant are among the current unsolved mysteries regarding gravity, all of which signal the collapse of the general theory of relativity at different scales and highlight the need for a gravitational theory that goes into the quantum realm. At distances close to the Planck length , like those near the center of a black hole, quantum fluctuations of spacetime are expected to play an important role. Finally,

2376-435: The event horizon of a black hole has been hypothesized by physicist Stephen Hawking to be a mechanism for the eventual "evaporation" of black holes . If one of the pair is pulled into the black hole before this, then the other particle becomes "real" and energy/mass is essentially radiated into space from the black hole. This loss is cumulative and could result in the black hole's disappearance over time. The time required

2442-407: The bulk (spacetime) resistance to localized curvature. It illustrates the association of the field strength of vacuum energy to the curvature of the background, where this concept challenges the traditional understanding of gravity and suggests that the gravitational constant, G, may not be a universal constant, but rather a parameter dependent on the field strength of vacuum energy. Determination of

2508-495: The charge and mass of the electron, as measured at a particular energy scale. On the other hand, in quantizing gravity there are, in perturbation theory , infinitely many independent parameters (counterterm coefficients) needed to define the theory. For a given choice of those parameters, one could make sense of the theory, but since it is impossible to conduct infinite experiments to fix the values of every parameter, it has been argued that one does not, in perturbation theory, have

2574-429: The concept of potential energy has been treated in classical mechanics for centuries. This argument is the underpinning of the theory of renormalization . In all practical calculations, this is how the infinity is handled. Vacuum energy can also be thought of in terms of virtual particles (also known as vacuum fluctuations) which are created and destroyed out of the vacuum. These particles are always created out of

2640-430: The conception of time in quantum theory, at the macroscopic level. There are a number of proposed quantum gravity theories. 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

2706-499: The development of quantum electrodynamics in the early part of the 20th century (when physicists considered quantum mechanics in classical electromagnetic fields), the consideration of quantum field theory on a curved background has led to predictions such as black hole radiation. Phenomena such as the Unruh effect , in which particles exist in certain accelerating frames but not in stationary ones, do not pose any difficulty when considered on

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2772-475: The discrepancies between the predicted value for the vacuum energy and the observed values (which, depending on considerations, can be of 60 or 120 orders of magnitude) highlight the necessity for a quantum theory of gravity. The field of quantum gravity is actively developing, and theorists are exploring a variety of approaches to the problem of quantum gravity, the most popular being M-theory and loop quantum gravity . All of these approaches aim to describe

2838-454: The existence of the graviton. The Weinberg–Witten theorem places some constraints on theories in which the graviton is a composite particle . While gravitons are an important theoretical step in a quantum mechanical description of gravity, they are generally believed to be undetectable because they interact too weakly. General relativity, like electromagnetism , is a classical field theory . One might expect that, as with electromagnetism,

2904-561: 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 . 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

2970-457: The field be quantized at each point in space. Canonically, if the field at each point in space is a simple harmonic oscillator , its quantization places a quantum harmonic oscillator at each point. Excitations of the field correspond to the elementary particles of particle physics . Thus, according to the theory, even the vacuum has a vastly complex structure and all calculations of quantum field theory must be made in relation to this model of

3036-757: 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. Thus, at least in the low-energy regime, the model is a predictive quantum field theory. Furthermore, many theorists argue that the Standard Model should be regarded as an effective field theory itself, with "nonrenormalizable" interactions suppressed by large energy scales and whose effects have consequently not been observed experimentally. 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

3102-427: The gravitational field while it is kept separate from the other forces. Other lesser-known but no less important theories include causal dynamical triangulation , noncommutative geometry , and twistor theory . One of the difficulties of formulating a quantum gravity theory is that direct observation of quantum gravitational effects is thought to only appear at length scales near the Planck scale , around 10 meters,

3168-481: The gravitational force should also have a corresponding quantum field theory . However, gravity is perturbatively nonrenormalizable . For a quantum field theory to be well defined according to this 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

3234-541: The late 1990s. However, since the 2000s, physicists have realized that evidence for quantum gravitational effects can guide the development of the theory. Since theoretical development has been slow, the field of phenomenological quantum gravity , which studies the possibility of experimental tests, has obtained increased attention. The most widely pursued possibilities for quantum gravity phenomenology include gravitationally mediated entanglement, violations of Lorentz invariance , imprints of quantum gravitational effects in

3300-558: The other hand, quantum mechanics has depended since its inception on a fixed background (non-dynamic) 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. String theory can be seen as a generalization of quantum field theory where instead of point particles, string-like objects propagate in

3366-414: The photons of the electromagnetic field) also affects the structure of space. The main result of loop quantum gravity is the derivation of a granular structure of space at the Planck length. This is derived from the following considerations: In the case of electromagnetism, the quantum operator representing the energy of each frequency of the field has a discrete spectrum. Thus the energy of each frequency

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3432-411: The polarization was due to interstellar dust interference. Vacuum energy Vacuum energy is an underlying background energy that exists in space throughout the entire universe . The vacuum energy is a special case of zero-point energy that relates to the quantum vacuum . The effects of vacuum energy can be experimentally observed in various phenomena such as spontaneous emission ,

3498-480: The quantum behavior of the gravitational field , which does not necessarily include unifying all fundamental interactions into a single mathematical framework. However, many approaches to quantum gravity, such as string theory , try to develop a framework that describes all fundamental forces. Such a theory is often referred to as a theory of everything . Some of the approaches, such as loop quantum gravity, make no such attempt; instead, they make an effort to quantize

3564-402: The same renormalization problems. Additional contributions to the vacuum energy come from spontaneous symmetry breaking in quantum field theory . Vacuum energy has a number of consequences. In 1948, Dutch physicists Hendrik B. G. Casimir and Dirk Polder predicted the existence of a tiny attractive force between closely placed metal plates due to resonances in the vacuum energy in

3630-401: The same term [REDACTED] This disambiguation page lists articles associated with the title QG . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=QG&oldid=1139523053 " Category : Disambiguation pages Hidden categories: Short description

3696-433: 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. The theory is successful in that one mode will always correspond to a graviton , the messenger particle of gravity; however, the price of this success is unusual features such as six extra dimensions of space in addition to

3762-530: The space between them. This is now known as the Casimir effect and has since been extensively experimentally verified. It is therefore believed that the vacuum energy is "real" in the same sense that more familiar conceptual objects such as electrons, magnetic fields, etc., are real. However, alternative explanations for the Casimir effect have since been proposed. Other predictions are harder to verify. Vacuum fluctuations are always created as particle–antiparticle pairs. The creation of these virtual particles near

3828-416: The spacetime geometry is dynamic. While simple to grasp in principle, this is a complex 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 spacetime. On

3894-413: The strength of the field is like the displacement of a ball from its rest position. The theory requires "vibrations" in, or more accurately changes in the strength of, such a field to propagate as per the appropriate wave equation for the particular field in question. The second quantization of quantum field theory requires that each such ball–spring combination be quantized, that is, that the strength of

3960-510: The theory is today constructed in several versions. One version starts with the canonical quantization of general relativity. The analogue of the Schrödinger equation is a Wheeler–DeWitt equation , which can be defined within the theory. In the covariant, or spinfoam formulation of the theory, the quantum dynamics is obtained via a sum over discrete versions of spacetime, called spinfoams. These represent histories of spin networks. There are

4026-430: The ultraviolet infinities of quantum field theory. The quantum state of spacetime is described in the theory by means of a mathematical structure called spin networks . Spin networks were initially introduced by Roger Penrose in abstract form, and later shown by Carlo Rovelli and Lee Smolin to derive naturally from a non-perturbative quantization of general relativity. Spin networks do not represent quantum states of

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4092-444: The usual three for space and one for time. 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 form part of a hypothesized eleven-dimensional model known as M-theory , which would constitute a uniquely defined and consistent theory of quantum gravity. As presently understood, however, string theory admits

4158-412: The vacuum in particle– antiparticle pairs, which in most cases shortly annihilate each other and disappear. However, these particles and antiparticles may interact with others before disappearing, a process which can be mapped using Feynman diagrams . Note that this method of computing vacuum energy is mathematically equivalent to having a quantum harmonic oscillator at each point and, therefore, suffers

4224-426: The vacuum. The theory considers vacuum to implicitly have the same properties as a particle, such as spin or polarization in the case of light , energy, and so on. According to the theory, most of these properties cancel out on average leaving the vacuum empty in the literal sense of the word. One important exception, however, is the vacuum energy or the vacuum expectation value of the energy. The quantization of

4290-418: The value of G has been a topic of extensive research, with numerous experiments conducted over the years in an attempt to measure its precise value. These experiments, often employing high-precision techniques, have aimed to provide accurate measurements of G and establish a consensus on its exact value. However, the outcomes of these experiments have shown significant inconsistencies, making it difficult to reach

4356-437: Was able to place a limit in the granularity of space that is less than 10  m, or 13 orders of magnitude below the Planck scale. The BICEP2 experiment detected what was initially thought to be primordial B-mode polarization caused by gravitational waves in the early universe. Had the signal in fact been primordial in origin, it could have been an indication of quantum gravitational effects, but it soon transpired that

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