Graviton

Last updated
Graviton
Composition Elementary particle
Statistics Bose–Einstein statistics
Familyspin-2 boson
Interactions Gravitation
Status Hypothetical
SymbolG [1]
Antiparticle Self
Theorized1930s [2]
The name is attributed to Dmitrii Blokhintsev and F. M. Gal'perin in 1934 [3]
Mass 0
<6×10−32  eV/c2 [4]
Mean lifetime stable
Electric charge 0  e
Color charge No.
Spin 2  ħ

In theories of quantum gravity, the graviton is the hypothetical quantum of gravity, an elementary particle that mediates the force of gravitational interaction. There is no complete quantum field theory of gravitons due to an outstanding mathematical problem with renormalization in general relativity. In string theory, believed by some to be a consistent theory of quantum gravity, the graviton is a massless state of a fundamental string.

Contents

If it exists, the graviton is expected to be massless because the gravitational force has a very long range, and appears to propagate at the speed of light. The graviton must be a spin-2 boson because the source of gravitation is the stress–energy tensor, a second-order tensor (compared with electromagnetism's spin-1 photon, the source of which is the four-current, a first-order tensor). Additionally, it can be shown that any massless spin-2 field would give rise to a force indistinguishable from gravitation, because a massless spin-2 field would couple to the stress–energy tensor in the same way gravitational interactions do. This result suggests that, if a massless spin-2 particle is discovered, it must be the graviton. [5]

Theory

It is hypothesized that gravitational interactions are mediated by an as yet undiscovered elementary particle, dubbed the graviton. The three other known forces of nature are mediated by elementary particles: electromagnetism by the photon, the strong interaction by gluons, and the weak interaction by the W and Z bosons. All three of these forces appear to be accurately described by the Standard Model of particle physics. In the classical limit, a successful theory of gravitons would reduce to general relativity, which itself reduces to Newton's law of gravitation in the weak-field limit. [6] [7] [8]

History

General relativity models gravity as a curvature of spacetime akin to that of a two-dimensional plane, however lacks a basis for any form of quantum gravity One-sided spacetime curvatures.png
General relativity models gravity as a curvature of spacetime akin to that of a two-dimensional plane, however lacks a basis for any form of quantum gravity

The term graviton was originally coined in 1934 by Soviet physicists Dmitrii Blokhintsev  [ ru; de ] and F. M. Gal'perin. [3] Paul Dirac reintroduced the term in a number of lectures in 1959, noting that the energy of the gravitational field should come in quanta. [9] [10] A mediation of the gravitational interaction by particles was anticipated by Pierre-Simon Laplace. [11] Just like Newton's anticipation of photons, Laplace's anticipated "gravitons" had a greater speed than the speed of light in vacuum , the speed of gravitons expected in modern theories, and were not connected to quantum mechanics or special relativity, since these theories didn't yet exist during Laplace's lifetime.

Gravitons and renormalization

When describing graviton interactions, the classical theory of Feynman diagrams and semiclassical corrections such as one-loop diagrams behave normally. However, Feynman diagrams with at least two loops lead to ultraviolet divergences. [12] These infinite results cannot be removed because quantized general relativity is not perturbatively renormalizable, unlike quantum electrodynamics and models such as the Yang–Mills theory. Therefore, incalculable answers are found from the perturbation method by which physicists calculate the probability of a particle to emit or absorb gravitons, and the theory loses predictive veracity. Those problems and the complementary approximation framework are grounds to show that a theory more unified than quantized general relativity is required to describe the behavior near the Planck scale.

Comparison with other forces

Like the force carriers of the other forces (see photon, gluon, W and Z bosons), the graviton plays a role in general relativity, in defining the spacetime in which events take place. In some descriptions energy modifies the "shape" of spacetime itself, and gravity is a result of this shape, an idea which at first glance may appear hard to match with the idea of a force acting between particles. [13] Because the diffeomorphism invariance of the theory does not allow any particular space-time background to be singled out as the "true" space-time background, general relativity is said to be background-independent. In contrast, the Standard Model is not background-independent, with Minkowski space enjoying a special status as the fixed background space-time. [14] A theory of quantum gravity is needed in order to reconcile these differences. [15] Whether this theory should be background-independent is an open question. The answer to this question will determine the understanding of what specific role gravitation plays in the fate of the universe. [16]

Energy and wavelength

While gravitons are presumed to be massless, they would still carry energy, as does any other quantum particle. Photon energy and gluon energy are also carried by massless particles. It is unclear which variables might determine graviton energy, the amount of energy carried by a single graviton.

Alternatively, if gravitons are massive at all, the analysis of gravitational waves yielded a new upper bound on the mass of gravitons. The graviton's Compton wavelength is at least 1.6×1016  m , or about 1.6 light-years, corresponding to a graviton mass of no more than 7.7×10−23  eV/c 2. [17] This relation between wavelength and mass-energy is calculated with the Planck–Einstein relation, the same formula that relates electromagnetic wavelength to photon energy.

Experimental observation

Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, is impossible with any physically reasonable detector. [18] The reason is the extremely low cross section for the interaction of gravitons with matter. For example, a detector with the mass of Jupiter and 100% efficiency, placed in close orbit around a neutron star, would only be expected to observe one graviton every 10 years, even under the most favorable conditions. It would be impossible to discriminate these events from the background of neutrinos, since the dimensions of the required neutrino shield would ensure collapse into a black hole. [18]

LIGO and Virgo collaborations' observations have directly detected gravitational waves. [19] [20] [21] Others have postulated that graviton scattering yields gravitational waves as particle interactions yield coherent states. [22] Although these experiments cannot detect individual gravitons, they might provide information about certain properties of the graviton. [23] For example, if gravitational waves were observed to propagate slower than c (the speed of light in vacuum), that would imply that the graviton has mass (however, gravitational waves must propagate slower than c in a region with non-zero mass density if they are to be detectable). [24] Recent observations of gravitational waves have put an upper bound of 1.2×10−22 eV/c2 on the graviton's mass. [19] Astronomical observations of the kinematics of galaxies, especially the galaxy rotation problem and modified Newtonian dynamics, might point toward gravitons having non-zero mass. [25] [26]

Difficulties and outstanding issues

Most theories containing gravitons suffer from severe problems. Attempts to extend the Standard Model or other quantum field theories by adding gravitons run into serious theoretical difficulties at energies close to or above the Planck scale. This is because of infinities arising due to quantum effects; technically, gravitation is not renormalizable. Since classical general relativity and quantum mechanics seem to be incompatible at such energies, from a theoretical point of view, this situation is not tenable. One possible solution is to replace particles with strings. String theories are quantum theories of gravity in the sense that they reduce to classical general relativity plus field theory at low energies, but are fully quantum mechanical, contain a graviton, and are thought to be mathematically consistent. [27]

See also

Related Research Articles

In physics, the fundamental interactions or fundamental forces are the interactions that do not appear to be reducible to more basic interactions. There are four fundamental interactions known to exist:

<span class="mw-page-title-main">Elementary particle</span> Subatomic particle having no known substructure

In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. The Standard Model presently recognizes seventeen distinct particles—twelve fermions and five bosons. As a consequence of flavor and color combinations and antimatter, the fermions and bosons are known to have 48 and 13 variations, respectively. Among the 61 elementary particles embraced by the Standard Model number: electrons and other leptons, quarks, and the fundamental bosons. Subatomic particles such as protons or neutrons, which contain two or more elementary particles, are known as composite particles.

<span class="mw-page-title-main">Particle physics</span> Study of subatomic particles and forces

Particle physics or high-energy physics is the study of fundamental particles and forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the scale of protons and neutrons, while the study of combination of protons and neutrons is called nuclear physics.

A photon is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that always move at the speed of light when in vacuum. The photon belongs to the class of boson particles.

<span class="mw-page-title-main">Quantum field theory</span> Theoretical framework

In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines classical field theory, special relativity, and quantum mechanics. QFT is used in particle physics to construct physical models of subatomic particles and in condensed matter physics to construct models of quasiparticles.

<span class="mw-page-title-main">Quantum gravity</span> Description of gravity using discrete values

Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics. It deals with environments in which neither gravitational nor quantum effects can be ignored, such as in the vicinity of black holes or similar compact astrophysical objects, such as neutron stars as well as in the early stages of the universe moments after the Big Bang.

<span class="mw-page-title-main">Standard Model</span> Theory of forces and subatomic particles

The Standard Model of particle physics is the theory describing three of the four known fundamental forces in the universe and classifying all known elementary particles. It was developed in stages throughout the latter half of the 20th century, through the work of many scientists worldwide, with the current formulation being finalized in the mid-1970s upon experimental confirmation of the existence of quarks. Since then, proof of the top quark (1995), the tau neutrino (2000), and the Higgs boson (2012) have added further credence to the Standard Model. In addition, the Standard Model has predicted various properties of weak neutral currents and the W and Z bosons with great accuracy.

In quantum field theory, a force carrier is a type of particle that gives rise to forces between other particles. These particles serve as the quanta of a particular kind of physical field.

A virtual particle is a theoretical transient particle that exhibits some of the characteristics of an ordinary particle, while having its existence limited by the uncertainty principle, which allows the virtual particles to spontaneously emerge from vacuum at short time and space ranges. The concept of virtual particles arises in the perturbation theory of quantum field theory (QFT) where interactions between ordinary particles are described in terms of exchanges of virtual particles. A process involving virtual particles can be described by a schematic representation known as a Feynman diagram, in which virtual particles are represented by internal lines.

<span class="mw-page-title-main">Subatomic particle</span> Particle smaller than an atom

In physics, a subatomic particle is a particle smaller than an atom. According to the Standard Model of particle physics, a subatomic particle can be either a composite particle, which is composed of other particles, or an elementary particle, which is not composed of other particles. Particle physics and nuclear physics study these particles and how they interact. Most force carrying particles like photons or gluons are called bosons and, although they have discrete quanta of energy, do not have rest mass or discrete diameters and are unlike the former particles that have rest mass and cannot overlap or combine which are called fermions.

<span class="mw-page-title-main">Gauge boson</span> Elementary particles that are force carriers

In particle physics, a gauge boson is a bosonic elementary particle that acts as the force carrier for elementary fermions. Elementary particles whose interactions are described by a gauge theory interact with each other by the exchange of gauge bosons, usually as virtual particles.

In particle physics, a massless particle is an elementary particle whose invariant mass is zero. At present the only confirmed massless particle is the photon.

Fuzzballs are hypothetical objects in superstring theory, intended to provide a fully quantum description of the black holes predicted by general relativity.

<span class="mw-page-title-main">History of quantum field theory</span>

In particle physics, the history of quantum field theory starts with its creation by Paul Dirac, when he attempted to quantize the electromagnetic field in the late 1920s. Major advances in the theory were made in the 1940s and 1950s, leading to the introduction of renormalized quantum electrodynamics (QED). The field theory behind QED was so accurate and successful in predictions that efforts were made to apply the same basic concepts for the other forces of nature. Beginning in 1954, the parallel was found by way of gauge theory, leading by the late 1970s, to quantum field models of strong nuclear force and weak nuclear force, united in the modern Standard Model of particle physics.

This timeline lists significant discoveries in physics and the laws of nature, including experimental discoveries, theoretical proposals that were confirmed experimentally, and theories that have significantly influenced current thinking in modern physics. Such discoveries are often a multi-step, multi-person process. Multiple discovery sometimes occurs when multiple research groups discover the same phenomenon at about the same time, and scientific priority is often disputed. The listings below include some of the most significant people and ideas by date of publication or experiment.

<span class="mw-page-title-main">Gravitational wave</span> Propagating spacetime ripple

Gravitational waves are waves of the intensity of gravity that are generated by the accelerated masses of binary stars and other motions of gravitating masses, and propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1893 and then later by Henri Poincaré in 1905 as the gravitational equivalent of electromagnetic waves. Gravitational waves are sometimes called gravity waves, but gravity waves typically refer to displacement waves in fluids. In 1916 Albert Einstein demonstrated that gravitational waves result from his general theory of relativity as ripples in spacetime.

<span class="mw-page-title-main">Superfluid vacuum theory</span> Theory of fundamental physics

Superfluid vacuum theory (SVT), sometimes known as the BEC vacuum theory, is an approach in theoretical physics and quantum mechanics where the fundamental physical vacuum is considered as a superfluid or as a Bose–Einstein condensate (BEC).

In quantum field theory, a contact term is a radiatively induced point-like interaction. These typically occur when the vertex for the emission of a massless particle such as a photon, a graviton, or a gluon, is proportional to . This factor cancels the of the Feynman propagator, and causes the exchange of the massless particle to produce a point-like -function effective interaction, rather than the usual long-range potential. A notable example occurs in the weak interactions where a W-boson radiative correction to a gluon vertex produces a term, leading to what is known as a "penguin" interaction. The contact term then generates a correction to the full action of the theory.

The GRSI model is an attempt to explain astrophysical and cosmological observations without dark matter, dark energy or modifying the laws of gravity as they are currently established. This model is an alternative to Lambda-CDM, the standard model of cosmology.

References

  1. ↑ G is used to avoid confusion with gluons (symbol g)
  2. ↑ Rovelli, C. (2001). "Notes for a brief history of quantum gravity". arXiv: gr-qc/0006061 .
  3. 1 2 Blokhintsev, D. I.; Gal'perin, F. M. (1934). "Гипотеза нейтрино и закон сохранения энергии" [Neutrino hypothesis and conservation of energy]. Pod Znamenem Marxisma (in Russian). 6: 147–157. ISBN   978-5-04-008956-7.
  4. ↑ Zyla, P.; et al. (Particle Data Group) (2020). "Review of Particle Physics: Gauge and Higgs bosons" (PDF). Progress of Theoretical and Experimental Physics. Archived (PDF) from the original on 2020-09-30.
  5. ↑ For a comparison of the geometric derivation and the (non-geometric) spin-2 field derivation of general relativity, refer to box 18.1 (and also 17.2.5) of Misner, C. W.; Thorne, K. S.; Wheeler, J. A. (1973). Gravitation. W. H. Freeman. ISBN   0-7167-0344-0.
  6. ↑ Feynman, R. P.; Morinigo, F. B.; Wagner, W. G.; Hatfield, B. (1995). Feynman Lectures on Gravitation . Addison-Wesley. ISBN   0-201-62734-5.
  7. ↑ Zee, Anthony (2003). Quantum Field Theory in a Nutshell. Princeton, New Jersey: Princeton University Press. ISBN   0-691-01019-6.
  8. ↑ Randall, L. (2005). Warped Passages: Unraveling the Universe's Hidden Dimensions . Ecco Press. ISBN   0-06-053108-8.
  9. ↑ Farmelo, Graham (2009). The Strangest Man : The Hidden Life of Paul Dirac, Quantum Genius. Faber and Faber. pp. 367–368. ISBN   978-0-571-22278-0.
  10. ↑ Debnath, Lokenath (2013). "A short biography of Paul A. M. Dirac and historical development of Dirac delta function". International Journal of Mathematical Education in Science and Technology. 44 (8): 1201–1223. doi:10.1080/0020739X.2013.770091. ISSN   0020-739X.
  11. ↑ Zee, Anthony (2018-04-24). On Gravity: A Brief Tour of a Weighty Subject. Princeton, New Jersey: Princeton University Press. ISBN   978-0-691-17438-9.
  12. ↑ Zvi Berna; Huan-Hang Chib; Lance Dixonb; Alex Edisona. "Two-Loop Renormalization of Quantum Gravity Simplified" (PDF). www.slac.stanford.edu. Bhaumik Institute for Theoretical Physics – Department of Physics and Astronomy.
  13. ↑ See the other Wikipedia articles on general relativity, gravitational field, gravitational wave, etc.
  14. ↑ Colosi, D.; et al. (2005). "Background independence in a nutshell: The dynamics of a tetrahedron". Classical and Quantum Gravity . 22 (14): 2971–2989. arXiv: gr-qc/0408079 . Bibcode:2005CQGra..22.2971C. doi:10.1088/0264-9381/22/14/008. S2CID   17317614.
  15. ↑ Witten, E. (1993). "Quantum Background Independence In String Theory". arXiv: hep-th/9306122 .
  16. ↑ Smolin, L. (2005). "The case for background independence". arXiv: hep-th/0507235 .
  17. ↑ Abbott, B. P.; et al. (LIGO Scientific Collaboration and Virgo Collaboration) (1 June 2017). "GW170104: Observation of a 50-Solar-Mass Binary Black Hole Coalescence at Redshift 0.2". Physical Review Letters . 118 (22): 221101. arXiv: 1706.01812 . Bibcode:2017PhRvL.118v1101A. doi:10.1103/PhysRevLett.118.221101. PMID   28621973. S2CID   206291714.
  18. 1 2 Rothman, T.; Boughn, S. (2006). "Can Gravitons be Detected?". Foundations of Physics . 36 (12): 1801–1825. arXiv: gr-qc/0601043 . Bibcode:2006FoPh...36.1801R. doi:10.1007/s10701-006-9081-9. S2CID   14008778.
  19. 1 2 Abbott, B. P. et al. (LIGO Scientific Collaboration and Virgo Collaboration) (2016). "Observation of Gravitational Waves from a Binary Black Hole Merger". Physical Review Letters. 116 (6): 061102. arXiv: 1602.03837 . Bibcode:2016PhRvL.116f1102A. doi:10.1103/PhysRevLett.116.061102. PMID   26918975. S2CID   124959784.
  20. ↑ Castelvecchi, Davide; Witze, Witze (February 11, 2016). "Einstein's gravitational waves found at last". Nature News. doi:10.1038/nature.2016.19361. S2CID   182916902.
  21. ↑ "Gravitational waves detected 100 years after Einstein's prediction | NSF – National Science Foundation". www.nsf.gov. Retrieved 2016-02-11.
  22. ↑ Senatore, L.; Silverstein, E.; Zaldarriaga, M. (2014). "New sources of gravitational waves during inflation". Journal of Cosmology and Astroparticle Physics. 2014 (8): 016. arXiv: 1109.0542 . Bibcode:2014JCAP...08..016S. doi:10.1088/1475-7516/2014/08/016. S2CID   118619414.
  23. ↑ Dyson, Freeman (8 October 2013). "Is a Graviton Detectable?". International Journal of Modern Physics A . 28 (25): 1330041–1–1330035–14. Bibcode:2013IJMPA..2830041D. doi:10.1142/S0217751X1330041X.
  24. ↑ Will, C. M. (1998). "Bounding the mass of the graviton using gravitational-wave observations of inspiralling compact binaries" (PDF). Physical Review D . 57 (4): 2061–2068. arXiv: gr-qc/9709011 . Bibcode:1998PhRvD..57.2061W. doi:10.1103/PhysRevD.57.2061. S2CID   41690760. Archived (PDF) from the original on 2018-07-24.
  25. ↑ Trippe, Sascha (2012). "A Simplified Treatment of Gravitational Interaction on Galactic Scales". Journal of the Korean Astronomical Society. 46 (1): 41–47. arXiv: 1211.4692 . Bibcode:2013JKAS...46...41T. doi:10.5303/JKAS.2013.46.1.41.
  26. ↑ Platscher, Moritz; Smirnov, Juri; Meyer, Sven; Bartelmann, Matthias (2018). "Long range effects in gravity theories with Vainshtein screening". Journal of Cosmology and Astroparticle Physics. 2018 (12): 009. arXiv: 1809.05318 . Bibcode:2018JCAP...12..009P. doi:10.1088/1475-7516/2018/12/009. S2CID   86859475.
  27. ↑ Sokal, A. (July 22, 1996). "Don't Pull the String Yet on Superstring Theory". The New York Times . Retrieved March 26, 2010.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.