Graviton

Source: Wikipedia, the free encyclopedia.
This article is about the hypothetical particle. For other uses, see Graviton (disambiguation).
Graviton
Hypothetical
SymbolG[1]
AntiparticleSelf
Theorized1930s[2]
The name is attributed to Dmitrii Blokhintsev and F. M. Gal'perin in 1934[3]
Mass0
< 6×10−32 
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.

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

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

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

Planck scale
.

Comparison with other forces

Like the

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

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,

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

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

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. ^ a b 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. ISBN 0-7167-0344-0
    .
  6. ^ Feynman, R. P.; Morinigo, F. B.; Wagner, W. G.; Hatfield, B. (1995). Feynman Lectures on Gravitation.
    ISBN 0-201-62734-5
    .
  7. ^ Zee, Anthony (2003). Quantum Field Theory in a Nutshell. Princeton, New Jersey:
    ISBN 0-691-01019-6
    .
  8. ^ Randall, L. (2005). Warped Passages: Unraveling the Universe's Hidden Dimensions.
    ISBN 0-06-053108-8
    .
  9. ISBN 978-0-571-22278-0
    .
  10. ISSN 0020-739X
    .
  11. 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".
    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. S2CID 206291714
    .
  18. ^ a b Rothman, T.; Boughn, S. (2006). "Can Gravitons be Detected?".
    S2CID 14008778
    .
  19. ^
    S2CID 124959784
    .
  20. S2CID 182916902
    .
  21. ^ "Gravitational waves detected 100 years after Einstein's prediction | NSF – National Science Foundation". www.nsf.gov. Retrieved 2016-02-11.
  22. S2CID 118619414
    .
  23. doi:10.1142/S0217751X1330041X
    .
  24. ^ Will, C. M. (1998). "Bounding the mass of the graviton using gravitational-wave observations of inspiralling compact binaries" (PDF).
    S2CID 41690760. Archived
    (PDF) from the original on 2018-07-24.
  25. doi:10.5303/JKAS.2013.46.1.41
    .
  26. 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.

External links

Wikiversity has learning resources about Graviton
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