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U teorijama kvantne gravitacije, graviton je hipotetički kvant gravitacije, elementarna čestica koja posreduje gravitacionu silu. Ne postoji potpuna teorije kvantnog polja gravitona usled nerešenog matematičkog problema vezanog za renormalizaciju u opštoj teoriji relativnosti. U teoriji struna, za koju se smatra da je konzistentna teorija kvantne gravitacije, graviton je bezmaseno stanje fundamentalne strune.

Graviton
KompozicijaElementarna čestica
StatistikeBoze-Ajnštajnova statistika
InterakcijeGravitacija
StatusHipotetičan
SimbolG[1]
AntičesticaSelf
Teorije1930-e[2]
Naziv se pripisuje Dmitriju Blokhintsevu i F. M. Galperinu u 1934. godini[3]
Masa0
Srednji poluživotStabilan
Naelektrisanje0 e
Spin2

Ako postoji, graviton očekuje se da je bez mase jer gravitacione sile deluju na veoma dugim opsezima i šire se brzinom svetlosti. 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 that gravitational interactions do. This result suggests that, if a massless spin-2 particle is discovered, it must be the graviton.[4]

Teorija

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.[5][6][7]

The term graviton was originally coined in 1934 by Soviet physicists Dmitrii Blokhintsev and F. Gal'perin.[3]

Gravitoni i renormalizacija

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. These infinite results cannot be removed because quantized general relativity is not perturbatively renormalizable, unlike quantum electrodynamics 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.

Upoređenje sa drigim silama

Like the force carriers of the other forces (see charged black hole), gravitation 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.[8] 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.[9] A theory of quantum gravity is needed in order to reconcile these differences.[10] Whether this theory should be background-independent is an open question. The answer to this question will determine our understanding of what specific role gravitation plays in the fate of the universe.[11]

Gravitoni u spekulativnim teorijama

Teorija struna predicts the existence of gravitons and their well-defined interactions. A graviton in perturbative string theory is a closed string in a very particular low-energy vibrational state. The scattering of gravitons in string theory can also be computed from the correlation functions in conformal field theory, as dictated by the AdS/CFT correspondence, or from matrix theory.

A feature of gravitons in string theory is that, as closed strings without endpoints, they would not be bound to branes and could move freely between them. If we live on a brane (as hypothesized by brane theories), this "leakage" of gravitons from the brane into higher-dimensional space could explain why gravitation is such a weak force, and gravitons from other branes adjacent to our own could provide a potential explanation for dark matter. However, if gravitons were to move completely freely between branes, this would dilute gravity too much, causing a violation of Newton's inverse-square law. To combat this, Lisa Randall found that a three-brane (such as ours) would have a gravitational pull of its own, preventing gravitons from drifting freely, possibly resulting in the diluted gravity we observe, while roughly maintaining Newton's inverse square law.[12] See brane cosmology.

A theory by Ahmed Farag Ali and Saurya Das adds quantum mechanical corrections (using Bohm trajectories) to general relativistic geodesics. If gravitons are given a small but non-zero mass, it could explain the cosmological constant without need for dark energy and solve the smallness problem.[13] The theory received an Honorable Mention in the 2014 Essay Competition of the Gravity Research Foundation for explaining the smallness of cosmological constant.[14] Also the theory received an Honorable Mention in the 2015 Essay Competition of the Gravity Research Foundation for naturally explaining the observed large-scale homogeneity and isotropy of the universe due to the proposed quantum corrections.[15]

Vidi još

Reference

  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. ^ а б Blokhintsev, D. I.; Gal'perin, F. M. (1934). „Гипотеза нейтрино и закон сохранения энергии” [Neutrino hypothesis and conservation of energy]. Pod Znamenem Marxisma (на језику: руски). 6: 147—157. 
  4. ^ 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. 
  5. ^ Feynman, R. P.; Morinigo, F. B.; Wagner, W. G.; Hatfield, B. (1995). Feynman Lectures on Gravitation. Addison-Wesley. ISBN 0-201-62734-5. 
  6. ^ Zee, A. (2003). Quantum Field Theory in a Nutshell. Princeton University Press. ISBN 0-691-01019-6. 
  7. ^ Randall, L. (2005). Warped Passages: Unraveling the Universe's Hidden Dimensions. Ecco Press. ISBN 0-06-053108-8. 
  8. ^ See the other articles on General relativity, Gravitational field, Gravitational wave, etc
  9. ^ Colosi, D.; et al. (2005). „Background independence in a nutshell: The dynamics of a tetrahedron”. Classical and Quantum Gravity. 22 (14): 2971—2989. Bibcode:2005CQGra..22.2971C. arXiv:gr-qc/0408079  . doi:10.1088/0264-9381/22/14/008. 
  10. ^ Witten, E. (1993). „Quantum Background Independence In String Theory”. arXiv:hep-th/9306122  . 
  11. ^ Smolin, L. (2005). „The case for background independence”. arXiv:hep-th/0507235  . 
  12. ^ Kaku, Michio (2006) Parallel Worlds – The science of alternative universes and our future in the Cosmos. Doubleday. pp. 218–221. ISBN 978-0385509862.
  13. ^ Ali, Ahmed Farag (2014). „Cosmology from quantum potential”. Physics Letters B. 741: 276—279. Bibcode:2015PhLB..741..276F. arXiv:1404.3093v3  . doi:10.1016/j.physletb.2014.12.057. 
  14. ^ Das, Saurya (2014). „Cosmic coincidence or graviton mass?”. International Journal of Modern Physics D. 23 (12): 1442017. Bibcode:2014IJMPD..2342017D. arXiv:1405.4011  . doi:10.1142/S0218271814420176. 
  15. ^ Das, Saurya (2015). „Bose–Einstein condensation as an alternative to inflation”. International Journal of Modern Physics D. 24 (12): 1544001—219. Bibcode:2015IJMPD..2444001D. arXiv:1509.02658  . doi:10.1142/S0218271815440010. 

Spoljašnje veze

 
Graviton na Vikimedijinoj ostavi.
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