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Graviton

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This article is about the hypothetical particle. For other uses, see Graviton (disambiguation).
Graviton
CompositionElementary particle
StatisticsBosonic
FamilyGauge boson
InteractionsGravitation
Statustheoretical
SymbolG[1]
AntiparticleSelf
Theorized1930s[2]
The name is attributed to Dmitrii Blokhintsev and F.M. Gal'perin in 1934[3]
Discoveredhypothetical
Mass0
Mean lifetimeStable
Electric charge0 e
Spin2

In physics, the graviton is a hypothetical elementary particle that mediates the force of gravitation in the framework of quantum field theory. If it exists, the graviton must be massless (because the gravitational force has unlimited range) and must have a spin of 2. This is because the source of gravitation is the stress-energy tensor, a second-rank tensor, compared to electromagnetism, the source of which is the four-current, a first-rank tensor. Additionally, it can be shown that any massless spin-2 field would be indistinguishable from gravitation, because a massless spin-2 field must couple to (interact with) the stress-energy tensor in the same way that the gravitational field does.[4] This result suggests that if a massless spin-2 particle is discovered, it must be the graviton, so that the only experimental verification needed for the graviton may simply be the discovery of a massless spin-2 particle.[5]

Gravitons are postulated because of the great success of quantum field theory (in particular, the Standard Model) at modeling the behavior of all other known forces of nature as being mediated by elementary particles: electromagnetism by the photon, the strong interaction by the gluons, and the weak interaction by the W and Z bosons. The hypothesis is that the gravitational interaction is likewise mediated by a – yet undiscovered – elementary particle, dubbed the graviton. In the classical limit, the theory would reduce to general relativity and conform to Newton's law of gravitation in the weak-field limit.[6][7][8]

However, attempts to extend the Standard Model with gravitons have run into serious theoretical difficulties at high energies (processes with energies close to or above the Planck scale) because of infinities arising due to quantum effects (in technical terms, gravitation is nonrenormalizable). Since classical general relativity and quantum mechanics are incompatible at such energies, from a theoretical point of view the present situation is not tenable.[9] Some proposed models of quantum gravity[10] attempt to address these issues, but these are speculative theories.

Gravitons and renormalization

When describing graviton interactions, the classical theory (i.e. the tree diagrams) and semiclassical corrections (one-loop diagrams) behave normally, but Feynman diagrams with two (or more) loops lead to ultraviolet divergences; that is, infinite results that cannot be removed because the quantized general relativity is not renormalizable, unlike quantum electrodynamics. That is, the usual ways physicists calculate the probability that a particle will emit or absorb a graviton give nonsensical answers and the theory loses its predictive power. These problems, together with some conceptual puzzles, led many physicists to believe that a theory more complete than just general relativity must regulate the behavior near the Planck scale.

Experimental observation

Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, is impossible with any physically reasonable detector.[11] 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.[11]

However, experiments to detect gravitational waves, which may be viewed as coherent states of many gravitons, are underway (e.g. LIGO and VIRGO). Although these experiments cannot detect individual gravitons, they might provide information about certain properties of the graviton. For example, if gravitational waves were observed to propagate slower than c (the speed of light in a vacuum), that would imply that the graviton has mass.[12]

Comparison with other forces

Unlike the force carriers of the other forces, gravitation plays a special role in general relativity in defining the spacetime in which events take place. Because it does not depend on a particular space-time background, general relativity is said to be background independent. In contrast, the Standard Model is not background independent.[13] A theory of quantum gravity is needed in order to reconcile these differences.[14] Whether this theory should be background independent or not is an open question. The answer to this question will determine if gravitation plays a special role in the universe.[15]

Gravitons in speculative theories

String theory 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.

An interesting 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 some theorists) 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. See brane cosmology.

In the Star Trek universe, Federation ships have a graviton field generator, or simply graviton generator—a piece of equipment utilized in such technologies as tractor beams, shields, and warp drives. The Borg also use gravimetric torpedoes, which utilize the force of graviton particles to tear ships apart.

In the show Quantum Leap (1989–1993), the accelerator into which physicist Sam Beckett steps accelerates supercharged gravitons which then emanates energy and thus mass gravitational fields that then are able to warp space and time into interconnecting loops. These particles then allow displacement into past or future, however because we are embodied by fermions, whatever energy (mass) is hyper accelerated into time will then be replaced by whoever it is Sam leaps into (i.e. Pauli Exclusion Principle).

In the Dead Space (series), graviton manipulation is a key technology that allows corporations to mount planetary mining operations by literally lifting sections of a planet's crust into the sky.

In the RTS Starcraft II, one can research Graviton Catapults to launch Interceptors from Carriers at a faster rate. Additionally the Phoenix unit has an ability called Graviton Beam, a spell that lifts ground enemies into the air.

In the FOX television show, Fringe, the alternate opening at the start of Season 4 lists the graviton among its other fringe science topics.

See also

References

  1. ^ G is used to avoid confusion with gluons (symbol g)
  2. ^ Rovelli, C. (2000). Notes for a brief history of quantum gravity. 9th Marcel Grossmann Meeting in Roma. p. p.5. arXiv:gr-qc/0006061. {{cite conference}}: |page= has extra text (help); Unknown parameter |month= ignored (help)
  3. ^ Blokhintsev, D.I.; Gal'perin, F.M. (1934). "Gipoteza neitrino i zakon sokhraneniya energii". Pod Znamenem Marxisma (in Russian). 6: 147–157. {{cite journal}}: Unknown parameter |trans_title= ignored (|trans-title= suggested) (help)CS1 maint: multiple names: authors list (link)
  4. ^ Lightman, Alan P. (1975). "Problem 12.16". Problem book in Relativity and Gravitation. Princeton University Press. ISBN 0-691-08162-X. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  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 Gravitation by Misner et. al.: Misner, Charles W.; Thorne, Kip. S.; Wheeler, John A. (1973). Gravitation. W. H. Freeman. ISBN 0-7167-0344-0.
  6. ^ Feynman, R. P. (1995). Feynman lectures on gravitation. Addison-Wesley. ISBN 0201627345. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  7. ^ Zee, A. (2003). Quantum Field Theory in a Nutshell. Princeton University Press. ISBN 0-691-01019-6.
  8. ^ Randall, Lisa (2005). Warped Passages: Unraveling the Universe's Hidden Dimensions. Ecco. ISBN 0-06-053108-8.
  9. ^ Alan Sokal (July 22, 1996). "Don't Pull the String Yet on Superstring Theory". The New York Times. Retrieved March 26, 2010. {{cite news}}: Italic or bold markup not allowed in: |publisher= (help)
  10. ^ Roger Penrose (1975). "The Non-Linear Graviton".
  11. ^ a b Rothman, Tony (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. {{cite journal}}: |access-date= requires |url= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  12. ^ Will, Clifford M. (1998). "Bounding the mass of the graviton using gravitational-wave observations of inspiralling compact binaries". Physical Review D. 57 (4): 2061–2068. arXiv:gr-qc/9709011. Bibcode:1998PhRvD..57.2061W. doi:10.1103/PhysRevD.57.2061. {{cite journal}}: Unknown parameter |month= ignored (help)
  13. ^ C. Rovelli et al., Background independence in a nutshell, Class.Quant.Grav. 22 (2005) 2971-2990, gr-qc/0408079
  14. ^ Edward Witten, Quantum Background Independence In String Theory, hep-th/9306122
  15. ^ L. Smolin, The case for background independence, hep-th/0507235
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