Graviton

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Graviton (G)

classification
Elementary particle
boson
gauge boson
properties
electric charge neutral
Dimensions 0  kg
Spin 2
average lifespan ∞
Interactions Gravity

The hypothetical gauge boson of a quantum theory of gravitation is called a graviton . According to this assumption, it is the carrier of the gravitational force.

designation

The name Graviton was chosen based on the photon of the electromagnetic interaction . It appeared for the first time in 1934 in an essay by Dmitri Iwanowitsch Blochinzew and F. M. Galperin, but it was only accepted when Paul Dirac suggested a name in 1959, who probably had no knowledge of Blochintsev's essay.

properties

Some of the properties of the graviton (speed of propagation, masslessness) also correspond to those of a photon.

The spin of the graviton is postulated to be . This happens due to the following considerations from quantum field theory :

In quantum electrodynamics  (QED), bosons with an even-numbered spin between the same charges have an attractive effect , while bosons with an odd-numbered spin between the same charges have a repulsive effect. So acts z. B. repulsing the photon with spin 1 between two electrons , each with a charge of −e. In analogy to this, one assumes in the case of gravitation that there are only particles with the same charge (in accordance with the experience that gravitation always has an attractive effect) and therefore postulates the graviton as a spin-2 particle.

Formally, this results from the fact that the source of the gravitational field is a symmetrical tensor of the 2nd level ( energy-momentum tensor with spin 2), while the source is e.g. B. in electromagnetism is a vector (spin 1). This was already established by Wolfgang Pauli and Markus Fierz in the 1930s.

Just as electromagnetic radiation is described by Maxwell's equations of classical electrodynamics , gravitational radiation results from Einstein's field equations of general relativity .

Gravitons are their own antiparticles . An anti-graviton would be the same as a graviton and has nothing to do with a hypothetical antigravity .

Supersymmetric properties

In supersymmetric models of quantum gravity , the ordinary graviton receives massive bosonic partners with spin 0 (graviskalar) and spin 1 (gravivector or graviphoton). Depending on their masses and thus on their ranges, these new particles could result in a change in the normal 1 / r 2 force law of gravitation .

Fermionic partners in these models, the gravitino (Super partner of the graviton) with spin 1½ and Goldstino with spin ½ (whose Super partner with the spin 0 Sgoldstino ).

Quantum gravity

Analogous to the quantization of the electromagnetic radiation in QED by photons, it was speculated early on that a corresponding quantization of the gravitational radiation by gravitons exists in a previously unknown theory of quantum gravity. However, this quantization is made more difficult by the fact that, in contrast to all other known radiations, gravitation cannot be shielded and acts on all masses, regardless of where they are in the universe. (This is a compelling consequence of the interpretation of gravity as the curvature of space-time .) Thus, objects that are far apart also attract each other when something is between them. Also, no smallest amount of gravitation can be detected, it apparently takes on arbitrarily small values, and even very light elementary particles are subject to it. However, this does not mean that there is no minimum amount of gravity is .

All previous attempts at a renormalizable quantum field theory of gravity have failed: The ultraviolet divergences of the theories could not be eliminated, not even by transitioning to the supersymmetrical formulation of supergravity , in which gravitino was also introduced, or by allowing more than three spatial dimensions. Stanley Deser brought these negative results to a certain conclusion in 1999 with references to the perturbation-theoretical non - renormalization of supergravity in the maximum permitted 11 dimensions (the theory already showed divergences with two loops). The cause of the non-renormalizability is ultimately due to the fact that the coupling constant is dimensional, as Werner Heisenberg pointed out in 1938.

In the case of the two so far purely hypothetical candidates of a theory of quantum gravity, string theory and loop quantum gravity, the existence of a graviton in the case of string theory inevitably results, the position in loop quantum gravity is less clear. Both theories have not yet been developed so far that they could be tested experimentally and possibly refuted. So the question of the existence of a particle that carries the gravitational force is still open.

Experimental observations

The possibility of detecting gravitons was controversial.

Freeman Dyson distinguished several aspects of this question, including the question of the detectability under the conditions (interference effects) in the present universe, the delimitation from theories in which gravity is only a statistical, emergent effect similar to entropy, and the delimitation of predictions from calculations , in which gravity enters as a classic field.

Although idealized thought experiments for the detection of gravitons have been proposed on various occasions, there is no physically reasonable detector for this . The reason lies in the extremely small cross-section for the interaction of gravitons with matter. If, for example, a detector with the mass of Jupiter and an efficiency of 100% were placed in a close orbit of a neutron star , a graviton would only be detected every 10 years.

The LIGO and Virgo observatories observed gravitational waves directly , but in principle cannot detect gravitons (Dyson). Others have postulated that graviton scattering produces gravitational waves, since particle interactions lead to coherent states .

Although these experiments cannot detect individual gravitons, they can provide information about certain properties of the graviton. If, for example, gravitational waves were observed slower than  c (the speed of light in a vacuum), it would mean that gravitons have a mass (however, gravitational waves must propagate more slowly than c in a region with a mass density greater than zero in  order for them to be detected at all) . More recent observations of gravitational waves have determined an upper limit of for the graviton mass corresponding to a limit for the Compton wavelength of the graviton of (1  light year ). Astronomical observations of the galaxy's movements, especially the rotation curve and Modified Newtonian Dynamics , could indicate that gravitons have a mass greater than zero. A non-zero mass of the graviton can not be explained by the Higgs mechanism .

That are developing nanotechnology measurement setups can be possibly used in the future to quantum gravity effects to measure directly.

Web links

Wiktionary: Graviton  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ Helge Kragh, Quantum Generations, Princeton University Press 1999, p. 411
  2. ↑ Chris C. King: Dual-Time Supercausality . In: Physics Essays 2/2 . 1989, p. 128–151 ( math.auckland.ac.nz [PDF]).
  3. ↑ Andrea Brignole, Ferruccio Feruglio and Fabio Zwirner: Four-fermion interactions and goldstino masses in models with a superlight gravitino . In: CERN-TH / 98-149, DFPD-98 / TH / 20 . August 31, 1998, p. 9 , arxiv : hep-ph / 9805282v2 .
  4. ↑ Deser, Infinities in Quantum Gravities, Annalen der Physik, Volume 9, 2000, pp. 299-307, Arxiv
  5. ^ A b T. Rothman, S. Boughn: Can Gravitons be Detected? . In: Foundations of Physics . 36, No. 12, 2006, pp. 1801-1825. arxiv : gr-qc / 0601043 . bibcode : 2006FoPh ... 36.1801R . doi : 10.1007 / s10701-006-9081-9 .
  6. ↑ B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration): Observation of Gravitational Waves from a Binary Black Hole Merger . In: Physical Review Letters . 116, No. 6, 2016. arxiv : 1602.03837 . bibcode : 2016PhRvL.116f1102A . doi : 10.1103 / PhysRevLett.116.061102 .
  7. ↑ Davide Castelvecchi, Jokes Jokes: Einstein's gravitational waves found at last . In: Nature News . February 11, 2016. doi : 10.1038 / nature.2016.19361 . Retrieved February 11, 2016.
  8. ↑ Gravitational waves detected 100 years after Einstein's prediction | NSF - National Science Foundation .
  9. ↑ Senatore, L., Silverstein, E., & Zaldarriaga, M. (2014). New sources of gravitational waves during inflation. Journal of Cosmology and Astroparticle Physics, 2014 (08), 016.
  10. ↑ Freeman Dyson: Is a graviton detectable? . In: International Journal of Modern Physics A . 28, No. 25, October 8, 2013, pp. 1330041-1-1330035-14. bibcode : 2013IJMPA..2830041D . doi : 10.1142 / S0217751X1330041X .
  11. ↑ CM Will: Bounding the mass of the graviton using gravitational-wave observations of inspiralling compact binaries . In: Physical Review D . 57, No. 4, 1998, pp. 2061-2068. arxiv : gr-qc / 9709011 . bibcode : 1998PhRvD..57.2061W . doi : 10.1103 / PhysRevD.57.2061 .
  12. ↑ B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration): GW170104: Observation of a 50-Solar-Mass Binary Black Hole Coalescence at Redshift 0.2 . In: Physical Review Letters . 118, No. 22, 2017. doi : 10.1103 / PhysRevLett.118.221101 .
  13. ↑ Trippe, S. (2013), “A Simplified Treatment of Gravitational Interaction on Galactic Scales”, J. Kor. Astron. Soc. 46 , 41. arxiv : 1211.4692
  14. ↑ Nima Arkani-Hamed et al .: Scattering Amplitudes For All Masses and Spins . 2017, arxiv : 1709.04891 .
  15. ↑ Jonas Schmöle, Mathias Dragosits, Hans Hepach, Markus Aspelmeyer: A micromechanical proof-of-principle experiment for measuring the gravitational force of milligram masses . In: Instrumention and Detectors . 2, No. 2, 2016, p. 20. arxiv : 1602.07539v2 . doi : 10.1088 / 0264-9381 / 33/12/125031 .
  16. ↑ Researchers propose experiment to measure the gravitational force of milli-gram objects, reaching almost into the quantum realm .