Einstein–Cartan theory

In theoretical physics, the Einstein–Cartan theory, also known as the Einstein–Cartan–Sciama–Kibble theory, is a classical theory of gravitation similar to general relativity. The theory was first proposed by Élie Cartan in 1922^[1] and expounded in the following few years.^[2]

Overview

The theory relaxes the assumption that the affine connection has vanishing antisymmetric part (torsion tensor), so that the torsion can be coupled to the intrinsic angular momentum (spin) of matter, much in the same way in which the curvature is coupled to the energy and momentum of matter. In fact, the spin of matter in curved spacetime requires that torsion is not constrained to be zero but is a variable in the principle of stationary action.

Regarding the metric and torsion tensors as independent variables gives the correct generalization of the conservation law for the total (orbital plus intrinsic) angular momentum to the presence of the gravitational field.

History

Albert Einstein became affiliated with the theory in 1928 during his unsuccessful attempt to match torsion to the electromagnetic field tensor as part of a unified field theory. This line of thought led him to the related but different theory of teleparallelism.^[3]

Dennis Sciama^[4] and Tom Kibble^[5] independently revisited the theory in the 1960s, and an important review was published in 1976.^[6]

Einstein–Cartan theory has been historically overshadowed by its torsion-free counterpart and other alternatives like Brans–Dicke theory because torsion seemed to add little predictive benefit at the expense of the tractability of its equations. Since the Einstein–Cartan theory is purely classical, it also does not fully address the issue of quantum gravity. In the Einstein–Cartan theory, the Dirac equation becomes nonlinear^[7] and therefore the superposition principle used in usual quantization techniques would not work. Recently, interest in Einstein–Cartan theory has been driven toward cosmological implications, most importantly, the avoidance of a gravitational singularity at the beginning of the universe.^[8]^[9] The theory is considered viable and remains an active topic in the physics community.^[10]

Field equations

The Einstein field equations of general relativity can be derived by postulating the Einstein–Hilbert action to be the true action of spacetime and then varying that action with respect to the metric tensor. The field equations of Einstein–Cartan theory come from exactly the same approach, except that a general asymmetric affine connection is assumed rather than the symmetric Levi-Civita connection (i.e., spacetime is assumed to have torsion in addition to curvature), and then the metric and torsion are varied independently.

Let ${\mathcal {L}}_{\mathrm {M} }$ represent the Lagrangian density of matter and ${\mathcal {L}}_{\mathrm {G} }$ represent the Lagrangian density of the gravitational field. The Lagrangian density for the gravitational field in the Einstein–Cartan theory is proportional to the Ricci scalar:

{\mathcal {L}}_{\mathrm {G} }={\frac {1}{2\kappa }}R{\sqrt {|g|}}

S=\int \left({\mathcal {L}}_{\mathrm {G} }+{\mathcal {L}}_{\mathrm {M} }\right)\,d^{4}x,

where $g$ is the determinant of the metric tensor, and $\kappa$ is a physical constant $8\pi G/c^{4}$ involving the gravitational constant and the speed of light. By Hamilton's principle, the variation of the total action $S$ for the gravitational field and matter vanishes:

\delta S=0.

The variation with respect to the metric tensor $g^{ab}$ yields the Einstein equations:

{\frac {\delta {\mathcal {L}}_{\mathrm {G} }}{\delta g^{ab}}}-{\frac {1}{2}}P_{ab}=0

$R_{ab}-{\frac {1}{2}}Rg_{ab}=\kappa P_{ab}$

where $R_{ab}$ is the Ricci tensor and $P_{ab}$ is the canonical stress-energy-momentum tensor. The Ricci tensor is no longer symmetric because the connection contains a nonzero torsion tensor; therefore, the right-hand side of the equation cannot be symmetric either, implying that $P_{ab}$ must include an asymmetric contribution that can be shown to be related to the spin tensor. This canonical energy-momentum tensor is related to the more familiar symmetric energy-momentum tensor by the Belinfante–Rosenfeld procedure.

The variation with respect to the torsion tensor ${T^{ab}}_{c}$ yields the Cartan equations

{\frac {\delta {\mathcal {L}}_{\mathrm {G} }}{\delta {T^{ab}}_{c}}}-{\frac {1}{2}}{\sigma _{ab}}^{c}=0

${T_{ab}}^{c}+{g_{a}}^{c}{T_{bd}}^{d}-{g_{b}}^{c}{T_{ad}}^{d}=\kappa {\sigma _{ab}}^{c}$

where ${\sigma _{ab}}^{c}$ is the spin tensor. Because the torsion equation is an algebraic constraint rather than a partial differential equation, the torsion field does not propagate as a wave, and vanishes outside of matter. Therefore, in principle the torsion can be algebraically eliminated from the theory in favor of the spin tensor, which generates an effective "spin-spin" nonlinear self-interaction inside matter.

Avoidance of singularities

The Einstein–Cartan theory eliminates the general-relativistic problem of the unphysical singularity at the Big Bang.^[9] The minimal coupling between torsion and Dirac spinors generates an effective nonlinear spin-spin self-interaction, which becomes significant inside fermionic matter at extremely high densities. Such an interaction replaces the singular Big Bang with a cusp-like Big Bounce at a minimum but finite scale factor, before which the observable universe was contracting. This scenario also explains why the present Universe at largest scales appears spatially flat, homogeneous and isotropic, providing a physical alternative to cosmic inflation.^[8]

Torsion allows fermions to be spatially extended^[11] instead of "pointlike", which helps to avoid the formation of singularities such as black holes and removes the ultraviolet divergence in quantum field theory. According to general relativity, the gravitational collapse of a sufficiently compact mass forms a singular black hole. In the Einstein–Cartan theory, instead, the collapse reaches a bounce and forms a regular Einstein–Rosen bridge (wormhole) to a new, growing universe on the other side of the event horizon.

References

↑ Élie Cartan. "Sur une généralisation de la notion de courbure de Riemann et les espaces à torsion." C. R. Acad. Sci. (Paris) 174, 593–595 (1922).
↑ Élie Cartan. "Sur les variétés à connexion affine et la théorie de la relativité généralisée." Part I: Ann. Éc. Norm. 40, 325–412 (1923) and ibid. 41, 1–25 (1924); Part II: ibid. 42, 17–88 (1925).
↑ Hubert F. M. Goenner. "On the History of Unified Field Theories." Living Rev. Relativity, 7, 2 (2004).
↑ Dennis W. Sciama. "The physical structure of general relativity", Rev. Mod. Phys. 36, 463-469 (1964).
↑ Tom W. B. Kibble. "Lorentz invariance and the gravitational field", J. Math. Phys. 2, 212-221 (1961).
↑ Friedrich W. Hehl, Paul von der Heyde, G. David Kerlick, and James M. Nester. "General relativity with spin and torsion: Foundations and prospects." Rev. Mod. Phys. 48, 393–416 (1976). http://link.aps.org/doi/10.1103/RevModPhys.48.393
↑ F. W. Hehl and B. K. Datta. "Nonlinear spinor equation and asymmetric connection in general relativity", J. Math. Phys. 12, 1334–1339 (1971).
1 2 Nikodem J. Popławski, (2010). "Cosmology with torsion: An alternative to cosmic inflation". Phys. Lett. B. 694 (3): 181–185. Bibcode:2010PhLB..694..181P. arXiv:1007.0587 . doi:10.1016/j.physletb.2010.09.056.
1 2 Nikodem Popławski, (2012). "Nonsingular, big-bounce cosmology from spinor-torsion coupling". Phys. Rev. D. 85 (10): 107502. Bibcode:2012PhRvD..85j7502P. arXiv:1111.4595 . doi:10.1103/PhysRevD.85.107502.
↑ Friedrich W. Hehl. "Note on the torsion tensor." Letter to Physics Today. March 2007, page 16.
↑ Nikodem J. Popławski, (2010). "Nonsingular Dirac particles in spacetime with torsion". Phys. Lett. B. 690 (1): 73–77. Bibcode:2010PhLB..690...73P. arXiv:0910.1181 . doi:10.1016/j.physletb.2010.04.073.

Cartan, E. (1922). Comptes Rendus 174, 437–439, 593–595, 734–737, 857–860, 1104–1107.
Gronwald, F. and Hehl, F. W. (1996). "On the Gauge Aspects of Gravity" arXiv:gr-qc/9602013.
Hammond, R. T. (2002). Rep. Prog. Phys. 65, 599.
Hehl, F. W. (1973). Gen. Rel. Grav. 4, 333.
Hehl, F. W. (1974). Gen. Rel. Grav. 5, 491.
Hehl, F. W., von der Heyde, P., and Kerlick, G. D. (1974). Phys. Rev. D 10, 1066.
Kleinert, H. (2000). Gen. Rel. Grav. 32, 769.
Kuchowicz, B. (1978). Gen. Rel. Grav. 9, 511.
Lord, E. A. (1976). "Tensors, Relativity and Cosmology" (McGraw-Hill).
Petti, R. J. (1976). "Some Aspects of the Geometry of First Quantized Theories", Gen. Rel. Grav. 7, 869-883.
Petti, R. J. (1986). "On the Local Geometry of Rotating Matter", Gen. Rel. Grav. 18, 441-460.
Petti, R. J. (2006). "Translational spacetime symmetries in gravitational theories", Class. Quantum Grav. 23, 737-751.
Petti, R. J. (2013). "Derivation of Einstein-Cartan theory from general relativity", http://arxiv.org/abs/1301.1588.
Poplawski, N. J. (2009). "Spacetime and fields", arXiv:0911.0334.
de Sabbata, V. and Gasperini, M. (1985). "Introduction to Gravitation" (World Scientific).
de Sabbata, V. and Sivaram, C. (1994). "Spin and Torsion in Gravitation" (World Scientific).
Shapiro, I. L. (2002). Phys. Rep. 357, 113.
Trautman, A. (1973). "Spin and torsion may avert gravitational singularities", Nature (Phys. Sci.) 242, 7.
Trautman, A. (2006). "Einstein-Cartan Theory", arXiv:gr-qc/0606062.

Theories of gravitation

Standard

Newtonian gravity (NG)	Newton's law of universal gravitation History of gravitational theory
General relativity (GR)	Introduction History Mathematics Resources Tests Post-Newtonian formalism Linearized gravity ADM formalism Gibbons–Hawking–York boundary term

Alternatives to
general relativity

Paradigms	Classical theories of gravitation Quantum gravity Theory of everything
Classical	Einstein–Cartan Bimetric theories Gauge theory gravity Teleparallelism Composite gravity f(R) gravity Massive gravity Modified Newtonian dynamics (MOND) Nonsymmetric gravitation Scalar–tensor theories Brans–Dicke Scalar–tensor–vector Conformal gravity Scalar theories Nordström Whitehead Geometrodynamics Induced gravity Tensor–vector–scalar Chameleon Pressuron
Quantisation	Euclidean quantum gravity Canonical quantum gravity Wheeler–DeWitt equation Loop quantum gravity Spin foam Causal dynamical triangulation Causal sets DGP model Rainbow gravity theory
Unification	Kaluza–Klein theory Dilaton Supergravity
Unification and quantisation	Noncommutative geometry Semiclassical gravity Superfluid vacuum theory Logarithmic BEC vacuum String theory M-theory F-theory Heterotic string theory Type I string theory Type 0 string theory Bosonic string theory Type II string theory Little string theory Twistor theory Twistor string theory
Generalisations / Extensions of GR	Scale relativity Liouville gravity Lovelock theory (2+1)-dimensional topological gravity Gauss–Bonnet gravity Jackiw–Teitelboim gravity

Pre-Newtonian theories and toy models

Albert Einstein
Physics	Special relativity General relativity Mass–energy equivalence (E=MC²) Brownian motion Photoelectric effect Einstein coefficients Einstein solid Equivalence principle Einstein field equations Einstein radius Einstein relation (kinetic theory) Cosmological constant Bose–Einstein condensate Bose–Einstein statistics Bose–Einstein correlations Einstein–Cartan theory Einstein–Infeld–Hoffmann equations Einstein–de Haas effect EPR paradox Bohr–Einstein debates Teleparallelism Unsuccessful investigations
Works	Annus Mirabilis papers (1905) "Investigations on the Theory of Brownian Movement" (1905) Relativity: The Special and the General Theory (1916) The World as I See It (1934) "Why Socialism?" (1949) Russell–Einstein Manifesto (1955)
Family	Pauline Koch (mother) Hermann Einstein (father) Maja Einstein (sister) Mileva Marić (first wife) Elsa Einstein (second wife) Lieserl Einstein (daughter) Hans Albert Einstein (son) Eduard Einstein (son) Bernhard Caesar Einstein (grandson) Evelyn Einstein (granddaughter) Thomas Martin Einstein (great-grandson)
Einstein Prizes	Albert Einstein Award Albert Einstein Medal Albert Einstein Peace Prize Albert Einstein World Award of Science Einstein Prize (APS) Einstein Prize for Laser Science
Related	Political views Religious views Albert Einstein House Einstein refrigerator Brain In popular culture Einsteinium Awards and honors List of things named after Albert Einstein Einstein Papers Project Die Grundlagen der Einsteinschen Relativitäts-Theorie (1922 documentary) The Einstein Theory of Relativity (1923 documentary) Relics: Einstein's Brain Genius (2017 series)

This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.