Kretschmann scalar

The Kretschmann invariant is[1][2]

${\displaystyle K=R_{abcd}\,R^{abcd}}$

where ${\displaystyle R_{abcd}}$ is the Riemann curvature tensor (in this equation the Einstein summation convention was used, and it will be used throughout the article). Because it is a sum of squares of tensor components, this is a quadratic invariant.

For the use of a computer algebra system a more detailed writing is meaningful:

${\displaystyle K=R_{abcd}\,R^{abcd}=\sum _{a=0}^{3}\sum _{b=0}^{3}\sum _{c=0}^{3}\sum _{d=0}^{3}R_{abcd}\,R^{abcd}{\text{ with }}R^{abcd}=\sum _{i=0}^{3}g^{ai}\,\sum _{j=0}^{3}g^{bj}\,\sum _{k=0}^{3}g^{ck}\,\sum _{\ell =0}^{3}g^{d\ell }\,R_{ijk\ell }.\,}$

Another possible invariant (which has been employed for example in writing the gravitational term of the Lagrangian for some higher-order gravity theories) is

${\displaystyle C_{abcd}\,C^{abcd}}$

where ${\displaystyle C_{abcd}}$ is the Weyl tensor, the conformal curvature tensor which is also the completely traceless part of the Riemann tensor. In ${\displaystyle d}$ dimensions this is related to the Kretschmann invariant by[3]

${\displaystyle R_{abcd}\,R^{abcd}=C_{abcd}\,C^{abcd}+{\frac {4}{d-2}}R_{ab}\,R^{ab}-{\frac {2}{(d-1)(d-2)}}R^{2}}$

where ${\displaystyle R^{ab}}$ is the Ricci curvature tensor and ${\displaystyle R}$ is the Ricci scalar curvature (obtained by taking successive traces of the Riemann tensor). The Ricci tensor vanishes in vacuum spacetimes (such as the Schwarzschild solution mentioned above), and hence there the Riemann tensor and the Weyl tensor coincide, as do their invariants.

The Kretschmann scalar and the Chern-Pontryagin scalar

${\displaystyle R_{abcd}\,{{}^{\star }\!R}^{abcd}}$

where ${\displaystyle {{}^{\star }R}^{abcd}}$ is the left dual of the Riemann tensor, are mathematically analogous (to some extent, physically analogous) to the familiar invariants of the electromagnetic field tensor

${\displaystyle F_{ab}\,F^{ab},\;\;F_{ab}\,{{}^{\star }\!F}^{ab}}$

• Carminati-McLenaghan invariants, for a set of invariants.
• Classification of electromagnetic fields, for more about the invariants of the electromagnetic field tensor.
• Curvature invariant, for curvature invariants in Riemannian and pseudo-Riemannian geometry in general.
• Curvature invariant (general relativity).
• Ricci decomposition, for more about the Riemann and Weyl tensor.

• Grøn, Øyvind; Hervik, Sigbjørn (2007), Einstein's General Theory of Relativity, New York: Springer, ISBN 978-0-387-69199-2CS1 maint: ref=harv (link)
• B. F. Schutz (2009), A First Course in General Relativity (Second Edition), Cambridge University Press, ISBN 978-0-521-88705-2CS1 maint: ref=harv (link)
• Misner, Charles W.; Thorne, Kip. S.; Wheeler, John A. (1973), Gravitation, W. H. Freeman, ISBN 978-0-7167-0344-0CS1 maint: ref=harv (link)