Curvature invariant (general relativity)

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Curvature invariants in general relativity are a set of scalars called curvature invariants that arise in general relativity. They are formed from the Riemann, Weyl and Ricci tensors - which represent curvature - and possibly operations on them such as contraction, covariant differentiation and dualisation.

Certain invariants formed from these curvature tensors play an important role in classifying spacetimes. Invariants are actually less powerful for distinguishing locally non-isometric Lorentzian manifolds than they are for distinguishing Riemannian manifolds. This means that they are more limited in their applications than for manifolds endowed with a positive definite metric tensor.

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[edit] Principal invariants

The principal invariants of the Riemann and Weyl tensors are certain quadratic polynomial invariants (i.e., sums of squares of components).

The principal invariants of the Riemann tensor of a four-dimensional Lorentzian manifold are

  1. the Kretschmann scalar K_1 = R_{abcd} \, R^{abcd}
  2. the Chern-Pontryagin scalar K_2 = {{}^\star R}_{abcd} \, R^{abcd}
  3. the Euler scalar K_3 = {{}^\star R^\star}_{abcd} \, R^{abcd}

These are quadratic polynomial invariants (sums of squares of components). (Some authors define the Chern-Pontryagin scalar using the right dual instead of the left dual.)

The first of these was introduced by Erich Kretschmann. The second two names are somewhat anachronistic, but since the integrals of the last two are related to the instanton number and Euler characteristic respectively, they have some justification.

The principal invariants of the Weyl tensor are

  1. I_1 = C_{abcd} \, C^{abcd}
  2. I_2 = {{}^\star C}_{abcd} \, C^{abcd}

(Because {{}^\star C^\star}_{abcd} = -C_{abcd}, there is no need to define a third principal invariant for the Weyl tensor.)

[edit] Relation with Ricci decomposition

As one might expect from the Ricci decomposition of the Riemann tensor into the Weyl tensor plus a sum of fourth-rank tensors constructed from the second rank Ricci tensor and from the Ricci scalar, these two sets of invariants are related:

K_1 = I_1 + 2 \, R_{ab} \, R^{ab} - \frac{1}{3} \, R^2
K_3 = -I_1 + 2 \, R_{ab} \, R^{ab} - \frac{2}{3} \, R^2

[edit] Relation with Bel decomposition

Recall that (in four dimensions), the Bel decomposition of the Riemann tensor, with respect to a timelike unit vector field \vec{X}, not necessarily geodesic or hypersurface orthogonal, consists of three pieces

  1. the electrogravitic tensor E[\vec{X}]_{ab} = R_{ambn} \, X^m \, X^n
  2. the magnetogravitic tensor B[\vec{X}]_{ab} = {{}^\star R}_{ambn} \, X^m \, X^n
  3. the topogravitic tensor L[\vec{X}]_{ab} = {{}^\star R^\star}_{ambn} \, X^m \, X^n

Because these are all transverse (i.e. projected to the spatial hyperplane elements orthogonal to our timelike unit vector field), they can be represented as linear operators on three dimensional vectors, or as three by three real matrices. They are respectively symmetric, traceless, and symmetric (6,8,6 linearly independent components, for a total of 20). If we write these operators as E, B, L respectively, the principal invariants of the Riemann tensor are obtained as follows:

  • K1 / 4 is the trace of E2 + L2 - 2 B BT,
  • − K2 / 8 is the trace of B ( E - L ),
  • K3 / 8 is the trace of E L - B2.

[edit] Expression in Newman-Penrose formalism

In terms of the Weyl scalars in the Newman-Penrose formalism, the principal invariants of the Weyl tensor may be obtained by taking the real and imaginary parts of the expression

I_1 - i \, I_2 = 16 \, \left( 3 \Psi_2^2 + \Psi_0 \, \Psi_4 - 4 \, \Psi_1 \Psi_3 \right)

(But note the minus sign!)

The principal quadratic invariant of the Ricci tensor, R_{ab} \, R^{ab}, may be obtained as a more complicated expression involving the Ricci scalars (see the paper by Cherubini et al. cited below).

[edit] See also

  • Bach tensor, for a sometimes useful tensor generated by I2 via a variational principle.
  • Carminati-McLenaghan invariants, for a set of polynomial invariants of the Riemann tensor of a four-dimensional Lorentzian manifold which is known to be complete under some circumstances.
  • Curvature invariant, for curvature invariants in a more general context.

[edit] References

  • Cherubini, C.; Bini, D.; Capozziello, S.; and Ruffini R. (2002). "Second order scalar invariants of the Riemann tensor: applications to black hole spacetimes". Int. J. Mod. Phys. D 11: 827-841. See also the eprint version.
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