pseudo-Riemannian metric


Riemannian geometry

Differential geometry

differential geometry

synthetic differential geometry








The notion of pseudo-Riemannian metric is a slight variant of that of Riemannian metric.

Where a Riemannian metric is governed by a positive-definite bilinear form, a Pseudo-Riemannian metric is governed by an indefinite bilinear form.

This is equivalently the Cartan geometry modeled on the inclusion of a Lorentz group into a Poincaré group.


A pseudo-Riemannian “metric” is a nondegenerate quadratic form on a real vector space n\mathbb{R}^n. A Riemannian metric is a positive-definite quadratic form on a real vector space. The data of such a quadratic form may be equivalently given by a nondegenerate symmetric bilinear pairing ,\langle\, , \, \rangle on n\mathbb{R}^n.

A pseudo-Riemannian metric

Q: nQ: \mathbb{R}^n \to \mathbb{R}

can always be diagonalized: there exists a basis e 1,,e ne_1, \ldots, e_n such that

Q( 1inx ie i)=x 1 2++x p 2x p+1 2x n 2Q(\sum_{1 \leq i \leq n} x_i e_i) = x_1^2 + \ldots + x_p^2 - x_{p+1}^2 - \ldots - x_n^2

where the pair (p,np)(p, n-p) is called the signature of the form QQ. Pseudo-Riemannian metrics on n\mathbb{R}^n are classified by their signatures; thus we have a standard metric of signature (p,np)(p, n-p) where {e 1,,e n}\{e_1, \ldots, e_n\} is the standard basis of n\mathbb{R}^n.

More generally, there is a notion of pseudo-Riemannian manifold (of type (p,np)(p, n-p), which is an nn-dimensional manifold MM equipped with a global section

σ:MS 2(T *M)\sigma: M \to S^2(T^* M)

of the bundle of symmetric bilinear forms over MM, such that each σ(x)\sigma(x) is a nondegenerate form on the tangent space T x(M)T_x(M).

Certain theorems of Riemannian geometry carry over to the more general pseudo-Riemannian setting; for example, pseudo-Riemannian manifolds admit Levi-Civita connections, or in other words a unique notion of covariant differentiation of vector fields

:(X,Y) X(Y)\nabla: (X, Y) \mapsto \nabla_X(Y)

such that

XY,Z= XY,Z+Y, XZX \cdot \langle Y, Z\rangle = \langle \nabla_X Y, Z\rangle + \langle Y, \nabla_X Z\rangle
[X,Y]= XY YX[X, Y] = \nabla_X Y - \nabla_Y X

In that case, one may define a notion of geodesic in pseudo-Riemannian manifolds MM, and we have a notion of “distance squared” between the endpoints along any geodesic path α:[0,1]M\alpha: [0, 1] \to M (which might be a negative number of course). The term “pseudo-Riemannian metric” may refer to such distances in general pseudo-Riemannian manifolds. (I guess.)

A typical example of pseudo-Riemannian manifold is a Lorentzian manifold, where the metric is of type (1,n1)(1, n-1). This is particularly so in the case n=4n = 4, where such manifolds are the mathematical backdrop for studying general relativity and cosmological models.

  • The terminology “metric” is not optimal of course: the values of the quadratic form would need to be nonnegative to avoid terminological conflict with metric as it is more commonly understood (and even in that case, the values of “metric” refer to the square of the metric rather than the metric itself). Caveat lector.
geometric contextgauge groupstabilizer subgrouplocal model spacelocal geometryglobal geometrydifferential cohomologyfirst order formulation of gravity
differential geometryLie group/algebraic group GGsubgroup (monomorphism) HGH \hookrightarrow Gquotient (“coset space”) G/HG/HKlein geometryCartan geometryCartan connection
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Poincaré group Iso(d1,1)Iso(d-1,1)Lorentz group O(d1,1)O(d-1,1)Minkowski spacetime d1,1\mathbb{R}^{d-1,1}Lorentzian geometrypseudo-Riemannian geometryspin connectionEinstein gravity
anti de Sitter group O(d1,2)O(d-1,2)O(d1,1)O(d-1,1)anti de Sitter spacetime AdS dAdS^dAdS gravity
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supergeometrysuper Lie group GGsubgroup (monomorphism) HGH \hookrightarrow Gquotient (“coset space”) G/HG/Hsuper Klein geometrysuper Cartan geometryCartan superconnection
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Revised on February 27, 2015 14:57:03 by Urs Schreiber (