Killing vector field
In mathematics, a Killing vector field (often just Killing field), named after Wilhelm Killing, is a vector field on a Riemannian manifold (or pseudo-Riemannian manifold) that preserves the metric. Killing fields are the infinitesimal generators of isometries; that is, flows generated by Killing fields are continuous isometries of the manifold. More simply, the flow generates a symmetry, in the sense that moving each point on an object the same distance in the direction of the Killing vector field will not distort distances on the object.
Specifically, a vector field X is a Killing field if the Lie derivative with respect to X of the metric g vanishes:
In terms of the Levi-Civita connection, this is
for all vectors Y and Z. In local coordinates, this amounts to the Killing equation
This condition is expressed in covariant form. Therefore it is sufficient to establish it in a preferred coordinate system in order to have it hold in all coordinate systems.
- The vector field on a circle that points clockwise and has the same length at each point is a Killing vector field, since moving each point on the circle along this vector field simply rotates the circle.
- If the metric coefficients in some coordinate basis are independent of , then is automatically a Killing vector, where is the Kronecker delta.
To prove this, let us assume
Now let us look at the Killing condition
The Killing condition becomes
- The physical meaning is, for example, that, if none of the metric coefficients is a function of time, the manifold must automatically have a time-like Killing vector.
- In layman's terms, if an object doesn't transform or "evolve" in time (when time passes), time passing won't change the measures of the object. Formulated like this, the result sounds like a tautology, but one has to understand that the example is very much contrived: Killing fields apply also to much more complex and interesting cases.
A Killing field is determined uniquely by a vector at some point and its gradient (i.e. all covariant derivatives of the field at the point).
The Lie bracket of two Killing fields is still a Killing field. The Killing fields on a manifold M thus form a Lie subalgebra of vector fields on M. This is the Lie algebra of the isometry group of the manifold if M is complete.
For compact manifolds
- Negative Ricci curvature implies there are no nontrivial (nonzero) Killing fields.
- Nonpositive Ricci curvature implies that any Killing field is parallel. i.e. covariant derivative along any vector j field is identically zero.
- If the sectional curvature is positive and the dimension of M is even, a Killing field must have a zero.
The divergence of every Killing vector field vanishes.
Each Killing vector corresponds to a quantity which is conserved along geodesics. This conserved quantity is the metric product between the Killing vector and the geodesic tangent vector. That is, along a geodesic with some affine parameter , the equation is satisfied. This aids in analytically studying motions in a spacetime with symmetries.
- Killing vector fields can be generalized to conformal Killing vector fields defined by for some scalar The derivatives of one parameter families of conformal maps are conformal Killing fields.
- Killing tensor fields are symmetric tensor fields T such that the trace-free part of the symmetrization of vanishes. Examples of manifolds with Killing tensors include the rotating black hole and the FRW cosmology.
- Killing vector fields can also be defined on any (possibly nonmetric) manifold M if we take any Lie group G acting on it instead of the group of isometries. In this broader sense, a Killing vector field is the pushforward of a left invariant vector field on G by the group action. If the group action is effective, then the space of the Killing vector fields is isomorphic to the Lie algebra of G.
- Affine vector field
- Curvature collineation
- Homothetic vector field
- Killing form
- Killing horizon
- Killing spinor
- Killing tensor
- Matter collineation
- Spacetime symmetries
- Misner, Thorne, Wheeler (1973). Gravitation. W H Freeman and Company. ISBN 0-7167-0344-0.
- Carrol, Sean (2004). An Introduction to General Relativity Spacetime and Geometry. Addison Wesley. pp. 133–139.
- Carrol, Sean (2004). An Introduction to General Relativity Spacetime and Geometry. Addison Wesley. pp. 263,344.
- Choquet-Bruhat, Yvonne; DeWitt-Morette, Cécile (1977), Analysis, Manifolds and Physics, Amsterdam: Elsevier, ISBN 978-0-7204-0494-4