Jacobi fields can be obtained in the following way: Take a smooth one parameter family of geodesics with, then is a Jacobi field, and describes the behavior of the geodesics in an infinitesimal neighborhood of a given geodesic. A vector field J along a geodesic is said to be a Jacobi field if it satisfies the Jacobi equation: where D denotes the covariant derivativewith respect to the Levi-Civita connection, R the Riemann curvature tensor, the tangent vector field, and t is the parameter of the geodesic. On a complete Riemannian manifold, for any Jacobi field there is a family of geodesics describing the field. The Jacobi equation is a linear, second order ordinary differential equation; in particular, values of and at one point of uniquely determine the Jacobi field. Furthermore, the set of Jacobi fields along a given geodesic forms a real vector space of dimension twice the dimension of the manifold. As trivial examples of Jacobi fields one can consider and. These correspond respectively to the following families of reparametrisations: and. Any Jacobi field can be represented in a unique way as a sum, where is a linear combination of trivial Jacobi fields and is orthogonal to, for all. The field then corresponds to the same variation of geodesics as, only with changed parameterizations.
Motivating example
On a sphere, the geodesics through the North pole are great circles. Consider two such geodesics and with natural parameter,, separated by an angle. The geodesic distance is Computing this requires knowing the geodesics. The most interesting information is just that Instead, we can consider the derivative with respect to at : Notice that we still detect the intersection of the geodesics at. Notice further that to calculate this derivative we do not actually need to know rather, all we need do is solve the equation for some given initial data. Jacobi fields give a natural generalization of this phenomenon to arbitrary Riemannian manifolds.
Solving the Jacobi equation
Let and complete this to get an orthonormal basis at. Parallel transport it to get a basis all along. This gives an orthonormal basis with. The Jacobi field can be written in co-ordinates in terms of this basis as and thus and the Jacobi equation can be rewritten as a system for each. This way we get a linear ordinary differential equation. Since this ODE has smooth coefficients we have that solutions exist for all and are unique, given and, for all.
Examples
Consider a geodesic with parallel orthonormal frame,, constructed as above.
The vector fields along given by and are Jacobi fields.
In Euclidean space Jacobi fields are simply those fields linear in.
For Riemannian manifolds of constant negative sectional curvature, any Jacobi field is a linear combination of, and, where.
For Riemannian manifolds of constant positive sectional curvature, any Jacobi field is a linear combination of,, and, where.
The restriction of a Killing vector field to a geodesic is a Jacobi field in any Riemannian manifold.
