# Geodesic

In mathematics, a geodesic is a generalization of the notion of a "straight line" to "curved spaces". Definition of geodesic depends on the type of "curved space". If the space carries a natural metric then geodesics are defined to be (locally) the shortest path between points on the space.

The term "geodesic" comes from geodesy, the science of measuring the size and shape of the earth; in the original sense, a geodesic was the shortest route between two points on the surface of the earth, namely, a segment of a great circle.

## Introduction

The shortest path between two points in a curved space can be found by writing the equation for the length of a curve, and then minimizing this length using standard techniques of calculus and differential equations. Equivalently, a different quantity may be defined, termed the energy of the curve; minimizing the energy leads to the same equations for a geodesic. Intuitively, one can understand this second formulation by noting that an elastic band stretched between two points will contract its length, and in so doing will minimize its energy; the resulting shape of the band is a geodesic.

Geodesics are commonly seen in the study of Riemannian geometry and more generally metric geometry. In physics, geodesics describe the motion of point particles; in particular, the path taken by a falling rock, an orbiting satellite, or the shape of a planetary orbit are all described by geodesics in the theory of general relativity. More generally, the topic of sub-Riemannian geometry deals with the paths that objects may take when they are not free, and their movement is constrained in various ways.

This article presents the mathematical formalism involved in defining, finding, and proving the existence of geodesics, in the case of Riemannian and pseudo-Riemannian manifolds. The article geodesic (general relativity) discusses the special case of general relativity in greater detail.

### Examples

The most familiar examples are the straight lines in Euclidean geometry. On a sphere, the geodesics are the great circles. The shortest path from point A to point B on a sphere is given by the shorter piece of the great circle passing through A and B. If A and B are antipodal points (like the North pole and the South pole), then there are many shortest paths between them.

## Metric geometry

In metric geometry, a geodesic is a curve which is everywhere locally a distance minimizer. More precisely, a curve γ: IM from the unit interval I to the metric space M is a geodesic if there is a constant v ≥ 0 such that for any tI there is a neighborhood J of t in I such that for any t1, t2J we have

${\displaystyle d(\gamma (t_{1}),\gamma (t_{2}))=v|t_{1}-t_{2}|.\,}$

This generalizes the notion of geodesic for Riemannian manifolds. However, in metric geometry the geodesic considered is almost always equipped with natural parametrization, i.e. in the above identity v = 1 and

${\displaystyle d(\gamma (t_{1}),\gamma (t_{2}))=|t_{1}-t_{2}|.\,}$

If the last equality is satisfied for all t1, t2I, the geodesic is called a minimizing geodesic or shortest path.

In general, a metric space may have no geodesics, except constant curves.

## (pseudo-)Riemannian geometry

On a (pseudo-)Riemannian manifold M a geodesic is defined as a smooth curve γ(t) that parallel transports its own tangent vector. That is,

${\displaystyle {\frac {D}{dt}}{\dot {\gamma }}(t)=\nabla _{{\dot {\gamma }}(t)}{\dot {\gamma }}(t)=0}$.

where ∇ stands for the Levi-Civita connection on M.

In the case of a Riemannian manifold, the geodesics that one obtains this way are identical to geodesics for the induced metric space.

In terms of local coordinates on M the geodesic equation can be written (using the summation convention):

${\displaystyle {\frac {d^{2}x^{a}}{dt^{2}}}+\Gamma _{bc}^{a}{\frac {dx^{b}}{dt}}{\frac {dx^{c}}{dt}}=0}$

where xa(t) are the coordinates of the curve γ(t) and ${\displaystyle \Gamma _{bc}^{a}}$ are the Christoffel symbols.

Geodesics can also be defined as extremal curves for the following energy functional

${\displaystyle E(\gamma )={\frac {1}{2}}\int g({\dot {\gamma }}(t),{\dot {\gamma }}(t))\,dt,}$

where ${\displaystyle g}$ is a Riemannian (or pseudo-Riemannian) metric. This "energy functional" should be called action, but few in mathematics use this term; the geodesic equation can then be obtained as the Euler-Lagrange equations of motion for this action.

Therefore, from a mechanical point of view, geodesics can be thought of as trajectories of free particles in a manifold (at least in the Riemannian case).

In a similar manner, one can obtain geodesics as a solution of the Hamilton-Jacobi equations, with (pseudo-)Riemannian metric taken as Hamiltonian. See Riemannian_manifolds in Hamiltonian_mechanics for further details.

### Existence and uniqueness

The local existence and uniqueness theorem for geodesics states that geodesics exist, and are unique; this is a variant of the Frobenius theorem. More precisely, for any point p in M and for any vector V in TpM (the tangent space to M at p) there exists a unique geodesic γ : IM such that ${\displaystyle \gamma (0)=p}$ and ${\displaystyle {\dot {\gamma }}(0)=V}$. Here I is a maximal open interval in R containing 0. In general, I may not be all of R as for example for an open disc in R². The proof of this theorem follows from the theory of ordinary differential equations, by noticing that the geodesic equation is a second-order ODE. Existence and uniqueness then follow from the Picard-Lindelöf theorem for the solutions of ODEs with prescribed initial conditions. γ depends smoothly on both p and V.

### Geodesic flow

Geodesic flow is an ${\displaystyle \mathbb {R} }$-action on tangent bundle ${\displaystyle T(M)}$ of a manifold ${\displaystyle M}$ defined in the following way

${\displaystyle G^{t}(V)={\dot {\gamma }}_{V}(t)}$

where ${\displaystyle t\in \mathbb {R} }$, ${\displaystyle V\in T(M)}$ and ${\displaystyle \gamma _{V}}$ denotes geodesic with initial data ${\displaystyle {\dot {\gamma }}_{V}(0)=V}$.

It defines a Hamiltonian flow on (co)tangent bundle with (pseudo-)Riemannian metric as Hamiltonian. In particular it preserves the (pseudo-)Riemannian metric ${\displaystyle g}$, i.e.

${\displaystyle g(G^{t}(V),G^{t}(V))=g(V,V).}$

That makes possible to define geodesic flow on unit tangent bundle of Riemannian manifold ${\displaystyle UT(M)}$.

### Geodesic spray

The geodesic flow defines a family of curves on the tangent bundle. The derivatives of these curves define a vector field on the tangent bundle, known as the geodesic spray.