The noungeodesic and the adjectivegeodetic come fromgeodesy, the science of measuring the size and shape ofEarth, though many of the underlying principles can be applied to anyellipsoidal geometry. In the original sense, a geodesic was the shortest route between two points on the Earth'ssurface. For aspherical Earth, it is asegment of agreat circle (see alsogreat-circle distance). The term has since been generalized to more abstract mathematical spaces; for example, ingraph theory, one might consider ageodesic between twovertices/nodes of agraph.
A locally shortest path between two given points in a curved space, assumed[b] to be aRiemannian manifold, can be defined by using theequation for thelength of acurve (a functionf from anopen interval ofR to the space), and then minimizing this length between the points using thecalculus of variations. This has some minor technical problems because there is an infinite-dimensional space of different ways to parameterize the shortest path. It is simpler to restrict the set of curves to those that are parameterized "with constant speed" 1, meaning that the distance fromf(s) tof(t) along the curve equals |s−t|. Equivalently, a different quantity may be used, termed the energy of the curve; minimizing the energy leads to the same equations for a geodesic (here "constant velocity" is a consequence of minimization).[citation needed] Intuitively, one can understand this second formulation by noting that anelastic band stretched between two points will contract its width, and in so doing will minimize its energy. The resulting shape of the band is a geodesic.
It is possible that several different curves between two points minimize the distance, as is the case for two diametrically opposite points on a sphere. In such a case, any of these curves is a geodesic.
A contiguous segment of a geodesic is again a geodesic.
In general, geodesics are not the same as "shortest curves" between two points, though the two concepts are closely related. The difference is that geodesics are onlylocally the shortest distance between points, and are parameterized with "constant speed". Going the "long way round" on agreat circle between two points on a sphere is a geodesic but not the shortest path between the points. The map from the unit interval on the real number line to itself gives the shortest path between 0 and 1, but is not a geodesic because the velocity of the corresponding motion of a point is not constant.
Geodesics are commonly seen in the study ofRiemannian geometry and more generallymetric geometry. Ingeneral relativity, geodesics inspacetime describe the motion ofpoint particles under the influence of gravity alone. In particular, the path taken by a falling rock, an orbitingsatellite, or the shape of aplanetary orbit are all geodesics[c] in curved spacetime. More generally, the topic ofsub-Riemannian geometry deals with the paths that objects may take when they are not free, and their movement is constrained in various ways.
Ageodesic on a triaxial ellipsoid.If an insect is placed on a surface and continually walks "forward", by definition it will trace out a geodesic.
The most familiar examples are the straight lines inEuclidean geometry. On asphere, the images of geodesics are thegreat circles. The shortest path from pointA to pointB on a sphere is given by the shorterarc of the great circle passing throughA andB. IfA andB areantipodal points, then there areinfinitely many shortest paths between them.Geodesics on an ellipsoid behave in a more complicated way than on a sphere; in particular, they are not closed in general (see figure).
Ageodesic triangle is formed by the geodesics joining each pair out of three points on a given surface. On the sphere, the geodesics aregreat circle arcs, forming aspherical triangle.
Geodesic triangles in spaces of positive (top), negative (middle) and zero (bottom) curvature.
Inmetric geometry, a geodesic is a curve which is everywherelocally adistance minimizer. More precisely, acurveγ :I →M from an intervalI of the reals to themetric spaceM is ageodesic if there is aconstantv ≥ 0 such that for anyt ∈I there is a neighborhoodJ oft inI such that for anyt1, t2 ∈J we have
This generalizes the notion of geodesic for Riemannian manifolds. However, in metric geometry the geodesic considered is often equipped withnatural parameterization, i.e. in the above identityv = 1 and
If the last equality is satisfied for allt1,t2 ∈I, the geodesic is called aminimizing geodesic orshortest path.
In general, a metric space may have no geodesics, except constant curves. At the other extreme, any two points in alength metric space are joined by a minimizing sequence ofrectifiable paths, although this minimizing sequence need not converge to a geodesic. Themetric Hopf-Rinow theorem provides situations where a length space is automatically a geodesic space.
The distance between two points and of is defined as theinfimum of the length taken over all continuous, piecewise continuously differentiable curves such that and. In Riemannian geometry, all geodesics are locally distance-minimizing paths, but the converse is not true. In fact, only paths that are both locally distance minimizing and parameterized proportionately to arc-length are geodesics.
Another equivalent way of defining geodesics on a Riemannian manifold, is to define them as the minima of the followingaction orenergy functional
All minima of are also minima of, but is a bigger set since paths that are minima of can be arbitrarily re-parameterized (without changing their length), while minima of cannot.For a piecewise curve (more generally, a curve), theCauchy–Schwarz inequality gives
with equality if and only if is equal to a constant a.e.; the path should be travelled at constant speed. It happens that minimizers of also minimize, because they turn out to be affinely parameterized, and the inequality is an equality. The usefulness of this approach is that the problem of seeking minimizers of is a more robust variational problem. Indeed, is a "convex function" of, so that within each isotopy class of "reasonable functions", one ought to expect existence, uniqueness, and regularity of minimizers. In contrast, "minimizers" of the functional are generally not very regular, because arbitrary reparameterizations are allowed.
Techniques of the classicalcalculus of variations can be applied to examine the energy functional. Thefirst variation of energy is defined in local coordinates by
In an appropriate sense, zeros of the second variation along a geodesic arise alongJacobi fields. Jacobi fields are thus regarded as variations through geodesics.
at each point along the curve, where is the derivative with respect to. More precisely, in order to define the covariant derivative of it is necessary first to extend to a continuously differentiablevector field in anopen set. However, the resulting value of (1) is independent of the choice of extension.
where are the coordinates of the curve and are theChristoffel symbols of the connection. This is anordinary differential equation for the coordinates. It has a unique solution, given an initial position and an initial velocity. Therefore, from the point of view ofclassical mechanics, geodesics can be thought of as trajectories offree particles in a manifold. Indeed, the equation means that theacceleration vector of the curve has no components in the direction of the surface (and therefore it is perpendicular to the tangent plane of the surface at each point of the curve). So, the motion is completely determined by the bending of the surface. This is also the idea of general relativity where particles move on geodesics and the bending is caused by gravity.
Thelocal existence and uniqueness theorem for geodesics states that geodesics on a smooth manifold with anaffine connection exist, and are unique. More precisely:
For any pointp inM and for any vectorV inTpM (thetangent space toM atp) there exists a unique geodesic :I →M such that
The proof of this theorem follows from the theory ofordinary differential equations, by noticing that the geodesic equation is a second-order ODE. Existence and uniqueness then follow from thePicard–Lindelöf theorem for the solutions of ODEs with prescribed initial conditions. γ dependssmoothly on bothp and V.
In general,I may not be all ofR as for example for an open disc inR2. Anyγ extends to all ofℝ if and only ifM isgeodesically complete.
Geodesicflow is a localR-action on thetangent bundleTM of a manifoldM defined in the following way
wheret ∈ R,V ∈ TM and denotes the geodesic with initial data. Thus, is theexponential map of the vectortV. A closed orbit of the geodesic flow corresponds to aclosed geodesic on M.
On a (pseudo-)Riemannian manifold, the geodesic flow is identified with aHamiltonian flow on the cotangent bundle. TheHamiltonian is then given by the inverse of the (pseudo-)Riemannian metric, evaluated against thecanonical one-form. In particular the flow preserves the (pseudo-)Riemannian metric, i.e.
In particular, whenV is a unit vector, remains unit speed throughout, so the geodesic flow is tangent to theunit tangent bundle.Liouville's theorem implies invariance of a kinematic measure on the unit tangent bundle.
The geodesic flow defines a family of curves in thetangent bundle. The derivatives of these curves define avector field on thetotal space of the tangent bundle, known as thegeodesicspray.
The geodesic spray is the unique horizontal vector fieldW satisfying
at each pointv ∈ TM; hereπ∗ : TTM → TM denotes thepushforward (differential) along the projectionπ : TM → M associated to the tangent bundle.
More generally, the same construction allows one to construct a vector field for anyEhresmann connection on the tangent bundle. For the resulting vector field to be a spray (on the deleted tangent bundle TM \ {0}) it is enough that the connection be equivariant under positive rescalings: it need not be linear. That is, (cf.Ehresmann connection#Vector bundles and covariant derivatives) it is enough that the horizontal distribution satisfy
for everyX ∈ TM \ {0} and λ > 0. Hered(Sλ) is thepushforward along the scalar homothety A particular case of a non-linear connection arising in this manner is that associated to aFinsler manifold.
Equation (1) is invariant under affine reparameterizations; that is, parameterizations of the form
wherea andb are constant real numbers. Thus apart from specifying a certain class of embedded curves, the geodesic equation also determines a preferred class of parameterizations on each of the curves. Accordingly, solutions of (1) are called geodesics withaffine parameter.
An affine connection isdetermined by its family of affinely parameterized geodesics, up totorsion (Spivak 1999, Chapter 6, Addendum I). The torsion itself does not, in fact, affect the family of geodesics, since the geodesic equation depends only on the symmetric part of the connection. More precisely, if are two connections such that the difference tensor
isskew-symmetric, then and have the same geodesics, with the same affine parameterizations. Furthermore, there is a unique connection having the same geodesics as, but with vanishing torsion.
Geodesics without a particular parameterization are described by aprojective connection.
A ribbon "test" is a way of finding a geodesic on a physical surface.[6] The idea is to fit a bit of paper around a straight line (a ribbon) onto a curved surface as closely as possible without stretching or squishing the ribbon (without changing its internal geometry).
For example, when a ribbon is wound as a ring around a cone, the ribbon would not lie on the cone's surface but stick out, so that circle is not a geodesic on the cone. If the ribbon is adjusted so that all its parts touch the cone's surface, it would give an approximation to a geodesic.
Mathematically the ribbon test can be formulated as finding a mapping of aneighborhood of a line in a plane into a surface so that the mapping "doesn't change the distances around by much"; that is, at the distance from we have where and aremetrics on and.
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While geometric in nature, the idea of a shortest path is so general that it easily finds extensive use in nearly all sciences, and in some other disciplines as well.
In a surface with negativeEuler characteristic, any (free) homotopy class determines a unique (closed) geodesic for ahyperbolic metric. These geodesics contribute significantly to the geometric understanding of the action ofmapping classes.
The study of how thenervous system optimizes muscular movement may be approached by endowing aconfiguration space of the body with aRiemannian metric that measures the effort, so that the problem can be stated in terms of geodesy.[7]
Geodesic distance is often used to measure the length of paths for signal propagation in neurons.[8]
The structures of geodesics in large molecules plays a role in the study ofprotein folds.[9]
Geodesics Revisited — Introduction to geodesics including two ways of derivation of the equation of geodesic with applications in geometry (geodesic on a sphere and on atorus), mechanics (brachistochrone) and optics (light beam in inhomogeneous medium).