- Geodesics as Hamiltonian flows
mathematics, the geodesic equations are second-order non-linear differential equations, and are commonly presented in the form of Euler–Lagrangeequations of motion. However, they can also be presented as a set of coupled first-order equations, in the form of Hamilton's equations. This later formulation is developed in this article.
It is frequently said that
geodesicsare "straight lines in curved space". By using the Hamilton-Jacobi approach to the geodesic equation, this statement can be given a very intuitive meaning: geodesics describe the motions of particles that are not experiencing any forces. In flat space, it is well known that a particle moving in a straight line will continue to move in a straight line if it experiences no external forces; this is Newton's first law. The Hamiltonan describing such motion is well known to be with "p" being the momentum. It is the conservation of momentumthat leads to the straight motion of a particle. On a curved surface, exactly the same ideas are at play, except that, in order to measure distances correctly, one must use the metric. To measure momenta correctly, one must use the inverse of the metric. The motion of a free particle on a curved surface still has exactly the same form as above, i.e. consisting entirely of a kinetic term. The resulting motion is still, in a sense, a "straight line", which is why it is sometimes said that geodesics are "straight lines in curved space". This idea is developed in greater detail below.
Geodesics as an application of the principle of least action
Given a (pseudo-)
Riemannian manifold"M", a geodesicmay be defined as the curve that results from the application of the principle of least action. A differential equation describing their shape may be derived, using variational principles, by minimizing (or finding the extremum) of the energyof a curve. Given a smooth curve:
that maps an interval "I" of the
real number lineto the manifold "M", one writes the energy
where is the
tangent vectorto the curve at point .Here, is the metric tensoron the manifold "M". Using the energy given above as the action, one may choose to solve either the Euler–Lagrange equations, or the Hamilton-Jacobi equations. Both methods give the geodesic equationas the solution; however, the Hamilton–Jacobi equations provide greater insight into the structure of the manifold, as shown below. In terms of the local coordinateson "M", the (Euler–Lagrange) geodesic equation is
Here, the "x""a"("t") are the coordinates of the curve γ("t") and are the
Christoffel symbols. Repeated indices imply the use of the summation convention.
Hamiltonian approach to the geodesic equations
Geodesics can be understood to be the
Hamiltonian flows of a special Hamiltonian vector fielddefined on the cotangent spaceof the manifold. The Hamiltonian is constructed from the metric on the manifold, and is thus a quadratic formconsisting entirely of the kinetic term.
The geodesic equations are second-order differential equations; they can be re-expressed as first-order ordinary differential equations taking the form of the Hamiltonian–Jacobi equations by introducing additional independent variables, as shown below. Start by finding a chart that trivializes the
cotangent bundle"T"∗"M" ("i.e." a " local trivialization"):
where "U" is an open
subsetof the manifold "M", and the tangent space is of rank "n". Label the coordinates of the chart as ("x"1, "x"2, …, "x""n", "p"1, "p"2, …, "p""n"). Then introduce the Hamiltonian as
Here, "g""ab"("x") is the inverse of the
metric tensor: "g""ab"("x")"g""bc"("x") = . This inverse almost always exists for a broad class of metric manifolds. The behavior of the metric tensor under coordinate transformations implies that "H" is invariant under a change of variable. The geodesic equations can then be written as
The second order geodesic equations are easily obtained by substitution of one into the other. The flow determined by these equations is called the cogeodesic flow. The first of the two equations gives the flow on the tangent bundle "TM", the geodesic flow. Thus, the geodesic lines are the integral curves of the geodesic flow onto the manifold "M". This is a
Hamiltonian flow, and that the Hamiltonian is constant along the geodesics:
Thus, the geodesic flow splits the cotangent bundle into
level sets of constant energy
for each energy "E" ≥ 0, so that
Hopf-Rinow theoremguarantees the completeness of the manifold. The positivity of the energy follows from the positivity of the metric tensor; this analysis is modified on pseudo-Riemannian manifolds.
* Ralph Abraham and Jarrold E. Marsden, "Foundations of Mechanics", (1978) Benjamin-Cummings, London ISBN 0-8053-0102-X "See section 2.7".
* Jurgen Jost, "Riemannian Geometry and Geometric Analysis", (2002) Springer-Verlag, Berlin ISBN 3-540-42627-2 "See section 1.4".
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