Cotangent bundle

In mathematics, especially differential geometry, the cotangent bundle of a smooth manifold is the vector bundle of all the cotangent spaces at every point in the manifold. It may be described also as the dual bundle to the tangent bundle.

Contents

The cotangent sheafEdit

Smooth sections of the cotangent bundle are differential one-forms.

Definition of the cotangent sheafEdit

Let M be a smooth manifold and let M×M be the Cartesian product of M with itself. The diagonal mapping Δ sends a point p in M to the point (p,p) of M×M. The image of Δ is called the diagonal. Let   be the sheaf of germs of smooth functions on M×M which vanish on the diagonal. Then the quotient sheaf   consists of equivalence classes of functions which vanish on the diagonal modulo higher order terms. The cotangent sheaf is the pullback of this sheaf to M:

 

By Taylor's theorem, this is a locally free sheaf of modules with respect to the sheaf of germs of smooth functions of M. Thus it defines a vector bundle on M: the cotangent bundle.

See also: bundle of principal parts (which generalizes the above constructions to higher orders.)

Contravariance in manifoldsEdit

A smooth morphism   of manifolds, induces a pullback sheaf   on M. There is an induced map of vector bundles  .

ExamplesEdit

Given a smooth manifold   embedded into   such that   is the vanishing locus of a smooth function  , we can define the cotangent bundle of   as the set

 

For example, consider the 3-sphere given by the vanishing locus of   in  . Then, we can compute a presentation of its cotangent bundle as the set

 

The cotangent bundle as phase spaceEdit

Since the cotangent bundle X=T*M is a vector bundle, it can be regarded as a manifold in its own right. Because of the manner in which the definition of T*M relates to the differential topology of the base space M, X possesses a canonical one-form θ (also tautological one-form or symplectic potential). The exterior derivative of θ is a symplectic 2-form, out of which a non-degenerate volume form can be built for X. For example, as a result X is always an orientable manifold (meaning that the tangent bundle of X is an orientable vector bundle). A special set of coordinates can be defined on the cotangent bundle; these are called the canonical coordinates. Because cotangent bundles can be thought of as symplectic manifolds, any real function on the cotangent bundle can be interpreted to be a Hamiltonian; thus the cotangent bundle can be understood to be a phase space on which Hamiltonian mechanics plays out.

The tautological one-formEdit

The cotangent bundle carries a tautological one-form θ also known as the Poincaré 1-form or Liouville 1-form. (The form is also known as the canonical one-form, although this can sometimes lead to confusion.) This means that if we regard T*M as a manifold in its own right, there is a canonical section of the vector bundle T*(T*M) over T*M.

This section can be constructed in several ways. The most elementary method is to use local coordinates. Suppose that xi are local coordinates on the base manifold M. In terms of these base coordinates, there are fibre coordinates pi: a one-form at a particular point of T*M has the form pi dxi (Einstein summation convention implied). So the manifold T*M itself carries local coordinates (xi,pi) where the x are coordinates on the base and the p are coordinates in the fibre. The canonical one-form is given in these coordinates by

 

Intrinsically, the value of the canonical one-form in each fixed point of T*M is given as a pullback. Specifically, suppose that π : T*MM is the projection of the bundle. Taking a point in Tx*M is the same as choosing of a point x in M and a one-form ω at x, and the tautological one-form θ assigns to the point (x, ω) the value

 

That is, for a vector v in the tangent bundle of the cotangent bundle, the application of the tautological one-form θ to v at (x, ω) is computed by projecting v into the tangent bundle at x using dπ : TT*MTM and applying ω to this projection. Note that the tautological one-form is not a pullback of a one-form on the base M.

Symplectic formEdit

The cotangent bundle has a canonical symplectic 2-form on it, as an exterior derivative of the tautological one-form, the symplectic potential. Proving that this form is, indeed, symplectic can be done by noting that being symplectic is a local property: since the cotangent bundle is locally trivial, this definition need only be checked on  . But there the one form defined is the sum of  , and the differential is the canonical symplectic form, the sum of  .

Phase spaceEdit

If the manifold   represents the set of possible positions in a dynamical system, then the cotangent bundle   can be thought of as the set of possible positions and momenta. For example, this is a way to describe the phase space of a pendulum. The state of the pendulum is determined by its position (an angle) and its momentum (or equivalently, its velocity, since its mass is not changing). The entire state space looks like a cylinder. The cylinder is the cotangent bundle of the circle. The above symplectic construction, along with an appropriate energy function, gives a complete determination of the physics of system. See Hamiltonian mechanics for more information, and the article on geodesic flow for an explicit construction of the Hamiltonian equations of motion.

See alsoEdit

ReferencesEdit

  • Abraham, Ralph; Marsden, Jerrold E. (1978). Foundations of Mechanics. London: Benjamin-Cummings. ISBN 0-8053-0102-X.
  • Jost, Jürgen (2002). Riemannian Geometry and Geometric Analysis. Berlin: Springer-Verlag. ISBN 3-540-63654-4.
  • Singer, Stephanie Frank (2001). Symmetry in Mechanics: A Gentle Modern Introduction. Boston: Birkhäuser.