In mathematics, specifically in symplectic geometry, the moment map (or momentum map) is a tool associated with a Hamiltonian action of a Lie group on a symplectic manifold, used to construct conserved quantities for the action. The moment map generalizes the classical notions of linear and angular momentum. It is an essential ingredient in various constructions of symplectic manifolds, including symplectic (Marsden-Weinstein) quotients, discussed below, and symplectic cuts and sums.
the pairing between the two. Any in induces a vector field on M describing the infinitesimal action of . To be precise, at a point x in M the vector is
A moment map for the -action on is a map such that
for all in . Here is the function from M to defined by . The moment map is uniquely defined up to an additive constant of integration.
A moment map is often also required to be G-equivariant, where G acts on via the coadjoint action. If the group is compact or semisimple, then the constant of integration can always be chosen to make the moment map coadjoint equivariant; however in general the coadjoint action must be modified to make the map equivariant (this is the case for example for the Euclidean group).
Clearly, for a moment map to exist must be not merely closed but also exact. In practice it is useful to make an even stronger assumption. The -action is said to be Hamiltonian if and only if the following conditions hold. First, for every in the one-form is exact, meaning that it equals for some smooth function
If this holds, then one may choose the to make the map linear. The second requirement for the -action to be Hamiltonian is that the map be a Lie algebra homomorphism from to the algebra of smooth functions on M under the Poisson bracket.
If the action of G on is Hamiltonian in this sense, then a moment map is a map such that writing defines a Lie algebra homomorphism satisfying . Here is the vector field of the Hamiltonian , defined by
In the case of a Hamiltonian action of the circle , the Lie algebra dual is naturally identified with , and the moment map is simply the Hamiltonian function that generates the circle action.
Another classical case occurs when M is the cotangent bundle of and G is the Euclidean group generated by rotations and translations. That is, G is a six-dimensional group, the semidirect product of and . The six components of the moment map are then the three angular momenta and the three linear momenta.
Suppose that the action of a compact Lie group G on the symplectic manifold is Hamiltonian, as defined above, with moment map . From the Hamiltonian condition it follows that is invariant under G.
Assume now that 0 is a regular value of and that G acts freely and properly on . Thus and its quotient are both manifolds. The quotient inherits a symplectic form from M; that is, there is a unique symplectic form on the quotient whose pullback to equals the pullback of to . Thus the quotient is a symplectic manifold, called the Marsden-Weinstein quotient, symplectic quotient or symplectic reduction of M by G. Its dimension equals the dimension of M minus twice the dimension of G.