cotangent bundle symplectic reduction and fibre bundles - MathOverflow most recent 30 from http://mathoverflow.net 2013-06-20T07:17:33Z http://mathoverflow.net/feeds/question/12847 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/12847/cotangent-bundle-symplectic-reduction-and-fibre-bundles cotangent bundle symplectic reduction and fibre bundles Orbicular 2010-01-24T15:34:51Z 2012-01-04T03:56:40Z <p>Suppose a compact Lie group $G$ acts on a manifold $M$ with only one orbit type $G/H$ ($H$ denotes the stabiliser group). Then the manifold $M$ becomes a fibre bundle over the quotient manifold $X:=M/G$ with typical fibre $G/H$ and structure group $G$.<br /> On the one hand one could look at the cotangent bundle $T^* X$ of the quotient (which carries a natural symplectic structure).<br /> On the other hand consider the lifted action of $G$ on the cotangent bundle $T^* M$ with moment map $\mu: T^* M\to \mathfrak{g}^* .$ The symplectic quotient $T^* M//G:=\mu^{-1}(0)/G$ inherits the structure of a symplectic manifold. Here comes the question: Are $T^* X$ and $T^* M//G$ (canonically) symplectomorphic?</p> http://mathoverflow.net/questions/12847/cotangent-bundle-symplectic-reduction-and-fibre-bundles/12849#12849 Answer by Gil Bor for cotangent bundle symplectic reduction and fibre bundles Gil Bor 2010-01-24T16:23:49Z 2010-01-24T16:23:49Z <p>No. Take $M=G$, with action by left translations. Then $T^*G//G$ gives the coadjoint orbits with the Kirilov symplectic str (eg for G=U(n) the complex flag manifolds with the standard Kahler str), while $X$ is just a point. </p> http://mathoverflow.net/questions/12847/cotangent-bundle-symplectic-reduction-and-fibre-bundles/12868#12868 Answer by Dmitri for cotangent bundle symplectic reduction and fibre bundles Dmitri 2010-01-24T21:26:41Z 2010-01-24T22:59:23Z <p>These two symplectic manifolds are canonically symplectomorphic. </p> <p>Notice first, that the map $\mu$ vanishes on the sub-bundle of $T^* M$ of 1-forms vanishing on the fibers of the fibration $M\to X$. Let us call this sub-bundle by $T_h ^* M$ (h- for horizontal).</p> <p>To construct the symplectomorphism notice that there is an obvious projection $\pi: T_h^* M \to T^* X$. The restriction of the symplectic form of $T^* M$ to $T_h^* M$ equals to the pullback of the symplectic form of $T^* X$ under $\pi$. The projection $\pi$ commutes with the action of $G$ and $G$ preserves the symplectic form on $T^* M$. Since the projection $\pi$ just produces the quotient of $T_h^*M$ by the action of $G$, now everything follows from definitions.</p> http://mathoverflow.net/questions/12847/cotangent-bundle-symplectic-reduction-and-fibre-bundles/14804#14804 Answer by jvkersch for cotangent bundle symplectic reduction and fibre bundles jvkersch 2010-02-09T20:03:06Z 2010-02-09T20:03:06Z <p>In addition to Dmitri's answer: when you're doing reduction at non-zero momentum some interesting things happen: the symplectic reduced space $J^{-1}(\mu)/G_\mu$ then becomes a fiber bundle over $T^\ast X$ with typical fiber the co-adjoint orbit $\mathcal{O}_\mu$. In the case $\mu = 0$, this reduces to the case discussed previously. This realization is not canonical, though, and depends on the choice of a connection in $M \to M/G$. </p> <p>The idea of the proof is to relate $J^{-1}(\mu)$ with $J^{-1}(0)$ using the connection one-form, and then to use the isomorphism mentioned previously. Afterwards, the curvature shows up in the reduced symplectic form on $J^{-1}(\mu)/G_\mu$, which is then the sum of the canonical form on $T^\ast X$ and a two-form constructed from the curvature.</p> <p>In the case where $M = G$, you can choose the Maurer-Cartan form as your connection, and then the isomorphism is $J^{-1}(\mu)/G_\mu = \mathcal{O}_\mu$ with the KKS symplectic form, which is in some sense the "curvature" of the Maurer-Cartan "connection".</p> http://mathoverflow.net/questions/12847/cotangent-bundle-symplectic-reduction-and-fibre-bundles/84851#84851 Answer by Gourishankar for cotangent bundle symplectic reduction and fibre bundles Gourishankar 2012-01-04T02:59:52Z 2012-01-04T02:59:52Z <p>Hello Jvkersch Could you please elaborate on the method of choosing a connection one form in order to get an expression for the reduced symplectic form Thanks </p> http://mathoverflow.net/questions/12847/cotangent-bundle-symplectic-reduction-and-fibre-bundles/84859#84859 Answer by Richard Montgomery for cotangent bundle symplectic reduction and fibre bundles Richard Montgomery 2012-01-04T03:56:40Z 2012-01-04T03:56:40Z <p>This is more a comment towards Gourishankar than an answer to the original question. It was part of my thesis, (UCB, about 1986), so, apologies, I chime in. For simplicity, I take the case $G$ Abelian, and $H =$ trivial. To map $J^{-1} (\mu)$ equivariantly to $J^{-1}(0)$ subtract $\mu \cdot A$ where $A$ is any $G$-connection for $\pi: X \to X/G$. $J^{-1} (0)/G = T^* (X/G)$ canonically, independent of connection. The map momentum shift map of subtracing $\mu \cdot A$ from co-vectors is not symplectic, relative to the standard structure, but it becomes symplectic if you subtract $\mu \pi^* F_A$, where $F_A = curv(A)$, from the standard structure. So the reduced space at $\mu$ is $T^*(X/G)$ with the standard structure minus the `magnetic term'' $\mu F_A$. </p> <p>For non-Abelian $G$ ($H$ still trivial), it is easier to explain things in Poisson terms.<br> $T^* X/G$ is a Poisson manifold whose symplectic leaves are the reduced spaces in question. The momentum shift trick turns it into $T^* (X/G) \oplus Ad^* (X)$ where $Ad^* (X) \to X/G$ is the co-adjoint bundle associated to $X \to X/G$ -- its fibers are the dual Lie algebras for $G$. This direct sum bundle admits coordinates $s_i, p_i, \xi_a$ where $s_i, p_i$ are canonical coordinates on $T^*(X/G)$ induced by coordinates $s_i$ on $X/G$ and where $\xi_a$ are fiber-linear coordinates on the co-adjoint bundle induced by a choice of local section of $\pi$. Then the main tricky part of the bracket is that the bracket of $p_i$ with $p_j$ is $\Sigma \xi_a F^a _{ij}$, $F$ being the curvature of the connection relative to the choice of local section. The symplectic leaves = reduced spaces are of the form $T^*(X/G) \oplus$(co-adjoint orbit bundle). </p>