Symplectic manifold explained

In differential geometry, a subject of mathematics, a symplectic manifold is a smooth manifold,

M , equipped with a closed nondegenerate differential 2-form \omega , called the symplectic form. The study of symplectic manifolds is called symplectic geometry or symplectic topology. Symplectic manifolds arise naturally in abstract formulations of classical mechanics and analytical mechanics as the cotangent bundles of manifolds. For example, in the Hamiltonian formulation of classical mechanics, which provides one of the major motivations for the field, the set of all possible configurations of a system is modeled as a manifold, and this manifold's cotangent bundle describes the phase space of the system.

Motivation

Symplectic manifolds arise from classical mechanics; in particular, they are a generalization of the phase space of a closed system.^[1] In the same way the Hamilton equations allow one to derive the time evolution of a system from a set of differential equations, the symplectic form should allow one to obtain a vector field describing the flow of the system from the differential

dH of a Hamiltonian function H .^[2] So we require a linear map TM → T ^{*M} from the tangent manifold TM to the cotangent manifold T ^{*} M , or equivalently, an element of T ^{*M} ⊗ T ^{*M} . Letting \omega denote a section of T ^{*M} ⊗ T ^{*} M , the requirement that \omega be non-degenerate ensures that for every differential dH there is a unique corresponding vector field V _{H} such that dH = \omega (V _{H,} ⋅ ) . Since one desires the Hamiltonian to be constant along flow lines, one should have \omega(V _{H,} V _{H)} = dH(V _{H)} = 0 , which implies that \omega is alternating and hence a 2-form. Finally, one makes the requirement that \omega should not change under flow lines, i.e. that the Lie derivative of \omega along V _{H} vanishes. Applying Cartan's formula, this amounts to (here \iota _{X} is the interior product): l{L} _{V _{H}} (\omega) = 0 \Leftrightarrow d (\iota _{V _{H}} \omega) + \iota _{V _{H}} d\omega= d ( dH) + d\omega(V _{H)} = d\omega(V _{H)=0}

so that, on repeating this argument for different smooth functions

H such that the corresponding V _{H} span the tangent space at each point the argument is applied at, we see that the requirement for the vanishing Lie derivative along flows of V _{H} corresponding to arbitrary smooth H is equivalent to the requirement that ω should be closed.

Definition

M is a closed non-degenerate differential 2-form \omega .^{[3] [4]} Here, non-degenerate means that for every point p \in M , the skew-symmetric pairing on the tangent space T _{p} M defined by \omega is non-degenerate. That is to say, if there exists an X \in T _{p} M such that \omega( X, Y ) = 0 for all Y \in T _{p} M , then X = 0 . Since in odd dimensions, skew-symmetric matrices are always singular, the requirement that \omega be nondegenerate implies that M has an even dimension.^{[3] [4]} The closed condition means that the exterior derivative of \omega vanishes. A symplectic manifold is a pair (M, \omega) where M is a smooth manifold and \omega is a symplectic form. Assigning a symplectic form to M is referred to as giving M a symplectic structure.

Examples

Symplectic vector spaces

See main article: Symplectic vector space.

Let

\{v _{1,} \ldots, v _{2n} \} be a basis for \R ^{2n} . We define our symplectic form ω on this basis as follows:\omega(v _{i,} v _{j)} = \begin{cases} 1 & j - i = n with 1 \leqslant i \leqslant n \ -1 & i - j = n with 1 \leqslant j \leqslant n \ 0 & otherwise \end{cases}

In this case the symplectic form reduces to a simple quadratic form. If I_n denotes the n × n identity matrix then the matrix, Ω, of this quadratic form is given by the block matrix:

\Omega = \begin{pmatrix} 0 & I _{n} \ - I _{n} & 0 \end{pmatrix}.

Cotangent bundles

Let

Q be a smooth manifold of dimension n . Then the total space of the cotangent bundle T ^{*} Q has a natural symplectic form, called the Poincaré two-form or the canonical symplectic form\omega = \sum ^{n} _{i =1} dp _{i} \wedge dq ^{i}

Here

(q ^{1,} \ldots, q ^{n)} are any local coordinates on Q and (p _{1,} \ldots, p _{n)} are fibrewise coordinates with respect to the cotangent vectors dq ^{1,} \ldots, dq ^{n} . Cotangent bundles are the natural phase spaces of classical mechanics. The point of distinguishing upper and lower indexes is driven by the case of the manifold having a metric tensor, as is the case for Riemannian manifolds. Upper and lower indexes transform contra and covariantly under a change of coordinate frames. The phrase "fibrewise coordinates with respect to the cotangent vectors" is meant to convey that the momenta p _{i} are "soldered" to the velocities dq ^{i} . The soldering is an expression of the idea that velocity and momentum are colinear, in that both move in the same direction, and differ by a scale factor.

Kähler manifolds

V \subset CP ^{n} has a symplectic form which is the restriction of the Fubini—Study form on the projective space CP ^{n} .

Almost-complex manifolds

Riemannian manifolds with an

\omega -compatible almost complex structure are termed almost-complex manifolds. They generalize Kähler manifolds, in that they need not be integrable. That is, they do not necessarily arise from a complex structure on the manifold.

Lagrangian and other submanifolds

There are several natural geometric notions of submanifold of a symplectic manifold

(M, \omega) :

• Symplectic submanifolds of

M (potentially of any even dimension) are submanifolds S \subset M such that \omega| _{S} is a symplectic form on S .

• Isotropic submanifolds are submanifolds where the symplectic form restricts to zero, i.e. each tangent space is an isotropic subspace of the ambient manifold's tangent space. Similarly, if each tangent subspace to a submanifold is co-isotropic (the dual of an isotropic subspace), the submanifold is called co-isotropic.
• Lagrangian submanifolds of a symplectic manifold

(M,\omega) are submanifolds where the restriction of the symplectic form \omega to L\subset M is vanishing, i.e. \omega| _{L=0} and dim L = \tfrac{ 1 }{ 2 }\dim M . Lagrangian submanifolds are the maximal isotropic submanifolds.

One major example is that the graph of a symplectomorphism in the product symplectic manifold is Lagrangian. Their intersections display rigidity properties not possessed by smooth manifolds; the Arnold conjecture gives the sum of the submanifold's Betti numbers as a lower bound for the number of self intersections of a smooth Lagrangian submanifold, rather than the Euler characteristic in the smooth case.

Examples

Let

\R ^{2 n} _{bf{x} , bf{y}} have global coordinates labelled (x _{1,} ... c, x _{n,} y _{1,} ... c, y _{n)} . Then, we can equip \R _{bf{x} , bf{y}} ^{2n} with the canonical symplectic form \omega = d x _{1\wedge} d y _{1} + ... b + d x _{n\wedge} d y _{n.}

There is a standard Lagrangian submanifold given by

\R ^{n} _{x} \to \R ^{2 n} _{x , y} . The form \omega vanishes on \R ^{n} _{x} because given any pair of tangent vectors X = f _{i(bf{x})\partial} ^{x _{i}} , Y = g _{i(bf{x})\partial} ^{x _{i}} , we have that \omega(X,Y) = 0 . To elucidate, consider the case n =1 . Then, X = f(x)\partial _{x,} Y = g(x)\partial _{x,} and \omega = d x\wedge d y . Notice that when we expand this out\omega(X,Y) = \omega(f(x)\partial _{x,g(x)\partial} _{x)} = (1)/(2) f(x)g(x)( d x(\partial _{x)dy(\partial} _{x)} - d y(\partial _{x)dx(\partial} _{x))}

both terms we have a

d y(\partial _{x)} factor, which is 0, by definition.

Example: Cotangent bundle

The cotangent bundle of a manifold is locally modeled on a space similar to the first example. It can be shown that we can glue these affine symplectic forms hence this bundle forms a symplectic manifold. A less trivial example of a Lagrangian submanifold is the zero section of the cotangent bundle of a manifold. For example, let

X = \{(x,y) \in \R ^{2} : y ^{2} - x = 0 \}.

Then, we can present

T ^{*X} asT ^{*X} = \{(x,y, d x, d y) \in \R ^{4} : y ^{2} - x = 0 , 2 y d y - d x = 0 \}

where we are treating the symbols

d x, d y as coordinates of \R ^{4} = T ^{*\R} ^{2} . We can consider the subset where the coordinates d x =0 and d y =0 , giving us the zero section. This example can be repeated for any manifold defined by the vanishing locus of smooth functions f _{1,...c,f} _{k} and their differentials d f _{1,...c,df} _{k} .

Example: Parametric submanifold

Consider the canonical space

\R ^{2n} with coordinates (q _{1,...c} ,q _{n,p} _{1,...c} ,p _{n)} . A parametric submanifold L of \R ^{2n} is one that is parameterized by coordinates (u _{1,...c,u} _{n)} such that q _{i=q} _{i(u} _{1,...c,u} _{n)} p _{i=p} _{i(u} _{1,...c,u} _{n)} [u _{i,u} _{j]} vanishes for all i,j . That is, it is Lagrangian if [u _{i,u} _{j]=\sum} _{k} (\partial q _{k})/(\partial u _{i}) (\partial p _{k})/(\partial u _{j}) - (\partial p _{k})/(\partial u _{i}) (\partial q _{k})/(\partial u _{j}) = 0

for all

i,j . This can be seen by expanding(\partial)/(= \partial u _{i}) (\partial q _{k})/(\partial u _{i}) (\partial)/(\partial q _{k}) + (\partial p _{k})/(\partial u _{i}) (\partial)/(\partial p _{k}) in the condition for a Lagrangian submanifold L . This is that the symplectic form must vanish on the tangent manifold TL ; that is, it must vanish for all tangent vectors:\omega\left( (\partial)/(\partial u _{i}) , (\partial)/(\partial u _{j}) \right) =0 for all i,j . Simplify the result by making use of the canonical symplectic form on \R ^{2n} :\omega\left( (\partial)/(\partial q _{k}) , (\partial)/(\partial p _{k}) \right) = - \omega\left( (\partial)/(\partial p _{k}) , (\partial)/(\partial q _{k}) \right) = 1 and all others vanishing.

As local charts on a symplectic manifold take on the canonical form, this example suggests that Lagrangian submanifolds are relatively unconstrained. The classification of symplectic manifolds is done via Floer homology—this is an application of Morse theory to the action functional for maps between Lagrangian submanifolds. In physics, the action describes the time evolution of a physical system; here, it can be taken as the description of the dynamics of branes.

Example: Morse theory

f:M\to\R and for a small enough \varepsilon one can construct a Lagrangian submanifold given by the vanishing locus V (\varepsilon ⋅ d f) \subset T ^{*M} . For a generic Morse function we have a Lagrangian intersection given by M \cap V (\varepsilon ⋅ d f) = Crit (f) .

See also: symplectic category.

Special Lagrangian submanifolds

In the case of Kähler manifolds (or Calabi–Yau manifolds) we can make a choice

\Omega = \Omega _{1+i\Omega} _{2} on M as a holomorphic n-form, where \Omega _{1} is the real part and \Omega _{2} imaginary. A Lagrangian submanifold L is called special if in addition to the above Lagrangian condition the restriction \Omega _{2} to L is vanishing. In other words, the real part \Omega _{1} restricted on L leads the volume form on L . The following examples are known as special Lagrangian submanifolds,

1. complex Lagrangian submanifolds of hyperkähler manifolds,
2. fixed points of a real structure of Calabi–Yau manifolds.

The SYZ conjecture deals with the study of special Lagrangian submanifolds in mirror symmetry; see .

The Thomas–Yau conjecture predicts that the existence of a special Lagrangian submanifolds on Calabi–Yau manifolds in Hamiltonian isotopy classes of Lagrangians is equivalent to stability with respect to a stability condition on the Fukaya category of the manifold.

Lagrangian fibration

T ^{*\R} ^{n,} and the Lagrangian fibration as the trivial fibration \pi: T ^{*\R} ^{n} \to \R ^{n.} This is the canonical picture.

Lagrangian mapping

Let L be a Lagrangian submanifold of a symplectic manifold (K,ω) given by an immersion (i is called a Lagrangian immersion). Let give a Lagrangian fibration of K. The composite is a Lagrangian mapping. The critical value set of π ∘ i is called a caustic.

Two Lagrangian maps and are called Lagrangian equivalent if there exist diffeomorphisms σ, τ and ν such that both sides of the diagram given on the right commute, and τ preserves the symplectic form.^[4] Symbolically:

\tau \circ i _{1} = i _{2} \circ \sigma, \nu \circ \pi _{1} = \pi _{2} \circ \tau, \tau ^{*\omega} _{2} = \omega _{1} , where τ^∗ω₂ denotes the pull back of ω₂ by τ.

Special cases and generalizations

• A symplectic manifold

(M, \omega) is exact if the symplectic form \omega is exact. For example, the cotangent bundle of a smooth manifold is an exact symplectic manifold. The canonical symplectic form is exact.

• A symplectic manifold endowed with a metric that is compatible with the symplectic form is an almost Kähler manifold in the sense that the tangent bundle has an almost complex structure, but this need not be integrable.
• Symplectic manifolds are special cases of a Poisson manifold.
• A multisymplectic manifold of degree k is a manifold equipped with a closed nondegenerate k-form.^[5]
• A polysymplectic manifold is a Legendre bundle provided with a polysymplectic tangent-valued

(n +2 ) -form; it is utilized in Hamiltonian field theory.^[6]

See also

• • —an odd-dimensional counterpart of the symplectic manifold.

General and cited references

• Book: Dusa . McDuff . Dusa McDuff . D. . Salamon . Introduction to Symplectic Topology . 1998 . Oxford Mathematical Monographs . 0-19-850451-9 .
• Web site: Denis . Auroux . Denis Auroux . Seminar on Mirror Symmetry .
• Web site: Eckhard . Meinrenken . Eckhard Meinrenken . Symplectic Geometry .
• Book: Ralph . Abraham . Ralph Abraham (mathematician) . Jerrold E. . Marsden . Jerrold E. Marsden . Foundations of Mechanics . 1978 . Benjamin-Cummings . London . 0-8053-0102-X . See Section 3.2 .
• Book: de Gosson, Maurice A. . Maurice A. de Gosson . Symplectic Geometry and Quantum Mechanics . 2006 . Birkhäuser Verlag . Basel . 3-7643-7574-4 .
• Alan Weinstein . Symplectic manifolds and their lagrangian submanifolds . Alan Weinstein . . 6 . 3 . 1971 . 329–46 . 10.1016/0001-8708(71)90020-X . free .
• Book: Arnold, V. I. . Singularities of Caustics and Wave Fronts . 1990 . Ch.1, Symplectic geometry . Springer Netherlands . 978-1-4020-0333-2 . Mathematics and Its Applications . 62 . Dordrecht . 10.1007/978-94-011-3330-2 . 22509804.

Further reading

• Petr . Dunin-Barkowski . 2022 . Symplectic duality for topological recursion . math-ph . 2206.14792 .
• Web site: How to find Lagrangian Submanifolds . . December 17, 2014 .
  • Gennadi Sardanashvily . Sardanashvily . G. . 2009 . Fibre bundles, jet manifolds and Lagrangian theory . Lectures for Theoreticians . 0908.1886 .
• Web site: Dusa McDuff . McDuff . D. . Symplectic Structures—A New Approach to Geometry . Notices of the AMS . November 1998 .
• Nigel . Hitchin . 1999 . Lectures on Special Lagrangian Submanifolds . math/9907034 .

Notes and References

1. Web site: Ben . Webster . What is a symplectic manifold, really? . 9 January 2012 .
2. Web site: Henry . Cohn . Why symplectic geometry is the natural setting for classical mechanics .
3. Book: de Gosson, Maurice . Symplectic Geometry and Quantum Mechanics . 2006 . Birkhäuser Verlag . Basel . 3-7643-7574-4 . 10 .
4. Book: V. I.. Arnold. A. N.. Varchenko. S. M.. Gusein-Zade. Vladimir Arnold. Sabir Gusein-Zade. Alexander Varchenko. The Classification of Critical Points, Caustics and Wave Fronts: Singularities of Differentiable Maps, Vol 1. Birkhäuser. 1985. 0-8176-3187-9.
5. F. . Cantrijn . L. A. . Ibort . M. . de León . On the Geometry of Multisymplectic Manifolds . J. Austral. Math. Soc. . Ser. A . 66 . 1999 . 3 . 303–330 . 10.1017/S1446788700036636 . free .
6. G. . Giachetta . L. . Mangiarotti . G. . Sardanashvily . Gennadi Sardanashvily . Covariant Hamiltonian equations for field theory . Journal of Physics . A32 . 1999 . 38 . 6629–6642 . 10.1088/0305-4470/32/38/302 . hep-th/9904062 . 1999JPhA...32.6629G . 204899025 .