A Quick Glimpse of Symplectic Topologyucahmko/GlimpseOfSympTop2.pdf · Applications Gromov’s...

A Quick Glimpse of Symplectic Topology

Momchil Konstantinov

London School of Geometry and Number TheoryUniversity College London

Momchil Konstantinov (LSGNT/UCL) Symplectic Topology 1 / 17

Two immediate questions:

Topology has simplices...are you sure you don’t mean simplicialtopology?

Ok then, what does symplectic mean?

Why Symplectic?

On page 165 of his book “The Classical Groups” Hermann Weyl starts achapter on the Symplectic Group. In a footnote he writes:

Some Definitions

Definition 1:

A symplectic form on a real vector space V is a non-degenerateantisymmetric bilinear form

ω : V × V −→ R

Exercise: Such a thing exists if and only if the dimension of V is even!Here is (the) one on R4:

ω((v1, v2, v3, v4), (w1,w2,w3,w4)) = v1w2 − v2w1 + v3w4 − v4w3

Definition 2:

A symplectic structure on a manifold M is a 2-form ω, which is everywherenon-degenerate and closed. A symplectic manifold is a manifold with afixed symplectic structure.

Some Examples

R2n, ωstd = dx1 ∧ dy1 + · · ·+ dxn ∧ dyn

The cotangent bundle π : T ∗M → M of any manifold M. It isendowed with its canonical symplectic form ωcan = dλcan, whereλcan(m, ξ) = π∗ξ.

Any orientable surface with fixed volume form.

Complex projective varieties

many more...

Some Examples

many more...

Some Examples

many more...

Some Examples

many more...

Some Examples

many more...

Why people do it:

Cotangent bundles are models for phase space of classical mechanicalsystems.

Modern theoretical physics - string theory and mirror symmetry

It is fun and mysterious!

Why I like it:

Flexibility vs. rigidity

Employs differential geometry, analysis, homological algebra, sheaftheory...

Why people do it:

Why I like it:

Why people do it:

Why I like it:

Why people do it:

Why I like it:

Why “topology”?

Darboux Theorem

Let (M2n, ω) be a symplectic manifold and p ∈ M be a point. Then thereexists an open set U ⊆ M, containing p, an open set V ⊆ R2n and adiffeomorphism ϕ : V → U such that

ϕ∗ω = d x1 ∧ d y1 + · · ·+ d xn ∧ d yn.

Lagrangian submanifolds

Definition 3

A Lagrangian subspace of (V 2n, ω) is an n−dimensional subspace L ≤ Vsuch that for every v ,w ∈ L one has ω(v ,w) = 0.

Definition 4

A Lagrangian submanifold of (M2n, ω) is an n−dimensional submanifoldL ⊆ M such that for every x ∈ L the tangent space TxL is a Lagrangiansubspace of TxM.

Two immediate questions

1 What are the closed Lagrangian submanifolds of R2?

2 What are the compact Lagrangian submanifolds of R4?a) Orientable:

· · ·

b) Non-orientable

RP2 RP2#RP2

· · ·

b) Non-orientable

RP2 RP2#RP2

· · ·

2 What are the compact Lagrangian submanifolds of R4?

a) Orientable:

· · ·

b) Non-orientable

RP2 RP2#RP2

· · ·

b) Non-orientable

RP2 RP2#RP2

· · ·

b) Non-orientable

RP2 RP2#RP2

RP2#RP2#RP2 · · ·

· · ·

b) Non-orientable

RP2#RP2

RP2# · · ·#RP2︸︷︷︸4k+2

· · ·

b) Non-orientable

RP2 RP2#RP2

RP2# · · ·#RP2︸︷︷︸4k+2

· · ·

Rigidity: Gromov’s Pseudoholomorphic Curves

Definition 5

An almost complex structure on a manifold M is a sectionJ ∈ Γ(End(TM)) such that J2 = −1. A pseudoholomorphic curve in(M, J) is a map f : (Σ, j)→ M from a surface Σ to M which satisfies theequation

df ◦ j = J ◦ df

These things are good, but when J is not integrable they can be quitewild. But if there is a symplectic structure ω compatible with J (i.e. suchthat gJ = ω(·, J·) is a Riemannian metric) things work out great!

mainly because...

∫Σf ∗ω =

∫ω(∂s f , ∂t f ) ds dt

∫ √ω(∂s f , J∂s f )ω(∂t f , J∂t f ) ds dt

∫ √det({gJ(∂i f , ∂j f )}i ,j) ds dt

dVolf ∗gJ

that is...

Symplectic area is actual area!

Applications

Gromov’s non-squeezing theorem

Suppose there exists an embedding F : B2n(R) ↪→ B2(r)× R2n−2 whichrespects the standard symplectic structures. Then R ≤ r .

Lagrangians in R2n

There are no compact simply-connected Lagrangians in R2n.

Applications

Gromov’s non-squeezing theorem

Suppose there exists an embedding F : B2n(R) ↪→ B2(r)× R2n−2 whichrespects the standard symplectic structures. Then R ≤ r .

Lagrangians in R2n

There are no compact simply-connected Lagrangians in R2n.

Input from many walks of mathematics

For the past 30 years people have been working hard and have come upwith things like: Quantum Cohomology, Floer homology, Symplectichomology, Fukaya categories, Symplectic Field Theory.... All these thingsare machines which take information about the complicated ways in whichpseudoholomorphic curves bend and twist and repackage it into algebra.Then one can do some homological algebra and deduce more and moresurprising rigidity facts.

More recently people have applied microlocal sheaf theory to try andanswer the same questions without pseudoholomorphic curves.

Open Problems

Symp(M , ω)

What is the topology of the group of diffeomorphisms of (M, ω) whichpreserve ω?

Arnold’s Nearby Lagrangian Conjecture

Is it true that every compact exact Lagrangian in the cotangent bundle ofa compact manifold can be mapped to the zero section by a Hamiltonianisotopy?

Open Problems

Symp(M , ω)

What is the topology of the group of diffeomorphisms of (M, ω) whichpreserve ω?

Arnold’s Nearby Lagrangian Conjecture

Is it true that every compact exact Lagrangian in the cotangent bundle ofa compact manifold can be mapped to the zero section by a Hamiltonianisotopy?

a weaker question...

Is it true that if M and N are compact manifolds and there exists adiffeomorphism F : (T ∗M, ωcan)→ (T ∗N, ωcan) which respects thesymplectic structures, then M is diffeomorphic to N?

a rephrasing...

Is all of differential topology just a part of symplectic topology?

a weaker question...

Is it true that if M and N are compact manifolds and there exists adiffeomorphism F : (T ∗M, ωcan)→ (T ∗N, ωcan) which respects thesymplectic structures, then M is diffeomorphic to N?

a rephrasing...

Is all of differential topology just a part of symplectic topology?

Disclaimer!

This last thing was a massive piece of propaganda and exaggeration! Thisis a good time for you to stop listening to me!

Thank you!

A Quick Glimpse of Symplectic Topologyucahmko/GlimpseOfSympTop2.pdf · Applications Gromov’s...

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