# What is the role of contact geometry in the hamiltonian mechanics?

Let us assume someone is interested in the study of Hamiltonian mechanics.
What are good examples to illustrate him of the usefulness of contact geometry in this context?

On one hand the Hamiltonian mechanics was time ago expressed in the language of symplectic geometry, but, on the other hand, the contact geometry is often presented like the brother of the symplectic geometry.

My question is:

In the hamiltonian mechanics, not necessarily only for Hamiltonian of mechanical type, what is the role played by the contact geometry?

Any kind of suggestion is welcome.

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In mechanics you often want to study systems whose Hamiltonian function depends on time (explicitly). For example, you can look at the motion of a charged particle in a time-dependent electric field. In such cases you are solving an ODE in the "extended phase space" ($\mathbb{R}^7$ in the above example), and not in $\mathbb{R}^6\simeq T^\vee \mathbb{R}^3$. Also, the translation between Hamiltonian and Lagrangian formulation of mechanics goes via Legendre transform, which fits very nicely in the framework of contact geometry. Contact geometry also enters mechanics through Hamilton-Jacobi theory and the "method of characteristics".

So you can think of contact geometry as the odd-dimensional ("non-stationary") analogue of symplectic geometry. You can go from one to the other by "symplectisation". For example, if you start with a manifold, $M$, then you have a tautological contact structure on $X=\mathbb{P}T^\vee_M$. The symplectisation of $X$ is $T^\vee _M-\{ 0 \}$, with the canonical symplectic form. Symplectisation maps contact diffeomorphisms to symplectomorphisms, etc. etc. In the opposite direction, if you are given a symplectic manifold$(M,\omega)$ with $[\omega]=0\in H^2(M,\mathbb{R})$, you can build a line bundle $E\to M$ with a contact structure.

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Dear Dalakov, thanks for your answer. A question: About the role of contact geometry in the H--J theory, are you considering also its formulation for (systems) of generic $1^{\text{st}}$-order pdes $H\circ j^1f=0$ on $M$ (not necessarily H--J eqns)? Infact, in such a case the H--J theory speaks about the connection between the solutions of the system and the integral manifolds of the characteristic distribution for the corank 1 distribution on the level set $H^{-1}(0)$ which is induced by the natural contact structure on $J^1M$ (the space of 1-jets on $M$). –  Giuseppe Tortorella Aug 10 '11 at 10:02

I think the basic example is when you have a symplectic manifold $M$ with a Hamiltonian $H : M \to \mathbb{R}$. Then take a regular value $a$ of $H$, and look at the hypersurface $N := H^{-1}(a)$, which will be a smooth submanifold of $M$ of odd dimension. Then (probably with some more hypotheses that I forget now), $N$ will have a contact structure, and the corresponding Reeb vector field will agree with the Hamiltonian vector field $X_H$ corresponding to $H$. Recall that the value of the Hamiltonian function $H$ is constant along the flows of $X_H$. In terms of physics this is interpreted as conservation of energy or something like that. So in this basic example, contact geometry can be thought of as the study of Hamiltonian mechanics for a fixed value of energy.

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Dear Kevin Lin, your answer has been very interesting. Starting from it I have reviewed how to give a geometric proof of the Maupertius principle. Thanks. –  Giuseppe Tortorella Aug 10 '11 at 11:47

Form the contact one-form $\Theta = pdq -Hdt$ on extended phase space $T^* Q \times {\mathbb R}$, the secont factor being time and parameterized by $t$ the function $H = H(q,p,t)$ being the time-dependent energy or Hamiltonian,
and $pdq$ denotes the usual canonical one-form on $T^* Q$ pulled back to extended phase space by the projection onto the first factor. (Assume $H \ne 0$ to get $\Theta$ contact.) Then the Reeb vector field for this one-form, i.e the kernel of $d \Theta$, is the time-dependent Hamiltonian vector field, up to scale.

Arnol'd has a nice discussion of this in his Mathematical Methods in Classical Mechanics.

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Do you know of a citable reference containing this remark? Arnol'd doesn't explicitly make the connection between time-dependent Hamiltonian mechanics and contact geometry, and I would love to have a source that does. This answer is exactly what I'm looking for, but BibTeX doesn't seem to have a MathOverflow template yet... –  Vectornaut Jan 29 '14 at 8:20

Starting from Kevin Lin's answer I find that contact geometry gives a proof of the Maupertuis principle which is geometric and doesn't appeal at the variational principle of Least Action.

Let be given an Hamiltonian system with configuration space $M$, potential energy $V$ and kinetic energy $K$.
For any regular value $h$ of the Hamilton function $H:=K+V\circ\tau_M^\ast$, let us introduce:

• W the open subset of $M$ where $h-V$ is positive, and
• $N:=H^{-1}(h)\setminus K^{-1}(0)$, a codimension-1 submanifold of $T^\ast W$.

Let us define $\tilde{K}=(h-V\circ\tau_M^\ast)^{-1}K|_{T^\ast W}$, a metric on $W$, seen as a smooth function on $T^\ast W$, which is a fiberwise positive definite quadratic form.

Let us denote by $X$ and $\tilde{X}$ the Hamiltonian vector fields on $(T^\ast W,d\lambda)$ havind as Hamilton functions $H$ and $\tilde{K}$ respectively.
By definition, $N:=H^{-1}(h)\setminus K^{-1}(0)$ coincides with $\tilde{K}^{-1}(1)$, and the Liouville $1$-form $\lambda$ induces a contact form on it.
From the following pair of identities:

• $i(X)\lambda=2K$, $i(X)d\lambda=-dH$, and
• $i(\tilde{X})\lambda=2\tilde{K}$, $i(\tilde{X})d\lambda=-d\tilde{K}$,

we deduce that:

• $X$ and $\tilde{X}$ are tangent to $N$,
• both $(2K)^{-1}X|_N$ and $2\tilde{K})^{-1}\tilde{X}|_N\equiv 1/2\tilde{X}|_N$ satisfy the defining equations for the Reeb vector field on the strictly contact manifold $(N,j_N^\ast\lambda)$.

So on $N:=H^{-1}(h)\setminus K^{-1}(0)$, the Hamiltonian vector field $X$ of $H:=K+V\circ\tau_M^ast$ coincides with $2K\tilde{X}$, being $\tilde{X}$ the geodesic vector field for the Jacobi metric on $W$ given by $(h-V\circ\tau_M^\ast)^{-1}K$.

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