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Physics LibreTexts

14.2: Contact Transformations and General Perturbation Theory

( \newcommand{\kernel}{\mathrm{null}\,}\)

(Before reading this section, it may be well to re-read section 10.11 of Chapter 10.)

Suppose that we have a simple problem in which we know the hamiltonian H0 and that the Hamilton-Jacobi Equation has been solved:

H0(q1,Sq1,t)+St=0.

Now suppose we have a similar problem, but that the hamiltonian, instead of being just H0 is H=H0R, and K=H+St.

Let us make a contact transformation from (pi, qi) to (Pi, Qi), where ˙Qi=KPi and ˙Pi=KQi. In the orbital context, following Section 10.11, we identify Qi with αi and Pi with βi, which are functions (given in Section 10.11) of the orbital elements and which can serve in place of the orbital elements. The parameters are constants with respect to the unperturbed problem, but are variables with respect to the perturbing function. They are given, as functions of time, by the solution of Hamilton’s Equations of motion, which retain their form under a contact transformation.

˙αi=Rβi and ˙βi=Rαi.

Perturbation theory will show , then, how the αi and βi will vary with a given perturbation. The conventional elements a, \ e, \ i, \ Ω, \ ω, \ T are functions of α_i , \ β_i, and our aim is to find how the conventional elements vary with time under the perturbation R.

We can do that as follows. Let A_i be an orbital element, given by

A_i = A_i ( α_i , β_i ) . \label{14.2.3} \tag{14.2.3}

Then \dot{A}_i = \sum_j \frac{\partial A_i}{\partial α_j} \dot{α}_j + \sum_j \frac{\partial A_i}{\partial β_j} \dot{β}_j . \label{14.2.4} \tag{14.2.4}

By Equations 14.2.2a,b, this becomes

\dot{A}_i = \sum_j \frac{\partial A_i}{\partial α_j} \frac{\partial R}{\partial β_j} - \sum_j \frac{\partial A_i}{\partial β_j} \frac{\partial R}{\partial β_j} . \label{14.2.5} \tag{14.2.5}

But \frac{\partial R}{\partial α_j} = \sum_k \frac{\partial R}{\partial A_k} \frac{\partial A_k}{\partial α_j} \quad \text{and} \quad \frac{\partial R}{\partial β_j} = \sum_k \frac{\partial R}{\partial A_k} \frac{\partial A_k}{\partial β_j} . \label{14.2.6a,b} \tag{14.2.6a,b}

\therefore \quad \dot{A}_i = \sum_j \sum_k \frac{\partial R}{\partial A_k} \left( \frac{\partial A_i}{\partial a_j} \frac{\partial A_k}{\partial β_j} - \frac{\partial A_i}{\partial β_j} \frac{\partial A_k}{\partial α_j} \right) \label{14.2.6} \tag{14.2.6}

That is \dot{A}_i = \sum_k \frac{\partial R}{\partial A_k} \sum_j \left( \frac{\partial A_i}{\partial a_j} \frac{\partial A_k}{\partial β_j} - \frac{\partial A_i}{\partial β_j} \frac{\partial A_k}{\partial α_j} \right) \label{14.2.7} \tag{14.2.7}

This can be written, in shorthand:

\dot{A}_i = \sum_k \frac{\partial R}{\partial A_k} \{ A_i , A_k \}_{α_j , β_j} . \label{14.2.8} \tag{14.2.8}

Here the symbol \{ A_i , A_k \}_{α_i ,β_i} is called the Poisson bracket of A_i , \ A_k with respect to α_j, β_j. (In the language of the typographer, the symbols (), [] and {} are, respectively, parentheses, brackets and braces; you may refer to Poisson braces if you wish, but the usual term, in spite of the symbols, is Poisson bracket.)

Note the property \{ A_i , A_k \}_{α_j ,β_j} = - \{ A_k , A_i \}_{α_j ,β_j}.


This page titled 14.2: Contact Transformations and General Perturbation Theory is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Jeremy Tatum via source content that was edited to the style and standards of the LibreTexts platform.

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