Poisson Brackets

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# Poisson Brackets - PowerPoint PPT Presentation

Poisson Brackets. The dynamic variables can be assigned to a single set. q 1 , q 2 , …, q n , p 1 , p 2 , …, p n z 1 , z 2 , …, z 2 n Hamilton’s equations can be written in terms of z a Symplectic 2 n x 2 n matrix Return the Lagrangian. Matrix Form.

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### Poisson Brackets

The dynamic variables can be assigned to a single set.

q1, q2, …, qn, p1, p2, …, pn

z1, z2, …, z2n

Hamilton’s equations can be written in terms of za

Symplectic 2n x2n matrix

Return the Lagrangian

Matrix Form
A dynamical variable F can be expanded in terms of the independent variables.

This can be expressed in terms of the Hamiltonian.

The Hamiltonian provides knowledge of F in phase space.

Dynamical Variable

S1

Angular Momentum

Example

• The two dimensional harmonic oscillator can be put in normalized coordinates.
• m = k = 1
• Find the change in angular momentum l.
• It’s conserved
The time-independent part of the expansion is the Poisson bracket of F with H.

This can be generalized for any two dynamical variables.

Hamilton’s equations are the Poisson bracket of the coordinates with the Hamitonian.

Poisson Bracket

S1

The Poisson bracket defines the Lie algebra for the coordinates q, p.

Bilinear

Antisymmetric

Jacobi identity

Bracket Properties

{A + B, C} ={A, C} + {B, C}

S1

{kA, B} = k{A, B}

{A, B} = -{B, A}

{A, {B, C}}+ {B, {C, A}}+ {C, {A, B}} = 0

In addition to the Lie algebra properties there are two other properties.

Product rule

Chain rule

The Poisson bracket acts like a derivative.

Poisson Properties
Let za(t) describe the time development of some system. This is generated by a Hamiltonian if and only if every pair of dynamical variables satisfies the following relation:Poisson Bracket Theorem
Equations of motion must follow standard form if they come from a Hamiltonian.

Consider a pair of equations in 1-dimension.

Not Hamiltonian

Not consistent with H

Not consistent with motion

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