Canonical coordinates

From Academic Kids

In mathematics and physics, the canonical coordinates are a special set of coordinates on the cotangent bundle of a manifold. They are usually written as a set of <math>(q^i,p_j)<math> or <math>(x^i,p_j)<math> with the x 's or q 's denoting the coordinates on the underlying manifold and the p 's denoting the conjugate momentum, which are 1-forms in the cotangent bundle at point q in the manifold. This article defines the canonical coordinates as they appear in classical physics. A closely related concept also appears in quantum mechanics; see the Stone-von Neumann theorem and canonical commutation relations for details. In the following exposition, we assume that the manifolds are real manifolds, so that cotangent vectors acting on tangent vectors produce real numbers.

This article attempts to provide a rigorous definition of the looser, simpler idea presented in the article canonical conjugate variables.

Definition

Given a manifold Q, a vector field X on the tangent bundle TQ can be thought of as a function acting on the cotangent bundle, by the duality between the tangent and cotangent spaces. That is, define a function

<math>P_X:T^*Q\to \mathbb{R}<math>

such that

<math>P_X(q,p)=p(X_q)<math>

holds for all cotangent vectors p in <math>T_q^*Q<math>. Here, <math>X_q<math> is the vector in <math>T_qQ<math>, the tangent space to the manifold Q at point q. The function <math>P_X<math> is called the momentum function corresponding to X.

In local coordinates, the vector field X at point q may be written as

<math>X_q=\sum_i X^i(q) \frac{\partial}{\partial q^i}<math>

where the <math>\partial /\partial q^i<math> are the coordinate frame on TQ. The conjugate momentum then has the expression

<math>P_X(q,p)=\sum_i X^i(q) \;p_i<math>

where the <math>p_i<math> are defined as the momentum functions corresponding to the vectors <math>\partial /\partial q^i<math>:

<math>p_i = P_{\partial /\partial q^i}<math>

The <math>q^i<math> together with the <math>p_j<math> together form a coordinate system on the cotangent bundle <math>T^*Q<math>; these coordinates are called the canonical coordinates.

Generalized coordinates

In Lagrangian mechanics, a different set of coordinates are used, called the generalized coordinates. These are commonly denoted as <math>(q^i,\dot{q}^i)<math> with <math>q^i<math> called the generalized position and <math>\dot{q}^j<math> the generalized velocity. When a Hamiltonian is defined on the cotangent bundle, then the generalized coordinates are related to the canonical coordinates by means of the Hamilton-Jacobi equations.

See also

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