# Conjugate variables

(Redirected from Canonical conjugate)
For conjugate variables in context of thermodynamics, see Conjugate variables (thermodynamics).

Conjugate variables are pairs of variables mathematically defined in such a way that they become Fourier transform duals of one another,[1][2] or more generally are related through Pontryagin duality. The duality relations lead naturally to an uncertainty in physics called the Heisenberg uncertainty principle relation between them. In mathematical terms, conjugate variables are part of a symplectic basis, and the uncertainty principle corresponds to the symplectic form.

## Examples

There are many types of conjugate variables, depending on the type of work a certain system is doing (or is being subjected to). Examples of canonically conjugate variables include the following:

• Time and frequency: the longer a musical note is sustained, the more precisely we know its frequency (but it spans more time). Conversely, a very short musical note becomes just a click, and so one can't know its frequency very accurately.[citation needed]
• Doppler and range: the more we know about how far away a radar target is, the less we can know about the exact velocity of approach or retreat, and vice versa. In this case, the two dimensional function of doppler and range is known as a radar ambiguity function or radar ambiguity diagram.
• Surface energy: γdA (γ = surface tension ; A = surface area).
• Elastic stretching: FdL (F = elastic force; L length stretched).

### Derivatives of action

In classical physics, the derivatives of action are conjugate variables to the quantity with respect to which one is differentiating. In quantum mechanics, these same pairs of variables are related by the Heisenberg uncertainty principle.

### Fluid Mechanics

In Hamiltonian fluid mechanics and quantum hydrodynamics, the action itself (or velocity potential) is the conjugate variable of the density (or probability density[disambiguation needed]).