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* The Newtonian ''[[gravitational potential]]'' at an event is the negative of the derivative of the action of the Newtonian gravitation field with respect to the ''[[mass density]]'' at that event. {{citation needed|date=April 2013}}
===Quantum
In [[quantum mechanics]], conjugate variables are realized as pairs of observables whose operators do not commute. In conventional terminology, they are said to be ''incompatible observables''. Consider, as an example, the measurable quantities given by position <math> \left (x \right) </math> and momentum <math> \left (p \right) </math>. In the quantum mechanical formalism, the two observables <math> x </math> and <math> p </math> correspond to the operators <math> \hat{x} </math> and <math> \hat{p} </math>, which necessarily satisfy the canonical commutation relation:
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