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In physics, an operator is almost always either a square matrix or a linear mapping between two function spaces (defined on, say, $\mathbb R^n$). Operators serve as *observables* and as *time evolution operators* in Quantum Mechanics. This tag will most often find valid use in quantum mechanics; don't use this tag just because your equations contain "everyday operations" like $\times$, $+$!

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So let's remember that $X$ is an operator, and $\langle X \rangle$ is just a number, and we can use the definition of the expectation value $\langle O \rangle = \langle \psi | O | \psi \rangle$ to wor …
answered Nov 5 '14 by Jason B.