Operators (or variables in quantum mechanics) do not necessarily commute. We can compute the commutator of two variables, for example

\begin{displaymath}\bgroup\color{black}[p,x]\equiv px-xp={\hbar\over i}.\egroup\end{displaymath}

Later we will learn to derive the uncertainty relation for two variables from their commutator. We will also use commutators to solve several important problems.

Jim Branson 2013-04-22