  The von Neumann theory of measurement, based on an entanglement of the
quantum observable with a classical machine followed by decoherence or
collapse, does not readily apply to most measurements of momentum. Indeed, how
we measure the momentum of a quantum particle is not even discussed in most
quantum mechanics textbooks. Instead, we often teach the lore that position and
momentum cannot be measured at the same time. Yet, most ways to measure
momentum actually involve measuring position to infer momentum. In this
tutorial review, I examine real experiments that measure momentum and describe
how one can improve our teaching of the theory of measurement when we focus on
real experiments, rather than abstract mathematical models of measurement.
