  We point out that many wavepacket discussions for the coherence properties of
particle beams are unnecessary since they deal with stationary sources; and
when the problem is stationary, essentially all information is in the energy
spectrum. This recognition allows a simple answer to a number of long-debated
points, usually framed in terms of "the length of the wavepacket". In
particular we discuss neutrino oscillations, and some issues in neutron
physics. The question as to whether two simple beams with the same energy
spectrum are distinguishable is answered negatively for stationary situations.
The question as to whether neutrino oscillations should be thought of as taking
place between states of the same energy or the same momentum is answered in
favor of energy for stationary situations. Consequences for proposals involving
the 7Be neutrino line of the sun, the observation of oscillations in supernova
neutrinos and wavepacket studies with neutrons are briefly discussed, as well
as the connection with the coherence notions of quantum optics.
