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. The consequences for proposals involving the ${}^7\mathrm{Be}$ 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.

• Received 4 March 1998

DOI:https://doi.org/10.1103/PhysRevD.58.036006