  Why are different mass states coherent? What is the correct formula for the
oscillation phase? How can textbook formulas for oscillations in time describe
experiments which never measure time? How can we treat the different velocities
and different transit times of different mass eigenstates and avoid incorrect
factors of two? How can textbook forumulas which describe coherence between
energy states be justified when Stodolsky's theorem states there is no
coherence between different energies? Is covariant relativistic quantum field
theory necessary to describe neutrino oscillations? How important is the
detector, which is at rest in the laboratory and cannot be Lorentz tranformed
to other frames?
  These questions are answered by a simple rigorous calculation which includes
the quantum fluctuations in the position of the detector and in the transit
time between source and detector. The commonly used standard formula for
neutrino oscillation phases is confirmed. An "ideal" detector which measures
precisely the energy and momentum of the neutrino destroys all phases in the
initial wave packet and cannot observe oscillations. A realistic detector
preserves the phase differences between neutrinos having the same energy and
different momenta and confirms the standard formula. Whether phase differences
between neutrinos with different energies are observable or destroyed by the
detector is irrelevant.
