  We perform a calculation in quantum field theory of neutrino oscillation
probabilities, where we include simultaneously the source, detector, and
neutrino fields in the Hamiltonian. Within the appropriate limits associated
with current neutrino oscillation experiments, we recover the standard
oscillation formula. On the other hand, we find that the dominant contributions
to the amplitude are associated with different neutrino mass eigenstates being
emitted at different times, such that they arrive at the detector at the same
time. This is contrary to the neutrino wave packet picture, where they are
emitted simultaneously and separate as they travel to the detector. This has
direct consequences regarding the mechanisms that lead to a damping of neutrino
oscillations for very long baselines. Our analysis also provides a pedagogical
example of a measurement process in quantum mechanics.
