  The oscillations of ultra-relativistic neutrinos are realized by the
propagation of assumed zero-mass on-shell neutrinos with the speed of light in
vacuum combined with the phase modulation by the small mass term
$\exp[-i(m^{2}_{\nu_{k}}/2|\vec{p}|)\tau]$ with a time parameter $\tau$. This
picture is realized in the first quantization by the mass expansion and in
field theory by the use of $\delta(x^{0}-y^{0}-\tau) \langle 0|T^{\star}\nu_{L
k}(x)\overline{\nu_{L k}(y)}|0\rangle$ with the neutrino mass eigenstates
$\nu_{L k}$ and a finite positive $\tau$ after the contour integral of the
propagating neutrino energies.
  By noting that the conventional detectors are insensitive to neutrino masses,
the measured energy-momenta of the initial and final states with assumed
zero-mass neutrinos are conserved. The propagating neutrinos preserve the
three-momentum in this sense but the energies of the massive neutrinos are
conserved up to uncertainty relations and thus leading to oscillations.
Conceptual complications in the case of Majorana neutrinos due to the charge
conjugation in $d=4$ are also discussed.
