  Lorentz-covariant harmonic oscillator wave functions are constructed from the
Lorentz-invariant oscillator differential equation of Feynman, Kislinger, and
Ravndal for a two-body bound state. The wave functions are not invariant but
covariant. As the differential equation contains the time-separation variable,
the wave functions contain the same time-separation variable which does not
exist in Schr\"odinger wave functions. This time-separation variable can be
shown to belong to Feynman's rest of the universe, and can thus be eliminated
from the density matrix. The covariant probability interpretation is given.
This oscillator formalism explains Feynman's decoherence mechanism which is
exhibited in Feynman's parton picture.
