  Based on Feynman's lifetime efforts on quantum mechanics and relativity, it
is concluded that the basic difference between field theory and string theory
is that field theory is based on running waves while string theory should deal
with standing waves in the Lorentz-covariant regime.
  At the 1970 spring meeting of the American Physical Society held in
Washington, DC, R. P. Feynman stunned the audience by proposing harmonic
oscillators for relativistic bound states, instead of Feynman diagrams. His
talk was later published in the paper of Feynman, Kislinger, and Ravndal [Phys.
Rev. D, Vol. 3, 2706 (1971)]. These authors noted that the hadron mass spectra
can be predicted by the degeneracy of the three-dimensional harmonic
oscillators. In so doing, they started with the Lorentz-invariant differential
equation for the harmonic oscillator, and obtained Lorentz invariant solutions.
However, their solutions are not normalizable in the time-separation variable
and cannot carry probability interpretation. It is pointed out that there are
solutions normalizable in the time-separation variable within the framework of
Wigner's little-group representation of the Poincar\'e group. These solutions
are not invariant but covariant under Lorentz transformations. These solutions
give a covariant bound-state model which gives the quark model and the parton
model as two different limiting cases, in the low- and high-speed limits
respectively.
