  Since quarks are regarded as the most fundamental particles which constitute
hadrons that we observe in the real world, there are many theories about how
many of them are needed and what quantum numbers they carry. Another important
question is what keeps them inside the hadron, which is known to have
space-time extension. Since they are relativistic objects, how would the hadron
appear to observers in different Lorentz frames? The hadron moving with speed
close to that of light appears as a collection of Feynman's partons. In other
words, the same object looks differently to observers in two different frames,
as Einstein's energy-momentum relation takes different forms for those
observers. In order to explain this, it is necessary to construct a quantum
bound-state picture valid in all Lorentz frames. It is noted that Paul A. M.
Dirac studied this problem of constructing relativistic quantum mechanics
beginning in 1927. It is noted further that he published major papers in this
field in 1945, 1949, 1953, and in 1963. By combining these works by Dirac, it
is possible to construct a Lorentz-covariant theory which can explain hadronic
phenomena in the static and high-speed limits, as well as in between. It is
shown also that this Lorentz-covariant bound-state picture can explain what we
observe in high-energy laboratories, including the parton distribution function
and the behavior of the proton form factor.
